Multifunctional and Nanoreinforced Polymers for Food Packaging (Woodhead Publishing in Materials)

Multifunctional and nanoreinforced polymers for food packaging

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Related titles: Innovations in food labelling (ISBN 978-1-84569-676-4) Increasingly, consumers desire information about the health, safety, environmental and socioeconomic characteristics of food products. These traits often cannot be detected by sight, smell or taste; therefore, consumers must use food labels to select products that meet their needs and preferences. The growing consumer and industry interest in food labels presents challenges for governments, which must ensure that the product information is accurate, truthful and not misleading to consumers. With the increase in global trade in food, there is also a need to harmonize food labels so that product information is relevant to foreign markets. Innovations in food labelling provides information about the principles and requirements of food labelling and reviews the latest trends in this important area. Development of packaging and products for use in microwave ovens (ISBN 978-1-84569-420-3) Improving the quality and safety of microwavable convenience food products is a priority for manufacturers. Development of packaging and products for use in microwave ovens provides a comprehensive review of this important area. Written by a distinguished team of international contributors, the text discusses the principles, properties of ingredients, materials issues, design, product development and safety of packaging for use in microwaves. Passive and active packaging is explored in detail with an emphasis on practical issues, in addition to the computer simulation of microwave heating of foods in both types of container. Environmentally compatible food packaging (ISBN 978-1-84569-194-3) Food packaging performs an essential function, but packaging materials can have a negative impact on the environment. This collection reviews bio-based, biodegradable and recycled materials and their current and potential applications for food protection and preservation. The first part of the book focuses on environmentally-compatible food packaging materials. The second part discusses drivers for using alternative packaging materials, such as legislation and consumer preference, environmental assessment of food packaging and food packaging eco-design. Chapters on the applications of environmentally-compatible materials for particular functions, such as active packaging, and in particular product sectors then follow. Details of these and other Woodhead Publishing materials books can be obtained by: · visiting our web site at www.woodheadpublishing.com · contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 832819; tel.: +44 (0) 1223 499140 ext. 130; address: Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; email: francis.dodds@woodhead publishing.com). Please confirm which subject areas you are interested in.

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Multifunctional and nanoreinforced polymers for food packaging Edited by JoseÂ-MarõÂa LagaroÂn

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Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi ± 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited ß Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publisher cannot assume responsibility for the validity of all materials. Neither the authors nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-84569-738-9 (print) ISBN 978-0-85709-278-6 (online) The publisher's policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acidfree and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Godiva Publishing Services Limited, Coventry, West Midlands, UK Printed by TJI Digital, Padstow, Cornwall, UK

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Contents

1

Contributor contact details

xv

Preface

xix

Multifunctional and nanoreinforced polymers for food packaging LAGAROÂN,

J.-M. Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction Structural factors governing barrier properties Novel polymers and blends Nanocomposites Future trends References Appendix: Abbreviations

1

1 7 15 21 25 25 28

Part I Nanofillers for plastics in food packaging 2

Multifunctional nanoclays for food contact applications

J.-M. LAGAROÂN and M.-A. BUSOLO, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain 2.1 2.2 2.3 2.4 2.5

Introduction Antimicrobial nanoclays Oxygen-scavenging nanoclays Future trends References

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31 33 37 39 39

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Contents

3

Hydrotalcites in nanobiocomposites

3.1 3.2 3.3

U. COSTANTINO and M. NOCCHETTI, University of Perugia, Italy and G. GORRASI and L. TAMMARO, University of Salerno, Italy

3.5 3.6

Introduction Hydrotalcite-like compounds (HTlc): basic chemistry Organically modified biocompatible hydrotalcite-like compounds (HTlc) Nanocomposites of biodegradable polymeric matrices and modified hydrotalcites Conclusions and future trends References and further reading

4

Cellulose nanofillers for food packaging

3.4

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

5

R. T. OLSSON and L. FOGELSTROÈM, Royal Institute of Technology, Sweden, M. MARTIÂNEZ-SANZ, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain and M. HENRIKSSON, Royal Institute of Technology, Sweden and SP Technical Research Institute of Sweden, Sweden Introduction Morphological and structural characteristics of cellulose nanofillers Extraction and refining of cellulose nanofillers Mechanical properties of cellulose nanofillers Surface modification of cellulose nanofillers Preparation of cellulose-reinforced nanocomposites Future trends and applications of cellulose nanofillers References

Electrospun nanofibers for food packaging applications

S. TORRES-GINER, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

5.1 5.2 5.3 5.4 5.5 5.6

Electrospinning Functional nanofibers Nanoencapsulation Electrospinning in packaging applications Future trends References

Part II High barrier plastics for food packaging

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43 45 52 67 75 77

86

86 87 91 95 96 99 101 102

108

108 113 116 119 121 123

Contents

6

Mass transport and high barrier properties of food packaging polymers

F. NILSSON and M. S. HEDENQVIST, Royal Institute of Technology, Sweden 6.1 6.2 6.3 6.4 6.5 6.6

Introduction: the basics of mass transport Diffusivity Solubility What makes a barrier a barrier? Characterisation techniques References

7

Ethylene±norbornene copolymers and advanced single-site polyolefins T. J. DUNN, formerly at Printpack, Inc., USA

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

8

Introduction Synthesis and molecular structure: advanced single-site polyolefins Macromolecular structure: advanced single-site polyolefins Macromolecular structure: ethylene±norbornene copolymers Nanocomposite preparation: advanced single-site polyolefins Future trends Sources of further information and advice References

Advances in polymeric materials for modified atmosphere packaging (MAP)

T. K. GOSWAMI, Indian Institute of Technology, India and S. MANGARAJ, CIAE, India

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12

Introduction Modified atmosphere packaging (MAP) Physiological factors affecting shelf-life of fresh produce Post-harvest pathology of fruits and vegetables Response of fresh produce to modified atmosphere packaging Polymeric films for application in modified atmosphere packaging (MAP) Cellulose-based plastics Biodegradable polymers Multilayer plastic films Gas permeation or gas transmission Water vapor permeability Packaging systems in modified atmosphere packaging (MAP)

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129

129 130 131 143 146 149

152 152 153 154 155 156 160 161 161

163

163 167 173 188 189 197 204 204 205 208 211 214

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Contents

8.13

8.18 8.19

Advanced technology for efficient modified atmosphere packaging (MAP) Package management Design of modified atmosphere packaging (MAP) Mathematical modeling of gaseous exchange in modified atmosphere packaging (MAP) systems Current application of polymeric films for modified atmosphere packaging (MAP) of fruits and vegetables Future trends References and further reading

9

Nylon-MXD6 resins for food packaging

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Structure and general overview Processing Gas barrier properties Other properties Applications Nylon-MXD6 nanocomposites Future trends References

10

Ethylene±vinyl alcohol (EVOH) copolymers

8.14 8.15 8.16 8.17

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

11

A. AMMALA, CSIRO Materials Science and Engineering, Australia

A. LOÂPEZ-RUBIO, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain Introduction Structure and general properties of ethylene±vinyl alcohol (EVOH) copolymers Ethylene±vinyl alcohol (EVOH) versus aliphatic polyketones Processing in packaging Improving retorting of ethylene±vinyl alcohol (EVOH) Nanocomposites of ethylene±vinyl alcohol (EVOH) and poly(vinyl) alcohol (PVOH) Future trends References

High barrier plastics using nanoscale inorganic films

V. TEIXEIRA, J. CARNEIRO, P. CARVALHO, E. SILVA, S. AZEVEDO and C. BATISTA, University of Minho, Portugal 11.1

Introduction

215 220 221 222 223 226 228

243

243 244 246 250 253 255 258 259

261

261 262 265 266 271 276 280 281

285

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11.2 11.3 11.4 11.5 11.6 11.7 11.8

12

Contents

ix

Nanotechnologies of thin films for advanced food packaging Thin film technologies for polymer coating using vacuum processes Physical vapour deposition (PVD) processes Inorganic thin film systems Functional properties of diffusion barrier coated polymers Future trends References

287 290 294 299 303 310 311

Functional barriers against migration for food packaging

316

Introduction Food safety issues related to migration Functional barriers Nanostrategies for functional barriers Future trends Sources of further information and advice References and further reading

316 317 319 335 338 339 340

C. JOHANSSON, Karlstad University, Sweden

12.1 12.2 12.3 12.4 12.5 12.6 12.7

Part III Active and bioactive plastics 13

Silver-based antimicrobial polymers for food packaging

A. MARTIÂNEZ-ABAD, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain 13.1 13.2 13.3 13.4 13.5 13.6

Introduction Incorporation of silver into coatings and polymer matrices Antimicrobial silver in food packaging Future trends Sources of further information and advice References and further reading

14

Incorporation of chemical antimicrobial agents into polymeric films for food packaging

BALDEV RAJ, R. S. MATCHE and R. S. JAGADISH, Central Food Technological Research Institute, India

14.1 14.2 14.3 14.4

Introduction Antimicrobial agents Chemical antimicrobial agents Natural antimicrobial agents

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347 350 356 359 361 362

368

368 371 372 380

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Contents

14.5 14.6 14.7 14.8 14.9 14.10 14.11

Polymers (synthetic or natural) Nano-antimicrobial agents Antimicrobial films and coatings Antimicrobial activity Future trends References Appendix: Abbreviations

15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8

16

16.1 16.2 16.3 16.4 16.5 16.6

Natural extracts in plastic food packaging

P. SUPPAKUL, Kasetsart University, Thailand

Introduction Natural plant extracts as antimicrobials and antioxidants Designing active plastic packaging systems from natural plant extracts Packaging films based on natural extracts Factors to consider in designing active systems Future trends Sources of further information and advice References and further reading

Bioactive food packaging strategies

A. LOÂPEZ-RUBIO, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain Introduction Definition and technologies Nanotechnologies Controlled release of bioactives Future trends References and further reading

389 390 393 403 404 404 420

421 421 422 430 434 445 448 449 450

460

460 461 470 473 475 476

Part IV Nanotechnology in sustainable plastics for food packaging 17

Polylactic acid (PLA) nanocomposites for food packaging applications

J.-M. LAGAROÂN, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain 17.1 17.2 17.3 17.4

Introduction and properties of polylactic acid (PLA) Nanobiocomposites of polylactic acid (PLA) for monolayer packaging Future trends References and further reading ß Woodhead Publishing Limited, 2011

485

485 486 493 494

Contents

18

18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8

19

xi

Polyhydroxyalkanoates (PHAs) for food packaging 498

D. PLACKETT and I. SIROÂ, Technical University of Denmark, Denmark

Introduction Commercial developments Polyhydroxyalkanoates (PHAs) and their nanocomposite films Polyhydroxyalkanoate (PHA) foams and paper coatings Conclusions Future trends Sources of further information and advice References

Starch-based polymers for food packaging

R. M. GONZAÂLEZ and M. P. VILLANUEVA, Technological Institute of Plastic (AIMPLAS), Spain

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9

Introduction Market for starch-based materials and potential applications Structure and properties of native and plasticized starch Processing in packaging Mechanical and barrier performance of starch-based systems Nanocomposites Future trends Sources of further information and advice References

20

Chitosan polysaccharide in food packaging applications

P. FERNANDEZ-SAIZ, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8

Introduction Structure and properties Processing in packaging Antimicrobial chitosan Barrier performance Nanocomposites Future trends References

21

Carrageenan polysaccharides for food packaging

21.1

Introduction

M. D. SANCHEZ-GARCIA, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

498 500 502 515 516 517 518 518

527

527 528 531 537 542 546 557 559 560

571

571 572 573 574 582 584 586 587

594

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Contents

21.2 21.3 21.4 21.5 21.6

Structure and properties of carrageenan Processing in packaging Barrier performance Nanocomposites References and further reading

595 597 598 601 606

22

Protein-based resins for food packaging

610

Materials (sources, extraction, structure and properties) Structure and properties Packaging materials characterization (barrier performance, mechanical properties) Applications Future trends References

610 618

22.1 22.2 22.3 22.4 22.5 22.6

23

A. A. VICENTE, M. A. CERQUEIRA and L. HILLIOU, University of Minho, Portugal and C. M. R. ROCHA, University of Porto, Portugal

Wheat gluten (WG)-based materials for food packaging H. ANGELLIER-COUSSY, V. GUILLARD, C. GUILLAUME and N. GONTARD, University of Montpellier II, France

23.1 23.2 23.3 23.4 23.5 23.6 23.7

24

Introduction Preparation of wheat gluten-based materials Mechanical and barrier properties of wheat gluten-based materials Wheat gluten-based nanocomposites Example of integrated approach for the packaging of fresh fruits and vegetables Future trends References

Safety and regulatory aspects of plastics as food packaging materials

BALDEV RAJ and R. S. MATCHE, Central Food Technological Research Institute, India 24.1 24.2 24.3 24.4 24.5

Introduction Indirect food additives Nanotechnology in food contact materials Migration of additives Indian Standards for overall migration (IS:9845-1998)

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622 634 638 638

649

649 650 652 658 661 664 664

669

669 670 673 674 677

Contents 24.6 24.7 24.8 24.9 24.10 24.11 24.12

xiii

US Food and Drug Administration (US FDA), Code of Federal Regulations (CFR) 681 European Commission Directives on plastic containers for foods 682 Specific migration of toxic additives 684 Recent problems in specific migration 687 Future trends 687 References and further reading 689 Appendix: Abbreviations 691 Index

692

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Contributor contact details

Chapter 4

(* = main contact)

Editor and Chapters 1, 2 and 17 Professor Dr JoseÂ-MarõÂa LagaroÂn Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 3 Umberto Costantino* and Morena Nocchetti Department of Chemistry University of Perugia 06123 Perugia Italy E-mail: [email protected] Giuliana Gorrasi and Loredana Tammaro Chemical and Food Engineering Department University of Salerno 84084 Fisciano (SA) Italy

Assistant Professor Richard T. Olsson* and Dr Linda FogelstroÈm Department of Fibre and Polymer Technology School of Chemical Science and Technology Royal Institute of Technology Teknikringen 56±58 SE-100 44 Stockholm Sweden E-mail: [email protected] Marta MartõÂnez-Sanz Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain Dr Marielle Henriksson Department of Fibre and Polymer Technology School of Chemical Science and Technology Royal Institute of Technology Teknikringen 56±58 SE-100 44 Stockholm Sweden

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Contributor contact details

Chapter 8

and SP Technical Research Institute of Sweden P.O. Box 5609 SE-114 86 Stockholm Sweden

Chapter 5 Dr Sergio Torres-Giner Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 6 Fritjof Nilsson and Professor Michael S. Hedenqvist* School of Chemical Science and Engineering Fiber and Polymer Technology Royal Institute of Technology SE-100 44 Stockholm Sweden E-mail: [email protected]

Tridib Kumar Goswami* Department of Agricultural and Food Engineering Indian Institute of Technology Kharagpur West Bengal 721302 India E-mail: [email protected] Shukadev Mangaraj CIAE Nabibagh Berasia Road Bhopal 462038 (MP) India E-mail: [email protected] [email protected]

Chapter 9 Dr Anne Ammala CSIRO Materials Science and Engineering Private Bag 33 Clayton South MDC Victoria 3169 Australia E-mail: [email protected]

Chapters 10 and 16

Chapter 7 Thomas J. Dunn Flexpacknology LLC 2526B Mt Vernon Road Atlanta GA 30338 USA E-mail: [email protected]

Dr Amparo LoÂpez-Rubio Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

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Contributor contact details

Chapter 11 Vasco Teixeira*, Joaquim Carneiro, Pedro Carvalho, Emanuel Silva, Sofia Azevedo and Carlos Batista University of Minho Physics Department GRF-Functional Coatings Group Campus de AzureÂm 4800-058 GuimaraÄes Portugal E-mail: [email protected]

Chapter 12 Associate Professor Caisa Johansson Karlstad University Faculty of Technology and Science Department of Chemical Engineering SE-651 88 Karlstad Sweden E-mail: [email protected]

Chapter 13 Antonio MartõÂnez-Abad Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 14 Baldev Raj*, Rajeshwar S. Matche and R. S. Jagadish Food Packaging Technology Department Central Food Technological Research Institute Mysore 570020

xvii

India E-mail: [email protected] [email protected] [email protected]

Chapter 15 Assistant Professor Dr Panuwat Suppakul Department of Packaging and Materials Technology Faculty of Agro-Industry Kasetsart University Agro-Industry Building V 50 Phaholyouthin Road Ladyao Chatuchak Bangkok 10900 Thailand E-mail: [email protected]

Chapter 18 David Plackett* and IstvaÂn Siro Solar Energy Programme Risù National Laboratory for Sustainable Energy Technical University of Denmark 4000 Roskilde Denmark E-mail: [email protected]

Chapter 19 R. M. GonzaÂlez* and M. P. Villanueva Extrusion Department Technological Institute of Plastic (AIMPLAS) Calle Gustave Eiffel 4 (Parque TecnoloÂgico) 46980 Paterna Valencia Spain E-mail: [email protected]

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Contributor contact details

Chapter 20 P. Fernandez-Saiz Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 21 M. D. Sanchez-Garcia Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 22 AntoÂnio A. Vicente* and Miguel A. Cerqueira IBB ± Institute for Biotechnology and Bioengineering Centre of Biological Engineering Universidade do Minho Campus de Gualtar 4710-057 Braga Portugal E-mail: [email protected] [email protected] LoõÈc Hilliou Institute for Polymers and Composites/I3N University of Minho Campus de AzureÂm 4800-058 GuimaraÄes

Portugal E-mail: [email protected] Cristina M. R. Rocha REQUIMTE Departamento de Engenharia QuõÂmica Faculdade de Engenharia Universidade do Porto Rua Dr Roberto Frias 4200-465 Porto Portugal

Chapter 23 Dr H. Angellier-Coussy, Dr V. Guillard, Dr C. Guillaume and Pr N. Gontard* Unite Mixte de Recherche IngeÂnierie des AgropolymeÁres et Technologies Emergentes INRA/ENSA.M/UMII/CIRAD Universite Montpellier II CC023, pl. E Bataillon 34095 Montpellier Cedex France E-mail: [email protected]

Chapter 24 Baldev Raj* and Rajeshwar S. Matche Food Packaging Technology Department Central Food Technological Research Institute Mysore 570 020 India Email: [email protected] [email protected]

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Preface

The current book intends to review the latest developments in the functionalization of high performance plastic materials for food packaging applications. Various polymers, biopolymers and their composites `reinforced' with various organic, inorganic or hybrid engineered nano- or biomaterials, are described which help ensure, or even enhance, the quality and safety of packaged foods. Extending the shelf-life of foods has become of primary interest across the food chain in order to facilitate logistics during production, handling, storage, transportation, presentation by the retailer and even disposal, and to avoid substantial losses due to the deterioration of packaged food quality and safety. An extensive review of the most advanced packaging technologies based on the use of polymers, with special emphasis on polymer-based nanocomposites is presented. In the first chapters of the book several `natural' nanotechnologies of promising value in the food packaging area such as passive and active nanoclays and hydrotalcites, cellulose nanowhiskers and electrospun nanofibres and nanocapsules are presented. These are later discussed in regard to their value in enhancing the physical (chiefly barrier) properties against the transport of low molecular weight components and UV light, their role in modified atmosphere packaging, heat sterilization or retorting, active (antimicrobial, oxygen scavenging, antioxidant, etc.) and bioactive (consumer health promoting) packaging and to provide functional barriers against migration. Finally, an updated chapter on legislation completes the book. JoseÂ-M. LagaroÂn

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Multifunctional and nanoreinforced polymers for food packaging  N , Novel Materials and Nanotechnology Group, J.-M. LAGARO IATA-CSIC, Spain

Abstract: The packaging industry has been implementing at a rapidly expanding rate the number of packaging elements made of plastics over recent decades. Plastics, in contrast to more traditional packaging materials like glass and metals, (1) are permeable to the exchange of low molecular weight compounds such as gases and vapours, (2) undergo sorption, so-called scalping, of packaged food constituents, and (3) are amenable to migration into foodstuffs of packaging constituents. Despite these drawbacks, the availability of shapes and forms in which plastics can be conformed, their ease of processing and handling, their low price, their excellent chemical resistance, etc., have made them very attractive in packaging applications. Consequently, a lot of industrial and academic research has been devoted to understanding the mechanisms of mass transport in polymers in order to design new materials and composites with balanced physical properties in general and with improved barrier properties in particular, and to add additional functionalities which may take advantage of their permeability characteristics to positively actuate on the product. This chapter first highlights the factors that make polymers become more impermeable, putting special emphasis on nanotechnology approaches, and then reviews some of the general advances made in the field. Key words: nanotechnology, high barrier polymers/plastics, biopolymers/ bioplastics, packaging, food technology, transport properties.

1.1

Introduction

1.1.1

High barrier concept

High barrier is without doubt a highly desirable property of polymeric materials intended to be used in many packaging applications. The term high barrier usually refers to the low to very low permeability of a material to the transport of low molecular weight chemical species, like gases and vapours. Usually, the lower-limit definition for high barrier typically refers to the performance of PET polymers. However, this property has perhaps never attracted so much attention from industry as over the last decades, when it began to be pursued by some modern food and beverage packaging technologies making use of plastic materials.1±3 In this respect, high barrier has attracted a great deal of recent

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Multifunctional and nanoreinforced polymers for food packaging

attention from industry as it has become associated with primary objectives such as commercialization of perishable foods far away from their origin, food shelflife extension and maintaining food quality and safety. Furthermore, it has also become very relevant to a number of other applications including gas separation membranes, packaging of healthcare products, pharmaceuticals and chemicals, and housing of fuels and oxygenated fuels in fuel tanks and lines in the automotive field. The reason for the more recent interest in the development of high barrier polymers and polymer-based structures rests on a widespread trend to implement polymeric materials in an ever-increasing number of applications, in many cases aiming to substitute them for other, more traditional packaging materials. It is common knowledge that the attractiveness of plastics lies in their versatility and ability to offer a broad variety of properties and yet be cheap and easily processed and conformed into a myriad of shapes and sizes. However, polymers do have a number of limitations for certain applications when compared with more traditional materials like metals and alloys or ceramics. Among some of these limitations relevant to the purpose of this chapter are their permeability and comparatively low thermal resistance, and the strong interdependence between these two properties. The permeability of plastics to the exchange of gases and vapours imposes a number of challenges in those applications where high barrier, ideally impermeability, is required. These applications were, for instance, traditionally assumed by tinplate and glass in the food packaging field. However, polymer scientists, engineers and technologists in industry and academia have pulled together a great deal of effort and resources to push the limits of plastics performance towards impermeability, chiefly due to the overwhelming pressure exerted by the numerous other advantages associated with the use of plastics in high barrier applications. Table 1.1 gives typical oxygen permeability and water permeability values for a number of commercial polymers and structures used in food packaging applications.4

1.1.2

Functional packaging

The concept of functional or active/bioactive/intelligent packaging for food applications has been recently exploited, obtaining for the package an active role in the preservation, health-promoting capacity and provision of information concerning the products. Among these, active packaging is perhaps the area that has steered more research and industrial interest. Packages may be termed active when they perform some desired role in food preservation other than providing an inert barrier to external conditions. The opportunity of modifying the inner atmosphere of the package or even the product by simply incorporating certain substances in the package wall has made this group of technologies very attractive, representing an increasingly productive research area. Even though

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Multifunctional and nanoreinforced polymers for food packaging

3

Table 1.1 Water permeability (at 38ëC and 90% RH) and oxygen permeability (at 23ëC) of a number of commercial plastics and multilayer structures Material

PVOH EVOH PAN PAN (70% AN) PVDC PA6 aPA (amorphous) PET PP PC LDPE LCP PET/PVDC PA/PVDC PP/PVDC PET-met. PET/AlOx/PE PET/SiOx/PE PA/SiOx/PE PP/SiOx/PE PLA PLA PHB PHB PHBV PCL PCL PCL

Water permeability 1018 kg m/(m2 s Pa)

Oxygen permeability 1021 m3 m/(m2 s Pa) 0% RH

75% RH

485 000 17 000 2420 8250 30.53 20 600 2420 2300 726 19 400 1200 10 170 160 43 58 21 16 32 13 12 600

0.17 0.77 1.9 10.5 4.5 52 83 135 6750 10 500 21 500 0.42 17.5 18.2 25 3.5 7 4.9 7.7 81 2250

900 91

1689

230

6900 26 600

1590 4380 934 1960

31 225 60

15

2209 1750 5100 3010 7850

Reference of data source

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 6 7 8 7 7 9 10

the first active packaging developments and most of the commercialized technologies consist of sachet technologies, which make use of a small permeable pouch (sachet) containing the active compound that is inserted inside the package, current trends tend towards the incorporation of active ingredients directly into the packaging wall. This strategy is associated with a number of advantages, such as reduction in package size, higher effectiveness of the active principles (which are now completely surrounding the product), and, in many cases, higher throughput in packaging production, since the additional step of incorporating the sachet is eliminated.7 Polymers, and in particular biomassderived polymers, are the preferred materials for active packaging because of their intrinsic properties, constituting an ideal carrier for active principles, with the advantage of being tuneable in terms of controlled release and the possibility

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Multifunctional and nanoreinforced polymers for food packaging

of combining several polymers through blending or multilayer extrusion to tailor the application. Active packaging has been used with many products and is under investigation for numerous others. These new food packaging technologies have been developed as a response to trends in consumer preferences towards mildly preserved, fresh, tasty, healthier, and convenient food products with prolonged shelf-life. These novel packaging technologies can also be used to compensate for shortcomings in the packaging design, for instance in order to control the oxygen, water or carbon dioxide levels in the package headspace. In addition, changes in retail practices, such as globalization of markets resulting in longer distribution distances, present major challenges to the food packaging industry, which finally act as driving forces for the development of new and improved packaging concepts that extend shelf-life while maintaining the safety, quality and health aspects of the packaged foods. The combinations of polymers and active substances that can be studied for potential use as active packages are in principle unlimited and it is forecast that the number of applications will increase in the near future. Among the existing active packaging technologies, oxygen scavengers and antimicrobial packaging stand out over the other developments. Both technologies were initially based on the sachet concept, using reducing and inhibitory substances, respectively. Lately, the growth in both areas has been enormous, especially in the case of antimicrobials. Other active packaging applications include systems capable of absorbing carbon dioxide, phase-changing materials, moisture, ethylene and/or flavour/odour taints; releasing carbon dioxide and/or flavour/odour. Traditionally, plastic food packaging has been related to negative food safety issues, due mainly to problems with migration of packaging components. In more recent trends, packaging is being designed more favourably to impact on consumer health by integrating functional ingredients in the packaging structure, through so-called bioactive packaging strategies.8 Novel active and bioactive packaging technologies, combined with bioplastics and nanotechnology, can best help do this. Therefore, proper combination of these technological cornerstones will provide innovation in the food packaging sector over the next few years. Furthermore, due to the shortage of oil resources and waste-management issues, research focus is shifting from synthetic oil-based plastics to biomassderived biodegradable and environmentally friendly polymers. The drawbacks that initially characterized these biopolymers in terms of poor barrier properties and high instability have, in turn, resulted in novel applications, making highly permeable and water-plasticizable biopolymers an ideal partner for active and bioactive packaging where the package is no longer a passive barrier, but actively contributes to the preservation of food by controlled release of the substances. Biopolymers are, thus, the ideal matrix for the incorporation and controlled release of a number of substances to be added to the food. Probably

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the area that is evolving more quickly is the antimicrobial packaging one, but it is foreseen that biopackages will also serve as reservoirs for vitamins, antioxidants, and pre- and probiotics.

1.1.3

Phenomenology of transport in polymers

According to the above, barrier properties in polymers are necessarily associated with their inherent ability to permit the exchange, to a higher or lower extent, of low molecular weight substances through mass-transport processes like permeation. The phenomenology of permeation of low molecular weight chemical species through a polymeric matrix is generally envisaged down to the molecular level as a combination of two processes, i.e. solution of the solutes and molecular diffusion.11 A permeating gas is first dissolved into the upstream face of the polymer film, and then undergoes a molecular diffusion to the downstream face of the film through typically the polymer amorphous phase, where it evaporates into the external phase again. A solution±diffusion mechanism is thus applied, which can be formally expressed in terms of permeability (P), solubility (S) and diffusion (D) coefficients as follows: P ˆ DS

1:1

This permeability coefficient derives from application of Henry's law of solubility to Fick's first law of diffusion as follows: Jˆ

q @c S
p
p ql ˆ ÿD ˆD ˆ DS ) P ˆ DS ˆ At @x l l At
p

1:2

The solubility coefficient S is thermodynamic in nature and is defined as the ratio of the equilibrium concentration of the dissolved penetrant in the polymer to its partial pressure (p) in the gas phase (Henry's law). In polymers, Henry's law is usually obeyed at low penetrant concentrations, i.e. when S is independent of concentration (or of the partial pressure). D characterizes the average ability of the sorbed permeate to move through the polymer chain segments and is typically governed by Fick's first law of diffusion, i.e. the flux of the permeant (J) is proportional to the local gradient of concentration (c) through the thickness of the polymer film (l). During sorption kinetic experiments, if Fickian transport (case I) is assumed, linear behaviour in the penetrant uptake vs. the t1/2 (t being time) curve at small times is usually observed.12 Case II diffusion is defined when linear behaviour is observed in the uptake vs. t curve. This behaviour is observed in a number of systems and is associated with large uptakes and plasticization of the structure by the penetrant. When complex sorption behaviours like sigmoidal shapes are observed it is usually assumed that an `anomalous' or non-Fickian transport occurs. Nevertheless, from recent works a better rationalization of these `anomalous' behaviours has been achieved, in which contributions from the effect of macroscopic elastic constraints arising

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during the swelling process (geometrical effects) in adsorption experiments have been pointed out.13,14 Concerning the mechanisms of the mass-transport process through polymeric materials, two general approaches can be found, namely (1) molecular models studying the specific penetrant and chain motions in conjunction with the corresponding intermolecular forces, and (2) `free-volume' models which pay attention to the relations between the transport coefficients and the free volume existing in the polymeric matrix, without considering molecular-scale mechanisms. It is also relevant to emphasize here that the mass transport mechanisms, as well as their dependence on permeant partial pressure and testing temperature, are thought to be generally different depending on whether the polymer is in a rubbery or glassy state. Rubbery polymers are above their glass transition temperature (Tg) and, therefore, have very short relaxation times and respond quickly to physical changes. Thus, absorption of small molecules or penetrants causes immediate adjustments to a new equilibrium state and, consequently, there appears to exist a unique mode of penetrant transport for these polymers. Moreover, rubbery polymers are more amenable to show upwardly inflecting permeability responses with increasing penetrant partial pressure due to plasticization. This is typically the case in D-limonene, a common flavour component in fruit juices, in polymers like polyethylene and polypropylene. By comparison, glassy polymers are below their Tg and hence require on average long timescales to fully relax. Gas transport then typically occurs in glassy polymers under nonthermodynamic equilibrium conditions. In this case, penetrant molecules can allocate in holes or irregular cavities with very different diffusional mobility and, consequently, more than one mode of transport may be accessible. A `dual-mode sorption' model satisfactorily describes the dependence of transport properties on penetrant partial pressure in glassy polymers. This model postulates the existence of two different molecular populations dissolved in a glass: one dissolved by an ordinary dissolution process which can typically follow Henry's law (c ˆ Sp), and the other dissolved in a limited amount of fixed microcavities which can be described by a Langmuir-like isotherm: cˆ

cH bp 1 ‡ bp

1:3

In equation 1.3, cH is the hole saturation constant and b is the hole affinity constant. More complex sorption behaviours have also been postulated for other glassy materials. For instance, a modified dual-mode model requiring Langmuir and Flory±Huggings equations was suggested to explain the sorption of water in an amorphous polyamide.15 In what follows, we first overview some relevant structural factors defining and/or altering high gas barrier properties in polymers, and then comment on recent material developments in the field, i.e. blends, coatings and nanocomposites.

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1.2

7

Structural factors governing barrier properties

The structural factors determining inherent high barrier properties in polymers are fundamentally the chemistry, but there are also other relevant factors making a significant impact on barrier properties for a given chemistry, including polymer morphology (crystallinity, thermal history, amorphous density, molecular orientation, etc.), polymer molecular architecture (branches, molecular weight and tacticity), polymer plasticization, temperature, penetrant type and chemistry, and others.

1.2.1

Polymer chemistry

Nowadays, very many chemical combinations and high throughput and selective catalyst technologies are accessible via cutting-edge polymer chemistry, to generate polymeric materials with tailor-made structures and properties. As would be reasonable to expect, then chemistry is the basic and main defining factor determining barrier properties in polymeric materials. Thus, by varying the chemistry of the macromolecule, often by just adjusting the pendant group along the polymer chain, a significantly large variation in barrier properties can be achieved (see Table 1.2). Some commonly employed abbreviations applied to both well-known and new commercial plastics are listed in the Appendix. Behind the significant changes in barrier properties resulting from variations of chemistry are, for instance, the introduction of apolar voluminous groups at the low barrier side of the permeability spectrum, or the incorporation of small and strongly self-interacting chemical groups at the high barrier side of the permeability spectrum. The permeability of a polymer can change by up to six orders of magnitude depending on the grafted chemical groups attached to the polymer backbone. As is well known, most polymeric materials comprise exceedingly long high molecular weight molecules (called polymer chains) which for the case of the most widely used plastics, the thermoplastics family, do not have intermolecular links in the amorphous state other than secondary forces of, for instance, the van der Waals type. Consequently, the presence of Table 1.2 Relative oxygen permeability of polymer materials based on the repetition of CH2±CHX Polymer PVOH PAN PVC PP PS PE

Pending X unit

Relative O2 permeability

±OH ±CN ±Cl ±CH3 ±C6H5 ±H

1 4 800 15 000 42 000 48 000

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these different pendant groups can either disrupt or enhance the high intermolecular cohesion necessary to maintain high barrier efficiency against the transport of low molecular weight substances. Moreover, chemistry also defines the affinity between a potential permeant and the polymer matrix. As the process of permeation is a bimodal process comprising solution and diffusion, low solubility based on chemical disparity of a permeant and the polymer matrix will also result in low permeability, irrespective of whether the kinetics of diffusion are going to be favourable to the permeant transport. In this chapter, we will rather concentrate, due to their relevance and ease of generalization, on the barrier properties of non-interacting chemicals as is usually the case of the permanent gases. A physical magnitude called the cohesive energy density can be useful in helping to explain, quantify or even predict the behaviour in terms of barrier properties of polymeric materials. The cohesive energy of a substance in a condensed state is defined as the increase in internal energy per mole of substance if all the intermolecular forces are eliminated. For low molecular weight substances this energy can be experimentally calculated from the heat of evaporation. However, for polymers the cohesive energy density (defined as the cohesive energy per unit of volume) can be estimated using additive group contribution models like those devised by, for instance, Van Kreveland for cohesive energy and Traube for molar volume.16 These models propose contribution values for each of the chemical entities building up the polymer chain. Consequently, this parameter tells us about the strength of the interaction between molecules, and how this interaction changes when different chemical groups are added to the polymer chain. The cohesive energy density is often referred to as the square of the solubility parameter. Another important factor strongly associated with barrier properties is the free volume. The free volume comprehends the microcavities present in a polymeric material. Permeants make use of these cavities ± whether permanent or transient ± to diffuse through the polymer matrix. The transport properties of a permeant are therefore dependent on the number and size of these microcavities. This concept is usually expressed through the so-called fractional free volume parameter (Vf) and is indeed strongly related to chemistry (cohesive energy density), but it is also related to a number of other relevant factors having an impact on barrier properties like thermal history, polymer Tg, crystallinity and/or conformational order, etc. The fractional free volume Vf can be easily determined by the following simple equation: V ÿ V0 1:4 V where V is the specific volume of a particular polymer sample determined by density, and V0 is the specific volume at zero solubility (volume exclusively occupied by polymer chains). The latter parameter can be experimentally Vf ˆ

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determined by, for instance, extrapolation of experimental data17 or can be estimated from additive group contributions models. A very useful concept for free volume is that proposed by Cohen and Turnbull18,19 and Fujita20 through a general expression as follows: D / eÿBd =Vf

1:5

In this expression, D is the diffusion coefficient and Bd is a constant that depends only on the size of the penetrant molecule. This model has been shown to adequately describe the transport kinetics of organic vapours and small gas molecules in a number of polymers. More recent efforts have led to the development of an experimental methodology based on a technique called positronium annihilation spectroscopy. This methodology provides an experimental approach to determining free volume, as it enables one to measure hole size on a nanoscale and its fraction.21 Nevertheless, the absolute value of the fractional free volume cannot be directly obtained from only positron lifetime measurements. In spite of that, a study making use of positronium annihilation spectroscopy showed that there exists an excellent correlation between the oxygen permeability and a relative fractional free volume parameter as determined by this technique in a number of EVOH copolymers.22 From the experiments, it was clear that the fractional free volume in these materials does mainly concern the free volume size, as only the free volume size and not the orthopositronium o-Ps lifetime intensity, i.e. the number of holes, varied across composition in these polymers. It is, therefore, relevant to realize that high barrier polymers are the result of a permeable structure (amorphous phase) with a high cohesive energy density and very low fractional free volume. Figure 1.1 plots the oxygen permeability of a number of plastics, superimposed with the performance of bioplastics, vs. the ratio of the cohesive energy density to the fractional free volume. From this figure, it can be seen that EVOH copolymers (with 32 mol% ethylene) are one of the most efficient oxygen barrier materials due to their high intermolecular cohesion and low fractional free volume. Consequently, this material is being increasingly introduced in packaging applications where high barrier properties to gases are required. On the contrary, polymers like HDPE have much lower gas barrier properties due to low intermolecular cohesion and large fractional free volume. High intermolecular cohesion can, however, be distorted by for instance chemical alterations in the material (polymer degradation) due to thermal treatments.23 Polymer chain rigidity or polymer Tg also plays a relevant role in barrier properties since, as explained earlier, penetrant transport mechanisms are greatly altered depending on whether the permeation process occurs above (rubbery state) or below (glassy state) the polymer glass transition temperature. There is a very general trend that indicates that the higher the polymer Tg the lower the gas permeability and the better the permselectivity. However, this does not apply to common polymers like PS or PC which are very rigid glassy materials with

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1.1 PO2 (cm3 mm/m2 day atm) vs. the fractional free volume/cohesive energy density ratio for a number of polymers typically used in food packaging applications. References to the typical oxygen barrier properties of biopolymers are also included.

values of Tg above 100ëC and very high permeability. This is of course a consequence of the voluminous side groups which indeed reduce chain segment mobility due to steric hindrance but in turn generate large fractional free volumes. On the other hand, polymers like EVOH copolymers, PK copolymers or PVDC have lower values of Tg than for instance PS, PC or other materials like PET and yet have outstanding barrier properties. This is again due to the very high cohesive energy density and low fractional free volume exhibited by the former materials.

1.2.2

Polymer morphology

An important issue that has been implicit in all the previous considerations is the well-known characteristic that polymers are not able to fully crystallize due to metastability, some being in fact totally amorphous. Many polymers used in packaging applications have, therefore, a semicrystalline nature and hence are, from a structural viewpoint, heterogeneous materials. These polymers contain, under the most simplistic two-phase model visualization, both a fraction of chain segments constituting highly packed and conformationally ordered threedimensional structures ± polymer crystalline fraction () ± and another fraction in an amorphous state without conformational regularity and lateral order. As a large body of experimental evidence suggests that polymer crystals are impermeable to the transport of most low molecular weight substances, it is broadly accepted that the amorphous phase is the only phase available for permeation of these substances.

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It is therefore this particular structural feature, i.e. polymer crystallinity, together with a low intermolecular cohesion between polymer chains in the amorphous phase that best defines many of the most characteristic polymer properties, including permeability. However, polymer crystals not only fill the molecular structure of semicrystalline materials with microscopic impermeable blocks but also affect the surrounding amorphous phase. To begin with, the presence of crystallinity, its morphology (for instance, crystal width-to-thickness ratio) and orientation bring in additional considerations in terms of permeability as the penetrant molecules have to circumvent the crystallites, and thereby travel through a more tortuous diffusive path than in a fully amorphous material. This effect is usually accounted for in the calculations of the transport coefficients (see equation for diffusion below) by the so-called tortuosity or geometrical impedance factor ( ). Thus, the tortuosity factor is in essence the path length that a permeant has to travel across a film thickness divided by its actual thickness. Furthermore and as commented above, the presence of these crystalline blocks also affects the surrounding conformationally disordered amorphous phase. The constraining effects imposed by crystals to the chain segments in the amorphous phase typically depend on factors like crystal surface area and penetrant size. This phenomenon is substantiated from extensive mechanical and transport data, which clearly indicate that the segmental mobility of the non-crystalline fraction is much less than that in the fully amorphous polymer.24,25 This effect is accounted for in the calculations of the transport coefficients (see equation below) by the so-called chain immobilization factor (): Dsemicrystalline ˆ

Damorphous …1 ÿ † 

1:6

As a result of this, being aware of the implications of the crystallinity and its morphology on the barrier properties is, as a matter of fact, a relevant issue, because by adequate processing (thermal history) of polymers these parameters can be optimized to obtain specimens, based on the same chemistry, with enhanced permeability. Polymer molecular orientation due to drawing or processing generally leads to an increase in barrier properties. This is usually attributed to (1) orientationinduced crystallization, (2) fractionation and alignment (perpendicular to the permeant transport) of the crystals in the straining direction (increase in the tortuosity factor), and (3) densification (reduction in free volume) of the amorphous phase due to an increase in conformational order in the non-crystalline chain segments. The oxygen permeability, diffusivity and solubility parameters have been found to decrease with the amount of uniaxial orientation in PET due to conformational transformations of glycol linkages from gauche to trans. However, for a given uniaxial orientation in PET, biaxial drawing results in increased permeability, reducing the barrier performance. Orientation is then generally seen as the process of decreasing excess free volume bringing the nonequilibrium glassy polymer closer to the equilibrium condition.

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A special case in barrier properties is that of liquid crystal polymers and PVDC. These materials can have gas barrier properties as good as those of high barrier EVOH copolymers. Liquid crystal polymers are often termed `mesomorphic' because they have structures between those of amorphous polymers with no regular order and those with a three-dimensional crystal lattice. The unique packing arrangement of these polymeric systems has raised some fundamental questions about the permeation mechanisms of low molecular weight molecules, i.e. whether they behave more like glasses or conventional crystals. PVDC also shows high barrier properties to gases and water vapour, attributed to high lateral molecular order and hence density. Although the barrier properties of PVDC are somewhat inferior to those of dry EVOH, the former has the advantage that unlike EVOH it is not plasticized by sorption of moisture in medium to high humidity ranges due to its high molecular lateral packing.

1.2.3

Polymer molecular architecture

Some relevant routes to modifying the molecular architecture of polymers, and hence their barrier properties, are copolymerization, i.e. introducing a few side groups or branches along the main chain, and modification of the molecular weight or the stereoisomerism. Linear polyethylene (HDPE) is more crystalline than both branched polyethylenes (e.g. LLDPEs and LDPE) and ultra-high molecular weight polyethylenes and is, therefore, found to be more dense, less permeable and stiffer, albeit less tough. Moreover, the homogeneous or heterogeneous character of the incorporation of the branches along the polymer backbone has a large impact on properties, including barrier properties.26,27 The more recently developed polyolefins obtained by single site catalyst technologies can lead to very low density materials with unprecedented very low barrier properties, which in thin film form can serve as excellent packaging materials for products that have breathing necessities like fruits and vegetables. A significant effect is also the stereoisomerism (tacticity). This is due to the different stereochemical arrangements that can be present along the polymer backbone and that cannot be changed by rotation along the C±C bond. A polymer for which the pendant groups contain the same configuration is said to be isotactic. Polymers for which alternate carbon atoms have the same configuration are called syndiotactic and when the configuration is at random are called atactic. The atactic configuration is in principle more permeable as it usually yields amorphous polymers (e.g. PS or PMMA).

1.2.4

Polymer plasticization

In this context, it is relevant to add here that polymer plasticization (Tg depletion) due to polymer/permeant interactions or due to polymer and surrounding media chemical interactions has very detrimental effects, which

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usually lead to losses in intermolecular cohesion and decrease in overall barrier performance.28 Relative humidity has a tremendously detrimental impact on the outstanding gas barrier capacity of EVOH polymers, proteins and polysaccharides. This is also the case, albeit to a lesser extent, for other polar polymers like those in the polyamide family. Thus, it is often the case that polymers that are high barrier to gases have very low barrier performance to polar solvents like water, except PVDC. This behaviour is associated with the disruption by moisture of the existing polymer intermolecular self-association promoted by, for instance, hydrogen bonding in EVOH, PVOH and PA.29±31 As opposed to this behaviour, polymers like polyolefins, PE and PP have low barrier properties to gases due to weak self-association but are extremely good barrier materials to water due to their olefinic hydrophobic character. An exceptional case is that of the amorphous polyamide (aPA) and some polyimides, for which oxygen permeability decreases with increasing relative humidity.11 For this aPA, even though the presence of moisture greatly decreases the polymer Tg, the oxygen permeability does not decrease but surprisingly increases (see Table 1.1). Recent spectroscopic work suggests that moisture has a specific interaction with this particular polymer.32 The results indicate that moisture molecules do not disrupt the originally existing hydrogen bonding intermolecular interactions between amide groups, but rather link to the few remaining free amide groups, and most of the sorbed water molecules selfassociate forming clusters, which altogether act as a free volume blocking mechanism to the diffusion of oxygen molecules. This behaviour also occurs in EVOH copolymers but in the low humidity range. For these copolymers, dry EVOH at 0% RH is a lower barrier than EVOH at 30% RH, due to sorbed moisture at low water activities acting as adsorbed blocking elements to the solubility and diffusion of gas molecules.

1.2.5

Temperature

It is well known that temperature affects many of the properties of polymers. Temperature-induced changes in barrier properties are of an exponential nature. In the case of diffusion, the D value increases exponentially with temperature, in agreement with the Arrhenius law (equation 1.7), since activation energies (ED) are always positive. This phenomenon is related to the greater mobility of polymer chains at higher temperatures, which reduces the energy needed by the permeant molecules to jump to the next active site, and with an increase in the free volume of the polymer:33 D ˆ D0 eÿED =RT

1:7

In the case of the solubility coefficient, the exponential dependence on T is described by Van't Hoof's Law (equation 1.8). The enthalpy of solution (
H S)

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values is usually positive, although negative values have also been reported.34 In this case, in spite of the larger number of molecules that can be accommodated in the active sites produced by the greater mobility of the polymer chains and the bigger free volume size, the volatility of the sorbates also affects their partition equilibrium between the polymer and the outer medium.35 S ˆ S0 eÿAHS =RT

1:8

Finally, as permeability combines sorption and diffusion, its changes with temperature depend on the values of ED and AHS as shown in equation 1.9. Since the values of ED are usually greater than the absolute value of AHS, the permeation equation is considered to be an Arrhenius-type expression, the temperature dependence being described through the activation energy of permeation (EP):    P ˆ D0 eÿED =RT S0 eÿAHS =RT ˆ D0 S0 e…ÿED ÿAHS †=RT ˆ P0 eÿEP =RT 1:9 The temperature also affects the state of the polymer, the transport properties of the polymer being affected by it. In the melted polymer, the crystalline regions disappear and transport takes place across the entire matrix, which behaves like a liquid. In this case, all the polymer volume is available for the permeant, which increases its solubility, and the blocking effect of the crystals disappears, which reduces tortuosity and makes diffusion easier. Also, the polymer chains are in constant movement, which facilitates the jumps of the permeant molecules. Changes associated with the glass transition, i.e. with the passage of the polymer from the glassy to the rubbery state, take place as a result of the relaxation or increased mobility of the chain segments in the amorphous phase of the polymer. Above the glass transition temperature (Tg) the amorphous phase of the polymer is in the rubbery state; below this temperature it is in the glassy state. In the rubbery state, relaxation times are shorter and, after the sorption of permeant molecules, a new equilibrium state is reached more quickly. As a result, diffusion is faster when the polymer is in the rubbery state.

1.2.6

The permeant

Characteristics of the permeant like molecular size, shape and chemical nature usually affect its transport properties. Increasing the molecular size in homologous series of permeants (alkanes, esters, aldehydes or alcohols) generally reduces the diffusion and solubility coefficient values of the permeants, mainly for steric reasons. Only when solutes are in the form of vapour do the higher solubilities correspond to the larger molecules, as a consequence of their greater condensabilities.36 The shape of the permeant molecules is also important, as flattened or elongated molecules will diffuse more quickly through the polymer

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than spherical ones with the same molecular volume.37 The nature of the permeant also affects its transport properties, as described above in the effect of chemistry. If the affinity between the permeant and the polymer is very high it can sometimes cause plasticization of the polymer. In this case, sorption leads to a decrease in the self-association between adjacent macromolecules in the amorphous region. The initial hydrogen bonding and van der Waals forces are replaced by polymer±sorbate interactions, increasing chain mobility and free volume, reducing the Tg and raising the diffusion and solubility coefficients of the solute. Plasticization depends on the penetrant concentration, which has to be above a certain limit for it to take place. However, while outstanding affinity between the sorbate and the polymer and large uptakes are necessary, sometimes they are not sufficient to produce plasticization of the polymer, as described in the case of aPA. When a complex matrix like a foodstuff is placed inside a polymeric package, the polymer will be in contact with a large number of solvents simultaneously and the transport properties of one solute are often affected by the presence of the other co-solvents. Water is the main component of many foodstuffs and also the most frequently reported co-solvent. In hydrophilic polymers like the EVOH copolymers, waterinduced plasticization at high moisture levels has been reported to increase the permeability to hydrophobic and apolar solvents like limonene and oxygen.38 However, as described before in the case of the aPAs, the presence of water can also have a positive effect on the barrier properties of the material. Another co-solute whose effect has been widely described in the literature is limonene, the main component of orange juice flavour. The effect of this terpene on the barrier performance of apolar polyolefins is similar to that of water on polar EVOH copolymers. The presence of high concentrations of limonene has been reported to double the permeability of ethyl-butyrate through HDPE and to increase that of ethyl acetate through biaxially oriented polypropylene by up to 40 times.39 The simultaneous transport of a group of co-solvents with similar transport properties has usually been described as a competition between them for the active sites, resulting in the transport of certain compounds being reduced and that of the rest increased.40 However, positive synergistic effects have also been reported, as in the case of toluene/methanol mixtures.41

1.3

Novel polymers and blends

Novel developments in high barrier plastics mainly come from three sources, namely (1) new polymers including biopolymers, (2) polymer blends including nanocomposites, and (3) inorganic coatings such as aluminium obtained by vacuum deposition technologies and oxides (AlOx or SiOx). Polymeric materials

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for high barrier applications are challenged today by a broad range of stringent property requirements including ease of processing, higher barrier properties to permanent gases, to moisture and to low molecular weight organic compounds, excellent chemical resistance, permselectivity, low relative humidity dependence for the barrier performance, and ease of recycling and biodegradability. Among the novel high barrier polymers that have been more recently developed are materials like the PK copolymers (aliphatic polyketones).42,43 These semicrystalline materials have an outstanding range of mechanical, thermal and high barrier properties (comparable to some EVOH copolymers, see Fig. 1.1), chemical resistance and reduced relative humidity dependence for barrier properties, which give them significant commercial potential in a broad range of engineering, barrier packaging, fibre and blend application. Another novel, extremely high barrier material that has been recently developed is polyglycolic acid (PGA). This biodegradable polymer is claimed to have very low O2 and CO2 permeabilities, one hundredth that of PET (see Fig. 1.2). Additionally, and as opposed to EVOH and PVOH, the barrier properties of commercial PGA resins are said to be largely insensitive to humidity conditions, making it ideally suited for a variety of beverage and perishable food packaging applications.44 Another family of resins that have been recently developed and are currently making their way into the market are the amorphous vinyl alcohol resins (AVOH).45 Water-soluble but melt-compoundable AVOH is said to have, in addition to excellent gas barrier properties and good chemical resistance compared to PVOH and EVOH, superior extrusion properties, orientability, shrinkability and transparency. This polymer can be used in all extrusion processes such as melt-spinning, oriented film, transparent container and injection, and because it is biodegradable, it lends itself to a variety of applications such as new packaging materials that reduce the burden on the environment.

1.2 OTR/WVTR of some polymers vs. the properties claimed for PGA.

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Aromatic polyamides such as Ny-MXD6, i.e. polyamide resins produced from meta-xylenediamine and adipic acid, are currently being considered in packaging applications since they provide a transparent high gas barrier at high humidity properties (see Fig. 1.2 and Chapter 9) and can be functionalized to achieve oxygen scavenging properties. Another new range of promising materials that have already been developed and in some cases marketed with success in packaging applications are a number of resins derived from biomass and, therefore, to a higher or lower extent easily biodegradable or compostable.6,46 Among these materials, it is possible to find (1) polymers synthesized from bio-derived monomers such as polylactic acid resins (PLA); (2) polymers produced directly by microorganisms like PHAs, bacterial cellulose, etc.; and (3) polymers extracted directly from biomass such as polysaccharides (plant cellulose, starch, chitosan), proteins (soy protein, gluten, zein) and lipids. These biopolymers can have excellent barrier properties to gases such as for instance plasticized chitosan, although their barrier performance is dramatically reduced in the presence of moisture. However, other polymers like PLA and PHAs have relatively good water barrier properties and their relatively good oxygen barrier, lower than for PET, is largely insensitive to moisture sorption. So in principle, one could devise a bio-based derived high barrier multiplayer system where an inner layer of plasticized chitosan could be sandwiched between high moisture barrier PLA or PHA layers. An interesting property of some of these bio-based polymers, e.g. PLA and starch, is that their permeability to carbon dioxide compared to oxygen (permselectivity) is higher than that of most conventional mineral oil based plastics. This is, for instance, of interest for some food packaging applications where a high barrier to oxygen is required, but CO2 generated by the product should be allowed to exit the package headspace to avoid package swelling. These materials, however, still suffer from high production costs compared to polyolefins but are now competitive with, for instance, PET. An interesting development based on cellulose has been recently published.47 In this study, softwood and hardwood celluloses were oxidized by 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation. The TEMPO-oxidized cellulose fibres were converted to transparent dispersions in water, which consisted of cellulose nanofibres 3±4 nm in width. Films derived from this material were seen to consist of randomly assembled nanofibres, were transparent and flexible, and had extremely low coefficients of thermal expansion caused by the high crystallinity. Moreover, the oxygen permeability of a polylactic acid (PLA) film drastically decreased by a factor of about 750 by forming a thin layer of the cellulose material on the PLA film. Hydrophobization of the originally hydrophilic films was achieved by treatment with alkylketene dimer. Blending polymers is a feasible route to accessing the desired balance of properties by controlling the polymer phase interaction and/or the morphology

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1.3 Modelling of oxygen permeability for various dispositions of EVOH/aPA blend components facing the transport of oxygen gas and as a function of the volume fraction of EVOH. Experimental data (see arrow) for 80/20 EVOH/PA and EVOH/ionomer melt-mixed blends recently developed in our labs are also provided.

in monolayer barrier systems.48 The most commonly used case is to blend polymers with other polymers that have higher barrier properties. The barrier properties of these blends seem to follow a relationship (see equation 1.10) in good general agreement with that proposed by Maxwell and extended by Roberson (see equation 1.1049) for spheres of a low oxygen barrier phase (aPA in Fig. 1.3), but with higher water resistance, dispersed in a high oxygen barrier (EVOH in Fig. 1.3) continuous matrix which has a lower water resistance.50 This simple model would appear to closely reflect, albeit with a slight positive deviation (due to orientation, see Fig. 1.3), the case of the dispersed morphology found for this EVOH/PA blend. The EVOH/ionomer blend even presents a considerably better barrier than is predicted from equation 1.10 due to the fact that the morphology of the particles is elongated (higher aspect ratio) in the machine direction and normal to the permeation direction.   PaPA ‡ 2PEVOH ÿ 2VaPA …PEVOH ÿ PaPA † 1:10 PEVOH=aPA ˆ PEVOH PaPA ‡ 2PEVOH ‡ VaPA …PEVOH ÿ PaPA † The permeability of blends following the above equation would then approach the permeability of a co-extruded multilayer (see equation 1.11) system comprising two layers, one made of a lower barrier disperse phase and the other of a high barrier matrix; therefore, the overall permeability will be close to the

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permeability of the neat high barrier matrix for a sufficiently high volume fraction of the matrix (VEVOH). Equation 1.11 presents a very favourable situation in terms of permeability for a non-miscible blend. PEVOH=aPA ˆ

PEVOH PaPA VaPA PEVOH ‡ VEVOH PaPA

1:11

The circles on the graph in Fig. 1.3 represent the values of permeability obtained by application of a simple additive rule (layers parallel to permeant flow: see equation 1.12). This case would clearly represent a very unfavourable situation in terms of permeability for blends. PEVOH=aPA ˆ PEVOH VEVOH ‡ PaPA VaPA

1:12

Figure 1.3 shows, as an example, some modelling for the barrier properties of EVOH/aPA blends as a function of blend composition and the orientation of the blend constituents in relation to the direction of oxygen transport. High barrier blends of EVOH with an ionomer and an amorphous polyamide have also been developed.30,31 These blends show excellent barrier properties to gases compared to neat EVOH (see experimental values for EVOH 80/20 blends in Fig. 1.3), and yet much better thermoformability than EVOH alone for the production of thermoformed multilayer rigid food containers. Curiously, the EVOH/aPA blends, that under dry conditions present a lower barrier to oxygen, when submitted to typical packaged food water vapour sterilization (at 120ëC for 20 minutes) processes, have a better oxygen barrier than EVOH due to the decreased water sensitivity of the system. There are also a relatively large number of blends reported in the literature in which a high gas barrier polymer like EVOH was added to improve the barrier properties of a low gas barrier material and, conversely, in which a high water barrier polymer is added to a high gas barrier material to reduce relative humidity dependence in the barrier properties of the latter. In a recent paper, a PVOH-based interpolymer complex stabilized by hydrogen bonding with enhanced gas barrier was reported.51 Thus, hydrogen bonding between poly(methyl vinyl ether-co-maleic acid) (PMVE±MA) and PVOH resulted in films with lower oxygen transmission rates (OTR) than pure PVOH. In the range 20±30% (w/w) PMVE±MA, complexation between the two polymers was maximized. The improved oxygen barrier properties were believed to result from a combination of the relatively intact PVOH crystalline regions and a higher degree of hydrogen bonding in the amorphous regions of the PVOH and PMVE±MA films. This leads to denser amorphous regions that reduce the rate of gases diffusing through the polymer film, hence reducing oxygen permeability. Some other successful blending routes are achieved by blending PET with polyamides. Thus, in a recent study52 PET was blended with an aromatic

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polyamide, either poly(m-xylylene adipamide) (Ny-MXD6) or a copolyamide based on Ny-MXD6 in which 12 mol% adipamide was replaced with isophthalamide (Ny-MXD6-12I). Incorporating a small amount of sodium 5-sulfoisophthalate into the PET matrix was needed to compatibilize the blends and was seen to reduce the polyamide domain size to 100±300 nm. Blending PET with 10 wt% Ny-MXD6 or Ny-MXD6-12I reduced oxygen permeability of PET by a factor of about 0.8 (P/PPET) when measured at 43% relative humidity (RH), in accordance with the Maxwell model prediction. However, after biaxial orientation, oxygen permeability of blends with 10 wt% Ny-MXD6 was reduced by 0.3 at 43% RH, and permeability of blends with 10 wt% Ny-MXD6-12I was reduced by 0.4. Even at 85% RH, oxygen permeability was reduced by 0.4 and 0.6 for blends with Ny-MXD6 and Ny-MXD6-12I, respectively. The blends were even more effective in reducing carbon dioxide permeability of oriented PET. Transformation of spherical polyamide domains into platelets of high aspect ratio was thought to cause the barrier increase. The platelet aspect ratio predicted by the Nielsen model was confirmed by atomic force microscopy. The higher aspect ratio of Ny-MXD6 domains was ascribed to a lower Tg compared to Ny-MXD6-12I. More interestingly, similar reduction in oxygen permeability was achieved in bottle walls blown from PET blends with Ny-MXD6 or NyMXD6-12I. A very interesting blending technique with high potential is the `layer multiplying co-extrusion' technique, which enables the production of layered films with tens to thousands of alternating layers of two or three different polymers with individual layer thicknesses in the 10 nm to 100 m range and various arrangements.53 Using this technology, polymers with widely dissimilar solid state morphologies and properties can be combined into unique layered and gradient structures. Micro- and nanolayers with up to 4096 layers and individual layer thicknesses less than 20 nm have been successfully produced with the technology. As the layer thickness approaches the micro- and nanometre length scales, useful and interesting changes in gas transport, mechanical and optical properties occur. This technology therefore offers an attractive approach for creating designed architectures from particulate-filled polymers such as alternating filled/unfilled layers with varying thickness and composition. Coupling of carefully chosen inorganic/organic barrier systems with multilayering technology offers the potential for generating tens or hundreds of individual, high aspect ratio barrier domains through which oxygen, carbon dioxide, water vapour or any permeant of interest would have to traverse. Finally, inorganic coatings or nanocoatings such as metallized layers, silicon oxide (SiOx) and aluminium oxide (Al2O3) layers are also being used or developed to reduce permeability in packaging structures. Thus, coating plastics with vacuum-deposited aluminium seeks to increase barrier properties to gases, moisture and organic vapours, and results in better flexibility, greater consumer appeal and lower environmental impact due to reduction in metal consumption

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and better recyclability than conventional lamination with aluminium foil.54 On the other hand, the metal coating of polymeric films imposes reductions in flexibility, stretchability and thermoformability compared to the performance of the polymer films alone. SiOx coatings possess highly desirable properties, such as transparency, recyclability, retortability and microwave use, and are superior in these regards to the thin metal (generally aluminium-based) coatings currently employed commercially on various polymer substrates. For the SiOx coatings to compete effectively against more established, as well as concurrently emerging barrier technologies, they must demonstrate time and temperature stability and promote substantially reduced oxygen and water vapour permeability. Recent studies of SiOx coatings produced by different processing routes have, in fact, shown that these criteria are usually satisfied. One of the benefits of SiOx coatings lies in the flexibility by which they can be deposited on polymer surfaces. Thus far, sputtering, electronbeam deposition, and plasma-enhanced chemical vapour deposition (PECVD) have all been utilized successfully to produce SiOx barrier coatings on polymer substrates. Of these methods, the last one has become the most popular due to its operational ease and efficacy.55 Thin aluminium oxide (Al2O3) layers have also been considered as high barrier coatings and were trialled on various uncoated papers, polymer-coated papers and boards and plain polymer films using the atomic layer deposition (ALD) technique.56 This study demonstrated that such ALD-grown Al2O3 coatings efficiently enhanced the gas-diffusion barrier performance of the studied porous and non-porous materials against oxygen, water vapour and aromas.

1.4 Nanocomposites Over the last few years there has been a significant increase in the number of research works devoted to enhancing relevant polymer properties, mainly mechanical and barrier properties, but also surface hardness, control released, active and intelligent functionalizations, UV±Vis (ultraviolet±visible light) protection, thermal stability and fire retardancy, in existing polymers by means of nanotechnology. Nanotechnology is by definition the creation and utilization of structures with at least one dimension in the nanometre length scale, typically below 100 nm, that creates novel properties and phenomena otherwise not displayed by either isolated molecules or bulk materials. Among the various existing nanotechnologies available such as metallic antimicrobial and UV light protecting nanoparticles,57 carbon nanotubes and nanofibres,58 the very recently developed grapheme-based materials,59 cellulose nanowhiskers,60 electrospun nanofibres and nanocapsules,61 the one that has attracted more attention in the food packaging field is the use of inorganic layered nanoclays. It has been broadly reported in the scientific literature that the addition of low loadings of nanoclay particles, with thickness in the nanometre scale and with high aspect ratios, to a raw polymer to generate the so-called

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1.4 Typical modelling examples of permeability reductions in nanocomposites as a result of the application of the Nielsen and Fricke models to layered particles.

nanocomposites can have a profoundly enhancing effect over some material properties, such as mechanical properties, thermal stability, UV±Vis protection,62 active properties, conductivity and gas and vapour barrier properties. Figure 1.4 shows typical modelling examples of permeability reductions in nanocomposites as a result of the application of the Nielsen and Fricke models to layered particles. The model of Nielsen63 (see equation 1.13), and other ulterior refinements such as that of Fredrickson and Bicerano,64 describe systems in which the layered, i.e. thin, flat and squared, particles are perfectly oriented with length and width perpendicular to the permeant transport direction and are homogeneously diluted in the polymer matrix: 1 ÿ Vclay Pnano ˆ Pneat 1 ‡ …L=2W †
Vclay

1:13

In the above equation, L=W is the aspect ratio of the platelets, Vclay is the volume fraction of the clay filler, Pnano is the permeability of the nanocomposite, and Pneat is the permeability of the pure material. A more realistic system to consider is one in which a discontinuous lowpermeability phase is present in a high-permeability matrix. Maxwell developed a model to describe the conductivity of a two-phase system in which permeable spheres are dispersed in a continuous permeable matrix.50 Fricke extended Maxwell's model to describe the conductivity of a two-phase system in which permeable ellipsoids are dispersed in a more permeable continuous matrix.65

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This model describes the conductivity of a two-phase system in which lower permeability elongated ellipsoids (Pd) are dispersed in a more permeable continuous matrix (Pm). According to this model, the permeability of a composite system consisting of a blend of the two materials in which the dispersed phase (2 is the volume fraction of the dispersed phase) is distributed as ellipsoids can be expressed as follows:48 Pˆ

Pm ‡ Pd F 1‡F

where  Fˆ



1:14 2

3

6 7 2 1 6  7 4 5 P 1 ÿ 2 d 1 ‡ …1 ÿ M† ÿ1 Pm

1:15

M ˆ ‰cos =sin 3Š‰ ÿ 12 sin 2Š and cos  ˆ W =L where W is the dimension of the axis of the ellipsoid parallel to, and L the dimension perpendicular to, the direction of transport, and  is in radians. In this regard, gas and water vapour permeabilities have been found to decrease, in some cases, to a large extent in the nanocomposites due to, among other factors, increased tortuosity factors.66 For example, an EPDM±clay nanocomposite with a 4 wt% loading was found to decrease N2 permeability by 30% compared to EPDM alone.67 Oxygen permeability decreased by a factor of 3 in polyester±clay nanocomposites at 2.5 wt% loading. A 60% reduction in the water permeability was measured in a 5 wt% loaded poly(vinyl alcohol)/sodium montmorillonite nanocomposite and the material still retained its optical clarity.68 In EVOH nanocomposites, reductions in oxygen permeability of more than 70%, over a range of relative humidity values, have been reported69,70 and reductions in water permeability beyond 90% in some proteins and polysaccharides have also been reported.71 Table 1.3 reports the interesting behaviour of EVOH nanocomposites containing a recently developed kaolinite-based grade complying with food contact legislation,72 in which the oxygen permeability reduction due to the nanoclay is higher with increasing relative humidity with minimum impact on transparency. EVOH resins are known to be strongly sensitive to moisture sorption and hence EVOH nanocomposites are the only efficient technology that can overcome this drawback while retaining transparency and film integrity. Additionally, a higher retorting, i.e. humid heat sterilization resistance is observed in EVOH nanocomposites compared to EVOH alone (see Fig. 1.5). This may have considerable implications in retortable packaged foods, where thick layers of hydrophobic

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Table 1.3 PO2 of extruded films of EVOH29 and of EVOH29 nanocomposites as a function of relative humidity Material

PO2 (cm3mm)/(m2day)

EVOH29 EVOH29 EVOH29 containing 4 wt% nanoclay EVOH29 containing 4 wt% nanoclay

4.2 (50% RH) 1470.6 (90% RH) 3.0 (50% RH) (28% reduction) 427.8 (90% RH) (71% reduction)

polymers are needed to protect EVOH from significant barrier and structural deterioration. In fact, reducing the water sensitivity of EVOH by blending without significant losses in transparency, with higher barrier properties and with enhanced retorting resistance can only be achieved, to the best of our knowledge, by the nanocomposites technology. Moreover, nanocomposites containing specific nanoclays can also be used as UV-light barrier materials for protection of UV-sensitive packaged products.73 A very recent development is the use of nanoclays as carriers of novel functionalizations such as for the controlled release of antimicrobials, antioxidants and oxygen scavengers of value in, for instance, active food packaging technologies.74,75 Notwithstanding the above, in general, the experimentally measured reductions in permeability have not been in full agreement with the values expected from modelling work for most systems, due to lack of complete exfoliation, insufficient compatibility, morphological alterations, solubility effects and other factors.

1.5 Retorting (humid heat sterilization) resistance experiments at 120ëC for 20 minutes of similar food packaging multilayer systems containing in the intermediate layer (a) pure EVOH and (b) an EVOH nanocomposite with 4 wt% nanoclay.

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Future trends

Great efforts have been made by researchers in multidisciplinary fields over the last decades to develop new, high-performance polymeric materials or novel technological solutions for existing materials. The overall objective has been to extend the shelf-life of packaged foodstuffs, retaining or even enhancing their quality and safety attributes. The technological `holy grails' have been both (1) to procure glass-tight barrier performance and to make plastics more functional and versatile while retaining their positive attributes, and (2) to provide property-tailoring solutions for the newly developed and poorly performing renewable and biodegradable first generations of biopolymeric resins. To do so, new materials, but more importantly selected nanotechnology and functionalization tools, have been implemented from simple research ideas into fully functional commercial applications. In the years to come, new nanomaterials and functionalities with property-balancing capacity will continue to make their way from research centres across application fields into the food packaging area to additionally provide more efficiency for innovative food packaging strategies such as emerging preservation, active, bioactive and intelligent technologies. Thus, several cutting-edge nanotechnologies and novel functionalities are currently being trialled by an increasing number of material manufacturers and packaging converters. Nevertheless, for their wide commercial implementation and success they need to comply with current and future legislation and be specifically designed to reach specific targets in materials and properties. It is also clear that there is still a lot of missing information in the food packaging sector regarding their use and potentialities in finished articles and we, the authors and editors, really hope that this book can help steer the mind of the readers towards filling this gap.

1.6

References

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Multifunctional and nanoreinforced polymers for food packaging foods into healthier foods through biomaterials. Trends Food Sci. Tech., 17, 567± 575. S.K. Young, G.C. Gemeinhardt, J.W. Sherman, R.F. Storey, K.A. Mauritz, D.A. Schiraldi, A. Polyakova, A. Hiltner, E. Baer (2002). Polymer, 43(23), 6101±6114. I. Olabarrieta, D. ForsstroÈm, U.W. Gedde, M.S. Hedenqvist (2001). Polymer, 42, 4401±4408. J. Crank, G.S. Park (1968). Diffusion in Polymers, Academic Press, New York. Z. Zhang, I.J. Britt, M.A. Tung (1999). J. Polym. Sci.: Part B: Polym. Phys., 37, 691. M.A. Samus, G. Rossi (1996). Macromolecules, 29, 2275. G. Rossi (1996). Trends Polym. Sci., 4, 337. R.J. Hernandez, J.R. Giacin, E.A. Grulke (1992). J. Membrane Sci., 65, 187. D.W. Van Krevelen (1990). Properties of Polymers, Elsevier, New York. R.Y.F. Liu, D.A. Schiraldi, A. Hiltner, E. Baer (2002). J. Polym. Sci., Part B: Polym. Phys., 40, 862. M.H. Cohen, D. Turnbull (1959). J. Chem. Phys., 31, 1164. D. Turnbull, M.H. Cohen (1970). J. Chem. Phys., 52, 3038. H. Fujita (1961). Fortschr. Hochpolym. Forsch., 3, 1. K. Tanaka, T. Kawai, H. Kita, K. Okamoto, Y. Ito (2000). Macromolecules, 33, 5513. K. Ito, Y. Saito, T. Yamamoto, Y. Ujihira, K. Nomura (2001). Macromolecules, 34, 6153. J.M. LagaroÂn, E. Gimenez, J.J. Saura (2001). Polym. International, 50, 635. R.H. Boyd (1979). Polym. Eng. Sci., 19, 1010. M. Hedenqvist, A. Angelstok, L. Edsberg, P.T. Larsson, U.W. Gedde (1996). Polymer, 37, 2887. J.M. LagaroÂn, S. Lopez-Quintana, J.C. Rodriguez-Cabello, J.C. Merino, J.M. Pastor (2000). Polymer, 41, 2999. B. Neway, M.S. Hedenqvist, V.B.F. Mathod, U.W. Gedde (2001). Polymer, 42, 5307. S Aucejo, R. Catala, R. Gavara (2000). Food Sci. Technol. Int., 6, 159±164. J.M. LagaroÂn, A.K. Powell, J.G. Bonner (2001). Polymer Testing, 20/5, 569. J.M. LagaroÂn, E. Gimenez, R. Gavara, J.J. Saura (2001). Polymer, 42, 9531. J.M. LagaroÂn, E. Gimenez, J.J. Saura, R. Gavara (2001). Polymer, 42, 7381. J.M. LagaroÂn, E. Gimenez, R. Catala, R. Gavara (2003). Macrom. Chem. Phys., 202(4). L. Nicolas, E. Drioli, H.B. Hopfenbergand, D. Tidone (1977). Polymer, 18, 1137. J. Brandrupand, E.H. Immergut (1989). Polymer Handbook, 3rd edn, John Wiley & Sons, New York. Q. Zhou, K.R. Cadwallader (2004). J. Agric. Food Chem., 52, 6271. G.W. Halek, J.P. Luttmann (1991). In Food Packaging Interactions 2, ed. J.H. Hotchkiss, ACS, Washington, DC. A.R. Berens, H.B. Hopfenberg (1982). J. Membrane Sci., 10(2±3), 283. Z. Zhang, I.J. Britt, M.A. Tung (2001). J. Appl. Polym. Sci., 82, 1886. T.J. Nielsen, J.R. Giacin (1994). Packaging Technol. Sci., 7(5), 247. J. Letinski, G.W. Halek (1992). J. Food Sci., 57(2), 481. C. Gagnard, Y. Germain, P. Keraudren, B. Barriere (2004). J. Appl. Polym. Sci., 92, 676. J.G. Bonner, A.K. Powell (1997). Proc. 213th National American Chemical Society Meeting, ACS Materials Chemistry Publications, Washington, DC. J.M. LagaroÂn, A.K. Powell (2000). Patent WO 0042089. http://www.kureha.com/pdfs/Kureha-KUREDUX.pdf

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45. http://www.g-polymer.com 46. C.J. Weber, V. Haugaard, R. Festersen, G. Bertelsen (2002). Food Additives and Contaminants, 19, 172. 47. H. Fukuzumi, T. Saito, T. Iwata, Y. Kumamoto, A. Isogai (2009). Biomacromolecules, 10, 162±165. 48. D.R. Paul, C.B. Bucknall, editors (2000). Polymer Blends, Volume 2: Performance, John Wiley & Sons, New York. 49. H.B. Hopfenberg, D.R. Paul (1978). In Polymer Blends, ed. D.R. Paul and S. Newman, Academic Press, New York. 50. J.M. LagaroÂn, E. Gimenez, V. Del-Valle, B. Altava, R. Gavara (2003). Macromolecular Symposia, 198, 473. 51. P.W. Labuschagne, W.A. Germishuizen, S.M.C. Verryn, F.S. Moolman (2008). Eur. Polym. J., 44, 2146±2152. 52. Y.S. Hua, V. Prattipatia, S. Mehtab, D.A. Schiraldia, A. Hiltnera, E. Baera (2005). Polymer, 46, 2685±2698. 53. M. Gupta, Y. Lin, T. Deans, E. Baer, A. Hiltner, D.A. Schiraldi (2010). Macromolecules, 43, 4230±4239. 54. R.S.A. Kelly (1992). I+D Packaging Conference, Sevilla, Spain. 55. A.G. Erlat, R.J. Spontak, R.P. Clarke, T.C. Robinson, P.D. Haaland, Y. Tropsha, N.G. Harvey, E.A. Vogler (1999). J. Phys. Chem. B, 103, 6047±6055. 56. T. Hirvikorpi, M. VaÈhaÈ-Nissi, T. Mustonen, E. Iiskola, M. Karppinen (2010). Thin Solid Films, 518, 2654±2658. 57. A. Travan, C. Pelillo, I. Donati, E. Marsich, M. Benincasa, T. Scarpa, S. Semeraro, G. Turco, R. Gennaro, S. Paoletti (2009). Biomacromolecules, 10(6), 1429±1435. 58. M.D. Sanchez-Garcia, J.M. LagaroÂn, S.V. Hoa (2010). Comp. Sci. Technol., 70(7), 1095±1105. 59. T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso, R.D. Piner, D.H. Adamson, H.C. Schniepp, X. Chen, R.S. Ruoff, S.T. Nguyen, I.A. Aksay, R.K. Prud'homme, L.C. Brinson (2008). Nature Nanotechnology, 3, 327±331. 60. M.D. Sanchez-Garcia, J.M. LagaroÂn (2010). Cellulose, 17, 987±1004. 61. A. Fernandez, S. Torres-Giner, J.M. LagaroÂn (2009). Food Hydrocolloids, 23(5), 1427±1432. 62. M.D. Sanchez-Garcia, J.M. LagaroÂn (2010). J. Appl. Polym. Sci., 118(1), 188-199. 63. L.E. Nielsen (1967). Models for the permeability of filled polymer systems. J. Macromol. Sci. (Chem.), A1, 929±942. 64. G.H. Fredrickson, J. Bicerano (1999). Barrier properties of oriented disk composites. J. Chem. Phys., 110, 2181±2188. 65. M. Krook, G. Morgan, M.S. Hedenqvist (2005). Barrier and mechanical properties of injection molded montmorillonite/polyesteramide nanocomposites. Polym. Eng. Sci., 45, 136±140. 66. R.K. Bharadwaj, A.R. Hehrabi, C. Hamilton, C. Trujillo, M. Murga, R. Fan, A. Chavira, A.K. Thompson (2002). Structure-property relationships in cross-linked polyester-clay nanocomposites. Polymer, 43, 3699. 67. A. Usuki, A. Tukigase, M. Kato (2002). Polymer, 43, 2185. 68. K.E. Strawhecker, E. Manias (2000). Chem. Mater., 12, 2943. 69. J.M. LagaroÂn, D. Cava, L. Cabedo, R. Gavara, E. Gimenez (2005). Food Additives and Contaminants, 22(10), 994±998. 70. L. Cabedo, E. GimeÂnez, J.M. LagaroÂn, R. Gavara, J.J. Saura (2004). Polymer, 45/15, 5233±5238. 71. J.M. LagaroÂn, E. Gimenez, M.D. SaÂnchez-GarcõÂa, M.J. Ocio, A. Fendler (2007).

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72. 73. 74. 75.

Multifunctional and nanoreinforced polymers for food packaging Food Contact Polymers, Rapra Conference Proceedings, Chapter 19, ISBN 978-184735-012-1. www.nanobiomatters.com J.-M. LagaroÂn-Cabello, M.D. Sanchez-Garcia, E. Gimenez-Torres (2009). Patent WO/2009/065986. M.A. Busolo, P. Fernandez, M.J. Ocio, J.-M. LagaroÂn (2010). Food Additives and Contaminants: Part A, 27(11), 1617±1626. M.A. Busolo, A. Aouad, J.-M. LagaroÂn (2010). Conference Proceedings, ANTEC2010, 2044±2047.

1.7

Appendix: Abbreviations

aPA AVOH EPDM EVOH HDPE LCP LDPE LLDPE Ny-MXD6 PA PA6 PAN PC PCL PE PET PGA PHA PK PLA PMMA PMVE±MA PP PS PVC PVDC PVOH

Amorphous polyamide Amorphous vinyl polymers Ethylene propylene diene monomer Ethylene±vinyl alcohol copolymers High density polyethylene Liquid crystal polymer Low density polyethylene Linear low density polyethylene Aromatic polyamide, poly(m-xylylene adipamide) Polyamide Polyamide 6 (Nylon) Polyacrylonitrile Polycarbonate Polycaprolactone Polyethylene Polyethylene terephthalate Polyglycolic acid Polyhydroxyalkanoates Aliphatic polyketone copolymers Polylactic acid Polymethyl methacrylate Poly(methyl vinyl ether-co-maleic acid) Polypropylene Polystyrene Polyvinyl chloride Polyvinylidene chloride Polyvinyl alcohol

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Multifunctional nanoclays for food contact applications  N and M .-A . B U S O L O , Novel Materials and J.-M. L A G A R O Nanotechnology Group, IATA-CSIC, Spain

Abstract: This chapter introduces a novel type of nanomaterials based on nanoclays, which provide in addition to the well-known benefits associated with the reinforcing effect of layered nanoclays, the capacity to deliver active new functionalities to packaging materials. More specifically, it is shown how active metals or their compounds can be nanoscaled and stabilized on the surface of nanoclays to provide antimicrobial and oxygenscavenging capacity while being able to nicely disperse within packaging polymers to deliver both enhanced physical performance and active functionalities. Key words: active packaging, antimicrobials, nanoclays, nanotechnology, oxygen scavengers.

2.1 Introduction There is a current trend to incorporate into packaging materials active agents that will maintain and enhance the quality and safety of packaged goods. These concepts are generally termed active packaging technologies. Thus, active packaging has been defined as a system in which the product, the package and the environment interact in a synergistic manner to extend shelf-life or to achieve some characteristics that cannot be obtained otherwise.1±6 Among these, antimicrobial performance and oxygen scavengers are two of the most desired functionalities in plastic packaging. The main aim of active packaging is thus to respond to changes in the conditions of packaged foods in order to extend packaged product shelf-life. This practice can improve food safety and sensorial properties, while maintaining the quality of packaged foods. Active packaging techniques for preserving or even improving the quality and safety of foods can be divided into three classes: (1) absorbing systems; (2) releasing systems; and (3) other speciality systems for temperature, ultraviolet light and microwave control systems.7 Active packaging materials that can absorb or release active compounds for enhancing the quality and safety of a wide range of foods during extended storage are particularly important.

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Traditionally, active technologies have been commercially implemented within separate sachet units, but there is now more interest in integrating them within the packaging material to enhance functionality and design.8 For example, the active substances in the polymer permit the absorption of oxygen, control the concentration of carbon dioxide or ethylene, stabilize temperature, control the release of ethanol or antioxidant or antimicrobial substances, and control the humidity and the growth of microorganisms.3,9 Antimicrobial activity can be realized by adding AM agents to a packaging system during manufacture or by using AM polymeric materials.10 The absorption systems remove the essential factors of microbial growth from the food and inhibit the growth of microorganisms. The immobilization systems are not intended to release AM agents and hence limit the biocide action to microorganisms existing at the contact surface. The release systems allow the migration of the AM agent (to the liquid or gas phase) into the food or the headspace inside the package to inhibit the growth of microorganisms. Whereas a gaseous AM agent can penetrate through any space, a solute AM agent cannot migrate through the air space between the food and the packaging material. The release kinetics of packaging systems are typically studied by measuring the release rate of the AM agent into a food simulant or by measuring the effectiveness in inhibiting microbial growth and extending the shelf-life of foods. Controlled-release packaging is thus a new generation of packaging materials that can release active compounds at different controlled rates suitable for enhancing the quality and safety of foods during extended storage. The substances that are being considered for inclusion in release packaging are, among others, nutrients, antimicrobials, antioxidants, enzymes, flavours and nutraceuticals. The antimicrobial substances in the release packaging permit the gradual migration to the food during storage and use. These technologies are very effective in minimizing the superficial contamination of the foods and for that reason the application of this antimicrobial packaging to foods like meat, fruits and vegetables is very attractive. The antimicrobial substances used in food packaging that can migrate to the food should be food additives and need to comply with the new legislation related to active and intelligent packaging.11 As was introduced above, oxygen scavengers also constitute one of the more interesting `active packaging' technologies as they contribute to keeping the optimal concentration of oxygen inside the packaging in order to preserve the quality (appearance, smell, taste and texture) and prolong the shelf-life of oxygen-sensitive products. An excess of oxygen in packaging can cause undesirable changes in foods such as fat oxidation or growth of bacteria and moulds. Oxygen molecules can remain in the packaging headspace as well as permeate through the packaging film, hence reducing the product shelf-life. Unlike traditional or passive packages, which cannot remove or reduce the oxygen present, the use of active packaging with oxygen scavengers can reduce the oxygen concentration to

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levels below 0.01%, maintaining those levels during food storage.5 The use of oxygen-free atmospheres in food packaging has to be designed with caution, as anaerobic microorganisms can now break out, leading to potentially serious safety issues. In general, oxygen-scavenging commercial technologies make use of iron powder oxidation; however, a minority of systems are based on ascorbic acid oxidation, catechol oxidation, metallic salts and photosensitive dyes, among others.12 Iron-based scavengers are based on the oxidation of iron into Fe(OH)3: 4Fe + 3O2 + 6H2O ÿ! 4Fe(OH)3 ÿ! 2Fe2O3
3H2O Iron-based scavenging systems are mostly marketed as sachets (to prevent imparting colour, odour and taste to the food), and more recently some oxygenscavenging laboratory prototype films have been developed by incorporation of commercial iron systems into polymer matrices.5 Considering that the sachets mentioned above have the potential risk of being misused by the consumer and eventually being ingested, as well as the risk of contamination of the product by leakage from the sachet, the use of other types of oxygen-scavenging systems is desirable. The incorporation of active systems into packaging materials allows some advantages such as the potential use with retort packaging, elimination of food product distortion that may occur when a sachet contacts the food, and potential cost savings due to increased production efficiency and convenience. This chapter deals with the introduction of a new nanotechnological toolbox based on the natural dispersability and good properties of nanoclays to impart new active functionalities to plastics and bioplastics of interest in food packaging applications.

2.2

Antimicrobial nanoclays

Nanotechnology in the form of nanocomposites can be designed to control the release of, for instance, antimicrobial natural components from packaging materials. One recent example is the release of natural antimicrobial agents such as thymol and linalool. Thymol is a phenolic monoterpene that has received considerable attention as an antimicrobial agent with very high antifungal activity and very low MIC values13 and as a possible food antioxidant.14 Linalool is another essential oil that has been previously reported to have effective antibacterial15 and antifungal16 properties that would make it suitable for the development of antimicrobial films. The combination of active technologies such as antimicrobials and nanotechnologies such as clay-based nanocomposites can synergistically lead to bioplastic formulations with balanced properties and functionalities for their implementation in packaging applications. As an example of bioactive packaging, the formulation of novel antimicrobial nanocomposites of polycaprolactone (PCL) was presented as a way to control solubility and diffusion of natural biocides such as thymol.17 The

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2.1 Schematics of the functioning of active nanoclays.

antimicrobial nanocomposites of biodegradable PCL were processed by a solution casting method. The diffusion kinetics of the released biocide were determined by Attenuated Total Reflection Fourier Transformed Infrared (ATRFTIR) spectroscopy. The enhancement of antimicrobial solubility as a result of the presence of the nanoplatelets of mica was possibly due to retention of the apolar biocide agent over the engineered nanofiller surface (see Fig. 2.1). On the other hand, the thymol diffusion coefficient was seen to decrease (from ca. 2.8  10±15 to 1.1  10±15 m2/s) with the addition of the nanoadditive in the biocomposite. This is probably the result of the larger tortuosity effect imposed on the diffusion of the biocide by the dispersed nanoclay. As a result, the incorporation of nanoclays led not only to enhancing the solubility of natural biocides into polymeric matrices but also to controlling the release of natural antimicrobials with interest in the design of novel active antimicrobial film and coating systems. With the exposure of the first commercial active packaging materials, certain concerns were raised by authorities, legislators and consumers with respect to the release of chemical antimicrobial agents such as triclosan or other organic molecules from packaging to, for instance, foods. For this reason, there has been a strong push towards the development of natural antimicrobial technologies derived from mineral, plant or animal sources.18±20 Besides the use of natural extracts, silver is a mineral with very efficient biocide properties known since ancient times. The use of silver-based antimicrobial additives for plastics used in food production and medical equipment is today permitted and regulated.21

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Thus, silver nanoparticles as well as silver compounds are widely used as efficient biocides. In fact, many commercial antimicrobial products include silver in their formulations as the active ingredient. In this context, many products have been developed for specific applications in quite different areas, i.e. medical devices, liquid disinfectants for large surfaces, personal care products, electronics, food and water storage materials to extend shelf-life, etc. Recent technical innovations and findings facilitate the availability and incorporation of silver products in a wide range of materials at the manufacturing stage, providing novel antimicrobial formulations. Nevertheless, a specific form of efficient silver does not exist for every application, procedure or matrix. In this sense, nanotechnology is becoming a key factor due to the capability of modulating metals, compounds and materials into the nanosize, which often changes their chemical, physical and optical properties, as well as those of the matrices in which they are incorporated. Stable silver nanoparticles can be obtained by using soluble starch as both the reducing and the stabilizing agent22 or by being synthesized via the regular borohydride reduction of Ag+ ions.23,24 Silver nanoparticles were synthesized in the interlamellar space of kaolin by UV radiation or chemical-induced reduction,25,26 in layered laponite suspensions via photoreduction,27 or supported on micro and mesoporous structures after ion exchange followed by in situ reduction.28,29 Silver(I) nitrate adducts with diverse electronic and steric characteristics can be synthesized with N- and P-donor ligands.30 Thus, Ag/SiO2 coating solutions have been prepared to serve for antimicrobial refinement of temperaturesensitive materials like fabrics or wood.31 Moreover, a suspension of silver nitrate in an ammonium salt medium has been reported as a precursor of stable nanoscale AgBr particles.32 In another line of work, many efforts have also been made to develop inorganic materials, such as zeolites, for supporting Ag+ ions due to their ability to incorporate and release ionic species. Coleman et al.33 prepared Ag+- and Zn+-exchanged tobermorites and demonstrated that they have a marked bacteriostatic effect and can be potentially used as antimicrobial materials for in situ bone tissue regeneration. The thermal stability of Ag+-supported È lkuÈ.34 clinoptilolite and possible applications were tested by Akdeniz and U Some reports based on silver-modified clays by a cation exchange method have been published. Oya et al.35 reported the antimicrobial properties of Ag+exchanged montmorillonite in 1991. Keller-Besrest et al.36 prepared a silverloaded montmorillonite for possible topic uses in the treatment of burns. They obtained the coexistence of both silver metal particles and Ag+ ions, and they also observed significant differences in the final silver content in clays using an exchange resin procedure (of up to 10 wt%) with regard to the standard cation exchange capacity (CEC) methodology done in solution (of up to 1 wt%). Quintana et al.37 studied the effects of calcination and mechanical grinding on silver-exchanged Na-MMT and its antimicrobial performance. They reported

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metallic silver nanoparticles on the clay aggregates, and pointed out that the antibacterial performance is affected by the availability of the ionic silver to be in contact with the bacteria. Praus et al.38 compared the antimicrobial activities of some chemical compounds, silver ions and elemental silver immobilized on montmorillonite. They demonstrated that antibacterial compounds are effective just when they are released from the inorganic carrier, and they concluded that intercalated silver ions are the most effective antibacterial elements while elemental silver does not show any antibacterial effects. In any case, silver species provide colour when incorporated into inorganic carriers and are rather unstable against temperature. However, a recent new patented technology39 that makes use of silver strongly stabilized on nanoclays either in the elemental nanoform or in ionic form (see Fig. 2.2) and that is aimed at dispersion in food contact plastic has been developed, which has been scaled up and is commercially available under the trademark of BactiblockÕ (NanoBioMatters Ltd, Paterna, Spain). This is a white powder material, heat stable and readily dispersable in all kinds of plastics with strong biocide capacity at low dosages (see Table 2.1). Regarding nanobiocomposites, the value of this technology was additionally demonstrated in PLA films.40 From the results, the silver-based nanoclay showed a strong antimicrobial effectiveness against Gram-negative Salmonella spp. with minimum inhibitory concentration and minimum bactericide concentration below 1 mg per 10 ml. PLA nanobiocomposites with different antimicrobial nanofiller loadings were trialled by casting or by melt compounding, showing excellent optical properties. An improved barrier to water was measured for the nanobiocomposites due to the presence of the nanoclay, which also exhibited strong antimicrobial performance (see Table 2.2). In this context, the European Food Safety Authority (EFSA) has recently evaluated the use of several silver-based substances intended to come into contact with foods, and defined a general specific migration limit of 0.05 mg of silver per kg of food (EFSA Journals). The

2.2 Typical TEM pictures of commercial BactiblockÕ nanoclays (a) containing elemental silver nanoparticles and (b) with ionically exchanged silver.

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Table 2.1 Typical dosages (%) of active BactiblockÕ in nanocomposites of various plastic materials required to overcome the standards JIS Z 2801 or ISO 22196:2007 for antimicrobial performance in surfaces Biocide dosage required to comply with standard JIS Z 2801 or ISO 22196:2007 Thermosets Epoxy based Polyester based

1% 3%

Thermoplastics Polypropylene Polyethylene Polystyrene Polycarbonate

0.5% 0.5% 0.5% 1%

Elastomers EVA

1%

Coatings Solvent based

1%

Source: Unpublished results by the authors.

Table 2.2 Viable cell counts before and after 24 h incubation in antimicrobial activity tests and water permeability of PLA-BactiblockÕ nanocomposite Sample Control without film PLA control film PLA-BactiblockÕ nanocomposite film

Initial CFU/mL

CFU/mL after 24 h incubation

WVTR (g m/m2 s Pa)

2:0  105 2:0  105 2:0  105

4:7  108 6:6  108 3:5  102

± 1:90  10ÿ14 1:28  10ÿ14

study of Busolo et al.40 also proved that those levels of permitted migration can be sufficient to exert strong biocide performance.

2.3

Oxygen-scavenging nanoclays

As mentioned above, supporting scavenging systems on nanoclays is a convenient strategy to develop new materials with multiple functionalities. An example of this technology based on iron is presented below.39 The incorporation of iron into nanoclays has been reported before for several applications such as water treatment and remediation processes41 and for the removal of aqueous Cu2+ and Co2+ ions in waste.42 Iron in organomodified montmorillonite has been previously prepared for the production of flameretardant materials,43 and iron nanoparticles were synthesized in the presence of

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2.3 Headspace (40 ml) %O2 reduction as a function of time caused by 1.5 g of commercial scavenging technologies.

montmorillonite as an effective protective reagent and support.44 In spite of this, there are many difficulties associated with developing iron-based systems that can lead to efficient oxygen-scavenging materials and that disperse well into packaging plastics with minimum impact on optical and mechanical properties. A feasible proprietary technology that does so, marketed under the trademark of O2BlockÕ (NanoBioMatters Ltd, Paterna, Spain) and based on nanoclays containing iron, results in a highly plastics-dispersable nanomaterial that produces a strong decrease in the headspace oxygen concentration.45 As an example, Fig. 2.3 shows the variation of oxygen content in the headspace of vials containing two commercial scavenging systems as a function of time. Taking into account that oxygen-sensitive products deteriorate relatively quickly, the kinetics of oxygen depletion may become very important, especially in the early stages, but in some other cases it may not be advisable to consume oxygen completely (see later). 46 Regarding this, the O2BlockÕ nanoclay-based grade reported in the study seems to act somewhat more slowly compared to the very efficient commercial sachet material. The reason is that the sachet most likely contains a higher mass fraction of the scavenging principle. Figure 2.4 shows the results for a LDPE containing 5 wt% of an O2BlockÕ grade, indicating that a significant reduction in oxygen content occurs in the nanocomposite. An even higher reduction in the oxygen headspace concentration was also reported in a PLA-FeMMT nanocomposite film.45 In this study, it was seen that a reduction in the oxygen content from 20.9% to 6.8% was seen to occur after six days in solution casting films. In a similar experiment, the commercial sachet system AgelessÕ reduced the oxygen content to 0.5%. Nevertheless, it is relevant to note that the commercial scavenging system AgelessÕ contains ca. 2.8 g of solid inside each sachet, this being mostly elemental iron. Considering that the mass of the nanocomposite film evaluated

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2.4 Oxygen-scavenging capacity of 1.5 g of LDPE-O2Block nanocomposite film in 40 ml headspace.

in the scavenging tests was 1.7 g, of which only 10 wt% corresponded to the iron-based clay that in turn contained ca. 25 wt% of Fe, the active material equivalent in each vial was only ca. 0.04 g. This means that if equivalent quantities of the active component were to be used the efficiency of the nanocomposite should have been higher compared to the sachet. As a result, these nanocomposites, once they are optimized for the purpose and tailored for specific packaging materials and applications, should provide great interest in the packaging of oxygen-sensitive products.

2.4

Future trends

In summary, the addition of active (antioxidant, antimicrobial, oxygenscavenging, etc.) layered engineered silicates complying with food contact regulations to biodegradable polymers through innovative technology is now available as a formidable tool for improving the properties of polymers and biopolymers and, therefore, to enhance packaged food quality and safety aspects. The fact that these technologies have become commercially available makes them even more interesting for their widespread implementation. Thus, with the advent of this new generation of nanomaterials providing multiple functionalities, i.e. combined physical reinforcement and active performance, to plastics, the plastic packaging field becomes consolidated in its own right as a high-tech area of development.

2.5

References

1. Miltz, J., Passy, N. and Mannheim, C.H. (1995). Trends and applications of active packaging systems. In: Food and Packaging Materials ± Chemical Interaction, Ackerman, P., JaÈgerstad, M. and Ohlsson, P. (eds), The Royal Society of Chemistry, London, pp. 201±210.

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2. Yam, K.L., Takhistov, P.T. and Miltz, J. (2005). Intelligent packaging: concepts and applications. Journal of Food Science, Concise Reviews and Hypotheses, 70, R1± R10. 3. Vermeiren, L., Devlieghere, F., Van Beest, M., de Kruijf, N. and Debevere, J. (1999). Developments in the active packaging of foods. Trends in Food Science and Technology, 10, 77±86. 4. Suppakul, P., Miltz, J., Sonneveld, K. and Bigger, S.W. (2003). Active packaging technologies with emphasis on antimicrobial packaging and its applications. Journal of Food Science, 68, 408±442. 5. LoÂpez-Rubio, A., LagaroÂn, J.M. and Ocio, M.J. (2008). Active polymer packaging of non-meat food products. In: Smart Packaging Technologies for Fast Moving Consumer Goods, Kerry, J. and Butler, P. (eds), John Wiley & Sons, Chichester, UK, pp. 19±32. 6. Ahvenainen, R. (2003). Active and intelligent packaging. In: Ahvenainen, R. (ed.), Novel Food Packaging Techniques, Woodhead Publishing, Cambridge, pp. 5±21. 7. Han, J.H. (2003). Antimicrobial food packaging. In: Ahvenainen, R. (ed.), Novel Food Packaging Techniques, Woodhead Publishing, Cambridge, pp. 50±70. 8. Appendini, P. and Hotchkiss, J.H. (2002). Review of antimicrobial food packaging. Innovative Food Science & Emerging Technologies, 3, 113±126. 9. Gennadios, A., Hanna, M.A. and Kurth, L.B. (1997). Application of edible coatings on meats, poultry and seafoods: a review. Lebensmittel-Wissenschaft und -Technologie, 30, 337±350. 10. Hotchkiss, J.H. (1997). Food packaging interactions influencing quality and safety. Food Additives and Contaminants, 14, 601±607. 11. Commission Directive 2002/72/EC for Food Contact Applications (EFSA), http:// www.efsa.europa.eu/ 12. Ahvenainen, R. (2002). Novel Food Packaging Techniques. CRC Press, Boca Raton, FL, pp. 27±30. 13. Thompson, D.P. (1989). Fungitoxic activity of essential oil components on food storage fungi. Mycologia, 81, 151±153. 14. Youdim, K.A. and Deanes, S.G. (2000). Effect of thyme oil and thymol dietary supplementation on the antioxidant status and fatty acid composition of the ageing rat brain. Journal of Nutrition, 83, 87±93. 15. Onawunmi, G.O., Yisak, W.A. and Ogunlana, E.O. (1984). Antibacterial constituent in essential oil of cymbopogon citratus. Journal of Ethnopharmacology, 12, 279± 286. 16. Reuveni, R., Fleischer, A. and Putievsk, E. (1984). Fungistatic activity of essential oils from Ocimum basilicum. Journal of Essential Oil, 110, 20±22. 17. Sanchez-Garcia, M.D., Ocio, M.J., Gimenez, E. and LagaroÂn, J.M. (2008). Novel polycaprolactone nanocomposites containing thymol of interest in antimicrobial film and coating applications. Journal of Plastic Film and Sheeting, 24(3±4), 239±250. 18. Sun Lee, D. (2005). Packaging containing natural antimicrobial or antioxidative agents. In: Han, J.H. (ed.), Innovations in Food Packaging, Part 2, Elsevier, New York, pp. 108±122. 19. Fernandez-Saiz, P., Ocio, M.J. and LagaroÂn, J.M. (2006). Biopolymers, 83, 577±583. 20. Sanchez-Garcia, M.D., Gimenez, E., Ocio, M.J. and LagaroÂn, J.M. (2008). Technical Papers, Regional Technical Conference, Society of Plastics Engineers, 4, 2084± 2088. 21. Simpson, K. (2003). Plastics, Additives and Compounding, 5, 32. 22. Vigneshwaran, N., Nachane, R.P., Balasubramanya, R.H. and Varadarajan, P.V.

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23. 24. 25. 26. 27. 28. 29. 30.

31. 32. 33. 34. 35. 36. 37.

38. 39.

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(2006). A novel one-pot `green' synthesis of stable silver nanoparticles using soluble starch. Carbohydrate Research, 34, 2012±2018. Lok, C., Ho, C., Chen, R., He, Q., Yu, W., Sun, H., Kwong-Hang Tam, P., Chiu, J. and Che, C. (2007). Silver nanoparticles: partial oxidation and antibacterial activities. Journal of Biology and Inorganic Chemistry, 12, 527±534. Oh, S.G., Lee, G.J., Shin, S.I. and Kim, I.C. (2004). Preparation of silver nanorods through the control of temperature and pH of reaction medium. Materials Chemistry and Physics, 84, 197±204. DeÂkaÂny, I., Patakfalvi, R. and OszkoÂ, A. (2003). Synthesis and characterization of silver nanoparticle/kaolinite composites. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 220, 45±54. DeÂkaÂny, I. and Patakfalvi, R. (2004). Synthesis and intercalation of silver nanoparticles in kaolinite/DMSO complexes. Applied Clay Science, 25, 149±159. Huang, H. and Yang, Y. (2007). Preparation of silver nanoparticles in inorganic clay suspensions. Composite Science and Technology, 68(14), 2948±2953. Yang, X., Yang, L., Wang, X. and Yang, F. (2008). Excellent antimicrobial properties of mesoporous anatase TiO2 and Ag/TiO2 composite films. Microporous and Mesoporous Materials, 114, 431±439. Lv, L., Luo, Y., Ng, W.J. and Zhao, X.S. (2009). Bactericidal activity of silver nanoparticles supported on microporous titanosilicate ETS-10. Microporous and Mesoporous Materials, 120, 304±309. Pettinari, C., Di Nicola, C., Effendy, Marchetti, F., Skelton, B.W. and White, A.H. (2007). Synthesis and structural characterization of adducts of silver(I) nitrate with ER3 (E = P, As, Sb; R = Ph, cy, o-tolyl, mes) and oligodentate aromatic bases derivative of 2,2-bipyridyl, L, AgNO3:ER3:L (1:1:1). Inorganica Chimica Acta, 360, 1433±1450. Mahltig, B., Gutmann, E., Meyer, D.C., Reibold, M., Bund, A. and BoÈttcher, H. (2009). Thermal preparation and stabilization of crystalline silver particles in SiO2based coating solutions. Journal of Sol-Gel Science and Technology, 49, 202±208. Zhang, J., Liu, X., Luo, X., Lu, S., Cao, W. (2007). A novel cetyltrimethyl ammonium silver bromide complex and silver bromide nanoparticles obtained by the surfactant counterion. Journal of Colloid and Interface Science, 307, 94±100. Coleman, N.J., Bishop, A.J., Booth, S.E. and Nicholson, J.W. (2009). Ag+- and Zn2+Ê tobermorites. Journal of the exchange kinetics and antimicrobial properties of 11A European Ceramic Society, 29, 1109±1117. È lkuÈ, S. (2008). Thermal stability of Ag-exchanged clinoptilolite Akdeniz, Y. and U rich mineral. Journal of Thermal Analysis and Calorimetry, 3, 703±710. Oya, A., Banse, T., Ohashi, F. and Otani, S. (1991). An antimicrobial agent derived from montmorillonite. Applied Clay Science, 6, 135±142. Keller-Besrest, F., BeÂnazeth, S. and Souleau, C. (1995). EXAFS structural investigation of a silver-added montmorillonite clay. Materials Letters, 24, 17±21. Quintana, P., MaganÄa, S.M., Aguilar, D.H., Toledo, J.A., Angeles-Chavez, C., CorteÂs, M.A., LeoÂn, L., Freile-PelegrõÂn, Y., LoÂpez, T. and Torres SaÂnchez, R.M. (2008). Antibacterial activity of montmorillonites modified with silver. Journal of Molecular Catalysis A: Chemical, 281, 192±199. Praus, P., MalachovaÂ, K., PavlõÂcÏkovaÂ, Z. and TuricovaÂ, M. (2009). Activity of antibacterial compounds immobilised on montmorillonite. Applied Clay Science, 43, 364±368. LagaroÂn, J.M., Busolo, M. and Fernandez-Saiz, P. (2010). Patent application ES2331640.

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40. Busolo, M.A., Fernandez, P., Ocio, M.J. and LagaroÂn, J.M. (2010). Novel silverbased nanoclay as an antimicrobial in polylactic acid food packaging coatings. Food Additives and Contaminants, 27(11), 1617±1626. 41. Frost, R.L., Xi, Y. and He, H.J. (2009). Colloid Interface Science, doi: 10.1016/ j.jcis.2009.09.027. È zuÈm, C., ErogÏlu, A.E., Hallam, K.R., Scott, T.B. and Lieberwirth, I. 42. Shahwan, T., U (2009). Applied Clay Science, 43, 172±181. 43. Wei, Q., Cai, Y., Wu, N., Zhang, K., Xu, Q., Gao, W., Song, L. and Hu, Y. (2008). Surface and Coating Technologies, 203, 264±270. 44. Yuan, P., Fan, M., Zhu, J., Chen, T., Yuan, A., He, H., Chen, K. and Liu, D. (2009). Journal of Magnetism and Magnetic Materials, 321, 3515±3519. 45. Busolo, M.A. and Lagaron, J.M., (2010). ANTEC 2010 Conference Papers, SPE Publications, Society of Plastics Engineers, Newtown, CT. 46. Miltz, J. and Perry, M. (2005). Packaging Technology and Science, 18, 21±27.

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3

Hydrotalcites in nanobiocomposites

U . C O S T A N T I N O and M . N O C C H E T T I , University of Perugia, Italy and G . G O R R A S I and L . T A M M A R O , University of Salerno, Italy

Abstract: This chapter deals with the preparative methods, structural aspects and chemical±physical characteristics of hydrotalcite-like compounds (HTlc), an emerging class of layered solids with anion exchange and intercalation properties. Biocompatible HTlc can be modified with molecular anions having pharmaceutical, antimicrobial or antioxidant activity to obtain materials that can release the active anions in different environments with a de-intercalation process. Moreover, the organic±inorganic hybrids can exfoliate when dispersed in polymeric matrices and act as active fillers of biocompatible and biodegradable polymers. The fillers could enhance the mechanical and barrier properties of the polymer and confer on it biological activity for application in food packaging, particularly in active packaging technologies and in biomedical devices. Key words: biocompatible hydrotalcite-like compounds (HTlc), intercalation of biologically active species in HTlc, modified release of drugs and active species, exfoliation of modified HTlc in biocompatible polymers, modified HTlc as active fillers of nanobiocomposites.

3.1

Introduction

Hydrotalcite is the name of a rare mineral discovered in Sweden around 1842. Its chemical formula proposed by Manasse (1915) is magnesium aluminium hydroxycarbonate, Mg6Al2(OH)16CO34H2O, while its layered structure was elucidated independently by Allmann (1968) and Taylor (1969). For a long time hydrotalcite and other isomorphous minerals (i.e. piroaurite, sjogrenite and takovite) were mainly the object of mineralogical studies, but starting from the 1970s it was realized that these rare minerals, called also anionic clays, can be easily and economically prepared on a laboratory scale and have a number of interesting chemical properties (Miyata and Kumura, 1973; Miyata, 1980, 1983). The materials obtained were named hydrotalcite-like compounds (HTlc) or layered double hydroxides (LDH) and are generally represented by the empirical formula [M(II)1±xM(III)x(OH)2]x+[An±x/n]x±
mH2O where M(II) and M(III) are bi- and trivalent metal cations with suitable ionic radius, A is the interlayer

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exchangeable anion with charge ÿn, x is the molar ratio M(III)/[M(III) + M(II)] which ranges between 0.2 and 0.4, and m is the mol of co-intercalated water (Cavani et al., 1991; TrifiroÁ and Vaccari, 1996). It was also realized that a large number of materials with different properties can be obtained by changing the nature of the divalent and trivalent cations, and the type of interlayer molecular anions opening the way for a wide range of applications. At present, HTlc find application as heterogeneous catalysts, support of catalysts (Cavani et al., 1991; Turco et al., 2004; Busca et al., 2006; Costantino et al., 2008a), adsorbents, anion exchangers, anion scavengers (Newman and Jones, 1998; PreÂvot et al., 2001; Khan and O'Hare, 2002), components and/or active principles in pharmaceutical and cosmetic formulations (Costantino and Nocchetti, 2001; Carretero et al., 2007; Choy et al., 2009a) and additives of polymeric blends (Leroux and Taviot-GueÂho, 2005; Evans and Duan, 2006; Costantino et al., 2009a). Recent progress concerns modification of HTlc by intercalation of functional species bearing anionic groups (i.e. carboxylate, phosphonate and sulfonate). Among these species, the following may be mentioned: (1) dyes and chromophors to produce new materials with photochemical and photophysical properties (Ogawa and Kuroda, 1995; Bauer et al., 2003; Latterini et al., 2007); (2) drugs and anions with biological activity and even biomolecules to obtain systems for drug release and for biomedical applications, whenever biocompatible HTlc are used as layered hosts (Choy et al., 2000, 2009b; Hwang et al., 2001; Desigaux et al., 2006; Costantino et al., 2008b); and (3) anions having hydrophobic or hydrophilic tails to render HTlc layers compatible with different polymeric chains and produce novel nanofillers of polymeric nanocomposites (Xu et al., 2004; Costantino et al., 2009a; Xu and Braterman, 2010). This last application is typical of some inorganic layered materials that, when dispersed at low loading (less than 5%) in polymeric blends, are able to exfoliate into single layers each having a thickness of the order of 1 nm, the surface of each layer being functionalized, by ion exchange or grafting reactions, with organic groups that increase the compatibility with the polymers. In addition these layered solids may intercalate polymeric chains into their interlayer regions. In this context, much work has been reported on the use of organically modified smectite clays, in particular montmorillonites, as fillers of polymeric composites, while scarce attention has been paid to anionic clays of hydrotalcite type (Camino et al., 2001; Costantino et al., 2005, 2007; Leroux, 2006; Illaik et al., 2008; Costa et al., 2008; Nyambo et al., 2008; Kovanda et al., 2010). These latter materials compare favourably with natural clays in terms of purity, wellknown stoichiometry, higher ion exchange capacity, and a wider possibility of functionalization with a variety of organic anions generally much more numerous than organic cations, commonly involved in the modification of smectite clays. When biocompatible HTlc modified with organic anions possessing biological activity are exfoliated and homogeneously dispersed in biocompatible

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and, if possible, biodegradable polymers, an interesting new class of nanobiocomposites is obtained. In these systems the active molecules, fixed by ionic bonds to the inorganic lamellae, can not only improve the compatibility with the polymeric matrix but also carry out the biological activity (i.e. pharmaceutical, antimicrobial or antioxidant) being anchored to the lamellae, or being slowly released in particular environments. The modified HTlc nanofillers thus provide active release systems, simultaneously improving the mechanical and barrier properties of the biopolymer. The present chapter will be concerned mainly with the preparation, characterization, properties and potential use of these new nanobiocomposites. It is divided into three parts: the first part will recall the structural aspect, the preparative methods and the reactivity of HTlc; the second part will deal with the properties of intercalation compounds of biocompatible HTlc with anions having biological activity; and the third part will show the preparation and properties of nanobiocomposites with biodegradable polymers. The chapter will close with a commentary and future trends.

3.2

Hydrotalcite-like compounds (HTlc): basic chemistry

In the last two decades there has been a rapid growth in the number of scientific papers and industrial patents on HTlc, because of their broad possibility of manipulation to obtain materials of interest in many different fields that involve physics and physical chemistry, chemistry and industrial chemistry, medicinal chemistry, pharmaceutical technology and cosmetics. The rich harvest of information obtained has been collected in monographs and reviews to which the reader is referred for a study in depth (Rives, 2001; Braterman et al., 2004; Duan and Evans, 2006; Williams and O'Hare, 2006; Latterini et al., 2007; Perioli et al., 2008; Choy et al., 2009a; Costantino et al., 2009a). However, to make the present contribution self-consistent, in the following sections the fundamental aspects of composition, structure, preparative methods, morphology and thermal behaviour will be recalled.

3.2.1

Composition and structural aspects of HTlc

As already pointed out, this emerging class of compounds, also known as layered double hydroxides or anionic clays, gathers natural and synthetic layered solids commonly represented by the general formula [M(II)1±xM(III)x(OH)2] [Ax/n]
mH2O, where M(III) cations are typically Al, Cr, Fe or Ga, M(II) can be Mg, Zn, Ni, Co or Cu, and A is an anion of ionic valence n. The cations have an ionic radius similar to that of Mg2+ (0.065 nm) and prefer the octahedral coordination. Therefore, despite the nature of the cations present, the structure of these compounds is similar to that of hydrotalcite mineral, having composition

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3.1 (a) View along the ab crystallographic axis of the brucite (Mg(OH)2) sheet; (b) Schematic representation of the packing of four sheets of the MgAl±HTlc in carbonate form.

[Mg0.75Al0.25(OH)2](CO3)0.125
0.5H2O. The hydrotalcite structure is clearly described by considering that of brucite, Mg(OH)2, arising from the packing of layers built up of Mg(OH)6 octahedral units with shared edges (see Fig. 3.1a). In the mineral, 25% of Mg(OH)6 units of the brucite layer are substituted by Al(OH)6 octahedral units, the excess of positive charge being balanced by carbonate anions accommodated in the interlayer region (Taylor, 1973). In a similar way, the structure of hydrotalcite-like compounds originates from the packing of brucite layers containing M(II) cations, partially replaced by M(III) cations, surrounded by six OH± ions. Note that the notation M(II) may indicate the presence of more than one type of divalent cation and M(III) of more than one trivalent cation, but the molar ratio x ˆ M(III)/[M(III) ‡ M(II)] should remain confined between 0.2 and 0.4. Figure 3.1b shows, as an example, the sequence of four layers of a Mg±Al HTlc in which x is equal to 0.33. The presence in the layer of M(III) cations gives rise to positive electrical charges balanced by exchangeable anions (An±) accommodated in the interlayer region, where m mol of water for formula weight are also located. The x value determines the charge density of the layers and hence the ion exchange capacity (IEC) of the materials (Costantino et al., 1998). The IEC is much higher than that of smectite clays and, obviously, depends also on the empirical formula, generally ranging between 2 and 4 mmol of monovalent anion per gram. In natural compounds the brucite-type sheets can stack one to another with two different symmetries, one is rhombohedral (3R) with an ABC ABC . . . stacking sequence, and is typical of pyroaurite mineral (see Fig. 3.1b); the other symmetry is hexagonal (2H) with an AB AB . . . stacking sequence, and is typical of the sjogrenite phase (Taylor, 1973). On the other hand, structural analyses and refinements reported by several authors showed that synthetic HTlc

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3.2 Rietveld plot for [Mg0.67Al0.33(OH)2](CO3)0.165
0.48H2O. Experimental (‡), calculated (ÿ) and difference (lower) profiles. Inset shows the XRD patterns.

crystallize in the 3R symmetry, although a change in stacking sequence to the 2H polytype has been observed for a Zn±Al HTlc upon dehydration at 150ëC (Hines et al., 2000). Early structural determinations were carried out on natural single crystals. Synthetic HTlc are obtained as a microcrystalline powder (see later) not suitable for single crystal structure analysis, and crystal data have been recently obtained with an X-ray powder diffraction method in which the ab initio crystal structure is refined with the Rietveld procedure. By way of example, Fig. 3.2 and Table 3.1 report the Rietveld refinement of a Mg±Al HTlc Table 3.1 Crystallographic data for [Mg0.67Al0.33(OH)2](CO3)0.165
0.48H2O Crystal system Space group aˆb c

V Z Density Rwp (background subtracted) Rp (background subtracted) RF2

Trigonal* R-3m 0.304535(9) nm 2.2701(1) nm 120ë 0.18232(1) nm3 3 2.12 g/cm3 10.37 7.98 5.56

* Numerals in parentheses represent the standard deviation of the crystallographic data obtained from the retrieved refinement.

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Table 3.2 Structural parameters of indicated HTlc Sample [Mg0.67Al0.33(OH)2](CO3)0.165
0.48H2O [Zn0.67Al0.33(OH)2](CO3)0.165
0.51H2O [Co0.68Al0.32(OH)2](CO3)0.16
0.52H2O [Ni0.68Al0.32(OH)2](CO3)0.16
0.52H2O [Zn0.52Al0.37Cu0.11(OH)2](CO3)0.176
0.47H2O [Ni0.52Zn0.18Al0.30(OH)2](CO3)0.15
0.55H2O [Ni0.55Mg0.13Al0.32(OH)2](CO3)0.16
0.52H2O

a (nm)

c (nm)

V (nm3)

0.30454(1) 0.30748(1) 0.30738(1) 0.30749(1) 0.30728(1) 0.30564(1) 0.30622(1)

2.2701(1) 2.2769(1) 2.2840(1) 2.3707(1) 2.2686(1) 2.3148(1) 2.3763(1)

0.18233(1) 0.18642(1) 0.18689(1) 0.19413(1) 0.18551(1) 0.18726(2) 0.19297(1)

* Numerals in parentheses represent the standard deviation of the crystallographic data obtained from the retrieved refinement.

(Costantino et al., 1998), while Table 3.2 reports the structural parameters, obtained with this procedure, of several HTlc having different composition. Recently, structural and thermodynamic parameters have been obtained from molecular modelling (MM) procedures using different force field approaches (Lombardo et al., 2005, 2008).

3.2.2

Methods of preparation of HTlc

Traditional, simple procedures used in gravimetric analysis for the precipitation of insoluble metal hydroxides have been suitably modified to obtain synthetic hydrotalcites in carbonate, chloride or nitrate form. The most common procedures concern, in fact, the co-precipitation of the metal ions (at a given concentration and given molar ratio) and the charge-balancing anions dissolved in a solution maintained at room temperature or at 60±80ëC, under vigorous stirring, with a precipitating alkaline solution. The precipitation may be carried out at almost constant pH value, using as precipitating reagent buffer solutions, i.e. a NaHCO3/Na2CO3 solution, or at variable pH by titrating the metal ion solution with NaOH solution. Furthermore, the precipitation may be carried out at a low or high supersaturation degree according to the solution concentration and the rate of addition of the precipitating reagent. To improve the crystalline degree and the particle size, often the precipitate is aged for some days or hydrothermally treated (Cavani et al., 1991; Rives, 2001). Other preparative routes consider the so-called precipitation from a `homogeneous' solution or the sol gel technique. In the first case, a clear solution containing M(II) and M(III) salts (chloride or nitrate) at a concentration of 0.5±1.0 mol/dm3, and with molar ratio M(II)/M(III) ranging from 2 to 3, has urea added (molar ratio of urea/M(III) about 10). The solution is brought to 90± 100ëC under stirring. The urea hydrolysis generates ammonium carbonate and a pH of about 9 that affords the formation of HTlc in carbonate form. Wellcrystallized powders with a narrow distribution of crystal size are generally

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obtained (Costantino et al., 1998; Adachi-Pagano et al., 2003). The use of esamethylentetramine, which upon hydrolysis generates ammonia, has also been proposed (Choy et al., 2002; Iyi et al., 2004). In the sol-gel technique the M(II) and M(III) sources are alkoxides or acetylacetonates hydrolysed at a given temperature (Prinetto et al., 2000; Paredes et al., 2006), in some instances also in the presence of microwave irradiation (Rives et al., 2006). Recently, for niche application, methods of obtaining HTlc nanocrystals of dimension 50±250 nm have been proposed. Most are based on the control of the two steps of the precipitation process, that is, nucleation (formation of seeds) and crystal growth (ageing) (Choy et al., 2002; Xu et al., 2006; Duan and Evans, 2006; Rives et al., 2006; Okamoto et al., 2006; Liu et al., 2007; Ma et al., 2007; Gunawan and Xu, 2008). This control has been applied to both the coprecipitation and urea methods. In the former case a fast nucleation process is followed by a hydrothermal treatment at a temperature of 100±120ëC for different times. With the increase of time, particles with increasing crystal size and a sufficiently uniform size distribution are obtained. In the urea method, the addition of ethylene glycol and short refluxing times allows one to obtain particles of nanometric dimensions. It is also worth mentioning methods based on the formation of nanoparticles inside the water pool of reverse micellae (O'Hare and Hu, 2005; Hu et al., 2006; O'Hare et al., 2007; Liu et al., 2008). Colloidal dispersions of Mg±Al, Zn±Al and Ni±Al HTlc in bromide form, having dimensions of 50±100 nm, have been prepared with the double water-inoil microemulsions technique, which consists of mixing two microemulsions, one containing the M(II) and M(III) nitrate salt and the other with ammonia as precipitating reagent. Collisions between the two different micellae allow the formation of HTlc nanocrystals inside the water pool (Bellezza et al., 2009a). For the convenience of the reader, Fig. 3.3 summarizes the synthetic procedure discussed above. It should be clear that the co-precipitation methods are the most appropriate for the preparation of large amounts of HTlc fillers for polymer nanocomposites, and the urea methods for producing materials suited to fundamental studies and for pharmaceutical and cosmetic application; whereas methods for the preparation of nanocrystals produce materials that are used in the formation of thin films or as non-viral transfer vectors in cells and cellular tissue (Choy et al., 2009a).

3.2.3

Physical±chemical characterization of HTlc

Hydrotalcite-like microcrystals, once their chemical composition is known, are characterized by means of the most common techniques used in solid state and material chemistry. The solubility of HTlc in water obviously depends on the composition, though they are generally considered insoluble in the pH interval 4±10 in the absence of complexing agents of the metals. The X-ray powder diffraction (XRPD) patterns furnish various important information: (1) whether

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3.3 Steps involved in HTlc preparation by the double water-in-oil microemulsions technique.

a single phase or more than one phase is present; (2) the pattern may be indexed to have structural information, the intensity of the XRPD reflections being sensitive to the crystalline degree; and (3) the crystallite size along a given direction can be calculated from the broadening at half-height of the corresponding diffraction peaks by using the Debye±Scherrer equation. Fourier transform-infrared (FT-IR) spectroscopy provides information on the bonded water, the presence of hydrogen bonds, the nature of the intercalated anions, and the presence of impurity charge-balancing anions, such as carbonate and nitrate. The FT-IR spectrum may be considered a fingerprint of a given sample. The thermal properties are commonly studied by performing a coupled thermogravimetric±differential thermal analysis (TG-DTA) (Palmer et al., 2009). In certain cases these techniques are associated with an evolved gas analyser or recording high-temperature XRPD patterns for the identification of the thermally induced phase transitions. In general the thermal decomposition of HTlc can be divided into three endothermic stages, the first stage corresponds to the loss of physisorbed and co-intercalated water and occurs between room temperature and approximately 200ëC; the second stage sees the loss of constitutional water because of the dehydroxylation of brucite layers and occurs in the 250±400ëC range; and the third stage corresponds to the elimination of the charge-balancing anion. If organic anions are present and the TG-DTA analysis is performed in air, their combustion is observed with a strong exothermic effect. Often, the second and third stages overlap and at the end of the decomposition process a mixture of M(II) and M(III) oxides is obtained. At 800±1000ëC spinel

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3.4 TG±DTA curves of [Mg0.67Al0.33(OH)2](CO3)0.165
0.42H2O (operative conditions: heating rate 10ëC/min, air flow).

phases are formed. By way of example, Fig. 3.4 shows the TG-DTA curves of a Mg±Al HTlc in carbonate form. At 1000ëC a mixture of MgO and MgAl2O4 is obtained. The different preparative methods give rise to materials with the same composition but with different specific surface area and morphology of the microcrystals. The surface area is generally calculated from the N2 absorption isotherms obtained at 79 K, according to the B.E.T. method. It depends on composition and crystalline degree. Materials obtained with co-precipitation methods have a surface area (60±100 m2/g) (Yun and Pinnavaia, 1995) higher than that of materials obtained with urea methods (20±40 m2/g) (Costantino et al., 1998). Scanning electron microscopy (SEM) and sometime transmission electron microscopy (TEM) are used to analyse the morphology of the microcrystals. More or less regular platelets of hexagonal shape and dimension of the order of micrometres are generally found, again depending on the preparative methods. HTlc prepared by co-precipitation and aged and/or subjected to hydrothermal treatment show a rather small crystal size, less than 1 m, which is desirable for catalytic application. The urea method generally affords uniform and well crystallized powders of micron order and well-defined hexagonal shape (see Fig. 3.5a). For use as a filler for polymers, large platelet crystals having, when exfoliated, a high aspect ratio, are looked for. Therefore, studies have been published on the control of the crystal size of HTlc obtained with homogeneous precipitation methods (Choy et al., 2002; O'Hare and Hu, 2005; Xu et al., 2006; Evans and Duan, 2006; Rives et al., 2006; Okamoto et al., 2006; Hu et al., 2006; Liu et al., 2007; Ma et al., 2007; O'Hare et al., 2007; Gunawan and Xu, 2008; Liu et al., 2008).

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3.5 Micrographs of ZnAl±HTlc obtained by (a) urea method and (b) double water-in-oil microemulsions technique.

In addition, several procedures to exfoliate these microcrystals have been developed with a view to their application in nanotechnology (thin films, layerby-layer stacking) (Adachi-Pagano et al., 2000; Hibino and Jones, 2001; O'Leary et al., 2002; Hibino, 2004; Li et al., 2005; Wu et al., 2005; Hibino and Kobayashi, 2005; Jobbagy and Regazzoni, 2006; Jaubertie et al., 2006; Liu et al., 2006). Nanocrystals, when withdrawn from the colloidal dispersion, tend to aggregate, and very interesting nest-like or globular particles are generally observed (see Fig. 3.5b) (Bellezza et al., 2009a). Many other chemical±physical characterizations performed, for example, with XPS and ESCA (electron spectroscopy for chemical analysis) (Lakshmi Kantam et al., 2006; Fang et al., 2010), solid state nuclear magnetic resonance (MAS-NMR) spectrometry (Sideris et al., 2008), UV-vis spectrophotometry, fluorimetry, confocal fluorescence microscopy (Latterini et al., 2007) and impedance bridges to determine the ionic conductance have been reported to study particular properties and correlated applications of HTlc (Costantino et al., 1997; Mignani et al., 2009).

3.3

Organically modified biocompatible hydrotalcite-like compounds (HTlc)

In the previous section the general characteristics and properties of hydrotalcites have been discussed. The present section will deal with techniques of modification and functionalization of HTlc with different anions and, in line with the present contribution, biocompatible Mg±Al or Zn±Al HTlc and molecular anions with biological activity will be considered. Such association gives rise, in fact, to inorganic±organic hybrid materials in which bioactive species are stored in the interlayer region, often protected from light and oxygen, and potentially being released after a chemical signal. These hybrids have been proposed as systems for modified drug release (Costantino and Nocchetti, 2001; Ambrogi et al., 2001, 2002, 2003; Del Arco et al., 2004, 2009; Dupin et al., 2004; Li et al.,

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2004; Mohanambe and Vasudevan, 2005; Del Hoyo, 2007; Costantino et al., 2008b, 2009a, 2009b; Ay et al., 2009) and vectors for gene delivery. For example, DNA has been intercalated, protected from nuclease degradation and transfected into nuclei (Choy et al., 2000). Intercalation of molecular anions used in pharmaceutical care (emollients, surfactants, skin nutrients, vitamins and sunscreens) produces new materials to be used in cosmetics (Perioli et al., 2006a, 2006b, 2008). Other interesting products have recently been obtained after intercalation of antimicrobial and antioxidant species (Costantino et al., 2009a, 2009c). Especially these latter hybrids, when homogeneously and efficiently dispersed in polymeric film, may find application in the active packaging of food. In the following, as well as discussion on the procedures to modify the biocompatible HTlc, the composition and properties of the obtained hybrids, divided according to the nature of the intercalated molecular anions, will be reported.

3.3.1

Synthetic routes to obtain biocompatible HTlc intercalated with molecular anions with biological activity

Hydrotalcite-like compounds based on Mg±Al and Zn±Al are biocompatible materials reported in different pharmacopeias and already used in medicine as antacid and antipepsinic agents (Lin et al., 1998; Linares et al., 2004; Konturek et al., 2007) and in many ointments and poultices for the protection of damaged skin. However, the most promising aspect of their development is the use of intercalation compounds with drugs or anions with biological activity to obtain sustained release formulations and active fillers of polymers (Sammartino et al., 2006; Tammaro et al., 2007; Costantino et al., 2009b, 2009c). The conversion of the original HTlc, generally obtained in carbonate form, into intercalation compounds with these species is reached with different procedures; the most used are based on anion exchange reaction, reconstruction of calcinated hydrotalcite and co-precipitation. In designing the anion exchange reaction the nature of the counterion originally present in the HTlc should be considered. The diffusion of bulky anionic species into the interlayer region will be facilitated if the counterions originally present have a low affinity for the matrix and determine a large gallery height. If the known selectivity scale, CO32± > SO42±  OH± > F± > Cl± > Br± > NO3± > ClO4±, is taken into account (Miyata, 1983), HTlc containing chloride, or better, nitrate anions are to be considered the most suitable precursors for the uptake of biologically active species. Hence, HTlc containing the strongly held carbonate anions should be converted in chloride form by titration with 0.1M HCl at constant pH of 5; moreover, the HTlc±Cl can be equilibrated with an aqueous solution of 0.5M NaNO3 (molar ratio NO3±/Cl± ˆ 10) to obtain the nitrate form of the hydrotalcite (HTlc±NO3). The intercalation

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3.6 (a) Anion exchange isotherms of MgAl±NO3 towards TIAP. Experimental conditions: concentration 0.1M, temperature 25ëC, reaction time 3 days. (b) Xray powder diffraction patterns of the MgAl±HTlc at different exchange percentages of TIAP: (1) 24.2%, (2) 46.8%, (3) 64.9%, (4) 94.1%.

mechanism and the relative selectivity coefficient can be studied both by determining the anion exchange isotherm and by following the structural changes by taking the XRD patterns of samples at different degrees of exchange (Costantino and Nocchetti, 2001). By way of example, Fig. 3.6a shows the anion exchange isotherm of Mg±Al±HTlc±NO3 towards tiaprofenic anion (TIAP), while Fig. 3.6b shows the X-ray diffraction patterns of the Mg±Al±HTlc at different exchange percentages of TIAP. It may be seen that the drug is exchanged with high selectivity and that the ion exchange process occurs with a first-order phase transition from the NO3 phase to the TIAP phase (Costantino et al., 2008b). The reconstruction procedure, typical of Mg±Al±CO3 and in some instances of Zn±Al±CO3, takes advantage of the so-called `memory effect' of the hydrotalcite heated at 300±500ëC. The calcinated solid, consisting of a mixture of magnesium (or zinc) and aluminium oxides, is able to reconstruct the lamellar structure in water or in aqueous solution of given anions (Rey and Fornes, 1992; Rocha et al., 1999). When the regeneration occurs in CO2-free distilled water, the positive charge of the lamellae will be balanced by OH± ions. The interlayer OH± groups can be replaced by other anions via an acid±base reaction with the corresponding species in acid form. Otherwise, the reconstruction should be carried out in a solution containing the guest in acid form in order to have the direct intercalation of the guest. The direct synthesis by co-precipitation requires the precipitation of the HTlc in the presence of the anionic form of the guests. The chloride or nitrate M(II) and M(III) salts are often used and dissolved in a solution containing the selected guest. Co-precipitation is performed at pH between 9 and 10 by addition of NaOH solution. Well-crystallized samples are formed when the guests have a high selfassembly tendency. In other cases a hydrothermal treatment of the obtained intercalates may improve the crystallinity of the products (Reichle, 1986).

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3.7 Experimental routes to obtain HTlc intercalation compounds.

Figure 3.7 summarizes the experimental routes described above to obtain intercalation compounds.

3.3.2

HTlc hybrids containing anti-inflammatory and antibiotic drugs

Microcrystals of Mg±Al and Zn±Al hydrotalcites have been used as a reservoir of different non-steroidal anti-inflammatory drugs (NSAID) and of some antibiotics to obtain systems able to release the drugs in different biological fluids (Costantino et al., 2008b, 2009a) The chemical nature and reactivity of hydrotalcites allow one to design drug-intercalated layered materials for sustained release of the drug or for improving solubility and bioavailability of poorly soluble drugs. Drug-intercalated HTlc dispersed in biological fluids with pH around 7 can release the guest species via ion-exchange reactions. The release rate is affected by many factors, such as drug shape and size, arrangement of the drug anions into the interlayer region, selectivity of the HTlc towards the anions present in the release medium, and the dimensions of the HTlc particles (Williams and O'Hare, 2006). Moreover, HTlc are not simply acting as delivery matrices, but can also improve the apparent solubility of the drug; indeed, if the intercalation compounds are in a medium at acid pH (less than 4), the matrix slowly dissolves and the drug is released anion by anion in the medium. Moreover, the hydrotalcite matrix showed barrier properties similar to those of gastric mucus and may provide mucosal protection to the side-effect of the drug (GruÈbel et al., 1997). Thanks to the particular interaction of a drug

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3.8 Structural formulae and acronyms of NSAID and antibiotics used as guests of HTlc.

with the mucus network, its co-administration with hydrotalcite can not only ensure a protective effect but also improve the drug permeability through gastric mucus (Del Arco et al., 2004; Shaw et al., 2005; Perioli et al., 2010b). Drugs belonging to the NSAID class such as ibuprofen (IBU), diclofenac (DIK), indomethacin (IND), ketoprofen (KET), tiaprofenic acid (TIAP) and

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Table 3.3 Composition, interlayer distance and drug loading of NSAID-intercalated HTlc NSAID

Intercalation compounda

IND KET TIAP DIK IBU FLU

[MgAl]0.33IND0.20Cl0.13
0.3H2O [MgAl]0.33KET0.27Cl0.06
0.4H2O [MgAl]0.33TIAP0.27Cl0.06
0.4H2O [MgAl]0.33DIK0.33
1H2O [MgAl]0.33IBU0.33
0.5H2O [MgAl]0.40FLU0.31Cl0.09
0.8H2O

d (nm)

Drug loading (%)

2.57 2.27 2.27 2.36 2.17 2.42

50.4 50.0 50.3 55.0 50.0 49.3

a

[MgAl]x indicates [Mg1ÿxAlx(OH)2].

flurbiprofen (FLU) have been chosen as guests of Mg±Al±HTlc. The structural formulae of the selected NSAID are reported in Fig. 3.8. These bioactive species, containing carboxylate groups, have been intercalated both by ionexchange reactions, starting from HTlc±Cl, and by reconstruction of the HTlc structure. The best results, in terms of crystallinity and loading of the intercalation compounds, have been obtained with the former procedure. Composition, drug-loading and interlayer distance of the obtained hybrid materials are reported in Table 3.3. The knowledge of drug anion dimensions and shape as well as of the gallery height of the intercalation compounds allows one to predict the arrangement of the guest species into the interlayer region. In general, the drug anions are packed as a monolayer of partially interdigitated anions, with their principal axis at a slanting angle with respect to the layer plane. The ionogenic groups (±COO±) interact with the positive charges of the sheets, and the organic residues point to the interlayer region. As an example, the computer-generated disposition of the TIAP anions into the interlamellar region of MgAl±HTlc is shown in Fig. 3.9. The high tendency of the guest species to aggregate as a compact monofilm justifies the marked preference of the HTlc for these species (see the isotherm of Fig. 3.6a). Intercalation compounds containing DIK and IBU have been submitted to in vitro drug release studies in simulated intestinal fluid at pH 7.5 and in a solution designed to mimic the ionic conditions of the small intestine (pH 7.0) (Ambrogi et al., 2001, 2002). In the intestinal tract the drug released from intercalated product is due to exchange of drug ions with the phosphates, hydroxides and carbonates present in the intestinal medium. HTlc±DIK and HTlc±IBU have shown a sustained drug release; in particular, at pH 7.5 the dissolution rate of DIK from HTlc±DIK was 38% after 15 min, 60% after 90 min and 90% after 9 h; at pH 7.0, the DIK release from HTlc±DIK was 20% after 15 min, 40% after 2 h, 50% after 4 h, up to a maximum of 70% at the end of the experiment (24 h). In order to study the effect of particle size on the drug release rate, Zn±Al nanosized hydrotalcite, with dimensions of about 350 nm, has been used as host

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3.9 Computer-generated representation of HTlc±TIAP: TIAP ions are arranged in the interlayer region to form a monolayer partially interdigitated, with their principal axis at a slanting angle with respect to the layer plane.

of DIK and submitted to in vitro drug release studies, and its profile has been compared to that obtained from Zn±Al±HTlc±DIK microparticles (2 m), as shown in Fig. 3.10. The release profiles at the higher pH value (7.5) show different guest release times within the first hours (DIK released from nano- and micro-HTlc: 55% and 38% after 15 min, 80% and 53% after 60 min, respectively). The decrease of particle size determines the increase of the crystal edges and of the amount of intercalated species in the nanocrystal external part. Moreover, the diffusion of the anions through the ZnAl nanoparticles is faster than that through the microparticles due to the decrease of the length of the HTlc galleries. The above considerations affirm that the guest release time from HTlc, within the first hour, depends on the particle size. After the burst effect, the nano- and micro-HTlc±DIK profiles are similar (Perioli et al., 2010a). Some NSAID intercalation compounds have been tested to improve the solubility of poorly water-soluble drugs such as INDO, KET, TIAP and FLU. The solubility measurements of drugs from the intercalate were determined in a gastric juice with pH 1.2 (USP 25 at 37ëC) in which the hydrotalcite quickly dissolves, releasing the drug in molecular form promptly suitable for absorption.

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3.10 DIK release in phosphate buffer at pH 7.5 from nano- and micro-HTlc± DIK intercalation compounds.

The best results were obtained for indomethacin; indeed, the apparent solubility enhancement was seven times higher than that from the crystalline drug (Ambrogi et al., 2003). Recently, good results have been obtained with FLU and the hypoglycemic gliclazide too: an improvement of the drug dissolution rate in gastric medium and of the permeability through gastric mucus has been observed (Ambrogi et al., 2009; Perioli et al., 2010b). The design of formulations able to maintain pharmacologically active drug levels for long periods, avoiding repeated administrations, and to deliver and release the drug in its pharmaceutical target, and of formulations able to improve the apparent solubility of very insoluble drugs, has been extended to some antibiotic and antibacterial species. Drugs having antibacterial activity belonging to the quinolones (Nalidixic acid), fluoro-quinolones (Ciprofloxacin) and -lactam (Amoxicillin) classes and a bacteriostatic antibiotic (choramphenicol hemisuccinate) have been used as guests of HTlc (Costantino et al., 2009a). The structural formulae and acronyms of the selected drugs are shown in Fig. 3.8. The presence of the carboxylic group makes this species suitable to interact with the positive charges of the hydrotalcite lamellae. Intercalation compounds have been obtained by taking advantage of ion-exchange reactions. Because of its high steric hindrance, hydrotalcite in nitrate form has been used as starting material in order to favour the intercalation of the drugs. In fact the low affinity of nitrate anions for the HTlc and the relatively high interlayer distance of the intercalation compound, 0.87 nm, may promote the diffusion of the big drug anions. Table 3.4 shows the compositions, the interlayer distance and the drug loading (worded as grams of drug per 100 g of hybrid) of the intercalation

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Table 3.4 Composition, interlayer distance and drug loading of antibioticintercalated HTlc Antibiotic

Intercalation compounda

Cfs Nal Cipro Amox

[MgAl]0.35Cfs0.26(NO3)0.09
1H2O [MgAl]0.33Nal0.29(NO3)0.04
0.9H2O [MgAl]0.37Cipro0.35(CO3)0.01
1.2H2O [MgAl]0.33Amox0.13(CO3)0.1
0.4H2O

d (nm)

Drug loading (%)

2.47 2.11 1.39 1.87

57.0 46.3 58.6 39.5

a

[MgAl]x indicates [Mg1ÿxAlx(OH)2].

compounds. On the other hand, the very low-soluble Cipro has been intercalated by reconstructing the HTlc structure. In particular, a stoichiometric amount of solid Cipro, in acidic form, has been added to an aqueous slurry of HTlc in OH± form; an acid±base reaction occurs between the intercalated hydroxyl groups and the Cipro, resulting in intercalation of the drug in anionic form. As an example, Fig. 3.11 illustrates the computer-simulated model, obtained with the Hyperchem program, of the probable arrangement of CFS anions in the HTlc±CFS. The model has been obtained on the basis of the dimensions of the guest, the structural data of the host, and the composition and interlayer distance

3.11 Computer-generated representation of HTlc±CFS.

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of the intercalation compound. The CFS species are arranged into the interlayer region as a bi-film in which the ± interactions between the benzene rings occur. Using ion exchange, many novel hybrids may be synthesized, and beside anti-inflammatory drugs and antibiotics, tranexamic acid (Trx, trans-4(aminomethyl)cyclohexanecarboxylic acid) has been incorporated into HTlc to obtain nanohybrids that can slowly release these active guest molecules (Costantino et al., 2009b). Tranexamic acid, a synthetic derivative of the amino acid lysine, is an antifibrinolytic molecule also used as a haemostatic agent. It acts against breakdown of clots (by inhibiting or stopping plasminogen activation and fibrinolysis), so it is useful in stopping severe blood loss as it increases clot formation. It is also used in surgical procedures and dental extractions for people with haemophilia.

3.3.3

HTlc hybrids containing amino acids and proteins

Several recent papers have reported the intercalation of amino acids, oligopeptides (Aisawa et al., 2001, 2006; Hibino, 2004; Yasutake et al., 2008; Gao et al., 2009) and proteins into hydrotalcite-like compounds. Aromatic and bicarboxylic amino acids have been incorporated into hydrotalcites via ionexchange reactions starting from Mg±Al±HTlc in nitrate form (Costantino et al., 2009a). Table 3.5 shows the composition and the interlayer distance of intercalation compounds with selected amino acids. Note that incomplete intercalation has been obtained and the amino acid amount has always been insufficient to compensate for the sheet positive charges; anions such as hydroxyl or carbonate have been co-intercalated because of the high pH value of the equilibrating solutions (pH = 9), while in other cases some unexchanged nitrate remained. Studies on the preferential intercalation of pure enantiomers have been carried out. In particular, intercalation reactions have been performed starting both from phenylalanine (Phe) racemic solution and from the L-Phe enantiomeric solution. The specific rotatory power of the DL-phenylalanine solution, Table 3.5 Composition and basal spacing (d) of the indicated intercalation compounds (amino acids) dried at 75% of relative humidity Amino acid

Compositiona

DL-Phe

[MgAl]0.32Phe0.22OH0.1
0.52H2O [MgAl]0.32Tyr0.15OH0.17
0.2H2O [MgAl]0.32DOPA0.16OH0.16
0.32H2O [MgAl]0.37Glu0.24(NO3)0.13
0.34H2O [MgAl]0.37Asp0.25(NO3)0.12
0.45H2O

DL-Tyr

L-DOPA L-Glu

DL-Asp a

[MgAl]x indicates [Mg1ÿxAlx(OH)2].

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3.12 Structural model of the Mg±Al±DL±Phe.

after equilibration with the hydrotalcite, was about zero, showing that the Phe was intercalated as DL dimers. Moreover, the composition and the interlayer distance were not affected by the type of enantiomer present in solution. Figure 3.12 reports a tentative structural model of the compound containing DL-Phe, achieved with the program Hyperchem after having optimized the Phe anions with the MM+ force field. The microcrystals seem to be constituted by assembling two lamellae, one bearing the L enantiomers and the other bearing the D enantiomers, so that the Phe anions make an ordered monolayer of interdigitated moieties into the interlayer region. On the other hand, in the Mg± Al±L±Phe compound the L species can be intercalated with high steric hindrance. Studies on the thermal behaviour of Mg±Al±DL±Phe and Mg±Al±L±Phe, performed by thermogravimetric analysis and in situ high-temperature powder diffraction, have shown that upon water loss the Phe species rearrange their disposition into the interlayer region, reaching an interlayer distance of 2.7 nm at 150ëC. FT-IR spectra of the Mg±Al±DL±Phe and Mg±Al±L±Phe recorded before and after thermal treatment at 150ëC are superimposable, suggesting that the samples do not undergo chemical reactions as polymerizations or grafting. The presence of an OH group in the para position on the Phe phenyl ring, to obtain the aromatic amino acid tyrosine (Tyr), is responsible for the air- and photo-instability of the Tyr. The intercalation of Tyr was performed in order to protect the amino acid from oxygen and light. Similarly to the systems containing Phe, intercalation compounds with DL- or L-Tyr have been prepared (Table 3.5). The compositions and the interlayer distances are independent of the type of enantiomer. However, the OH group in the para position on the phenyl ring causes a contraction of the basal spacing with respect to Mg±Al±DL±Phe which is very probably due to the formation of hydrogen bonds between the guest OH

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groups and the OH groups of the lamellae through hydration water molecules, as already observed in similar systems. It was of interest to investigate the ability of Mg±Al±HTlc to intercalate L-DOPA (L-3,4-dihydroxyphenylalanine), which is structurally similar to the previously studied amino acids and is used as a drug in Parkinson's disease. The catecholic group of the L-DOPA makes these molecules easily oxidizable to quinone. The intercalation was achieved with success at pH 9 and in the presence of hydrazine as antioxidant. More complex bioactive species such as oligopeptides have also been incorporated into Zn±Al±HTlc by coprecipitation reaction (Aisawa et al., 2006). The interfacial behaviour and the adsorption of biological macromolecules such as proteins on solid inorganic surfaces are two of the major interesting topics in the biotechnology area (Gray, 2004). The adsorption of a protein onto a non-biological solid surface is an important phenomenon, not only because it may affect the biological functioning of the macromolecules (Haynes and Norde, 1995) but also because it is the key to several important applications such as artificial implants, protein purification strategies, biosensors, drug delivery systems, catalysts and catalyst supports (Nakanishi et al., 2001; He et al., 2006; Martinez Martinez et al., 2008). Protein adsorption is a complex process involving many events such as conformational changes, hydrogen bonding and/or hydrophobic and electrostatic interactions. Although surface±protein interaction is not well understood, the chemical nature of the surface has been shown to play a fundamental role in protein adsorption (Bellezza et al., 2002, 2006). Proteins adsorb in different quantities, conformations and orientations, depending on the chemical and physical characteristics of both protein and support surfaces. In the biomaterials field, much research has been devoted to methods that modify the size and textural surface of existing materials in order to achieve more desirable biological integration (Caruso, 2001). HTlc has scarcely been exploited for the adsorption of biological macromolecules such as proteins and enzymes at the solid±liquid interface. Recently, delamination/restacking and co-precipitation methods have been employed to immobilize and adsorb several proteins such as porcine pancreatic lipase (PPL), haemoglobin (Hb), bovine serum albumin (BSA) (An et al., 2009; Charradi et al., 2009), urease (Vial et al., 2008), alkaline phosphatase (AlP) (Mousty et al., 2008) and horseradish peroxidase (HRP) (Chen et al., 2008) into HTlc of micrometric size to develop novel biosensors (Mousty, 2010). Many researchers have indicated that an important factor in determining the biological response of solid materials is the particle size. Attention has been focused on nanomaterials, which offer a new pathway for regulating protein behaviour through surface interactions because they can provide large surface areas for efficient protein binding, and multivalent functionalities can be grafted on their surfaces to meet the structural complexity of biomolecules (Katz and Willner, 2004; Bellezza et al., 2005, 2007).

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The adsorption of myoglobin (Mb) onto Ni±Al±HTlc nanoparticles has been investigated in terms of structural properties and enzymatic activity. The nanostructured biocomposite is active in the oxidation of 2-methoxyphenol by hydrogen peroxide, and the observed enzyme kinetics follow the Michaelis± Menten mechanism. The catalytic turnover (kcat) and the Michaelis constant (KM) values of adsorbed Mb are lower than those of the native protein, while the catalytic efficiency (kcat/KM) of the adsorbed protein is slightly decreased. In order to explain the decrease of Mb catalytic activity, IR, fluorescence and Raman spectra were collected; the adsorption of protein molecules onto the nanoparticle surface alters the tertiary structure without changing the secondary structure. The absence of catalytic activity for Mb adsorbed onto Ni±Al±HTlc prepared with the urea hydrolysis method, together with the low adsorption capacity of these large HTlc particles, is evidence for the importance of the surface dimensions in the modulation of protein activity (Bellezza et al., 2009b).

3.3.4

HTlc hybrids containing antimicrobial and antioxidant species

Recently, increasing attention has been paid to developing and testing films with antimicrobial properties in order to improve food safety and shelf-life. In this context the preparation of inorganic filler organically modified with antimicrobial species has gained academic interest. The obtained hybrids can be finely dispersed into polymeric matrices and can slowly release the active species. Benzoate derivatives having antimicrobial activity, such as benzoate (Bz), 2,4-dichlorobenzoate (BzDC) and para- and ortho-hydroxybenzoate (pBzOH and o-BzOH), have been chosen as guest model species for HTlc (Costantino et al., 2009c). Benzoate and benzoate derivatives are used as food preservatives and show toxicity at very high levels (maximum acceptable daily intake 5 mg/kg body weight). Intercalation compounds have been obtained by an anion exchange procedure starting from the nitrate form of the hydrotalcite and their chemical compositions and interlayer distances are summarized in Table 3.6. It may be noted that the molecular anions replace almost completely the HTlc nitrate counteranions. FT-IR absorption spectroscopy of the Zn±Al±HTlc±Bz sample suggested the presence of a monodentate carboxylate coordination with the brucite-type layer. This experimental information, together with knowledge of the chemical composition, interlayer distance and van der Waals dimensions of the guests, has allowed structural models of the intercalation compounds to be proposed. Generally, the anions are arranged with the Ph±COO± bond almost perpendicular to the layer and form a partially interdigitated monolayer (see Fig. 3.13). However, the position and nature of the aromatic ring substituents affect the gallery height. Guest species that are ortho-substituted (o-BzOH) are

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Table 3.6 Composition and basal spacing (d) of the indicated intercalation compounds (antimicrobials) dried at 75% of relative humidity Antimicrobial

Compositiona

ZnAl-Bz ZnAl-o-BzOH ZnAl-p-BzOH ZnAl-BzDC

[ZnAl]0.35Bz0.35
1H2O [ZnAl]0.35o-BzOH0.27(NO3)0.08
1H2O [ZnAl]0.35p-BzOH0.33(NO3)0.02
0.85H2O [ZnAl]0.35BzDC0.32(NO3)0.03
1H2O

a

d (nm) 1.55 1.55 1.53 1.68

[ZnAl]0.35 indicates [Zn0.65Al0.35(OH)2].

3.13 Computer-generated models showing the most probable arrangement of: (left) Bz and (right) p-BzOH anions between the LDH layers.

arranged into the interlayer region like the unsubstituted species (Bz) because the long axis of the guest anions is unvaried. On the other hand, the presence of a para-substituent (BzDC) causes an increase of the BzDC long axis dimension and then an increase of the interlayer distance of about 0.13 nm with respect to the Zn±Al±Bz. However, the nature of the substituent plays a fundamental role in the interlayer distance value. When the para substituent is the OH group, a network of hydrogen bonds between this group and the OH group of the sheet through the hydration water should occur, bringing the lamellae nearer. Indeed, the interlayer distance of Zn±Al±p-BzOH is smaller (1.53 nm) than that of Zn±Al±Bz (1.55 nm). Another very important topic in food stability and human health is the prevention of lipid oxidation. Many studies have been carried out to search for and develop antioxidants having a natural origin to be used in the food industry to delay the oxidation process. Hydroxycinnamic acid (CA) and its derivatives have drawn attention because they are very diffuse in nature and are potent antioxidants. The aim of a recent work has been to intercalate into the Mg±Al± HTlc the anionic form of CA and ferulic acid; in addition the ascorbate (Asc) has also been considered. Intercalation compounds have been used as active fillers of polycaprolactone (see Section 3.4.4).

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Multifunctional and nanoreinforced polymers for food packaging Table 3.7 Composition and basal spacing (d) of the indicated intercalation compounds (antioxidants) dried at 75% of relative humidity Antioxidant

Compositiona

CA Fer Asc

[MgAl]0.39(CA)0.17(NO3)0.22
0.46H2O [MgAl]0.36(Fer)0.19(NO3)0.17
0.89H2O [MgAl]0.36(Asc)0.11Cl0.25
0.29H2O

d (nm) 1.47 1.73 1.28

a

[MgAl]x indicates [Mg1ÿxAlx(OH)2].

The anions cinnammate (CA) and ferulate (Fer) have been intercalated via ion exchange starting from Mg±Al in nitrate form, while the Asc has been intercalated in the presence of hydrazine as antioxidant and using the chloride form of the HTlc (Costantino et al., 2009a). Table 3.7 reports the composition and the interlayer distance of the intercalation compounds. CA, Fer and the Asc exchange were about 45%, 53% and 31% respectively. The CA arrangement in the interlayer region is shown in Fig. 3.14; the interlayer distance value is in agreement with the formation of a monofilm partially interdigitate of CA species. Fer anions, very likely, are arranged in the same way; the increasing of the interlayer distance with respect to the Mg±Al±CA is probably due to the presence of the OCH3 group instead of the OH group. The free-radical scavenging activities of the intercalation compounds have been tested and compared with those of the neat antioxidant (see Section 3.4.4).

3.14 Computer-generated models showing the most probable arrangement of CA between the HTlc layers.

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Nanocomposites of biodegradable polymeric matrices and modified hydrotalcites

The development of polymer±clay nanocomposites is one of the latest revolutionary steps in polymer technology. Preparations of blends or nanocomposites using inorganic or natural fibres are the route to improving some of the properties of biodegradable polymers. The addition of low percentages of clay to polymers may increase mechanical strength, reduce weight, increase heat resistance, and improve the barrier properties of food packaging materials against moisture, oxygen, carbon dioxide, ultraviolet radiation and volatiles in comparison to the barrier properties of traditional composites. Hence, synthetic polymer nanocomposites have emerged as an area of research in recent years and their development represents a very attractive way to improve and diversify physical and chemical properties of polymers (Messersmith and Giannelis, 1993; Vaia et al., 1994; Giannelis, 1996; Ren et al., 2000; Strawhecker and Manias, 2000). In an ideal nanocomposite structure, all of the inorganic particles must be completely separated into individual layers, forming an exfoliated structure. Therefore, most of the polymer is located at the nanofiller±polymer interface, and the conversion of bulk polymer into interfacial polymer represents the key to impart new and diversified polymer properties. To increase the compatibility between the polymer and the filler, thus favouring the exfoliation, the inorganic compound has to be modified with an organic molecular anion, able to create physical and intermolecular bonds with the polymeric chains. Many methods are used to allow a good dispersion of the inorganic compound into the polymer (Oswald and Asper, 1977; Pinnavaia and Beall, 2000; Alexandre and Dubois, 2000; Kaempfer et al., 2002). In the melt compounding method, polymer nanocomposites can be prepared by conventional compounding techniques (twin-screw extruder or melt compounder). If the compatibility between the polymer chains and the organic modification of the nanoparticles is sufficiently high, polymer chains penetrate into the galleries of the layered materials, and intercalation or exfoliation of the layered clay can occur. The solution-blending method consists of dissolving polymer and organically modified clay in a mutual solvent with subsequent solvent removal. Another interesting possibility is to directly intercalate or exfoliate the clay with a charged polymer that can constitute the counterbalancing ion in the clay galleries. In this case, the organic modification of the clay is not necessary, because the polymer in the charged form can penetrate into the clay galleries by a simple exchange reaction, and intercalate or exfoliate the inorganic solid. The most common nanocomposites investigated so far are composed of polymers and organically modified silicates. Hydrotalcite-like compounds represent a different and interesting class of nanofillers for polymers. As

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already discussed in Section 3.2, HTlc can be prepared with simple procedures, at a high level of purity. They are cheap and eco-compatible and can be organically modified with a variety of organic anions that may confer to the obtained hybrids special functionalities (see Section 3.3). This latter characteristic will make these layered compounds a very attractive class of lamellar solids because the release of active guest anions from intercalated layered materials is potentially controllable. The new trend of the research is based on the fact that the active molecules, fixed by ionic bonds to the inorganic lamellae, not only can improve the compatibility with the polymer matrix but also can carry out the specific activity being anchored to the lamellae, or being slowly released in a particular environment (Oriakhi et al., 1996; Rives, 2001; Leroux et al., 2003). Recent results obtained using different, mainly biodegradable, polymeric matrices are reported below.

3.4.1

The case of poly(-caprolactone) (PCL)

The need for biodegradable plastics has increased during recent decades, not only due to growing environmental concerns, but also for their biomedical applications. Biodegradable polymers have been extensively investigated for packaging and agricultural products, in order to reduce the environmental pollution caused by plastic wastes (Scott and Gilead, 1995; Mecking, 2004). In the family of synthetic biodegradable polymers, poly(-caprolactone) (PCL), which is a linear, hydrophobic and partially crystalline polyester, is very attractive, not only as a substitute for non-biodegradable polymers for commodity applications, but also for specific applications in medicine and agriculture (Jarrett et al., 1984). The development of new nanohybrid composites obtained by intercalating PCL and active molecular anions into the interlayer region of HTlc is a very promising field for application of PCL in controlled release. The polymeric composite can release the active molecular anion with controlled kinetics, depending on the electronic structure of the active species, the interaction of the species with the matrix component, the concentration of the acceptor medium, and the morphology and polymorphism of the polymeric matrix. At least, the presence of the inorganic compound can improve either mechanical or transport polymer properties. The ability of the composites to release active species makes them useful for many applications as active food packaging films or controlled drug release membranes and scaffolds.

3.4.2

Procedures to obtain films, membranes and fibres of PCL±HTlc composites

PCL±HTlc nanocomposites have been synthesized by in situ ring-opening polymerization of -caprolactone (Tammaro et al., 2005). The polymerization is

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induced by the alcoholic OH group belonging to 12-hydroxydodecanoate (HD), previously intercalated in Mg±Al±HTlc via an anion-exchange procedure. In particular, 1 g of the intercalation compound of formula [Mg0.65Al0.35(OH)2] (NO3)0.08(HD)0.28 and with interlayer distance of 2.27 nm was dispersed into 22.4 ml of -caprolactone. PCL oligomers were formed in the interlayer region, as evidenced by the increase of the interlayer distance. Finally, nanocomposites containing exfoliated HTlc lamellae were obtained by the solution mixing of high molecular weight PCL with the oligomers of PCL partially intercalated into HTlc±HD. The latter hybrid probably acted as a compatibilizer between the organically modified hydrotalcite and polycaprolactone. The HD-modified hydrotalcite was also used to prepare novel composites based on poly(-caprolactone) with different procedures. Microcomposite systems were obtained by the solution mixing of modified Mg±Al±HTlc with PCL. Other composites of PCL and HTlc±HD have been prepared using meltextrusion processing, a versatile, cheap and environmentally friendly technique (Pucciariello et al., 2007). Although exfoliation has not been achieved and despite the very low content of filler (from 1 to 3% by weight), significant enhancements have been obtained in the physical and mechanical properties of the composites with respect to neat PCL. An alternative and innovative strategy to produce nanocomposites relies on solid-state mixing at near room temperature, which ought to involve an efficient mixing of two or more species by mechanical milling, avoiding high temperatures and solvents. High energy ball milling (HEBM) is an effective unconventional technique currently used in material synthesis and processing. It consists of repeated events of energy transfer, promoted by the milling device, from the milling tools (generally balls) to the milled powder. During the milling the powder particles crack, clean surfaces are produced, and atom diffusion and `intimate mixing' are promoted. As a consequence of the prolonged milling action, when the energy transferred during the hit is enough to overcome the activation barrier, chemical reactions may occur. It has been proved that HEBM on polymeric materials can help to obtain materials with new characteristics that can be barely achieved through other conventional processes. HEBM of powders constituted of organic polymers and fillers has been proved to be an alternative and efficient technique to produce novel composites. This technique may support the more conventional and more utilized techniques for producing nanocomposites, which are based mainly on in situ polymerization and melt extrusion. Sorrentino et al. (2005) used HEBM to prepare nanocomposites of PCL and an organically modified Mg±Al±HTlc. The molecular weight of PCL decreased and its distribution increased as a consequence of milling. The mechanical parameters derived from the stress±strain curves improved in the composite samples containing up to 2.8 wt% of inorganic filler, with respect to the pure polymer, in spite of the molecular weight decrease. The thermodynamic

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diffusion coefficient of water vapour in composite samples was lower than in pure PCL, indicating an improvement of the barrier effect. Recently the electrospinning technique, which is able to produce non-woven membranes of micro/nanofibres characterized by high surface area and high porosity, has been demonstrated as a successful method to produce scaffolds having many of the desirable and controllable properties. It is applicable to a wide variety of polymers and composite polymers, both natural and synthetic, already widely used in tissue engineering (Teo and Ramakrishna, 2006; Travis and Horst, 2008; Agarwal et al., 2008). Romeo et al. (2007) reported, for the first time, the successful fabrication of hydrotalcites (Mg±Al±HTlc)-reinforced polycaprolactone (PCL) nanofibres by electrospinning. Either the HTlc in carbonate form or an HTlc organically modified with 12-hydroxydodecanoic acid (HTlc±HA) were incorporated into PCL and electrospin using a voltage of 20 kV. The HTlc±HA was prepared by an ionic exchange reaction from pristine HTlc and encapsulated into PCL from acetone solutions at 15 wt%. The morphological analysis showed pure PCL fibres with an average diameter of 600  50 nm, and this dimension was maintained in the fibres with HTlc, with the inorganic component residing outside the fibres and not exfoliated. At variance, the fibres with the HTlc-HA showed a significantly lower average diameter in the range of 350  50 nm, indicating the improved electrospinnability of PCL. Moreover, the inorganic lamellae were exfoliated, as shown by XRPD, and residing inside the nanofibres, as demonstrated by energy-dispersive X-ray (EDX) spectroscopy analysis. The structural parameters, such as degradation temperature and crystallinity, were investigated for all the samples and correlated with the electrospinning parameters.

3.4.3

PCL nanobiocomposites for modified drug release

The development of controlled-release technology needs materials with more specific drug-delivery properties, and therefore many efforts are being made to develop retarded and tunable drug-release systems. A remarkable innovation in this field is currently coming from nanoscience and nanotechnology ± the aim is to produce polymeric nanobiocomposites for controlled release of a wide variety of pharmaceuticals, or in general more `active' products. Nanobiocomposites have been prepared by employing as nanoscale reinforcement layered materials functionalized with biologically active molecules that can be successively released by a chemical signal, i.e. exchange reactions. Hydrotalcite containing diclofenac, chloramphenicol hemisuccinate and tranexamic acid has been incorporated into PCL by solvent casting and HEBM procedures (Sammartino et al., 2006; Tammaro et al., 2007; Costantino et al., 2009b). Composites containing different weight percentages of modified hydrotalcites have been processed as films or threads. The composite materials

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3.15 In vitro release tests of HTlc±Cfs/PCL film composites in a physiological saline solution (0.9% NaCl). The content of HTlc±Cfs in the composites was 5% w/w (l) and 20% w/w (n).

have been analysed by X-ray diffractometry, thermogravimetry and mechanical properties. Studies of the mechanical properties of these composites showed that the presence of the inorganic filler in the polymeric matrix led to an improvement of mechanical parameters except for fracture toughness. Moreover, the composites processed as films were submitted to in vitro release tests in a physiological saline solution (0.9% NaCl). Samples having different HTlc loading show the same qualitative release profile. The typical time-dependent profile of each sample is a fast release in an early period, followed by a reduced release (Fig. 3.15). The drug release consists of two stages: a first stage, very rapid as a burst, in which a small fraction of the drug is released from the surface of the lamellae, and a second stage that is much slower, extending for a longer and longer time, due to the drug de-intercalation from the interlayer region of HTlc inside the polymeric film. The amount of drug released from composite materials depends on both the nature of the incoming counter-anion that will replace the anionic drug, and the counter-diffusion of anions through the polymer. These composites are very promising in the preparation of new hybrid polymeric materials to be used for the controlled molecular delivery of drugs in topical applications, as suture threads or medical scaffolds. In the last few years considerable effort has been made to develop biocompatible scaffolds for tissue engineering. The scaffold should mimic the structure and biological function of native extracellular matrix (ECM) as much as possible, in terms of both chemical composition and physical structure. Native ECM does far more than just provide a physical support for cells. It also provides a substrate with specific ligands for cell adhesion and migration, and

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regulates cellular proliferation and function by providing various growth factors. In a typical connective tissue, structural protein fibres such as collagen fibres and elastin fibres have diameters ranging from several ten to several hundred nanometres. Polymeric nanofibre non-woven matrix is among the most promising biomaterials for native ECM analogists. Tammaro et al. (2009) reported the incorporation of an Mg±Al hydrotalcitelike compound intercalated with diclofenac anions (HTlc-DIC) into poly(caprolactone) in different concentrations by the electrospinning technique, and the comparison of the obtained non-woven fibres to the pristine pure electrospun PCL. The fibres, characterized by X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, showed an exfoliated clay structure up to 3 wt%, a good thermal stability of the diclofenac molecules and a crystallinity of PCL comparable to the pure polymer. The scanning electron microscopy revealed electrospun PCL and PCL composite fibres diameters ranging between 500 nm to 3.0 m and a generally uniform thickness along the fibres. As the results suggested, the in vitro drug release from the composite fibres is markedly slower than the release from the corresponding control-spun solutions of PCL and diclofenac sodium salt. Thus, HTlc-DIC/PCL fibrous membranes can be used as an anti-inflammatory scaffold for tissue engineering.

3.4.4

PCL nanobiocomposites for potential food packaging applications

Research and development of nanocomposite materials for food applications such as packaging and other food contact surfaces is expected to grow in the next decade with the advent of new polymeric materials and composites with inorganic nanoparticles. The rationale for incorporating antimicrobials into the packaging is to prevent surface growth in foods where a large proportion of spoilage and contamination occurs (Appendini and Hotchkiss, 2002; LaCoste et al., 2005). This approach can reduce the addition of larger quantities of antimicrobials that are usually incorporated into the bulk of the food. A controlled release from packaging film to the food surface has numerous advantages over dipping and spraying. Hydrotalcite intercalated with benzoate and benzoate derivative anions with antimicrobial activity have been used as fillers of PCL (Costantino et al., 2009c). The composites have been prepared by HEBM and processed, on a laboratory scale, as thin films. According to the nature of the guest, microcomposites and intercalated and/or exfoliated polymeric composites have been obtained and studied. X-ray diffraction analysis and scanning electron microscopy of the composites indicate that the HTlc samples containing BzDC anions are exfoliated into the polymeric matrix, whereas those containing pBzOH anions largely maintain the crystal packing and give rise to microcomposites. Intermediate behaviour was found for HTlc modified with Bz and o-

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BzOH anions, since exfoliated and partly intercalated nanocomposites have been obtained. When the filler exfoliates into the polymer, the guest anions not only improve the compatibility of the inorganic layer with the polymeric matrix, and hence the mechanical and barrier properties of the composite, but also confer to it their typical biological activity. Preliminary antimicrobial tests indicate that the composites are able to inhibit the growth of Saccharomyces cerevisiae of 40% in comparison with the growth in a pure culture medium. In other words, the growth of the microorganisms in the presence of composites is only 60% of the growth found in the pure culture medium. Such a result gives evidence of the feasibility of the composites as `active packaging' materials because of the antimicrobial properties of the anions anchored to the HTlc layer. Mechanical and barrier properties of water vapour have been studied for all the nanocomposite films, showing the influence of the morphology on the physical properties. A preliminary study on the release kinetics of the Bz anions bound to HTlc has also been performed, revealing very good perspectives in the field of controlled release of active species (Bugatti et al., 2010). Films with antioxidant activity have been prepared by incorporating hydrotalcite modified with ferulate and ascorbate anions by solvent casting. Microcomposites or exfoliated and partly intercalated nanocomposites have been obtained for HTlc-Asc/PCL and HTlc-Fer/PCL systems respectively (Costantino et al., 2009a). The film antioxidative activity has been evaluated by the scavenging of the stable 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical. The stable DPPH radical absorbs at 517 nm and the antioxidant activity of the species can be determined by monitoring the decrease in absorbance as a function of time (Brand-Williams et al., 1995). The variation of the absorbance of 10±4M methanol solution of DPPH radical and antioxidant (molar ratio antioxidant/DPPH = 0.5) has been compared with that of the pure DPPH 10±4M methanol solution. In order to investigate the effect of the microenvironment on the antioxidant properties, the radical scavenger ability has been measured for the free species in acid and salt forms, for the anions intercalated into the Mg±Al±HTlc and after dispersing the intercalation compounds into the PCL. In Fig. 3.16 the percentage of the remaining DPPH is plotted against time. The antioxidant activities of the ferulic acid and of its sodium salt are comparable, being attributed to the hydroxyl group of the aromatic ring. The ferulate anions trapped between the sheets preserve the antiradical power, albeit with lower kinetics, probably due to the time of diffusion of the guests from the interlayer region of HTlc to the solution. Within the first hours the antioxidant activity of the composite is higher than that of the Mg±Al±Fer. As observed by X-ray diffraction analysis, a part of the hybrid containing Fer is able to exfoliate into the polymeric matrix, increasing the amount of active

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3.16 Percentage reduction of DPPH absorbance with time in the presence of Fer and Asc in acid form (only for Fer), as sodium salt, intercalated into the Mg±Al±HTlc and in the PLC±HTlc composites.

anions that can be promptly released. With the increase of time the activity of the above two materials is reversed: the activity of the composite is lower than that of the Mg±Al±Fer since the Fer anions coming from the Mg±Al±Fer, not exfoliated into the PCL have to diffuse through the inorganic sheet and the polymer. Similar results have been obtained for the system containing the ascorbate. Ascorbic acid immediately discoloured the DPPH solution, while the sodium ascorbate reduced by 50% the DPPH concentration. In this case the

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activity of the PCL composite is always better than that of the intercalation compound. The obtained composites can be considered as promising active food packaging systems due to the presence of antioxidant agents that can control the oxidation of the food.

3.4.5

The case of poly(hydroxyalkanoates) and hydrocolloids

Among biodegradable thermoplastic polyesters, polymers such as poly(3hydroxybutyrate) (PHB) and poly(butylene succinate) (PBS) are promising biomaterials that can be used in packaging, automotive and biomedical fields. However, PHB and PBS present some drawbacks such as low hydrolysis resistance, low barrier to gases and water vapour, low melt stability, and meltviscosity not sufficient for processing for practical end-use applications. In order to improve the thermal and barrier properties, melt blending nanobiocomposites of PHB and different layered phyllosilicates have been prepared. Composites containing kaolinite showed enhanced crystallinity and barrier properties (Sanchez-Garcia et al., 2008). Hydrotalcites grafted on the surface with poly(ethylene glycol) phosphonate have been dispersed into PHB to improve the crystallization kinetics of the nanocomposites (Hsu et al., 2006). Recently, Mg± Al hydrotalcite modified with oleate anion, an `environmentally friendly' guest, has been used as filler of PBS. Composites quasi-exfoliated and with improved rheological properties have been obtained for HTlc loading lower than 5% w/w (Zhou et al., 2010). The water vapour permeability and mechanical properties of glycerol plasticized dextrin±alginate films, filled with stearate intercalated hydrotalcite (HTlc-SA), have been investigated (Landman and Focke, 2006). The total filler content, comprising both the stearic acid (SA) and the [Mg4Al2(OH)12CO3
3H2O](HTlc), was fixed at 16.6% w/w of the dried films. The two filler components were allowed to react in the film casting solution for one hour. Sodium alginate acted as a dispersant and facilitated the intercalation of the stearic acid into HTlc that was suspended in the water±alcohol film solution. The resultant cast film properties were not affected when either neat SA or HTlc was the filler. However, superior mechanical and barrier properties were realized at intermediate filler compositions.

3.5

Conclusions and future trends

At present, exfoliated and organically modified smectite clays are the key fillers of different polymeric matrices. However, among other inorganic materials proposed as nanofillers (i.e. carbon nanotubes, perovskites, nanoparticles of silica and modified silica (POSS siloxanes), modified TiO2 nanoparticles), hydrotalcite-like compounds compare favourably with smectite clays for many features. HTlc have a well-known stoichiometry and composition, a higher level

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of purity, may be synthesized with simple and reproducible methods, have a wider possibility of layer modification, have higher ion exchange capacity and show better ability to store and release biologically active species. This last feature allows the design and synthesis of functional HTlc nanofillers for biodegradable and biocompatible polymeric matrices, and the preparation of nanobiocomposites of interest for biomedical and active packaging applications. In this context, the content of the present chapter has been worded to introduce the reader to the chemistry of hydrotalcites, starting from their synthesis and their physical±chemical properties, to their manipulation via anion exchange and intercalation reactions to obtain organic±inorganic hybrids hosting selected drugs, amino acids, proteins or species with antimicrobial or antioxidant properties. It has been shown that these hybrids can release the guest species in a modified way, in different environments, and can be dispersed at nanometric level in biodegradable polymers, conferring to them additional and useful properties. For concerning the potential of hydrotalcites for enhancing barrier properties, it is well known that inorganic fillers may increase the barrier properties of the nanocomposites by creating a more `tortuous path' that retards the progress of the small molecules through the polymeric matrix. The direct benefit of the formation of such a path is clearly observed in all the prepared nanocomposites by dramatically improved barrier properties. There is also evidence that the nanosized platelets restrict the molecular dynamics of the polymer chains surrounding the inorganic, thus retarding the relaxation of polymer chains. The effect of the hydrotalcites on the barrier properties of polymers has been scarcely studied. Film composites constituted by PCL and HTlc functionalized with antimicrobial species have been characterized also for their barrier properties of water vapour (Bugatti et al., 2010). The barrier properties have been investigated by measuring the isotherms of sorption and the diffusion of water vapour for all the composites. The isotherms of the composites follow the same trend as PCL, although showing a higher sorption in all the activity range, due to the higher hydrophilicity of the inorganic lamellae. At variance, the thermodynamic diffusion parameter, at zero vapour concentration, is significantly lower and decreases on increasing the inorganic concentration for all the composites. However, the most effective reduction was found for the exfoliated samples. This chapter will have achieved its aim if the reader acquires the conviction that hydrotalcite-like compounds are an extremely versatile class of materials that can be produced at low cost and can be easily modified with simple procedures, and if the reader envisages the preparation of novel products for unforeseen new applications. This conviction is well established in many academic and industrial laboratories, as documented by the large number of research articles, reviews and patents available. Some chemical firms are producing and selling hydrotalcites as fillers of polymers and this will probably favour an extended and widened interest in these materials in the near future.

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exchange, and delamination of Co±Al layered double hydroxide: assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies', J. Am. Chem. Soc., 128, 4872±4880. Liu Z, Ma R, Ebina Y, Iyi N, Takada K and Sasaki T (2007), `General synthesis and delamination of highly crystalline transition-metal-bearing layered double hydroxides', Langmuir, 23, 861±867. Lombardo G M, Pappalardo G C, Punzo F, Costantino F, Costantino U and Sisani M (2005), `A novel integrated approach X-ray powder diffraction (XRPD) and molecular dynamics (MD) for modelling mixed-metals (Zn, Al) layered double hydroxides (LDH)', Eur. J. Inorg. Chem., 5026±5034. Lombardo G M, Pappalardo G C, Costantino F, Costantino U and Sisani M (2008), `Thermal effects on mixed metal (Zn/Al) layered double hydroxides (LDHs): direct modelling of the X-ray powder diffraction (XRPD) line-shape through molecular dynamics (MD) simulation', Chem. Mater., 20, 5585±5592. Ma R, Liu Z, Takada K, Iyi N, Bando Y and Sasaki T (2007), `Synthesis and exfoliation of Co2+±Fe3+ layered double hydroxides: an innovative topochemical approach', J. Am. Chem. Soc., 129, 5257±5263. Manasse E (1915), `Rocce eritree e di aden della collezione issel', Atti Soc. Toscana Sc. Nat., Proc. Verb., 24, 92. Martinez Martinez V, De Cremer G, J. Roeffaers M B, Sliwa M, Baruah M, De Vos D E, Hofkens J and Sels B F (2008), `Exploration of single molecule events in a haloperoxidase and its biomimic: localization of halogenation activity', J. Am. Chem. Soc., 130, 13192±13193. May Y W and Yu Z Z (2006), Polymer Nanocomposites, Woodhead Publishing, Cambridge, UK. Mecking S (2004), `Nature or petrochemistry? ± Biologically degradable materials', Angew. Chem. (Int. Ed.), 43, 1078±1085. Messersmith P B and Giannelis E P (1993), `Polymer-layered silicate nanocomposites: in situ intercalative polymerization of -caprolactone in layered silicates', Chem. Mater., 5, 1064±1066. Mignani A, Scavetta E, Guadagnini L and Tonelli D (2009), `Comparative study of protective membranes for glucose biosensors based on electrodeposited hydrotalcites', Sensors and Actuators B, 136, 196±202. Miyata S (1980), `Physico-chemical properties of synthetic hydrotalcites in relation to composition', Clays and Clay Minerals, 28, 50±56. Miyata S (1983), `Anion-exchange properties of hydrotalcite-like compounds', Clays and Clay Minerals, 31, 305±311. Miyata S and Kumura T (1973), `Synthesis of new hydrotalcite-like compounds and their physico-chemical properties', Chem. Lett., 2, 843±848. Mohanambe L and Vasudevan S (2005), `Anionic clay containing anti-inflammatory drugs molecules: comparison of molecular dynamics simulations and measurements', J. Phys. Chem. B, 109, 15651±15658. Mousty C (2010), `Biosensing applications of clay-modified electrodes: a review', Anal. Bioanal. Chem., 396, 315±325. Mousty C, Kaftan O, PreÂvot V and Forano C (2008), `Alkaline phosphatase biosensors based on layered double hydroxides matrices: Role of LDH composition', Sensors and Actuators B, 133, 442±448. Nakanishi K, Sakiyama T and Imamura K (2001), `On the adsorption of proteins on solid surfaces, a common but very complicated phenomenon', J. Biosci. Bioeng., 91, 233±244.

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Newman S P and Jones W (1998), `Synthesis, characterization and applications of layered double hydroxides containing organic guests', New J. Chem., 22, 105±115. Nyambo C, Songtipya P, Manias E, Jimenez-Gasco M M and Wilkie C A (2008), `Effect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fire-retardancy of PMMA and PS', J. Mater. Chem., 18, 4827±4838. O'Hare D and Hu G (2005), `Unique layered double hydroxide morphologies using reverse microemulsion synthesis', J. Am. Chem. Soc., 127, 17808±17813. O'Hare D, Hu G, Wang N and Davis J (2007), `Synthesis of magnesium aluminium layered double hydroxides in reverse microemulsions', J. Mater. Chem., 17, 2257± 2266. O'Leary S, O'Hare D and Seeley G (2002), `Delamination of layered double hydroxides in polar monomers: new LDH-acrylate nanocomposites', Chem. Commun., 14, 1506±1507. Ogawa M and Kuroda K (1995), `Photofunctions of intercalation compounds', Chem. Rev., 95, 399±438. Okamoto K, Sasaki T, Fujita T and Iyi N (2006), `Preparation of highly oriented organic± LDH hybrid films by combining the decarbonation, anion-exchange, and delamination processes', J. Mater. Chem., 16, 1608±1616. Oriakhi C O, Farr I V and Lerner M (1996), `Incorporation of poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) within layered double hydroxides', J. Mater. Chem., 6, 103±107. Oswald H R and Asper R (1997), in Physics and Chemistry of Materials with Layered Structures, Vol. 1, Lieth R M A (ed.), D. Reidel Publishing Co., Dordrecht, The Netherlands. Palmer S J, Spratt H J and Frost R L (2009), `Thermal decomposition of hydrotalcites with variable cationic ratios', Journal of Thermal Analysis and Calorimetry, 95, 123±129. Paredes S P, Fetter G, Bosch P and Bulbulian S (2006), `Sol-gel synthesis of hydrotalcitelike compounds', J. Mater. Sci., 41, 3377±3382. Perioli L, Ambrogi V, Bertini B, Ricci M, Giovagnoli S, Nocchetti M, Latterini L and Rossi C (2006a), `Anionic clays for sunscreen agent safe use: photoprotection, photostability and prevention of their skin penetration', Eur. J. Pharm. Biopharm., 62, 185±193. Perioli L, Ambrogi V, Rossi C, Latterini L, Nocchetti M and Costantino U (2006b), `Use of anionic clays for photoprotection and sunscreen photostability: hydrotalcites and phenylbenzimidazole sulfonic acid', J. Phys. Chem. Solids, 67, 1079±1083. Perioli L, Nocchetti M, Ambrogi V, Latterini L, Rossi C and Costantino U (2008), `Sunscreen immobilization on ZnAl-hydrotalcite for new cosmetic formulations', Micropor. Mesopor. Mater., 107, 180±189. Perioli L, Posati T, Nocchetti M, Bellezza F, Costantino U and Cipiciani A (2010a), `Intercalation and release of antiinflammatory drug diclofenac into nanosized ZnAl hydrotalcite-like compound', Appl. Clay Sci., in press. Perioli L, Ambrogi V, di Nauta L, Nocchetti M and Rossi C (2010b), `Hydrotalcite as matrix for double modified release of flurbiprofen', Pharm. Res., submitted. Pinnavaia T J and Beall G W (2000), Polymer±Clay Nanocomposites, Wiley Series in Polymer Science, Wiley, New York. PreÂvot V, Forano C and Besse J P (2001), `Hybrid derivatives of layered double hydroxides', Appl. Clay Sci., 18, 3±15. Prinetto F, Ghiotti G, Graffin P and Tichit D (2000), `Synthesis and characterization of sol-gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-

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precipitated samples', Micropor. Mesopor. Mater., 39, 229±247. Pucciariello R, Tammaro L, Villani V and Vittoria V (2007), `New nanohybrids of poly(-caprolactone) and a modified Mg/Al hydrotalcite: mechanical and thermal properties', J. Polym. Sci. Part B: Polym. Phys., 45, 945±954. Reichle W T (1986), `Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite)', Solid State Ionics, 22, 135±141. Ren J, Silva A S and Krishnamoorti R (2000), `Linear viscoelasticity of disordered polystyrene±polyisoprene block copolymer based layered-silicate nanocomposites', Macromolecules, 33, 3739±3746. Rey F and Fornes V (1992), `Thermal decomposition of hydrotalcites. An infrared and nuclear magnetic resonance spectroscopic study', J. Chem. Soc. Faraday Trans., 88, 2233±2238. Rives V (ed.) (2001), Layered Double Hydroxides: Present and Future, Nova Science Publishers, New York. Rives V, Benito P and Labajos F M (2006), `Uniform fast growth of hydrotalcite-like compounds', Cryst. Growth Des., 6, 1961±1966. Rocha J, del Arco M, Rives V and Ulibarri M A (1999), `Reconstruction of layered double hydroxides from calcined precursors: a powder XRD and 27Al MAS NMR study', J. Mater. Chem., 9, 2499±2503. Romeo V, Gorrasi G, Vittoria V and Chronakis I S (2007), `Encapsulation and exfoliation of inorganic lamellar compounds into polycaprolactone by electrospinning', Biomacromolecules, 8(10), 3147±3152. Sammartino G, Marenzi G, Tammaro L, Bolognese A, Calignano A, Costantino U, Califano L, Mastrangelo F, TeteÁ S and Vittoria V (2006), `Anti-inflammatory drug incorporation into polymeric nano-hybrids for local controlled release', Int. J. Immunopathol. Pharmacol., 18, 55±62. Sanchez-Garcia M, Gimenez E and LagaroÂn J M (2008), `Morphology and barrier properties of nanobiocomposites of poly(3-hydroxybutyrate) and layered silicates', J. Appl. Polym. Sci., 108, 2787±2801. Scott G and Gilead D (1995), in Degradable Polymers. Principles and Applications, Chapman & Hall, London. Shaw L R, Irwin W J, Grattan T J and Conway B R (2005), `The role of gastric mucus as a barrier to the absorption of ibuprofen or paracetamol and the effects of coadministered antacids and modified pH', Int. J. Pharm., 290, 145±154. Sideris P J, Nielsen U G, Gan Z and Grey C P (2008), `Mg/Al ordering in layered double hydroxides revealed by multinuclear NMR spectroscopy', Science, 321, 113±117. Sorrentino A, Gorrasi G, Tortora M, Vittoria V, Costantino U, Marmottini F and Padella F (2005), `Incorporation of Mg±Al hydrotalcite into a biodegradable poly(3caprolactone) by high energy ball milling', Polymer, 46, 1601±1608. Strawhecker K E and Manias E (2000), `Structure and properties of poly(vinyl alcohol)/ Na+ montmorillonite nanocomposites', Chem. Mater., 12, 2943±2949. Tammaro L, Tortora M, Vittoria V, Costantino U and Marmottini F (2005), `Methods of preparation of novel composites of poly(-caprolactone) and a modified Mg/Al hydrotalcite', J. Polym. Sci.: Part A: Polym. Chem., 43, 2281±2290. Tammaro L, Vittoria V, Costantino U, Bolognese A, Sammartino G, Marenzi G, Califano L, Calignano A and TeteÁ S (2007), `Nanohybrids for controlled antibiotic release in topical applications', Int. J. Antimicrob. Agents, 29, 417±423. Tammaro L, Vittoria V and Russo G (2009), `Encapsulation of diclofenac molecules into poly(-caprolactone) electrospun fibers for delivery protection', J. Nanomater., doi: 10.1155/2009/238206.

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Taylor H F W (1969), `Segregation and cation-ordering in sjoÈgrenite and pyroaurite', Min. Mag., 37, 338±342. Taylor H F W (1973), `Crystal structures of some double hydroxide minerals', Min. Mag., 39, 377±389. Teo W E and Ramakrishna S (2006), `A review on electrospinning design and nanofiber assemblies', Nanotechnology, 17, R89±R106. Travis J S and Horst A R (2008), `Electrospinning: applications in drug delivery and tissue engineering', Biomaterials, 29, 1989±2006. TrifiroÁ F and Vaccari A (1996), `Hydrotalcite-like anionic clays (layered double hydroxides)', in Solid-State Supramolecular Chemistry: Two and Threedimensional Inorganic Networks, of Comprehensive Supramolecular Chemistry, Alberti G and Bein T (eds), Vol. 7, Pergamon Press, Elsevier Science, Oxford, pp. 251±291. Turco M, Bagnasco G, Costantino U, Marmottini F, Montanari T, Ramis G and Busca G (2004), `Production of hydrogen from oxidative steam reforming of methanol. I. Preparation and characterization of Cu/ZnO/Al2O3 catalysts from a hydrotalcitelike LDH precursor', J. Catalysis, 228, 43±55. Vaia R A, Teukolsky R K and Giannelis E P (1994), `Interlayer structure and molecular environment of alkylammonium layered silicates', Chem. Mater., 6, 1017±1022. Vial S, PreÂvot V, Leroux F and Forano C (2008), `Immobilization of urease in ZnAl layered double hydroxides by soft chemistry routes', Micropor. Mesopor. Mater., 107, 190±201. Williams G R and O'Hare D (2006), `Towards understanding, control and application of layered double hydroxide chemistry', J. Mater. Chem., 16, 3065±3074. Wu Q, Olafsen A, Vistad é B, Roots J and Norby P (2005), `Delamination and restacking of a layered double hydroxide with nitrate as counter anion', J. Mater. Chem., 15, 4695±4700. Xu Z P and Braterman P S (2010), `Synthesis, structure and morphology of organic layered double hydroxide (LDH) hybrids: Comparison between aliphatic anions and their oxygenated analogs', Appl. Clay Sci., 48, 235±242. Xu Z P, Braterman P S, Yu K, Xu H, Wang Y and Brinker C J (2004), `Unusual hydrocarbon chain packing mode and modification of crystallite growth habit in the self-assembled nanocomposites zinc±aluminum±hydroxide oleate and elaidate (cisand trans-[Zn 2 Al(OH) 6 (CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COO ± )] and magnesium analogues', Chem. Mater., 16, 2750±2756. Xu Z P, Stevenson G S, Lu C Q, Lu G Q, Bartlett P F and Gray P P (2006), `Stable suspension of layered double hydroxide nanoparticles in aqueous solution', J. Am. Chem. Soc., 128, 36±37. Yasutake A, Aisawa S, Takahashi S, Hirahara H and Narita E (2008), `Synthesis of biopolymer intercalated inorganic-layered materials: Intercalation of collagen peptide and soybean peptide into Zn±Al layered double hydroxide and layered zinc hydroxide', J. Phys. Chem. Solids, 69, 1542±1546. Yun S K and Pinnavaia T (1995), `Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides', J. Chem. Mater., 7, 348±354. Zhou Q, Verney V, Commereuc S, Chin I and Leroux F (2010), `Strong interfacial attrition developed by oleate/layered double hydroxide nanoplatelets dispersed into poly(butylene succinate)', J. Colloid Interface Sci., 349, 127±133.

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Cellulose nanofillers for food packaging È M , Royal Institute of R. T. OLSSON and L. FOGELSTRO Technology, Sweden, M . M A R T IÂ N E Z - S A N Z , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain and M . H E N R I K S S O N , Royal Institute of Technology, Sweden and SP Technical Research Institute of Sweden, Sweden

Abstract: This chapter presents a review of methods for the extraction of cellulose nanofillers, as well as the most important characteristic features related to the exploration of these nanofillers in composite applications. Various methods for the extraction and surface modification of cellulose crystals are presented for the adaption of cellulose crystals in composite applications. A brief review of the different morphological characteristics as well as mechanical properties of different cellulose nanofillers are also presented. Key words: cellulose, microfibrils, extraction, nanocomposite, processing.

4.1

Introduction

Cellulose is the most abundant renewable polymer on earth and is responsible for the structural integrity of wood, plants and algae, as well as some sea animals and microbial cellulose. Ultimately, this structural integrity has been related to rod-like, load-bearing crystal units composed of poly- (1,4)-D-glucopyranoside chains organized parallel in a highly ordered manner.1±3 The crystal units were originally referred to as `cellulose microfibrils' or `elementary fibrils of cellulose', though the terms `nanowhiskers', `protofibrils', `nanofibrils', etc., have also been used to designate cellulose nanofillers (CNFs) as the topic has become the subject of intense research.4,5 Lately, the intrinsic mechanical properties (strength and stiffness) of CNFs have been in focus, and modulus values in excess of 130 GPa have been reported, whereas the mechanical strength may exceed 7±10 GPa.6±13 The mechanical properties are not far from those of some grades of steel, which suggest that CNFs may eventually find use as reinforcement agents in composite formulations with engineering polymers.14 In addition, CNFs also show many other useful characteristics such as high sound attenuation,15 high gas impermeability,16 non-abrasive nature in combination with very high specific surface area17 and low density (ca. 1500 kg/m3),18 which can be considered unique for an inexpensive biodegradable material.

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However, although significant research advances have been reported concerning the specific characteristics of CNFs, a number of hurdles still exist before these materials can be fully exploited in commercial applications. Primarily, the processing of CNFs into polymer composites has proven to be challenging due to the hydrophilic surface of CNFs, often resulting in severe agglomerations caused by CNF surface incompatibility with the polymer host matrix. Another hurdle is related to the procedures used to isolate CNFs from the cellulose source material and the tailoring of their surface properties to improve the CNF surface solubility in different polymers. Extraction procedures need to be cost-efficient and performed with low CNF degradation, whereas efficient surface coatings are necessary for a predictable dispersion of the CNF in polymer matrices.19 Provided that systematic investigations on these topics are performed, CNFs may become important polymer fillers in commercial plastics, and potentially their load-bearing qualities can be taken advantage of in plastics in similar ways as in nature. In this chapter, we will try to survey some of the important features related to CNFs and their potential use in composite applications.

4.2

Morphological and structural characteristics of cellulose nanofillers

Cellulose nanofillers are typically long and slender micron-sized crystal units that show a whisker-like, rectangular cross-sectional area in the nanoscale with dimensions depending on the cellulose source. The cross-sectional dimensions of the plant parenchyma are ca. 2±3 nm, those of CNFs from wood sources 2±4 nm, from bacterial cellulose 4±7 nm, and for cotton linters and ramie 7±9 nm and 10± 15 nm, respectively.20±28 Marine resources yield CNFs with larger diameters: for tunicate marine animals ca. 20 nm, whereas algae contain 10±70 nm wide CNFs.6,29,30 Figure 4.1 shows a selection of CNFs derived from different cellulose sources. It can be approximated that a 3 nm thick and 4 nm wide CNF contains ca. 150  200 poly- (1,4)-D-glucopyranoside chains aligned parallel along the longitudinal direction of the CNF. The chains can be configured slightly differently depending on how the chains are twisted around their axis and interact by intramolecular hydrogen bonding with neighboring chains, thereby creating different allomorphs. It was recently demonstrated that the cellulose crystal unit is a composite of two crystalline phases (allomorphs), I and I , which have been assigned to triclinic and monoclinic unit cells, respectively.35±38 The allomorphs vary in proportion depending on the origin of the CNF. The cellulose I is an unstable phase and tends to transform into the I phase upon thermal heating.39,40 This transformation works best in polar media such as dilute alkali solutions, and it was suggested that medium interaction with the cellulose chains renders the chains more flexible and prone to reconfiguration.41

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4.1 (a) Micrograph of wood pulp cellulose microcrystals, from Battista [31]. (b) Micrograph of freeze-dried microfibrillated cellulose from wood pulp, from Henriksson et al. [32]. (c) Micrograph of microfibrils from Valonia ventricosa (alga), from Horikawa and Sugiyama [33]. (d) Micrograph of rod-like cellulose microcrystals extracted from the mantle of the tunicate Microcosmus fulcatus, from Favier et al. [34].

An explanation for this morphological transformation phenomenon has been proposed as a break-slip model based on molecular dynamics simulation, whereas Wada et al. experimentally related this transformation to heat-induced thermal expansion of the crystal lattice, allowing for rearrangements of the hydrogen bonds between hydroxyl groups.41,42 The morphology of microfibrils extracted by acidic hydrolysis from microbial cellulose and absorbed on a silicon wafer from an aqueous suspension is displayed in Fig. 4.2. Whereas the hydroxyl groups inside the crystal units link the poly- (1,4)-Dglucopyranoside chains by creating hydrogen bonds with oxygen molecules on neighboring chains, the hydroxyl groups are also present on the surface of the CNF and serve to interconnect the CNFs in the formation of bundles (Fig. 4.2a). However, depending on the configuration of the surface-located hydroxyl groups on the poly- (1,4)-D-glucopyranoside chains, the surface reactivity and their capability for inter-nanofiller condensation reactions vary, and have been expressed as an availability ratio between O(2)H:O(3)H:O(6)H groups (Fig. 4.3).43±46 Rowland, Verlac and others43±46 showed that among the three groups, the availability of the O(3)H group is particularly sensitive to the surface perfection of the crystals. If the CNF surface is highly ordered as inside the cellulose

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4.2 (a) FE-SEM micrograph displaying CNF derived from bacterial cellulose by acid hydrolysis (coated with 1±2 nm thick gold/palladium layer). (b) FE-SEM micrograph displaying CNF derived from bacterial cellulose by acid hydrolysis (not coated).

crystals, the O(3)H group is unreactive due to its strong intramolecular bonds with O(50 ), whereas substantial reactivity of the O(3)H groups has been reported for more disorganized cellulose crystals. The CNF crystalline units with the highest perfection have been reported to originate from the green alga Valonia.47 From an interpretive perspective, the crystals from this source may be considered less prone to surface-modification reactions than those from cellulose from cotton, for example, which exhibit less internal chain order and a more disorganized surface, i.e., leaving a larger amount of O(3)H available for modification. The surface reactivity can also be related to induced functional surface molecules remaining from the extraction procedure (Section 4.4). Whereas the cross-sectional dimensions (thickness and width) of CNFs show generic values depending on the source of the crystals, the lengths of CNFs have received less attention. However, it can be presumed that generic length dimensions also can be related to the CNF source material, but due to the inherent difficulty of ascertaining that the extracted CNFs retain their natural length (post extraction) as related to the source, very little systematic information has been reported on this topic. The complications lie in that the CNFs occasionally break at imperfections (possibly less organized crystal regions) along the crystals during extraction and therefore show a distribution of different lengths.48,49

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4.3 Molecular structure of the poly- (1,4)-D-glucopyranoside chain with central repeating cellubiose unit.

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4.3

91

Extraction and refining of cellulose nanofillers

Cellulose nanofillers (CNFs) can be extracted from the source biomaterial by chemical hydrolysis and/or by applying large mechanical shear forces onto a cellulose suspension.

4.3.1

Extraction by chemical hydrolysis

A commonly used extraction methodology of CNFs is acidic hydrolysis of the amorphous regions surrounding the embedded CNFs and cleavage of the bundles, followed by filtration or centrifugation to exclude dissolved noncrystalline elements.50±57 The methodology is beneficial in that it can be performed on very small quantities of cellulose, it requires only the simplest laboratory equipment, and the CNFs can be obtained without any induced imperfections caused by mechanical processing. The conditions typically involve the use of aqueous solutions of sulfuric acid, stirred at 50±60ëC at atmospheric pressure until a homogeneous beige solution is obtained. This procedure results in cellulose nanocrystals having anionic groups on the surfaces (leading to electrostatic stabilization of the nanocrystals in suspension) with the ability to form chiral nematic liquid crystalline phases in concentrated solutions.54,57,58 The obtained form of cellulose was denoted microcrystalline cellulose, MCC, by Battista.31 With chemical hydrolysis the yield of CNF can be high (>80%), provided the original source is highly crystalline.49 It can, however, be expected that the yield is strongly influenced by the conditions used, since excess hydrolysis results in degradation of the CNF. Exaggerated hydrolysis can typically be noted as the solutions turn dark or black in color as the degradation of the CNF occurs. This phenomenon was reported by Roman et al. who assigned the crystal degradation to potential induced thermal degradation related to the sulfate groups introduced as a functional surface on the CNF when sulfuric acid is used for the hydrolysis.59 However, no mechanism for the degradation related to exaggerated hydrolysis has been presented. So far, the literature provides relatively scarce systematic information on optimized extraction conditions as related to different sources of CNF in terms of acidic strength, temperature and pressure, and how these conditions relate to the intrinsic properties of the CNF. It is notable, however, from earlier literature that temperature and acidic strength may provide efficient tools worth considering for successful extraction procedures, and an increase of 10ëC in temperature has a much greater effect on the rate of hydrolysis than doubling the acid concentration.60 Figure 4.4 shows an example of two solutions of extracted CNFs from bacterial cellulose networks, differing only in their respective hydrolysis temperature. In addition to sulfuric acid, hydrochloric acid has also been used for hydrolysis extraction, leading to less stable suspensions due to smaller amounts

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4.4 (a) Suspension of CNFs (white) at optimized acidic hydrolysis conditions from bacterial cellulose networks. (b) Exaggerated acidic hydrolysis conditions for the same bacterial cellulose networks result in a darker solution.

of induced anionic groups on the crystal surfaces.61±63 The smaller amount of surface charges results in the solutions not exhibiting the same gel-like properties and not dispersing in polar solvents as well as cellulose nanocrystals extracted by sulfuric acid solutions. However, cellulose nanocrystals extracted by hydrochloric acid can be dispersed in protic solvents such as formic acid and m-cresol, since these solvents are able to disrupt the hydrogen bonds in aggregated crystals.64 Furthermore, owing to their reported less elongated shape, the hydrochloric acid-hydrolyzed crystals are sometimes easier to disperse and implement as reinforcement in composite materials.65,66

4.3.2

Extraction by mechanical force

The mechanical methods to extract CNFs from wood pulp and parenchyma cells typically involve a high-pressure homogenizer treatment,67±70 a microfluidizer,19,71 a high-pressure refiner, a super-grinder treatment72±75 or ultrasonication.76 The obtained form of cellulose was denoted microfibrillated cellulose, MFC, by Herrick et al.67 and Turbak et al.68 These processing methodologies have in common that they rely on applying large shear forces on cellulose fiber suspensions in order to mechanically liberate the CNFs from the original plant cell wall structure. In a high-pressure homogenizer this is achieved by allowing a cellulose suspension to pass under high pressure through a thin slit where it is subjected to large shear forces. The shear forces serve to disintegrate the microfibrils or microfibril bundles in the plant cell wall, resulting in nanofibers with diameters of about 5±100 nm. During this homogenization, the

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viscosity of the cellulose suspension increases in relation to the increase in the Einstein coefficient, which increases with length per diameter ratio of the suspended particles.77 Practically, this sets a limit on the original cellulose suspension concentration to approximately 2 wt% fibers, as greater concentrations become overly viscous to force through the system due to the limitations on the pump system. However, the character of the original plant cell wall also influences the number of cycles the suspension has to be passed through the slit, and the procedure is usually experimentally evaluated (by microscopy) and optimized to favor the complete disintegration of cellulose nanofibers. The functional part of a slit homogenizer and the general principle of a microfludizer are illustrated in Fig. 4.5. The function of the high-pressure homogenizer is reviewed in detail by Rees.78 Various pretreatment methods have been developed to facilitate the extraction if the flocculation of the cellulose fibers is severe and causes problems during processing, or if the nanofibers are not sufficiently disintegrated to yield individual nanofibers.32,79 These methods include reduction of the pulp fiber length by mechanical cutting,67 acid hydrolysis,80 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated fiber oxidation,70 swelling,79 enzymatic hydrolysis in combination with beating,32,71 and cryocrushing.12,69,81 Herrick et al.67 showed that pre-cut fibers facilitated the disintegration of the nanofibers by increasing the stability of the pulp fiber suspension, preventing it from sedimentation and interfering with the pumping system in the homogenizer. They also suggested that the degree of fibrillation increased with a more significant exposure of the fiber cross-sectional area. In the case of acid hydrolysis, it was suggested that the more facile disintegration stems from a more brittle cell wall resulting from the hydrolysis reaction. 80 Enzymatic treatment with endoglucanase has been suggested to facilitate microfibrillation by swelling of the pulp fibers32 and in combination with processing in a microfluidizer, which results in nanofibers with dimensions of approximately 10±40 nm.32 TEMPOmediated oxidation introduces negative charges on the surface of the microfibrils, resulting in very efficient microfibrillation during light mechanical treatment, and diameters of 3±5 nm have been reported for nanofibers from wood, cotton and tunicates (see Fig. 4.6).70,82 Mechanical extraction has been applied to several types of cellulose sources, such as wood,19,32,67,68,71,79 sugar beet,69,83 potato tuber,84 banana rachis,85 and wheat straw and soy hulls.86 The disintegration of nanofibers from plant sources normally requires less energy and is easier to liberate from the fiber matrix as compared to isolating fine microfibrils from multilayered structures such as wood pulp fibers, especially if hydrogen bonds have formed between the nanofibers, as in dried pulp.87

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ß Woodhead Publishing Limited, 2011 4.5 (a) Functional valve causing disruption of agglomerates in a typical homogenizer, from Rees [78]. (b) Principal sketch showing a microfluidizer with interaction chamber for disruption of agglomerates, from Microfluidics Corp.

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4.6 (a) Ca. 20 nm wide TEMPO-mediated oxidized CNF from tunicates, from Habibi et al. [82]; (b) 3±5 nm wide oxidized CNF from wood, from Saito et al. [70]. Both dispersed on TEM grids.

4.4

Mechanical properties of cellulose nanofillers

The true value of the crystal modulus of cellulose is an important property, since it sets an upper limit to what is achievable in terms of reinforcing capacity in polymers. Several values have been suggested in the literature, both theoretical and experimental. Meyer and Lotmar reported the first theoretically modeled value of about 120 GPa in 1936.88 Their structure was found to be incorrect and was later corrected by Treloar, who reported a lower value of 56 GPa.89 The modulus for cellulose I crystals was first determined experimentally by Sakurada et al. to 134 GPa from the observation of the change in the c-spacing measured by X-ray diffraction of deformed fiber bundles.90 This crystal modulus is significantly higher than theoretical values, which may be explained by the fact that most theoretical calculations presume uniform stress in the cellulose crystals. In addition, the orientation and distribution of amorphous and crystalline segments also affect the measurement of the elastic modulus by X-ray diffraction. It was pointed out that experimental and theoretical crystal modulus values differed when a parallel coupling between crystalline and amorphous regions is present in the cellulose structure. The significance of this morphological dependence decreased when the degree of molecular orientation and the crystallinity increased. Therefore, the use of fibers or films with high molecular orientation and crystallinity was recommended for the determination of the crystal modulus by X-ray diffraction.91 This highlights the difficulty of accurately determining the crystal modulus of less crystalline cellulose samples. An alternative method is the determination of the crystal modulus by means of Raman spectroscopy. The shift in the 1095 cm±1 band, which is characteristic for cellulose, as a function of tensile deformation, is measured and related to the modulus of cellulose crystals. This technique was applied to measure the micromechanical properties of microcrystalline cellulose, and an elastic modulus of 25  4 GPa was reported.92 By means of this method, a modulus

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of 143 GPa was determined for tunicate whiskers,93 while a lower value of 114 GPa was determined for bacterial cellulose nanofibers.6 A more recent determination of the cellulose crystal modulus using inelastic X-ray scattering (IXS) reported a value for the elastic modulus of 220 GPa.94 This technique exhibits better selectivity than traditional X-ray diffraction, since it is much less sensitive to the contributions of amorphous regions compared with the crystalline regions. A strong anisotropy was observed, with the elastic modulus of 220 GPa in the fiber direction while the modulus in the perpendicular direction was 15 GPa. Single-fiber measurements of the nanofiber stiffness have been conducted by AFM on bacterial cellulose95 and tunicate whiskers.8 In this case the reported modulus is the macroscopic modulus for the nanofibers in comparison with the previously mentioned measurement on the cellulose crystal stiffness. The modulus of bacterial cellulose was experimentally determined as 78  17 GPa,95 while the modulus for the tunicate whiskers was 145 GPa or 150 GPa depending on the extraction method ± TEMPO-oxidation or acid hydrolysis, respectively.8 The modulus for the tunicate whiskers is comparable with the reported values for the cellulose crystal while the modulus for bacterial cellulose is significantly lower. The reason for this is believed to be due to the differences in crystallinity. The crystallinity for the bacterial cellulose was determined to be about 60%,95 while the tunicate whiskers are highly crystalline. Regardless of the method used for measuring the crystal modulus of cellulose, the obtained values are comparable with those of high-performance synthetic fibers such as aramid (130 GPa).96 The crystal modulus is also well above the modulus for aluminum (70 GPa) and glass fibers (76 GPa).96 The ultimate tensile strength of cellulose is estimated to be 17.8 GPa, which is seven times higher than that of steel per weight.96

4.5

Surface modification of cellulose nanofillers

Cellulose nanofillers have a high tendency for self-association due to their strongly interacting surface hydroxyl groups. These interactions lead to the aggregation of CNFs, which often is undesirable for the preparation of nanocomposites. A uniform dispersion of the CNFs, and adhesion between the nanofillers and the polymer host matrix generally are prerequisites for obtaining improved mechanical properties of the resulting nanocomposites. In fact, the main drawbacks of using cellulose nanofillers as functional elements for nanocomposite preparations is their polar and hydrophilic nature, which causes incompatibility issues with most organic solvents and hydrophobic thermoplastic matrices. However, to achieve a controlled dispersion of cellulose nanofillers within the polymeric matrix, several strategies have been developed. One method to enable dispersion in organic media is to coat the surface of the nanofillers with a surfactant.97±103 The surfactant migrates within the organic

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medium and adsorbs onto the hydrophilic surface of the CNFs due to its amphiphilic characteristics, leaving its hydrophobic molecule section in the organic phase of the suspension. Stable suspensions of cotton, tunicate and wood crystals coated by surfactants were obtained in tetrahydrofuran (THF), toluene and cyclohexane,97,98,103 and by small angle neutron scattering (SANS) experiments it was revealed that the surfactant layer showed a thickness of Ê covering the crystals.98 The use of a surfactant is a very convenient ca. 15 A method for improving the dispersion in organic solvents. However, successful use of surfactants relies on selecting amphiphilic molecules with a moderately hydrophobic nature adjusted for the intended organic medium. This is due to the fact that a too-optimized solubility match for the organic medium tends to hinder migration so that the surfactant stays in the organic phase. For this reason, sometimes a very high amount of surfactant is required to coat a high specific surface filler such as cellulose crystals (four times the weight of the crystals),97 which occasionally leads to crystals consisting mostly of surfactant (1.6 times surfactant to the weight of the crystals).99 It is also notable that if the cellulose crystals are surfactant-modified within a solvent medium and further transferred by solvent-exchange procedures, or drying, into a composite application, it is difficult to ascertain to which quantity the surfactant remains on the surface of the crystals within the composite. The lack of covalent bonds between the hydroxyl groups on the crystals' surface and hydrocarbon-functional groups on the surfactant may also pose limitations on the use of this technique in composite applications, since covalent bonds in general are more efficient in providing adequate strength to the composites.104±107 As an alternative, covalent modification of the reactive hydroxyl groups on the surface of the CNFs has gained attention and has been widely studied over the last decade. These modifications include silylation, acetylation, esterification and graft-polymerization reactions. Silylation generally relies on the condensation reaction between the hydrolyzed alkoxy, acetoxy or chloro groups of a hydrocarbon-functional silane coupling agent and the hydroxyl groups on the CNFs' surfaces, leading to the formation of a condensed hydrocarbon-functional silsesquioxane cover on the crystals.108 The condensation reaction (the dissociation of the silane molecules) is energy-driven and depends strongly on the amount of water in the solutions.109 The reactions are normally catalyzed by either acids or bases and proceed at a minimum rate at pH 7.110 For example, tunicin whiskers were stabilized in organic solvents of low polarity such as acetone and THF by a partial silylation of their surface, using alkylchloro silanes as silylation agents. Interestingly, the partially modified whiskers appeared to be significantly more flexible than the unmodified crystals. However, strong silylation conditions or excess reaction times resulted in destruction of the whisker morphology; thus, the outcome relied on the compromise between extent of silylation and preservation of the cellulose morphology.111 This phenomenon was later confirmed for the same type of silane

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by Andresen et al. on microfibrillated cellulose from softwood pulp.112 Alkylalkoxy silanes have also been used for the modification of cellulose crystals.113±116 In general, the silylation route can be regarded as very versatile. However, the main drawback with the silylation reactions relates to sensitivity of the reaction conditions, which are strongly influenced by pH, temperature, and ± in particular ± traces of water in the crystal suspensions. Excess access to water affects the dissociation and condensation of the silane molecules, which immediately reacts and starts to produce dimers and larger oligomers also in absence of the cellulose.117 The outcome of the reaction is therefore much related to the competing polymerization reactions in the suspension and the adsorption of condensed smaller and larger complexes on the surface of the crystals.110 For inorganic nanoparticle systems these phenomena have resulted in coatings of agglomerates rather than the individual nanoconstituents.118,119 Surface acetylation or acylation of CNFs is performed by reacting the reactive hydroxyl groups on the surface of the nanofillers with either acid or anhydride groups, leading to the transformation of hydroxyl groups into acetyl groups (acetylation) or more generally into acyl groups (acylation). The gradual conversion of cellulose into cellulose triacetate (CTA) by adding a mixture of acetic anhydride and acetic acid in the presence of a small amount of catalyst was studied in order to elucidate the mechanism of the acetylation. The reaction appears to start within the amorphous, less organized regions and is followed by the acetylation of the cellulose crystals.120 Highly hydrophobic cellulose crystals were obtained by acylation with alkenyl succinic anhydride.121 A single-step process was developed in which surface acetylation, through Fischer esterification, occurred simultaneously with the hydrolysis of the amorphous cellulose, yielding acetylated cellulose nanowhiskers in a one-pot reaction.122 Surface acetylation has also been used to modify the physical properties of bacterial cellulose, while preserving its microfibrillar morphology.123 It is noteworthy that the degree of acetyl substitution has a significant effect on the properties of the obtained material, and that excess acetylation could have detrimental effects on the final properties in end-use applications regarding, for example, optical transmittance and hygroscopicity.124 Cellulose nanocrystals have been esterified by reaction with organic fatty acid chlorides, with varying aliphatic chain lengths (C12 to C18).125 With the applied method, the obtained degree of substitution was high enough so that the long-chain fatty acids (C18) could crystallize on the surface of the cellulose nanocrystals. Another method for surface esterification is the gas-phase process, in which the surface of cellulose nanocrystals can be almost completely reacted with fatty acid chains, while maintaining the original morphology of the crystal, and also leaving the core of the crystal unmodified.126 Polymer grafting can be conducted through two main routes, `grafting-to' and `grafting-from' the cellulose crystals. The `grafting-to' route involves attachment of pre-synthesized polymers to the CNF surface using a coupling agent.

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The possibility of controlling the polymer size and size distribution is a significant advantage with the `grafting-to' approach, which is possible since the polymer is pre-synthesized and can be readily characterized prior to the reaction. However, the approach shows limitations with regard to the grafting density due to steric hindrance because there is a potential for the grafted polymer chains to block the reactive sites on the cellulose surface. The `grafting-from' technique is based on in situ surface-initiated polymerization from immobilized initiators on the substrate. This method may allow access to a higher graft density and better control of the overall structure due to organized growth of the polymer functionality from the surface of the crystals. Grafting of poly(-caprolactone) (PCL) chains onto the surface of cellulose and starch nanocrystals has been performed by previously subjecting the polymeric matrix to reaction with isocyanate functionalities,127 and PCL-grafted nanoparticles were combined with a PCL matrix to obtain films by casting/ evaporation. The grafting of PCL chains on the surface resulted in higher mechanical modulus and ductility of films, thus indicating the formation of a percolating network owing to chain entanglements and co-crystallization. Octadecyl isocyanate has also been used as grafting agent in order to improve the compatibility of MFC with PCL.128 Microfibrillated cellulose was successfully grafted with PCL by means of ring-opening polymerization (ROP) in order to obtain stable suspensions of MFC in non-polar solvents and to improve the compatibility with PCL.129,130 Grafting of cellulose nanocrystals with poly(styrene) (PS) was performed through atom transfer radical polymerization (ATRP). The hydroxyl groups on the cellulose nanocrystals were esterified with 2-bromoisobutyrylbromide to yield 2-bromoisobutyryloxy groups, which were used to initiate the polymerization of styrene.131,132 Similarly, surface-initiated single electron living radical polymerization (SI-SET-LRP) was employed to polymerize N-isopropylacrylamide from the surface of cellulose nanocrystals to produce thermo-responsive substrates.133 It is important to note that aqueous suspensions of CNFs are stable at lower pH values when they have been extracted by means of sulfuric acid treatment, since negatively charged sulfate groups are introduced on the surface, thus inducing electrostatic repulsion between the CNFs. Accordingly, for watersoluble polymers that allow for uniform mixing with aqueous suspensions of CNFs, it may prove unnecessary to surface-modify the nanofillers for obtaining high dispersion of the cellulose component in composite applications.

4.6

Preparation of cellulose-reinforced nanocomposites

The formation of strong hydrogen bonds between cellulose nanocrystals as the water was removed from a suspension of microcrystalline cellulose (CNF) was

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originally demonstrated by Battista.31 The strong hydrogen bonds between adjacent CNFs allowed for the formation of a dry, stiff and strong network of CNFs, which was impossible to disperse in water. The high stiffness and strength of the cellulose CNFs in combination with the vast sources of cellulose promoted further research efforts. However, the ability of CNFs to condense into a hard and dense material can be very useful, as well as completely disastrous in the process of preparing cellulose-based nanocomposites. In the composites the CNF network improves the mechanical and thermal properties of the composite material. Favier et al. prepared composites by mixing tunicate whiskers and a polymer latex, followed by solution casting.33 The stiffness of the composite was increased compared with the unfilled polymer with additions of only a few percent of whiskers. The thermal stability of the composite was also increased. At an addition of 6 wt% of whiskers the storage modulus was stabilized at temperatures well above the glass transition temperature. This improvement was described as being due to the formation of a whisker network within the nanocomposite. The mechanical potential of CNF networks can also be demonstrated by cellulose nanopapers, i.e., cellulose nanofiber networks. Moduli for these cellulose nanopapers are reported to be in the range of 1±16 GPa19,69,75,134±137 and as high as 30 GPa for bacterial cellulose-based nanopapers.138 There are different approaches reported in the literature on how to prepare cellulose-reinforced nanocomposites. Due to the strong hydrogen bonds formed between adjacent CNFs during drying procedures, the most successful practice in nanocomposite preparation methodologies relies on maintaining the CNFs in the never-dried dispersed state (with or without surface modification) prior to the incorporation in the polymer matrix. Some methods are based on casting where a water-soluble polymer is mixed with the CNF water suspensions and cast. The composite is then formed after water evaporation.19,139,140 Nonsoluble polymers can be used as water-borne latex and directly mixed with the nanofiller suspension in a similar manner as water-soluble polymers. During drying, the polymer particles coalesce and a reinforced polymer composite is formed.33 Solvent-exchange procedures allow for maintaining the CNFs in their non-agglomerated wet state in some organic solvents, thereby omitting drying of the CNFs post extraction. Unmodified CNFs have been reported to successfully disperse in different aprotic solvents,141,142 whereas dispersion in other solvents can be facilitated by the use of surfactants, by chemical modifications, and by polymer grafting. However, in many systems the CNFs still show limited surface solubility regardless of surface modifications, and additional energy is required to maintain the dispersion at a reasonably high level. This energy could be supplied in the form of ultrasound, high-shear mixing or equivalent, which potentially will provide sufficient energy for instant disruption of the aggregated nanoclusters, and thereby open a processing window.

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A different approach to nanocomposite preparation is to impregnate the porous nanopaper structures with a monomer or polymer, which is then cured. The nanopapers are prepared by filtration of fiber suspensions. The wet nanofiber networks can be directly impregnated with the matrix solution143,144 or dried, after which the dry nanopaper is immersed in the solution.13,18,145 Attempts have also been made to process CNFs by melt extrusion.146,147 Provided that sufficient dispersion can be achieved and that the nanocomposite material is formed with a uniform dispersion on a macroscopic as well as a nanoscopic level, there are a number of parameters that can be considered important in terms of evaluating the enhancement in mechanical properties of cellulose nanocomposites: · Matrix/filler and filler/filler interactions. The predominance of one of these interactions over the other depends on the matrix structure and its affinity for the CNF. In the case of cellulose whisker composite materials, it has been observed that too high an affinity between the cellulose whiskers and the polymeric matrix may not always favor the mechanical properties.148,149 Accordingly, matrix±CNF as well as CNF±CNF interactions play a crucial role in the reinforcement effect, since hydrogen bonding among the whiskers leads to the formation of rigid whisker networks, which facilitate the stress transfer from the matrix. · The geometrical aspect ratio (L/d) of the filler, i.e., the ratio between the length L and the diameter d of the nanofillers. This parameter is directly influenced by the source of cellulose and the preparation conditions of the CNFs. It is advantageous to have an aspect ratio larger than 50 in order to have a considerable reinforcement effect when compared with micron-sized filaments.150 Nevertheless, for aspect ratios larger than 100, Young's modulus reaches a plateau which corresponds to the maximum point of reinforcement.150 · The processing method. The solvent-casting technique was found to give higher mechanical performance nanocomposites than those obtained by freeze-drying/molding.12,151 This behavior was ascribed to the sedimentation of the filler during evaporation of the solvent and to the whisker/whisker interactions. A decrease in the apparent aspect ratio of whiskers was thought to take place when hot pressing or extrusion is used, due to a gradual breakage and/or orientation of the whiskers.152

4.7

Future trends and applications of cellulose nanofillers

CNFs are likely to be of high relevance in future polymer composite formulations provided that means for their extraction, isolation and refinement, as well as surface modifications, can be systematically developed. It can be

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presumed that exploitations of the CNFs initially will find use in high-end applications, where their use presents significant advantages in limited quantities. Possibly, these applications include membranes/filters and wound dressings, dental implants, advanced glue systems, strong adhesive tapes and products requiring high transparency in combination with improved mechanical properties. It was recently demonstrated that crystals from cellulose not only provide improved mechanical properties but can also be associated with a high level of optical transparency, provided that polymer matrix host material is selected with care.143,153 As inexpensive large-scale production of CNFs are continuously being developed, the main hurdle for their implementation on an industrial level will undoubtedly be related to finding the extraction procedures and simple methodologies to surface-modify CNFs as integrated in a system. It is noteworthy that some recent literature shows improved barrier properties as related to the cellulose crystal contents, which implies that CNFs may eventually find use in more environmentally friendly packaging materials.154±159 Implementation of CNFs as barrier agents in packaging films would present a significant leap forward in the direction of creating a more sustainable environment. Currently, layered silicates are the most commonly used barrier modifiers in the plastics industry; however, the natural layered silicates are not from a renewable resource or biodegradable, whereas CNFs possess these characteristics.160±165 A promising area relates to the implementation of CNFs in current industry-emerging biopolymers, such as poly(lactic acid) (PLA), polyhydroxylalkanoates (PHA) and polycaprolactones (PCL), of which PLA and PHA are fully renewable and biodegradable plastics.

4.8 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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153. Nogi, M.; Iwamoto, S.; Nakagaito, A. N.; Yano, H. Adv. Mater., 2009, 21, 1595± 1598. 154. Sanchez-Garcia, M. D.; Lopez-Rubio, A.; LagaroÂn, J. M. Trends Food Sci. Tech., 2010, 21, 528±536. 155. Fendler, A.; Villanueva, M. P.; Gimenez, E.; LagaroÂn, J. M. Cellulose, 2007, 14, 427±438. 156. Hult, E.-L.; Iotti, M.; Lenes, M. Cellulose, 2010, 17, 575±586. 157. Bilbao-SaÂinz, C.; Avena-Bustillos, R. J.; Wood, D. F.; Williams, T. G.; McHugh, T. H. J. Agric. Food Chem., 2010, 58, 3753±3760. 158. Spence, K. L.; Venditti, R. A.; Rojas, O. J.; Habibi, Y.; Pawlak, J. J. Cellulose, 2010, 17, 835±848. 159. Petersson, L.; Oksman, K. Compos. Sci. Technol., 2006, 66, 2187±2196. 160. Ray, S. S.; Maiti, P.; Okamoto, M.; Yamada, K.; Ueda, K. Macromolecules, 2002, 35, 3104±3110. 161. Maiti, P.; Yamada, K.; Okamoto, M.; Ueda, K.; Okamoto, K. Chem. Mater., 2002, 14, 4654±4661. 162. Di, Y.; Iannac, S.; Sanguigno, L.; Nicolais, L. Macromol. Symp., 2005, 228, 115± 124. 163. Sanchez-Garcia, M. D.; Gimenez, E.; LagaroÂn, J. M. J. Plast. Film Sheet., 2007, 23, 133±148. 164. Chowdhury, S. R. Polym. Int., 2008, 57, 1326±1332. 165. Hamad, W. Cellulosic Materials; Fibers, Networks, and Composites. Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002.

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Electrospun nanofibers for food packaging applications S . T O R R E S - G I N E R , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: Electrospinning is a novel fabrication technology based on high electric fields that can be used to produce polymer- and biopolymer-based mats composed of nanofibers or other nanostructures. Electrospun mats have been shown to possess unique functionalities originating from the very large surface-to-volume ratios of the nanofibers, the use of functional and/or renewable polymers or the encapsulation of bioactive non-polymer substances. The functional electrospun mats can be used as tools for the development of nanocomposite fabrics from a wide variety of plastics with improved performance for packaging applications. They could serve, for example, as a reinforcement to enhance the physical properties of plastics and bioplastics, as transparent layers to a gas barrier, and even as an emerging technology to design bioactive packaging with antimicrobial protection or delivery of nutraceuticals to foods. Key words: electrospinning, ultrathin fibers, nanocomposites, active and bioactive packaging.

5.1

Electrospinning

Looking genuinely at nature, nanofibers often serve as a basic platform on which either organic or inorganic components are built. For instance, cellulose nanofibers would represent the building blocks in plants while collagen nanofibers would do so in the animal body. The fiber structure exhibits, from a structural point of view, a certain ability to transmit forces along its length, thus reducing the amount of materials required. While strong enough for their designed purpose, nanofibers have the added advantage of giving high porosity to the natural supports, which allows faster diffusion of chemicals to the inner structure. To follow this extraordinary natural design, a process that is able to fabricate fiber nanostructures from a variety of materials and mixtures is an indispensable pre-requisite. Control of the nanofiber arrangement is also necessary to optimize such structural requirements. Electrospinning is a physical process used for the formation of ultrathin fibers by subjecting a polymer solution to high electric fields. A schematic representation of a typical laboratory electrospinning setup is shown in Fig. 5.1. In this configuration, a polymer solution is placed inside a syringe lying

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5.1 Typical electrospinning setup where a non-woven nanofiber mat is collected.

horizontally on a digitally controlled pump which permits adjustment and precise control of the solution flow-rate. The polymer solution is then pumped to a metallic millimeter-size needle which is connected to a high voltage power supply operated in positive DC mode and with low current intensity. At a critical high voltage (5±25 kV), the polymer solution droplet at the tip of the needle distorts and forms a Taylor cone to be ejected as a charged polymer jet. This stretches and is accelerated by the electrical field towards a grounded and oppositely charged collector. As the electrospun jet travels through the electrical field, the solvent completely evaporates while the entanglements of the polymer chains prevent it from breaking up. This results in the deposition of ultrathin polymer fibers on a metallic collector to habitually assemble the fibers as nonwoven mats. As elongation is accomplished via a contactless scheme, electrospun fibers are considerably thinner in diameter and thus higher in surface-to-volume ratio than fibers fabricated using conventional mechanical extrusion or a classical spinning process. For instance, while electrospun fiber diameters are habitually under the micron and also within the nanometric range, synthetic fibers produced via extrusion and fibers of biological origin such as cotton, wool, or silk are characterized by diameters in the range of various micrometers and above. Moreover, since the electrospinning is a continuous process, fibers when wound can be as long as several meters or even kilometers. The formed fibers are not only ultrathin and relatively long but also fully interconnected to form a three-dimensional network. Such small dimensions generally lead to very high ratios of specific surface area to mass and provide the electrospun products with extraordinary functionalities which are not found in similar materials of larger sizes. Fiber formation via electrospinning basically requires the materials to be processed to display specific viscoelastic properties, electrical conductivities in

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a limited range of values, and specific surface energies. Such requirements can only be met by polymers provided that molecular weights are chosen in an appropriate range. In order to carry out the process, the polymer must first be in a liquid form, either as molten polymer or as polymer solution. Melting electrospinning is limited by the fact that fibers are generally above the micron size and electrospinning from polymer solutions is then habitually preferred. In this last case, solvent properties such as boiling point and conductivity play a significant part in the electrospinning process and in the resultant electrospun morphology (Torres-Giner et al., 2008a). In particular, round-like fibers from less than 40 nm to a few microns can be fabricated by electrospinning of more than 50 different types of polymers (Li and Xia, 2004). In addition to observational changes in the fiber diameter, through the modification of the electrospinning setup other fiber and non-fiber assemblies with novel remarkable features can be produced, such as uniform or variable flat and round-like fibers, bead-like or round particles, core±sheath or multilayer coaxial structures, hollow tubes or porous fibers, aligned fibers, crosslinked fibers, and multi-jet fibers. This adjustability certainly enhances the performance of the electrospun materials, allowing application-specific modifications. Electrospun shapes are particularly developed by changing the polymer solution and process conditions: polymer concentration, solvent nature, tip-tocollector distance, voltage and flow-rate. Among all process parameters, the polymer concentration of the solution for electrospinning is the most relevant factor determining the fiber diameter (Torres-Giner et al., 2008b). Thus, higher concentrations generally result in the formation of fibers with larger diameters. Reduction of the polymer concentration below a threshold value would result in beads or in extremely thin fibers with beads along their length. This variation within the electrospinning technology is also called `electrospraying' because the process acts like an atomization procedure to form nearly mono-dispersed and non-fibrillar ultrathin particles (Torres-Giner et al., 2010). Figure 5.2 shows the most common structures that may be fabricated by the electrospinning technology. Fibers composed of blends of different polymers can be prepared by electrospinning from solutions containing different polymer species in a common solvent. The resulting morphology can be either of a matrix-dispersed phase type or co-continuous, depending on the thermodynamic and kinetic properties of the electrospinning solutions. Another interesting characteristic of the electrospinning procedure is the ability to form porous nanostructures, which may imply a tremendous increase of the fiber surface area (see Fig. 5.3). These pores can be produced when, in the electrospinning of a blend consisting of two polymers, one of the polymers is partially removed after the fiber formation by dissolving it in a solvent in which the other polymer is insoluble (Torres-Giner et al., 2008a). It is even possible to vary the pore size and density by further controlling the processing parameters (Ramakrishna et al., 2006). Pore

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5.2 Selected SEM images of electrospun zein networks for: (a) thick tubular fibers obtained from concentrated solutions; (b) thin tubular nanofibers obtained using long tip-to-collector distances; (c) nanobeads obtained from diluted solutions; (d) ribbon-like ultrathin fibers obtained from the acidified solution. Scale markers of 5 m in all cases (Torres-Giner et al., 2008b).

formation can also be induced in the presence of high humidity, where condensation processes lead to the formation of water islands within the fibers, subsequently causing pore formation. Electrospinning onto very cold substrates such as liquid nitrogen can also result in highly nanoporous fibers. On the other hand, similar to this, fiber surfaces may not always be smooth, depending mainly on the solvent volatility used in the electrospinning. For instance, fibers may fuse together if the solvents are not completely evaporated, to yield threedimensional networks that have foam or sponge-like structures (Gomes et al., 2007). Coaxial electrospinning, also habitually called co-electrospinning, can be further applied for the preparation of polymer core±shell fibers and hollow

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5.3 Chitosan porous nanofibers resulting from specific solvent post-treatment. Scale marker of 2.5 m (Torres-Giner et al., 2008a).

fibers. This is a frequent solution for those cases in which the material cannot be electrospun to produce fibers, such as oils and low molecular weight polymers. It consists of the same electrospinning setup as for polymer blends with the exception of the use of two needles which are arranged in a concentric configuration and connected to two different reservoir solutions. If the two solutions are immiscible and contain two different polymers, a core±shell nanofiber is formed, while if the inner solution is free of polymer a hollow fiber is produced from the outer polymer solution. In the latter case, the inner solution usually consists of a non-polymer fluid or an immiscible solvent with the solvent used for the outer solution. It is frequently preferable to apply the shell from the immiscible solvent which can be easily removed by a vapor phase separation rather than from a fluid phase, which may give rise to mechanical forces causing a disruption or a swelling of the fibers. Furthermore, typical electrospinning results in planar random non-wovens, that is, with an isotropic orientation of the fibers, if planar electrodes are used. However, a parallel arrangement of the fibers can be obtained by employing cylindrically shaped electrodes which rotate and collect the fibers during the process (Xu et al., 2004). Tubular arrangements become accessible in this way, with the fibers being either parallel or random depending, among other parameters, on the rotation speed. A further approach consists in the use of the so-called split electrodes which are composed of, for instance, a set of two parallel electrodes or a set of four electrodes arranged in a rectangular pattern.

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The expansion of the technology has also brought alternative methods to produce the nanofibers in a more controlled and efficient manner. For instance, similar nanofibers can be made from solution blow spinning, which does not require high voltage equipment or any electrically conductive collector and, more importantly, avoids the use of aggressive solvents. This setup consists of a source of compressed gas such as nitrogen, argon and air, equipped with a pressure regulator system of concentric nozzles: an inner nozzle through which the polymer solution is pumped, and an outer nozzle through which a high pressure stream of the gas passes (Medeiros et al., 2009). This particular nozzle geometry creates a region of low pressure around the inner nozzle that helps draw the polymer solution into a cone, which then elongates into a fiber. The certain ability to fabricate polymer-based nanofibers with controllable size and porous structure in the form of non-woven mats or three-dimensional porous structures could provide virtually unlimited novel sources for the development of natural polymer-based applications (Torres-Giner et al., 2008a, 2008b). The highest potential role that the electrospinning process can play is possibly the construction and reproduction of multi-level materials. Unlike the bottom-up methods, electrospun fibers are produced through a top-down process, which results in continuous and low-cost fibers that are also relatively easy to align, assemble, and process into applications. Also, as previously mentioned, deposited structures can also be particulates or mixtures of fibers and particles. The use of such ultrathin structures as components in subsequent largescale derived products is attracting considerable interest in the industry of functionalized materials for their utilization as tissue engineering scaffolds, pharmaceutical drug release dosages, highly functional food and ingredients, and active and bioactive packaging materials, and in general for the manufacture of advanced functional materials.

5.2

Functional nanofibers

The electrospinning technique has mainly dealt with synthetic polymers due to their low cost, high availability, and well-defined chemical properties that allow for more uniform behavior during the electrospinning process. However, instead, the technology also opens up enormous possibilities for the implementation of bio-based materials and food hydrocolloids such as proteins and polysaccharides, to make novel biodegradable and renewable structures of interest in various functional applications. In the context of the literature review, there are numerous studies that focus on the use and production of electrospun biopolymer-based ultrathin fibers. The main advantage of biopolymers is that in a natural way they usually compromise bioactive properties without including for this function any other kind of compounds. The reason for this is that biopolymers are macromolecules produced in nature by living organisms and plants, and because of their participation in the natural biocycles, they can

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compromise certain functionalities. Biopolymers are of great interest in various industries, chief amongst them being the food industry, because they may be (a) non-toxic, edible, and digestible, (b) biocompatible and biodegradable, and (c) renewable and sustainable, giving rise to a broader utilization, especially in fields such as biomedical sciences, pharmaceuticals, cosmetics, and other related fields. Further, biopolymer uses could allow for the creation of value-added products if currently under-utilized but abundantly available biomass were to be used as raw material for electrospinning. Naturally occurring polysaccharides and proteins have been known to be biocompatible and safe for many different applications. A wide range of them can be electrospun into ultrathin fiber mats with a specific fiber arrangement, structural integrity, and full biocompatibility. Many studies have been conducted using polysaccharides and proteins for an electrospun fabrication that could be potentially useful for functionalizing materials. Table 5.1 gathers the most widespread researches on electrospun biopolymers with their expected applications. Till now, polysaccharides including, but not limited to, alginates (Nie et al., 2008), cellulose (Zhang et al., 2008; Ma and Ramakrishna, 2008), and chitosan (Torres-Giner et al., 2008a; Neamnark et al., 2008) have been electrospun as good examples of novel functional materials. For instance, alginate nanofibers have excellent biocompatibility, low toxicity, non-immunogenicity, relatively low cost and simple gelation behavior with divalent cations which have been studied for many biomedical applications. On the other hand, cellulose-based electrospun nanofibers have been largely proposed in the pharmaceutical and biomedical fields, including for applications as adsorbent beads, filters and barrier membranes, artificial tissue/skin, antimicrobial membranes and protective clothing. Furthermore, studies on antibacterial activities of Table 5.1 Summary of promising properties related to the electrospinning of functional polymers Polymer nanofibers

Functional properties

Alginates Celluloses

Wound dressings for tissue engineering Ion-exchange medium for protein separations in biomembranes Cell support, antimicrobial performance Enlarges cell proliferation and osteoblastic activity Proliferation of muscle, bone and skin cells Improved frictional surface and cell adhesion Biofillers of mechanically improved packaging Water entrapper, thermally resistant, and biologically valuable Increases cell biocompatibility and antibacterial activity Enhances Cu2+ adsorption capacity in water purification filters

Chitosan Starch Collagen/gelatin Silk fibroin Gluten Zein Collagen and chitosan Wool keratose and silk

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chitosan nanofibers have shown that the salts of this biopolymer inhibit the growth of S. aureus bacteria in the presence of very low fiber amounts (TorresGiner et al., 2008a, 2009a). The polycationic nature of migrated glucosamine fractions of chitosan is considered to interfere with the negatively charged residues of macromolecules at the cell membrane surface, which results in the death of the microorganisms. In this way, composite materials which contain the chitosan fibers can be used in far more applications with respect to their biodegradability and unusual properties that could help control the microorganisms. Other polysaccharides, such as hyaluronic acid, starch, dextran, and heparin have also shown potential in the electrospinning process, with or without polymer additives (Lee et al., 2009). Some proteins are original counterparts of the animal body and their electrospun mats can produce specific biological responses. In the tissue engineering field, electrospun ultrathin fiber interfaces made of animal proteins can easily approximate the nanostructural morphology of natural tissues by assessing appropriate levels and sizes of porosity to allow cell migration, sufficient surface area and a variety of surface chemistries to encourage cell adhesion, growth, migration, and differentiation, and adjust the degradation rate to match tissue regeneration (Lannutti et al., 2007). In this context, collagen can be considered as the most promising natural polymer for tissue restoration processes and, therefore, the electrospinning of collagen has been widely reported (TorresGiner et al., 2009b). Another interesting protein, which shows not only cellular responsive features but also potential uses in functional textiles and clothing design, is silk fibroin. The use of silk for active surfaces as electrospun nanofiber assemblies with other polymers is confirming very encouraging results due to improved frictional properties (Akada et al., 2007). Electrospun ultrathin structures of zein prolamine from corn also have good potential in the food technology area, not only as a reinforcing fiber in, for instance, plastic food packaging applications, but also as an edible carrier for encapsulation of food additives (Torres-Giner et al., 2010) or to modify food properties and in the design of novel active and bioactive packaging technologies (Torres-Giner et al., 2008b). As a most recent and natural approach to increase the bioactivity of the electrospun polymers, the latest research is being focused on blending two biological polymers. Probably the clearest example that shows benefits of this biological combination in the electrospinning is the collagen±chitosan complex, which has been proposed as optimal wound dressings because it combines the enhanced cell biocompatibility of collagen with the antibacterial activity of chitosan (Chen et al., 2008). Apart from the in tissue engineering field, other electrospun bioblends are promising candidates for different functional applications: wool keratose and silk fibroin as metallic particle filters in water treatment (Ki et al., 2007) or zein and chitosan for antibacterial and functional packaging (Torres-Giner et al., 2009a).

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Multifunctional and nanoreinforced polymers for food packaging

Nanoencapsulation

Electrospun fibers may be given additional functionality by incorporating functional nanoscalar compounds, e.g. drugs, into the electrospinning solution. Further investigations on this approach permit one to functionalize or add extra functionalities to electrospun materials to display specific properties. The process generally consists of the introduction of very diverse non-polymeric agents into the polymer solution to electrospin. This results in hybrid ultrathin fibers in which the incorporated agents remain packed into the polymer matrix as individual nanodroplets of liquid or solid nanoparticles. Such nanostructural distribution can overcome the issue of low stability of encapsulated ingredients as well as improve the distribution of the functional components. This approach also becomes crucial when synthetic polymers, such as poly(lactic acid) (PLA), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), poly(ethylene oxide) (PEO), poly(-caprolactone) (PCL), poly(styrene) (PS), and their copolymers, are employed because this constitutes their unique current way to functionalization. In particular, the electrospun morphology can provide high efficiency in drug-loading and, therefore, has been proposed to provide novel functional carriers in pharmaceutical compositions by nanoencapsulating specific therapeutic compounds (Sawicka and Gouma, 2006). The rate profiles can also be controlled, obtaining desired adjustable releases, such as rapid, immediate, or delayed dissolution, or a modified release profile, for instance the sustained and/ or pulsatile release characteristic. This is specifically modulated by changing the morphology, porosity, and hydrophilic/hydrophobic composition of the electrospun fiber membrane. For instance, the application of electrospraying to substance-loading polymer solutions results in the formation of nanocapsules of high substance encapsulation efficiency and performance because of the ultrathin particle sizes which can reduce the losses of the entrapped substances and enhance the bioavailability (Torres-Giner et al., 2010). Several examples can be found in the literature regarding encapsulation of substances using electrospinning with practical application in the packaging field. Table 5.2 includes different electrospun encapsulation works that report uses in the design of functional nanostructures of additives, coatings or interlayers. Improving the surface functionality of electrospun fibers with bioactive molecules could be very important for specific bioactive applications. For example, clays and other minerals have been proposed to be introduced in electrospun nanofibers of zein, such as organo-modified and unmodified mica, kaolinite, montmorillonite (MMT) and zeolite (Torres-Giner and LagaroÂn, 2010). These zein±clay nanofibers are proposed to reinforce the mechanical, thermal, barrier and control release properties of both plastic and bioplastic matrices without implying losses in biodegradability and optical properties. For active food packaging, incorporation of enzymes and bioactive molecules into a matrix is usually challenging, as they are generally sensitive to heat and may

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Table 5.2 Summary of promising properties related to nanoencapsulation using electrospinning Polymer nanofibers

Functional properties

Zein±clays

Improved mechanical and thermal properties, controlled release Enhancement and control of water resistance

PVA±polyhedral oligosilsesquioxane PLA±lysozyme PVA±lipase PLA±paracetamol Chitosan±ibuprofen PVA±acetylsalicylic acid PCL±resveratrol and gentamicin sulfate PLA±tetracycline Cellulose±vitamin A, E PLA±biotin Cellulose±erythromycin Zein± -carotene PVA±Ag PVP±Iron particles PEO±DNA PVA±bacteria

Antibacterial, controlled release Fast transesterification activity Analgesic sustained release Sustained release of an anti-inflammatory Controlled release of analgesic Sustained release of antioxidant and antibiotic substances Controlled release of antibiotic Controlled release of vitamins as transdermal and dermal therapeutic agents Release of vitamin H Artificial gastric juice of acid protection Antioxidant activity Films of broad-spectrum antibacterial and antifungal functionability Offers enhanced protection to prevent from oxidation Fluorescently labeled proteins Protection and release of living organisms

lose their activity when in contact with certain chemicals. Electrospinning can be carried out in room temperature conditions and this allows such molecules to be satisfactorily incorporated. Bioactive molecules can include enzymes such as lysozyme and lipase, pharmaceuticals such as paracetamol, ibuprofen, and acetylsalicylic acid, or antibiotics such as gentamicin sulfate or tetracycline. Nutraceuticals can also be nanoencapsulated as functional food, which promotes health beyond providing basic nutrition and can be included in bioactive food packaging applications. For example, electrospinning technology has been presented as novel route to stabilization of vitamins or antioxidants such as carotene (Fernandez et al., 2009) and omega-3 fatty acids (Torres-Giner et al., 2010). They can bioact on the containing food from the incorporated package or be released from it when it is open. Figure 5.4 shows zein/ -carotene ultrathin fibers which have been proved to present a remarkably good protection against oxidation when exposed to UV±vis irradiation. The technology is therefore reported here as capable of producing added-value biopolymer nanofibers that can have good potential in food and nutraceutical formulations and coatings, bioactive food packaging and food processing industries. In other more complex cases such as with some metals, semiconductors or inorganic compounds, it can be difficult to be able to fulfill the physical

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5.4 Fluorescence image of electrospun zein/ -carotene nanofibers (Fernandez et al., 2009).

electrospinning requirements. The answer to this problem is to add their low molar mass precursor materials such as metal salts, for instance, in high concentrations to the polymer solution. This, in turn, is electrospun to yield fibers in which the precursor molecules are dispersed within the polymer carrier. The precursor species are subsequently subjected to further chemical modifications such as, for instance, a reduction process. A clear example for this case is the inclusion of silver salts in natural and synthetic polymer solutions. Silvercontaining electrospun fibers are then thermally activated and the resulting materials are highly antimicrobial because they can function either as release systems for Ag+ ions or as contact-active materials (Dong et al., 2009). Other cases include the dispersion of ferrofluids into the fibers to produce materials which display ferromagnetism and supermagnetism functionalities. Fe3O4, for example, is a well-known ferromagnetic material which, however, becomes superparamagnetic as the size of the particles is reduced down to the nanometer scale (Graeser et al., 2007). The coaxial electrospinning method has a number of additional advantages that make it more attractive for the production of functional nanofibers. From the point of view of the nanoencapsulated objects, first of all this provides a more natural environment, and secondly the electric charges are located practically only at the outer surface, so that the objects dispersed in the inner parts are not charged at all. It may also reduce the contact between the nanoencapsulated components and the organic solvents, which is expected to improve the bioactivity of the resultant nanofibers. They would only be affected mechanically by the predominantly viscous stresses in the droplet during electrospinning. Apart from requiring milder conditions, the coaxial method also

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has the advantage of exerting greater control over the release behavior, which increases the loading efficiency. This method effectively protected fragile biological agents, enabling the incorporation of proteins, enzymes, and DNA (Yarin et al., 2007). This can be applied even to obtaining `living' membranes of non-wovens made from fibers containing biological objects, for instance viruses, bacteria and cells, in such a way that these objects keep their specific functions to adapt to their new environment (Lopez-Rubio et al., 2009).

5.4

Electrospinning in packaging applications

The ease and versatility of the electrospinning technique has offered new opportunities for researchers to investigate the effectiveness of nanofibers as a reinforcement to enhance the properties of a matrix material (Huang et al., 2003). Composites are combinations of two distinct material phases, the bulk phase or matrix, and the reinforcing phase. From a mechanical point of view, it is the combination of the strength of the reinforcement and the toughness of the matrix that gives composites their superior properties that are not present in single conventional materials. When fiber reinforcements close to 100 nm in cross-section are used, the final materials are called nanocomposites, and they generally maintain their original optical properties. Also as anticipated, nanofibers not only can have better mechanical properties than microfibers of the same materials but also may possess some additional merits which cannot be shared by traditional composites. Additionally, their application to renewable biopolymers represents an emerging new class of nanocomposites, also referred to as nanobiocomposites, with reduced environmental impact and high functionality, which are the wave of the future and are considered as the materials of the next generation (Pandey et al., 2005). In the fiber reinforcement context, depending on the type of application, the arrangement of nanofibers can basically take two different forms: aligned or randomly distributed. For load-bearing applications, the most effective arrangement is to have the nanofibers aligned in the stress direction to form a laminated nanocomposite (see Fig. 5.5a). Such a material is strong in the direction coinciding with the aligned nanofibers but weak in the perpendicular direction. Randomly distributed nanofibers, in the form of non-woven nanocomposites (Fig. 5.5b), can also be effective in other areas, for instance as gas barrier layers. The introduction into the matrix of nanometer-sized spaces produced by the nanofiber mesh can lower the gas permeability because of a tortuosity effect in the gas diffusion. Recently, electrospinning has been presented as a potential technology for use as a platform for multifunctional, hierarchically organized nanocomposites (Teo and Ramakrishna, 2009). Electrospun nanofibers can be incorporated as reinforcements to plastic matrices by different methods, such as melt mixing or solvent casting, to generate novel nanocomposites consisting of nanofiber layers intercalated into a

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5.5 Basic nanocomposite reinforcements based on electrospun fibers: (a) laminated nanocomposites; (b) non-woven nanocomposites.

plastic matrix. In other cases, with a greater emphasis on functional capability, bioactive nanofibers can be exposed on the matrix surface to the environment to modify external stimuli or to the food as drug vehicle. Figure 5.6 presents these different fiber dispositions in the electrospun nanocomposites according to the desired functionality for food packaging applications. Nevertheless, to date, little or no research has been done on this topic and only a few researchers have tried to make nanocomposites reinforced with electrospun polymer nanofibers. Some works have proved that the mechanical performance of certain epoxy composites can be expanded when impregnation with electrospun nanofibers is realized (Kim and Reneker, 1999; Fong, 2004). Nanofibers can thus penetrate through the resin matrix to form a stronger and stiffer network, in which their very high surface-to-volume ratio may improve the resultant material toughness. Such results in long-term materials have triggered research on the exploitation of nanofibers for biodegradable plastics. As an example, it is reported that electrospun PVA fibers can be coffined into a PS matrix to produce a reinforced film with enhanced mechanical properties (Tsutsumi and Hara, 2008). In other work (Chen and Liu, 2008), electrospun cellulose mats composed of fibers 200±

5.6 Electrospun nanocomposites in the design of novel food packaging materials.

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800 nm long have been incorporated in soybean protein isolate to improve the mechanical stress at yield and Young's modulus by 13 and 6 times, respectively. More interestingly, this composite exhibited high visible light transmittance of ca. 75%. Regarding this optical characteristic, a nanocomposite using electrospun Nylon-4,6 nanofibers as reinforcement and phenolic epoxy resin as matrix was shown to be also transparent (Bergshoef and Vancso, 1999). This is because light was able to transmit over the entire range of wavelength of the visible spectrum (400±700 nm). Since light neither reflects nor refracts at air± nanomaterial interfaces if the material size is less than one-tenth of the wavelength of visible light, the reinforcing nanofibers serve greatly in the manufacture of transparent reinforced materials. Using nanofibers as composite reinforcements has shown a few interesting optical and mechanical results, but they have not been so well studied in regard to the simultaneous reinforcement of the barrier properties of plastics and bioplastics in packaging and membrane applications. The gas and vapor permeability of biopolymers can be enhanced dramatically by incorporating nanofiber layers of fairly low thickness as interlayers or coatings. For instance, the incorporation of a thin interlayer of electrospun zein ultrathin fibers into PLA-based films has shown to reduce up to 71% the oxygen permeability in comparison to the same unreinforced matrix (Busolo et al., 2009). As can be seen by optical microscopy, in the interior of the PLA film of Fig. 5.7 (top image) the thin reinforcement results in completely transparent and colorless sheets similar to the original one (bottom images).

5.5

Future trends

To conclude, electrospinning allows extensive tunability in material properties and functions through specific selection of the solution composition. Although electrospun materials are predominantly polymer-based, ceramic, metallic and other bioactive particles can also be introduced into the fibers and subsequently be part of the final nanocomposites. At first, non-polymer particles or a second polymer can be mixed into the primary polymer solution and electrospun to form hybrid ultrathin fibers. As an example, results of the nanodispersion of commercial minerals into electrospun ultrathin zein fibers have shown a considerable increase in thermal resistance at mineral contents below 10 wt% (Torres-Giner and LagaroÂn, 2010). Further modifications of the electrospinning technique can be performed to increase the number of functional materials and to broaden the range of potential applications. This can be represented in the modification of the morphology or surface of nanofibers, the use of coaxial electrospinning technology to produce a second layer of polymer material, and the orientation and organization of the nanofibers by modification of the collector to optimize its performance. The development of new electrospinning configurations such as solution blow spinning can provide novel nanofibers with greater potential for

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5.7 Top: PLA±zein nanocomposite micrographs in top view by optical microscopy (scale marker is 100 m); bottom: images of the original film (left) and resultant composite (right) with electrospun nanofibers (Busolo et al., 2009).

commercial scale-up. Such additional adaptations will allow creating advanced multi-functional nanocomposites, in which various functions are incorporated for plastics in multi-sectorial applications. In this sense, future hybrid nanostructures will be applied as functional reinforcing fillers in uses such as coatings, packaging, and other applications. In the future it will be important to focus research on gaining a better fundamental understanding of the electrospinning process, but even more importantly on how this technique can be used as a tool in developing new materials. The studies described above indicate that the production of nanocomposites from electrospun fibers is feasible. However, some more essential

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studies are still required and many challenges remain to be faced. In particular, the ability to integrate the nanofibers into packaging materials in an efficient and reproducible manner remains a major challenge. Although many works have reported on the control, orientation, size, and other target characteristics, further advances concerning the reproducibility of locating the nanofibers in specific positions and orientations will be necessary. The encapsulation and posterior release of bioactives such as nutraceuticals or antimicrobials will also require further studies to prove the expected bioactive properties in the resultant material. Regarding fiber productivity, scale-up and commercial production are other general challenges which need to be addressed. The design and construction of process equipment for controllable and reproducible electrospinning will act as a stimulus to provide novel products based on electrospinning technology.

5.6

References

Akada M, Kotaki M, Sato M, Sukigara S (2007) `Surface frictional properties of silk/ nylon blended nanofiber assemblies' J. Textile Eng. 53 245±248. Bergshoef M, Vancso G (1999) `Transparent nanocomposites with ultrathin, electrospun nylon-4,6 fiber reinforcement' Adv. Mater. 11 1362±1365. Busolo MA, Torres-Giner S, LagaroÂn JM (2009) `Enhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers' Annual Technical Conference ± ANTEC, Conference Proceedings 5 2763±2767. Chen G, Liu H (2008) `Electrospun cellulose nanofiber reinforced soybean protein isolate composite film' J. Appl. Polym. Sci. 110 641±646. Chen Z, Mo X, He C, Wang H (2008) `Intermolecular interactions in electrospun collagen±chitosan complex nanofibers' Carbohyd. Polym. 72 410±418. Dong G, Xiao X, Liu X, Qian B, Liao Y, Wang C, Chen D, Qiu J (2009) `Functional Ag porous films prepared by electrospinning' Appl. Surf. Sci. 255 7623±7626. Fernandez A, Torres-Giner S, LagaroÂn JM (2009) `Novel route to stabilization of bioactive antioxidants by encapsulation in electrospun fibers of zein prolamine' Food Hydroc. 23 1427±1432. Fong H (2004) `Electrospun nylon 6 nanofiber reinforced BIS-GMA/TEGDMA dental restorative composite resins' Polymer 45 2427±2432. Gomes DS, da Silva ANR, Morimoto NI, Mendes LTF, Furlan R, Ramos I (2007) `Characterization of an electrospinning process using different PAN/DMF concentrations' PolõÂmeros: CieÃncia e Tecnologia 17 206±211. Graeser M, Bognitzki M, Massa W, Pietzonka C, Greiner A, Wendorff JH (2007) `Magnetically anisotropic cobalt and iron nanofibers via electrospinning' Adv. Mater. 19 4244±4247. Huang Z, Zhang Y, Kotaki M, Ramakrishna S (2003) `A review on polymer nanofibers by electrospinning and their applications in nanocomposites' Comp. Sci. Tech. 63 2223±2253. Ki CS, Gang EH, Um IC, Park YH (2007) `Nanofibrous membrane of wool keratose/silk fibroin blend for heavy metal ion adsorption' J. Memb. Sci. 302 20±26. Kim JS, Reneker DH (1999) `Mechanical properties of composites using ultrafine electrospun fibers' Polym. Comp. 20 124±131.

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Lannutti J, Reneker D, Ma T, Tomasko D, Farson D (2007) `Electrospinning for tissue engineering scaffolds' Mater. Sci. Eng. C27 504±509. Lee KY, Jeong L, Kang YO, Lee SJ, Park WH (2009) `Electrospinning of polysaccharides for regenerative medicine' Adv. Drug Deliv. Reviews 61 1020± 1032. Li D, Xia Y (2004) `Electrospinning of nanofibers: Reinventing the wheel?' Adv. Mater. 16 1151±1170. Lopez-Rubio A, Sanchez E, Sanz Y, LagaroÂn JM (2009) `Encapsulation of living bifidobacteria in ultrathin PVOH electrospun fibers' Biomacromolecules 10 2823± 2829. Ma Z, Ramakrishna S (2008) `Electrospun regenerated cellulose nanofiber affinity membrane functionalized with protein A/G for IgG purification' J. Membr. Sci. 319 23±28. Medeiros ES, Glenn GM, Klamczynski AP, Orts WJ, Mattoso LHC (2009) `Solution blow spinning: A new method to produce micro- and nanofibers from polymer solutions' J. Appl. Polym. Sci. 113 2322±2330. Neamnark A, Sanchavanakit N, Pavasant P, Rujiravanit R, Supaphol P (2008) `In vitro biocompatibility of electrospun hexanoyl chitosan fibrous scaffolds towards human keratinocytes and fibroblasts' Eur. Polymer J. 44 2060±2067. Nie H, He A, Zheng J, Xu S, Li J, Han CC (2008) `Effects of chain conformation and entanglement on the electrospinning of pure alginate' Biomacromolecules 9 1362± 1365. Pandey JK, Reddy KR, Kumar AP, Singh RP (2005) `An overview on the degradability of polymer nanocomposites' Polym. Degrad. Stab. 88 234±250. Ramakrishna S, Fujihara K, Teo W-E, Yong T, Ma Z, Ramakrishna R (2006) `Electrospun nanofibers: Solving global issues' Mater. Today 9 40±50. Sawicka K, Gouma P (2006) `Electrospun composite nanofibers for functional applications' J. Nanopart. Res. 8 769±781. Teo W-E, Ramakrishna S (2009) `Electrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite' Comp. Sci. Tech. 69 1804±1817. Torres-Giner S, LagaroÂn JM (2010) `Zein-based ultrathin fibers containing ceramic nanofillers obtained by electrospinning. I. Morphology and thermal properties' J. Appl. Polym. Sci. 118 778±789. Torres-Giner S, Ocio MJ, LagaroÂn JM (2008a) `Development of active antimicrobial fiber based chitosan polysaccharide nanostructures using electrospinning' Eng. Life Sci. 8 303±314. Torres-Giner S, Gimenez E, LagaroÂn JM (2008b) `Characterization of the morphology and thermal properties of zein prolamine nanostructures obtained by electrospinning' Food Hydroc. 22 601±614. Torres-Giner S, Ocio MJ, LagaroÂn JM (2009a) `Novel antimicrobial ultrathin structures of zein-chitosan blends obtained by electrospinning' Carbohyd. Polym. 77 261± 266. Torres-Giner S, Gimeno-AlcanÄiz JV, Ocio MJ, LagaroÂn JM (2009b) `Comparative performance of electrospun collagen cross-linked by means of different methods' Appl. Mat. Inter. 1 218±223. Torres-Giner S, Martinez-Abad A, Ocio MJ, LagaroÂn JM (2010) `Stabilization of a nutraceutical omega-3 fatty acid by encapsulation in ultrathin electrosprayed zein prolamine' J. Food Sci. doi: 10.1111/j.1750-3841.2010.01678.x. Tsutsumi H, Hara C (2008) `Characterization of new type polymer composites prepared

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by in situ coffining electrospun fibers into polymer matrixes' Technical Proceedings of the 2008 NSTI Nanotechnology Conference and Trade Show, NSTI-Nanotech, Nanotechnology 2 733±736. Xu CY, Inai R, Kotaki M, Ramakrishna S (2004) `Aligned biodegradable nanofibrous structure: A potential scaffold for blood vessel engineering' Biomaterials 25 877± 886. Yarin AL, Zussman E, Wendorff JH, Greiner A (2007) `Material encapsulation and transport in core±shell micro/nanofibers, polymer and carbon nanotubes and micro/ nanochannels' J. Mater. Chem. 17 2585±2599. Zhang L, Menkhaus TJ, Fong H (2008) `Fabrication and bioseparation studies of adsorptive membranes/felts made from electrospun cellulose acetate nanofibers' J. Membr. Sci. 319 176±184.

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Electrospun nanofibers for food packaging applications S . T O R R E S - G I N E R , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: Electrospinning is a novel fabrication technology based on high electric fields that can be used to produce polymer- and biopolymer-based mats composed of nanofibers or other nanostructures. Electrospun mats have been shown to possess unique functionalities originating from the very large surface-to-volume ratios of the nanofibers, the use of functional and/or renewable polymers or the encapsulation of bioactive non-polymer substances. The functional electrospun mats can be used as tools for the development of nanocomposite fabrics from a wide variety of plastics with improved performance for packaging applications. They could serve, for example, as a reinforcement to enhance the physical properties of plastics and bioplastics, as transparent layers to a gas barrier, and even as an emerging technology to design bioactive packaging with antimicrobial protection or delivery of nutraceuticals to foods. Key words: electrospinning, ultrathin fibers, nanocomposites, active and bioactive packaging.

5.1

Electrospinning

Looking genuinely at nature, nanofibers often serve as a basic platform on which either organic or inorganic components are built. For instance, cellulose nanofibers would represent the building blocks in plants while collagen nanofibers would do so in the animal body. The fiber structure exhibits, from a structural point of view, a certain ability to transmit forces along its length, thus reducing the amount of materials required. While strong enough for their designed purpose, nanofibers have the added advantage of giving high porosity to the natural supports, which allows faster diffusion of chemicals to the inner structure. To follow this extraordinary natural design, a process that is able to fabricate fiber nanostructures from a variety of materials and mixtures is an indispensable pre-requisite. Control of the nanofiber arrangement is also necessary to optimize such structural requirements. Electrospinning is a physical process used for the formation of ultrathin fibers by subjecting a polymer solution to high electric fields. A schematic representation of a typical laboratory electrospinning setup is shown in Fig. 5.1. In this configuration, a polymer solution is placed inside a syringe lying

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5.1 Typical electrospinning setup where a non-woven nanofiber mat is collected.

horizontally on a digitally controlled pump which permits adjustment and precise control of the solution flow-rate. The polymer solution is then pumped to a metallic millimeter-size needle which is connected to a high voltage power supply operated in positive DC mode and with low current intensity. At a critical high voltage (5±25 kV), the polymer solution droplet at the tip of the needle distorts and forms a Taylor cone to be ejected as a charged polymer jet. This stretches and is accelerated by the electrical field towards a grounded and oppositely charged collector. As the electrospun jet travels through the electrical field, the solvent completely evaporates while the entanglements of the polymer chains prevent it from breaking up. This results in the deposition of ultrathin polymer fibers on a metallic collector to habitually assemble the fibers as nonwoven mats. As elongation is accomplished via a contactless scheme, electrospun fibers are considerably thinner in diameter and thus higher in surface-to-volume ratio than fibers fabricated using conventional mechanical extrusion or a classical spinning process. For instance, while electrospun fiber diameters are habitually under the micron and also within the nanometric range, synthetic fibers produced via extrusion and fibers of biological origin such as cotton, wool, or silk are characterized by diameters in the range of various micrometers and above. Moreover, since the electrospinning is a continuous process, fibers when wound can be as long as several meters or even kilometers. The formed fibers are not only ultrathin and relatively long but also fully interconnected to form a three-dimensional network. Such small dimensions generally lead to very high ratios of specific surface area to mass and provide the electrospun products with extraordinary functionalities which are not found in similar materials of larger sizes. Fiber formation via electrospinning basically requires the materials to be processed to display specific viscoelastic properties, electrical conductivities in

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a limited range of values, and specific surface energies. Such requirements can only be met by polymers provided that molecular weights are chosen in an appropriate range. In order to carry out the process, the polymer must first be in a liquid form, either as molten polymer or as polymer solution. Melting electrospinning is limited by the fact that fibers are generally above the micron size and electrospinning from polymer solutions is then habitually preferred. In this last case, solvent properties such as boiling point and conductivity play a significant part in the electrospinning process and in the resultant electrospun morphology (Torres-Giner et al., 2008a). In particular, round-like fibers from less than 40 nm to a few microns can be fabricated by electrospinning of more than 50 different types of polymers (Li and Xia, 2004). In addition to observational changes in the fiber diameter, through the modification of the electrospinning setup other fiber and non-fiber assemblies with novel remarkable features can be produced, such as uniform or variable flat and round-like fibers, bead-like or round particles, core±sheath or multilayer coaxial structures, hollow tubes or porous fibers, aligned fibers, crosslinked fibers, and multi-jet fibers. This adjustability certainly enhances the performance of the electrospun materials, allowing application-specific modifications. Electrospun shapes are particularly developed by changing the polymer solution and process conditions: polymer concentration, solvent nature, tip-tocollector distance, voltage and flow-rate. Among all process parameters, the polymer concentration of the solution for electrospinning is the most relevant factor determining the fiber diameter (Torres-Giner et al., 2008b). Thus, higher concentrations generally result in the formation of fibers with larger diameters. Reduction of the polymer concentration below a threshold value would result in beads or in extremely thin fibers with beads along their length. This variation within the electrospinning technology is also called `electrospraying' because the process acts like an atomization procedure to form nearly mono-dispersed and non-fibrillar ultrathin particles (Torres-Giner et al., 2010). Figure 5.2 shows the most common structures that may be fabricated by the electrospinning technology. Fibers composed of blends of different polymers can be prepared by electrospinning from solutions containing different polymer species in a common solvent. The resulting morphology can be either of a matrix-dispersed phase type or co-continuous, depending on the thermodynamic and kinetic properties of the electrospinning solutions. Another interesting characteristic of the electrospinning procedure is the ability to form porous nanostructures, which may imply a tremendous increase of the fiber surface area (see Fig. 5.3). These pores can be produced when, in the electrospinning of a blend consisting of two polymers, one of the polymers is partially removed after the fiber formation by dissolving it in a solvent in which the other polymer is insoluble (Torres-Giner et al., 2008a). It is even possible to vary the pore size and density by further controlling the processing parameters (Ramakrishna et al., 2006). Pore

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5.2 Selected SEM images of electrospun zein networks for: (a) thick tubular fibers obtained from concentrated solutions; (b) thin tubular nanofibers obtained using long tip-to-collector distances; (c) nanobeads obtained from diluted solutions; (d) ribbon-like ultrathin fibers obtained from the acidified solution. Scale markers of 5 m in all cases (Torres-Giner et al., 2008b).

formation can also be induced in the presence of high humidity, where condensation processes lead to the formation of water islands within the fibers, subsequently causing pore formation. Electrospinning onto very cold substrates such as liquid nitrogen can also result in highly nanoporous fibers. On the other hand, similar to this, fiber surfaces may not always be smooth, depending mainly on the solvent volatility used in the electrospinning. For instance, fibers may fuse together if the solvents are not completely evaporated, to yield threedimensional networks that have foam or sponge-like structures (Gomes et al., 2007). Coaxial electrospinning, also habitually called co-electrospinning, can be further applied for the preparation of polymer core±shell fibers and hollow

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5.3 Chitosan porous nanofibers resulting from specific solvent post-treatment. Scale marker of 2.5 m (Torres-Giner et al., 2008a).

fibers. This is a frequent solution for those cases in which the material cannot be electrospun to produce fibers, such as oils and low molecular weight polymers. It consists of the same electrospinning setup as for polymer blends with the exception of the use of two needles which are arranged in a concentric configuration and connected to two different reservoir solutions. If the two solutions are immiscible and contain two different polymers, a core±shell nanofiber is formed, while if the inner solution is free of polymer a hollow fiber is produced from the outer polymer solution. In the latter case, the inner solution usually consists of a non-polymer fluid or an immiscible solvent with the solvent used for the outer solution. It is frequently preferable to apply the shell from the immiscible solvent which can be easily removed by a vapor phase separation rather than from a fluid phase, which may give rise to mechanical forces causing a disruption or a swelling of the fibers. Furthermore, typical electrospinning results in planar random non-wovens, that is, with an isotropic orientation of the fibers, if planar electrodes are used. However, a parallel arrangement of the fibers can be obtained by employing cylindrically shaped electrodes which rotate and collect the fibers during the process (Xu et al., 2004). Tubular arrangements become accessible in this way, with the fibers being either parallel or random depending, among other parameters, on the rotation speed. A further approach consists in the use of the so-called split electrodes which are composed of, for instance, a set of two parallel electrodes or a set of four electrodes arranged in a rectangular pattern.

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The expansion of the technology has also brought alternative methods to produce the nanofibers in a more controlled and efficient manner. For instance, similar nanofibers can be made from solution blow spinning, which does not require high voltage equipment or any electrically conductive collector and, more importantly, avoids the use of aggressive solvents. This setup consists of a source of compressed gas such as nitrogen, argon and air, equipped with a pressure regulator system of concentric nozzles: an inner nozzle through which the polymer solution is pumped, and an outer nozzle through which a high pressure stream of the gas passes (Medeiros et al., 2009). This particular nozzle geometry creates a region of low pressure around the inner nozzle that helps draw the polymer solution into a cone, which then elongates into a fiber. The certain ability to fabricate polymer-based nanofibers with controllable size and porous structure in the form of non-woven mats or three-dimensional porous structures could provide virtually unlimited novel sources for the development of natural polymer-based applications (Torres-Giner et al., 2008a, 2008b). The highest potential role that the electrospinning process can play is possibly the construction and reproduction of multi-level materials. Unlike the bottom-up methods, electrospun fibers are produced through a top-down process, which results in continuous and low-cost fibers that are also relatively easy to align, assemble, and process into applications. Also, as previously mentioned, deposited structures can also be particulates or mixtures of fibers and particles. The use of such ultrathin structures as components in subsequent largescale derived products is attracting considerable interest in the industry of functionalized materials for their utilization as tissue engineering scaffolds, pharmaceutical drug release dosages, highly functional food and ingredients, and active and bioactive packaging materials, and in general for the manufacture of advanced functional materials.

5.2

Functional nanofibers

The electrospinning technique has mainly dealt with synthetic polymers due to their low cost, high availability, and well-defined chemical properties that allow for more uniform behavior during the electrospinning process. However, instead, the technology also opens up enormous possibilities for the implementation of bio-based materials and food hydrocolloids such as proteins and polysaccharides, to make novel biodegradable and renewable structures of interest in various functional applications. In the context of the literature review, there are numerous studies that focus on the use and production of electrospun biopolymer-based ultrathin fibers. The main advantage of biopolymers is that in a natural way they usually compromise bioactive properties without including for this function any other kind of compounds. The reason for this is that biopolymers are macromolecules produced in nature by living organisms and plants, and because of their participation in the natural biocycles, they can

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compromise certain functionalities. Biopolymers are of great interest in various industries, chief amongst them being the food industry, because they may be (a) non-toxic, edible, and digestible, (b) biocompatible and biodegradable, and (c) renewable and sustainable, giving rise to a broader utilization, especially in fields such as biomedical sciences, pharmaceuticals, cosmetics, and other related fields. Further, biopolymer uses could allow for the creation of value-added products if currently under-utilized but abundantly available biomass were to be used as raw material for electrospinning. Naturally occurring polysaccharides and proteins have been known to be biocompatible and safe for many different applications. A wide range of them can be electrospun into ultrathin fiber mats with a specific fiber arrangement, structural integrity, and full biocompatibility. Many studies have been conducted using polysaccharides and proteins for an electrospun fabrication that could be potentially useful for functionalizing materials. Table 5.1 gathers the most widespread researches on electrospun biopolymers with their expected applications. Till now, polysaccharides including, but not limited to, alginates (Nie et al., 2008), cellulose (Zhang et al., 2008; Ma and Ramakrishna, 2008), and chitosan (Torres-Giner et al., 2008a; Neamnark et al., 2008) have been electrospun as good examples of novel functional materials. For instance, alginate nanofibers have excellent biocompatibility, low toxicity, non-immunogenicity, relatively low cost and simple gelation behavior with divalent cations which have been studied for many biomedical applications. On the other hand, cellulose-based electrospun nanofibers have been largely proposed in the pharmaceutical and biomedical fields, including for applications as adsorbent beads, filters and barrier membranes, artificial tissue/skin, antimicrobial membranes and protective clothing. Furthermore, studies on antibacterial activities of Table 5.1 Summary of promising properties related to the electrospinning of functional polymers Polymer nanofibers

Functional properties

Alginates Celluloses

Wound dressings for tissue engineering Ion-exchange medium for protein separations in biomembranes Cell support, antimicrobial performance Enlarges cell proliferation and osteoblastic activity Proliferation of muscle, bone and skin cells Improved frictional surface and cell adhesion Biofillers of mechanically improved packaging Water entrapper, thermally resistant, and biologically valuable Increases cell biocompatibility and antibacterial activity Enhances Cu2+ adsorption capacity in water purification filters

Chitosan Starch Collagen/gelatin Silk fibroin Gluten Zein Collagen and chitosan Wool keratose and silk

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chitosan nanofibers have shown that the salts of this biopolymer inhibit the growth of S. aureus bacteria in the presence of very low fiber amounts (TorresGiner et al., 2008a, 2009a). The polycationic nature of migrated glucosamine fractions of chitosan is considered to interfere with the negatively charged residues of macromolecules at the cell membrane surface, which results in the death of the microorganisms. In this way, composite materials which contain the chitosan fibers can be used in far more applications with respect to their biodegradability and unusual properties that could help control the microorganisms. Other polysaccharides, such as hyaluronic acid, starch, dextran, and heparin have also shown potential in the electrospinning process, with or without polymer additives (Lee et al., 2009). Some proteins are original counterparts of the animal body and their electrospun mats can produce specific biological responses. In the tissue engineering field, electrospun ultrathin fiber interfaces made of animal proteins can easily approximate the nanostructural morphology of natural tissues by assessing appropriate levels and sizes of porosity to allow cell migration, sufficient surface area and a variety of surface chemistries to encourage cell adhesion, growth, migration, and differentiation, and adjust the degradation rate to match tissue regeneration (Lannutti et al., 2007). In this context, collagen can be considered as the most promising natural polymer for tissue restoration processes and, therefore, the electrospinning of collagen has been widely reported (TorresGiner et al., 2009b). Another interesting protein, which shows not only cellular responsive features but also potential uses in functional textiles and clothing design, is silk fibroin. The use of silk for active surfaces as electrospun nanofiber assemblies with other polymers is confirming very encouraging results due to improved frictional properties (Akada et al., 2007). Electrospun ultrathin structures of zein prolamine from corn also have good potential in the food technology area, not only as a reinforcing fiber in, for instance, plastic food packaging applications, but also as an edible carrier for encapsulation of food additives (Torres-Giner et al., 2010) or to modify food properties and in the design of novel active and bioactive packaging technologies (Torres-Giner et al., 2008b). As a most recent and natural approach to increase the bioactivity of the electrospun polymers, the latest research is being focused on blending two biological polymers. Probably the clearest example that shows benefits of this biological combination in the electrospinning is the collagen±chitosan complex, which has been proposed as optimal wound dressings because it combines the enhanced cell biocompatibility of collagen with the antibacterial activity of chitosan (Chen et al., 2008). Apart from the in tissue engineering field, other electrospun bioblends are promising candidates for different functional applications: wool keratose and silk fibroin as metallic particle filters in water treatment (Ki et al., 2007) or zein and chitosan for antibacterial and functional packaging (Torres-Giner et al., 2009a).

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Nanoencapsulation

Electrospun fibers may be given additional functionality by incorporating functional nanoscalar compounds, e.g. drugs, into the electrospinning solution. Further investigations on this approach permit one to functionalize or add extra functionalities to electrospun materials to display specific properties. The process generally consists of the introduction of very diverse non-polymeric agents into the polymer solution to electrospin. This results in hybrid ultrathin fibers in which the incorporated agents remain packed into the polymer matrix as individual nanodroplets of liquid or solid nanoparticles. Such nanostructural distribution can overcome the issue of low stability of encapsulated ingredients as well as improve the distribution of the functional components. This approach also becomes crucial when synthetic polymers, such as poly(lactic acid) (PLA), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), poly(ethylene oxide) (PEO), poly(-caprolactone) (PCL), poly(styrene) (PS), and their copolymers, are employed because this constitutes their unique current way to functionalization. In particular, the electrospun morphology can provide high efficiency in drug-loading and, therefore, has been proposed to provide novel functional carriers in pharmaceutical compositions by nanoencapsulating specific therapeutic compounds (Sawicka and Gouma, 2006). The rate profiles can also be controlled, obtaining desired adjustable releases, such as rapid, immediate, or delayed dissolution, or a modified release profile, for instance the sustained and/ or pulsatile release characteristic. This is specifically modulated by changing the morphology, porosity, and hydrophilic/hydrophobic composition of the electrospun fiber membrane. For instance, the application of electrospraying to substance-loading polymer solutions results in the formation of nanocapsules of high substance encapsulation efficiency and performance because of the ultrathin particle sizes which can reduce the losses of the entrapped substances and enhance the bioavailability (Torres-Giner et al., 2010). Several examples can be found in the literature regarding encapsulation of substances using electrospinning with practical application in the packaging field. Table 5.2 includes different electrospun encapsulation works that report uses in the design of functional nanostructures of additives, coatings or interlayers. Improving the surface functionality of electrospun fibers with bioactive molecules could be very important for specific bioactive applications. For example, clays and other minerals have been proposed to be introduced in electrospun nanofibers of zein, such as organo-modified and unmodified mica, kaolinite, montmorillonite (MMT) and zeolite (Torres-Giner and LagaroÂn, 2010). These zein±clay nanofibers are proposed to reinforce the mechanical, thermal, barrier and control release properties of both plastic and bioplastic matrices without implying losses in biodegradability and optical properties. For active food packaging, incorporation of enzymes and bioactive molecules into a matrix is usually challenging, as they are generally sensitive to heat and may

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Table 5.2 Summary of promising properties related to nanoencapsulation using electrospinning Polymer nanofibers

Functional properties

Zein±clays

Improved mechanical and thermal properties, controlled release Enhancement and control of water resistance

PVA±polyhedral oligosilsesquioxane PLA±lysozyme PVA±lipase PLA±paracetamol Chitosan±ibuprofen PVA±acetylsalicylic acid PCL±resveratrol and gentamicin sulfate PLA±tetracycline Cellulose±vitamin A, E PLA±biotin Cellulose±erythromycin Zein± -carotene PVA±Ag PVP±Iron particles PEO±DNA PVA±bacteria

Antibacterial, controlled release Fast transesterification activity Analgesic sustained release Sustained release of an anti-inflammatory Controlled release of analgesic Sustained release of antioxidant and antibiotic substances Controlled release of antibiotic Controlled release of vitamins as transdermal and dermal therapeutic agents Release of vitamin H Artificial gastric juice of acid protection Antioxidant activity Films of broad-spectrum antibacterial and antifungal functionability Offers enhanced protection to prevent from oxidation Fluorescently labeled proteins Protection and release of living organisms

lose their activity when in contact with certain chemicals. Electrospinning can be carried out in room temperature conditions and this allows such molecules to be satisfactorily incorporated. Bioactive molecules can include enzymes such as lysozyme and lipase, pharmaceuticals such as paracetamol, ibuprofen, and acetylsalicylic acid, or antibiotics such as gentamicin sulfate or tetracycline. Nutraceuticals can also be nanoencapsulated as functional food, which promotes health beyond providing basic nutrition and can be included in bioactive food packaging applications. For example, electrospinning technology has been presented as novel route to stabilization of vitamins or antioxidants such as carotene (Fernandez et al., 2009) and omega-3 fatty acids (Torres-Giner et al., 2010). They can bioact on the containing food from the incorporated package or be released from it when it is open. Figure 5.4 shows zein/ -carotene ultrathin fibers which have been proved to present a remarkably good protection against oxidation when exposed to UV±vis irradiation. The technology is therefore reported here as capable of producing added-value biopolymer nanofibers that can have good potential in food and nutraceutical formulations and coatings, bioactive food packaging and food processing industries. In other more complex cases such as with some metals, semiconductors or inorganic compounds, it can be difficult to be able to fulfill the physical

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5.4 Fluorescence image of electrospun zein/ -carotene nanofibers (Fernandez et al., 2009).

electrospinning requirements. The answer to this problem is to add their low molar mass precursor materials such as metal salts, for instance, in high concentrations to the polymer solution. This, in turn, is electrospun to yield fibers in which the precursor molecules are dispersed within the polymer carrier. The precursor species are subsequently subjected to further chemical modifications such as, for instance, a reduction process. A clear example for this case is the inclusion of silver salts in natural and synthetic polymer solutions. Silvercontaining electrospun fibers are then thermally activated and the resulting materials are highly antimicrobial because they can function either as release systems for Ag+ ions or as contact-active materials (Dong et al., 2009). Other cases include the dispersion of ferrofluids into the fibers to produce materials which display ferromagnetism and supermagnetism functionalities. Fe3O4, for example, is a well-known ferromagnetic material which, however, becomes superparamagnetic as the size of the particles is reduced down to the nanometer scale (Graeser et al., 2007). The coaxial electrospinning method has a number of additional advantages that make it more attractive for the production of functional nanofibers. From the point of view of the nanoencapsulated objects, first of all this provides a more natural environment, and secondly the electric charges are located practically only at the outer surface, so that the objects dispersed in the inner parts are not charged at all. It may also reduce the contact between the nanoencapsulated components and the organic solvents, which is expected to improve the bioactivity of the resultant nanofibers. They would only be affected mechanically by the predominantly viscous stresses in the droplet during electrospinning. Apart from requiring milder conditions, the coaxial method also

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has the advantage of exerting greater control over the release behavior, which increases the loading efficiency. This method effectively protected fragile biological agents, enabling the incorporation of proteins, enzymes, and DNA (Yarin et al., 2007). This can be applied even to obtaining `living' membranes of non-wovens made from fibers containing biological objects, for instance viruses, bacteria and cells, in such a way that these objects keep their specific functions to adapt to their new environment (Lopez-Rubio et al., 2009).

5.4

Electrospinning in packaging applications

The ease and versatility of the electrospinning technique has offered new opportunities for researchers to investigate the effectiveness of nanofibers as a reinforcement to enhance the properties of a matrix material (Huang et al., 2003). Composites are combinations of two distinct material phases, the bulk phase or matrix, and the reinforcing phase. From a mechanical point of view, it is the combination of the strength of the reinforcement and the toughness of the matrix that gives composites their superior properties that are not present in single conventional materials. When fiber reinforcements close to 100 nm in cross-section are used, the final materials are called nanocomposites, and they generally maintain their original optical properties. Also as anticipated, nanofibers not only can have better mechanical properties than microfibers of the same materials but also may possess some additional merits which cannot be shared by traditional composites. Additionally, their application to renewable biopolymers represents an emerging new class of nanocomposites, also referred to as nanobiocomposites, with reduced environmental impact and high functionality, which are the wave of the future and are considered as the materials of the next generation (Pandey et al., 2005). In the fiber reinforcement context, depending on the type of application, the arrangement of nanofibers can basically take two different forms: aligned or randomly distributed. For load-bearing applications, the most effective arrangement is to have the nanofibers aligned in the stress direction to form a laminated nanocomposite (see Fig. 5.5a). Such a material is strong in the direction coinciding with the aligned nanofibers but weak in the perpendicular direction. Randomly distributed nanofibers, in the form of non-woven nanocomposites (Fig. 5.5b), can also be effective in other areas, for instance as gas barrier layers. The introduction into the matrix of nanometer-sized spaces produced by the nanofiber mesh can lower the gas permeability because of a tortuosity effect in the gas diffusion. Recently, electrospinning has been presented as a potential technology for use as a platform for multifunctional, hierarchically organized nanocomposites (Teo and Ramakrishna, 2009). Electrospun nanofibers can be incorporated as reinforcements to plastic matrices by different methods, such as melt mixing or solvent casting, to generate novel nanocomposites consisting of nanofiber layers intercalated into a

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5.5 Basic nanocomposite reinforcements based on electrospun fibers: (a) laminated nanocomposites; (b) non-woven nanocomposites.

plastic matrix. In other cases, with a greater emphasis on functional capability, bioactive nanofibers can be exposed on the matrix surface to the environment to modify external stimuli or to the food as drug vehicle. Figure 5.6 presents these different fiber dispositions in the electrospun nanocomposites according to the desired functionality for food packaging applications. Nevertheless, to date, little or no research has been done on this topic and only a few researchers have tried to make nanocomposites reinforced with electrospun polymer nanofibers. Some works have proved that the mechanical performance of certain epoxy composites can be expanded when impregnation with electrospun nanofibers is realized (Kim and Reneker, 1999; Fong, 2004). Nanofibers can thus penetrate through the resin matrix to form a stronger and stiffer network, in which their very high surface-to-volume ratio may improve the resultant material toughness. Such results in long-term materials have triggered research on the exploitation of nanofibers for biodegradable plastics. As an example, it is reported that electrospun PVA fibers can be coffined into a PS matrix to produce a reinforced film with enhanced mechanical properties (Tsutsumi and Hara, 2008). In other work (Chen and Liu, 2008), electrospun cellulose mats composed of fibers 200±

5.6 Electrospun nanocomposites in the design of novel food packaging materials.

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800 nm long have been incorporated in soybean protein isolate to improve the mechanical stress at yield and Young's modulus by 13 and 6 times, respectively. More interestingly, this composite exhibited high visible light transmittance of ca. 75%. Regarding this optical characteristic, a nanocomposite using electrospun Nylon-4,6 nanofibers as reinforcement and phenolic epoxy resin as matrix was shown to be also transparent (Bergshoef and Vancso, 1999). This is because light was able to transmit over the entire range of wavelength of the visible spectrum (400±700 nm). Since light neither reflects nor refracts at air± nanomaterial interfaces if the material size is less than one-tenth of the wavelength of visible light, the reinforcing nanofibers serve greatly in the manufacture of transparent reinforced materials. Using nanofibers as composite reinforcements has shown a few interesting optical and mechanical results, but they have not been so well studied in regard to the simultaneous reinforcement of the barrier properties of plastics and bioplastics in packaging and membrane applications. The gas and vapor permeability of biopolymers can be enhanced dramatically by incorporating nanofiber layers of fairly low thickness as interlayers or coatings. For instance, the incorporation of a thin interlayer of electrospun zein ultrathin fibers into PLA-based films has shown to reduce up to 71% the oxygen permeability in comparison to the same unreinforced matrix (Busolo et al., 2009). As can be seen by optical microscopy, in the interior of the PLA film of Fig. 5.7 (top image) the thin reinforcement results in completely transparent and colorless sheets similar to the original one (bottom images).

5.5

Future trends

To conclude, electrospinning allows extensive tunability in material properties and functions through specific selection of the solution composition. Although electrospun materials are predominantly polymer-based, ceramic, metallic and other bioactive particles can also be introduced into the fibers and subsequently be part of the final nanocomposites. At first, non-polymer particles or a second polymer can be mixed into the primary polymer solution and electrospun to form hybrid ultrathin fibers. As an example, results of the nanodispersion of commercial minerals into electrospun ultrathin zein fibers have shown a considerable increase in thermal resistance at mineral contents below 10 wt% (Torres-Giner and LagaroÂn, 2010). Further modifications of the electrospinning technique can be performed to increase the number of functional materials and to broaden the range of potential applications. This can be represented in the modification of the morphology or surface of nanofibers, the use of coaxial electrospinning technology to produce a second layer of polymer material, and the orientation and organization of the nanofibers by modification of the collector to optimize its performance. The development of new electrospinning configurations such as solution blow spinning can provide novel nanofibers with greater potential for

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5.7 Top: PLA±zein nanocomposite micrographs in top view by optical microscopy (scale marker is 100 m); bottom: images of the original film (left) and resultant composite (right) with electrospun nanofibers (Busolo et al., 2009).

commercial scale-up. Such additional adaptations will allow creating advanced multi-functional nanocomposites, in which various functions are incorporated for plastics in multi-sectorial applications. In this sense, future hybrid nanostructures will be applied as functional reinforcing fillers in uses such as coatings, packaging, and other applications. In the future it will be important to focus research on gaining a better fundamental understanding of the electrospinning process, but even more importantly on how this technique can be used as a tool in developing new materials. The studies described above indicate that the production of nanocomposites from electrospun fibers is feasible. However, some more essential

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studies are still required and many challenges remain to be faced. In particular, the ability to integrate the nanofibers into packaging materials in an efficient and reproducible manner remains a major challenge. Although many works have reported on the control, orientation, size, and other target characteristics, further advances concerning the reproducibility of locating the nanofibers in specific positions and orientations will be necessary. The encapsulation and posterior release of bioactives such as nutraceuticals or antimicrobials will also require further studies to prove the expected bioactive properties in the resultant material. Regarding fiber productivity, scale-up and commercial production are other general challenges which need to be addressed. The design and construction of process equipment for controllable and reproducible electrospinning will act as a stimulus to provide novel products based on electrospinning technology.

5.6

References

Akada M, Kotaki M, Sato M, Sukigara S (2007) `Surface frictional properties of silk/ nylon blended nanofiber assemblies' J. Textile Eng. 53 245±248. Bergshoef M, Vancso G (1999) `Transparent nanocomposites with ultrathin, electrospun nylon-4,6 fiber reinforcement' Adv. Mater. 11 1362±1365. Busolo MA, Torres-Giner S, LagaroÂn JM (2009) `Enhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers' Annual Technical Conference ± ANTEC, Conference Proceedings 5 2763±2767. Chen G, Liu H (2008) `Electrospun cellulose nanofiber reinforced soybean protein isolate composite film' J. Appl. Polym. Sci. 110 641±646. Chen Z, Mo X, He C, Wang H (2008) `Intermolecular interactions in electrospun collagen±chitosan complex nanofibers' Carbohyd. Polym. 72 410±418. Dong G, Xiao X, Liu X, Qian B, Liao Y, Wang C, Chen D, Qiu J (2009) `Functional Ag porous films prepared by electrospinning' Appl. Surf. Sci. 255 7623±7626. Fernandez A, Torres-Giner S, LagaroÂn JM (2009) `Novel route to stabilization of bioactive antioxidants by encapsulation in electrospun fibers of zein prolamine' Food Hydroc. 23 1427±1432. Fong H (2004) `Electrospun nylon 6 nanofiber reinforced BIS-GMA/TEGDMA dental restorative composite resins' Polymer 45 2427±2432. Gomes DS, da Silva ANR, Morimoto NI, Mendes LTF, Furlan R, Ramos I (2007) `Characterization of an electrospinning process using different PAN/DMF concentrations' PolõÂmeros: CieÃncia e Tecnologia 17 206±211. Graeser M, Bognitzki M, Massa W, Pietzonka C, Greiner A, Wendorff JH (2007) `Magnetically anisotropic cobalt and iron nanofibers via electrospinning' Adv. Mater. 19 4244±4247. Huang Z, Zhang Y, Kotaki M, Ramakrishna S (2003) `A review on polymer nanofibers by electrospinning and their applications in nanocomposites' Comp. Sci. Tech. 63 2223±2253. Ki CS, Gang EH, Um IC, Park YH (2007) `Nanofibrous membrane of wool keratose/silk fibroin blend for heavy metal ion adsorption' J. Memb. Sci. 302 20±26. Kim JS, Reneker DH (1999) `Mechanical properties of composites using ultrafine electrospun fibers' Polym. Comp. 20 124±131.

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Lannutti J, Reneker D, Ma T, Tomasko D, Farson D (2007) `Electrospinning for tissue engineering scaffolds' Mater. Sci. Eng. C27 504±509. Lee KY, Jeong L, Kang YO, Lee SJ, Park WH (2009) `Electrospinning of polysaccharides for regenerative medicine' Adv. Drug Deliv. Reviews 61 1020± 1032. Li D, Xia Y (2004) `Electrospinning of nanofibers: Reinventing the wheel?' Adv. Mater. 16 1151±1170. Lopez-Rubio A, Sanchez E, Sanz Y, LagaroÂn JM (2009) `Encapsulation of living bifidobacteria in ultrathin PVOH electrospun fibers' Biomacromolecules 10 2823± 2829. Ma Z, Ramakrishna S (2008) `Electrospun regenerated cellulose nanofiber affinity membrane functionalized with protein A/G for IgG purification' J. Membr. Sci. 319 23±28. Medeiros ES, Glenn GM, Klamczynski AP, Orts WJ, Mattoso LHC (2009) `Solution blow spinning: A new method to produce micro- and nanofibers from polymer solutions' J. Appl. Polym. Sci. 113 2322±2330. Neamnark A, Sanchavanakit N, Pavasant P, Rujiravanit R, Supaphol P (2008) `In vitro biocompatibility of electrospun hexanoyl chitosan fibrous scaffolds towards human keratinocytes and fibroblasts' Eur. Polymer J. 44 2060±2067. Nie H, He A, Zheng J, Xu S, Li J, Han CC (2008) `Effects of chain conformation and entanglement on the electrospinning of pure alginate' Biomacromolecules 9 1362± 1365. Pandey JK, Reddy KR, Kumar AP, Singh RP (2005) `An overview on the degradability of polymer nanocomposites' Polym. Degrad. Stab. 88 234±250. Ramakrishna S, Fujihara K, Teo W-E, Yong T, Ma Z, Ramakrishna R (2006) `Electrospun nanofibers: Solving global issues' Mater. Today 9 40±50. Sawicka K, Gouma P (2006) `Electrospun composite nanofibers for functional applications' J. Nanopart. Res. 8 769±781. Teo W-E, Ramakrishna S (2009) `Electrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite' Comp. Sci. Tech. 69 1804±1817. Torres-Giner S, LagaroÂn JM (2010) `Zein-based ultrathin fibers containing ceramic nanofillers obtained by electrospinning. I. Morphology and thermal properties' J. Appl. Polym. Sci. 118 778±789. Torres-Giner S, Ocio MJ, LagaroÂn JM (2008a) `Development of active antimicrobial fiber based chitosan polysaccharide nanostructures using electrospinning' Eng. Life Sci. 8 303±314. Torres-Giner S, Gimenez E, LagaroÂn JM (2008b) `Characterization of the morphology and thermal properties of zein prolamine nanostructures obtained by electrospinning' Food Hydroc. 22 601±614. Torres-Giner S, Ocio MJ, LagaroÂn JM (2009a) `Novel antimicrobial ultrathin structures of zein-chitosan blends obtained by electrospinning' Carbohyd. Polym. 77 261± 266. Torres-Giner S, Gimeno-AlcanÄiz JV, Ocio MJ, LagaroÂn JM (2009b) `Comparative performance of electrospun collagen cross-linked by means of different methods' Appl. Mat. Inter. 1 218±223. Torres-Giner S, Martinez-Abad A, Ocio MJ, LagaroÂn JM (2010) `Stabilization of a nutraceutical omega-3 fatty acid by encapsulation in ultrathin electrosprayed zein prolamine' J. Food Sci. doi: 10.1111/j.1750-3841.2010.01678.x. Tsutsumi H, Hara C (2008) `Characterization of new type polymer composites prepared

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by in situ coffining electrospun fibers into polymer matrixes' Technical Proceedings of the 2008 NSTI Nanotechnology Conference and Trade Show, NSTI-Nanotech, Nanotechnology 2 733±736. Xu CY, Inai R, Kotaki M, Ramakrishna S (2004) `Aligned biodegradable nanofibrous structure: A potential scaffold for blood vessel engineering' Biomaterials 25 877± 886. Yarin AL, Zussman E, Wendorff JH, Greiner A (2007) `Material encapsulation and transport in core±shell micro/nanofibers, polymer and carbon nanotubes and micro/ nanochannels' J. Mater. Chem. 17 2585±2599. Zhang L, Menkhaus TJ, Fong H (2008) `Fabrication and bioseparation studies of adsorptive membranes/felts made from electrospun cellulose acetate nanofibers' J. Membr. Sci. 319 176±184.

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Mass transport and high barrier properties of food packaging polymers F . N I L S S O N and M . S . H E D E N Q V I S T , Royal Institute of Technology, Sweden

Abstract: Polymers are to various extents permeable towards gases and liquids. For proper material selection it is therefore important to be able to predict and assess their permeation properties in actual/realistic environments. This chapter deals with the basics of the transport properties of polymers and their barrier properties, with the fundamental equations of mass transport. The second and third parts describe the physics behind the two parameters governing the transport: solute diffusivity and solubility. Since the focus of this chapter is on the prediction of solubility, the diffusivity description is very brief. The fourth part shows ways of obtaining high barrier properties of polymers by limiting the diffusivity and/or the solubility. Finally, the fifth part exemplifies ways of measuring the mass transport properties of polymers. Key words: transport properties, diffusivity, solubility, barrier, prediction.

6.1

Introduction: the basics of mass transport

The amount of solute transferred through unit cross-section per unit time is called the flux (F). According to Fick's first law, the flux depends only on the diffusion rate (D) and the concentration gradient (c=x) (Crank, 1986). Consider the plate in Fig. 6.1 which is subjected to an unlimited amount of nitrogen gas on the left side. At steady state, it is possible to calculate the flux as:   @c c2 ÿ c1 6:1 F ˆ ÿD ˆ ÿD x2 ÿ x1 @x where c1 and c2 are the solute concentrations in the plate at the two boundaries. Henry's law (c ˆ Sp) gives a relationship between the solute vapour pressure (p) and the solute equilibrium concentration (c) through the solubility coefficient (S). The law is, at least at low pressures, valid for most non-glassy gas/polymer combinations. Assuming that Henrys law holds, eq. 6.1 can be rewritten as:   p 1 ÿ p2 6:2 F ˆ DS x2 ÿ x1 This equation can be further simplified by introducing the permeability coefficient (P), defined as:

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6.1 A plate subjected to a steady-state gas transport from left to right.

P ˆ DS

6:3

In essence this means that the solute permeation rate depends on two factors, the diffusivity (D) and the solubility (S). By controlling these factors it is possible to steer towards high barrier properties or specific membrane characteristics. A useful expression is Fick's second law, which describes the solute concentration (uptake or loss) with time:   @c @ @c ˆ D 6:4 @t @x @x This equation is easily obtained for a plate geometry by a one-dimensional mass balance and the use of eq. 6.1.

6.2

Diffusivity

The diffusivity depends on the size and shape of the solute and on the mobility and structure of the polymer chain network. The diffusivity will be reduced in the presence of polymer crystals and will increase with solute concentration if the solute plasticises the polymer. Thornton et al. (2009) were recently able to model a large set of permeability and diffusivity data over a broad range of free volumes. Data included both conventional polymers and those with extra large free volume where diffusion also occurred in percolated channels. The diffusivity could be expressed as an exponential function depending on the fractional free volume ( f ) and two empirical constants ( and ): D ˆ  exp … f † A corresponding expression for the solubility was also derived.

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6.2 as a function of the square of the kinetic diameter obtained from permeability ( ) and diffusivity (l) data. Lines are best fits using a linear relationship. Drawn after Thornton et al. (2009).

The fractional free volume ( f ) for a specific polymer can be obtained directly by Bondi's group contribution method (Bondi, 1964) without considering that the accessible free volume may be different for different gases. An alternative approach for obtaining f (Park and Paul, 1997; Greenfield and Theodorou, 1993) is to consider that the accessible fractional free volume is different for different gases. For a polymer with a total specific volume v, the fractional free volume f of gas n is dependent on the specific free gas volume (v0 ):   fn ˆ v ÿ …v0 †n =v 6:6 For a polymer with K repeating units, where each segment has a van der Waals volume vw and an empirically determined gas±polymer interaction parameter

nk , the variable v0 can be calculated as a summation over all repeating units: …v 0 †n ˆ

K X kˆ1

nk …nw †n

6:7

The constants  and in eq. 6.5 can both be experimentally determined. For example, is an approximately linear function of the square of the kinetic gas diameter, which is obtained from diffusivity or permeability experiments: see Fig. 6.2.

6.3

Solubility

The solute (gas/vapour) solubility depends on a number of factors, including the size and shape of the solute and polymer molecules and their polarity and

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hydrogen-bonding capacities. In addition, molecular crosslinking, orientation and crystallisation are important factors. Several models, both semi-empirical and theoretical, have been developed to predict gas, vapour and even liquid solubilities in polymers. Some examples of models based on statistical thermodynamics will be given here. In these methods, relationships between pressure, temperature and volume of the pure components are usually developed first and a rule of mixing is then used to determine the properties of the mixture. Note that this chapter should not be considered as a complete survey of existing models, but rather as a presentation of interesting examples for systems above and below the glass transition temperature Tg. For a comprehensive review on the topic (equation-of-state models before 2000) please consult Wei and Sadus (2000). Examples of models that, for sake of space, we have not considered include the UNIFAC and/or free-volume-based models (Rolker et al., 2007; Radfarnia et al., 2005; Wibawa and Widyastuti, 2009; Wang 2007; Serna et al., 2008).

6.3.1

The Sanchez±Lacombe equation-of-state model (SL-EOS)

Sanchez and Lacombe presented in an early pioneering work a lattice-fluid equation of state model for polymers (Sanchez and Lacombe, 1976; Lacombe and Sanchez, 1976; Challa and Visco, 2005). The starting point in the development of the theory was the relationship between the free energy (G) and the configurational partition function (Z) in the pressure ensemble: G ˆ ÿkT ln Z…T; p†

6:8

where k, p and T are respectively the Boltzmann's constant, temperature and pressure. Z is, in turn, a function of the number of configurations …E; V ; N † available in a system of N molecules having configurational/potential energy E and volume V: XX Z…T; p† ˆ

…E; V ; N † exp …ÿ 1 …E ‡ pV †† 6:9 V

E

In the ensemble studied, T and p were constant and 1  1=kT. The main problem here was to determine , and the approach was to use the Guggenheim solution with a mean field approximation. Consider first a binary mixture of N0 empty sites and N linear r-mers (molecule-chains) giving the total number of sites as Nr ˆ N0 ‡ rN . The interior r-mer is surrounded by z ÿ 2 nearest nonbonded and two bonded neighbours, where z is the coordination number. Consider an orthogonal lattice with z ˆ 6 (Fig. 6.3). One of the middle mers is surrounded by two bonded neighbours and four non-bonded neighbours/ vacancies. The corresponding numbers for the end-mers are 1 and z ÿ 1 ˆ 5. In the general case, an r-mer is surrounded by qz nearest neighbours:

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6.3 The fluid lattice.

qz ˆ …r ÿ 2†…z ÿ 2† ‡ 2…z ÿ 1† ˆ r…z ÿ 2† ‡ 2

6:10

The total number of nearest neighbour pairs is …z=2†Nr and the number of nonbonded pairs is …z=2†Nq , where Nq ˆ N0 ‡ qN . In the derivation, a symmetry number  and a flexibility parameter  were introduced. The first parameter was equal to 2 if the two chain ends were indistinguishable and was equal to 1 if they were different. The variable  described the internal degrees of freedom of the rmer. Its maximum value is  ˆ z…z ÿ 1†rÿ2 for a flexible linear chain. Hence, for a 2-mer component, d ˆ z holds. In obtaining a useful solution for , Sanchez and Lacombe made use of, e.g., Guggenheim's findings/derivations which at large z yielded:   N0   N 1 w 6:11 lim ˆ z!1 f0 f where w ˆ  =erÿ1 and the fractions of empty and occupied sites are f0 ˆ N0 =Nr and f ˆ rN =Nr . In the following, it is assumed that d and the closepacked volume (rv ) are independent of pressure and temperature. The closepacked volume of a mer is the same as that of an (empty) lattice site (Fig. 6.3) and can be obtained from the close-packed mass density (r ) through knowledge of the molar mass M and the close-packed mass density  : rv ˆ M= , where the energy of the system depends only on the nearest neighbour interactions. It can be written: XX E ˆ ÿ…z=2†Nr p…i; j†ij 6:12 i

j

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where p…i; j† is the joint-pair probability between components i and j and ij is the corresponding interaction energy. As stated before, the only non-zero contributions to the energy are the mer-to-mer interactions. Assuming random mixing of r-mers and holes, the probability term becomes, for large z, p…mer, mer† ˆ …rN =Nr †2  f 2 which yields an energy equal to: E ˆ ÿNr …z=2† f

2

6:13

or using the fact that the close-packed volume of an r-mer system is V  ˆ N …rv † and that the total volume is V ˆ …N0 ‡ rN †v ˆ Nr v ˆ V  =f

6:14

the equation: E=rN ˆ ÿ…z=2†…V  =V † ˆ ÿ …V  =V † ˆ  f

6:15

The variable  can be considered as the energy to make a hole and r is the molecular energy in the absence of holes. Both energy and volume are solely functions of the number of holes, and this leads to a simplified form of the configurational partition function: Z…p; T† ˆ

1 X

exp … 1 …E ‡ pV ††

6:16

N0 ˆ0

This can be solved by approximating the above sum by its maximum term. This is the same as inserting the generic term of the partition function in eq. 6.7 and finding the minimum of the Gibbs free energy: G ˆ E ‡ pV ÿ kT ln

6:17

Using eqs 6.11, 6.14 and 6.15, eq. 6.17 can, be expressed in dimensionless variables:    ~ 1 1 p ~ ˆ ÿ~  ‡ ‡ T~ G=…Nr † ˆ G ÿ 1 ln…1 ÿ ~† ‡ ln…~ =w† 6:18 ~ r ~ where ~p ˆ p=p , ~ ˆ = and T~ ˆ T=T  are, respectively, the reduced pressure, mass density and temperature. Further, p ˆ  =v , T  ˆ  =k and ~ ˆ 1=~v ˆ V  =V . In other words, p and  are the `hypothetical' pure component cohesive energy density and mass density. T  is proportional to the depth of the potential energy well. The occupied fraction can be written f ˆ =  ~  1=~v. By obtaining the minimum of the Gibbs free energy with respect to the volume ~v, the following expression is finally obtained:     1 2 ~ p ‡ T ln…1 ÿ ~† ‡ 1 ÿ ~ ˆ 0 ~ ‡ ~ 6:19 r The characteristic fluid length is r ˆ p M=…RT   †, where M is the pure component molar mass. For n ˆ 1 mol solute, the Sanchez-Lacombe equation

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can be written in the standard form Z ˆ F…p; V ; T† by expanding eq. 6.19 in the virial form:   pV 1 1 r r Zˆ 6:20 ˆ1‡r ÿ ~ ‡ ~2 ‡ ~3 ‡ . . . RT 2 T~ 3 4 Challa and Visco (2005) used the SL-EOS model to predict the solubilities of blowing agent in polyols. First, the three parameters p , T  and  of the pure systems were determined through a minimisation of an objective function including experimental p, T and  data. The binary system (i, j) with blowing agent and polyol were subsequently predicted using the following combining rules, which differed slightly from the mixing formulae used by Sanchez-Lacombe: XX p ˆ i j pij 6:21 i

where

j

0:5 ÿ
 pij ˆ 1 ÿ fij Pi Pj

6:22

and fij is the SL-EOS binary interaction parameter. Further, T  ˆ p

X 0 T 0 i

i

and

pi

i

6:23

  1 X 0i ˆ r ri0 i

6:24

with 0i ˆ

i pi =Ti j pj =Tj

and

!i   i ˆ X i  !j  j

6:25

j

The zero subscript refers to the pure state and i is the segment fraction of component i. For a binary solute±polymer mixture, j is the volume fraction of solute. !i is the mass fraction of component i. The solubility was defined as the mole-fraction (x1 ) of the blowing agent divided by the pressure when the pressure approaches zero, and the mole-fraction x1 is related to the volume fraction 1 through 1 ˆ r1 x1 =r:   x1 6:26 S ˆ lim p!0 p The solubilities estimated with the SL-EOS model and with a `variable-range statistical associating fluid theory' (VR-SAFT) by Challa and Visco (2005) are compared with experimental data in Table 6.1. In the analysis, it was assumed

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Multifunctional and nanoreinforced polymers for food packaging Table 6.1 Henry's law solubility constants for blowing agents in PluracolÕ 975* Blowing agent HFC 32 HFC 134a HFC 143a HFC 125 HFC 152a

SSL (bar/mol)

SVR-SAFT (bar/mol)

Sexp (bar/mol)

0.1267 0.2364 0.1558 0.1170 0.3519

0.0605 2.1853 2.0899 2.1263 2.1340

0.1220 0.1761 0.0649 0.0976 0.1647

*Data obtained from Challa and Visco (2005).

that the vapour temperature and pressure were the same as the liquid temperature and pressure, that the chemical potential of the blowing agent was the same in the liquid and vapour phases, and that the vapour pressure of the polyol was zero. It was noticed that the SL-EOS model gave good results compared to experimental data, and clearly better than the VR-SAFT model (which will be described later in this chapter).

6.3.2

Statistical associated fluid theory (SAFT) models

An important class of statistical thermodynamic methods are the statistical associated fluid (SAFT) models. An early development of the original SAFT model was the `variable range statistical associating fluid theory' (VR-SAFT), which was mentioned briefly in Section 6.3.1 (Gil-Villegas et al., 1997; Galindo et al., 1998; McCabe et al., 2001). Here it is considered that the Helmholtz free energy (A) consists of the ideal-gas free energy (AIDEAL), the monomer free energy (AMONO), the free energy due to the formation of chains of monomers (ACHAIN) and the free energy due to the formation of association complexes (AASSOC): A ˆ AIDEAL ‡ AMONO ‡ ACHAIN ‡ AASSOC

6:27

When a square well is used to describe the interactions between the segments, the compressibility becomes:   pV @A=…kB T† A ˆ 6:28 Zˆ ÿ NkB T @N Nk BT T;V where kB is the Boltzmann constant. A more complete description of the model can be found in Gil-Villegas et al. (1997), Galindo et al. (1998) and McCabe et al. (2001). The most well-known of the successfully improved SAFT-type models is the perturbed chain SAFT model (PC-SAFT), presented by Gross and Sadowski (2001). This model involves a perturbation theory with a hard chain reference fluid rather than the spherical molecules used in earlier SAFT work. The compressibility was considered to consist of a hard chain and a dispersion component:

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Mass transport and high barrier properties Z ˆ 1 ‡ Z hc ‡ Z disp

137 6:29

The hard chain part can be expressed as Z

hc

! 3 31 2 323 ÿ 3 23  ˆm ‡ ‡ ÿ 1 ÿ 3 0 …1 ÿ 3 †2 0 …1 ÿ 3 †3 X i

xi …mi ÿ 1†…giihs †ÿ1 

@giihs @

6:30

where giihs is the radial distribution function of the hard sphere fluid, xi is the mole fraction and mi is the number of segments per chain of component i,  is P the total number density of molecules, and n ˆ =6 i xi mi din , where di is the segment diameter of component i. The dispersion term is   @…I1 † @…I2 † disp  C1 Z ˆ ÿ2 6:31 C3 ÿ m ‡ C2 I2 C4 @ @ where  is the packing fraction and I1 and I2 are integrals of density, segment number and temperature. For a complete description of how to calculate these terms, and the coefficients Ci , please refer to Gross and Sadowski (2001). Not surprisingly, since the chain feature was implemented in the model, the fit to PVT data of non-spherical molecules was superior to the original SAFT, as illustrated for toluene in Fig. 6.4. Interestingly, the prediction of the pressuredependent solubility of n-pentane in polyethylene was also good. The fitted binary interaction parameter was small (ÿ0:0195) using polyethylene parameters from the extrapolation of n-alkane parameters to high molar mass. It

6.4 Experimental saturated liquid and vapour densities for toluene ( ) and the corresponding fits using PC-SAFT (solid line) and SAFT (broken line). Drawn after Gross and Sadowski (2001).

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6.5 n-Pentane-polyethylene (l, left y-axis) and CO2-polyamide 11 ( , right y-axis) experimental data and corresponding fits using the simplified SAFT model (lines). The binary interaction parameters used were 0 for the polyethylene case and ÿ0.05 for the PA11 case. Drawn after von Solms et al. (2005).

should be noted that the assessment of the quality of the model was based on the fit and predictions of non-associating or only weakly polar substances. In improved versions of the PC-SAFT model, the same ideal and dispersion terms were used, but simplified terms for the hard-chain contribution were used (simplified PC-SAFT) and a term for associating energies was added (associating complexes) (von Solms et al., 2005). Figure 6.5 shows high-pressure data predicted with the simplified PC-SAFT method together with experimental data for concentration versus pressure for two very different systems: n-pentane±polyethylene and CO2±polyamide 11. In the polyethylene case, a finite binary interaction parameter was in fact unnecessary; the temperature dependence of the gas solubility was still correctly predicted. However, in order to obtain a good fit in the PA11 case, a small but finite kij was necessary. In fact, a temperature-dependent kij had to be included in some cases. In these cases (CH4/HDPE and CH4/PVDF), the kij was considered to increase linearly with increasing temperature. A group-contribution simplified PC-SAFT for the prediction of polymer systems was presented by Tihic et al. (2008) (GC-PC-SAFT). The methodology is similar to that described above, except that the mixing rules are different. Both first- and second-order interactions (for molecular length m, segment diameter  and the energy term ) are considered. For details refer to Tihic et al. (2008). Pedrosa et al. (2006) compared the PC-SAFT model with the soft-SAFT model, in which the reference term involves spherical Lennard±Jones species. The chain feature was, however, included; a perturbation to the reference term (using Wertheim's theory) was applied, followed by the use of a radial

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6.6 Experimental ( ) n-pentane mass concentration in polyethylene at 423.65 K and predicted values using the SOFT-SAFT (solid line) and the PCSAFT (dashed line). Drawn after Pedrosa et al. (2006).

distribution function for the Lennard±Jones fluid. Figure 6.6 illustrates good agreement between the experimental n-pentane solubility in polyethylene and predictions based on the two models.

6.3.3

The non-equilibrium lattice fluid (NELF) model

Doghieri, Baschetti, Sarti and coworkers in Bologna, Italy, have modelled the solubility of gases in glassy polymers using a non-equilibrium lattice fluid (NELF) model. The approach was to use the polymer density as an internal state variable describing the departure from equilibrium. In essence, only the PVT data of the pure components and the density of the solid mixture were needed for the calculations. Through a pseudo-equilibrium between the chemical potentials of the gas (G ) and of the solid (S ), it is possible to calculate the solute volume fraction (1 ) as a function of gas pressure (p) and temperature (T): S …T; ~S ; 1 † ˆ G …T; p† The left-hand side (S ) of eq. 6.32 is calculated from   S r1 ÿ r10 S 0 ln…1 ÿ ~S † ÿ r1 ÿ ˆ ln…~   1 † ÿ ri ‡ RT ~S ÿ 0  
 2  S r1 v1 …p1 ‡ p ÿ 2
p † ~ RT

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6:32

6:33

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where p1 and p2 are tabulated gas normalisation constants for the gas and the polymer, respectively. The density constants 1 and 2 of the pure components, as well as the constants v1 and v2 , are tabulated.ÿp p
2 The binary pressure parameter is
p ˆ p1 ÿ p2 , a first-order approximation where fij ˆ 1, the site molar volume is v ˆ v1 v2 =…v1 2 ‡ v2 1 †, the site occupation number is r10 ˆ M1 =…1 v1 † and the mixture occupation number is r1 ˆ r10 v1 =v . The reduced solid density ~S can be approximated by   02 w1 1 ÿ w1 6:34 ‡ ~S ˆ 1 ÿ w1 1 2 where 02 is the density of the pure polymer and w1 is the weight fraction of gas. The relation between weight fraction and volume fraction is: 1 ˆ

w1 =1 w1 =1 ‡ w2 =2

6:35

The right-hand side of eq. 6.32 can be derived from the Sanchez-Lacombe equation of state, resulting in: G ~Eq r10 v1 p1 ˆ ln…~ Eq † ÿ r10 ln…1 ÿ ~Eq † ÿ r10 ÿ RT RT

6:36

Finally, the equilibrium density Eq of the gas must be pre-calculated by minimising the chemical potential of the Sanchez±Lacombe equation of state, resulting in (cf. eq. 6.18):     ÿ Eq
2 ÿ
1 Eq Eq ~ ˆ0 6:37 ~ ‡~ p ‡ T ln 1 ÿ ~ ‡ 1 ÿ ~ r Examples included the prediction of the CO2 dual sorption isotherm of PC at 35ëC as well as the reduced dual character for the annealed material (Doghieri and Sarti, 1996). Different desorption isotherms for the same system but from different sorption pressures were also predicted (hysteresis). Later, the model was successfully tested on low-pressure sorption isotherms using a constant density equal to the pure unpenetrated polymer, i.e. without the need for dilation data. Wherever significant swelling was absent, the model yielded in general a satisfactory description of the sorption behaviour. Examples studied included vinyl chloride in PVC and CO2 in PMMA (Sarti and Doghieri, 1998; Doghieri and Sarti, 1998). If dilation data are missing, and eq. 6.34 is not assumed to be valid, it is still possible to obtain the polymer pseudo-equilibrium density by considering that it decreases linearly with increasing penetrant pressure (Giacinti Baschetti et al., 2001): S …p† ˆ 02 …1 ÿ kp†

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6.7 Solute (CO2)±polymer mass ratio as a function of solute pressure. Lines correspond to NELF one-parameter fitting to PMMA (33ëC, ) and PS (35ëC, l) data. Drawn after data of Giacinti Baschetti et al. (2001).

where k, the swelling coefficient, and the pure polymer density are nonequilibrium parameters. At least one of the two parameters k and 02 is usually unknown; the authors, however, presented convenient ways of obtaining these from a small amount of data. Figure 6.7 shows a fit where the polymer density is known and the swelling coefficient is determined from a single high-pressure solubility datum (one-parameter correlation). The estimated swelling coefficient values of 0.0097 (PS) and 0.0218 (PMMA) were indeed close to the experimental values: 0.0121 (PS) and 0.0243 (PMMA). It is useful to know that, at the limit of vanishing pressure, the solubility coefficient can be obtained from (De Angelis et al., 2007):          TSTP M1 p1 T1 p2 2 02 exp   1 ‡ ÿ 1 0 ln 1 ÿ  S0 ˆ TpSTP 1 RT1 T2 p1 2 2   T1 p2 02 T1    6:39 ‡   ÿ 1 ‡   ‰p1 ‡ p2 ÿ
p12 Š T2 p1  2 p1 T

6.3.4

The non-equilibrium perturbed hard-sphere chain (NE-PHSC) model

A non-equilibrium perturbed hard-sphere-chain (NE-PHSC) model has been used to predict the solubility of gases and vapours in glassy polymers (Doghieri et al., 2006). The non-equilibrium thermodynamics for glassy polymers (NETGP), previously used with lattice-fluid models, was here combined with the

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perturbed hard-sphere-chain (PHSC) theory. In the latter, the residual Helmholtz free energy is obtained as the sum of two contributions: ares ˆ aref ‡ aper

6:40

ref

where a accounts for the chain connectivity and a hard-sphere interaction and aper accounts for mean field forces. Two different approaches were used to describe the perturbation, either a van der Waals approach or a square well potential with a variable width. The NE-PHSC was solved, in the limit of low penetrant concentrations, considering the following pseudo-equilibrium condition: " #     M M NE…S† 0 sol T; p; PE ;  s ; p ; ; p ; ksp ˆ sol ; pol ; s ; r s r p     M EQ…G† T; p; s ; ; s sol 6:41 r s 0 where the suffixes s and p refer to the solute and polymer, respectively. PE sol , pol and ksp are the pseudo-equilibrium solute mass per polymer mass, the pure polymer density and the binary interaction parameter. The chemical potential in the solid (left-hand side of eq. 6.39) is obtained from NET-GP calculations and the solute chemical potential in the gas phase (right-hand side) is obtained from equilibrium EOS. The methodology was tested on different solute±polymer pairs. Figure 6.8 gives an example where experimental CO2 solubilities in PC were fitted with the square well potential type of EOS. It is interesting to note the large improvement in the fitting of the glassy data when PHSC was combined with NET-GP.

6.8 CO2 solubility ( ) as a function of the inverse temperature predicted by the PHSC model with the square well potential (constant relative width of 1.455) with (solid curve) and without (dashed curve) the NET-GP approach. The binary interaction parameter was 0.075. Drawn after data of Doghieri et al. (2006).

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6.4

143

What makes a barrier a barrier?

The starting point when aiming at developing materials with excellent barrier properties is to consider eq. 6.3. A low permeation is obtained by targeting a low diffusivity and/or a low solubility. There are several ways to achieve this and some of them will be briefly discussed in the following. For a more complete description, please consult Hedenqvist (2005). Crystals are impermeable to most solutes and the presence of crystals therefore lowers the solubility. The fact that the solutes have to circumvent the crystals also leads to a decrease in the diffusivity due to a tortuosity effect. The spherulitic morphology of polyethylene is complex and the prediction of the tortuosity is not straightforward. The radially growing, splining, splaying crystal lamellae form a network of crystals that the solute molecule has to pass during its journey through the material (Fig. 6.9). A Monte-Carlo-generated random walk yielded the tortuosity effect given in Fig. 6.10, which is compared with experimental data for four different solutes. Note that the tortuosity effect in the present cases leads to a reduction of the diffusivity by, at most, a factor of ca. 10 when going from 0 to ca. 80 vol% crystallinity.

6.9 Generated spherulite based on 100000 `crystalline' bricks having an aspect ratio of 10. The volume crystallinity is around 25%.

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6.10 The reciprocal of the tortuosity () as a function of volume crystallinity for different polyethylenes with the following penetrants: n-hexane at a specific penetrant concentration in the amorphous component (15 vol%), n CH4,
N2, s Ar. The latter three sets of data were obtained from permeation data taken at 80ëC and 10 MPa. The line predicted by simulation is displayed as a solid line ending with l. Drawn after Nilsson et al. (2009).

In Fig. 6.10, three of the solutes are small and in the case of n-hexane the polymer is swollen with a mobile amorphous interphase. Thus the solute mobility reduction near the crystal faces is expected to be reasonably small even though the self-diffusivity of the amorphous chains near the crystals is low. However, in the case of larger solutes or unswollen polymers, the constraint effect will most probably be more pronounced. The reduction in the diffusivity due to the constraint effect is larger for materials of greater crystallinity. The constraint of the amorphous component and hence the permeation can also be decreased by chemical crosslinking. Figure 6.11 shows the permeability of methanol through elastomeric crosslinkable polysilicon olefins. These contain unsaturations that oxidise during a high temperature (120ëC) treatment and produce intermolecular crosslinks. Interestingly, the crosslinking effect is greater for the larger solutes. It should, however, be noted that increasing the degree of crosslinking does not always lead to a lower permeability. It has been shown for poly(ethylene glycol) diacrylate (Lin et al., 2005) and for photocrosslinked polyethylene (Chen and RaÊnby, 1989) that the constraining effect may be small, absent or of less importance than other effects leading to opposite trends. The presence of strong secondary intermolecular bonds increases the barrier for non-polar solutes. For this reason, the hydrogen-bonded EVOH has a significantly lower oxygen permeability than polyethylene. The weak point is that moisture interacts with the hydrogen bonds in the material, and in the worst

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6.11 Methanol permeability through polyorganosilicone-containing elastomers. The treatment time corresponds to the extent of crosslinking. The data correspond to (from bottom to top) the permeation of CH4, C2H6, C3H8 and C4H10. Drawn after data of Kim et al. (2005).

case this effectively leads to the disappearance of the barrier. The high gas barrier component, in this case, has to be protected from the moist environment with a `waterproof' layer. In addition, the layers need good bonding that then requires a tie layer on both sides of the barrier. The resulting films are therefore often relatively complex and advanced. Numerous examples of multilayer films and their barrier properties are given in Massey (2003). Biaxial stretching in a sequential or simultaneous mode reduces the permeability. In a study on polypropylene, Lin et al. (2008) showed that, even though the amorphous content increased with drawing, the oxygen permeability decreased. They suggested that this was due to the reduced mobility of stretched tie chains, which reduced the frequency by which connecting channels form between neighbouring free-volume holes. Pinhole-free metals and defect-free inorganic glasses are considered to be `absolute' barriers. Consequently, when extra high barriers are needed, polymer layers may be combined with one or both of these. BarixTM is an interesting barrier solution (www.vitexsys.com). It consists of several layer-pairs of a thin polymer film and an AlOx layer. The thin film is produced from a liquid that is cured with UV and the ceramic is applied by physical vapour deposition. The polymer films are generally between 0.25 and 1 m thick and the ceramic films are significantly thinner. It has been shown that a 20 nm thick ceramic layer can give good barrier properties. The resulting multilayer BarixTM film is a very thin and thus transparent and flexible material that can, for instance, be used as a coating on thicker substrates. The key to success here is that the polymer film is smooth, continuous and clean, which gives a uniform nucleation and a more

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defect-free AlOx film during the deposition. In addition, a possible defect or crack in the ceramic layer cannot easily grow past the adjacent polymer film layers. The resulting multilayer film is claimed to be a million times tighter to water vapour and oxygen than conventional food packaging. The 3M ScotchpakTM is another example of a high barrier film obtained with an AlOx layer. There are also materials using SiOx vacuum-coated layers (see, e.g., the `TechbarrierÕ' material at www.techbarrier.com). With biaxially oriented PET as the base, the water vapour transmission rate is 0.3 g/(m2 24 h) and the oxygen transmission rate is 0.3 cm3/(m2 24 h atm). With SiOx-coated oriented polyvinylalcohol (OPVA), the values of the Techbarrier go down to, respectively, <0.1 and 0.1.

6.5

Characterisation techniques

The measuring techniques of mass uptake or loss and transfer can roughly be divided into those that involve, and those that do not involve, some type of gravimetric measurement. In the non-gravimetric methods there is typically a detector that relies on, e.g., an electric signal or a chemical reaction. In some cases, the gravimetric technique is combined with dilation measurements to enable the volume change to be observed as a function of the mass uptake or loss. Note that this chapter is not a complete listing of different techniques, merely a short overview of some interesting techniques. The weight uptake or weight loss can be monitored either directly with a balance or indirectly through changes in another property directly related to the mass or the change in mass. The quartz crystal microbalance (QCM) is an example where the mass change is recorded through the frequency change of a vibrating quartz crystal that is forced to vibrate due to an applied voltage (www.q-sense.com). This is probably the most sensitive of all gravimetric techniques currently available. It can detect nanogram differences over a 1 cm2 surface. By melting/casting a polymer film onto the quartz crystal, it is possible to measure mass uptake or loss due to changes in vapour pressure (gas applications) or liquid environments (ICM). Because of the viscoelastic nature of the polymers, there is a dissipation of energy during the vibration, which can, however, be measured by analysing the amplitude decay after a shutdown voltage. In the quartz spring balance system, the sample is suspended on a quartz spring with a known spring constant (www.ruska.com). The elongation or retraction of the spring then enables the mass change to be recorded. It is here assumed that it is only the sample that changes its mass. The spring is mounted in a tight glass column with openings to allow vapour to enter or leave. As the sample sorbs or loses vapour, the sample position changes and by careful reading through, e.g., a DVT-CCD camera it is possible to calculate the sample mass uptake or loss. By stepwise increasing the vapour pressure (that can explicitly be measured) and measuring the stepwise mass uptake and uptake

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rate, it is possible to obtain a sorption isotherm as well as the diffusion coefficient of the vapour as a function of vapour pressure (Hedenqvist and Doghieri, 2002). The differential (stepwise change) measurement can also be performed in desorption mode, i.e., by stepwise lowering the vapour pressure. In integral sorption or desorption measurements, a complete sorption or desorption experiment is performed with a given change in the environment. It can be the mass uptake of a dry polymer sample immersed in a liquid or exposed to a sudden change in vapour pressure. The accuracy of the balance needed for the experiments depends on the size of the uptake. The experiments can be made using, e.g., a sensitive TGA-type of gravimetric system or simply a bench-top balance. A large-capacity microbalance measures with 0.1 g resolution on a 5 g sample (hidenisochema.com). TGA-based systems can readily be used for differential sorption/desorption kinetic measurements and, with an attached mass spectrometry unit, it is possible to determine what species evolves from the sample. The cup method is very popular due to its simplicity, its low cost and the fact that it allows numerous samples to be measured simultaneously. The most common substance for study is water. The mass transfer (uptake or loss) is measured by recording the weight change of the whole cup. In the uptake measurement, a moisture absorber (e.g. phosphorus pentoxide) is placed inside the cup, whereafter the film is tightly mounted onto the cup rim using a rubber oring and clamps. The cup is subsequently placed in an environment of known humidity and water will start to diffuse into the dry cup. By recording the mass change as a function of time, it is possible to calculate the water vapour transmission rate, provided a `steady-state' is observed, i.e. a linear curve of mass change versus time. A desorption measurement is made by placing water or a salt solution in the cup and placing the cup in a low relative humidity environment (Fig. 6.12). It is possible to achieve different relative humidity gradients over the film by combining different salt solutions in or outside the cup. It is important to note that if there is a risk that air becomes stagnant between the liquid surface and the lower film surface (1 and 2 in Fig. 6.12) and/ or between the upper film surface and surrounding environment (3 and 4) the vapour pressures at the film surfaces, determining the mass transfer rate (p2 and p3), may not be the same as the vapour pressure above the liquid and in the environment (p1 and p4) (Gennadios et al., 1994). Corrections for the differences in pressure may, however, be estimated from the known water diffusivity in stagnant air. Consider, e.g., that dry air is circulating above the film but that there is a pocket with stagnant air between 1 and 2. Above the film it is possible to assume that the water vapour pressure is zero (p3 ˆ p4 ˆ 0). However, there will be a pressure drop from 1 to 2 leading to an underestimation of the WVTR if it is normalised with
p13 ˆ p1 ÿ p3 
23 , rather than
p23 ˆ p2 ÿ p3 . Magnetic suspension balances make `contactless' weighing possible (Blasig et al., 2007). An electromagnet is coupled to a balance, both being located

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6.12 Cup measurement, with liquid water (striped section) in the cup.

outside the measuring cell. The electromagnet is adjusted so that a permanent magnet that holds the sample inside the measuring cell is kept in suspension. The technique is very suitable in cases where the balance cannot be placed in the actual measuring cell, because of, e.g., liquid exposure. The pressure decay method is suitable when sorption at high pressures is studied (Davis et al., 2004). In its most simple form, a sample is placed together with a high-pressure gas in a closed cell. As the sample sorbs the gas, the pressure decays and, by using an equation-of-state, the pressure decay can be transformed into mass uptake, which then enables the solubility and diffusion properties to be evaluated. There are a number of techniques for measuring the mass flow of permeants through polymer films. In the case of water vapour transmission rates, a modulated infrared sensor can be used to detect the moisture in the gas transported from the lower dry side of the film (Stevens et al., n.d.). It produces an electric signal, the amplitude of which correlates with the moisture concentration. Alternatively, tritiated water can be used as permeant, which then requires a radioactivity detector. A high sensitivity is obtained if an absolute coulometric detector is used, measuring with an accuracy down to 0.0005 g/(m2 24 h). The calcium test is yet another principle. It is based on the fact that a calcium sample, behind the test film, changes appearance as moisture penetrates the film; the calcium becomes a transparent calcium salt. For oxygen transmission rates, an electrochemical (non-coulometric) or a coulometric oxygen sensor can be used, and for CO2 an infrared detector can be used (Mocon). If the transmission rate can be monitored from the onset of exposure until steady state is reached, the permeation experiment also provides

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the basis for calculation of the diffusivity. A range of other techniques have also been used over the years, including UV/vis spectroscopy (Krongauz et al., 1994), ATR-IR (Balik and Simendinger, 1998; van Alsten, 1995), imaging and pulsed field NMR (Harding et al., 1997; Fleischer, 1985), forced Rayleigh scattering (FRS), fluorescence recovery after pattern photobleaching (FRAP), elastic recoil detection (ERD), nuclear reaction analysis (NRA) (Green, 1996) and phosphorescence of singlet oxygen (Klinger et al., 2009).

6.6

References

Balik CM and Simendinger III WH (1998), `An attenuated total reflectance cell for analysis of small molecule diffusion in polymer thin films with Fourier-transform infrared spectroscopy', Polymer 39, 4723±4728. Blasig A, Tang J, Hu X, Shen Y and Radosz M (2007), `Magnetic suspension balance study of carbon dioxide solubility in ammonium-based polymerized ionic liquids: poly(p-vinylbenzyltrimethyl ammonium tetrafluoroborate) and poly([2(methacryloyloxy)ethyl] trimethyl ammonium tetrafluoroborate)', Fluid Phase Eq 256, 75±80. Bondi A (1964), `Van der Waals volumes and radii', J Phys Chem 68, 441±451. Challa VV and Visco Jr DP (2005), `Evaluating the SAFT-VR and the Sanchez±Lacombe EOS for modeling the solubility of blowing agents in polyols', J Cell Plast, 41 563± 588. Chen YL and RaÊnby B (1989), `Photocrosslinking of polyethylene. 1. Photoinitiators, crosslinking agents and reaction kinetics', J Polym Sci Part A: Polym Chem 27, 4051±4075. Crank J (1986), The Mathematics of Diffusion, Oxford Scientific Publishers, Oxford, UK. Davis PK, Lundy GD, Palamara JE, Duda JL and Danner RP (2004), `New pressuredecay techniques to study gas sorption and diffusion in polymers at elevated pressures', Ind Eng Chem Res 43, 1537±1542. De Angelis MG, Sarti GC and Doghieri F (2007), `NELF-model prediction of the infinite dilution gas solubility in glassy polymers', J Membr Sci 289, 106±122. Doghieri F and Sarti GC (1996), `Nonequilibrium lattice fluids: a predictive model for the solubility in glassy polymers', Macromolecules 29, 7885±7896. Doghieri F and Sarti GC (1998), `Predicting the low-pressure solubility of gases and vapors in glassy polymers by the NELF model', J Membr Sci 147, 73±86. Doghieri F, De Angelis MG, Baschetti MG and Sarti GC (2006), `Solubility of gases and vapors in glassy polymers modelled through non-equilibrium PHSC theory', Fluid Phase Eq 241, 300±307. Fleischer G (1985), `The effect of polydispersity on measuring polymer self-diffusion with the n.m.r. pulsed field gradient technique', Polymer 26, 1677±1682. Galindo A, Davies LA, Gil-Villegas A and Jackson G (1998), `The thermodynamics of mixtures and the corresponding mixing rules in the SAFT-VR approach for potentials of variable range', Mol Phys 93, 241±252. Gennadios A, Weller CL and Gooding CH (1994), `Measurement errors in water vapor permeability of highly permeable, hydrophilic edible films', J Food Eng 21, 395±409. Giacinti Baschetti M, Doghieri F and Sarti GC (2001), `Solubility in glassy polymers: correlations through the nonequilibrium lattice fluid model', Ind Eng Chem Res 40, 3027±3037.

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Gil-Villegas A, Galindo A, Whitehead PJ, Mills SJ, Jackson G and Burgess AN (1997), `Statistical associating fluid theory for chain molecules with attractive potentials of variable range', J Chem Phys 106, 4168±4186. Green PF (1996), `Translational dynamics of macromolecules in melts', in Diffusion in Polymers, ed. Neogi P, Marcel Dekker, New York, pp. 251±298. Greenfield ML and Theodorou DN (1993), `Geometric analysis of diffusion pathways in glassy and melt atactic polypropylene', Macromolecules 26 5461±5672. Gross J and Sadowski G (2001), `Perturbed-chain SAFT: an equation of state based on a perturbation theory for chain molecules', Ind Eng Chem Res 40, 1244±1260. Harding SG, Johns ML, Pugh SR, Fryer PJ and Gladden LF (1997), `Magnetic resonance imaging studies of diffusion in polymers', Food Additiv Contamin 14, 583±589. Hedenqvist MS (2005), `Barrier packaging materials', in Environmental Degradation of Materials, ed. Kutz M, William Andrew Publishing, Norwich, NY, pp. 547±563. Hedenqvist MS and Doghieri F (2002), `The significance of the zero-concentration diffusivity value obtained from integral desorption data', Polymer 43, 223±226. Kim T-J, Bryantseva IS, Borisevich OB, Syrtsova DA, Khotimsky VS, Roizard D and Teplyakov VV (2005), `Synthesis and permeability properties of crosslinkable elastomeric poly(vinyl allyl dimethylsilane)s', J Appl Polym Sci 96, 927±935. Klinger M, Poulsen Tolbud L, Gothelf KV and Ogilby PR (2009), `Effect of polymer cross-links on oxygen diffusion in glassy PMMA films', ACS Appl Mater Interfaces 1, 661±667. Krongauz VV, Mooney III WF and Schmelzer ER (1994), `Real-time monitoring of diffusion between laminated polymer films', Polymer 35, 929±934. Lacombe RH and Sanchez IC (1976), `Statistical thermodynamics of fluid mixtures', J Phys Chem 80, 2568±2580. Lin H, Kai T, Freeman BD, Kalakkunnath S and Kalika DS (2005), `The effect of crosslinking on gas permeability in cross-linked poly(ethylene glycol diacrylate)', Macromolecules 38, 8381±8393. Lin YJ, Dias P, Chen HY, Hiltner A and Baer E (2008), `Relationship between biaxial orientation and oxygen permeability of polypropylene film', Polymer 49, 2578± 2586. Massey LK (2003), Permeability Properties of Plastics and Elastomers ± A Guide to Packaging and Barrier Materials, 2nd edn, William Andrew Publishing/Plastics Design Library, Norwich, NY. McCabe C, Galindo A, GarciaÂ-Lisbona MN and Jackson G (2001), `Examining the adsorption (vapor-liquid equilibria) of short-chain hydrocarbons in low-density polyethylene with the SAFT-VR approach', Ind Eng Chem Res 40, 3835±3842. Nilsson F, Gedde UW and Hedenqvist MS (2009), `Penetrant diffusion in polyethylene spherulites assessed by a novel off-lattice Monte-Carlo technique', European Polym J 45, 3409±3417. Park JY and Paul DR (1997), `Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method', J Membr Sci 125 23±39. Pedrosa N, Vega LF, Coutinho JAP and Marrucho IM (2006), `Phase equilibria calculations of polyethylene solutions from SAFT-type equations of state', Macromolecules 39, 4240±4246. Radfarnia HR, Ghotbi C, Taghikhani V and Kontogeorgis GM (2005), `A modified freevolume-based model for predicting vapor±liquid and solid±liquid equilibria for size asymmetric systems', Fluid Phase Equilibria 234, 94±100. Rolker J, Seiler M, Mokrushina L and Arlt W (2007), `Potential of branched polymers in

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the field of gas absorption: experimental gas solubilities and modeling', Ind Eng Chem Res 46, 6572±6583. Sanchez IC and Lacombe RH (1976), `An elementary molecular theory of classical fluids. Pure fluids', J Phys Chem 80, 2352±2362. Sarti GC and Doghieri F (1998), `Predictions of the solubility of gases in glassy polymers based on the NELF model', Chem Eng Sci 53, 3435±3447. Serna LV, Becker JL, GaldaÂmez JR, Danner RP and Duda JL (2008), `Elastic effects on solubility in semicrystalline polymers', J Appl Polym Sci 107, 138±146. Stevens M, Tuomela S and Mayer D (n.d.), `Water Vapor Permeation Testing of UltraBarriers: Limitations of Current Methods and Advancements Resulting in Increased Sensitivity', technical report, MOCON, Inc., Minneapolis, MN, www.mocon.com. Thornton AW, Nairna KM, Hill AJ and Hill JM (2009), `New relation between diffusion and free volume: I. Predicting gas diffusion', J Membr Sci 338, 29±37. Tihic A, Kontogeorgis GM, von Solms N and Michelsen ML (2008), `A predictive groupcontribution simplified PC-SAFT equation of state: Application to polymer systems', Ind Eng Chem Res 47, 5092±5101. van Alsten JG (1995), `Diffusion measurements in polymers using IR attenuated total reflectance spectroscopy', Trends Polym Sci 3, 272±276. von Solms N, Michelsen ML and Kontogeorgis GM (2005), `Prediction and correlation of high-pressure gas solubility in polymers with simplified PC-SAFT', Ind Eng Chem Res 44, 3330±3335. Wang L-S (2007), `Calculation of vapor±liquid equilibria of polymer solutions and gas solubilities in molten polymers based on PSRK equation of state', Fluid Phase Equilibria 260, 105±112. Wei YS and Sadus RJ (2000), `Equations of state for the calculation of fluid-phase equilibria', AIChE J 46, 169±196. Wibawa G and Widyastuti A (2009), `Improvement of an entropic-FV model based on solubility parameters for prediction of vapor±liquid equilibria of solvent±polymer systems', Fluid Phase Equilibria 285, 105±111.

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7

Ethylene±norbornene copolymers and advanced single-site polyolefins T . J . D U N N , formerly at Printpack, Inc., USA

Abstract: The chapter addresses the polymerization and structure of advanced single-site polyolefins and ethylene±norbornene copolymers. The relevance of this information to compounding a variety of nanocomposite polymers is discussed. Data from the literature and unpublished experiments are summarized with an eye towards understanding the growing potential of these materials in commercial food packaging applications. Key words: advanced single-site polyolefins, ethylene±norbornene copolymers, food packaging, maleic anhydride modifications, nanocomposites, oxygen barrier, plastomers, water vapor barrier.

7.1

Introduction

Previous chapters have presented information on the composition, structure, and function of nano-compounded polymers for food packaging. Materials considered in this chapter demonstrate some of the nuance of how improved function is accomplished, of how it can be enhanced, and of where basic physical constraints preclude improvements. This chapter presents basic information on the structure and properties of ethylene±norbornene copolymers and advanced single-site polyolefins. It addresses polymerization reactions to the extent necessary to demonstrate similarities and differences between the two classes of polymers. To the extent that these reactions originated from related catalyst chemistry, attention is given to the historical and research context of the development of these materials. With this basic understanding of the materials, descriptions and data are provided to explain the unique role played by modified advanced single-site polyolefins as compatibilizers in compounding nanocomposites of other resins and resin blends. Resultant performance improvements, particularly oxygen and water vapor barrier, of the nanocomposites, compared to neat resins, blends and nanocomposites formulated without compatibilizers, are addressed. The molecular design versatility available for modified advanced single-site polyolefins combines with the emerging understanding of the geometric and physical optima for formulating nanopolymer composites to promise improved barrier performance. These same two factors also help to recognize hopeless proposals before resources are wasted in their pursuit.

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Synthesis and molecular structure: advanced single-site polyolefins

`Single-site polyolefins' refers to polyolefin resins polymerized with a `metallocene' molecule: a transition-metal atom sandwiched between cyclopentadienyl ring structures (Fig. 7.1). The large volume polyolefins, polyethylene and polypropylene, had historically been polymerized in high pressure processes leading to highly branched molecules using `Ziegler±Natta catalysts' (based on titanium halides and organo-aluminum compounds). In these products, branches result on a carbon chain where a carbon atom bonds to three other carbons rather than the two that characterize a straight carbon±carbon chain. Additional catalyst work in the 1960s and 1970s led to production of `linear polyolefins'. Birell (1972) at the time noted that `The low pressure polymerization of alpha olefins with catalyst systems composed of a partially reduced heavy transition metal component and an organometallic reducing component to form high density, high molecular weight, solid, relatively linear polymers [was] well known.' (`Alpha-olefins' are alkenes with a carbon1/carbon2 double bond.) His patent is representative of the great body of work done in Europe and North America in the decade to optimize catalyst systems for linear olefins. Ethylene and propylene are copolymerized with each other and with the common industrial comonomers butene, hexene, and octene (`C4, C6, and C8 alkenes' respectively) to make the linear (co-)polymers (e.g. Fig. 7.2). These linear polymers are properly considered to be copolymers of ethylene or propylene with C4 to C8 -alkene comonomers. Comonomer distribution along the C2 or C3 `backbone' results from the specific catalyst system, the ratio of the -alkene monomers in the input stream, and process conditions. In the chain, the carbon backbone results from the polymerization of the carbon± carbon double bond of the alkene, and pendent side chains of carbon length n ÿ 2 from the Cn alkene comonomer. Themselves an evolution of the linear polymer catalysts, metallocene catalysts provide a well-defined single catalytic site and a well-understood

7.1 General structure of a metallocene.

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7.2 Isotactic ethylene/butylene copolymer.

molecular structure, producing uniform polyolefins with unique structures and physical properties. Titanium, zirconium, and hafnium are the `Group 4 transition metals' used as industrial catalysts. Sinn and Kaminsky (1980) are credited with experimental determination of the underlying chemistry of the single-site polymerization process. The catalyst site, with its specific metal atom and surrounding cyclopentadienyl rings, provides a low pressure polymerization environment that produces a uniformly sequenced chain of comonomers in the polymer. The terms `single site' and `metallocene' olefins have come to be used interchangeably.

7.3

Macromolecular structure: advanced single-site polyolefins

The crystal structure properties of single-site polyolefins depend on variables such as the extent and type of comonomers and the molecular weight. Catalyst development provided the means of adjusting these variables to provide a sort of designer polymer range representing targeted properties (Welborn, 1987). This polymeric design latitude results in new classes of polyethylene copolymers with densities somewhat less to very much lower than the previously commercially offered (Table 7.1). These `ultra' and `very' low density polyethylenes offer the strength and elasticity features not available in other grades of traditional or linear low density polyethylene. The properties resemble rubbery, soft, elastic materials and have produced a new class of thermoplastic Table 7.1 Mechanical properties of single-site polyethylenes (http://www.ides.com/ generics/PE/PE_typical_properties.htm) Property

LLDPEa

mLLDPEb

ULDPEc

VLDPEd

Density (g/cm2) 0.916±0.937 0.912±0.927 0.904±0.912 0.884±0.911 Mass flow rate (g/10 min) 0.10±4.0 0.87±1.3 0.76±4.0 0.10±3.1 Flex modulus (MPa) 110±807 120±630 ± 29.6±152 Tensile strength (MPa) 0.0276±88.9 7.54±67.6 3.60±53.2 3.45±52.1 Tensile elongation (%) 1.0±1000 6.0±970 450±800 490±910 a

LLDPE: Linear low density PE. mLLDPE: metallocene linear low density PE. c ULDPE: (metallocene) ultra low density PE. d VLDPE: (metallocene) very low density PE. b

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7.3 Crystallinity and composition continuum for ethylene±propylene polymers (after Bhowmick, 2008).

elastomers termed olefin `plastomers'. Higher comonomer content serves as the primary means to these ends. Unlike natural and synthetic rubbers, plastomers retain their thermoplastic properties and lend themselves readily to industrial plastic processes. Crystallinity of the polymer depends on molecular weight distribution, comonomer content, and tacticity. Figure 7.3 provides an illustrative presentation of general crystallinity for different ethylene±propylene plastomers. The same molecular properties that determine crystallinity serve to influence greatly the ease and degree to which nanoclay particles can be incorporated into the polymers.

7.4

Macromolecular structure: ethylene± norbornene copolymers

Norbornene is a cyclic alkene with a dense three-dimensional structure: a cyclohexene ring with a bridging methylene in the para-position (Fig. 7.4a). When polymerizing, either the double bond provides the locus for covalent bonding with ethylene (or other norbornenes) (Fig. 7.4c) or the cyclohexene ring opens at the double bond, leaving a pentane ring co-planar with the polymer chain (Fig. 7.4b). Kaminsky (1988) discovered the latter ring opening reaction

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7.4 Norbornene and polymerization modes.

using an appropriate metallocene catalyst. The former mode was patented by Ishimaru et al. (1987). Work continues in order to understand the mechanisms and kinetics of norbornene copolymerization, allowing for more complex and structurally preferred coplymers (Boggioni et al., 2008). From this work have come commercial grades of mode II ethylene± norbornene copolymers, TopasÕ, marketed by TOPAS Advanced Polymers, Inc. in North America and by TOPAS Advanced Polymers GmbH in Europe and South America. The products are amorphous, high clarity products offering superior water vapor barrier (compared to other polyolefins), a relative density of 1.02, and a range of 65ë to 180ëC glass transition temperatures. Because they maintain the bridging methylene, the polymers exhibit chain stiffness (arising from the rigidity and steric interactions of the norbornene units). As a result, Corinne et al. (1999) reported limitations on the miscibility of blends of cyclic olefin copolymers with norbornene content greater than 50%.

7.5

Nanocomposite preparation: advanced singlesite polyolefins

Blends of nanoscale clay particles with polyolefins in general have produced only minor improvements in plastic performance features significant for food packaging applications. Advanced single-site polyolefins in contrast have found a special niche in plastic nanocomposites as compatibilizers (Small et al., 2006; Lai et al., 2003). When modified with maleic anhydride low molecular weight (<100,000) single-site polyolefins (e.g. EpoleneÕ) (www.epolene.com) they improve the dispersion of the minor component in the major component of many polymer blends. Small et al. (2006) evaluated the blend in Table 7.2 for blown film properties. They used a multi-step masterbatch sequence, summarized in Fig. 7.5, to achieve better clay intercalation and exfoliation than a direct dilution of the nanoclay in the LDPE. X-ray diffraction analysis of the blown film samples by Small et al. (2006) indicated that clay intercalation and exfoliation were greater in films prepared by masterbatch let-down than with direct dilution of organoclay into LDPE

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Table 7.2 Material characteristics Material

Supplier/grade

LDPE PE Wax compatibilizer Organoclay

Dow 300E Westlake/Epolene E16 Dioctohedral flouromica CO-OP Chemical Co. Ltd, Japan

Source: Small et al. (2006).

7.5 Blend preparation (after Small et al., 2006).

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MFI

Density (g/cm3)

0.08 >100

0.924 0.943

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7.6 1.5 BUR film properties.

nanocomposites. They attributed this to differences in blend compatibility and concentration during initial compounding stages. Data for film samples made at various blow up ratios indicated that this process condition did not have a significant influence on film properties. It should be pointed out that the low shear (15 rpm) conditions used in the initial stage of the directly diluted blend are not sufficiently productive for reasonable industrial processes. They lend themselves more properly to masterbatch production. In spite of the clay distribution difference, Fig. 7.6 suggests that the resultant film properties did not differ much between the masterbatch and direct diluted blends. The levels of OTR and CO2TR for nanocomposite films were about onehalf of (i.e. better than) those of the film made with neat polymer. While proportionately great, the starting points for LDPE are so high (ca. 200 and 1500 cm3 mm/m2 day, respectively) that the improvements are not functionally useful. Lai et al. (2003) demonstrated that modified single-site olefins also enhance blends of nanocomposite Nylon-6 with ABS (80/20). Their analysis focused on tensile properties of the blends, but SEM images of the fracture surface of impact consistently indicated decreased particle sizes. Data indicating that the compatibilizer had a larger effect in the absence of the nanoclay suggested to the researchers that clay competes with the compatibilizer for available reaction sites from amine groups on Nano-Nylon 6 and Nylon 6. Schirmer et al. (2008) reported water vapor transmission rate (WVTR) improvements in cast polypropylene (PP) film by careful compounding of montmorillonite platelets into PP resin in the presence of 2.6% Epolene G-3003. The oxygen permeation of nano-PP blown and cast films was found to be significantly less than for the neat PP films, although not as much as reported by Small et al. (2006) for nano-LDPE.

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Printpack (2010) under contract to the US Department of Defense attempted to reproduce and improve on the results of Schirmer et al. (2008). In particular, the research sought water vapor barrier improvements for all-plastic packaging structures intended to replace foil laminations. Lower WVTR sealant layers were sought by first trying to replicate the Schirmer results and then evaluating possible synergies of nanocomposites with polypropylene/ethylene±norbornene copolymer (TopasÕ blends). TopasÕ was selected based on its inherent water vapor barrier and reported suitability for high temperature sterilization processes (Tatarka, 2008). Two compounders produced nanocomposites of modified clay nanoplatelets with PP and with PP/COC blends. Preliminary assessments by the compounders were used to select two blends by each to evaluate as cast films. Rhetech (Whitmore Lake, MI) was chosen based on prior work (Karian et al., 2008) to produce modified montmorillonite nanoclay compounded into PP and PP±COC blends. Two levels of the nanoclay were used at 6% and 8% of total weight. In addition to the two levels of nanoclay, two different COC products were tested: Topas 5013X14 (230ëC, melt index (MI) ˆ 11) and 6013F-04 (MI ˆ 2.3). Six blends were produced by Rhetech (Table 7.2). Rhetech reported significant processability difficulties for blends containing COC. Nanobiomatters (NBM, Valencia, Spain) was also chosen to evaluate their proprietary treated nanoclays compounded into PP and PP±COC blends. They initially considered three different nanoclays: NanobioterÕ 202 A1.41, NanobioterÕ 202 A1.49, and NanobioterÕ 404 C1.33, and then added a fourth for consideration, NanobioterÕ 434 C1.33. Both of the 202-grade clays are food contact compliant organic-modified montmorillonite clays (oMMTs). The 400series grades are also food contact compliant and specifically developed for polyolefins so as to require no compatibilizers. The 404-grade shows extremely enhanced thermal stability in the products and has been designed for packaging applications in the food contact layer since it is believed not to affect the organoleptic properties of the packaged contents. Full characterization of the NBM products is proprietary at this writing (cf. European patent application EP 1,985,585 A1, Lagaron Cabello et al., 2008). NBM encountered similar processability difficulties for blends containing COC. Based on the process difficulties encountered by both compounders, the COC-blended resins were not evaluated for cat film MVTR. Table 7.3 summarizes those blends (nano-PP with compatibilizer) that were evaluated. With fundamentally identical experiences at two independent compounders, the conclusion is that ethylene±norbornene copolymers are not subject to compatibilization with PP using available materials. Indeed, the miscibility limitations of these copolymers as noted by Corinne et al. (1999) seem to apply to the experimental systems. The rigidity and steric interactions of the norbornene units along the copolymer chain prevent production of nanocomposites based on extrusion blending of such materials at this time.

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Table 7.3 Nano-polypropylene blends with compatibilizer Product type and supplier

PP Compatibilizer Clay (Basell) (Westlake) (S. Clay)

Grade

SA-861

Epolene G-3003

Cloisite 20A

92 89 89

0 5 6

6 8

Sample NBM-1 Sample Rhetech-1 Sample Rhetech-2

Clay MVTR (NBM) improvement 404 C1.33 8

‡27% ÿ5% ÿ1%

A literature search for reports of a successful nanocomposite material based on ethylene±norbornene copolymers discovered very few (Ravasio et al., 2009; Bredeau et al., 2007). Both used in situ polymerization to incorporate multiwalled carbon nanotubes. Work by Bredeau et al. proceeded to assess improved physical properties of EVA (ethylene vinyl acetate copolymers) achieved by melt-blending it with the nanocomposite, but no packaging-related properties were addressed. Composites from both teams demonstrated excellent dispersion of filler, suggesting future possibilities in the area of enhanced MVTR performance of ethylene±norbornene copolymers. Table 7.3 also indicates the limits of the compatibilizing Epolene. Rhetech blends with greater clay loadings than those used by Schirmer et al. (2008) have slightly higher MVTR rates than neat PP, while the NBM blend with the proprietary compatibilizer incorporated into the organoclay, had a 27% reduction in permeation at 8% clay loading.

7.6

Future trends

The future role for single-site polymers in nanofilled food packaging materials is promising. Nanocomposites are already finding a place in food packaging. They offer the promise of key benefits, oxygen barrier improvement and moisture barrier improvement, and they work with generally low-cost materials that process well in existing processing equipment for film extrusion. They also bring along necessary food contact compliance in most cases. The needs are relatively simple: compounding low-cost masterbatch materials and assuring reliable performance levels independent of processing equipment. And so, the opportunities are attractive. Ideally, families of compatibilizers appropriate to the performance expectations of various primary polymers will be offered. These would seem to be needed in relatively small percentages in a given functional packaging material, so the cost of managing and maintaining such a broad product line can be accommodated in the market prices. Compatibilizers may also provide other functions that enhance value to the packaging material. Better adhesion, stiffness, heat resistance, strength, and

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surface properties are well within reach. By way of example, the maleic anhydride modification of single-site polymers for increasing their compatibilizing function for nanocomposites is chemically identical to the modification of ethylene or propylene copolymers that act as `tie' resins. Coextruded films require such products to adhere disparate resins (e.g. Nylon-6 and LDPE) in their multilayer construction. Separate extruders and die channels direct the tie resins required to their designated position in the coextrusion. A compatibilizer that can enhance the barrier performance of a nanolayer while it binds it to an adjacent layer in a coextrusion not only eliminates a costly structural component, but also simplifies the manufacturing equipment and process needed to produce functional packaging material. Importantly, the main principles for using modified single-site polymers to enhance nano-composite performance are understood. The challenge becomes one of combining single-site polymer design latitude with an appreciation of working knowledge of the principles that control successful and repeatable compatibilization of nanoclays into target polymers.

7.7

Sources of further information and advice

Research on applications of related technology is presented annually by the Society of Plastics Engineers (Newtown, CT, USA: www.4spe.org) at its Annual Technical Conference (Antec) as well as various topical conferences sponsored by product and market groups (e.g. the Composites and Flexible Packaging Divisions). These reports from academia and industry are searchable and available from the website. The Technical Association of the Pulp and Paper Industry (TAPPI) has a focus group, the `PLACE' (Polymers. Laminations, Adhesives, Coatings and Extrusion) Division, with a more market-oriented approach to technology applications in food packaging. Papers presented at its semiannual conferences are available online at www.tappi.org. The book Polymeric Nanocomposites: Theory and Practice (S.N. Bhattacharya, R.K. Gupta, and M.R. Kamal, 2008, Hanser (Munich), 383 pp.) provides an excellent overview of the thermodynamic and physical constraints on nanocomposite chemistry.

7.8

References

Bhowmick, A. K. (2008), Current Topics in Elastomers Research, CRC Press, Taylor & Francis, Boca Raton, FL, 1104 pp. Boggioni, L., Zampa, C., Ravasio, A., Ferro, D. and Tritto, I. (2008), Propene± norbornene copolymers by C2-symmetric metallocene rac-Et(Ind)2ZrCl2: Influence of reaction conditions on reactivity and copolymer properties, Macromolecules, 41(14), 5107±5115. Bredeau, S., Boggioni, L., Bertini, F., Tritto, I., Monteverde, F., Alexandre, M., Dubois,

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P. (2007), Ethylene±norbornene copolymerization by carbon nanotube-supported metallocene catalysis: Generation of high-performance polyolefinic nanocomposites, Macromol. Rapid Commun., 28(7), 822±827. Birell, G. B. (1972), Preparation of linear alpha olefin polymers having broad molecular weight distribution, US Patent 3,678,025. Corinne, D., Dickinson, L.C., Freed, K. F. and Dudowicz, J. (1999), Molecular factors affecting the miscibility behavior of cycloolefin copolymers, Macromolecules, 32, 7781±7789. Ishimaru, N., Tsutsui, T., Toyota, A. and Kawashi, N. (1987), Cyclo-olefinic random copolymer, olefinic random copolymer, and process for producing cyclo-olefinic random copolymers, European Patent EP 0283164 (see also Patent JP 54152/97). Kaminsky, W. (1988), Process for producing an olefin, European Patent EP 0304671 B1, 30 July 1988. Karian, H., Lan, T., Logsdon, J. and Staropoli, V. (2008), A study of the interaction between nanocomposites and titanium dioxide pigment in thin-film applications, Society of Plastics Engineers Polyolefins and Flexible Packaging Conference, 23 pp. Lagaron Cabello, J. M., Gimenez Torres, E. and Cabedo Mas, L. (2008), Method for producing nanocomposite materials for multi-sectorial applications, European Patent Application EP 1985585 A1, 29 October 2008. Lai, S.-M., Chen, W.-C., Liao, Y.-C., Chen, T.-W., Shen, H. F. and Shiao, Y. K. (2003), Compatibilization of Nylon 6 nanocomposites/ABS blends using functionalized metallocene polyolefin elastomer, Society of Plastics Engineers, Antec Proceedings, pp. 2286±2291. Printpack (2010), Polyolefins Nanocomposites Report, US Army Contract W911QY-08C-0132, 21 pp. Ravasio, A., Boggioni, L., Tritto, I., D'Arrigo, C., Perico, A., Hitzbleck, J. and Okuda, J. (2009), A non-PFT (polymerization filling technique) approach to poly(ethyleneconorbornene)/MWNTs nanocomposites by in situ copolymerization with scandium half-sandwich catalyst. J. Polymer Sci.: Part A: Polymer Chemistry, 47, 5709± 5719. Schirmer, S., Ratto, J., Froio, D., Thellen, C. and Lucciarini, J. (2008), Nanocomposite polypropylene film for food packaging applications, Society of Plastics Engineers, Antec Proceedings, ANTEC-0369-2008.R2. Sinn, H. and Kaminsky, W. (1980), `Living polymers' on polymerizations with extremely productive catalysts, Angew. Chem. Int. Ed. Engl., 19, 390±392. Small, C. M., McNally, G. M., Murphy, W. R. and Lim, C.-S. (2006), Polyethylene nanocomposite films incorporating LDPE/nanoclay masterbatch, Society of Plastics Engineers, Antec Proceedings, pp. 1052±1056. Tatarka, P. D. (2008), Polyolefin film enhancement using cyclic olefin copolymers for retort applications, Society of Plastics Engineers, Polyolefins and Flexible Packaging Conference, 43 pp. Welborn, H. C. (1987), Supported polymerization catalyst, US Patent 4,701,432, 12 pp.

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Advances in polymeric materials for modified atmosphere packaging (MAP) T . K . G O S W A M I , Indian Institute of Technology, India and S . M A N G A R A J , CIAE, India

Abstract: Modified atmosphere packaging (MAP) is one of the methods used to preserve food materials for a short or a long time depending on the end use, food material, and selection of the packaging material. This chapter deals in detail with the different aspects of MAP, packaging materials and storage systems with packaging. Key words: modified atmosphere packaging, packaging materials, design of packaging materials, nanotechnology, quality of food.

8.1

Introduction

The annual world production of fruits and vegetables has reached about 1.4 billion tonnes. Approximately 900 Mt of vegetables and 500 Mt of fruits are produced in the world annually (http://www.lol.org.ua/eng/showart.php). Vegetable production is persistently increasing and the same trend is observed for fruit production. The average yearly growth of vegetables (4.2% p.a.) was almost double that of fruits (2.2% p.a.) during 1980±2004 (http:// faostat.fao.org). About 10% of fruits grown in the world are external trade items; this figure is much smaller for vegetables, around 3±4%. Since 2004, the fruit and vegetable market has been one of the fastest growing of all agricultural markets. Global fruit and vegetable consumption increased by an average of 4.5% per annum. This was higher than the world population growth rate, meaning that the global per capita consumption of fruit and vegetables has also increased. According to the World Health Organization, for the prevention of chronic diseases such as heart diseases, cancer, diabetes and obesity, fruit and vegetable consumption should be at least 400 g per day per capita and around 50% of the countries reached this level (http://www.who.int/dietphysicalactivity/ media/en/gsfs_fv.pdf). However, the post-harvest losses of fruits and vegetables are still very considerable. The estimated annual loss of fruits and vegetables due to inadequate facilities and improper technologies of handling, packaging and storage is in the range of 25±40% of the total production (Salunkhe and Kadam 1995).

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Fruits and vegetables are important sources of carbohydrate, protein, organic acid, dietary fibers, vitamins and minerals for human nutrition and are considered as integral part of the dietary system (Irtwange 2006). Hence fresh fruits and vegetables have always a good market demand. However, they are perishable commodities, which generally have a short storage life, and they lose their freshness shortly after harvest. The high post-harvest losses as well as the market demand for fresh fruits and vegetables even during lean periods have necessitated the development of various storage technologies to preserve these commodities in pristine condition for extended periods. Protection of fruits and vegetables from mechanical damage and microbial infection keeps them in sound condition; however, it does not increase the shelflife of the commodities beyond their normal season. This is because fresh fruits and vegetables are still living and their metabolic processes continue even after harvest. However, their metabolism is not identical with that of the parent plant growing in its original environment, and therefore they are subjected to physiological and pathological deterioration and losses (Rhodes 1980; Giusti et al. 2008; Martins et al. 2008). Loss means any change in the availability, edibility, wholesomeness or quality of the food that prevents it from being consumed by the public (Fallik et al. 2002). Causes of losses could be biological, microbiological, chemical or biochemical reactions, mechanical, physical, physiological and psychological. Microbiological, mechanical and physiological factors cause most of the losses in perishable crops (Kader 1992). Other causes of losses, according to Fallik et al. (2002), may be related to inadequate harvesting, packaging and handling skills; lack of adequate containers for the transport and handling of perishables; inadequate storage facilities to protect the food; inadequate transportation to move the food to the market before it spoils; inadequate refrigerated storage; or inadequate drying equipment or a poor drying season. Traditional processing and marketing systems can also be responsible for high losses, and legal standards can affect the retention or rejection of food for human use, being unduly lax or strict. Losses may occur anywhere from the point where the food has been harvested or gathered up to the point of consumption, that is, during harvest, preparation, preservation, processing, storage and transportation (Irtwange 2006). In this respect respiration is considered to be the major catabolic process, which leads to the natural ripening, senescence and subsequent deterioration (Saltveit 2005). Quality optimization and loss reduction in the post-harvest chain of fresh fruits and vegetables are the main objective of post-harvest technology. Temperature control and modified atmosphere are the two most important factors in prolonging shelf-life (Fonseca et al. 2002a). The primary factors in maintaining quality and extending the post-harvest life of fresh fruits and vegetables are harvesting at optimum maturity, minimizing mechanical injuries using proper sanitation procedures, and providing the optimum temperature and relative humidity during all marketing steps. Secondary factors include modification of

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oxygen, carbon dioxide and/or ethylene concentrations in the atmosphere surrounding the commodity to levels different from those in air. This is referred to as controlled atmosphere (CA) or modified atmosphere (MA) storage systems. CA implies a greater degree of precision than MA in maintaining specific levels of O2, CO2 and other gases (Mahajan 2001). The principal roles of food packaging are to protect the food products from outside influences and damage, to contain the food, and to provide consumers with ingredients and nutritional information. The goal of food packaging is to contain food in a cost-effective way that satisfies industry requirements and consumer desires, maintains food safety and minimizes environmental damage (Marsh and Bugusu 2007). In modified atmosphere packaging (MAP), a definite quantity of commodity is sealed in selected polymeric film packages. After a certain period of time, steady-state conditions are established inside an intact polymeric film package once the respiration rate of the produce matches with the permeability of packaging films to O2 and CO2. Oxygen inside the package is consumed by the produce as it respires and an approximately equal amount of CO2 is produced. The reduction in O2 concentration and increase in CO2 concentration create a gradient causing O2 to enter and CO2 to exit the package. Initially, however, the gradient is small and the flux across the package is not sufficient to replace the O2 that was consumed or to drive out all of the CO2 that was generated. Thus, inside the package, the O2 content decreases and the CO2 content increases. As this MA is created inside the package, respiration rates start to fall in response to reduction of the O2 content and elevation of the CO2 content. Thus, new equilibrium concentrations of the gases surrounding the fruit are established. When the rate of consumption of O2 by the commodity equals the rate of O2 permeation into the package, and the rate of evolution of CO2 equals the rate of CO2 permeation out of the package, steady-state equilibrium is achieved, which is called EMAP. Thus MAP of a commodity refers to the technique of sealing actively respiring produce in polymeric film packages to modify the O2 and CO2 levels within the package atmosphere. It is often desirable to generate an atmosphere low in O2 and/or high in CO2 to influence the metabolism of the product being packaged, or the activity of decay-causing organisms to increase storability and/or shelf-life (Kader et al. 1989; Gorris and Peppelenbos 1992; Church and Parsons 1995; Beaudry 2000; Iqbal et al. 2009). In addition to atmosphere modification, MAP vastly improves moisture retention, which can have a greater influence on preserving quality than O2 and CO2 levels. Furthermore, packaging isolates the product from the external environment and helps to ensure conditions that, if not sterile, at least reduce exposure to pathogens and contaminants (El-Goorani and Sommer 1981; Farber 1991; Kader 1997; Saltveit 1997; Mahajan et al. 2007; Lu Shengmin 2009). MAP is the replacement of air in a pack with a single gas or a mixture of gases, either naturally or artificially. The proportion of each component is fixed when the mixture is introduced to the package. No further control is exerted over

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the initial composition, and the gas composition is likely to change with time owing to the diffusion of gases into and out of the product, the permeation of gases into and out of the pack, and the effects of product and microbial metabolism (Cussler 1984; Tolle 1962; Kader et al. 1989; Church 1994; Church and Parsons 1995). MAP technology, which utilizes only the natural components of air, has achieved public acceptance due to these two trends. MAP has the advantages that synthetic chemicals are not used, no toxic residue is left, and there is little environmental impact, particularly if the plastic films used can be recycled. Recent advances in the design and manufacture of polymeric films with a wide range of gas-diffusion characteristics have also stimulated interest in MAP of fresh produce. In addition, the increased availability of various absorbers of O2, CO2 (Kader et al. 1989), water vapor (Shirazi and Cameron 1992) and C2H4 (Ben-Arie and Sonego 1985; Kader 1995; Saltveit 1997; Gorris and Tauscher 1999) provides possible additional tools for manipulating the microenvironment of MAP. Nearly all products are packaged at some point in their life-cycle. Plastic films are widely used in packaging, and continue to grow in use as more and more applications switch to flexible packages such as modified atmosphere packaging. In these packages, plastic films may be used alone or in combinations to serve the basic packaging functions of containment, protection, communication and utility in the delivery of quality products to the consumer. MAP is a dynamic process where respiration of the product and permeation of gases through the packaging film occur simultaneously. The composition of the atmosphere within a package results from the interaction of a number of factors that include the permeability characteristics of the package, the respiratory behavior of the plant material, and the environment (Smith et al. 1987a; Kader et al. 1989; Davies 1995; Ben-Yehoshua et al. 1994; Jayas and Jeyamkondan 2002; Mahajan et al. 2007). The films making up the package are selected to have specific permeability characteristics, and changes in these characteristics over time, temperature, and humidity follow known physical laws (Exama et al. 1993; Abdel-Bary 2003; Mahajan et al. 2007). The environment can be controlled to provide specific conditions. In contrast to these known and controllable factors are the often unknown and uncontrollable responses of the plant material. The plant species, cultivar, cultural practices, stage of development, manner of harvest, tissue type, and post-harvest handling all contribute to and influence the response of the material to the generated atmosphere. The scope of plant responses can be further modified by initial gas flush of the package before sealing and inclusion of chemical treatments to slow unwanted processes or reduce decay. Each of these components of the packaging process can be examined separately to better understand how each contributes to packaging strategies (Zagory and Kader 1988; Kader 1995; Irtwange 2006; Fernandez-Trujillo et al. 2008). Although the application of polymeric films for MAP is most often found in flexible package structures, they may also be used as components in rigid or

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semi-rigid package structures, for example as a liner inside a carton, or as lidding on a cup or tray. The plastic film used in modified atmosphere packaging may be low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), EVOH as high barrier interlayer, high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polyester, i.e. polyethylene terephthalate (PET), polyvinylidene chloride (PVDC), polyamide (nylon) and other suitable films (Karel et al. 1975; Sacharow and Griffin 1980; Crosby 1981; Salame 1986; Kader et al. 1989; Parry 1993; Exama et al. 1993; Prasad 1995; Abdel-Bary 2003; Ahvenainen 2003; Massey 2003; Del Nobile et al. 2007; Marsh and Bugusu 2007). Although an increasing choice of packaging materials is available to the MAP industry, most packs are still constructed from four basic polymers: polyvinyl chloride (PVC), polyethylene terephthalate (PET), polypropylene (PP) and polyethylene (PE) for packaging of fruits and vegetables (Kader et al. 1989; CalderoÂn and Barkai-Golan 1990; Exama et al. 1993; Prasad 1995; Ahvenainen 2003; Marsh and Bugusu 2007). Polystyrene has been used but polyvinylidene, polyester and nylon have such low gas permeabilities that they would be suitable only for commodities with very low respiration rates. However, perforating the films can expand their use to many commodities. Recent advances in the technology of manufacturing the polymeric films have permitted tailoring films for gas permeability needs for some fruits, vegetables and their products. Modified atmosphere packaging is most commonly used for highly perishable, high value commodities such as apple, cherry, strawberry, litchi, raspberry, broccoli, asparagus, mushroom, capsicum, fig, etc., and for freshly cut (minimally processed) fruits and vegetables (Church and Parsons 1995; Gorny 1997; Chen et al. 1981; Chen and Wang 1989; Roy et al. 1995; Granado-Lorencio et al. 2008; Sivakumar et al. 2008).

8.2

Modified atmosphere packaging (MAP)

8.2.1

History of MAP

MAP was first recorded in 1927 as a means of extending the shelf-life of apples by storing them in atmospheres with reduced O2 and increased CO2 concentrations. In the 1930s it was used as modified atmosphere storage to transport fruit and beef carcasses in the holds of ships by increasing the CO2 concentrations for long-distance transport, and it was observed to increase the shelf-life by up to 100% (Davies 1995). However, the technique was not introduced commercially for retail packs until the early 1970s in Europe. The primary limitation of MAP application in the early studies was technical in nature: specifically, the lack of consistent control of O2 concentration levels in the package. Since then, the types and properties of polymers have increased to provide a wider range of gas permeability, tensile strength, flexibility, printability, and clarity. As a result, successful MAP systems have been developed

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for a number of commodities. In the UK, Marks and Spencer introduced MAP for meat in 1979; the success of this product led years later to the introduction of MAP for bacon, fish, sliced cooked meats and cooked shellfish. Other food manufacturers and supermarket chains followed, resulting in a sharply increased availability of MAP food products reflecting the increase in consumer demand for food with a longer shelf-life and less use of preservatives. MAP techniques are now used on a wide range of fresh or chilled foods, including raw and cooked meat, fish and poultry, fresh pasta, fresh and cut fruits and vegetables, and more recently coffee, tea and bakery products (Church and Parsons 1995).

8.2.2

Definition of MAP

Modified atmosphere packaging emerged in the 1960s as a new technology that extends the storage life of perishable agricultural produce and reduces its spoilage and decay (Henig and Gilbert 1975). MAP is one of the food preservation methods that maintain the natural quality of food products in addition to extending the storage life. In other words MAP is a technique used for prolonging the shelf-life of fresh or minimally processed foods by changing the composition of the air surrounding the food in the package. The use of MAP reduces the respiration rate and activity of insects or microorganisms, provides control of fruit and vegetable ripening, retardation of senescence, or browning in cut produce, and ultimately prolongs the shelf-life (Kader 1995; Fonseca et al. 2002b). In the literature, the terms modified atmosphere and controlled atmosphere are used interchangeably. They both differ based on the degree of control exerted over the atmosphere composition. In MAP the modification of the atmosphere inside the package is achieved by the natural interplay between two processes, the respiration of the products and the permeation of gases through the packaging (Smith et al. 1987b; Mahajan et al. 2007). Also, the gas composition is modified initially and changes dynamically depending on the respiration rate of the food product and permeability of the film surrounding the food product (Jayas and Jeyamkondan 2002). In CA storage the atmosphere is modified and its composition is precisely controlled according to the specific requirements of the food product throughout the storage period (Raghavan and Gariepy 1985; Parry 1993; Mahajan 2001).

8.2.3

Principles of MAP

Modified atmosphere packaging of fresh produce relies on modification of the atmosphere inside the package, achieved by the natural interplay between two processes, the respiration of the product and the transfer of gases through the packaging, that leads to an atmosphere richer in CO2 and poorer in O2; and it depends on the characteristics of the commodity and the packaging film (Fonseca et al. 2002a; Mahajan et al. 2007). This atmosphere can potentially

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reduce respiration rate, ethylene sensitivity and production, ripening, softening and compositional changes, decay and physiological changes, namely oxidation (Kader et al. 1989; Parry 1993; Kader 1995; Saltveit 1997; Gorris and Tauscher 1999). MAP involves the exposure of produce to the atmosphere generated in a package by the interaction of the produce, the package and the external atmosphere. The initial atmosphere may be either air or a gas mixture. Different additives that may affect the atmosphere may be introduced into the package before it is sealed. Packaging fresh produce in polymeric films results a commodity-generated MA. Atmosphere modification within the package depends on film permeability, commodity respiration rate and gas diffusion characteristics of the commodity, and weight of commodity, surface area, initial free volume and atmospheric composition within the package. Temperature, relative humidity and air movement around the package can influence the permeability of the film. Temperature also affects the metabolic activity of the commodity and consequently the rate of attaining the desired MA. All these factors must be considered in developing a mathematical model for selecting the most suitable film for each commodity (Parry 1993).

8.2.4

Objective/goal of MAP

The objective of MAP design is to define conditions that will create the atmosphere best suited for the extended storage of a given produce while minimizing the time required to achieve this atmosphere. In other words, the goal of MAP is to achieve the equilibrium concentrations of O2 and CO2 within the package within the shortest possible time because of the interaction of the produce, the package and the external atmosphere; and these concentrations lie within the desired level required for maximum possible storage life of the commodity. The equilibrium concentration of O2 and CO2 achieved within the package for a packaging system needs to be remained constant throughout the period of storage to continue the respiration rate and the rate of all metabolic processes at a minimum possible rate for maintaining freshness and extending the shelf-life of stored commodity (Das 2005; Kader et al. 1989; Mahajan et al. 2007). Matching the film permeation rate for O2 and CO2 with the respiration rate of the packaged produce can do this. As different products vary in their behavior and as MA packages will be exposed to a dynamic environment, each package has to be optimized for specific demands (Saltveit 1993; Chau and Talasila 1994; Jacxsens et al. 2000; Mahajan et al. 2007). An improperly designed MAP system may be ineffective or may even shorten the storage life of a commodity. If the desired atmosphere is not established rapidly, the package will have no benefit; if O2 and/or CO2 are not within the recommended range, the product may experience serious alterations and its storage life may thereby be shortened.

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Effect of MAP

The effects of MAP are often based on the observed slowing of plant respiration in a low O2 environment. As the concentration of O2 inside the package falls below about 10±12%, respiration starts to slow (Saltveit 1993, 1997; Gorris and Tauscher 1999). This suppression of respiration continues until O2 reaches about 2±5% for most produce. If O2 gets lower than 2±5% (depending on product and temperature), fermentative metabolism replaces normal aerobic metabolism and off-flavors, off-odors and undesirable volatiles are produced (Kader et al. 1989; Farber 1991). Similarly, as the concentration of CO2 increases above the atmospheric level, a suppression of respiration rate, ethylene (C2H2) production and sensitivity to ethylene, and suppression of activities of microorganisms and fungal/bacterial growth, result for some commodities (Daniels et al. 1985; Dixon and Kell. 1989; Farber 1991). Reduced O2 and elevated CO2 concentrations together can reduce the rate of respiration more than by either alone. The diminution of enzymatic activities by providing low temperature and low O2 and high CO2 concentrations in general reduces the utilization rate of the substrate (i.e. carbohydrate, organic acid and other reserves) and increases the post-harvest life of the fruits and vegetables beyond its normal span (Mahajan and Goswami 2001). Others consider that elevated CO2 might inhibit C2H2 production and/or suppress plant tissue sensitivity to the effects of the ripening hormone ethylene rather than having a direct effect on the respiration process (Kubo et al. 1989). For those products that tolerate a high concentration of CO2, suppression of the growth of many bacteria and fungi results at greater than 10% CO2 (Kader et al. 1989).

8.2.6

Utility of MAP

MAP is a multidisciplinary technology of maintaining freshness and extending shelf-life that utilizes basic principles of chemistry, physics, plant physiology and pathology, microbiology, food science, engineering, and polymer chemistry. Better understanding of this wide scope will promote implementation of the technology. MAP technology has developed rapidly over the past decade (Lioutas 1988; Kader et al. 1989; Gorris and Peppelenbos 1992; Lu Shengmin 2009). This rapid development is due to two contradictory trends affecting modern post-harvest handling of fruits, vegetables, and other perishable produce. Firstly, food distribution in developed countries now involves many perishable food items, some of which are minimally processed, such as shredded lettuce, carrot or celery sticks, and fresh salad mixes, which increases the perishable nature and susceptibility to decay and desiccation, and consequently increases the need for quality and decay control measures (Stewart and Uota 1971; Siripanich and Kader 1985). Secondly, there is growing anxiety among consumers about the use of synthetic chemicals to protect food from pathogens and pests to extend the life of perishable produce. One of the consequences of

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this public anxiety is that more and more synthetic food protectants such as certain fungicides and pesticides are being banned. MAP technology, which utilizes only the natural components of air, has achieved public acceptance due to these two trends. MAP has the advantages that synthetic chemicals are not used, no toxic residue is left, and there is little environmental impact, particularly if the plastic films used can be recycled. Recent advances in the design and manufacture of polymeric films with a wide range of gas-diffusion characteristics have also stimulated interest in MAP of fresh produce. In addition, the increased availability of various absorbers of O2, CO2 (Kader et al. 1989), water vapor (Shirazi and Cameron 1992), and ethylene (C2H4) (Ben-Arie and Sonego 1985) provides possible additional tools for manipulating the microenvironment of MAP. There is extensive literature on the benefits of MAP and the dramatic extensions of shelf-life for various foods (Lioutas 1988; Kader et al. 1989). However, there are few papers dealing with the microbiological safety needed for successful MAP implementation (Genigeorgis 1985; Hintlian and Hotchkiss 1986). Future approaches must put consumer safety first and freshness second.

8.2.7

Applications of MAP

There are many advantages of MAP of fruits and vegetables, but the most obvious one must be the extension of shelf-life. By decreasing the amount of available O2 to the produce, the respiration rate and the rate of all metabolic processes are correspondingly decreased. This results in delayed ripening and senescence, which may be seen as chlorophyll retention, delayed softening and the prevention of discoloration (Gorris and Peppelenbos 1992). The extension of shelf-life is most noticeable with prepared products; this combined with ease of use for the consumer makes a MAP pack an attractive form of product presentation (Church 1994). Additionally, MAP packs reduce the quantity of water vapor lost from the produce (Church and Parsons 1995). Although fresh fruits and vegetables have been removed from the parent plant and from their normal nutrient supplies, they will continue to respire. Under normal aerobic conditions the rate of respiration of a product may be determined by either O2 uptake rate or CO2 production rate. A high respiration rate is usually associated with a short shelf-life. When the rate of packaging film transmission of O2 and CO2 equals the rate of respiration of the products, an equilibrium concentration of both gases is established (Church and Parsons 1995). The equilibrium value attained depends on the respiration rate of the product, the fill weight of the product, and the film surface available for gas exchange. The respiration rate of the product is influenced by storage temperature, produce variety, growing area and condition, and injury to the product (Parry 1993). Current applications of MAP technologies include long-term storage of apples, pears, kiwifruits, potatoes, sapota, oranges, cabbages and Chinese

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cabbages; temporary storage and/or transport of strawberries, bush berries, cherries, bananas, litchi, guavas, mushrooms, tomatoes, etc.; and other commodity and retailing of some cut or sliced (minimally processed) vegetables. MAP facilitates maintenance of the desired atmosphere during the entire postharvest handling time between harvest and consumption (Ayers and Pierce 1960; Daun et al. 1973; Zagory and Kader 1988; Church 1994; Talasila et al. 1992; Mahajan et al. 2007).

8.2.8

MAP gases

Various MAP systems have been developed and investigated for increasing the shelf-life of fresh commodities. Common gases used in MAP are CO2, O2 and N2. Carbon dioxide is bacteriostatic. Nitrogen is an inert gas; it does not possess any bacteriostatic effect. It is used as a filler gas in the MA gas mixture. The inhibitory effect of CO2 increases with a decrease in temperature (Mahajan and Goswami 2001).

8.2.9

Advantages and disadvantages of MAP

The beneficial and detrimental effects of MAP have been extensively reviewed (Day 1996; Isenberg 1979; Smock 1979; Kader 1980; Lioutas 1988; Kader et al. 1989; Dilley 1990; Parry 1993; Ben-Yehoshua et al. 1994; Phillips 1996; Zagory 1998; Irtwange 2006; Mahajan et al. 2007; Sivakumar et al. 2007) and are summarized below. Advantages 1. MAP maintains freshness and extends shelf-life from several days to several weeks, compared to conventional storage. 2. Reduction of O2 and increment of CO2 environment suppresses the respiration rate of the commodity, thereby slowing vital processes and prolonging the maintenance of post-harvest quality. 3. Reduction of respiration rate, loss of moisture, production of metabolic heat, yellowing/browning decay, and ethylene sensitivity and production. 4. Delay of ripening. 5. Reduction of weight loss, desiccation/water loss and shriveling. 6. Reduction of physiological injury, disorder and pathological deterioration. 7. Quality advantages such as color, moisture, flavors and maturity retention. 8. Reduction of fungal growth and diseases is common. 9. Retardation of softening and compositional changes. 10. Alleviation of chilling injury is common. 11. Increased shelf-life allowing less frequent loading of retail display in shelves.

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12. Improved presentation and clear visibility of product all around the package. 13. Little or no chemical preservatives used. 14. Centralized/semi-centralized packaging option is possible. 15. Expanded distribution area and reduced transport costs due to less frequent deliveries is possible. 16. Reduction of labor and waste at the retail level. 17. Excellent branding options. 18. Reduction of handling and distribution of unwanted or low-grade produce. 19. Quality advantages transferred to the consumer. 20. Reduction in production and storage cost due to better utilization of labor, space and equipment. 21. MAP has great advantage in developing countries because it can be done economically by hand, saving the high cost of new machinery. Additionally, the need there for such a technique is much greater because of the dearth of refrigerated storage. Disadvantages 1. Requirement for additional investment in machinery and labor in the packaging line. 2. Risks of spoilage of produce may occur due to improper packaging or temperature abuse. 3. Possible occurrence of new risks of microbiological safety due to possible development of anaerobic pathogenic flora. 4. Plastic films may be environmentally undesirable unless effective recycling is installed. 5. MAP technology is still unavailable for most produce. 6. No particular standard is available for MA packaging, because the intrinsic properties of the commodity vary greatly with cultivar, place of cultivation, maturity stage, etc., and the permeability of the films varies with the manufacturing company and process, etc.

8.3

Physiological factors affecting shelf-life of fresh produce

Shelf-life may be defined as the time period from harvest to manufacture to consumption throughout which a food product remains safe and maintains its desired/recommended harvest/production quality. Shelf-life is affected by intrinsic factors such as respiration rate, biological structure, ethylene production and sensitivity, transpiration, compositional changes, developmental stages and physiological breakdown (Wills et al. 1981, 1989; Irtwange 2006; Martinez-Romero et al. 2007).

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Respiration rate

The plant tissues in fresh-cut produce are still living and continue to respire even after harvest, deriving energy primarily through the process of respiration. Respiration is a catabolic process that involves the consumption, using atmospheric oxygen, of carbohydrates, fats, proteins and organic acids in the plant tissue to form various intermediate compounds and eventually CO2, water and metabolic energy (Meyer et al. 1973; Kader 1987; Zagory 1998; Fonseca et al. 2002b). The energy produced by the series of reactions comprising respiration can be captured as high-energy bonds in compounds used by the cell in subsequent reactions and biochemical processes, or lost as heat (Andres et al. 2007). The energy and organic molecules produced during respiration are used by other metabolic processes to maintain the health of the commodity. Heat produced during respiration is called vital heat and contributes to the refrigeration load that must be considered in designing storage rooms. For a biochemist, respiration is the oxidative breakdown of complex substrate molecules normally present in the plant cells such as starch, sugars and organic acids to simple molecules such as CO2 and water (Saltveit 2005). Under normal atmospheric conditions, aerobic respiration consists of oxidative breakdown of organic reserves (carbohydrate) to simple molecules, including carbon dioxide as described by the simplified equation C6H12O6 + 6O2 ! 6CO2 + 6H2O + energy (heat). During the respiration process there is a loss of stored food reserved in the commodity. This leads to hastening of senescence because the reserves that provide energy are exhausted. Also, use of substrates in the respiration can result in loss of food reserves in the tissue and loss of quality (especially sweetness) and food value to the consumer. The rate of deterioration of harvested commodities is proportional to the respiration rate (Fallik et al. 2002). In general, the storage life of commodities varies inversely with the rate of respiration. This is because respiration supplies compounds that determine the rate of metabolic processes directly related to quality parameters, e.g. firmness, sugar content, aroma, flavors, etc. Commodities and cultivars with higher rates of respiration tend to have shorter storage life than those with low rates of respiration. Storage life of asparagus, mushrooms, broccoli, lettuce, peas, spinach, and sweetcorn, all of which have high respiration rates, is short in comparison to that of apples, cranberries, limes, onions, and potatoes, all of which have low respiration rates. Commodities are classified according to their respiration rates (Church and Parsons 1995; Irtwange 2006; Saltveit 2005) and are so listed in Table 8.1. The respiration and ethylene production rates of fruits and vegetables at various temperatures are summarized in Tables 8.2 and 8.3. Furthermore, when fruits and vegetables are cut, sliced, shredded or otherwise processed, their respiration rates increase. This is probably due to the increased surface area exposed to the atmosphere after cutting, which allows oxygen to diffuse into the interior cells more rapidly, and to the increased metabolic activities of injured cells (Zagory 1998).

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Table 8.1 Classification of horticultural commodities according to respiration and ethylene production rates Class

Respiration rates Ranges at 5ëC (mg CO2/kg-h)

Very low

Commodities

Ethylene production rates Ranges at 20ëC (l C2H4/kg-h)

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<5

Dates, nuts, dried fruits and vegetables

< 0.1

Low

5±10

Apple, celery, citrus fruits, garlic, grape, kiwifruit, onion, persimmon, pineapple, potato, sweet potato, watermelon

0.1±1.0

Moderate

10±20

1.0±10.0

High

20±40

Apricot, banana, cabbage, cantaloupe, carrot, cherry, cucumber, fig, gooseberry, lettuce, nectarine, olive, peach, pear, pepper, plum, tomato, guava, mango, sapota Strawberry, litchi, blackberry, raspberry, avocado, cauliflower, lima bean

Very high

40±60

Extremely high

>60

Artichoke, beansprouts, broccoli, Brussels sprouts, cut flowers, green onion, snap beans Asparagus, broccoli, raspberry, mushroom, parsley, peas, spinach, sweetcorn

Sources: Deily and Rizvi 1981; Church and Parsons 1995; Saltveit 2005; Irtwange 2006.

10.0±100.0 >100

Commodities

Artichoke, asparagus, cauliflower, cherry, citrus, grape, jujube, strawberry, pomegranate, leafy vegetables, root vegetables, potato, most cut flowers Blueberry, cranberry, cucumber, eggplant, okra, olive, pepper, persimmon, pineapple, pumpkin, raspberry, tamarillo, watermelon Banana, fig, guava, honeydew melon, mango, plantain, tomato

Apple, apricot, avocado, cantaloupe, feijoa, kiwifruit (ripe), nectarine, papaya, peach, pear, plum Cherimoya, mammee apple, passion fruit, sapota

Table 8.2 Summary of respiration and ethylene production rates of some fruits at different temperatures ß Woodhead Publishing Limited, 2011

Commodity

Apple: Fall Summer Apricot Artichoke Asian pear Avocado Banana (ripe) Beets Blackberry Blueberry Cherry Grape, American Grape, Muscadine Grape, Table Grapefruit Guava

Respiration rate (mg CO2/kg-h) at a temperature of 0ëC

5ëC

10ëC

15ëC

20ëC

25ëC

3 5

6 8

9 17

15 25

20 31

na na

6 30 5 na na 5 19 6 8 3 10 3 na na

na 43 na 35 na 11 36 11 22 5 13 7 na na

16 71 na 105 80 18 62 29 28 8 na 13 na 34

na 110 na na 140 31 75 48 46 16 na na <10 na

40 193 25 190 280 60 115 70 65 33 51 27 na 74

na na na na na na na 101 na 39 na na na na

C2H4 production (l C2H4/kg-h)

Varies greatly Varies greatly <0.1 (0ëC) <0.1 Varies greatly >100 (ripe; 20ëC) 5.0 (15ëC) <0.1 (0ëC) Varies; 0.1±2.0 Varies; 0.5±10.0 <0.1 (0ëC) <0.1 (20ëC) <0.1 (20ëC) <0.1 (20ëC) <0.1 (20ëC) 10 (20ëC)

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Kiwifruit (ripe) Litchi Mammee apple Mandarin (tangerine) Mango Nectarine (ripe) Orange Papaya (ripe) Passion fruit Peach (ripe) Pineapple Plum (ripe) Pomegranate Raspberry Sapote Strawberry Tomato

3 na na na na 5 4 na na 5 na 3 na 17 na 16 na

6 13 na 6 16 na 6 5 44 na 2 na 6 23 na na na

12 24 na 8 35 20 8 na 59 20 6 10 12 35 na 75 15

Note: na = data not available; very low is considered to be <0.05 l C2H4 /kg-h. Sources: Biale andYoung 1981; Blanke 1991; Abeles et al.1992; Saltveit 2005.

na na na 16 58 na 18 19 141 na 13 na na 42 na na 22

19 60 na 25 113 87 28 80 262 87 24 20 24 125 na 150 35

na 102 35 na na na na na na na na na 39 na na na 43

75 Very low 400.0 (27ëC) <0.1 (20ëC) 1.5 (20ëC) 5.0 (0ëC) <0.1 (20ëC) 8.0 280.0 (20ëC) 5.0 (0ëC) <1.0 (20ëC) <5.0 (0ëC) <0.1 (10ëC) 12.0 (20ëC) <100 (20ëC) <0.1 (20ëC) 10.0 (20ëC)

Table 8.3 Summary of respiration and ethylene production rates of some vegetables at different temperatures ß Woodhead Publishing Limited, 2011

Commodity

Asparagus Beans: Snap Long Beets Broccoli Brussels sprouts Cabbage Carrot Cauliflower Coriander Cucumber Garlic: Bulbs Fresh peeled Ginger Lettuce: Head Leaf Okra

Respiration rate (mg CO2/kg-h) at a temperature of

C2H4 production (l C2H4/kg-h)

0ëC

5ëC

10ëC

15ëC

20ëC

25ëC

60

105

215

235

270

na

2.6 (20ëC)

20 40 5 21 40 5 15 17 22 na

34 46 11 34 70 11 20 21 30 na

58 92 18 81 147 18 31 34 na 26

92 202 31 170 200 28 40 46 na 29

130 220 60 300 276 42 25 79 na 31

na na na na na 62 na 92 na 37

<0.05 (5ëC) <0.05 (5ëC) <0.1 (0ëC) <0.1 (20ëC) <0.25 (7.5ëC) <0.1 (20ëC) <0.1 (20ëC) <1.0 (20ëC) Very low 0.6 (20ëC)

8 24 na

16 35 na

24 85 na

22 na na

20 na 6

na na na

Very low Very low Very low

12 23 21

17 30 40

31 39 91

39 63 146

56 101 261

82 147 345

Very low Very low 0.5

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Olive Onion Pea: Garden Edible pod Pepper Potato (cured) Radicchio Radish: Topped Bunched with tops Salad green: Rocket salad Lamb's lettuce Southern pea: Whole pods Shelled peas Spinach Sprout (mung bean) Sweet corn Turnip

na 3

15 5

28 7

na 7

60 8

na na

<0.5 (20ëC) <0.1 (20ëC)

38 39 na na 8

64 64 7 12 13

86 89 12 16 23

175 176 27 17 na

271 273 34 22 na

313 na na na 45

<0.1 (20ëC) <0.1 (20ëC) <0.2 (20ëC) <0.1 (20ëC) 0.3 (6ëC)

16 6

20 10

34 16

74 32

130 51

172 75

Very low Very low

42 12

113 67

na 81

na na

na 139

na na

Very low Very low

24 29 21 23 41 8

25 na 45 42 63 10

na na 110 96 105 16

na na 179 na 159 23

148 126 230 na 261 25

na na na na 359 na

Note: na = data not available; very low is considered to be <0.05 l C2H4/kg-h. Sources: Abeles et al.1992; Gorny 1997; Saltveit 2005.

na na Very low <0.1 (10ëC) Very low Very low

180

8.3.2

Multifunctional and nanoreinforced polymers for food packaging

Respiratory quotient

The composition of the commodity frequently determines which substrates are utilized in respiration, the nature of the respiratory process and consequently the value of the respiratory quotient (RQ). RQ is defined as the ratio of CO2 produced to O2 consumed during respiration (Kader et al. 1989; Fonseca et al. 2002a). The RQ value is normally assumed to be 1 if the metabolic substrates are carbohydrates. The total oxidation of 1 mol of hexose consumes 6 mol of O2 and produces 6 mol of CO2. If the substrate is a lipid, the RQ is always lower than unity, because lipids/fats are poor in oxygen and hence require greater amounts of O2 from the atmosphere for respiration. Also the ratio between C and O in lipids is lower than the ratio in carbohydrates. If the substrate is an organic acid, the RQ is greater than unity because organic acids are richer in O2, and hence require less O2 for respiration. Depending upon the substrate being oxidized, RQ values for fresh commodities range from 0.7 to 1.3 for aerobic respiration (Kader 1987; Beaudry et al. 1992; Renault et al. 1994a). C6H12O6 + 6O2 ÿ! 6CO2 + 6H2O Glucose

RQ = 1

C18H36O2 + 26O2 ÿ! 18CO2 + 18H2O Stearic acid RQ = 0.7 C4H6O5 + 3O2 ÿ! 4CO2 + 3H2O Malic acid

RQ = 1.33

The RQ is affected by CO2 fixation, incomplete oxidation and storage conditions. The RQ of apples increased with time from 1.02 to 1.25 at the climacteric peak and subsequently to 1.4. Within the biological range of temperature, the RQ falls with an increase in temperature. However, towards lower temperatures, low temperature breakdown (LTB) increases the RQ, while towards higher temperatures, denaturing of oxidative metabolism affects O2 consumption more than CO2 evolution, which in turn decreases the RQ (Forward 1960). Very high RQ values usually indicate anaerobic respiration in those tissues that produce ethanol. In such tissues, a rapid change in the RQ can be used as an indication of the shift from aerobic to anaerobic respiration. The RQ is much greater than 1.0 when anaerobic respiration takes place. In fermentative metabolism, ethanol production involves decarboxylation of pyruvate to CO2 without O2 uptake. The influence of CO2 concentration on RQ was not observed for apple (Jurin and Karel 1963), although Beaudry (1993) observed an RQ increase in high CO2 concentrations for blueberries. The RQ values depend on both O2 concentration and temperature (Beaudry et al. 1992; Lakakul et al. 1999). The RQ value of blueberries increased as O2 concentrations approached zero and the RQ breakpoint (the lowest O2 concentration that does not induce anaerobic respiration) increased with temperature (Fonseca et al. 2002a). Beaudry et al.

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(1992) explained this latter observation as being due to the fruit skin's permeability not rising as rapidly as O2 consumption for a given temperature change. Thus the risk of anaerobiosis increases with temperature. Various MAP studies have reported values of the RQ indicative of anaerobic respiration (Carlin et al. 1990; Beaudry et al. 1992; Joles et al. 1994). The RQ value for apples at 20ëC remained relatively constant up to 3.5% O2, and when the O2 concentration was decreased to less than 3.5% O2 the RQ increased rapidly (Jurin and Karel 1963). Beit-Halachmy and Mannheim (1992) found an RQ of approximately 1 for mushrooms at 20ëC and at O2 levels greater than 1.5± 2%; below this O2 level, the RQ increased rapidly to a value higher than 6. Kader et al. (1989) reported that the RQ might be affected by ambient gas concentration. That is, as the O2 and CO2 concentrations are changing in a package, the ratio of CO2 produced to O2 consumed may itself be changing. Henig and Gilbert (1975) used the regression analysis of gas concentration changes for tomato in PVC film packages to calculate O2 consumption and CO2 evolution rates under different O2 and CO2 concentrations. They found that RQ values remained constant at about 0.9 in the range of 0±9% CO2, and then a drop to 0.4 was observed with a later increase to 1.4 as CO2 concentration increased.

8.3.3

Factors affecting respiration rate

Factors affecting respiration are broadly classified as external or environmental factors and internal or commodity factors. Respiration is affected by a wide range of environmental factors that include light, chemical stress (fumigants), radiation stress, temperature, atmospheric composition, physical stress, water stress, growth regulators, and pathogen attack. The internal factors affecting rate of respiration are genetic make-up, type and maturity stage of the commodity. The most important factors affecting respiration are temperature, atmospheric composition, physical stress and stages of development (Kader 1987; Hagger et al. 1992; Pal and Buescher 1993; Saltveit 2005; Irtwange 2006). Temperature Without doubt the most important external factor affecting respiration is temperature. This is because temperature has a profound effect on the rates of biological reactions, e.g. metabolism and respiration (Fonseca et al. 2002b, Saltveit 2005). Over the physiological range of most crops, i.e. 0±30ëC (32± 86ëF), increased temperatures cause an exponential rise in respiration. The Van't Hoff Rule states that the velocity of a biological reaction increases two- to threefold for every 10ëC rise in temperature within the range of temperature normally encountered in the distribution and marketing chain (Burzo 1980; Zagory and Kader 1988). At higher temperatures, enzymatic denaturation may occur and reduce respiration rates. If temperatures are too low, physiological injury may

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Multifunctional and nanoreinforced polymers for food packaging Table 8.4 Variation of QR10 with 10ëC rise in temperature QR10

Temperature (ëC) 0 to 10 10 to 20 20 to 30 30 to 40

2.5 to 4.0 2.0 to 2.5 1.5 to 2.0 1.0 to 1.5

Sources: Wills et al.1981; Saltveit 1996, 2005.

occur, which may lead to an increase in respiration rate (Fidler and North 1967, Bhande et al. 2008). For distribution and retail temperatures (0±30ëC) at which MAP is suitable, the effect of low temperature in lowering the biochemical reaction rate is positive. The influence of temperature on respiration rate was first quantified with the Q10R (temperature quotient for respiration) values, which is the respiration rate increase for a 10ëC rise in temperature. It can be expressed as:  10=…T2 ÿT1 † R2 R Q10 ˆ R1 where R2 is the respiration rate at temperature T2 and R1 is the respiration rate at temperature T1 . The temperature quotient is useful because it allows us to calculate the respiration rates at one temperature from a known rate at another temperature. However, the respiration rate does not follow ideal behavior, and QR10 can vary considerably with temperature. For various products QR10 values may range from 1 to 4 depending on the temperature range (Kader 1987; Talasila 1992; Emond et al. 1993; Exama et al. 1993). At higher temperatures, QR10 is usually smaller than at lower temperatures. Typical figures for QR10 are presented in Table 8.4. These typical QR10 values allow us to construct a table showing the effect of different temperatures on the rates of respiration or deterioration and relative shelf-life of a typical perishable commodity (Table 8.5). This table shows that if Table 8.5 Effect of temperature on rate of deterioration Temperature (ëC)

0 10 20 30 40

Assumed QR10

Relative velocity of deterioration

Relative shelf-life

± 3.0 2.5 2.0 1.5

1.0 3.0 7.5 15.0 22.5

100 33 13 7 4

Sources: Wills et al.1981; Saltveit 1996, 2005.

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a commodity has a mean shelf-life of 13 days at 20ëC it can be stored for as long as 100 days at 0ëC, but will last no more than 4 days at 40ëC (Saltveit 2005). Atmospheric composition The other external factor that affects respiration is atmospheric composition, particularly O2 and CO2 concentrations. Respiration is widely assumed to be slowed down by decreasing available O2 as a consequence of reduction of overall metabolic activity (Isenberg 1979; Smock 1979; Kader 1987; Solomos and Kanellis 1989; McLaughlin and O'Beirne 1999). Reduced O2 level will decrease respiration rates (Burton 1975; Isenberg 1979) and ethylene biosynthesis (Konze et al. 1980; Yang 1985) as well as decrease sensitivity to ethylene (Burg and Burg 1965; Kader 1986; Kader et al. 1989). The reduction of respiration rate in response to low O2 levels is due to a decrease in the activities of other oxidases, such as polyphenoloxidase, ascorbic acid oxidase and glycolic acid oxidase activities whose affinity is much lower (Kader 1986; Jayas and Jeyamkondan 2002). Adequate O2 levels are required to maintain aerobic respiration. However, excessive depletion of O2 inside the package can lead to anaerobiosis accompanied by undesirable metabolic reactions such as tissue breakdown, off-odor and off-flavor production (Hulme 1971; Lipton 1975; Kader et al. 1989; Church 1994). At extremely low O2 levels, toxin production by anaerobic pathogenic organisms can occur (Farber 1991). The exact level of O2 that reduces respiration while still permitting aerobic respiration varies with commodity. In most crops, an O2 level around 2±3% produces a beneficial reduction in the rate of respiration and other metabolic reactions. Levels as low as 1% improve the storage life of some crops, e.g., apples, but only when the storage temperature is optimal. At higher storage temperatures, the demand for ATP may outstrip the supply and promote anaerobic respiration (Saltveit 2005). The influence of CO2 depends on the type and developmental stage of the commodity, CO2 concentration and exposure time. The idea of respiratory inhibition by CO2 was first supported by simple explanations, i.e., that CO2 was a product of the respiration process and caused simple feedback inhibition (Wolfe 1980; Herner 1987). Kader et al. (1989) and Lee et al. (1996) considered that elevated CO2 might affect the Krebs cycle intermediates and enzymes. Others considered that CO2 inhibits C2H4 production rather than having a direct effect on the respiration process (Kubo et al. 1989). However, increasing the CO2 level around some commodities reduces respiration, reduces ethylene sensitivity and production, delays ripening and senescence, retards bacterial and fungal growth and lowers the pH (Wolfe 1980; Kader 1986; Herner 1987; Kader et al. 1989; Kubo et al. 1989; Prasad 1995; Peppelenbos and Leven 1996; Mahajan and Goswami 2001, Bhande et al. 2008). The respiration rate increase due to the increase in CO2 concentration may be explained in terms of CO2 injury of tissues with a concomitant increase in C2H4 production. Some varieties

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of lettuce are very sensitive to CO2, and brown stain (browning of the epidermal tissue near the midrib) is a common CO2 injury when the product is exposed to levels above its tolerance limit (Kader et al. 1989; Ke and Saltveit 1989). Physical stress Even mild physical abuse can cause a substantial rise in respiration that is often associated with increased ethylene evolution. The signal produced by physical stress migrates from the site of injury and induces a wide range of physiological changes in adjacent, non-wounded tissue. Some of the more important changes include enhanced respiration, ethylene production, phenolic metabolism and wound healing. Wound-induced respiration is often transitory, lasting a few hours or days. However, in some tissues wounding stimulates developmental changes, e.g. promote ripening, that result in a prolonged increase in respiration. Ethylene stimulates respiration and stress-induced ethylene may have many physiological effects and induced detrimental effects on commodities besides stimulating respiration (Ryall and Lipton 1979; Kays 1991; Martinez-Romero et al. 2007, 2009). Stage of development/maturity stage of the commodity Respiration rates vary among and within commodities. Tissues with vegetative or floral meristems such as asparagus and broccoli have very high respiration rates. Vegetables include a great diversity of plant organs (roots, tubers, seeds, bulbs, fruits, sprouts, stems and leaves) that have different metabolic activities and consequently different respiration rates (Prince et al. 1986). Even different varieties of the same product can exhibit different respiration rates (Fidler and North 1967; Song et al. 1992). As plant organs mature, their rate of respiration typically declines. This means that commodities harvested during active growth, such as many vegetables and immature fruits, have high respiration rates. Mature fruits, dormant buds and storage organs have relatively low rates. After harvest, the respiration rate typically declines, slowly in non-climacteric fruits and storage organs, rapidly in vegetative tissues and immature fruits (Fernandez-Trujillo et al. 2008). The rapid decline presumably reflects depletion of respirable substrates that are typically low in such tissues. An important exception to the general decline in respiration following harvest is the rapid and sometimes dramatic rise in respiration during the ripening of climacteric fruit (Fig. 8.1). This rise, which has been the subject of intense study for many years, normally consists of four distinct phases: (1) pre-climacteric minimum, (2) climacteric rise, (3) climacteric peak, and (4) post-climacteric decline. In general, non-climacteric commodities have higher respiration rates in the early stages of development that steadily decline during maturation (Lopez-Galvez et al. 1997). Respiration rates of climacteric commodities also are high early in development and decline until a rise occurs that

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8.1 The climacteric pattern of respiration in ripening fruit.

coincides with ripening or senescence (Fig. 8.1). Climacteric products exhibit a peak of respiration and ethylene (C2H4) production associated with senescence or ripening. However, this does not imply that the respiratory response to MA/CA necessarily changes during the climacteric period. For example, Cameron et al. (1989) observed no influence of maturity or ripeness stage of tomatoes on O2 uptake as a function of O2 concentration. The division of fruits into climacteric and non-climacteric types has been very useful for post-harvest physiologists. However, some fruits, for example kiwifruit and cucumber, appear to blur the distinction between the groups. Respiratory rises also occur during stress and other developmental stages, but a true climacteric only occurs coincident with fruit ripening. Table 8.6 shows general classification of fruits according to their respiratory behavior during ripening. Care is necessary when packing in MAP due to alterations of respiration rate over time that are not normally considered in MAP design. The storage time period after harvest may influence the respiration curve due to the normal deterioration of the product with aging, ripening of climacteric products, and wound metabolism in fresh-cut products. In the senescent stage of climacteric plant organ development there is a rise in respiration, presumably in order to obtain more energy for metabolic processes. In non-climacteric tissues and climacteric tissues in the post-climacteric stage, increased respiration after some period of time in storage may be caused by the onset of decay by microorganisms (Rediers et al. 2009). Products in MAP are usually in short-term storage (distribution and retailing), thus the influence of storage time due to senescence may be considered negligible. Normally, climacteric changes are

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Table 8.6 Fruits classified according to respiratory behavior during ripening Climacteric fruits Apple Apricot Avocado Banana Blueberry Breadfruit Biriba Cherimoya Fig Guava Jackfruit Kiwifruit Mango Muskmelon Nectarine

Papaya Passion fruit Peas Pear Persimmon Plantain Plum Quince Rambutain Sadopilla Sapota Soursop Tomato Watermelon

Non-climacteric fruits Blackberry Blueberry Cacao Caju Carambola Cashew apple Cherry Cucumber Date Eggplant Grape Grapefruit Jujube Lemon Lime Longan

Loquat Litchi Okra Olive Orange Peas Pepper Pineapple Pomegranate Prickly pear Raspberry Strawberry Summer squash Tamarillo Tangerine

Sources: Kays 1991; Salunkhe and Kadam 1995; Mahajan 2001; Saltveit 2005; Irtwange 2006.

considered important only in the long term and not relevant to MAP (Fishman and Ben-Yehoshua 1996). MA conditions may control the timing of the climacteric rise as well as the magnitude of the peak. Young et al. (1962) observed a delay in the climacteric rise in avocados and bananas due to elevated CO2 levels, but only a reduction of O2 uptake at the climacteric peak in avocados. Fidler and North (1967) observed a delay in the onset of the climacteric rise in apples due to reduced O2 levels. Wounding plant cells and tissues causes the respiration rate to increase. Wounding induces elevated C2H4 production rates, which may stimulate respiration and consequently accelerate deterioration and senescence in vegetative tissues and promote ripening of climacteric fruit (Brecht 1995). The wounding may be due to mechanical damage or cutting of the product. The respiration rate may gradually increase over time until a maximum value is reached and then start decreasing again either to the value before the wounding or to a higher value (Lopez-Galvez et al. 1996). For example, the respiratory rate of apple slices was about 2±3 times that of the whole fruit (Lakakul et al. 1999).

8.3.4

Biological structure

The resistance of fruits and vegetables to diffusion of O2, CO2, ethylene and water vapor is dependent on the biological structure of the individual commodity. Resistance to gas diffusion varies depending on the type of commodity, cultivar, part of the plant, surface area and stage of maturity, but appears to be little affected by temperature (Ratti et al. 1996; Fallik et al. 2002).

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8.3.5

187

Ethylene production and sensitivity

Ethylene is the natural product of plant metabolism and is produced by all tissues of higher plants and by some microorganisms. It is the simplest of all the organic compounds and considerably affects the physiological processes of plants and the commodity after harvest (Abeles et al. 1992). Being a plant hormone, ethylene regulates many aspects of growth, development, and senescence and is physiologically active in trace amounts (less than 0.1 ppm). It also plays a major role in the abscission of plant organs (Tomas-Barberan et al. 1997). Ethylene production rates increase with maturity at harvest, physical injuries, disease incidence, increased temperatures up to 30ëC and water stress (Kader 1987; Kays 1991; Abeles et al. 1992; Saltveit 1996). On the other hand, ethylene production rates by fresh horticultural commodities are reduced by storage at low temperature, by reduced O2 levels, and by elevated CO2 levels around the commodity (Isenberg 1979; Kader 1986, 1987; Solomos and Kanellis 1989; Saltveit 1997). Exposure of climacteric fruits to ethylene advanced the onset of an irreversible rise in respiration rate and rapid ripening. Various packages can delay the onset of climacteric and prolong shelf-life of fruits by reducing ethylene production and sensitivity (Peleg 1985; Abeles et al. 1992). Even non-climacteric fruits and vegetables can benefit from reduced ethylene sensitivity and lower respiration rate under various conditions.

8.3.6

Transpiration

Transpiration is the process of evaporation of water from fruits and vegetables. Water loss is a very important cause of produce deterioration such as wilting/ shivering, with severe consequences (Ryall and Pentzer 1974). In fact water loss is, first, a loss of marketable weight and then adversely affects appearance (wilting and shriveling). Also, the textural quality is reduced by enhanced softening, loss of crispness and juiciness, and reduction in nutritional quality (Irtwange 2006). Transpiration is a result of morphological and anatomical characteristics, surface-to-volume ratio, surface injuries and maturity stage on the one hand, and relative humidity (R.H.), air movement and atmospheric pressure on the other. The transpiration can be controlled by applying waxes and packaging in plastic films to act as barriers between the produce and the environment, by manipulating R.H., temperature and air circulation (Herr 1991; Kader et al. 1989; Church and Parsons 1995; Irtwange 2006).

8.3.7

Compositional changes

Some changes in pigments of the commodity may continue after, or start only at harvest. These changes can occur as loss of chlorophyll, development of carotenoids (yellow orange and red colors) and development of anthocyanins

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and other phenolic compounds (Tomas-Barberan et al. 1997; Quevedo et al. 2008; Opara et al. 2008; Bureau et al. 2009). Changes in carbohydrates are generally desirable in fruits but are quite important in all commodities. In fruits starch is converted to sugars. In most commodities starch is used as a substrate for respiration. In the ripening process, softening occurs and polysaccharides such as pectins, cellulose and hemicellulose are degraded. There are changes in proteins, amino acids and lipids, which may affect the flavor of the commodity. Development of flavor and aroma volatiles is very important for eating quality. Loss of vitamins, particularly ascorbic acid (vitamin C), takes place during storage and thus adversely affects nutritional quality (Ryall and Pentzer 1974; Kays 1991; Irtwange 2006).

8.3.8

Developmental processes

Sprouting, seed germination and rooting of commodities during storage are undesirable processes and greatly reduce their commercial value (Kays 1991). Asparagus spears continue to elongate during storage and bend when held horizontally during transportation. Seed germination occurring inside fruit during storage is undesirable in tomato, pepper, avocado and lemon (Wills et al. 1981; Geeson et al. 1985; Yang and Chinnan 1988; Gong and Corey 1994; Irtwange 2006).

8.3.9

Physiological breakdown

Exposure of commodity to undesirable conditions such as freezing results in the collapse of tissues (Kays 1991). Chilling injury occurs in some tropical and subtropical commodities, which are stored at temperatures above their freezing point but still cause injury (below 15ëC or lower, depending on the commodity) (Adsule and Kadam 1995; Salunkhe and Kadam 1995). Chilling injuries are expressed as internal discoloration (browning), pitting, water-soaked areas, uneven ripening or failure to ripen, off-flavor development and accelerated incidence of decay (Adsule and Kadam 1995). Some physiological disorders originate from pre-harvest factors such as heat or chilling injuries in the field, which appear as bleaching, surface burning or scalding or nutritional imbalances. Very low oxygen or too-high carbon dioxide and the presence of excessive ethylene concentrations may exacerbate the severity of physiological disorders related to storage conditions (Wills et al. 1981; Kader et al. 1989; Saltveit 1996).

8.4

Post-harvest pathology of fruits and vegetables

Vegetables have more available water, less carbohydrates (sugars) and higher pH (near to neutral) than fruits (Manay and Shadaksharaswamy 2006). Due to

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having more available water and pH near to neutral, bacteria are the predominant microflora in vegetables. The common spoilage bacteria are Erwina spp., which cause bacterial rots in vegetables. The pH of the fruits is below the level to support bacterial growth. Molds and yeasts (fungi) are major spoilage microorganisms in fruits. Fungi and bacteria are the two main sources of infection that may occur during growing and post-harvest handling of produce (Hotchkiss 1989; Saltveit 1996; Rediers et al. 2009). Bacteria gain entry through wounds or natural openings (such as stomata, lenticels, or hydathodes) and multiply in the spaces between plant cells (Tomas-Barberan et al. 1997; Lu Shengmin 2009). Entry via wounds or natural openings is also characteristic of many fungi. Certain species of fungi, however, are capable of direct penetration of intact food commodity (Irtwange 2006). Fruits are resistant to fungal attack until unripe; the infection process at this stage is halted; however, the fungus remains alive, entering an inactive or dormant phase. The process of ripening is accompanied by weakening of cell walls and a decline in ability to synthesize anti-fungal substances, and during the ripening phase the fruit is no longer able to resist the advance of the fungus (Biale and Young 1981; Wills et al. 1981; Miller et al. 1986a). Decay can be reduced and disease controlled through sanitation, careful handling, storing at low temperature, use of approved chemicals (fungicides that prevent or delay the development of molds and rots in the products, chemicals that delay ripening or senescence, growth retardants that inhibit sprouting and growth, metabolic inhibitors that block certain biochemical reactions that normally occur, ethylene absorbants and fumigants to control insects or molds), physical treatments (heat, gamma-ray, UV irradiation), biological control and controlled and modified atmosphere (Biale and Young 1981; Farber 1991; Kader 1995; Ahvenainen 2003; Irtwange 2006).

8.5

Response of fresh produce to modified atmosphere packaging

MAP is the modification or replacement of air (N2 content 78%, O2 content 21%, CO2 content 0.035%, together with water vapor and traces of inert gases) in a pack with a mixture of gases achieved by the natural interplay between two processes, the respiration of the product and the transfer of gases through the packaging that leads to an atmosphere richer in CO2 and poorer in O2, or by the introduction of a fixed proportion of gas mixture into the package before it is sealed (Mahajan et al. 2007). No further control is exerted over the initial composition, and the gas composition is likely to change with time owing to the diffusion of gases into and out of the product, the permeation of gases into and out of the pack, and the effects of product and microbial metabolism. Storage in plastic films in all kinds of combinations (different materials, perforations, inclusions, individual seal packing ± shrunken and non-shrunken) gives addi-

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tional types of MA storage. Most perishable commodities require a R.H. of 90± 95% in order to avoid excessive moisture loss during storage. Cooling slows down the changes in the produce during ripening and subsequent deterioration, reduces water loss, and slows or stops the growth and spread of rots. Wilting, regrowth, ripening, senescence and decay can be postponed through good temperature and relative humidity management (Aradhya et al. 1993; Andrich et al. 1998). A modified atmosphere (MA) is created as a result of the respiratory activity of the produce, with consumption of oxygen (O2) and emanation of carbon dioxide (CO2) occurring within a sealed plastic package. Special film packages with suitable permeability to CO2 and O2 are used to ensure an optimal equilibrium of these gases during storage and shipment. The goal of MAP of fresh produce is to create an equilibrium package atmosphere or steady-state condition with O2% low enough and CO2% high enough to be beneficial to the produce and not injurious. The steady-state level depends on the rate of produce respiration, the produce weight and the package permeability (Exama et al. 1993; Del Nobile et al. 2007). The benefits of film packaging include ease of handling (consumer package); protection from injuries; reduction of water loss, shrinkage and wilting; reduction of decay by MA; reduction of physiological disorders (chilling injury); retardation of ripening and senescence processes; retardation of regrowth and sprouting (green-onion radishes), and control of insects in some commodities (Kawada and Kitagawa 1988; Moyls et al. 1998; Kader et al. 1989; Prasad and Singh 1994; Phillips 1996; Kim et al. 2006; Sivakumar and Korsten 2006). Harmful effects of film packaging include the enhancement of decay due to excess humidity; initiation and/or aggravation of physiological disorders; internal browning and/or irregular ripening in improper concentrations of CO2/O2; off-flavors and off-odors, and increased susceptibility to decay (Hotchkiss 1989; Phillips 1996; Koide and Shi 2007). The potential benefits of MA/CA for fruits and vegetables at various storage temperature and air compositions are presented in Tables 8.7 and 8.8, respectively (Parry 1993; Saltveit 1993, 1997; Kader 1997; Mahajan 2001; Irtwange 2006). Generally speaking, CA has its most beneficial effects on climacteric fruits at the pre-climacteric stage by prolonging this stage (Kader 1980; Raghavan and Gariepy 1985; Mahajan 2001). The effects are less marked in climacteric fruits at their ripening stage, and in non-climacteric fruits at any stage. Most importantly, the product to be stored must be of very good quality. CA storage can at best maintain quality, but cannot improve initially inferior quality of products. Also the crop to be stored must have been harvested at the correct stage of maturity. It must be free of disease and should have only minimal physical damage due to harvest and handling procedures (Ryall and Pentzer 1974; Wills et al. 1981). Recommended optimum conditions for CA/MA and refrigerated storage for climacteric and non-climacteric fruits are summarized in Table 8.9

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Table 8.7 Potential benefit of MA/CA for fresh fruits Commodity

Apple Apricot Avocado Banana Fig Grape Guava Honeydew melons Kiwifruit Lemon Lime Litchi Nectarine Olive Orange Mango Papaya Peas Pear, Asian Pear, European Persimmon Pineapple Plum and prune Raspberry Strawberry Sweet cherry Nuts and dried fruits

Temperature (ëC) 0±3 0±5 5±13 12±15 0±5 0±2 10±15 10±12 0±5 10±15 10±15 0±2 0±5 5±10 5±10 10±15 10±15 0±5 0±5 0±5 0±5 8±13 0±5 0±3 0±2 0±2 0±25

MA/CA % O2

% CO2

1±3 2±3 2±5 2±5 5±10 2±5 2±5 3±5 1±2 5±10 5±10 2±3 1±2 2±3 5±10 3±5 3±5 1±2 2±4 1±3 3±5 2±5 1±2 5±10 5±10 3±10 0±1

1±5 2±3 3±10 2±5 15±20 1±3 2±5 0±2 3±5 0±10 0±10 2±5 3±5 0±1 0±5 5±10 5±10 3±5 0±1 0±3 5±8 5±10 0±5 15±20 15±20 10±15 0±100

Potential for benefit Excellent Fair Good Excellent Good Fair Good Fair Excellent Good Good Good Good Fair Fair Fair Fair Good Good Excellent Good Fair Good Excellent Excellent Good Excellent

Sources: Parry 1993; Kader 1997; Mahajan 2001; Irtwange 2006.

(Saltveit 1993, 1997; Kader 1997; Mahajan 2001). Optimum levels of O2 are the levels below which it causes injuries and above which it is ineffective in extending storage life. However, different studies have shown great variations in the effective combination and ranges used in CA storage for both fruits and vegetables.

8.5.1

Modified atmosphere: favorable and injurious effects on fresh commodity

Several authors have reviewed the beneficial and detrimental effects of MA on fruits and vegetables (Kader 1980; Wolfe 1980; Zagory and Kader 1988; Kader et al. 1989; Ben-Yehoshua et al. 1994; Church 1994; Church and Parsons 1995; Irtwange 2006; Mahajan et al. 2007). Prevention of ripening and associated

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Table 8.8 Potential benefit of MA/CA for fresh vegetables Commodity

Artichokes Asparagus Beans Beets Broccoli Brussels sprouts Cabbages Cantaloupes Carrots Cauliflowers Celery Corn, sweet Cucumbers Leeks Lettuces Mushrooms Okra Onions, dry Onions, green Peppers, bell Peppers, chilli Potatoes Radishes Spinach Tomatoes

Temperature (ëC) 0±5 0±5 5±10 0±5 0±3 0±5 0±5 3±7 0±5 0±2 0±5 0±5 8±12 0±5 0±5 0±3 8±12 0±5 0±5 8±12 8±12 4±10 0±5 0±5 15±20

MA/CA % O2

% CO2

2±3 15±20 2±3 2±5 1±2 1±2 2±3 3±5 3±5 2±3 1±1 2±4 3±5 1±2 1±3 Air 3±5 1±2 1±2 3±5 3±5 2±3 1±5 18±21 3±5

2±3 5±10 4±7 2±5 5±10 5±7 3±7 10±15 2±5 2±5 0±5 5±10 0±2 3±5 0±3 10±15 0±2 0±5 10±20 0±2 0±3 2±5 2±3 10±20 0±3

Potential for benefit Good Excellent Fair Fair Excellent Good Excellent Good Fair Fair Good Good Fair Good Good Fair Fair Good Fair Fair Fair Fair Fair Good Good

Sources: Parry 1993; Saltveit 1993, 1997; Irtwange 2006.

changes in foods is one of the main benefits of MA. Oxygen concentration has to be lowered below 10% to have a significant effect on fruit ripening and the lower the O2 concentration the greater the effect (Kader et al. 1989; Davies 1995). Elevated CO2 levels (>1%) also retard fruit ripening and their effects are additive to those of reduced O2 atmosphere (Daniels et al. 1985; Dixon and Kell 1989; Kader et al. 1989). The effects of MA/CA on delay or inhibition of ripening are greater at higher temperature (Saltveit 2005). Thus use of MA may allow handling of ripening (climacteric-type) fruits at temperatures higher than their optimum temperature. This is especially beneficial for chill-sensitive fruits such as tomatoes, melons, avocados, bananas and mangoes to avoid their exposure to chilling temperature (Zagory and Kader 1988). MA conditions reduce respiration rates as long as the levels of O2 and CO2 are within those tolerated by the commodities. These, combined with the decreased C2H4 production and reduced sensitivity to C2H4 action, result in delayed senescence and extending shelf-life as indicated by retention of chlorophyll (green color), textural quality

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Table 8.9 Optimum conditions of MA/CA for some fruits and their shelf-life Commodity

Storage temperature (ëC)

Optimum MA/CA

Injurious atmosphere

Marketable life (days)

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% O2

% CO2

% O2

% CO2

RA storage

CA storage

0±3

3

3

2

10

200

300

Avocado Banana

7 12±15

2±5 2

3±10 5

1 1

15 8

12 21

56 60

Grape Guava

0±2 12±15

3±5 2±5

1±3 2±5

1 2

10 12

40 15±20

90±100 45

Lemon Litchi Mango Orange Papaya Pear

15 0±5 13 5±10 13 0±1

3±5 3±5 3±5 10 3±5 2±3

0±5 3±5 5±8 5 5±8 0±1

1 2 2 5 2 1

6 14 8 5 8 2

130 20±30 14±28 42 14±28 200

220 2230 21±45 84 21±35 300

Pineapple Strawberry

10±15 0

2±5 4±10

10 15±20

2 1

10 12

12 7

10±15 7±15

Apple

Sources: Saltveit 1993, 1997; Kader 1997; Mahajan 2001, Irtwange 2006.

Major benefit under MA/CA storage

Commercial potential

Maintains firmness and acidity Delays softening Suppresses climacteric pattern Controls disease Delays ripening and chilling injury Retains green color Delays ripening Delays ripening Maintains firmness Less decay Delays flesh and core browning Reduces chilling injury Less decay

Excellent Good Excellent Fair Good Good Good Fair Fair Excellent Fair Excellent

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Table 8.10 O2% limits below which injury can occur for some commodities Minimum O2 concentration tolerated (%) 0.5 or less 1

1.5 2

2.5 3 4 5 10 14

Commodities

Chopped greenleaf, redleaf, Romaine and iceberg lettuce, spinach, sliced pear, broccoli, mushroom Broccoli florets, chopped butterhead lettuce, sliced apple, Brussels sprouts, cantaloupe, cucumber, crisphead lettuce, onion bulbs, apricot, avocado, banana, cherimoya, atemoya, sweet cherry, cranberry, grape, kiwifruit, litchi, nectarine, peach, plum, rambutan, sweetsop Most apples, most pears Shredded and cut carrots, artichoke, cabbage, cauliflower, celery, bell and chilli pepper, sweet corn, tomato, blackberry, durian, fig, mango, olive, papaya, pineapple, pomegranate, raspberry, strawberry Shredded cabbage, blueberry Cubed or sliced cantaloupe, low permeability apples and pears, grapefruit, persimmon Sliced mushrooms Green snap beans, lemon, lime, orange Asparagus Orange sections

Sources: Bohling and Hansen 1984; Kader et al. 1989; Kays 1991, 1997; Gorny 1997; Kader 1997; Kupferman 1995; Richardson and Kupferman 1997; Saltveit 1997; Beaudry 2000.

(decreased lignification), and sensory quality of fruits and vegetables (Makhlouf et al. 1989; Pal and Buescher 1993; Saltveit 2005; Menon and Goswami 2008; Lu Shengmin 2009). Exposure of fresh fruits and vegetables to O2 levels below their tolerance limits or to CO2 levels above their tolerance limits (Tables 8.10 and 8.11) may increase anaerobic respiration and the consequent accumulation of ethanol and acetaldehyde, causing off-flavor (Bohling and Hansen 1984; Kader et al. 1989; Kays 1991, 1997; Gorny 1997; Kader 1997; Saltveit 1997; Beaudry 2000). Low O2 and/or high CO2 concentrations can reduce the incidence and severity of certain physiological disorders such as those induced by C2H4 (scald of apple and pear) and chilling injury of some commodities (Kader 1997; Saltveit 1997; Irtwange 2006). Besides, O2 and CO2 levels beyond those tolerated by the commodity can induce physiological disorder such as brown stain on lettuce, internal browning and surface pitting of pome fruits, and black heart of potato. CA/MA combinations have direct and indirect effects on post-harvest pathogens. El-Goorani and Sommer (1981) pointed out that delaying senescence, including fruit ripening, by MA/CA reduced the susceptibility of fruits and vegetables to pathogens. On the other hand, MA conditions unfavorable to a given commodity can induce its physiological breakdown and render it more

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Table 8.11 CO2% limits above which injury can occur for some commodities Maximum CO2 concentration tolerated (%) 2 3 5 7 8 10 15 20 25 30

Commodities

Lettuce (crisphead), pear Artichoke, tomato Apple (most cultivars), apricot, cauliflower, cucumber, grape, nashi, olive, orange, peach (clingstone), potato, pepper (bell) Banana, bean (green snap), kiwifruit Papaya Asparagus, Brussels sprouts, cabbage, celery, grapefruit, lemon, lime, mango, nectarine, peach (freestone), persimmon, pineapple, sweet corn Avocado, broccoli, litchi, plum, pomegranate, sweetsop Cantaloupe (muskmelon), durian, mushroom, rambutan Blackberry, blueberry, fig, raspberry, strawberry Cherimoya

Sources: Herner 1987; Gorny 1997; Kader 1997; Kays 1997; Kupferman 1995; Richardson and Kupferman 1997; Saltveit 1997.

susceptible to pathogens. Elevated CO2 concentrations inhibit the growth of some types of microorganisms, bacteria and fungi during storage. Oxygen levels below 1% and/or CO2 levels above 10% are needed to significantly suppress fungal growth (Farber 1991; Kader et al. 1989). Elevated CO2 levels (10±15%) can be used to provide fungistatic effects on commodities that tolerate such CO2 levels (Daniels et al. 1985; Dixon and Kell 1989).

8.5.2

Tolerance limit of commodities to modified atmosphere

The extent of benefits from the use of MA depends upon the commodity, cultivar, physiological age (maturity stage), initial quality, concentration of O2 and CO2, temperature and duration of exposure to such conditions. Subjecting a cultivar of a given commodity to an O2 level below and/or a CO2 level above its tolerance limit at a specific temperature±time combination will result in stress to the living plant tissue, which is manifested as various symptoms, such as irregular ripening, initiation and/or aggravation of certain physiological disorders, development of off-flavors and increased susceptibility to decay and fungal growth (Marcellin 1974; Lipton 1975; Isenberg 1979; Smock 1979; Kader et al. 1989; Ke and Saltveit 1989; Varoquaux et al. 1996; Beaudry 2000; Watkins 2000). Fruits and vegetables are classified according to their relative tolerance to low O2 or elevated CO2 concentration (Kader et al. 1989; Kader 1997; Saltveit 1997; Beaudry 2000; Watkins 2000) when kept at their optimum

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storage temperature and relative humidity ± see Tables 8.10 and 8.11. The limits of tolerance to low O2 levels would be higher than those indicated in Table 8.10 to maintain aerobic respiration if the storage temperature and/or duration are increased. For some commodities, susceptibility to low O2 and/or high CO2 stress is influenced by maturity stage. For example ripe fruits often tolerate higher levels of CO2 than mature green fruits. Minimally processed fruits and vegetables have fewer barriers to gas diffusion, and consequently they tolerate higher concentrations of CO2 and lower O2 levels than intact commodities. The effects of stress resulting from exposure to undesirable MA/ CA conditions (i.e. level of O2 and/or CO2) can be additive to other stresses (such as chilling injury, wounding, or ionizing radiation) in accelerating the deterioration of fresh produce. Successful MAP must maintain near-optimum O2 and CO2 levels to attain the beneficial effects of MA without exceeding the limits of tolerance which may increase the risk of physiological disorders and other detrimental effects (Watkins et al. 1998; Kader et al. 1989; Beaudry 2000; Watkins 2000).

8.5.3

Physiological and biochemical effects of modified atmosphere packaging

The respiration rate is considerably reduced by the low O2 and high CO2 atmosphere in MAP. The low respiration rate reduces the overall metabolic and biochemical activities (ethylene production and sensitivity to ethylene, rapid acid catabolism, changes of pectic substances in the cell wall leading to softening, etc.) in the cell, thereby reducing the rate of utilization of food reserves (Kader 1986; Kader et al. 1989). By reducing the respiration rates, MA also lowers the production of heat due to respiration. Carbon dioxide has an antagonistic effect on enzymes involved in ethylene biosynthesis. As O2 is required in the production of ethylene, low O2 concentrations suppress ethylene production. During MAP, a compound precursor to ethylene is accumulated in the products. Therefore, when the products are transferred to air, ethylene is rapidly produced and the products ripen faster (Wang 1990). Modified atmospheres delay the onset of the ripening process and increase firmness in fruits. By inhibiting the enzyme polyphenol oxidase in litchi, strawberries, lettuces and mushrooms, MA prevents browning of their tissues (Wang 1990; Renault et al. 1994b; Stewart et al. 2003; Sivakumar and Korsten 2006; Del Nobile et al. 2008). When O2 is not available, fruits and vegetables degrade glucose anaerobically by glycolysis to generate energy. In the glycolysis pathway, aldehydes, alcohols and lactates are produced (Kader 1986). Accumulation of these anaerobic by-products produces off-flavors associated with physiological disorders, leading to an unacceptable eating quality (Meyer et al. 1973; Herner 1987; Kader et al. 1989; Ke and Saltveit 1989; Varoquaux et al. 1996). Therefore, a minimum of 2±3% levels of O2 must be maintained

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in the MAP to prevent anaerobic respiration (Wang 1990; Jayas and Jeyamkondan 2002). Hence, the objective of MAP design is to define the conditions so as to achieve the optimum concentration of O2 and CO2 inside the package with the shortest possible time to preserve the quality and extend shelf-life.

8.5.4

Required characteristics of plastic films for MAP

The desirable characteristics of a polymeric film for modified atmosphere packaging depend on the respiration rate of the produce at the transit and storage temperature to be used and on the known optimum O2 and CO2 concentrations for the produce that will result in optimum MA conditions within a definite time period. For most produce, a suitable film must be much more permeable to CO2 than to O2 (Sacharow and Griffin 1980; Ben-Yehoshua 1985; Kader et al. 1989; Exama et al. 1993). The major factors to be taken into account when selecting the packaging materials are: · The type of package (i.e. flexible pouch or rigid or semi-rigid lidded tray) · The barrier properties needed (i.e. permeabilities of individual gases and gas ratios when more than one gas is used) · The physical properties of machinability, strength, clarity and durability · Integrity of closure (heat sealing), fogging of the film as a result of product respiration · Sealing reliability · Water vapor transmission rate · Resistance to chemical degradation · Non-toxic and chemically inert · Printability · Commercial suitability with economic feasibility.

8.6

Polymeric films for application in modified atmosphere packaging (MAP)

Flexible plastic packaging materials comprise nearly 90% of the materials used in MAP, with paper, paperboard, aluminum foil, metal and glass containers accounting for the remainder. This is largely due to changing consumer demand in which convenience, quality, safety and impact on the environment are of prime consideration. These materials provide a range of permeability to gases and water vapor together with the necessary package integrity needed for MAP (Tables 8.12 and 8.13). Sometimes the films are used alone, but often they are used in combinations that provide the benefits of multiple materials. The most commonly used polymeric films for modified atmosphere packaging are as follows.

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Table 8.12 Absorbers used for active MAP for extending shelf-life

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Packaging system

Example of working principle/mechanism/reagents

Purpose

Oxygen absorbers (sachet, labels, films, corks)

Ferro-compound (iron powders), ascorbic acid, metal salt, glucose oxidase, alcohol oxidase

Carbon dioxide absorbers (sachet)

Calcium hydroxide and sodium hydroxide or potassium hydroxide, calcium oxide, magnesium oxide, activated charcoal and silica gel Aluminum oxide and potassium permanganate (sachets), activated hydrocarbon (squalane, apiezon) + metal catalyst (sachets), builder-clay powders (films), zeolite (films), Japanese oya stone (films) and other compounds like silicones (phenylmethyl silicone) Polyacrylates (sheet), polypropylene glycol (film), silica gel (sachet), clays (sachet)

Reducing/preventing respiration rate, mold, yeast and aerobic bacteria growth; prevention of oxidation of fats, oils, vitamins, colors; prevention of damage by worms, insects and insect eggs Removing excess carbon oxide formed during storage to prevent fruit damage and bursting of package Prevention of too fast ripening and softening

Ethylene absorbers (sachets, films)

Humidity absorbers (drip absorbent sheets, films, sachets) Absorbers of off-flavors, amines and aldehydes (films, sachets) UV-light absorbers Reagents Preservative films

Cellulose acetate film containing narinaginase enzyme, ferrous salt and citric or ascorbic acid (sachet), specially treated polymer Polyolefins like polyethylene and propylene doped in the material with a UV-absorbent agent; crystallinity modification of nylon 6 Ferrous carbonate: 4FeCO3 + O2 + 6H2O ! 4Fe (OH)3 + 4CO2 Slowly diffuse preservatives such as nisin, sorbate, glycol, antioxidants, antibiotics, ethanol or ethylene into the package

Sources: Kader et al.1989; Labuza and Breene 1989; Ahvenainen 2003.

Control of excess moisture in packed produce, reduction of water activity on surface of food in order to prevent growth of molds, yeast and spoilage bacteria Reduction of bitterness in fruit, improving flavor, and oil-containing foods Restricting light induction oxidation For quickly developing an MA within a package. The reaction quickly builds up the CO2 content of the package while reducing the O2 content somewhat Control microbial growth; suppress unwanted biochemical reactions

Table 8.13 Permeability of polymeric films available for MAP Name of film

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Polyvinyl fluoride (PVF) Polyvinylidene fluoride (PVDF) Polyamide (nylon-6) Polycarbonate Polyethylene terephthalate (PET) Polyamide Low density polyethylene (LDPE) Linear low density polyethylene (LLDPE) Medium density polyethylene (MDPE) Linear medium density polyethylene (LMDPE) High density polyethylene (HDPE) Ethylene vinyl acetate copolymer (EVA) Ethylene vinyl alcohol copolymer (EVOH) Polypropylene (cast film) Polypropylene (BOPP) Polybutylene Polyvinyl alcohol (PVOH) Polystyrene (PS) Mylar (polyester) Oriented polystyrene (OPS) Polyvinyl chloride (PVC) ± plasticized Polyvinylidene chloride (PVDC) Cellulose acetate Rubber hydrochloride Ethylcellulose Methylcellulose Cellulose triacetate Vinylchloride acetate Natural rubber Silicone rubber

Permeability (cm3.m/m2.h. atm) O2

CO2

50 81.66 105.83 2829.17 50±100 416.66 11416.68 2916.66±8333.34 4083.33±8791.67 3666.66 1640.41±3280.83 7500 0.1 2458.33±2675 2675 6316.66 3.75 4875±6316.67 54.16±137.5 4100 422.5±32666.67 16.25 1919.58 623.33 32808.33 1312.08 2460.41 246.25 63500 1058333

179.16 408.33 423.33 18166.67 245.83±408.64 708.33 39958.33 ± 1625±41000 ± 9841.67±11482.92 45833.33 3.33 8166.67±13041.67 8833.33 23375 1.66 16375±23375 190.41±412.5 11500±21958.33 1633.33±49000 62 14107.50 4724.16 82020.83 6561.66 14435.66 902.5 370416.66 6350000

Sources: Karel et al.1975; Kader et al.1989; Abdel-Bary 2003; Massey 2003.

Permeability (g.m/m2.h) to water vapor

CO2/O2 ratio

54.16 8.16 640 62.5 16.25±21.25 20.83±56.25 18.75 12.5 11.66 9.16 6.66 187.5 33.33±100 ± 6.25±11.25 19.58±30.83 ± 32.5±162.5 ± 145.83 147.91±180.83 3.33 1230.84 8.25 328 3280.83 78.31 65.61 ± 72

3.58 5.00 4.00 6.42 4.91 1.70 3.49 ± 4.66 ± 5.99 6.11 33.3 3.32 3.30 3.70 2.00 3.35 3.51 2.80 3.86 3.81 7.34 7.57 2.5 5.00 5.86 3.66 5.83 6.00

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Multifunctional and nanoreinforced polymers for food packaging

Polyolefins

Polyolefin is a collective term for polyethylene and polypropylene, the two most widely used plastics in food packaging industries. Polyethylene and polypropylene both possess a successful combination of properties, including flexibility, strength, lightness, stability, moisture and chemical resistance, and easy processability, and are well suited for recycling and reuse (Karel et al. 1975; Abdel-Bary 2003; Marsh and Bugusu 2007).

8.6.2

Low-density polyethylene (LDPE)

The simplest and most inexpensive plastic made by addition polymerization of ethylene is polyethylene. Low-density polyethylene is the most commonly used packaging film. LDPE seals at a lower temperature and over a wider temperature range, and has better hot tack, all of which result, to a great extent, from its longchain branching (Prasad 1995; Moyls et al. 1998; Abdel-Bary 2003). LDPE forms a good barrier to water vapor but a poor barrier to oxygen, carbon dioxide and many odor and flavor compounds. Because LDPE is relatively transparent, it is predominantly used in film applications and in applications where heat sealing is necessary. Some properties and characteristics of LDPE are presented in Table 8.13. LDPE is generally the cheapest plastic film on a per-unit-mass basis.

8.6.3

Linear low-density polyethylene (LLDPE)

Linear low-density polyethylene is also one of the most commonly used packaging films in the packaging industry. The reduction of density comes about through the use of comonomers that put side groups on the main chain that act like branches in decreasing crystallinity. LLDPE is also a soft, flexible material with a hazy appearance. At equal density and thickness, LLDPE has higher impact strength, tensile strength, puncture resistance and elongation than LDPE. Like LDPE, LLDPE has good water vapor barrier properties but is a poor barrier to oxygen, carbon dioxide and many odor and flavor compounds (Abdel-Bary 2003; Massey 2003). Since LLDPE often permits considerable downgaging, it can be the lowest cost alternative on a per-use basis.

8.6.4

High-density polyethylene (HDPE)

High-density polyethylene is a linear addition polymer of ethylene, produced at temperatures and pressures similar to those used for LLDPE, and with only very slight branching. HDPE films are stiffer than LDPE films, though still flexible, and have poorer transparency. Their water vapor barrier is better, as is their gas barrier. However, permeability to oxygen and carbon dioxide is still much too

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high for HDPE to be suitable as a barrier for these permeants (Burton et al. 1987; Marsh and Bugusu 2007). Because of the distinctly cloudy appearance of HDPE film, a small amount of white pigment is commonly added to provide an attractive opaque white film. Typical HDPE properties are shown in Table 8.13.

8.6.5

Polypropylene (PP)

Polypropylene is a linear addition polymer of propylene; resins used in packaging are predominantly isotactic. PP has the lowest density of the commodity plastics, 0.89±0.91 g/cm3. Harder and more transparent than polyethylene, PP has good resistance to chemicals and is effective at barring water vapor. Its high melting point makes it suitable for application where thermal resistance is required. Barrier properties of PP are comparable to those of HDPE (Crosby 1981; Exama et al. 1993; Abdel-Bary 2003). In many applications, biaxially oriented film (BOPP) is preferred. BOPP film is explicitly used in modified atmosphere packaging of food commodities.

8.6.6

Polyvinyl chloride (PVC)

Polyvinyl chloride films are formed by combining PVC resin, produced by addition polymerization of vinyl chloride, with plasticizers and other additives to produce a flexible film. In general, the films are quite soft and flexible, easy to heat-seal, and have excellent self-cling, toughness, medium strength, excellent resistance to chemicals, resilience and clarity. Permeability is relatively high (Kader et al. 1989; Exama et al. 1993; Ahvenainen 2003; Massey 2003). Both oriented and unoriented films are available. The properties of PVC films are listed in Table 8.13.

8.6.7

Polyesters

Polyethylene terephthalate (PET), polycarbonate and polyethylene naphthalate (PEN) are polyesters, which are condensation polymers formed from ester monomers that result from the reaction between carboxylic acid and alcohol. The most commonly used polyester in food packaging is polyethylene terephthalate (Kader et al. 1989; Abdel-Bary 2003).

8.6.8

Polyethylene terephthalate (PET)

PET is commonly used in biaxially oriented form, and has excellent transparency and mechanical properties. PET provides a good barrier to gases (O2 and CO2), to moisture and especially to odors and flavors. The barrier properties can be enhanced by coating with PVDC. Coating or coextrusion is often used to provide good heat-seal properties. It can tolerate considerably higher

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temperatures for short periods, such as in dual ovenable packaging for frozen foods. The main reasons for its popularity in the food packaging industry are its glass-like transparency, adequate gas barrier properties, light weight and shatter resistance (Abdel-Bary 2003; Kirkland et al. 2008). Typical PET properties are listed in Table 8.13.

8.6.9

Polyvinylidene chloride (PVDC)

Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is heat-sealable and serves as an excellent barrier to oxygen, water vapor, odors and flavors (Kader et al. 1989). The PVDC copolymer can be heat-sealed and serves as an excellent barrier to gases. However, the best barrier films generally do not provide the best heat-seal capability, and vice versa, so when both heat-sealability and barrier properties are desired, sometimes two differently formulated PVDC copolymer coatings are applied. The major applications of PVDC include packaging of poultry, cured meats, cheese, snack foods, tea, coffee and confectionery and modified atmosphere packaging of food products.

8.6.10 Ethylene±vinyl alcohol (EVOH) Ethylene±vinyl alcohol is a copolymer of ethylene and vinyl alcohol. The presence of ±OH groups in the structure results in strong intermolecular hydrogen bonding. EVOH gives an excellent barrier to gases (especially O2), odors and flavors. However, the hydrogen bonds also make it a moisturesensitive material, and high humidity decreases its barrier capability (Marsh and Bugusu 2007). EVOH is most often used as an oxygen barrier. Typical EVOH properties are listed in Table 8.13.

8.6.11 Polyamide (nylon) Nylon films are used for specialty applications in packaging, where performance requirements justify their relatively high cost. Nylons have mechanical (excellent strength) and thermal properties (high-temperature performance) similar to those of PET and have similar usefulness. Nylons also provide an excellent odor and flavor barrier, and a reasonably good oxygen barrier (Crosby 1981; Kader et al. 1989). They are very poor water vapor barriers, and generally have a tendency to lose some barrier performance when exposed to large amounts of moisture. However, their performance is not as water-sensitive as that of EVOH. Owing to their relatively high cost, they are often coextruded with other plastics. Typical properties of some nylon films are given in Table 8.13. Nylon-6 tends to be the most-used nylon packaging film in industry.

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8.6.12 Polychlorotrifluoroethylene (PCTFE) These films are considered the best available transparent moisture barriers for flexible packaging; however, they are rather expensive. Aclar films can be laminated to paper, polyethylene, aluminum foil or other substrates. The film is heat-sealable and can be thermoformed. Aclar blister packages are often used for unit packages for highly moisture-sensitive pharmaceuticals.

8.6.13 Polyvinyl alcohol (PVOH) Polyvinyl alcohol polymers are produced by hydrolysis of polyvinyl acetate. Because PVOH degrades at temperatures well below melt, it cannot be processed by extrusion. Therefore, casting from a water solution is used to make the film. As produced, the film is amorphous, but orientation induces some crystallinity.

8.6.14 Ethylene±vinyl acetate (EVA) Ethylene±vinyl acetate is produced by addition copolymerization of ethylene and vinyl acetate. EVA has higher permeability to water vapor and gases than LDPE. These films have excellent transparency, and provide very good heat-seal and adhesive properties, with excellent toughness at low temperatures. In both lidding and base films, EVA is mainly used as a component of the sealant layer.

8.6.15 Ionomers The heat-seal performance of ionomers is outstanding. Ionomer films have excellent clarity, flexibility, strength and toughness, which make them suitable for modified atmosphere packaging of commodities. They can be used to package sharp objects, which break through many alternative materials when subject to vibration during distribution. Ionomers have relatively poor gas barrier properties, and tend to absorb water readily. They are also relatively costly compared to films such as ethylene±vinyl acetate (Massey 2003).

8.6.16 Polycarbonate films Polycarbonate films have excellent transparency, toughness and heat resistance, but high cost. They have some use in skin packaging, food packaging where exposure to high temperatures for in-bag preparation is required, and medical packaging.

8.6.17 Polystyrene Polystyrene is another thermoplastic film with excellent transparency, with a high tensile strength but giving a poor barrier to moisture vapor and gases

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(Benning 1983; Kader et al. 1989; Abdel-Bary 2003). It is often used in window envelopes and window cartons. Because of its low gas barrier, it can be used for produce where a `breathable' film is required. Polystyrene alone is brittle, but it can be blended or generally biaxially oriented to get the required properties. In heavier gages, polystyrene is widely used for transparent thermoformed trays.

8.7

Cellulose-based plastics

Cellulose-based plastics such as cellulose acetate, cellulose butyrate, cellulose propionate and copolymers are also used to a relatively small extent, most often as sheet rather than film. Their high price and water sensitivity limit their usefulness.

8.8

Biodegradable polymers

Biodegradable polymers are derived from replenishable agricultural feedstocks, animal sources, marine food processing industry wastes, or microbial sources. Biodegradable polymers are made from cellulose and starches. Cellophane is the most common cellulose-based biopolymer. Starch-based polymers include amylose, hydroxylpropylated starch and dextrin. Other starch-based polymers are polylactides (PLA), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), and a copolymer of PHB and valeric acids (PHB/V). Made from lactic acid formed from microbial fermentation of starch derivatives, polylactide does not degrade when exposed to moisture (Marsh and Bugusu 2007; Siracusa et al. 2008). In addition, biodegradable films can also be formed from chitosan, which is derived from the chitin of crustacean and insect exoskeletons. Chitin is a biopolymer with a chemical structure similar to that of cellulose. Edible films, in the form of a thin layer of edible materials applied to food as a coating or placed on or between food components, are another form of biodegradable polymer. They serve several purposes, including inhibiting the migration of moisture, gases and aromas and improving the food's mechanical integrity or handling characteristics, aiming to achieve modified atmosphere packaging conditions (Ben-Yehoshua et al. 1994; Marsh and Bugusu 2007). At present, bioplastics are more expensive than petroleum-based polymers, so substitution would likely result in increased packaging cost. Commercialization of bioplastics is underway. Polylactides are commercially produced from natural products (corn sugar). After the original use, the polymer can be hydrolyzed to recover lactic acid, thereby approaching the cradle-to-cradle objective (that is, imposing zero impact on future generations). In addition, Wal-Mart uses biopolymers by employing polylactides to package fresh and cut produce (Marsh and Bugusu 2007).

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Multilayer plastic films

In many cases, the best combination of packaging attributes at the lowest cost is achieved by using a combination of materials. Therefore, plastic packaging films are often combined with one another or with other materials such as paper or aluminum through processes such as coating, lamination, coextrusion and metalization.

8.9.1

Coating

Coating is commonly used to add a thin layer of a plastic on the surface of another plastic film or more commonly on a non-plastic substrate such as paper, cellophane or foil. The coating may be applied as a solution, a suspension, or a melt. Common reasons for using coating in flexible packaging are to impart heat-sealability for plastics that are not heat-sealed easily; to provide moisture protection for paper or cellophane; to improve barrier properties; and to provide protection from direct contact of the base material with the product (Banks 1984, 1985; Smith et al. 1987a; Ben-Yehoshua et al. 1994). PVDC copolymer coatings are often used to improve barrier properties and heat-sealability.

8.9.2

Lamination

Lamination is the process of combining two webs of film together (Prasad 1995; Marsh and Bugusu 2007). In flexible packaging applications, lamination is often used to combine a plastic film with another film, paper or foil. A variety of lamination methods are used. When plastic films are involved, either as a substrate or as an element in the finished structure, the laminating adhesive is often a low-density polyethylene, applied by extrusion, and the process is known as extrusion laminating. When paper is contained in a flexible package, it is most often being used for its excellent printability, along with its ability to impart substance and strength. Another significant use of lamination is to produce a web with buried printing.

8.9.3

Coextrusion

Coextrusion results in the production of a multilayer web without requiring initial production of individual webs and a separate combining step. The melted polymers are fed together carefully to produce a layered melt, which is then processed in conventional ways to produce a plastic film or sheet. When only plastics are being used in a flexible packaging structure, coextrusion is generally preferred to lamination, unless buried printing is involved. A major advantage of coextrusion over lamination is its ability to incorporate very thin layers of a material, much thinner than those that can be produced as a single web. This is

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particularly important for expensive substrates, such as those often used to impart barrier properties. The amount of the expensive barrier resin used need only be enough to provide the desired performance (Abdel-Bary 2003). The thinness of the layer is not limited by the need to produce an unsupported film and handle it in a subsequent lamination step.

8.9.4

Metalization

Metalization is a way of applying a thin metal layer on a plastic film. In commercial packaging practice, the metal being deposited is almost always aluminum. Metalized films have significantly enhanced barrier characteristics, and are usually chosen for this reason. In addition to a gas barrier, metalized film provides an essentially total light barrier (Hernandez et al. 2000).

8.9.5

Barriers and permeation

The mechanism by which substances travel through an intact plastic film is known as permeation. It involves dissolution of the penetrating substance, the permeant, in the plastic, followed by diffusion of the permeant through the film, and finally by evaporation of the permeant on the other side of the film, all driven by a partial pressure differential for the permeant between the two sides of the film (Karel et al. 1975; Nemphos et al. 1976; Koros 1989; Prasad 1995; Massey 2003). The barrier performance of the film is generally expressed in terms of its permeability coefficient or permeability. For one-dimensional steady-state mass transfer, the permeability coefficient is related to the quantity of permeant, which is transferred through the film as represented by the equation: Pˆ

Qx At
p

8:1

where P is the permeability coefficient, Q is the amount of permeant passing through the material, x is the thickness of the plastic film, A is the surface area available for mass transfer, t is time, and
p is the change in permeant partial pressure across the film. Hence the permeability coefficient (P) is the proportionality constant between the flow of the penetrant gas per unit film area per unit time and the driving force (partial pressure difference) per unit film thickness. The amount of gas penetrating through the film is expressed in terms of either moles per unit time (flux) or weight or volume of the gas at STP. Commonly, it is expressed in terms of volume. It can be shown that the permeability coefficient P, as defined by equation 8.1, is equal to the product of the Fick's law diffusion coefficient, D, and the Henry's law solubility coefficient, S (P ˆ DS) in situations where these laws

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adequately represent mass transfer (ideally dilute solutions, with diffusion independent of concentration). The permeability coefficient under these circumstances is a function of temperature but is not a function of film thickness or permeant concentration.

8.9.6

Concept and theoretical approach

Gases and vapors can permeate through materials by macroscopic or microscopic pores and pinholes or they may diffuse by a molecular mechanism, known as activated diffusion. In activated diffusion the gas is considered to dissolve in the film at one surface, to diffuse through the film by virtue of concentration gradient, and to reevaporate at the other surface of the packaging film. The equilibrium and kinetics considerations governing mass transfer have been applicable here as well. The gas transport properties through polymers can be described by three parameters: the diffusion coefficient, the permeability coefficient, and the solubility. These terms are interrelated, although the precise nature of the correlation is dependent on the type of diffusion that occurs. Generally the Fickian diffusion process is considered for gas transport in polymers. The rate of diffusion is the speed with which a gas molecule penetrates through the polymer. The diffusion coefficient D is based on Fick's first law of diffusion. It states that the flux J in the x direction is proportional to the concentration gradient …@c=@x†:   @c 8:2 J ˆ ÿD @x The flux, J, is the volume of substance diffusing across unit area in unit time, independent of the state of aggregation of the polymer. This first law is applicable to diffusion in the steady state, that is, where concentration is not varying with time. The change in concentration with time at a distance x into a thin film sheet, where the flux is in the x-direction only, is given by @c @Jx ˆÿ @x @x Substituting the values of equation 8.2 in equation 8.3, we have   @c @ @c ˆ D @x @x @x If D is independent of concentration, then equation 8.4 can be written as  2  @c @ c ˆD @x @x2

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8:4

8:5

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The permeability coefficient, P, concerns the steady-state flux, J, of gas passing through the polymer and the pressure difference across it, which gives the driving force: p ÿ p  1 2 J ˆP 8:6 x where p1 and p2 are the partial pressures on opposite sides of a film of thickness x. P is expressed in cm3 of gas at STP per cm2 of film, unit cm of film thickness per second for a pressure difference of 1 atm. The solubility, S, is defined as the amount of dissolved gas in the polymer divided by the volume of the sample for 1 atm of gas on the sample surface: c1 ÿ c2 ˆ S…p1 ÿ p2 † When p2  0 and c2  0, the above equation can be written as c1 ˆ Sp1

8:7

and when p1 ˆ 1 atm S ˆ c1 where c1 is the concentration in the sample when equilibrium is reached. Equation 8.6 obeys Henry's law when S is independent of p, and hence equations 8.2, 8.6 and 8.7 can be combined to be written as P ˆ DS

8:8

The solubility S is expressed in cm3 of gas at STP per cm3 of the solid at a pressure of 1 atm (cm3 STP/cm3.atm), The diffusivity or diffusion coefficient D is expressed as the diffusion of penetrants in cm2 across the film at STP per second (cm2/s). P is expressed in cm3 of gas at STP per cm2 of film, unit cm of film thickness per second for a pressure difference of 1 atm (cm3 STP-cm/ cm2.s.atm).

8.10

Gas permeation or gas transmission

Conceptually, the gas permeability coefficient is the same as the gas transmission rate (GTR). The GTR is defined as the volume of gas that passes through a sample of unit area under unit pressure differential, at a given temperature and film thickness, with the rate being determined after the gradient of the recorded volume±time curve has become constant. The gas transmission rate is usually expressed for the total thickness of the film, while gas permeability is expressed on the basis of per unit film thickness. For composite films, it is more appropriate to use gas transmission rate values since permeation in composite films does not usually vary linearly with film thickness. For some single material (polymer) films also, the relationship is not linear either. In such cases extrapolation may be erroneous (Nemphos et al. 1976; Laffin et al. 2009).

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8.10.1 Measurement of gas permeability There are many methods for measuring permeability of gases and it is not possible to review them in detail here. Two major types of methods used for the measurement of gas transmission rates are the pressure-increase method (or differential pressure principle) and the concentration-increase method (or equal pressure principle) (Karel et al. 1975; Prasad 1995). Pressure-increase method/differential pressure principle The test specimen is placed between the upper and lower chambers and clamped tightly. First the lower-pressure chamber (lower chamber) is vacuumized and then the whole system. When the specified degree of vacuum is reached, the lower test chamber is shut off and test gas of a certain pressure is fed to the upper test chamber (high-pressure chamber). It is ensured that a constant differential pressure (adjusted) is maintained across the specimen. Hence under the gradient of differential pressure the test gas permeates from the high-pressure side to the low-pressure side. By monitoring and measuring the pressure in the low-pressure side, the various barrier parameters (permeability coefficient) of the tested specimen are calculated (Labthink 2008). Karel et al. (1975) reported that in the pressure-increase method a membrane is mounted between the high-pressure and the low-pressure sides of a permeability cell. In this method, both sides are evacuated and the membrane is degassed. Then, at zero time a known constant pressure PH of the test gas is introduced on the high side, and PL (low-side pressure) is measured as a function of time. If the measurement is continued only as long as PH (high-side pressure) remains much larger than PL (low-side pressure),
P remains essentially constant and the permeability coefficient can be calculated as follows:        
pL VL 273
x   Pˆ  8:9
t 760 T A where P is the permeability coefficient (cm3-mm/cm2.s.cm Hg), PH is the pressure introduced at the high side, PL is the low-side pressure,
pL =
t is the steady gas pressure increment in the low-side pressure and is obtained from the slope of the increments of low-side pressure vs. time plot, VL is the calibrated volume of the low-side pressure of the cell, x is the thickness of the film, and A is the effective permeation area. Banerjee et al. (2004) measured the gas permeability of films using a laboratory-made high-vacuum apparatus with static permeation cell at 1 atm for different temperatures. The polymer film was degassed for 24 h within the permeation cell prior to the experiment. To start the measurement, desired gas pressure (Pi ˆ 1 bar) was applied instantaneously to the pressure side of the film. On the downstream side a reservoir of constant volume was connected with a pressure transducer, so that the total amount of gas that passed the polymer

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film could be monitored. The time-lag method was employed for the gas transport measurements. This technique allows the determination of the mean permeability coefficient P from the steady-state gas pressure increment (dp/dt)s in the calibrated volume V of the product side of the cell. The permeability coefficient is reported in barrer and was calculated from equation 8.10:     dp V T0 x Pˆ 8:10    dt S P0 P i T A where P is the permeability coefficient in barrer (cm3-cm/cm2.s.cm Hg), dp/dt is the steady-state gas pressure increment in the calibrated volume V of the of the cell and is obtained from the slope of the increments of downstream pressure vs. time plot, V is the calibrated volume of the product side of the cell in cm3, T0 is the standard temperature ˆ 273.15 K, P0 is the standard pressure ˆ 1.013 bar, Pi is the upstream side pressure (desired gas pressure to be applied at the highpressure side of the film, i.e. 1 bar), T is the temperature of measurement in K, x is the thickness of the film in cm, and A is the effective permeation area in cm2. Laffin et al. (2009) measured gas permeability of LDPE/LLDPE films under controlled conditions of pressure, temperature and relative humidity. The test consisted of placing the film sample between a partition test cell and an evacuated manometer, with the pressure across the film at 1 atm. As the gas passes through the film sample, the mercury in the capillary of the manometer is depressed. After a constant transmission rate was achieved, a plot of mercury height against time gave a constant gradient. The slope of the gradient was then used to calculate the gas transmission rates. The quantity of gas passing through the film is directly proportional to the difference in gas pressure on either side of the film, and inversely proportional to the thickness of the film. In addition, it is directly proportional to the time during which permeation has occurred, and to the exposed area of the sample (Karel et al. 1975). Hence, Q/

At…p1 ÿ p2 † x

8:11

PAt…p1 ÿ p2 † 8:12 x where Q is the quantity of gas which passes through the film, A is the surface area in contact with the gas, t is the time, p1 ÿ p2 is the partial pressure differential, and x is the thickness of film. Qˆ

Concentration-increase method/equal pressure principle The test principle is that high oxygen flows on one side of the film and high pure nitrogen flows on the other side of the film. Oxygen molecules pass through the film into the nitrogen on the other side, and are taken to the sensor by the flowing nitrogen. The transmission of oxygen is tested by analyzing the

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concentration of oxygen detected by the sensor. As for the packaging container, nitrogen flows in the container, and air or high pure oxygen covers the outside of the container. In most cases, gas transmission rates have been measured by employing the pressure gradient method (
p ˆ 1 atm). Normally, in MA packages a pressure gradient of 1 atm between the internal atmosphere of the package and the external atmosphere is a rare possibility. A certain degree of flexibility in the package free volume and the presence of a pressure-balancing gas, namely, N2, help in maintaining a low pressure gradient without causing considerable variation in the concentration gradient. Hence the concentration-increase or concentration gradient method facilitates close simulation of the conditions under which gas transmission takes place in MAP (Taylor et al. 1960; Yasuda et al. 1968; Prasad 1995).

8.11

Water vapor permeability

It is also important to calculate the water vapor transmission rate of the packaging system. In this case, the partial pressure difference for water vapor between the inside and the outside of the package is almost never constant. Simplifying equation 8.1, the rate of moisture gain or loss in the product is given by the resulting differential equation as follows (Abdel-Bary 2003): dQ 1 ˆ Pwv A…p2 ÿ p1 † 8:13 dt x where Q is the mass of permeant (water vapor) passing through the material in g, Pwv is the water vapor permeability of the packaging filom (g-mm/m2.day.Pa), p1 and p2 are the partial pressures of water vapor outside and inside the package respectively, in Pa, and p2 is function of Q. The principle involved is that saturated water vapor is transmitted through the test specimen in unit time under specified conditions of temperature and humidity. The transmitted mass is determined by testing the decreasing weight of distilled water as time passes. In a desiccant system of measurement, silica gel is used as the desiccant and is placed directly inside the film pouch whose Pwv is to be measured under controlled conditions of 38ëC and 90% relative humidity. Water vapor permeability is computed from the measured values of the change in weight of the packages with time, employing the following equation (Jaya 2005):     Pwv dw 1 ˆ  8:14 x dt Ap where Pwv is the water vapor permeability of the packaging film (g-mm/ m2.day.Pa), dw/dt is the weight gain by the desiccant with time and is obtained from the slope of the increments of weight vs. time plot, t is the time in days, w

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is the weight gain by the desiccant in g, x is the thickness of the film in mm, A is the area of the package in m2, and p is the water vapor pressure at 38ëC in Pa.

8.11.1 Effect of temperature on permeability The permeability of O2 and CO2 in polymeric films is temperature dependent and this dependence is commonly described by an exponential equation (Arrhenius-type equation) (Yam and Lee 1995; Exama et al. 1993). The relationship of O2 permeability and CO2 permeability with temperature can be depicted by this model. The generalized form and the specific form (permeability to O2 and CO2) of the Arrhenius equations are as follows:  P ÿEa P 8:15 P ˆ P exp RT EaP ˆ HS ‡ ED HS ˆ HC ‡ Hm where HS is the apparent heat of solution, ED is the activation energy for diffusion, HC is the heat of condensation, Hm is the heat of mixing, P is the permeability of gas at temperature T, PP is the permeability pre-exponential factor for gas, EaP is the activation energy of permeation for gas, R is the universal gas constant, and T is the absolute temperature When permeability coefficients are not available at the temperature of interest, an Arrhenius relationship can be used to determine the required value from the permeability coefficient at a nearby temperature and the activation energy. The equation used is the following:    Ea 1 1 P2 ˆ P1 exp 8:16 ÿ R T1 T 2 where T1 is the temperature at which P1 is known, T2 is the temperature at which P2 is to be calculated, Ea is the activation energy, and R is the gas constant. The permeability coefficient, as indicated, is a product of the diffusion coefficient and the Henry's law solubility constant. Since these vary in different ways with temperature, equations 8.15 and 8.16 are valid only over reasonably small temperature ranges. A particular concern is that permeation rates are much higher above the glass transition temperature (Tg) than below this temperature, and the rate of change with temperature differs. Generally, the above equations would accurately characterize a polymer's gas diffusivity/temperature behavior, except where there are strong interactions between the polymer and the gas molecules (e.g., water vapor and hydrophilic polymers). In addition, the above equations would only predict the effect of temperature above Tg, since most films show a discontinuity of diffusion at the transition. At or below Tg, the polymer conformation is set and rotational movements responsible for diffu-

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sional properties are blocked (Cowie 1973). Therefore, equation 8.15 should never be used to calculate the permeability coefficient across a temperature range that spans Tg of the plastic.

8.11.2 Temperature quotient for permeability The influence of temperature on permeability of polymeric films was quantified in Section 8.3.3 with the QP10 value, which is the permeability increase for a 10ëC rise in temperature and is given by the following equation:  10=…T2 ÿT1 † P2 P Q10 ˆ 8:17 P1 where QP10 is the temperature quotient for permeability, and P1 and P2 are the permeabilities at temperatures T1 and T2 .

8.11.3 Permeability coefficient of multiplayer films Permeability coefficients for multilayer plastic film or sheet, either laminations or coextrusions, can be calculated from the thickness and permeability coefficients of the individual layers, as follows (Abdel-Bary 2003): xt 8:18 Pt ˆ X n xi =Pi iˆ1

where the subscript t indicates the value for the total structure, i indicates the value for an individual layer, and there are n layers in the structure. When two films are combined to form a film laminate, equation 8.18 can be expressed as follows (Karel et al. 1975): 1 x1 x2 ˆ ‡ Pla xP1 xP2

8:19

where Pla is the permeability of the film laminate (cm3/m2.h.atm), P1 and P2 are the permeabilities of the individual films, i.e. of film 1 and film 2 respectively, x1 and x2 are the thicknesses of the individual films, and x is the thickness of the film laminate.

8.11.4 Effect of sub-zero temperature on permeability Lambden et al. (1985) investigated the effect of sub-zero temperatures on the oxygen transmission rate (OTR) of packaging films. They inferred that around 0ëC, a small variation in temperature greatly alters the permeabilities and thus their prediction is not possible with an Arrhenius relationship.

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8.11.5 Influence of polymer structure and morphology on permeability Salame (1986) correlated polymer structure and morphology with gas permeability. Based on cohesive energy density and fractional free-volume of the polymer, he derived a numerical scale of `permachor' values () to predict gas permeability for non-interacting polymer-penetrant systems as given below: P ˆ … A=T0 †eÿS

8:20

where A and S are constant and T0 is the tortuosity (oriented crystalline polymers).

8.12

Packaging systems in modified atmosphere packaging (MAP)

The polymeric films used for MAP are of three types: (1) polymeric films without perforations or microperforated, (2) macroperforated polymeric films, and (3) perforation-mediated packaging systems. Microperforated or non-perforated polymeric films yield low O2 and low CO2 concentrations because the CO2 permeability of these materials is generally three to six times that of O2 permeability (Yam and Lee 1995; Exama et al. 1993). These materials are suitable for less CO2-tolerant commodities such as mangoes, bananas, grapes and apples. The gas permeability in microperforated polymeric films is temperature dependent and this dependence is commonly described by Arrhenius-type equations (Exama et al. 1993; Mahajan et al. 2007) as follows:  P  ÿEa 1 1 8:21 ÿ P ˆ Pref exp R T Tref where P is the permeability at temperature T, EaP is the activation energy for permeation, R is the ideal gas constant, T is the absolute temperature, Tref is the reference temperature, and Pref is the permeability at the reference temperature. Perforated films have higher permeability rates but the ratio of CO2 to O2 permeability is much lower, approaching unity. Such films are, therefore, of great interest for commodities tolerating simultaneously low O2 and high CO2 levels such as fresh-cut products, strawberries and mushrooms ± commodities having a high respiration rate (Fonseca et al. 2000; Montero-CalderoÂn et al. 2008; Rediers et al. 2009). Macroscopic perforations in a polymeric film represent a parallel route for gas transport. An apparent permeability term is used for these films, which is a function of the film permeability and of the number and size of holes. For holes of equal size, the governing equation which describes the effect of temperature

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on permeability of macroscopic films is as follows (Fishman and Ben-Yehoshua 1996; Mahajan et al. 2007; Techavises and Hikida 2008): Pa ˆ P ‡

R2h 16:4  10ÿ6 Nh x ‡ Rh

8:22

where Pa is the apparent permeability of the macroscopic perforated film, Rh is the radius of the holes/macroperforations, Nh is the number of holes, x is the film thickness, and P is the permeability of the non-perforated films. In the perforation-mediated packaging, tubes are inserted in an airtight package (Fonseca et al. 2000, Mahajan et al. 2007). This system is also adequate for products requiring high CO2/low O2 concentrations and minimizes water accumulation inside the package. The perforation-mediated packaging system is a rigid one, which is suitable for bulk products and for products sensitive to mechanical damage. The gas exchange in perforation-mediated packages has been found to be independent of temperature within the biological range of temperature (0±25ëC). However, the permeability depends on dimensions, numbers and porosity of the tubes. The permeability of perforation-mediated packages can be represented by PAp/x as described in the following equation (Fonseca et al. 2000; Mahajan et al. 2007): PAP D1:45 ˆ 4:80  10ÿ6 NP  0:598 x LP

8:23

where NP is the number of perforations (tubes),  is the porosity ( ˆ 1 when the tubes have no packing), AP is the surface area of the package through which O2 and CO2 permeate, D is the diameter of perforation (tube) and LP is the length of perforation (tube). All variables in the above equation are in SI units.

8.13

Advanced technology for efficient modified atmosphere packaging (MAP)

The goal of MAP of fresh commodities is to create an equilibrium package atmosphere with %O2 low enough and %CO2 high enough to be beneficial to the produce and not injurious. This is accomplished through the proper balance of several variables that affect package atmosphere (Das 2005; Kader et al. 1989; Mahajan et al. 2007; Del-Valle et al. 2009). MAP has progressed in the past several years. Appropriate packaging materials have been developed for most commodities. Recent advances in polymer science and technology have made it possible to manufacture films with desired and well-designed gas transmission rates, especially for O2. There is consensus as to which films are appropriate for standard-sized packages of various food commodities. Knowledge of how to effectively seal packages and reduce incidence of leakages has developed and printing capabilities have provided ever more attractive packages. Technical

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challenges still exist in produce packaging. Some of the technologies currently available to meet those challenges are as follows.

8.13.1 Continuous films The movement of O2 and CO2 is usually directly proportional to the differences in gas concentration across the film. Steady-state (constant) O2 and CO2 levels are achieved in the package when the O2 uptake and CO2 production by the product are equal to that permeating through the film (Kader et al. 1989; Exama et al. 1993; Del-Valle et al. 2009).

8.13.2 Tailoring of film laminates Plastic films can be manufactured either as a single film or as a combination of more than one plastic. There are two ways of combining plastics: lamination and coextrusion. The tailoring of the film laminates is done when the gas permeability characteristics of any of the selected films do not match the gas permeability requirements of the MAP system satisfactorily. Thus various combinations of different films are taken and film laminates are prepared in order to bring the gas permeability characteristics as close as possible to the gas permeability requirements of the MAP (Burton et al. 1987; Prasad 1995; Jacomino et al. 2001). Based on the gas permeability characteristics of the individual films, two different films are combined to form the laminates. Lamination involves bonding together two or more plastics or bonding plastics to another material such as paper or aluminum. Bonding is commonly achieved by use of water-, solvent-, or solid-based adhesive. After the adhesive has been applied to one film, the two films are passed between rollers to pressure-bond them together. Lamination using lasers rather than adhesives has also been used for thermoplastics (Kirwan and Strawbridge 2003). Lamination enables reverse printing, in which the printing is buried between layers and thus is not subjected to abrasion and can add or enhance heat-sealability. Curwood has introduced laminations of 35±50 m polyester and linear LDPE film, which has been microcut. These microcuts permit better flow of oxygen and carbon dioxide, and thus minimize the probability of respiratory anaerobiosis.

8.13.3 Coextrusion In coextrusion, two or more layers of molten plastics are combined during film manufacture. This process is more rapid but requires materials that have thermal characteristics that allow coextrusion. Because coextrusion and lamination combine multiple materials, recycling is complicated. However, combining materials results in the additive advantage of properties from each individual material and often reduces the total amount of packaging material required.

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Therefore, coextrusion and lamination can be sources of packaging reduction (Marsh and Bugusu 2007).

8.13.4 Tailored oxygen transmission rate (OTR) The flexible packaging industry has become increasingly responsive to the specific gas requirements of fresh produce and is now providing films specifically designed for given produce items. Films for low, medium and high respiration rate commodities are now available from many package vendors and the process of matching OTR to product is being constantly refined. This has also allowed fresh-cut processors to begin to provide a much greater diversity of products, which now includes artichoke hearts, baby salad greens, sliced strawberries and many others. Very high respiration rate commodities such as litchi, strawberries, broccoli, asparagus and mushrooms have always presented a challenge to packagers. New technologies are now allowing the manufacture of very high OTR (>15,000 cm3/m2day) films for these applications (Zagory 1998).

8.13.5 Metallocene technology Technology developed independently by Dow Chemical Co. and Exxon Chemical Co. uses new single-site catalysts to produce desired polymer resins. These catalysts, when applied to the manufacture of polyethylene and other polymers, can provide a much narrower distribution of polymer chain length, molecular weight and density. This results in flexible plastic films with very high OTR, low moisture vapor transmission rate, enhanced clarity, superior strength, low seal initiation temperature and very rapid bonding of the seal. These stronger films with stronger seals are finding wide application in produce packaging (Zagory and Davis 1997).

8.13.6 Perforated films The rate of gas movement through a perforated film is a sum of gas diffusion through the perforation and gas permeation through the polymeric film. Generally, total gas flow through the perforations is much greater than gas movement through the film. Gas transmission through microperforations has been modeled (Fishman and Ben-Yehoshua 1996). The rate of gas exchange through perforations in a film is so much greater than through continuous films that a 1 mm perforation in a 0.0025 mm (1 mil) thick LDPE film has nearly the same gas flux as a half square meter area of the film. As might be surmised, perforated packages are more suitable for produce having a high O2 demand (Fonseca et al. 2002b).

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8.13.7 Microperforated films The concept of a packaging material with CO2/O2 transmissions compatible with the needs of the contained produce has been advanced. Two basic types of film materials have been proposed, tested and, to some extent, introduced on a commercial scale: microperforated and mineral filled. Alternative approaches to providing a high OTR, especially in applications where there is limited package surface area for gas exchange, have included films with holes or pores. P-Plus microperforated technology owned by Print Pak, and proprietary microperforation technologies owned by Respire Films in America and Sidlaw in England, are finding applications in the rapidly emerging fresh, lightly processed and cut fruit market (Cameron et al. 1993). The P-Plus film is manufactured by perforating a polyolefin film with very tiny orifices using laser beams. The gas permeabilities are designed to balance the respiration rate of the produce being packed. P-Plus films represent a range of base film substrates displaying permeabilities precisely matching the demands of the produce (Ben-Yehoshua et al. 1994). Most cut fruit is packaged in rigid, gas-impermeable trays with a permeable film lidstock sealed to the tray. Because the tray is impermeable to gases, there is reduced surface area for gas exchange. All the gas exchange must occur through the lidstock. Until recently, few films had a high enough OTR to be useful in these applications. Those films that had a high OTR often would not seal to the trays. However, these microperforated films display the properties required for MAP of highly respiring produce. Microporous and microperforated films allow much more rapid gas exchange than would normally be possible through plastic films (Geeson et al. 1994; Artes-Hernandez et al. 2006). Perforation retains many of the good results of sealing in reduction of water loss and alleviation of water stress without the possible deleterious effects of anaerobiosis such as off-flavors, fermentation or CO2 damage. Furthermore, perforation of polyolefin films enables the attainment of some of the advantages of seal packaging (Ben-Yehoshua et al. 1994). Microperforated packaging techniques have also proven effective in retarding deterioration in several other commodities (Geeson et al. 1985; Geeson and Smith 1989). Additionally, perforation enables MAP for highly respiring produce such as litchi, capsicums and mushrooms (Burton et al. 1987; Huang et al. 1990). Perforation may also enable MAP for produce that is sensitive to even small changes in concentrations of O2, CO2 and C2H4 (Ben-Yehoshua et al. 1994).

8.13.8 Microporous films Microporous films, which are engineered to pass low molecular weight gases such as O2, CO2, water vapor, nitrogen etc., specifically for the purpose of adjusting the gaseous concentration within the package, generally fall into two

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categories: those that are intentionally perforated with very small orifices which pass gases at a very slow rate relative to the total area; and those that contain intentional additives that interfere with the continuity of the plastic materials and thus alter their gas transmission rates (Ben-Yehoshua et al. 1994; Techavises and Hikida 2008). The two most popular microporous film technologies are those of van Leer (Belgium) and FreshHold, owned by Albert Fisher, plc. (USA), with the latter receiving major attention. In this type of material, the plastic polymer is admixed with an inert inorganic mineral such as crushed calcium carbonate or talc. The mineral fill is encapsulated in discrete particulates by the polymer and imparts a variety of properties such as stiffness. Those films exhibiting high gas permeability by virtue of their nature or by reason of being polymeric blends are technically not microporous. Among these are high (10±20%) EVA content polyethylene films such as Shields Bag or Cryovac, or polycyclic terepene film, Phillips K-resin block copolymer styrene film and Dow Chemical's Attane ultra-low-density ethylene octane copolymer films produced in the past by Bunzl in the UK. These films are being suggested as high gas permeability packaging materials for MAP of respiring produce (Abdel-Bary 2003).

8.13.9 Interactive package MAP application may require packaging materials capable of passing controlled quantities of water, oxygen, carbon dioxide and ethylene in order to control the concentrations of these gases in the internal package environment and to avoid anaerobiosis (Martinez-Romero et al. 2009). Thus was born the term, `smart' packaging, or packaging that could somehow sense the changing internal packaging environment and admit O2 from the outer atmosphere, allow excess CO2 to escape, or both. This terminology then translated into active packaging, which encompasses a broad spectrum of materials sensitive to the packaged produce requirements and its surrounding environment. The latter group includes families of package supplements such as in-package sachets of chemicals to absorb O2, CO2 or C2H4 and even to provide O2 and CO2 when the package environment has been depleted of desired gases (Ben-Yehoshua 1985; Ahvenainen 2003).

8.13.10 Tray/lidstock compatibility The high OTR requirements for lidstocks sealed to impermeable trays have often conflicted with poor sealing properties. Advances in coextrusion technology, coupled with single-site catalyst-based plastic resins, have provided better breathing, better sealing films just in time to meet the needs of the fresh-cut fruit industry (Zagory and Hurst 1996).

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8.13.11 Customizable packaging materials Because each produce item has differing, often unique, packaging requirements, the ability to customize the package to the product has been the aim of produce package development efforts. FreshHold labels can be customized to provide almost any desired OTR, as is true of microperforated films. Some film vendors provide an array of OTRs by varying the thickness of a given film. Thinner films have higher OTRs. Very thin films do not run well on modern automated packaging machinery and so this approach is limited. Landec Corporation, Inc. of California, USA, has developed side-chain polymer technology that allows the film OTR to increase rapidly as temperature increases, thereby avoiding anaerobic conditions subsequent to loss of temperature control. In addition, these polymers can provide very high OTRs, an adjustable CO2/O2 permeability ratio, and a range of moisture vapor transmission rates. These polymers are available as attachable patches that can go on bags or overwraps and represent the first truly customizable packaging system (Zagory and Davis 1997; Zagory 1998).

8.13.12 Antifog properties Potential buyers like to see fresh produce before they buy it. Therefore plastic packages need to be clear and the product visible. Condensation of water inside the package can often occlude the view of the product. Antifog compounds have been developed that, when included in coextruded films, migrate to the inner surface of the film and prevent large water drops from forming. This results in a more attractive package and a better view of the product. However, antifog coatings can interfere with seal integrity and so newer technology relies on register coatings that apply the antifog material only on selected areas of the film away from the seal.

8.14

Package management

There has emerged an increased appreciation that packaging can deliver its promised benefits to fresh produce only within a specific temperature range. In addition, an emphasis on shelf-life extension has shifted to an emphasis on quality preservation. As the marketplace for fresh and cut products becomes more competitive, it is the quality that sells, not shelf-life. This has resulted in increased attention to maintaining low temperatures and rapid distribution. Such changes in perspective have helped realize the benefits that modern packaging films can provide (Kablan et al. 2007).

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Design of modified atmosphere packaging (MAP)

The basis of MAP is that a reduced O2 environment suppresses respiration by the commodity, thereby slowing vital processes and prolonging the maintenance of post-harvest quality. A secondary but potentially important factor is a concomitant decrease in respiration in response to elevated CO2. So this modified atmosphere can potentially reduce respiration rate, ethylene biosynthesis and sensitivity to ethylene, decay and physiological changes, namely oxidation (Kader et al. 1989; Saltveit 1993; Mahajan et al. 2007). The objective of MAP design is to define conditions that will create the atmosphere best suited for the extended storage of a given produce while minimizing the time required to achieve this atmosphere. This can be done by matching the film permeation rate for O2 and CO2 with the respiration rate of the packaged produce. As different products vary in their behavior and as MA packages will be exposed to a dynamic environment, each package has to be optimized for specific demands (Chau and Talasila 1994; Jacxsens et al. 2000; Mahajan et al. 2007). MAP is a dynamic system during which respiration and permeation occur simultaneously. Factors affecting both respiration and permeation must be considered when designing a package (Cameron et al. 1989; Mannapperuma et al. 1989; Yam and Lee 1995; Jacxsens et al. 2000). The commodity mass kept inside the package, storage temperature, oxygen, carbon dioxide and ethylene partial pressures, and stage of maturity are known to influence respiration in a package (Kader et al. 1989; Beaudry et al. 1992; Ben-Yehoshua et al. 1994; Das 2005). Type, thickness, unintended holes, and surface area of the packaging film that is exposed to atmosphere and across which permeation of O2 and CO2 takes place, volume of void space present inside the package, as well as temperature, relative humidity, and gradient of oxygen and carbon dioxide partial pressures across the film, are known determinants of permeation (Ashley 1985; Beaudry et al. 1992; Kader 1997; Renault et al. 1994a; Das 2005). In a MAP packaging system, fresh fruits are sealed in perm selective polymeric film packages. Due to respiration of the packaged fruits, O2 starts depleting and CO2 starts accumulating within the package because of the consumption of O2 and the production of CO2 in the respiration process. Consequently, respiration begins to decrease while O2 and CO2 concentration gradients between package and ambient atmosphere begin to develop. The development of concentration gradients induced ingress of O2 and egress of CO2 through the packaging films (Cameron et al. 1989; Merts et al. 1993; Chau and Talasila 1994; Renault et al. 1994a; Mahajan et al. 2007). In a properly designed MAP, after a period of transient state an equilibrium state is established. At equilibrium, the amount of O2 entering the package and that of CO2 permeating out of the package become equal to the amount of O2 consumed and that of CO2 evolved by the packaged fruit, respectively (Jacxsens et al. 2000; Del Nobile et

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al. 2007; Techavises and Hikida 2008). The package atmosphere is then considered to be in dynamic equilibrium with the external atmosphere. An ideal package system will equilibrate and maintain the levels of O2 and CO2 that are known to be optimal for storage, transport and handling throughout the market chain for a specific commodity (Jacxsens et al. 1999; Van der Steen et al. 2001; Fonseca et al. 2000; Paul and Clarke 2002; Mahajan et al. 2007).

8.15.1 Design methodology MAP design requires the determination of intrinsic properties of the produce, i.e. respiration rate, optimum O2 and CO2 gas concentrations, and film permeability characteristics (Cameron et al. 1989; Talasila and Cameron 1997). The ultimate aim of this design process is to select suitable films for a given product, its area and thickness, filling weight, equilibrium time, and the equilibrium gas composition at isothermal and non-isothermal conditions. The design involves the mathematical modeling of the gaseous exchange in MAP, the respiration rate of the commodity, the permeability of the films and the optimization of package parameters (Exama et al. 1993; Mahajan et al. 2007; Das 2005; Kader et al. 1989).

8.16

Mathematical modeling of gaseous exchange in modified atmosphere packaging (MAP) systems

When fresh fruits and vegetables are sealed in a selected polymeric film package, it constitutes a dynamic system in which respiration of the product and gas permeation through the film take place simultaneously. In the respiration process O2 is consumed and the produce evolves CO2. The simplest concept is that the plastic film serves as the regulator of O2 flow into the package and of flow of CO2 out of the package. For a small length of transient period and at a given temperature, the rates of O2 consumption and CO2 evolution of the packaged commodity depend greatly on the O2 and CO2 concentrations. The differential mass balance equations that describe the O2 and CO2 concentration changes in a package containing respiring product are given in equations 8.24 and 8.25, respectively:     Wp Ap PO2 dYO2 a RO2 ‡ …YO2 ˆÿ ÿ YO2 † 8:24 dt Vfp Vfp     Wp Ap PCO2 dZCO2 a RCO2 ÿ …ZCO2 ÿ ZCO2 ˆÿ † 8:25 dt Vfp Vfp where Ap is the area of the package through which the O2 and CO2 permeate a a and ZCO2 are the O2 and CO2 concentrations in the atmospheric air (m2), YO2 3 3 (cm per cm of air) respectively, YO2 and ZCO2 are the O2 and CO2

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concentrations inside the package (cm3 per cm3 of air) respectively, PO2 and PCO2 are the O2 and CO2 permeabilities of the packaging material (cm3.m±2.h±1. [conc. diff. of O2 in volume fraction]±1) respectively, Wp is the weight of the fruit kept inside the package (kg), RO2 and RCO2 are the respiration rates for O2 consumption and CO2 evolution by the fruits (cm3.kg±1.h±1) respectively, Vfp is the free volume in the package (cm3), t is the storage time (h) and dYO2/dt and dZCO2/dt are the rates of change of O2 concentration (YO2) and CO2 concentration (ZCO2) within the package at time t of storage (cm3 per cm3 of air.h±1), respectively. Equations 8.24 and 8.25 coupled to the model that describes the dependence of respiration rate on gas composition, temperature (and eventually time) and models that describe the dependence of packaging material on temperature, constitute the basis of MAP design (Exama et al. 1993; Chau and Talasila 1994; Talasila et al. 1994; Talasila and Cameron 1997; Makino and Iwasaki 1997; Maneerat et al. 1997; Jacxsens et al. 2000; Das 2005; Del Nobile et al. 2007; Mahajan et al. 2007; Torrieri et al. 2007). The effect of gas composition on respiration rate is often described by the Michaelis±Menten equation with different types of inhibitions, while the effect of temperature is quantified by an Arrhenius-type equation (Lee et al. 1991; Peppelenbos and Leven 1996; Mangaraj and Goswami 2008a). The permeability of O2 and CO2 in polymeric films is temperature dependent and this dependence is commonly described by an exponential equation (Arrhenius-type equation) (Exama et al. 1993; Mahajan et al. 2007; Mangaraj and Goswami 2008b).

8.17

Current application of polymeric films for modified atmosphere packaging (MAP) of fruits and vegetables

Many plastic films have been used for MAP of varieties of produce (Table 8.3). The packaging of apples using polyethylene, PVC, PET, etc., films was found to be successful in increasing shelf-life and maintaining quality (Veeraju and Karel 1966; Anzueto and Rizvi 1985; Smith et al. 1987b, 1988; Prasad 1995; Kader 1997; Guan Wenqiang et al. 2004). Rocha et al. (2004) packed apple using polypropylene of 100 m during 6.5 months at 4ëC and 85% R.H. and found that apples packed in MA lost less weight, presented better color, and preserved better firmness than fruits stored in air. Prasad (1995) developed MA packages for apple, combining BOPP and PVC films by tailoring of film laminates. The MA packed apples were reported to have retained orchard freshness and increased shelf-life considerably. The incidence of bitter pit that developed during storage of apple was progressively reduced from 50% to less than 5% using LDPE packages (Hewett 1984). Packaging citrus fruits in polyethylene films maintains high R.H. inside the package and hence reduction of shrinkage (Oswin 1975; Barmore et al. 1983;

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Ben-Yehoshua 1985; Smith et al. 1987a). Guava packed with PVC, LDPE or PET and stored for two and three weeks at 5 and 8ëC hindered the development of peel color and the loss of firmness (Gaspar et al. 1997; Mohamed et al. 1994; Pal et al. 2002; Pereira et al. 2004). Sanjuka et al. (2003) observed that guava stored with the silicon membrane had good overall quality after storage and after ripening. Combrink et al. (2004) reported that unperforated polyethylene bags maintained guava fruit quality better than perforated bags. Most workers have used polyethylene and PVC films (Saguy and Mannheim 1975; Nakashi et al. 1991) to extend tomato shelf-life up to 21 days. Banana has greatly benefited from MAP using LDPE films due to reduced C2H4 sensitivity associated with high CO2 and low O2 (Banks 1984; Aradhya et al. 1993; Bhande 2007). MAP has been considered to be beneficial to maintain high humidity, essential for prevention of water loss and browning of litchi pericarp (Scott et al. 1982; Nip 1988; Underhill and Critchley 1992; Kader 1995; Ray 1998; Pesis et al. 2002; Duan et al. 2004; Tian et al. 2005; Sivakumar and Korsten 2006). Litchi treated with 1% HCl, packed with polyethylene films (Jiang Yueming et al. 2004) and with or without SO2 treatment, sealed with polyethylene and PVC films (Paull and Chen 1987; Paull et al. 1998; Chaiprasart 2003), prevented dehydration and pericarp browning. Sivakumar and Korsten (2006) reported that MAP of litchi fruits using BOPP film after post-harvest treatment minimized the rate of transpiration, preventing weight loss and deterioration of fruit quality. Microperforated LDPE (30 m) films provided effective, favorable atmospheres for Bramley apples during a simulated four-week marketing period under ambient conditions and the shelf-life benefit was observed (Geeson 1989). Artes-Hernandez et al. (2006) studied the quality of superior seedless table grapes under MAP using microperforated and oriented polypropylene films and reported that SO2-free MAP kept the overall quality of clusters close to that at harvest. Packaging of strawberries using LDPE, PVC and polypropylene films with or without perforations showed a considerable improvement in quality in terms of fruit firmness, weight loss, desiccation and decay (Pinatauro 1978; Aharoni and Barkai-Golan 1987; El-Ghaouth et al. 1992; Sanz et al. 2000; Van der Steen et al. 2001; Wu Ying et al. 2007; Zheng Yonghua 2008). A number of researchers have worked on MAP of cherries and other similarly perishable fruits including blueberries and raspberries (Beaudry and Lakakul 1992; Crisosto et al. 1993; Cameron et al. 1995; Reed 1995; Moyls et al. 1998; Van der Steen et al. 2001; Petracek et al. 2002) using PVC, polypropylene, LDPE and microperforated films or films that are unperforated but have a selective permeability to oxygen and carbon dioxide. Valero et al. (2006) developed active packaging by adding eugenol or thymol to table grapes stored for 56 days under MA condition. The sensory, nutritional and functional property losses were significantly reduced in packages with added eugenol or thymol. In addition, lower microbial spoilage counts were achieved with the active packaging (Labuza 1990).

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Jacomino et al. (2001) observed that multilayer coextruded polyolefinic film with selective permeability (PSP) can prolong storage of guava up to three weeks, while low-density polyethylene film with incorporated minerals (LDPEm) is suitable for guava storage at 10ëC with 85±90% relative humidity. The PSP film and LDPE film with mineral incorporation provided an atmosphere that kept the fruit with good sensorial characteristics for 28 days and 14 days, respectively. MAP was proven to extend the shelf-life of several vegetables. Both high CO2 and low O2 concentrations retard respiration and senescence of broccoli heads (Serrano et al. 2006). Broccoli maintains its quality longer in both perforated and sealed polyethylene, BOPP and PVC film packages (Aharoni et al. 1987; Rij and Ross 1987; Granado-Lorencio et al. 2008). Christie et al. (1995) successfully developed MA packages for broccoli using LDPE films impregnated with inorganic particles. It was observed that the overall quality of broccoli packaged in LDPE which contained an ethylene absorber was perceived to be the sample most similar to fresh broccoli (Martinez-Romero et al. 2007). A biodegradable laminate of a chitosan±cellulose was found to be suitable as a packaging material for MAP and storage of broccoli (Yoshio and Takashi 1997). Serrano et al. (2006) stored broccoli using macroperforated, microperforated and unperforated polypropylene films and observed that all changes related to loss of quality were significantly reduced and delayed with time, especially with perforated and unperforated films. Packaging of peppers with plastic films extended the shelf-life but the major benefit appeared to be mediated by the maintenance of high relative humidity inside the package which reduced the rates of transpiration (Maxie et al. 1974; Ben-Yehoshua et al. 1983; Miller et al. 1986b; Watada et al. 1987; Irtwange 2006). MAP is an economical and effective way of extending shelf-life of fresh mushrooms. Packaging of mushrooms using PVC wrap and polyolefin films increased shelf-life by retarding cap opening, reducing respiration, reducing internal browning and reducing weight loss, consequently resulting in higher quality (Nichols and Hammond 1975; Kim et al. 2006). In the perforated film packs the quality of mushroom varied according to film permeability and number of perforation holes (Nichols and Hammond 1975). Koide and Shi (2007) compared the storage quality of green peppers using PLA-based biodegradable film packaging with LDPE, and perforated LDPE film packaging. It is suggested that the biodegradable film with higher water vapor permeability can be used to maintain the quality and sanitary conditions of freshly harvested green peppers in modified atmosphere packaging. Almenar et al. (2008) packed highbush blueberries in polylactide (PLA) containers and stored them at 10ëC for 18 days and at 23ëC for nine days. Physicochemical and microbiological studies were carried out in order to know the efficacy of PLA packages. Results showed that the PLA containers prolonged blueberry shelf-life at different storage temperatures. Del Nobile et al.

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(2008) used four different films ± two polyester-based biodegradable films, a multilayer film made by laminating an aluminum foil with a polyethylene film, and an oriented polypropylene film ± to study the ability of biodegradable films to prolong the shelf-life of minimally processed lettuce stored at 4ëC. Results suggest that the gas permeability of the investigated films plays a major role in determining the quality of the packed produce. Moreover, it was observed that biodegradable films guarantee a shelf-life longer than with an oriented polypropylene film package. Jacxsens et al. (2000) designed equilibrium MA packages for fresh-cut vegetables such as bell peppers, broccoli, carrots, chicory, cucumbers, French beans, iceberg lettuce, mixed lettuce and mungbean sprouts subjected to changes in temperature that are similar to those encountered in the distribution chain. Higher respiration rates and temperature dependence of cut/shredded produce was observed compared to unprocessed vegetables. Hence, they developed new unperforated packaging films with high O2 and CO2 permeability. The EMA packages were designed by combining mathematical models that describe the effect of temperature, O2 and CO2 levels on produce respiration. The influence of temperature on respiration was described by an Arrhenius type of equation, while the influence of O2 and CO2 on respiration was modeled by Michaelis± Menten kinetics (Gonzalez-Aguilar et al. 2004; Mahajan et al. 2007; Del Nobile et al. 2007). Fresh-cut produce seemed to be more temperature sensitive than unprocessed fresh vegetables. The proposed packaging systems with the new polymer films of higher permeabilities provided a sufficient low headspace O2 level in a temperature range between 2 and 10ëC. Ultrathin SiO2 films in the range of 2±50 nm thickness were readily fabricated from inexpensive sodium silicate as starting material by its alternate adsorption with cationic polymer and subsequent treatment with O2 plasma and calcination. Film thickness can be controlled by adjusting the number of adsorption cycles and the pH value of silicate solution. The film surface is generally smooth (small roughness) and remains unchanged after O2 plasma treatment or calcination. Whereas a nanoporous thin film is obtained by O2 plasma treatment, a dense silica film is produced through calcination at 450ëC. These preparative methods prove that inexpensive sodium silicates are converted to advanced silica-based materials, such as functional ultrathin films, coatings, capsules, and catalyst, by a simple procedure.

8.18

Future trends

The use of MAP for fresh produce is quite restricted for a number of reasons. No single polymer offers all the properties required for MAP. In addition to barrier properties, properties like machinability/sealability must also be taken into account. One inherent requirement for all MAP packs is the ability to retain the desired atmosphere for as long as possible. This is achieved first by choosing a

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film or films to provide the required gas and moisture vapor permeability characteristics and second by ensuring seal integrity of the packs. To achieve the above film characteristics the different plastic films are either laminated or coextruded. The unavailability of appropriate films that provide a safe modified atmosphere, especially under the abusive temperature conditions that can occur in the handling chain, can be a significant problem. Packages that provide a safe atmosphere at one temperature may result in anaerobic conditions at higher temperature. As of today, the application of MAP is limited to certain produce and/or specific purposes that subsequently provide reduced profit margin. If MAP was applied commercially to a large number of crops, the profit margin would increase substantially. Some problems are associated with maintaining package integrity during storage and transportation. The plastic films used for MAP must be flexible and easy to use, but strong enough to survive normal handling operations. Based on the above points, some of the future research works that can be suggested are as follows. The conditions for MAP of all commodities should be standardized based on appropriate design steps which include matching the permeability requirements of films. This may lead to the commercial production of polymeric films of desirable properties for MAP and their availability in the market for sustainable growth and development. The polymeric films providing recommended gas transmission rates and other characteristics (strong, flexible, transparent, durable and food grade) required for MAP for all commodities should be produced commercially, as either single, coextruded or laminated polymers, for the success and popularization of MAP technology. When selecting polymer films for particular packaging applications, it is important that the film permeabilities be measured under the envisaged storage conditions and using a mixed gas technique to give realistic predictions in the MAP system. New trends of research are needed in the development of interactive or smart films. These new films may somehow sense the changing internal packaging atmosphere and admit O2 from the outer atmosphere or allow excess CO2 to escape. Package modeling can improve understanding of how package, plant and environmental factors interact and can be useful in package design. The creation and maintenance of an optimal atmosphere inside an MA package depends on the respiration rate of the product and the permeability of the films to O2 and CO2, both of which are affected by temperature. However, an increase in temperature has different effects on these two parameters: the increase in the respiration rate as a function of temperature described by QR10 is generally substantially greater than the increase in the permeability of the packaging material. Hence films should be produced to match the QR10 and QP10 to counter the effects of temperature fluctuations encountered during the transport, storage and distribution chain.

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Metallocene technology, ultrathin SiO2 films, O2 and CO2 tailored film, oxygen transmission rate, microperforated films, interactive packaging film, customizable packaging materials, mineral incorporated film, and various additives to preserve the quality of fresh produce are the future needs for the application of MAP. Modified atmosphere packaging is used as a supplement to low-temperature preservation of fruits and vegetables. It is mainly used to reduce respiration rate and retard the ripening process, thereby increasing resistance to diseases for the host. Commercially, MA is successful for storage of apples, pears, fresh cut (minimally processed) fruits and vegetables and for highly perishable and highvalue commodities, such as cherries, figs, raspberries, strawberries, litchi, capsicums, mushrooms, etc. (Gran and Beaudry 1992; Meheriuk et al. 1995). A marginal increase in storage life and quality by MA storage is not enough for the added cost of implementing MA technology commercially for most fruits and vegetables. An important problem in the commercial application of MA in fruits and vegetables is that the effect of MA is different for the same cultivar grown in different locations, under different cultural practices or in different seasons. Therefore, trial-and-error studies have to be conducted to determine the optimum atmosphere for each cultivar in a given place and season. Film packaging offering any desired values of permeability to O2, CO2 and water vapor is available in developed nations, but not in developing countries. Models describing the respiration rates of fresh fruits and vegetables and gas permeability need to be developed. Based on such models, and critical design of MAP systems, it is possible to maintain an optimum atmosphere recommended for safe storage and extended shelf-life of commodities. As fruits and vegetables are more sensitive to environmental conditions, accurate design of the MAP is essential to achieve superior product quality, and the development of models for different fruits and vegetables is a pre-requisite. Research is also needed in integrating active packaging with MAP to make this technology economically viable. Current ethylene-removing techniques (catalytic or chemical oxidation) are not commercially successful. Active packaging involving ethylene-absorbing substances should be studied. MAP and related technology can be selectively used in post-harvest handling of fresh fruits and vegetables with good results. There is a need for commercial application of the technology by processors and fresh produce retailers. There is also a need for research into MAP and related technologies for local crops under local conditions in developing countries.

8.19

References and further reading

Abeles FB, Morgan PW, Saltveit ME (1992) Ethylene in Plant Biology, 2nd edition, Academic Press, New York Abdel-Bary EM (2003) Handbook of Plastic Films, Rapra Technology Ltd, Shawbury, Shrewsbury, UK

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Manay N, Shadaksharaswamy M (2006) Foods: Facts and Principles, New Age International Publishers, New Delhi, India, p. 540 Maneerat C, Tongta A, Kanlayanarat S, Wongs-Aree C (1997) A transient model to predict O2 and CO2 concentrations in modified atmosphere packaging of bananas at various temperatures. In JR Gorny (ed.), Proc. 7th Int. Controlled Atmosphere Research Conf., Davis, CA, Vol. 5, pp. 191±197 Mangaraj S, Goswami TK (2008a) Respiration rate modelling of royal delicious apple at different temperature. Fresh Produce, 2(2), 72±80, published by Global Science Books, UK Mangaraj S, Goswami TK (2008b) Modelling of respiration rates of litchi fruit under aerobic condition. Food Bioprocess. Technol., in press, doi: 10.1007/s11947-0080145-z Mannapperuma JD, Zagory D, Singh RP, Kader AA (1989) Design of polymeric packages for modified atmosphere storage of fresh produce. In JK Fellman (ed.), Proc. 5th Int. Controlled Atmosphere Research Conf., Wenatchee, WA, Vol. 1, pp. 225±233 Marcellin P (1974) Conservation de leÂgumes en atmospheÁre controlleÂe dans des sacs en polyeÂthyleÁne avec feneÃtre de elastomeÁre de silicone. Acta Hortic. 38, 30±37 Marsh K, Bugusu B (2007) Food packaging ± roles, materials, and environmental issues. J. Food Sci. 72(3), R39±R54 Martinez-Romero D, Serrano M, Guillen F, Castillo S, Valero D (2007) Tools to maintain postharvest fruit and vegetable quality through the inhibition of ethylene action: a review. Crit. Rev. Food Sci. Nutr. 47, 543±560 Martinez-Romero D, Guillen F, Castillo S, Zapata PJ, Serrano M, Valero D (2009) Development of a carbon-heat hybrid ethylene scrubber for fresh horticultural produce storage purposes. Postharvest Biol. Technol. 51, 200±205 Martins RC, Lopes VV, Vicente AA, Teixeira JA (2008) Computational shelf-life dating: complex systems approaches to food quality and safety. Food Bioprocess Technol. 1, 207±222 Massey LK (2003) Permeability Properties of Plastics and Elastomers. A Guide to Packaging and Barrier Materials. Plastic Design Laboratory/William Andrew Publishing, Norwich, NY, p. 601 Maxie EC, Mitchell FG, Sommer NF, Synder RG, Rae HL (1974) Effect of elevated temperature on ripening of Bartlett pear, Pyrus communis L. J. Am. Soc. Hortic. Sci. 99, 344±149 McLaughlin CP, O'Beirne D (1999) Respiration rate of a dry coleslaw mix as affected by storage temperature and respiratory gas concentrations. J. Food Sci. 64(1), 116±119 Meheriuk M, Girard B, Moyls L, Beveridge HJT, McKenzie DL, Harrison J, Weintraub S, Hocking R (1995) Modified atmosphere packaging of `Lapins' sweet cherry. Food Res. Int. 28, 239±244 Menon RR, Goswami TK (2008) Modelling of respiration rates of green mature mango under aerobic conditions. Biosystems Engineering 99(2), 239±248 Merts I, Cleland DJ, Banks NH, Cleland AC (1993) Mathematical model of a modified atmosphere packaging system for horticultural produce. Sci. Technol. Froid. 3, 440±447 Meyer BS, Anderson DB, Bohling RH, Fratianne DG (1973) Introduction to Plant Physiology (2nd edn), Van Nostrand, Princeton, NJ Miller WR, Risee LA, McDonald RE (1986a) Deterioration of individually wrapped and non wrapped bell peppers during long-term storage. Trop. Sci. 26, 1±5 Miller WR, Spalding DH, Hale PW (1986b) Film wrapping mangoes at advancing stages

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of post harvest ripening. Trop. Sci. 26, 6±11 Mohamed S, Kyi KMM, Yusof S (1994) Effects of various surface treatments on the storage life of guava (Psidium guajava L.) at 10ëC. J. Sci. Food Agric. 66, 9±11 Montero-CalderoÂn M, Rojas-GrauÈ MA, MartõÂn-Belloso O (2008) Effect of packaging conditions on quality and shelf-life of fresh-cut pineapple (Ananas comosus). Postharvest Biol. Technol. 50, 82±89 Moyls AL, McKenzie DL, Hocking RP, Toivonen PMA, Delaquis P, Girard B, Mazza G (1998) Variability in O2, CO2, and H2O transmission rates among commercial polyethylene films for modified atmosphere packaging. Trans. Am. Soc. Agric. Eng. 41(5), 1441±1446 Nakashi S, Schlimme D, Solomos T (1991) Storage potential of tomatoes harvested at the breaker stage using modified atmosphere packaging. J. Food Sci. 56(1), 55±59 Nemphos SP, Salame M, Steingiser S (1976) Barrier polymers. In Encyclopedia of Polymer Science and Technology 1(Suppl.), 65±95 Nichols R, Hammond JBW (1975) The relationship between respiration, atmosphere and quality in intact and perforated mushroom prepacks. J. Food Technol. 10, 424±429 Nip WK (1988) Handling and perservation of lychee (Litchi chinensis Sonn.) with emphasis on color relation. Trop. Sci. 28, 5±11 Opara LU, Al-Ani MR, Al-Shuaibi YS (2008) Physico-chemical properties, vitamin C vontent, and antimicrobial properties of pomegranate fruit (Punica granatum L.). Food Bioprocess. Technol. doi 10.1007/s11947-008-0095-5 Oswin CR (1975) Plastic Films and Packaging. Applied Science Publishers, London Pal RK, Buescher RW (1993) Respiration and ethylene evolution of certain fruits and vegetables in response to carbon dioxide in controlled atmosphere storage. J. Food Sci. Technol. 30, 29±32 Pal RK, Singh SP, Singh CP, Asre R (2002) Response of guava fruits (Psidium guava L. cv. Lucknow-49) to control atmosphere storage. ISHS Acta Horticulturae 735: I, International Guava Symposium Parry RT (1993) In RT Parry (ed.), Principles and Application of Modified Atmosphere Packaging of Food, Blackie, Glasgow, UK, pp. 1±18 Paul DR, Clarke R (2002) Modeling of modified atmosphere packaging based on designs with a membrane and perforations. J. Membrane Sci. 208, 269±283 Paull RE, Chen JN (1987) Effect of storage temperature and wrapping on quality characteristics of litchi fruit. Sci. Hort. 33, 223±236 Paull RE, Reyes MEQ, Reyes MU (1998) Sulfite residues on litchi fruit treated with sulfur dioxide. Postharvest Biol. Technol. 14, 229±233 Peleg K (1985) In Produce Handling, Packaging and Distribution. AVI Publishing, Westport, CT, p. 625 Peppelenbos HW, Leven J (1996) Evaluation of four types of inhibition for modelling the influence of carbon dioxide on oxygen consumption of fruits and vegetables. Postharvest Biol. Technol. 7, 27±40 Pereira LM, Rodrigues ACC, Sarantopoulos CIGL, Junqueira VCA, Cunha RL, Hubinger MD (2004) Influence of modified atmosphere packaging and osmotic dehydration on the quality maintenance of minimally processed guavas. J. Food Sci. 69(4), 1107±1111 Pesis E, Dvir O, Feygenberg O, Arie RB, Ackerman M, Lichter A (2002) Production of acetaldehyde and ethanol during maturation and modified atmosphere storage of litchi fruit. Postharvest Biol. Technol. 26, 157±165 Petracek PD, Joles DW, Shirazi A, Cameron AC (2002) Modified atmosphere packaging of sweet cherry (Prunus avium L., cv. `Sams') fruit: metabolic responses to oxygen,

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carbon dioxide, and temperature. Postharvest Biol. Technol. 24, 259±270 Phillips CA (1996) Review: Modified atmosphere packaging and its effects on the microbiologcal quality and safety of produce. Int. J. Food Sci. Tech. 31, 463±479 Pinatauro ND (1978) Food Packaging. Noyes Data Corp., Park Ridge, NJ, pp. 379±387 Prasad M (1995) Development of modified atmosphere packaging system with permselective films for storage of red delicious apples. Unpublished Ph.D. thesis, Department of Agriculture and Food Engineering, Indian Institute of Technology, Kharagpur, India Prasad M, Singh RP (1994) Development of modified atmosphere packaging system for fruit storage through gas barrier polymeric films. In IS Bhardwaj (Ed.), Polymer Science ± Recent Advances, Vol. 2, Allied Publishers, New Delhi, India, pp. 971± 987 Prince TA, Herner RC, Lee J (1986) Bulb organ changes and influence of temperature on gaseous levels in a modified atmosphere package of precooled tulip bulbs. J. Am. Soc. Hortic. Sci. 111, 898±904 Quevedo RA, Aguilera JM, Pedreschi F (2008) Color of salmon fillets by computer vision and sensory panel. Food Bioprocess Technol. doi 10.1007/s11947-008-0106-6 Raghavan GSV, Gariepy Y (1985) Structure and instrumentation aspects of storage systems. Acta Hort. 157, 5±30 Ratti C, Raghavan GSV, Gariepy Y (1996) Respiration rate model and modified atmosphere packaging of fresh cauliflower. J. Food Eng. 28, 297±306 Ray PK (1998) Post-harvest handling of litchi fruits in relation to colour retention ± a critical appraisal. J. Food Sci. Technol. 35, 103±116 Rediers H, Claes M, Peeters L, Willems KA (2009) Evaluation of the cold chain of freshcut endive from farmer to plate. Postharvest Biol. Technol. 51, 257±262 Reed AN (1995) Commercial considerations for modified atmospheric packaging of cherries. Tree Fruit Postharvest J. 6(4), 11±14 Renault P, Souty M, Chambroy Y (1994a) Gas exchange in modified atmosphere packaging. 1. A new theoretical approach for micro-perforated packs. Int. J. Food Sci. Technol. 29, 365±378 Renault P, Houal L, Jackuemin G, Chambroy Y (1994b) Gas exchange in modified atmosphere packaging. 2. Experimental results with strawberries. Int. J. Food Sci. Technol. 29(4), 379±394 Rhodes MJC (1980) The physiological basis for the conservation of food crops. Prog. Food Nutr. Sci. 4, 11±20 Richardson DG, Kupferman E (1997) Controlled atmosphere storage of pears. In EJ Mitcham (ed.), Apples and Pears. Postharvest Hort. Series No. 16, University of California, Davis, CA, CA'97 Proc., Vol. 2, pp. 31±35 Rij RE, Ross SR (1987) Quality retention of fresh broccoli packaged in plastic films of defined CO2 transmission rates. Packag. Technol. 22, 14±18 Rocha AMCN, Barreiro MG, Morais AM MB (2004) Modified atmosphere package for apple `Bravo de Esmolfe'. J. Food Control 15(1), 61±64 Roy S, Anantheswaran RC, Beelman RB (1995) Fresh mushroom quality as affected by modified atmosphere packaging. J. Food Sci. 60(2), 334±340 Ryall AL, Lipton WJ (1979) Handling, Transportation and Storage of Fruits and Vegetables, Vol. 1, Vegetables and Melon, 2nd edition, AVI Publishing, Westport, CT, pp. 1±13 Ryall AL, Pentzer WT (1974) Handling, Transportation and Storage of Fruits and Vegetables, Vol. 2, Fruits, AVI Publishing, Westport, CT, pp. 4±12 Sacharow S, Griffin RC (1980) Principles of Food Packaging, 2nd edition, AVI

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Publishing, Westport, CT Saguy I, Mannheim CH (1975) The effect of selected plastic films and chemical dips on the shelf life of marmande tomatoes. J. Food Technol. 10, 544±549 Salame M (1986) Prediction of gas barrier properties of high polymers. Polym. Eng. Sci. 26(22), 1543±1546 Saltveit ME (1993) A summary of CA and MA requirements and recommendations for the storage of harvested vegetables. In GD Blanpied, JA Bartsch and JR Hicks (eds), Proc. 6th Int. Controlled Atmosphere Research Conf., Ithaca, NY, Vol. II, pp. 800±818 Saltveit ME (1996) Physical and physiological changes in minimally processed fruits and vegetables. In FA Tomas-Barberan (ed.), Psytochemistry of Fruits and Vegetables, Oxford University Press, Oxford, pp. 205±220. Saltveit ME Jr (1997) A summary of CA and MA requirements and recommendations for harvested vegetables. In ME Saltveit (ed.), Proc. 7th Int. Controlled Atmosphere Research Conf., Davis, CA, Vol. 4, pp. 98±117 Saltveit ME (2005) Commercial storage of fruits, vegetables and florist and nursery crops. Postharvest Technology Centre RIC, Department of Plant Science, 104 Mann Laboratories, University of California, USA Salunkhe DK, Kadam SS (1995) Handbook of Fruit Science and Technology, Marcel Dekker, New York Sanjuka PS, Nieuwenhof F, Raghavan GSV (2003) Extension of storage life of guava using silicon membrane system. Written for presentation at the CSAE/SCGR 2003 Meeting, Montreal, QueÂbec, 6±9 July, 2003 Sanz C, Perez AG, Olias R, Olias JM (2000) Modified atmosphere packaging of strawberry fruit: Effect of package perforation on oxygen and carbon dioxide. Food Sci. Technol. Int. 6(1), 33±38 Scott KJ, Brown BI, Chaplin GR, Wilcox ME, Bain JM (1982) The control of rotting and browning of litchi fruit by hot benomyl and plastic film. Sci. Hort. 16, 253±262 Serrano M, Martinez-Romero D, Guillen F, Castillo S, Valero D (2006) Maintenance of broccoli quality and functional properties during cold storage as affected by modified atmosphere packaging. Postharvest Biol. Technol. 39, 61±68 Shirazi A, Cameron AC (1992) Controlling relative humidity in modified atmosphere packages of tomato fruit. HortScience 27, 336±339 Siracusa V, Rocculi P, Romani S, Dalla Rosa M (2008) Biodegradable polymers for food packaging: a review. Trends Food Sci. Technol. 19, 634±643 Siripanich J, Kader AA (1985) Effect of CO2 on total phenolics, phenylalanine ammonia lyase and polyphenol oxidase in lettuce tissue. J. Am. Soc. Hortic. Sci. 110, 249± 253 Sivakumar D, Korsten L (2006) Influence of modified atmosphere packaging and post harvest treatments on quality retention of litchi cv. Mauritius. Postharvest Biol. Technol. 41, 135±142 Sivakumar D, Korsten L, Zeeman K (2007) Postharvest management on quality retention of litchi during storage. Fresh Produce, 1(1), 66±75 Sivakumar D, Arrebola E, Korsten L (2008) Postharvest decay control and quality retention in litchi (cv. McLean's Red) by combined application of modified atmosphere packaging and antimicrobial agents. Crop Protection 27, 1208±1214 Smith S, Geeson J, Stow J (1987a) Production of modified atmosphere in deciduous fruits by the use of films and coatings. HortScience 22, 772±776 Smith SM, Geeson JD, Browne KM, Genge PM, Everson HP (1987b) Modified atmosphere retail packaging of discovery apples. J. Sci. Food Agric. 40, 161±167

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Smith SM, Geeson JD, Genge PM (1988) Effects of harvest date on the responses of discovery apples to modified atmosphere retail packaging. Int. J. Food Sci. Technol. 23, 78±86 Smock RM (1979) Controlled atmosphere storage of fruits. In J Janick (ed.), Horticultural Reviews, AVI Publishing, Westport, CT, Vol. 1, pp. 301±336 Solomos T, Kanellis A (1989) Low oxygen and fruit ripening. Acta Hort. 258, 151±160 Song Y, Kim HK, Yam KL (1992) Respiration rate of blueberry in modified atmosphere at various temperatures. J. Am. Soc. Hortic. Sci. 117, 925±929 Stewart D, Oparka J, Johnstone C, Iannetta PPM, Davies HV (2003) Effect of modified atmosphere packaging (MAP) on soft fruit quality. Plant Biochem. Phytochem. 119±124 Stewart JK, Uota M (1971) Carbon dioxide injury and market quality of lettuce held in controlled atmosphere. J. Am. Soc. Hortic. Sci. 96, 27±30 Talasila PC (1992) Modeling of heat and mass transfer in a modified atmosphere package. Ph.D. dissertation, University of Florida, Gainesville, FL Talasila PC, Cameron AC (1997) Prediction equations for gases in flexible modifiedatmosphere packages of respiring produce are different than those for rigid packages. J. Food Sci. 62, 923±934 Talasila PC, Chau KV, Brecht JK (1992) Effects of gas concentrations and temperature on O2 consumption of strawberries. Trans. ASAE 35(1), 221±224 Talasila PC, Cameron AC, Joles DW (1994) Frequency distribution of steady-state oxygen partial pressures in modified atmosphere packages of cut broccoli. J. Am. Soc. Hortic. Sci. 119, 556±562 Taylor AA, Karel M, Proctor BE (1960) Measurement of O2 permeability. Mod. Package, 33(1), 131±136 Techavises N, Hikida Y (2008) Development of a mathematical model for simulating gas and water vapor exchanges in modified atmosphere packaging with macroscopic perforations. J. Food Eng. 85, 94±104 Tian SP, Li BQ, Xu Y (2005) Effect of O2 and CO2 concentrations on physiology and quality of litchi fruits in storage. Food Chem. 91, 659±663 Tolle WE (1962) Variables affecting film permeability requirements for modifiedatmosphere storage of apple. USDA Tech. Bull. 1418±1429 Tomas-Barberan FA, Loaiza-Velarde J, Bonfanti A, Saltveit ME (1997) Early woundand ethylene-induced changes in phenylpropanoid metabolism in harvested lettuce. J. Am. Soc. Hortic. Sci. 122(3), 399±404 Torrieri E, Cavella S, Masi P (2007) Modelling the respiration rate of fresh-cut Annurca apples to develop modified atmosphere packaging. Int. J. Food Sci. Technol. doi: 10.1111/j.1365-2621.2007.01615.x Underhill SJR, Critchley C (1992) Anthocyanin decolorisation and its role in lychee pericarp browning. Aust. J. Exp. Agric. 34, 115±122 Valero D, Valverde JM, MartõÂnez-Romero D, Guillen F, Castillo S, Serrano M (2006) The combination of modified atmosphere packaging with eugenol or thymol to maintain quality, safety and functional properties of table grapes. Postharvest Biol. Technol. 41, 317±327 Van der Steen C, Jacxsens L, Devlieghere F, Debevere J (2001) A combination of high oxygen atmosphere and equilibrium modified atmosphere packaging to improve the keeping quality of red fruits. Information Hyperlinked Over Protein (IHOP) Varoquaux P, Mazollier J, Albagnac G (1996) The influence of raw material characteristics on the storage life of fresh-cut butterhead lettuce. Postharvest Biol. Technol. 9, 127±139

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Veeraju P, Karel M (1966) Controlling atmosphere in a fresh-fruit package. Mod. Package 402, 164±172 Wang CY (1990) Physiological and biochemical effects of controlled atmosphere on fruits and vegetables. In M CalderoÂn M and R Barkai-Golan (eds), Food Preservation by Modified Atmospheres, CRC Press, Boca Raton, FL, pp. 197±224 Watada AE, Kim SD, Kim KS, Harris TC (1987) Quality of green beans, bell peppers and spinach stored in polyethylene bags. J. Food Sci. 526, 1635±1369 Watkins C (2000) Responses of horticultural commodities to high CO2 as related to modified atmosphere packaging. Hort. Technol. 10, 501±506 Watkins CB, Brookfield PL, Elgar HJ, McLeod SP (1998) Development of a modified atmosphere package for export of apple fruit. In S Ben-Yehoshua (ed.), Proc. 1997 Int. Congr. Plastics Agric., Laser Pages Pub. Ltd, Jerusalem, pp. 586±592 Wills RBH, Lee TH, Graham D, McGlasson WB, Hall EG (1981) Postharvest ± An Introduction to the Physiology and Handling of Fruits and Vegetables. AVI Publishing, Westport, CT, p. 163 Wills RBH, McGlasson WB, Graham D, Lee TH, Hall EG (1989) Postharvest: An Introduction to the Physiology and Handling of Fruits and Vegetables. Chapman & Hall, New York Wolfe SK (1980) Use of CO- and CO2-enriched atmospheres for meats, fish and produce. Food Technol. 34(3), 55±63 Wu Ying, Deng Yun, Li Yunfei (2007) Changes in enzyme activities in abscission zone and berry drop of `Kyoho' grapes under high O2 or CO2 atmospheric storage. LWT ± Food Sci. Technol. doi: 10.1016/j.lwt.2007.01.015 Yam KL, Lee DS (1995) Design of modified atmosphere packaging for fresh produce. In ML Rooney (ed.), Active Food Packaging, Blackie Academic and Professional, London, p. 55 Yang CC, Chinnan MS (1988) Modeling the effect of O2 and CO2 on respiration and quality of stored tomatoes. Trans. ASAE 30(3), 920±925 Yang SF (1985) Biosynthesis and mechanism of ethylene action. HortScience 20, 41±45 Yasuda H, Clark HG, Stannett V (1968) Permeability. In Encyclopedia of Polymer Science and Technology, 9: 794±807 Yoshio M, Takashi H (1997) Modified atmosphere packaging of fresh produce with a biodegradable laminate of chitosan-cellulose and polycaprolactone. Postharvest Biol. Technol. 10, 247±254 Young RE, Romani RJ, Biale JB (1962) Carbon dioxide effects on fruit respiration. II. Response of avocados, bananas and lemons. Plant Physiol. 37, 416±422 Zagory D (1998) An update on modified atmosphere packaging of fresh produce. Packaging International, http://www.davisfreshtech.com Zagory D, Davis CA (1997) Advances in modified atmosphere packaging (MAP) of fresh produce. Perishables Handling Newsletter, no. 90, 2±4 Zagory D, Hurst WC (eds) (1996) Food Safety Guidelines For The Fresh-cut Produce Industry, 3rd edition, International Fresh-cut Produce Association, p. 125 Zagory D, Kader AA (1988) Modified atmosphere packaging of fresh produce. Food Technol. 42(9), 70±77 Zheng Yonghua, Yang Zhenfeng, Chen Xuehong (2008) Effect of high oxygen atmospheres on fruit decay and quality in Chinese bayberries, strawberries and blueberries. Food Control 19, 470±474

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Nylon-MXD6 resins for food packaging A . A M M A L A , CSIRO Materials Science and Engineering, Australia

Abstract: This chapter reviews the properties and uses of Nylon-MXD6 as a high barrier polymer in food packaging applications. Nylon-MXD6 is a generic name given to a range of crystalline polyamides produced from meta-xylenediamine and adipic acid. Nylon-MXD6 has found widespread use in polymer blends and multilayer food packaging applications. Numerous examples of gas barrier performance are given, together with other properties such as aroma-retaining properties, mechanical properties and retortability. The chapter also discusses examples of commercially available systems employing Nylon-MXD6, including the use of oxygen scavenging systems and Nylon-MXD6 nanocomposites. Key words: Nylon-MXD6, poly(m-xylene adipamide), barrier packaging, gas barrier, nylon nanocomposites.

9.1

Structure and general overview

Nylon-MXD6 is a generic name given to a range of crystalline polyamides produced from meta-xylenediamine and adipic acid. Alternative names for Nylon-MXD6 include poly(m-xylene adipamide) and poly(m-xylylene adipamide). Compared to Nylon-6 and Nylon-6,6, Nylon-MXD6 contains an aromatic ring in its structure as shown in Fig. 9.1. The synthesis of Nylon-MXD6 as the meta polymer configuration was first reported in the 1950s.1,2 At the time, the application area of interest was for the textile industry and the production of superior synthetic fibres. The fact that the unsymmetrical meta isomer possessed a crystalline structure was of great interest, since it was previously thought this would result in an amorphous polymer. In addition, Carlston and Lum also reported that the use of aliphatic

9.1 Chemical structure of Nylon-MXD6.

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acids containing an even number of six to ten carbon atoms resulted in crystalline polymers, while an odd number of carbon atoms gave amorphous polymers. The polymer made from adipic acid resulted in the highest melting temperature of 243ëC. It was not until the 1980s that the Mitsubishi Gas Chemical Company (MGC), based in Japan, commercialized the production of Nylon-MXD6.3 This was made possible by previous work performed by MGC on purifying the xylene isomer and developing a patented process4,5 for preparing Nylon-MXD6 using direct polycondensation of the diamine and dicarboxylic acid at atmospheric pressure without the need for water, as was previously described by Lum and Carlston. The growing use of Nylon-MXD6 in food packaging applications stems largely from its excellent gas barrier properties as well as a number of other favourable characteristics. These properties are described in Sections 9.3 and 9.4. Due to its processability and similar moulding criteria to other polymers used in food packaging, Nylon-MXD6 has found widespread use in blends and multilayer applications. It is easy to combine Nylon-MXD6 with polymers such as polyethylene terephthalate (PET), polypropylene (PP) or polyethylene (PE) for co-extrusion and co-injection moulding to produce laminated films, sheets and bottles. Application examples are described in further detail in Section 9.5. The incorporation of nanoparticles into Nylon-MXD6 has been shown to further enhance the barrier properties while preserving the processing characteristics. Nylon-MXD6 nanocomposites are reviewed in Section 9.6.

9.2

Processing

9.2.1

Drying and handling

As a general class of polymers, nylons are susceptible to moisture absorption, which can potentially lead to hydrolysis during processing and alter the material properties. The aromatic structure and crystallinity of Nylon-MXD6 results in comparatively less moisture absorption than that observed for amorphous polyamides. Nylon-MXD6 has a water absorption value of 0.31% (24 h, ASTM D570) compared to 1.2% for Nylon-6,6 and 1.6% for Nylon-6.6 Nylon-MXD6 is usually packaged by the manufacturer in moisture-proof packaging as pellets with a moisture content of less than 0.1%. If the product is used immediately after opening then no further drying is recommended; however, Nylon-MXD6 will absorb moisture from the atmosphere, so it is important to process the polymer under dry conditions. The manufacturer recommends drying at 120±140ëC for 4±5 hours under reduced pressure (0.5±2.0 mm Hg). If a dehumidifier hopper dryer is used then it should be operated at 80ëC with dried air circulation (dewpoint ÿ20ëC) to prevent moisture absorption prior to processing.

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245

Extrusion

In addition to processing Nylon-MXD6 under dry conditions, it is also necessary to minimize residence time and to avoid any prolonged exposure of molten and hot nylon to air in order to prevent discolouration and yellowing. Typically, nylons are extruded under a nitrogen purge to minimize oxidation. A vacuum line attached to a vent in the barrel can also be used to remove volatiles and moisture. Processing temperatures of Nylon-MXD6 are usually in the range 250± 290ëC. Temperatures above this will start to degrade the polymer and should be avoided. The thermal stability of Nylon-MXD6 is discussed in more detail in Section 9.4.1.

9.2.3

Injection moulding

One of the more favourable characteristics of Nylon-MXD6 compared to other nylons is its ease of moulding. Its crystallization behaviour is similar to that of PET and it crystallizes most quickly at temperatures of 150±170ëC. Its favourable crystallization speed over other polymer resins allows thermal forming and drawing over a wide range of conditions. Nylon-MXD6 is available in several grades of varying molecular weight and melt flow index to match the desired application (Table 9.1).7 With melt indices varying from 0.5 to 7.0 g/10 min it is possible to select a grade to produce films, sheets, trays, bottles or cups. Application examples are presented in Section 9.5.

9.2.4

Biaxially drawn films

In the production of biaxially drawn Nylon-MXD6 film the polymer is drawn in both the extruded (longitudinal) and transverse directions. This process orientates the polymer chains and will generally result in improved physical properties compared to undrawn film. One of the properties that can be significantly improved due to orientation of the polymer through drawing or processing is gas barrier performance. LagaroÂn Table 9.1 Grades of Nylon-MXD6 Property Melting point (ëC) Relative viscosity Melt index (g/10min) Average molecular weight

6001

6007

6121

240  5 2.1 7.0 16,000

240  5 2.7 2.0 25,000

240  5 3.5 0.5 39,000

Sources: adapted from http://www.mgc.co.jp/eng/products/nop/nmxd6/grade.html and Mitsubishi Gas Chemical Catalogue ± Polyamide MXD6 (2003).

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et al.8 have noted that this is usually attributed to (1) orientation-induced crystallization, (2) fractionation and alignment (perpendicular to the permeant transport) of the crystals in the straining direction (increase in tortuosity), and (3) densification (reduction in free volume) of the amorphous phase owing to an increase in conformational order in the non-crystalline chain segments.

9.3

Gas barrier properties

9.3.1

Oxygen transmission rate

One of the most notable properties of Nylon-MXD6 is its excellent oxygen barrier properties. Table 9.2 compares the oxygen transmission rates to those of various other polymer films at different relative humidity. The oxygen barrier properties of Nylon-MXD6 exceed that of many other polymers used in packaging. It is particularly useful that Nylon-MXD6 can retain its excellent gas barrier properties at high humidity. In contrast, ethylene vinylalcohol copolymer (EVOH) films do not perform as well under a high humidity environment. Figure 9.2 shows the oxygen permeability dependence on relative humidity for Nylon-MXD6. This behaviour at both low and high relative humidity makes Nylon-MXD6 a very practical polymer for both dry and wet food packaging applications. Oxygen scavenging systems In the late 1980s the UK packaging company Carnaud MetalBox (CMB) found that the incorporation of small amounts of cobalt produced an oxygen scavenging system that enhanced the oxygen barrier performance of NylonTable 9.2 Oxygen transmission rates of various polymer films Film

Oxygen transmission rate (cm3/m2.day.atm), 20 m, 23ëC 60% RH

Nylon-MXD6 (oriented: 4  4) 2.8 Nylon-MXD6 (non-oriented) 4.3 EVOH (32 mol% ethylene) 0.5 EVOH (44 mol% ethylene) 2.0 PAN copolymer 17 Nylon-6 (oriented) 40 Nylon-6 (oriented, PVDC coated) 10 PET (oriented) 80 PP (oriented) 2500 PP (PVDC coated) 14

80% RH

90% RH

3.5 7.5 4.5 8.5 19 52 10 80 2500 14

5.5 20 50 42.5 22 90 10 80 2500 14

Source: http://www.mgc.co.jp/eng/products/nop/nmxd6/barrier.html

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9.2 Humidity dependence of oxygen permeability at 23ëC. Source: http:// www.mgc.co.jp/eng/products/nop/nmxd6/barrier.html

MXD6/PET systems.9 The cobalt salt catalyses the reaction of Nylon-MXD6 with oxygen passing through the package wall and prevents oxygen reacting with the food product. Bottles made using this technology were successful in demonstrating the inhibition of oxygen reacting with oxygen-sensitive products like orange juice, beer and wine. In today's market, there are two major producers of Nylon-MXD6/PET bottles that use the oxygen scavenging technology. Constar International market a product under the name OxbarTM for multilayer bottles and in addition they have monolayer systems, MonoxbarTM and the high clarity DiamondClearTM. Amcor market the system called Bind-OxTM, where the oxygen scavenger is incorporated in the middle layer of a three-layer multilayer bottle. In this system, the rate at which oxygen is bound is faster than the rate of oxygen permeation, so the Bind-OxTM layer acts like a sponge, capturing oxygen which has virtually no chance of reaching the product. Although oxygen scavenging systems are highly effective, they do have a limited lifetime and will only stay active until there is sufficient catalyst present to sustain the oxidation reaction. The barrier properties are usually a function of scavenging capacity and rate of consumption.10 The composition (MXD6 and cobalt content) as well as the container wall thickness are factors that can affect the scavenging capacity. Similarly, the barrier performance is dependent on

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package surface area/volume ratio.11 Large surface/volume ratios limit costeffective performance and for this reason oxygen scavengers have primarily been used for bottled products. An example of the oxygen barrier improvement observed with the use of OxbarTM has been reported by Brody et al. (2001).10 A 1-litre PET-only bottle showed an oxygen transmission rate of 3.5 cm3/m2/day (22ëC), while the equivalent OxbarTM bottle (PET/MXD6/Co) had a transmission value of less than 0.04 cm3/m2/day. This low level persisted for a period approaching two years.

9.3.2

Carbon dioxide transmission rate

Nylon-MXD6 exhibits excellent carbon dioxide (CO2) gas barrier properties. One of the main application areas of Nylon-MXD6 is for carbonated beverage containers. For carbonated soft drinks and beer, one of the primary concerns is the rate at which CO2 escapes from the bottle. If the rate of diffusion is too fast then the product will go flat on the shelf before it is consumed, so it is critical to have a high CO2 retention. The technology involving Nylon-MXD6 for improving CO2 retention is centred around the use of multilayer bottles and polymer blends. Figure 9.3a shows the change in retention of CO2 in a Nylon-MXD6/PET multilayer bottle. It can be seen that there is significant improvement in CO2 retention over a longer time with the use of MXD6. Similarly Fig. 9.3b shows how increasing MXD6 content in a MXD6/PET blend bottle can extend the shelf-life of a carbonated drink.

9.3.3

Aroma-retaining and odour-blocking properties

Aroma compound retention and the blocking of foreign odours is an important aspect of food packaging. Aroma compounds can be lost by chemical reactions within the food product or by sorption or permeation through the packaging. The loss of aroma compounds can be minimized through the use of a high barrier packaging material. Table 9.3 shows the aroma-retaining and odour-blocking properties of Nylon-MXD6 in comparison to various other gas barrier films. It can be seen that Nylon-MXD6 films perform very well at retaining aroma and flavour. The heat treatment of beverages such as extended shelf-life milk which undergoes ultra-high temperature (UHT) processing can cause aldehyde and ketone off-flavours. Suloff et al.12 have reported on the use of PET blends with Nylon-MXD6, D-sorbitol and -cyclodextrin as selective scalping agents which can be used to improve the flavour profile and remove these carbonyl compounds that create the stale flavours during storage. In the case of Nylon-MXD6, the free amino groups react with the carbonyls to form imines.

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9.3 (a) Carbonation retention of PET/Nylon-MXD6 multilayer bottle (500 cm3 bottle, average wall thickness 350 m, storage conditions 20ëC, inside 100% RH, outside 65% RH). Source: http://www.mgc.co.jp/eng/products/nop/ nmxd6/bottle.html; (b) Nylon-MXD6 content in Nylon-MXD6/PET blended bottles and the relationship between the carbon dioxide transmission coefficient and the shelf-life of a carbonated drink. Source: http:// www.mgc.co.jp/eng/products/nop/nmxd6/bottle.html

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Table 9.3 Aroma-retaining and odour-blocking properties of various barrier films Film (thickness 15 m)

Evaluated food

Nylon-MXD6 (oriented) Nylon-MXD6/Nylon-6 blend (oriented) Nylon-6 (oriented) PVDC-coated drawn Nylon-6 PET (oriented) PE

Soy sauce

Vinegar

Worcester sauce

l (one month) l (one month)

n n

l (three months) n

n l (one month) l (one month) u

u n s u

n n s u

Film (thickness 15 m)

Evaluated flavour

Nylon-MXD6 (oriented) Nylon-MXD6/Nylon-6 blend (oriented) Nylon-6 (oriented) EVOH PVDC-coated rolled Nylon-6 PET (oriented) PP (oriented) PE

D-limonene

Vanilla essence

L-menthol

l n

l s

l l

n n l n u u

u s l l u u

l l l l n u

Conditions: 23ëC, 50% RH, shaded from light; evaluation method: sensory analysis. Key: l no less than 2 weeks; n 1to 2 weeks; s 3 days to 1week; u within 3 days. Source: http://www.mgc.co.jp/eng/products/nop/nmxd6/blendfilm.html

9.4

Other properties

9.4.1

Thermal properties

Nylon-MXD6 is more thermally stable than other commonly used barrier packaging plastics. Table 9.4 summarizes some of the thermal properties. The glass transition temperature and heat distortion temperature are significantly higher than those of Nylon-6 and Nylon-6,6. Because of its excellent thermal stability, Nylon-MXD6 also exhibits superior recyclability. It is easy to recycle since it does not show any gel formation or decomposition.

9.4.2

Retortability

Retort packaging involves the use of steam or boiling water to cook food in its own package and extend shelf-life. Because plastic packaging is less bulky than traditional cans and glass jars, foods cook faster, providing a better-tasting product for the consumer. Nylon-MXD6 has excellent retortability due to its

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Table 9.4 Thermal properties of Nylon-MXD6 (injection-moulded specimens) Property Melting point (ëC) Heat distortion temperature (ëC) Glass transition temperature (ëC) Coefficient of thermal expansion (cm/cmëC)

NylonMXD6

Nylon-6,6

Nylon-6

PET

237 96 85 5  10ÿ5

260 75 50 10  10ÿ5

220 65 48 8  10ÿ5

255 85 77 7  10ÿ5

Source: adapted from http://www.mgc.co.jp/eng/products/nop/nmxd6/nature.html

resistance to boiling treatment and quick performance recovery. Its barrier and strength are also features that make Nylon-MXD6 an excellent candidate for retort packaging. An example of the retort properties of Nylon-MXD6 is illustrated in Table 9.5 which shows the change in oxygen transmission rate of a laminated multilayer PP/Nylon-MXD6/PP container. In comparison to the same laminated container made with EVOH instead of Nylon-MXD6, it is clear that the use of Nylon-MXD6 gives superior results. This is further illustrated in Fig. 9.4 showing the cumulative oxygen transmission rate of the containers. The patent JP7276582 (assigned to Sumitomo Bakelite Co. Ltd) refers to a multilayer film for retort pasteurization applications.13 The reported advantages of using Nylon-MXD6 include improved heat resistance, improved mechanical strength and no curling as well as enhanced gas barrier performance.

9.4.3

Mechanical properties

Compared to other polyamides, Nylon-MXD6 exhibits greater strength and stiffness. Some of the physical properties are compared in Table 9.6. Blended Table 9.5 Change in oxygen transmission rate of laminated containers after retorting Structure of laminated container

Thickness (m)

Oxygen transmission rate (cm3/m2.day.atm), 23ëC Initial

PP/Nylon-MXD6/PP PP/EVOH/PP (ethylene 32 mol%)

140/40/180 140/40/180

0.6 0.25

Time after retorting (days) 1

7

14

30

12.0 22.0

0.9 16.0

0.64 8.0

0.62 3.5

Retorting conditions: 121ëC, 30 min. Measuring conditions: 23ëC, 100% RH (inside), 50% RH (outside). Source: http://www.mgc.co.jp/eng/products/nop/nmxd6/container.html

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9.4 Cumulative oxygen transmission coefficient of Nylon-MXD6/polypropylene laminated containers after retorting. EVOH: ethylene 32 mol%. Retorting conditions: 121ëC, 30 min. Container specifications: volume 350 cm3, surface area 310 cm2, (inside)PP / Nylon-MXD6 / PP(outside) 140/ 10/40/10/180 m (average). Measuring conditions: 23ëC, 100% RH (inside), 50% RH (outside). Source: http://www.mgc.co.jp/eng/products/nop/ nmxd6/container.html

Table 9.6 Physical properties of Nylon-MXD6 (injection-moulded specimens) Property

Tensile strength

ASTM test method (units)

NylonMXD6

D638 1010 (kg/cm2) Tensile elongation D638 2.3 (%) Tensile modulus D638 48  103 (kg/cm2) Flexural strength D790 1600 (kg/cm2) Flexural modulus D790 45  103 (kg/cm2) Izod impact (notched) D256 2 (kg-cm/cm) Rockwell hardness D785 108 (M scale)

Nylon-6,6

Nylon-6

PET

780

630

800

60

200

5.8

32  103

26  103

31  103

1300

1250

1250

30  103

24  103

35  103

4

6

4

89

85

106

Source: adapted from http://www.mgc.co.jp/eng/products/nop/nmxd6/nature.html

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and oriented films of Nylon-MXD6 also exhibit good pin-hole resistance and easy tearing properties that further enhance the use of polymers in easy to open retort pouches.14

9.5

Applications

As illustrated in the examples in previous sections of this chapter, Nylon-MXD6 alone exhibits many useful properties for food packaging; however, its presence in a blend or multilayer structure can satisfy multiple performance criteria for bottle, film and sheet applications. This section will focus on further examples where the use of Nylon-MXD6 together with other polymers can achieve optimal properties for food packaging.

9.5.1

Polymer blends

The blending of polymers can result in improved physical properties and this is often done at a lower cost than developing a new polymer. In food packaging applications Nylon-MXD6 can be blended with other polymers such as Nylon-6, PET or PP to achieve higher gas barrier properties and greater thermal resistance. Recently, blends of Nylon-MXD6 with Nylon-6 and Nylon-6,6 have been reported as shrinkage films for food packaging.15 The Nylon-MXD6 blends achieved effective gas barrier properties and good controlled shrinkage, offering a halogen-free alternative to polyvinylidene chloride (PVDC) films, which have raised some environmental concerns. In addition, the use of Nylon-MXD6 blends is reported to give superior results compared with that of PET blends which can often have problems with rippling and appearance after boiling sterilization.15 As mentioned previously in an example of a blended product, Fig. 9.5 shows the relationship between carbon dioxide transmission rate and the percentage of Nylon-MXD6 in a Nylon-MXD6/PET blended bottle, as plotted against shelflife. It can be seen that as the percentage of Nylon-MXD6 in the blend increases then the carbon dioxide transmission decreases, resulting in a longer product shelf-life of a carbonated beverage. The compatibilization of Nylon-MXD6 blended with other polymers has been the subject of several studies. For instance, in PET/Nylon-MXD6 blends haze was found to increase as the level of Nylon-MXD6 increased.16 Stretching was also found to increase haze in the immiscible blends. Refractive index mismatch and particle size effects were identified as the contributing factors. In order to overcome the incompatibility, the use of a modified PET containing isophthalate to partially replace terephthalate has been reported to improve transparency after biaxial stretching while maintaining good barrier properties.17

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9.5 Image of a multilayer bottle preform showing cross-section of five layers (PET/Nylon-MXD6/PET/Nylon-MXD6/PET).

9.5.2

Multilayer products

Multilayer food packaging structures have been used for many decades to deliver multiple performance criteria in a single structure. Nylon-MXD6 can be incorporated into multilayer packaging with other polymers to optimize gas barrier properties and to improve retortability. An example is the patented process, known as SurshotTM, developed by Owens Illinois for co-injection moulding of a five-layer plastic bottle.18 The technology uses outer, middle and inner layers of PET. Sandwiched between them are two layers of proprietary SurshieldTM barrier which consists of Nylon-MXD6 and an oxygen scavenger. It is reported to improve CO2 gas barrier by 40%.18 The technology is now owned by Graham Packaging Company19 and is suitable for a range of products from beer to pasta sauces. A similar example of a five-layer bottle preform containing Nylon-MXD6 is shown in Fig. 9.5.20 Similarly, the OxbarTM technology described earlier uses a three-layer construction of PET and Nylon-MXD6 with oxygen scavenger to achieve improved barrier properties as presented in Fig. 9.6.

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9.6 Multilayer bottle showing three layers (PET/Nylon-MXD6/PET) with OxbarTM oxygen scavenging additive incorporated into centre layer. Source: adapted from http://www.constar.net/tech-barrier-oxbar.php

Apart from multilayer bottle applications, Nylon-MXD6 has also found applications in multilayer films and sheets suitable for food packaging. The patent WO 2001 092011 (assigned to Cryovac Inc.) describes the use of Nylon-MXD6 for co-extruded multilayer films that are able to maintain dimensional stability at high temperatures as well as having low oxygen transmission rates and good interlaminar bond strength.21 These multilayer films are suitable for both `hot fill' packaging products such as soups, sauces, beverages and other liquified foods as well as products that are not `hot filled' but still require heat sealing such as lidding films for lunch meats or dairy products, where, without dimensional stability, distortion of the package would otherwise occur. The patent EP 1529635 (US 2005 0100729)22 also describes the use of Nylon-MXD6 in a multilayer film. In this application Nylon-MXD6 is used in a multilayer film as a cover film for oven-ready meal trays. The heat-sealable product exhibits good oxygen barrier properties, interlayer adhesion and mechanical properties as well as easy-peel properties. Nylon-MXD6 has also found applications in multilayer laminated paper packaging materials. The patent WO 2001 19611 (assigned to Nippon Tetra Pak and Tetra Laval)23 describes the use of Nylon-MXD6 in such packaging for food and beverages including milk, soup, fruit juice, tea, wine and seasoning sauce. The package exhibits strength, barrier properties and taste retention.

9.6

Nylon-MXD6 nanocomposites

While Nylon-MXD6 alone is a proven gas barrier polymer, the addition of nanoclays can form nanocomposites with significantly enhanced barrier properties. The combination of Nylon-MXD6 and nanotechnology is ideal for food

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Table 9.7 Properties of ImpermTM (non-oriented, grade 105) Property OTR, 23ëC, 60% RH (cm3.mm/m2.day.atm) CO2TR, 23ëC, 60% RH (cm3.mm/m2.day.atm) WVTR, 40ëC, 90%RH (g.mm/m2.day) Haze (%) Tensile strength (MPa) Tensile modulus (GPa) Tensile elongation (%)

ImpermTM 105

Nylon-MXD6

0.02 0.11 0.43 1.5 89 4.4 2.6

0.09 0.30 1.36 1.4 85 3.1 3.3

Source: adapted from http://www.nanocor.com/tech_sheets/I105.pdf

packaging as shelf-life can be enhanced while transparency remains high and processing characteristics remain similar to those of Nylon-MXD6 itself. Currently, a commercially available nanocomposite Nylon-MXD6 resin is manufactured by Mitsubishi Gas Chemical Co. and Nanocor Inc. with the trademark ImpermTM. The uniform dispersion of clay platelets in ImpermTM is believed to enhance barrier by creating a `tortuous path' for gas molecule permeation.24 There are grades of ImpermTM suitable for multilayer bottles and for film and sheet applications. Table 9.7 shows some properties of non-oriented films of ImpermTM Grade 105. Oxygen barrier is improved by a factor of 4.5 times compared to the base resin. Similarly, the carbon dioxide barrier and water vapour barrier improve by a factor of 3. Figure 9.7 also shows the shelf-life improvement that can be achieved for carbonated beverages with the use of ImpermTM grade 103 (previous grade name M9). While a 5 wt% barrier layer of Nylon-MXD6 can extend

9.7 Carbon dioxide retention comparison of PET and multilayer PET bottles containing Nylon-MXD6 and nanocomposite Nylon-MXD6 (500 ml bottle, 28 g weight, 390 m thickness, 5 wt% barrier layer, test conditions 23ëC, inside 100% RH, outside 50% RH). Source: http://www.nanocor.com/tech_papers/ NOVAPACK03.pdf

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9.8 TEM micrographs of (a) MXD6±kaolinite nanocomposite, and (b) MXD6± montmorillonite nanocomposite.

the shelf-life of a monolayer PET bottle from 7 weeks to 14 weeks, ImpermTM can further extend the container shelf life to 21 weeks (using 90% CO2 retention as the cut-off). While the tortuous path theory is the most widely accepted model for gas barrier improvement of nanocomposites, other variables have also been shown to contribute to improvements in gas barrier. An example is the increase in crystallinity that results from nanoparticles acting as nucleating agents.25 Kaolinite and montmorillonite clays were dispersed in Nylon-MXD6 and then blow-moulded to form multilayer bottles. Both clays did not cause any haze in the final products, and despite the fact that kaolinite was only partially exfoliated compared to the montmorillonite clay (Fig. 9.8), both samples showed improvement in CO2 gas barrier over the neat Nylon-MXD6 resin alone. The nanoclay additives were shown to increase the crystallinity of the Nylon-MXD6, and because the crystalline regions are more impermeable to the transport of gases compared to amorphous regions, this can explain the improvement in gas barrier. Recently, it was also demonstrated that the choice of organic modifier used on montmorillonite clays can have a significant effect on the gas barrier properties of resulting Nylon-MXD6 nanocomposites.26 Positron annihilation lifetime spectroscopy (PALS) was used to study the free-volume changes occurring when different surface-modified montmorillonite clays were incorporated into Nylon-MXD6. The molecular transport of gases through a polymer depends on the amount of free volume present due to chain packing and chain segment rearrangement. The interactions between modified clay nanoparticles

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and polymer that result in lower free volumes are favourable and this information can be used to tailor lower gas permeability packaging materials. In the study described above, Cloisite 10A, a montmorillonite clay modified with a quaternary ammonium salt containing an aromatic functional group, was found to reduce free volume compared to other modified clays and neat NylonMXD6 resin. It also gave a 66% reduction in oxygen transmission rate over neat Nylon-MXD6. These results suggest that there are favourable interactions between the aromatic groups on the modified clay and the aromatic groups on the nylon-MXD6 chain. In addition to improving gas barrier properties, the use of nanoclays in Nylon-MXD6 can also improve mechanical properties of the polymer. Recently, it was reported that co-extruded multilayer films containing a montmorillonite clay in Nylon-MXD6 significantly enhanced oxygen barrier performance as well as decreasing film elongation while improving tear resistance of the films.27 The films are currently being investigated as potential replacements for foil-based packaging in the military food supply chain. Nanocor's ImpermTM products24 are also reported to enhance mechanical reinforcement, with the nanoclay additives acting to restrict the Brownian motion of the Nylon-MXD6 chains.24 In summary, the use of Nylon-MXD6 clay nanocomposites has already created a significant impact on food packaging technology and will most likely continue to expand commercially in the decades to come.

9.7

Future trends

Since its introduction into commercial markets, the demand for Nylon-MXD6 as a packaging material has expanded steadily. With increasing safetyconsciousness about foods, it is expected that the need for effective gas barrier packaging that enables long-term storage without damaging the freshness of foods will become higher in the future. Convenience continues to be a major driver in food packaging with consumers wanting products that are easy to handle and quick to prepare. The retortability of Nylon-MXD6 coupled with its high gas barrier properties makes it a polymer of choice for convenience food packaging. The further commercial development of nanotechnology with Nylon-MXD6 will be strongly dependent on the public perception of nanotechnology in food contact materials. While existing Nylon-MXD6 nanocomposite products like ImpermTM are fully approved for use as internal barrier layers in multilayer structures, they are still only approved for non-direct food contact applications. While clay nanoparticles are the most commonly used commercial application of nanoparticles in food packaging, the use of other innovative nanoparticle technologies with Nylon-MXD6 has yet to be fully explored. Possibilities could include nanoparticles for a wide range of applications, including antimicrobial functionality and UV protection which when combined

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with the previously discussed properties of Nylon-MXD6 have the potential to greatly influence food packaging markets in the future.

9.8

References

1. F. G. Lum and E. F. Carlston, Synthetic fiber-forming polymers from meta-xylylene diamine and adipic acid, US Patent 2,766,221 (1956). 2. E. F. Carlston and F. G. Lum, Ind Eng Chem 49: 1239±1240 (1957). 3. Mitsubishi Gas Chemical Co., Plant and Production History. Available from: http:// www.mgc.co.jp/eng/about/history/plant.html [accessed 25 February 2010]. 4. A. Miyamoto, S. Shimizu, M. Harada, T. Ajiro and H. Hara, Preparation of polyamide from meta-xylylene diamine and adipic acid by adding diamine to acid in two stages at atmospheric pressure, European Patent EP 84661-A1 (1982). 5. A. Miyamoto, S. Shimizu, K. Yamamiya and M. Harada, Polyamide preparation from m-xylylene di:amine and adipic acid by maintaining polycondensing mixture in uniformly fluidized state, US Patent 4,433,136-A (1984). 6. R. J. Palmer, `Polyamides, Plastics', in Kirk Othmer, Encylopedia of Chemical Technology (2005) John Wiley & Sons. Available from http://www.mrw. interscience.wiley.com/emrw/9780471238966/kirk/article/plaspalm.a01/current/html [accessed 25 February 2010]. 7. Mitsubishi Gas Chemical Co., Various Grades and Uses. Available from http:// www.mgc.co.jp/eng/products/nop/nmxd6/grade.html [accessed 25 February 2010]. 8. J. M. LagaroÂn, R. Catala and R. Gavara, Mater Sci Technol 20: 1±7 (2004). 9. A. Cochran, R. Folland, J. W. Nicholas, M. E. R. Robinson, M. A. Cochran and M. E. Riddell, Packaging material with oxygen-scavenging properties containing a polymer, metal oxidation catalyst and oxidisable polymer, European Patent EP 301719-A1 (1988). 10. A. L. Brody, E. R. Strupinksy, L. R. Kline, Active Packaging for Food Applications (2001), CRC Press. Available from http://books.google.com.au/books?id= 6z1QbQTmcukC&pg=PA56&lpg=PA56&dq=cobalt+mxd6&source=bl&ots=nGnEGrNG3b&sig=NTJo-GljE7LzKdkQzh7gSVEJA-0&hl=en&ei= XMN3SqzuA9eBkQXyo5mzBg&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q=cobalt%20mxd6&f=false2001] [accessed 25 February 2010]. 11. P. Maul, `Barrier enhancement using additives', Pira International Conference, Brussels, Belgium, 5±6 December 2005. Available from http://www.nanocor.com/ tech_papers/BARRIER%20ENHANCEMENT%20USING%20 ADDITIVES%20110605.pdf [accessed 25 February 2010]. 12. E. C. Suloff, J. E. Marcy, B. A. Blakistone, S. E. Duncan, T. E. Long and S. F. O'Keefe, J Food Sci 68: 2028±2033 (2003). 13. K. Susumu and O. Yoshiyuki, Multilayer film for packaging food to be retortpasteurised ± has outer and intermediate layers of a polycondensation product prepared from metaxylenediamine adipic acid and an aliphatic acid nylon, Japanese Patent JP 7276582-A (1995). 14. Mitsubishi Gas Chemical Co. brochure, Nylon MXD6 superior performance in barrier packaging. Available from http://www.idspackaging.com/Common/ exhib_349/Mxd6bro.1.pdf [accessed 25 February 2010]. 15. M. Takashige and T. Kanai, J Polym Eng 28: 179±201 (2008). 16. Y. Maruhashi and S. Iida, Polym Eng Sci 41: 1987±1995 (2001). 17. Y. S. Hu, V. Prattipati, A. Hiltner, E. Baer and S. Mehta, Polymer 46: 5202±5210

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(2005). 18. Handbook of Food Science, Technology and Engineering. Available from http:// books.google.com.au/books?id=Wt9QVCCLKOQC&pg=PT429&lpg=PT429&dq= mxd6+future+packaging&source=bl&ots=fLYSktweo3&sig=r-b2lb9 WIsod9A61_bgd6rBmJdM&hl=en&ei=uYZRSsqGMoamNoDwmFA&sa=X&oi= book_result&ct=result&resnum=3#v=onepage&q=mxd6%20future%20packaging& f=false2006 [accesssed 25 February 2010]. 19. Graham Packaging Company, New Technology. Available from http:// www.grahampackaging.com/technology/new-technology.asp [accessed 25 February 2010]. 20. A. Ammala, A. J. Hill, K. Lawrence and T. Tran, Nanocomposite Barrier Improvement for Plastics Packaging. Confidential Technical Report 169. CSIRO CMIT Australia (2004). 21. F. M. Hofmeister, P. J. Satterwhite, T. D. Kennedy, M. Hofmeister, J. Satterwhite and D. Kennedy, Multilayer film for packaging applications, has first layer having amorphous polyamide, second layer, and third layer having ethylene/vinyl alcohol copolymer, polyamide, polyvinylidene chloride and/or polyacrylonitrile, PCT International Patent WO 2001 92011-A (2001). 22. H. Pfeiffer, B. Janssen, G. Hilkert, M. Konrad, H. Peiffer and B. Janssens, Coextruded, biaxially-oriented polyester film for use e.g. as cover film for ovenready meal trays, has a base layer containing poly-m-xylylene-adipamide and a heatsealable outer layer of aromatic±aliphatic polyester, US Patent US 2005 0100729 (2005). 23. P. Frisk, N. Kobayashi, H. Ogita, K. Norio and O. Hiroaki, Laminated material for packaging foods and beverages as well as other liquid products, with strength, barrier properties and taste retention, as well as improved productivity and cost performance ratio, PCT International Patent WO200119611-A (2001) 24. Imperm Technical data sheet. Available from http://www.nanocor.com/tech_sheets/ I105.pdf [accessed 25 February 2010]. 25. A. Ammala, A. J. Hill, K. A. Lawrence and T. Tran, J Appl Polym Sci 104: 1377± 1381 (2007). 26. A. Ammala, S. J. Pas, K. A. Lawrence, R. Stark, R. I. Webb and A. J. Hill, J Mater Chem 18: 911±916 (2008). 27. C. Thellen, S. Schirmer, J. A. Ratto, B. Finnigan and D. Schmidt, J Membr Sci 340: 45±51 (2009).

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Ethylene±vinyl alcohol (EVOH) copolymers

 P E Z - R U B I O , Novel Materials and Nanotechnology A. LO Group, IATA-CSIC, Spain

Abstract: Ethylene±vinyl alcohol (EVOH) copolymers are excellent gasbarrier semicrystalline materials with very good chemical resistance and, as such, they are widely used in a number of packaging applications. One of the most widely implemented applications is that of an intermediate barrier layer in multilayer structures, to be used in various packaging designs for foodstuffs. The presence of EVOH in the packaging structure is key to food quality and safety because, for instance, it delays the ingress of oxygen, the agent responsible for a number of food deterioration processes. The effects of industrial processing on the structure and properties of these polymers are compiled, together with solutions to overcome the deleterious effects of industrial retorting processes commonly used in the food industry. Finally, the property improvements attained upon addition of different nanoclays to EVOH matrices and to the homopolymer poly(vinyl alcohol) (PVOH) will be described. Keywords: ethylene±vinyl alcohol (EVOH), nanocomposites, poly(vinyl alcohol) (PVOH), retorting, barrier properties.

10.1

Introduction

The use of polymers in the food packaging area has been steadily increasing over the last decades due to the numerous advantages, such as lightness, cost and versatility, which these materials present over the traditionally employed glass or tinplate. One of the main disadvantages arising from the use of polymers is probably the fact that they are not impermeable materials, thus allowing the passage of gases and aromas that could compromise food quality and, more importantly, food safety. In the specific case of oxygen-sensitive products, the use of high-barrier packaging materials is a requirement. A high-barrier material in food packaging refers to a material that has low oxygen permeability and, more specifically, an oxygen permeability lower than 1 cm3/m2.day.atm. Ethylene±vinyl alcohol (EVOH) copolymers are widely used as high barrier layers in multilayer food packaging structures due to their outstanding properties and very low permeability to oxygen and food aromas. The use of high barrier packaging concepts containing ethylene±vinyl alcohol copolymer resins actually began in the mid-1970s, although it was not until 1983 when their use in high barrier applications rapidly expanded (Foster, 1991).

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Today EVOH packages are used in low acid retortable applications, high acid hot fill applications, aseptic packages, and packages to prevent flavour and aroma scalping, and the resins have evolved into four different packaging segments ± flexible, formed, bottles and co-extrusion coating (Foster, 1991). In this chapter, the main characteristics and drawbacks of EVOH copolymers for food packaging applications are described together with some of the existing solutions, which include blending with other materials and addition of nanoclays to generate EVOH nanocomposites.

10.2

Structure and general properties of ethylene± vinyl alcohol (EVOH) copolymers

Ethylene±vinyl alcohol copolymers are a family of semicrystalline materials with excellent barrier properties to gases and hydrocarbons. The presence of hydroxyl groups from the vinyl alcohol fraction provides the materials with very high inter- and intramolecular cohesivity, reducing the free volume between the polymer chains available for the exchange of low molecular weight compounds such as gases and aromas. Figure 10.1 shows the chemical structure of the copolymers having a random distribution of the hydroxyl groups along their chains. EVOH copolymers are commonly produced via a saponification reaction of a parent ethylene-co-vinyl acetate copolymer, whereby the acetoxy group is converted into a secondary alcohol. These materials have been increasingly implemented in many pipe and packaging applications where stringent criteria in terms of chemical resistance and in gas, water, aroma, and hydrocarbon permeation are to be met. The composition of the copolymers can be varied by changing the ratio of the ethylene/vinyl alcohol fractions, which also causes a change in the physicochemical properties. In particular, the copolymers with low contents of ethylene (below 38 mol% ethylene) have outstanding barrier properties, under dry conditions, compared to other polymeric materials. The crystalline morphology of these copolymers is relatively well known across composition and has been the subject of several studies (Cerrada et al.,

10.1 Chemical structure of EVOH copolymers.

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1998; Takahashi et al., 1999). Both the melting point and the density of the materials vary as a function of the ethylene/vinyl alcohol ratio, indicating that the crystal structure itself changes continuously with the change in the copolymer composition. While the unit cell of EVOH samples with vinyl alcohol contents greater than 26 mol% has been observed to be monoclinic (like the polyvinyl alcohol ±PVOH± unit cell structure), for lower vinyl alcohol contents the crystal unit cell has been described as orthorhombic and, thus, more similar to the one from polyethylene (PE). A boundary of crystal structure transformation between the PVOH type and the PE type is considered to be located at vinyl alcohol contents between 27 and 14 mol% because the X-ray pattern can be interpreted almost equally on the basis of both the PVOH- and PE-type structure models (Takahashi et al., 1999). However, the crystal structure also depends on the thermal history of the polymeric samples, in a way that quenching EVOH copolymers with high vinyl alcohol contents leads to an orthorhombic crystal morphology (Cerrada et al., 1998). The changes in crystal structure as a function of the copolymer composition also result in changes in the oxygen permeability of the materials. Therefore, depending on the application, the copolymer grade can be chosen for the expected oxygen performance. The oxygen permeability of dry EVOH measured at 45ëC varies from 0.45 to 32 cm3/m2.day.atm for copolymers having an ethylene content of 26 and 48 mol%, respectively (Lopez-Rubio et al., 2005a). One of the main drawbacks of these materials is their high water sensitivity and, thus, the same hydroxyl groups that provide the high cohesivity between the polymer chains also make the copolymers very hydrophilic, so that in the presence of water or in high humidity environments, the structure of the material becomes plasticized and the barrier properties are greatly deteriorated. While under dry conditions, the glass transition temperature (Tg) is unaffected by the copolymer ethylene content, when increasing the relative humidity (RH) a drop in the Tg has been observed. Tg varies from around 50ëC (dry) to below room temperature in the presence of water vapour and, thus, EVOH copolymers are glassy polymers when dry and rubbery polymers at high RHs (Aucejo et al., 1999). This plasticization when increasing the RH, and especially above 75% RH, is reflected in significant increments of the oxygen permeability values. For instance, a ten-fold increase in oxygen permeability was observed for EVOH with 32 mol% of ethylene at 80% RH (Zhang et al., 2001). Zhang and coworkers (2001) also observed that at low relative humidity (up to 35%) an improvement in the oxygen permeability of the copolymers occurred, which was explained by strong adsorption of water molecules by the polymer, therefore occupying the free volume that would otherwise be available for oxygen. Exclusion of oxygen by water from the free volume of the polymer matrix reduced the available diffusive pathways for the oxygen. As a result, oxygen permeation decreased as the RH increased up to an intermediate RH. Concurrently, however, adsorbed water molecules interacted with polar hydroxyl

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10.2 Oxygen transmission rate versus relative humidity at 20ëC of three EVOH copolymer grades, in comparison with other commercial synthetic polymers. Graph extracted from EVAL Europe (www.eval.be).

groups of the polymer and weakened the intermolecular and intramolecular hydrogen bonding, thereby facilitating segmental motion and oxygen diffusion. At high RH, this bonding effect was more pronounced than the reduction of absorption sites because the polymer was increasingly plasticized by the sorbed water, resulting in the large increase in oxygen transmission rate observed (Zhang et al., 2001). Figure 10.2 shows the oxygen permeability of three grades of EVOH, in comparison with that of other commercial synthetic polymers, as a function of relative humidity (data extracted from EVAL Europe, Kuraray Co. Ltd). Mechanical performance of EVOH copolymers is also affected by humidity. Fourier transform infrared spectroscopy (FT-IR) was used to analyse both the transport properties of water through food packaging films made of EVOH with various ethylene contents, i.e. ranging from 26 to 48 mol% of ethylene, and the water±polymer interactions (Cava et al., 2006). From the results, an unreported Langmuir contribution was found at low relative humidity conditions for the copolymers, which was thought to be responsible for the unusual trend in oxygen permeability reported earlier for these materials. Furthermore, a distribution of water molecules with different hydrogen bonding strengths and different diffusion rates was encountered, which indicated that the interaction and transport properties of moisture in these polymers is far from being a simple process. A proper understanding of the above moisture sorption effect on the mechanical performance of EVOH copolymers is also of great importance to predict the effect of moisture uptake on the EVOH based packaging structures in service applications. In agreement with the mass transport results, a surprising

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anomalous antiplasticization regime for some mechanical properties below 30% RH was observed in EVOH materials, reflected in an increase of the modulus for all the copolymers analysed (Cabedo et al., 2006). Beyond this point, further moisture sorption acts as the well-reported effective plasticization agent for the mechanical properties (Cabedo et al., 2006). These results have a direct impact on food packaging, as water present in food products could affect EVOH behaviour in terms of barrier properties, resulting in reduced packaged food product shelf-life. For this reason, in most applications EVOH is used as an intermediate layer in multilayer packaging structures, protected by layers of hydrophobic materials such as polyethylene (PE) or polypropylene (PP). Other important characteristics of EVOH which make them very suitable for a variety of applications are their high chemical and thermal resistance, good optical characteristics and high crystallization velocity.

10.3

Ethylene±vinyl alcohol (EVOH) versus aliphatic polyketones

Aliphatic polyketones (PKs) are a family of semicrystalline thermoplastics prepared by the polymerization of -olefins and carbon monoxide in a perfectly 1:1 alternating sequence using palladium catalysts (Drent and Budzelaar, 1996; Sommazzi and Garbassi, 1997). The simplest member of the family of perfectly alternating polyketones is the copolymer of ethylene and carbon monoxide. This copolymer is a white powder of relatively high crystallinity (35±50% as determined by X-ray diffraction) and a melting temperature of 260ëC. Its alternating structure has been confirmed by elemental analysis, infrared and nuclear magnetic resonance spectra (Lai and Sen, 1984; Zhao and Chien, 1992; Drent et al., 1991). Lower melting point polymers can be produced by incorporation of propylene in addition to ethylene in order to avoid degradation phenomena induced by processing at such high temperatures. The lowering of the melting point is proportional to the number of propylene units present in the polymer chain (Sommazzi and Garbassi, 1997). When both ethylene and propylene are used as co-monomers, their distribution is statistically random. The general structure of this family of polymers is represented in Fig. 10.3.

10.3 Chemical structure of polyketone terpolymers.

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These polymers are reported to have a useful combination of mechanical, high-temperature, chemical resistance, wear resistance and barrier properties, giving them significant commercial potential in a broad range of engineering, barrier packaging, fibre and blend applications (Bonner and Powell, 1997). Their properties are at the border between commodity polymers (like polyethylene and polyvinyl chloride) and engineering polymers of medium performance like polyamides and polyesters. Ethylene/propylene/CO terpolymers show good chemical resistance. They are resistant to a wide range of chemicals including automotive fluids, solvents and other industrial chemicals. In contrast with EVOH copolymers, polyketones are particularly resistant to aqueous media, displaying minor swelling phenomena. Water absorption at 100ëC is not negligible, rising to a level of about 3.5%. However, the polymer exhibits good stability and water absorption does not produce hydrolytic degradation (Sommazzi and Garbassi, 1997). Furthermore, mechanical properties are only slightly affected by moisture sorption. The tensile strength at yield decreases from 60 MPa to 56 MPa, the elongation at yield increases from 25% to 28% and the elongation at break does not change at high RH. Polyketones are attractive polymers to be used in food-packaging applications. This class of polymers is reported to fulfil both processing and food preservation requirements. In fact, they are one-step processable, have good impact properties, are dimensionally heat-stable, are easily compoundable with other polymers used for food packaging (nylon, polycarbonate, and ethylene± vinyl alcohol copolymer), and have good barrier properties competitive with those of nylon and poly(ethylene terephthalate) (PET) (Del Nobile et al., 1993).

10.4

Processing in packaging

10.4.1 Retorting of EVOH and consequences on structure and barrier properties As mentioned in the first section, EVOH copolymers are widely used as a highbarrier layer in multilayer food packaging structures due to their outstanding properties and very low permeability to oxygen and food aromas. The interest for these materials in the food packaging area is based on the fact that many food products are to be packaged with high-barrier polymeric materials because oxygen is a ubiquitous element involved in many food deterioration reactions, such as fat oxidation, vitamin loss, etc. But furthermore, several food products are thermally treated within the food packages and, therefore, apart from the already mentioned high-barrier conditions, plastic packages must withstand such kinds of processes without suffering undesirable changes. Specifically, precooked foods (ready-to-eat products) for which there is a continuously increasing demand require a retorting treatment inside the package before

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being commercialized (typically 121ëC during 20 min in an industrial autoclave, i.e., in the presence of pressurized water vapour) (Ramesh, 1999). Ethylene±vinyl alcohol (EVOH) copolymers are commonly used in retortable packaging structures, but because of the above-mentioned high water sensitivity, these materials are used as intermediate high-barrier layer in multilayer structures protected from the external relative humidity by at least two layers of a hydrophobic material such as polypropylene (PP). However, it is common knowledge that even protected between these hydrophobic materials, retorting processes have a tremendous impact on the gas barrier performance of the copolymers partly due to extensive plasticization of the EVOH layer. The plasticizing effect of water on the barrier properties of EVOH is time-dependent, especially if the hydrophilic layer is protected by a water barrier such as polypropylene as in the case of packages for food retorting. When such retortable packages (containing aqueous foodstuffs) are subjected to steam retorting, water passing through the protective hydrophobic layer is thought to be sorbed on the EVOH layer in such quantities that the barrier layer becomes quite permeable to oxygen. The rate of water release through the outer polypropylene layer becomes very slow on cooling, so the oxygen permeability can remain elevated for many weeks (Tsai and Wachtel, 1990). Tsai and Jenkins (1988) reported that the oxygen barrier of retortable packages containing an EVOH barrier layer was initially reduced by two orders of magnitude when these containers were subjected to steam or pressurized water during thermal processing, and during long-term storage (>200 days) the barrier was partially recovered (by a factor of 10). In a more recent work (Lopez-Rubio et al., 2003) it was demonstrated that this huge increase in permeability was caused not only by the plasticization of the EVOH structure, as had been previously reported (Tsai and Jenkins, 1988; Zhang et al., 2001), but also by a deterioration of the copolymer crystallinity that takes place even in multilayer structures during the retorting process (LopezRubio et al., 2005a). It is worth noting that the retorting treatment is carried out at temperatures well below the copolymer's maximum of melting, albeit above its glass transition temperature. Nevertheless, polymer films, typically utilized in foodpackaging applications, did not withstand the treatment and irreversibly lost dimensional stability. Therefore, in order to evaluate the effects of retorting on pure EVOH matrices, thicker plates were prepared and submitted to this food preservation method (Lopez-Rubio et al., 2003). As can be observed in Fig. 10.4, after retorting, the samples appeared, by visual inspection, to be dramatically damaged, with the presence of voids and loss of transparency, i.e., intense whitening or hazing. These effects are attributed to pressurized water vapour having penetrated the amorphous phase but also diffusing through the crystal edges and inducing partial melting and fractionation of the crystalline morphology. Voiding and loss of transparency could arise from bubbles formed by water evaporation as

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10.4 EVOH32 (a) before and (b) after retorting in autoclave (121ëC, 20 min).

the pressure is released rapidly after retorting in the autoclave. These damaging effects do not occur upon water exposure or uptake at room temperature and evidently neither due to annealing. High temperature and humidity thus appear to be a very severe and aggressive combination of factors for these materials. Figure 10.5 shows the X-ray patterns as a function of temperature obtained after real-time synchrotron experiments of EVOH monolayers. Comparing the patterns of the dry and water-saturated specimens (the latter one simulating an in situ retorting process), it can be seen that while the dry sample melts around 183ëC (in accordance with DSC data), the X-ray patterns of the water-saturated EVOH specimen disappear around 100ëC, which implies that the polymer melts 83ëC earlier than expected in the presence of heated water vapour (Lopez-Rubio et al., 2003). The morphology and thermal characteristics of retorted specimens typically used in food-packaging structures (i.e. protected by polypropylene layers) were also seen to be greatly affected by the combination of temperature and humidity. An FTIR methodology was developed to ascertain the changes in crystallinity of EVOH films and a significant reduction in crystallinity was observed after retorting of the multilayer packaging structures (Lopez-Rubio et al., 2003). FTIR spectroscopy technique is an appropriate tool to study morphological alterations in these materials because of its high sensitivity to detect both crystallinity alterations through the use of the 1140 cmÿ1 band and the presence of humidity in the sample through observation of the OH in-plane bending band at 1650 cmÿ1. The 1140 cmÿ1 band is likely attributable to C±O±C stretching or to C±C stretching coupled with a C±O stretching mode. The absorbance of this band (divided by that of the internal standard at 1333 cmÿ1) can thus give us an indication of potential alterations in crystallinity after the various treatments, irrespective of differences in optical path and of minor thickness variations between different specimens (Lopez-Rubio et al., 2005a).

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10.5 WAXS patterns as a function of temperature of (a) dry and (b) watersaturated EVOH32 film specimens (from Lopez-Rubio et al., 2003).

10.4.2 Effects of other novel food preservation technologies on EVOH packaging structures Consumer preferences towards mildly preserved, high quality and more freshlike products are leading to the substitution of these traditional thermal treatments by other emerging preservation technologies based on the application of irradiation, microwave pasteurization, electric fields, high hydrostatic pressure and their combination with mild thermal treatments. Among these emerging technologies, high pressure processing (HPP) is receiving a great deal of attention due to its unique advantages over conventional thermal treatments, including application at low temperatures, which improves the retention of food quality. High pressure treatments are independent of product size and geometry and their effect is uniform and instantaneous (Palou et al., 1999). Most HPP equipment works in batch processes. Foodstuffs are packaged at the end of the production line and then pressurized at hundreds of MPa to produce a high

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nutritional and sensorial quality product, with more desirable texture and longer shelf-life (Ledward, 1995). It is then crucial to understand the effects of high pressure on relevant properties of plastic materials to assure the safety of the foodstuffs throughout their shelf-life. Irradiation of pre-packaged foodstuffs using gamma and electron beam radiation is also gaining ground as a method of food preservation. To enhance the protection offered by irradiation, foodstuffs are usually pre-packaged in flexible packaging materials prior to the treatment and, in that way, subsequent recontamination by microorganisms is prevented. Moreover, this technique is also being used for the sterilization of flexible packages utilized later on in aseptic packaging technology (Azuma et al., 1983). Plastics are affected in various ways when exposed to high-energy radiation and, thus, it is also important to characterize the effects of this preservation treatment on the structure and properties of polymers in order to ensure the adequate final performance, especially if they are going to be used for food packaging. The effects of these novel food preservation technologies on EVOH materials have also been studied. In contrast with the damaging effects of retorting, HPP and irradiation technologies do not alter significantly the morphology and physico-chemical properties of the packaging structures. Specifically, high hydrostatic pressures even lead to a slight improvement in the oxygen barrier properties of EVOH, which can be explained not only by a pressure-induced reduction of the free volume but also by a slight increase in the crystallinity of the materials (Lopez-Rubio et al., 2005b). Table 10.1 shows the oxygen transmission rate values for two different grades of EVOH that had been submitted to various high pressure treatments in comparison with the values obtained for the retorted specimens. In the case of irradiation, formation of radiolysis compounds was detected, which was observed to be dose-dependent (Riganakos et al., 1999). However, it Table 10.1 Oxygen transmission rate (cm3/m2.day) of multilayer structures PP// EVOH//PP high pressure processed, retorted and untreated

Untreated 400 MPa, 40ëC, 5 min 400 MPa, 75ëC, 5 min 800 MPa, 40ëC, 5 min 800 MPa, 75ëC, 5 min 400 MPa, 40ëC, 10 min 400 MPa, 75ëC, 10 min 800 MPa, 40ëC, 10 min 800 MPa, 75ëC, 10 min Retorted Retorted (after 150 h)

EVOH26

EVOH48

0.60 0.44 0.36 0.50 0.62 0.50 0.48 0.58 0.59 392.00 1.63

40.88 39.24 41.00 39.63 37.50 37.00 38.25 39.38 38.13 645.00 28.55

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was demonstrated that all the radiolysis and non-radiolysis products of ethylene± vinyl alcohol (EVOH) copolymers were within the threshold of regulation (Kothapalli and Sadler, 2003), so in principle, this confirms the safety of irradiated EVOH as a food contact material. Moreover, electron beam irradiation at doses of 30 and 90 kGy was seen to impart some oxygen scavenging capacity in an ethylene±vinyl alcohol copolymer (EVOH29, i.e., 29 mol% of ethylene). This oxygen-blocking activity is thought to arise from the reaction of oxygen with the free radicals formed during the irradiation process and was observed to be dependent on the irradiation dose, i.e., the higher the dose, the longer the time the polymer was able to react with oxygen (Lopez-Rubio et al., 2007). After exhaustion of this capacity the irradiated films are slightly more permeable to oxygen as a consequence of faster oxygen diffusion. From a food packaging application point of view, the e-beam irradiation of EVOH-containing structures can produce the desired reduction of microbial contamination, and an oxygenscavenging activity which might be of great interest for the packaging of oxygen-sensitive products.

10.5

Improving retorting of ethylene±vinyl alcohol (EVOH)

10.5.1 Strategies to improve EVOH resistance to the retorting treatment It is well known that the application of a thermal treatment below the melting point of semicrystalline polymeric materials ± particularly above the glass transition temperature of the material ± favours the mobility of chain segments at the crystals' interphase and within the crystals towards the development of a more stable and thicker crystalline morphology, a phenomenon known as annealing. Therefore, this phenomenon leads to the elimination of defects through partial melting and recrystallization of the most ill-defined (less metastable) crystals, generating a more regular stacking of the lamellae and higher crystallinity. Therefore, two of the strategies advocated for reducing the detrimental effects of retorting over EVOH structures were thermally treating the multilayer structures of PP/EVOH/PP either before or after the retorting treatment (Lopez-Rubio et al., 2005a). Different multilayer structures of various compositions of EVOH protected by PP layers were retorted and then dried at 70ëC for one week in a vacuum oven. Surprisingly, it was found that the oxygen transmission rate values of the retorted and then dried samples were found to be not only better than the values from the just retorted specimens, but also superior to those of the untreated ones, where this improvement was more pronounced for those copolymers with higher ethylene content. Analysing the FTIR spectra of the specimens, it was also found that drying is an effective process in reducing sorption-induced polymer plasticization, as the water band at 1650 cmÿ1 was not so clearly seen in the retorted and dried samples.

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10.6 FTIR spectra of EVOH32 specimens: from top to bottom, untreated, retorted, and retorted and then dried, delaminated from PP/EVOH/PP structures (from Lopez-Rubio et al., 2005a).

Figure 10.6 compares the FTIR spectra of EVOH32 specimens untreated, retorted, and retorted and then dried, delaminated from PP/EVOH/PP structures, with the 1650 cmÿ1 point arrowed. From this figure it can also be observed that the crystallinity band at 1140 cmÿ1 shows the highest absorbance, even higher than that of the unmodified sample, an observation that confirms that the crystallinity is the highest for this specimen. Thus, pressurized water vapour, which penetrated the multilayer structure during retorting, is thought to melt and disrupt part of the EVOH crystalline morphology. The subsequent annealing process in the vacuum oven at 70ëC for one week allowed the polymer chains to reorganize and anneal, giving rise to a significantly improved crystalline structure (Lopez-Rubio et al., 2005a). From the above results and from an applied problem-solving perspective, it is apparent that a drying step after sterilization of the package can restore or even improve the barrier performance of the materials by removing sorbed moisture and by rebuilding a more favourable morphology. Being aware of the implications of crystallinity and its morphology on barrier properties and of the ability of water sorption to modify the polymer morphology, the second strategy considered was to improve the initial crystallinity of the samples before retorting through the application of an annealing process. The different multilayer structures were first annealed for 20 min in an oven at a selected optimum temperature (beyond which crystallinity was seen to decrease through annealing) that had previously been determined through FTIR. Then, the specimens were retorted in the autoclave, delaminated, and the EVOH layer FTIR recorded. The rationale behind this annealing experiment was to provide the most adequate polymer morphology in terms of crystallinity content and robustness (i.e., higher crystalline density) before retorting.

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Table 10.2 FTIR absorbance of the 1440 cmÿ1 peak divided by the absorbance of the 1333 cmÿ1 reference peak Material

Not treated

Annealed

Retorted

Annealed and retorted

Retorted and dry

EVOH26 EVOH29 EVOH32 EVOH38 EVOH44

1.26 1.25 1.24 1.21 1.21

1.45 1.43 1.40 1.38 1.34

1.13 1.12 1.14 1.19 1.19

1.25 1.20 1.16 1.19 1.19

1.28 1.28 1.26 1.25 1.24

Variations in crystallinity were estimated through the ratio of the absorbance of the crystallinity band at 1140 cmÿ1 to that of the band at 1333 cmÿ1. The values of this ratio for the various copolymer grades and thermal histories are displayed in Table 10.2. From these results, it can be seen that pre-annealed EVOH26, EVOH29 and EVOH32 specimens can attain a higher level of crystallinity after retorting than that of retorted-only specimens. In fact, preannealed sample EVOH26 even shows a degree of crystallinity after retorting similar to that of the untreated specimen (dried under vacuum at 70ëC for one week). For EVOH38, EVOH44 and EVOH48, prior annealing of the specimens did not lead to improved morphology compared to that of untreated specimens (Lopez-Rubio et al., 2005a). The third strategy discovered to protect EVOH from the effects of retorting was by appropriate shielding of the EVOH layer between polypropylene layers of a critical thickness (40 m instead of the 10 m normally used in foodpackaging structures). From synchrotron radiation studies, it was observed that the integrity of the EVOH layer was largely maintained during a typical retorting process when using these sufficiently thick PP layers. Figure 10.7 shows the WAXS patterns of the PP, EVOH32 and PP/EVOH32/PP structures during different stages of the retorting process. In Fig. 10.7, the arrow indicates the presence of the (110) crystalline peak of EVOH in the structure during the whole retorting experiment, suggesting that the barrier layer does not melt in a multilayer structure (Lopez-Rubio et al., 2005c). Therefore, even though a plasticization of the EVOH amorphous structure will still take place, as the crystalline structure is not melted during the retorting process, the barrier effects are expected to be much lower in multilayer structures containing thicker layers of PP.

10.5.2 Blending EVOH with other materials Another strategy that showed promising results for palliating the deleterious effects of retorting treatments over EVOH structure was the use of blends. Specifically, blends of EVOH with amorphous polyamide (aPA) and nylon-

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10.7 WAXS patterns of PP, EVOH32 and PP/EVOH/PP structures during different stages of the retorting process (from Lopez-Rubio et al., 2005c).

containing ionomer were originally developed to diminish the barrier deterioration of EVOH when exposed to high-humidity environments (LagaroÂn et al., 2001, 2003a). The main drawback for blending EVOH with other polymers is the strong polymer self-association promoted by the hydroxyl groups, which leads to poor compatibility. Binary and ternary blends of amorphous polyamide (aPA) and nylon-containing ionomer with EVOH have proven to have beneficial effects such as improved processability during thermoforming. Even when these blends were found to be immiscible, good phase dispersion and adhesion at the interphase were generally found (LagaroÂn et al., 2001, 2003a). From a barrier perspective, the materials were found to provide a positive deviation from the Maxwell model in oxygen permeability, which led to better barrier properties than expected (LagaroÂn et al., 2003a). Furthermore, both the aPA and the nyloncontaining ionomer exhibited lower water sorption than the hydrophilic EVOH. This lower water sorption was probably the reason why, when these blends were exposed to a combination of temperature and pressurized water vapour (i.e. retorting), they did not experience detrimental structural changes as was the case in pure EVOH (Lopez-Rubio and LagaroÂn, 2008). In fact, improvements in the thermal properties, crystalline structure and water resistance of the blends were found upon retorting compared with neat EVOH. However, only the binary blend with aPA showed a real enhancement in the oxygen barrier properties immediately after retorting compared with neat EVOH. This effect was ascribed to the retorting-induced compatibilization between EVOH and aPA components of the blend as determined by SEM. In the blends with ionomer, increased oxygen permeability values were observed, probably due to the melting of the

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ionomer at the temperatures applied and, thus, upon cooling after the treatment, voids were left in the films, causing the drop in barrier properties (Lopez-Rubio and LagaroÂn, 2008).

10.5.3 Alternatives to EVOH for retortable packages As a consequence of the detrimental effects of common retorting processes over the structure and permeability of the EVOH copolymers, alternative high-barrier materials are being studied as potential substitutes in retortable food-packaging structures. Aliphatic polyketones and amorphous PA are high and medium-high barrier materials with potential in retortable applications. A very recent study has already proven that the aliphatic polyketones are adequate materials to withstand packaged food retorting conditions, as the deterioration in oxygen barrier suffered by these polymers even in monolayer structures is very small compared with the deterioration suffered by EVOH-based multilayer structures (Lopez-Rubio et al., 2006a). Moreover, the crystalline structure of polyketones withstands the retorting process as observed in the WAXS patterns during timeresolved in situ retorting of the specimens (cf. Fig. 10.8). Amorphous polyamides (aPAs), on the other hand, offer favourable properties such as dimensional stability, good dielectric and barrier properties, and low mould shrinkage (Granado et al., 2004) and, furthermore, exhibit an antiplasticization behaviour at high relative humidity conditions. Thus, it is known that the oxygen barrier performance of this material increases at high relative humidity conditions in contrast with the behaviour of most polar polymers, including most polyamides, aliphatic polyketones and EVOH (LagaroÂn et al., 2003b). When aPA was submitted to a retorting process, it was fortuitously found that the combination of heat and moisture was capable of inducing some crystallization in the otherwise amorphous polymer. The crystallization of the polymer began as characterized by wide angle X-ray scattering and differential scanning calorimetry in the presence of humidity at 90ëC and extended up to 120ëC under autoclave conditions, and is thought to be the result of heated

10.8 Synchrotron WAXS traces versus temperature (ëC) taken during a typical retorting run of PK specimens (from Lopez-Rubio et al., 2006a).

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moisture being able to disrupt the intense amide groups' self-association. Thus, the thermally activated molecular structure is thought to become plasticized by the combined presence of heat and water which, in turn, provoke sufficient segmental molecular mobility in the system to promote some degree of lateral order. Property-wise, the resulting consequences of this behaviour are an increase in the barrier properties to oxygen and a reduction in water sorption. From an applied viewpoint, it is suggested that this unexpected behaviour could make this polymer of significant interest in retortable food packaging applications (Lopez-Rubio et al., 2006b).

10.6

Nanocomposites of ethylene±vinyl alcohol (EVOH) and poly(vinyl) alcohol (PVOH)

A great deal of research has been put forward to minimize mass transport processes in polymeric materials for the packaging of, for instance, oxygensensitive foodstuffs and, thus, to guarantee food quality and safety during the extended shelf-life of these products (LagaroÂn et al., 2004). One of the most promising routes to accomplish this is through the development of `ultra-high' barrier materials by means of nanotechnology. Specifically, the generation of nanocomposites through the addition of low loadings of nanoparticles (generally nanoclays) to a raw polymer has been reported to have enhancing effects over some material properties, such as mechanical properties, thermal stability (Kotsilkova et al., 2001) and gas barrier properties, without significant reduction in other relevant characteristics such as toughness (Alexandre and Dubois, 2000) and transparency below a critical loading level (Wan et al., 2003). The addition of nanoclays to EVOH matrices is thought to result in ultra-high barrier properties mainly due to a tortuosity-driven decrease in molecular diffusion of gases and vapours and to increased thermal resistance. EVOH nanocomposites have been developed mainly using two types of layered silicates: commercial modified montmorillonites (MMT) and natural and modified kaolinites. The montmorillonites used in EVOH nanocomposites are organically modified by exchanging the interlayer cations, normally with alkylammonium ions. The kaolinite is a very common material in earth, very cheap and, hence, widely used as raw material in many industrial sectors (LagaroÂn et al., 2005). Surface chemical treatment of kaolinite platelets can also be applied to facilitate intercalation of the polymer chains and further exfoliation of the clay. The treatments of the platelets with chemical agents, such as dimethyl-sulfoxide, methanol and octadecylamine, lead to increases in the basal spacing of the clay as well as to increased compatibility with the polymer, thus facilitating mixing, exfoliation and dispersion (Cabedo et al., 2004). X-ray diffraction is a conventional method to characterize the gallery height in clay particles, which is indicative of the extent of intercalation. The (001) basal crystalline plane observed in the X-ray diffractogram is usually shifted

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towards lower angles upon chemical treatments (Cabedo et al., 2004; Villanueva et al., 2010). This displacement indicates that the clay interlayer distance increases as a result of the treatment. The shift in the basal crystalline plane is generally also observed after mixing with the polymer, indicating that intercalation takes place (Artzi et al., 2001; Lee et al., 2006). Further mixing can lead to the disappearance of the basal peak, which has been ascribed to exfoliated structure (Artzi et al., 2005). Because the silicate layers have hydroxyl groups, it is generally recognized that polymers with polar groups should be used to obtain a highly intercalated structure or exfoliation state. The presence of hydroxyl groups in EVOH thus makes these copolymers suitable for the interaction with the silicate layers (Jeong et al., 2005). However, some of the recent theoretical and experimental research results demonstrated that the adhesive role of a polar polymer between hydrophilic clay layers, the so-called glue effect, tends to strongly prohibit complete dissociation of the layered structure of clay, resulting in only an ordered intercalated state (Lyatskaya and Balazs, 1998; Lee et al., 2001). This has been confirmed by transmission electron microscopy (TEM) analysis of EVOH nanocomposites with 5 wt% montmorillonite organoclays, prepared by dynamic melt intercalation, where the silica platelets were observed to be highly associated, while the same nanocomposites prepared with a polymer with poor interfacial affinity with the clay showed much higher disintegration of the layered structure (Lee et al., 2006). The latter authors also demonstrated that even though there was lower dissociation in EVOH matrices, a dramatic increase in tensile strength and modulus was observed for these nanocomposites. Specifically, increases of 10% and 22% were observed for the tensile strength and modulus respectively (Lee et al., 2006). Based on these findings, it was concluded that the glue effect is important in determining the degree of intercalation, and the optimum value of interaction between the polymer and the clay surface is of critical consideration in designing high-performance clay nanocomposites with an intercalated structure (Lee et al., 2006). This has been further substantiated by several studies where through SEM and TEM a strong adhesion between EVOH and clay and intercalation (but not total exfoliation) was observed, respectively, which were correlated with 3 to 6ëC higher Tg values for the nanocomposites and, thus, higher rigidity (Cabedo et al., 2004; LagaroÂn et al., 2005). As an example, a TEM image of an EVOH±kaolinite composite is shown in Fig. 10.9. However, it is important to note that the amount of exfoliation appears to be strongly affected by the processing conditions. Generally, the level and mode of dispersion are governed by parameters such as matrix viscosity, level of shear field, and residence time in the process (Cho and Paul, 2001). Artzi and coworkers (2005) claimed to have both intercalation and delamination of organoclays in EVOH nanocomposites processed by melt mixing. The montmorillonite clays used by the previous authors had been modified using

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10.9 TEM image of an EVOH nanocomposite containing 2% of a modified kaolinite clay (from Cabedo et al., 2004).

octadecylamine instead of alkylammonium ions, the fact that could also influence the clay behaviour during processing of the nanocomposites. The fracturing process of the organoclay particles into smaller tactoids and posterior delamination during the mixing process with EVOH can be followed using a plastograph mixing cell and is indicated by the increase in processing torque. Residence mixing time has been found to strongly affect the structuring process of the composites, the level of exfoliation, the degree of EVOH crystallinity, and the resulting properties (Artzi et al., 2001). Subsequent processing of the nanocomposites by, for instance, extrusion results in higher delamination and platelet dispersion. Another factor contributing to the higher delamination level is the higher stress developed at the elevated rotational speed. Extrusion residence time, successive extrusion passes, screw rotational speed and processing temperature were all found to affect the morphology and the associated thermal and mechanical properties (Artzi et al., 2005). EVOH±clay nanocomposite extrusions processed at 200ëC and 40 rpm presented an exfoliated structure and, regardless of clay type or content, the tensile modulus and strength were observed to increase up to 40% and 30% respectively relative to the neat EVOH with only 0.5% clay addition, again highlighting the high level of interaction of the clays with the EVOH matrix (Artzi et al., 2004).

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The high interaction level in EVOH±clay systems has also been supported by DSC studies. Upon addition of montmorillonite clays, some authors observed a decrease in the melting temperature and enthalpy values, which could be explained by the interactions of the platelets with the polymeric chains, partially hindering segmental mobility and, thus, the crystallization process. The EVOH crystallization process is quite sensitive to crystallization conditions. The crystallization process in the presence of clay particles, especially clay nanoplatelets, can generate `smaller' EVOH crystals having a lower melting temperature. In this case, the clay is probably playing a role of hindering the crystallization process, owing to the high interaction level with EVOH (Artzi et al., 2004). However, these previous results contrast with the increase in both the melting temperature and enthalpy of fusion observed for the various EVOH nanocomposites based on kaolinite clays. In this case, the increase in the melting enthalpy was ascribed to the nucleating role of clay nanoplatelets on cooling from the melt (Cabedo et al., 2004). The latter observation is very relevant from a barrier perspective, as crystals are generally impermeable to the transport of low molecular weight substances and, thus, the combination of higher crystallinity and clay impermeable elements was seen to result in oxygen permeation rates for the nanocomposites below the experimental error of the instrument, i.e. below 10ÿ5 (cm3 m)/(m2 day atm) (Cabedo et al., 2004). Incorporation of kaolinite nanoclays to EVOH has been observed to result in significantly improved oxygen barrier properties, both under dry conditions and at high relative humidity. In this latter case, i.e. at high RH, reductions in the oxygen permeability of 70% were observed in the copolymer with 32 mol% of ethylene (LagaroÂn et al., 2005). Apart from the already mentioned improvement in mechanical and oxygen barrier properties of EVOH nanocomposites, addition of dispersed, intercalated nanoclays also results in better thermal stability. An increase in the temperature of the maximum weight loss rate of EVOH±kaolinite nanocomposites has been observed, with increases in the maximum rate of degradation temperature of more than 21ëC (Cabedo et al., 2004). The thermal stability increase could be explained by the fact that the clay, together with the solid degradation products, led to a dense coating that hinders the development of further degradation by opposing a strong mass transport resistance to the agents involved in the reaction, resulting in a decrease of the degradation kinetics (Zanetti et al., 2001). Nanocomposites of clays and the homopolymer poly(vinyl) alcohol (PVOH) have also been developed. PVOH has excellent film-forming characteristics and outstanding oxygen barrier properties in the dry state. As PVOH is even more hydrophilic than EVOH, its nanocomposites exhibit enhanced interfacial interactions through hydrogen bonding, due to adsorption of the polymer molecules on the clay surface (Grunlan et al., 2004; Ogata et al., 1997). The morphologies observed for these composites show both exfoliated and intercalated structures that are well known for a number of other polymer±clay systems. The macro-

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scopic composite properties are greatly affected by the size and morphology of the dispersed clay and, for instance, the thermomechanical properties of PVOH have been observed to increase or decrease with clay loadings, depending on the preparation conditions employed (Grunlan et al., 2004; Ogata et al., 1997; Strawhecker and Manias, 2000). It has been observed that sodium-exchanged montmorillonite clays are more easily dispersed in PVOH matrices than alkyl ammonium ion-exchanged clays (Chang et al., 2003), leading to significant increases in the Tg of the materials, and thus to increased rigidity, with just 1 wt% clay additions (Bandi and Schiraldi, 2006). The previous authors observed that increasing the amount of clay led to decreases in the glass transition temperature and explained the relative changes in Tg as being the result of two competing effects: on one hand the surface interaction between the polymer and the clay which strengthens the interface (decreasing chain mobility), and on the other hand, enhanced interfacial free volume due to the lower bulk crystallinity of polymer chains (increasing chain mobility) (Bandi and Schiraldi, 2006). A similar trend in Tg of PVOH nanocomposites has been reported by other authors (Ogata et al., 1997). However, as explained previously, the method of preparation is crucial in the final characteristics of the materials and, thus, some studies show improved properties of the hybrid materials at higher clay loadings (Chang et al., 2003; Grunlan et al., 2004). For instance, Grunlan and co-workers (2004) observed a significant reduction in oxygen permeability at 55% RH in PVOH with sodium montmorillonite with 10 wt% clay loadings. This low permeability was ascribed to the great interaction between the polymer and the clay as evidenced by an increase of more than 10ëC in the glass transition temperature at this clay concentration. Strawhecker and Manias (2000) observed significant enhancements in thermal, mechanical and barrier properties of PVOH with 5 wt% sodium montmorillonite clay addition. Interestingly, a 60% water permeability reduction was seen for these hybrid materials (Strawhecker and Manias, 2000). The ability to reduce the oxygen and water vapour permeability of PVOH-based systems at elevated humidity may prove advantageous for applications in food packaging, where moisture sensitivity currently prevents them from being used.

10.7

Future trends

The use of EVOH-based resins is expected to further expand in the food packaging sector, due both to the inherently excellent properties of these copolymers and to the newly developed solutions (including blending with other materials and generation of nanocomposites) to counteract the drawbacks that these materials present. Due to the generally poor miscibility of EVOH with other polymers, it is foreseen that the nanocomposite route would be the preferred one, which can also have an impact on the water barrier properties of the copolymers.

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On the other hand, the plasticization of EVOH in the presence of humidity can be exploited for the development of novel active packaging structures which contain active substances incorporated in the polymeric structure. Active packaging is certainly one of the most important innovations in the packaging field for food preservation and is covered in other corresponding chapters. Active packages are designed to perform a role other than to provide an inert barrier between the product and the outside environment, using the possible interactions between food and the packaging in a positive way to improve product quality and acceptability (Fernandez Alvarez, 2000). Contact of the EVOH with water or plasticization of the structure at elevated relative humidity can serve to trigger the active mechanism, either by favouring the release of the active agents towards the food product, or by facilitating the interaction of gases that need to be removed from package headspace with the scavenging elements incorporated into the packaging structure. Research into this area is steadily increasing and EVOH matrices have a great potential for the development of novel active packaging concepts. Therefore, in the food packaging area, EVOH materials have a very promising future for a wide variety of food products.

10.8

References

Alexandre, M., Dubois, P. (2000). Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Materials Science and Engineering 28, 1±63. Artzi, N., Nir, Y., Wang, D., Narkis, M. (2001). EVOH/clay nanocomposites produced by melt processing. Polymer Composites 22, 710±720. Artzi, N., Narkis, M., Siegmann, A. (2004). EVOH/clay nanocomposites produced by dynamic melt mixing. Polymer Engineering and Science 44, 1019±1026. Artzi, N., Tzur, A., Narkis, M., Siegmann, A. (2005). The effect of extrusion processing conditions on EVOH/clay nanocomposites at low organo-clay contents. Polymer Composites 26, 343±351. Aucejo, S., Marco, C., Gavara, R. (1999). Water effect on the morphology of EVOH copolymers. Journal of Applied Polymer Science 74, 1201±1206. Azuma, K., Hirata, T., Tsunoda, H., Ishitani, T., Tanaka, Y. (1983). Identification of the volatiles from low-density polyethylene film irradiated with an electron-beam. Agricultural and Biological Chemistry 47, 855±860. Bandi, S., Schiraldi, D.A. (2006). Glass transition behavior of clay aerogel/poly(vinyl alcohol) composites. Macromolecules 39, 6537±6545. Bonner, J.G., Powell, A.K. (1997). In: Proceedings of 213th National ACS Meeting, ACS Materials Chemistry Publications, Washington, DC and San Francisco, CA. Cabedo, L., GimeÂnez, E., LagaroÂn, J.M., Gavara, R., Saura, J.J. (2004). Development of EVOH±kaolinite nanocomposites. Polymer 45, 5233±5238. Cabedo, L., LagaroÂn, J.M., Cava, D., Saura, J.J., GimeÂnez, E. (2006). The effect of ethylene content on the interaction between ethylene-vinyl alcohol copolymers and water ± II: Influence of water sorption on the mechanical properties of EVOH copolymers. Polymer Testing 25, 860±867.

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Cava, D., Cabedo, L., GimeÂnez, E., Gavara, R., LagaroÂn, J.M. (2006). The effect of ethylene content on the interaction between ethylene±vinyl alcohol copolymers and water: (I) Application of FT-IR spectroscopy to determine transport properties and interactions in food packaging films. Polymer Testing 25, 254±261. Cerrada, M.L., Perez, E., PerenÄa, J.M., Benavente, R. (1998). Wide-angle X-ray diffraction study of the phase behavior of vinyl alcohol ethylene copolymers. Macromolecules 31, 2559±2564. Chang, J.-H., Jang, T.-G., Ihn, K.J., Lee, W.-K., Sur, G.S. (2003). Poly(vinyl alcohol) nanocomposites with different clays: pristine clays and organoclays. Journal of Applied Polymer Science 90, 3208±3214. Cho, J.W., Paul, D.R. (2001). Nylon 6 nanocomposites by melt compounding. Polymer 42, 1083±1094. Del Nobile, M.A., Mensitieri, G., Nicolais, L., Sommazzi, A., Garbassi, F. (1993). Gastransport properties of ethylene/propylene/carbon monoxide polyketone terpolymer. Journal of Applied Polymer Science 50, 1261±1268. Drent, E., Budzelaar, P.H.M. (1996). Palladium-catalyzed alternating copolymerization of alkenes and carbon monoxide. Chemical Reviews 96, 663±681. Drent, E., van Broekhoven, J.A.M., Doyle, M.J. (1991). Efficient palladium catalysts for the copolymerization of carbon monoxide with olefins to produce perfectly alternating polyketones. Journal of Organometallic Chemistry 417, 235±351. Fernandez Alvarez, M. (2000). Review: Active food packaging. Food Science and Technology International 6, 97±108. Foster, R.H. (1991). Improved ethylene vinyl alcohol copolymer (EVOH) resins for high barrier plastics packaging. Conference Proceedings of ANTEC 91, Montreal. Granado, A., Eguiazabal, J.I., Nazabal, J. (2004). Solid-state structure and mechanical properties of blends of an amorphous polyamide and a poly(amino-ether) resin. Macromolecular Materials and Engineering 289, 281±287. Grunlan, J.C., Grigorian, A., Hamilton, C.B., Mehrabi, A.R. (2004). Effect of clay concentration on the oxygen permeability and optical properties of a modified poly (vinyl alcohol). Journal of Applied Polymer Science 93, 1102±1109. Jeong, H.M., Kim, B.C., Kim, E.H. (2005). Structure and properties of EVOH/ organoclay nanocomposites. Journal of Materials Science 40, 3783±3787. Kothapalli, A., Sadler, G. (2003). Determination of non-volatile radiolytic compounds in ethylene co-vinyl alcohol. Nuclear Instruments and Methods in Physics Research B 208, 340±344. Kotsilkova, R., Petkova, V., Pelovski, Y. (2001). Thermal analysis of polymer-silicate nanocomposites. Journal of Thermal Analysis and Calorimetry 64, 591±598. LagaroÂn, J.M., Gimenez, E., Saura, J.J., Gavara, R. (2001). Phase morphology, crystallinity and mechanical properties of binary blends of high barrier ethylenevinyl alcohol copolimer and amorphous polyamide and a polyamide-containing ionomer. Polymer 42, 7381±7394. LagaroÂn, J.M., Gimenez, E., Altava, B., Del-Valle, V., Gavara, R. (2003a). Characterization of extruded ethylene-vinyl alcohol copolimer based barrier blends with interest in food packaging applications. Macromolecular Symposia 198, 473± 482. LagaroÂn, J.M., Gimenez, E., Catala, R., Gavara, R. (2003b). Mechanisms of moisture sorption in barrier polymers used in food packaging: Amorphous polyamide vs. high-barrier ethylene±vinyl alcohol copolymer studied by vibrational spectroscopy. Macromolecular Chemistry and Physics 204, 704±713. LagaroÂn, J.M., Catala, R., Gavara, R. (2004). Structural characteristics defining high

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barrier properties in polymeric materials. Materials Science and Technology 20, 1±7. LagaroÂn, J.M., Cabedo, L., Cava, D., Feijoo, J.L., Gavara, R., Gimenez, E. (2005). Improving packaged food quality and safety. Part 2: Nanocomposites. Food Additives and Contaminants 22, 994±998. Lai, T.W., Sen, A. (1984). Transition-metal catalyzed copolymerization of carbonmonoxide with olefins. 2. Palladium(II)-catalyzed copolymerization of carbon monoxide with ethylene ± direct evidence for a single mode of chain growth. Organometallics 3, 866±870. Ledward, A.D. (1995). High pressure processing ± the potential. In A.D. Ledward, D.E. Johnston, R.G. Earnshaw and A.P.M. Hasting (eds), High Pressure Processing of Foods. Nottingham, UK: Nottingham University Press, pp. 1±5. Lee, S.-S., Lee, C.S., Kim, M.-H., Kwak, S.Y., Park, M., Lim, S.H., Choe, C.R., Kim, J. (2001). Specific interaction governing the melt intercalation of clay with poly(styrene-co-acrylonitrile) copolymers. Journal of Polymer Science Part B ± Polymer Physics 39, 2430±2435. Lee, S.-S., Hur, M.H., Yang, H., Lim, S., Kim, J. (2006). Effects of interfacial attraction on intercalation in polymer/clay nanocomposites. Journal of Applied Polymer Science 101, 2749±2753. Lopez-Rubio, A., LagaroÂn, J.M. (2008). Improving the resistance to humid heat sterilization of EVOH copolymers through blending. Journal of Applied Polymer Science 109, 174±181. Lopez-Rubio, A., LagaroÂn, J.M., GimeÂnez, E., Cava, D., Hernandez-MunÄoz, P., Yamamoto, T., Gavara, R. (2003). Morphological alterations induced by temperature and humidity in ethylene±vinyl alcohol copolymers. Macromolecules 36, 9467±9476. Lopez-Rubio, A., Hernandez-MunÄoz, P., GimeÂnez, E., Yamamoto, T., Gavara, R., LagaroÂn, J.M. (2005a). Gas barrier changes and morphological alterations induced by retorting in ethylene vinyl alcohol-based food packaging structures. Journal of Applied Polymer Science 96, 2192±2202. Lopez-Rubio, A., LagaroÂn, J.M., HernaÂndez-MunÄoz, P., Almenar, E., Catala, R., Gavara, R., Pascall, M.A. (2005b). Effect of high pressure treatments on the properties of EVOH-based food packaging materials. Innovative Food Science and Emerging Technologies 6, 51±58. Lopez-Rubio, A., Hernandez-MunÄoz, P., Catala, R., Gavara, R., LagaroÂn, J.M. (2005c). Improving packaged food quality and safety. Part 1: Synchrotron X-ray analysis. Food Additives and Contaminants 22, 988±993. Lopez-Rubio, A., Gimenez, E., Gavara, R., LagaroÂn, J.M. (2006a). Gas barrier changes and structural alterations induced by retorting in a high barrier aliphatic polyketone terpolymer. Journal of Applied Polymer Science 101, 3348±3356. Lopez-Rubio, A., Gavara, R., LagaroÂn, J.M. (2006b). Unexpected partial crystallization of an amorphous polyamide as induced by combined temperature and humidity. Journal of Applied Polymer Science 102, 1516±1523. Lopez-Rubio, A., LagaroÂn, J.M., Yamamoto, T., Gavara, R. (2007). Radiation-induced oxygen scavenging activity in EVOH copolymers. Journal of Applied Polymer Science 105, 2676±2682. Lyatskaya, Y., Balazs, A.C. (1998). Modeling the phase behavior of polymer-clay composites. Macromolecules 31, 6676±6680. Ogata, N., Kawakage, S., Ogihara, T. (1997). Poly (vinyl alcohol)±clay and poly (ethylene oxide)±clay blends prepared using water as solvent. Journal of Applied

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Polymer Science 66, 573±581. Palou, E., LoÂpez-Malo, A., Barbosa-CaÂnovas, G.V., Swanson, B.G. (1999). High-pressure treatment in food preservation. In M.S. Rahman (ed.), Handbook of Food Preservation. New York: Marcel Dekker, pp. 533±576. Ramesh, M.N. (1999). In M.S. Rahman (ed.), Handbook of Food Preservation. New York: Marcel Dekker, p. 95. Riganakos, K.A., Koller, W.D., Ehlermann, D.A.E., Bauer, B., Kontominas, M.G. (1999). Effects of ionizing radiation on properties of monolayer and multilayer flexible food packaging materials. Radiation Physics and Chemistry 54, 527±540. Sommazzi, A., Garbassi, F. (1997). Olefin carbon monoxide copolymers. Progress in Polymer Science 22, 1547±1605. Strawhecker, K.E., Manias, E. (2000). Structure and properties of poly(vinyl alcohol)/ Na+ montmorillonite nanocomposites. Chemical Materials 12, 2943±2949. Takahashi, M., Tashiro, K., Amiya, S. (1999). Crystal structure of ethylene±vinyl alcohol copolymers. Macromolecules 32, 5860±5871. Tsai, B.C., Jenkins, B.J. (1988). Effect of retorting on the barrier properties of EVOH. Journal of Plastic Film and Sheeting 4, 63±71. Tsai, B.C., Wachtel, J.A. (1990). In: W.J. Koros (ed.), Barrier Polymers and Structures, American Chemical Society, Washington, DC, pp. 192±202. Villanueva, M.P., Cabedo, L., LagaroÂn, J.M., Gimenez, E. (2010). Comparative study of nanocomposites of polyolefin compatibilizers containing kaolinite and montmorillonite organoclays. Journal of Applied Polymer Science 115, 1325±1335. Wan, C., Qiao, X., Zhang, Y., Zhang, Y. (2003). Effect of different clay treatment on morphology and mechanical properties of PVC±clay nanocomposites. Polymer Testing 22, 453±461. Zanetti, M., Camino, G., Thomann, R., MuÈlhaupt, R. (2001). Synthesis and thermal behavior of layered silicate±EVA nanocomposites. Polymer 42, 4201±4207. Zhang, Z., Britt, I.J., Tung, M.A. (2001). Permeation of oxygen and water vapour through EVOH films as influenced by relative humidity. Journal of Applied Polymer Science 82, 1866±1872. Zhao, A.X., Chien, J.C.W. (1992). Palladium catalyzed ethylene±carbon monoxide alternating copolymerization. Journal of Polymer Science Part A ± Polymer Chemistry 30, 2735±2747.

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High barrier plastics using nanoscale inorganic films V. TEIXEIRA, J. CARNEIRO, P. CARVALHO, E . S I L V A , S . A Z E V E D O and C . B A T I S T A , University of Minho, Portugal

Abstract: Packaging materials for food containers need to fulfil extremely tight standards towards the fresh-like quality maintenance of packed products. Even though polymers are normally preferred for the production of packaging systems, their permeability to gases, water vapour and odours remains a concern. In this sense, new approaches using nanoscale effects are under development to design, create or model gas-barrier nanocoatings with significantly optimized properties. It is generally agreed that the final barrier performances of the deposited inorganic materials are strongly coupled with mechanical and morphological properties. Basically, a good barrier system should have a dense morphology without cracks, good adhesion to the substrate, low stress, uniform thickness and reproducibility. At the moment, researchers aim for development of cost effective mass production techniques. Key words: food industry, shelf-life, flexible polymers, nanotechnology, nanopackaging systems, gas-barrier nanocoatings, thin films, inorganic materials, deposition techniques, mechanical properties, diffusion mechanisms, WVTR, OTR.

11.1

Introduction

Food packaging is a constant presence in the daily life of all individuals who make up so-called modern societies in developed countries. In recent years, food preparation and consumption habits have changed with people's lifestyles, leading to a considerable increase in the market supply of pre-prepared and packaged food. Consequently, high quality packaging is an important tool for the safety and well-being of people as well as for the successful marketing of food products. Basically, the protective materials used in the food industry aim to extend shelf-life, preserving products from air (and oxygen), light (and UV radiation), microbial contamination, loss of gas (e.g. carbonated beverages), storage temperature variation, foreign aroma compounds, moisture loss/incorporation and mechanical influences [1]. As a conclusion, the packaging must ensure that the interaction between the environment and the packaged food is minimal.

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To get together all these requirements, different packaging concepts have been introduced into the food market. In order to understand the real importance of food packaging, it must be seen not only from the consumer's point of view but also from the market one. As stated, the market needs have changed: consumers are more selective, critical and rigorous in their concern for fresh-like qualities and, at the same time, there have been considerable changes in retail and distribution practices. The centralization of activities, new trends like shopping via the Internet and internationalization of markets have led to a considerable increase in distribution distances as well as to longer storage times of different products with different temperature requirements, which pushes the food packaging industry to higher limits in terms of product quality. For this reason novel nanomaterials for flexible packaging systems are being developed to increase the safety of foods and keep their natural characteristics, among them bioactive packaging, inorganic nanocoatings and smart labels based on nanotechnology concepts. Polymers are a common choice as protective materials since they combine flexibility, variable sizes and shapes, relative light weight, stability, resistance to breaking, barrier properties and perceived high-quality image with costeffectiveness. But still, traditional packaging concepts are limited in their ability to prolong the shelf-life of food products. Nanobiocomposites of biopolymers containing low additions (below 8%) of modified food-compliant nanolayered clays and bionanofibres can have a tremendous positive impact on a number of physical properties such as barrier properties to gases and vapours, UV protection and mechanical and thermal properties without significant losses in transparency and toughness, that can lead to enhance quality and safety of packaged foods. Moreover, these inorganic structures can also be surface modified to become active and bioactive nanoadditives, which by integration in biomaterials, can lead to novel packaging materials, coatings and encapsulates with active (antimicrobial, antioxidant or oxygen scavenging capacity) and bioactive properties (protection of functional ingredients such as body antioxidants, prebiotics and probiotics) which can more effectively enhance the quality, safety and health aspects of foods and packaged foods. In the field of nanotechnology-based thin films and nanostructured coating materials, new approaches using nanoscale effects will be used to design and create advanced nanocoating systems with significantly optimized or enhanced properties of high interest to the food, health and even biomedical industries. With the development of nanotechnology in various areas of materials science, there is great potential in the use of novel functional surfaces and more reliable nanomaterials by employing nanocomposite and nanostructured thin films in food packaging, security pharmaceutical labels and novel polymeric containers for food contact, as well as in applying these concepts to medical surface instruments, bio-implants, and even coated nanoparticles for bionanotechnology. During recent years, the food industry has been investing millions of dollars in

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11.1 Chart presenting some nanostructured materials used in food packaging.

R&D towards the application of nanoscience and nanotechnology. Some names like NestleÂ, Altria, H.J. Heinz and Unilever are at the top of the list, followed by hundreds of smaller companies. Currently, various technologies are the subject of intense investigation in both the food packaging and food security sectors. The incorporation of nanostructured materials can, for example, enhance the barrier properties of polymers while reducing the use of raw materials and, therefore, waste generation. Different applications of nanomaterials can be listed: (1) improved packaging (gas and moisture barriers, tensile strength); (2) shelf-life extension (via active packaging); (3) nanoadditives and nanofilms; (4) delivery and controlled release of nutraceuticals; (5) antibacterial (or self-cleaning) packaging and (6) monitoring product conditions during transportation using different nanosensing devices [2]. Figure 11.1 illustrates some of the nanotechnologies under intense research.

11.2

Nanotechnologies of thin films for advanced food packaging

Generally, untreated materials used in the design of flexible food packages are permeable to gases (oxygen, carbon dioxide, etc.), water vapour and odours [3, 4]. Packaging materials should present good barrier performance, highquality mechanical properties, sealability and cost-efficiency [5]. Microwave compatibility and package transparency are further important parameters pointed out by consumers.

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In vacuum coating of plastic films, which form the majority of roll-to-roll vacuum web coating, about 15 billion square metres per year are presently coated worldwide, with a continuous, long-term growth over the last decades. In this area, packaging films today represent about two-thirds of the overall volume. The rest is covered by technical and decorative applications. In packaging, the most important material used is still aluminium in the form of sheet or as thin film material thermally evaporated. However, it is not transparent and has a negative environmental impact. In addition, a new market perspective is seen in transparent and flexible barrier layers on the basis of oxides based on silicon and/or aluminium. In order to meet all the requirements listed, nowadays the package has a combination of different layers (each with a certain specification), the most critical barrier being the one that represents the highest fraction of the total cost of the laminate. In this way, R&D strategies have moved towards the development of coated polymers with thin films of transparent metal oxides to be used in advanced food package assembly. Silicon oxide (SiOx) coated polymers, e.g. poly(ethylene terephthalate), have been studied as particularly useful diffusion barrier films due to their low oxygen transmission rate and high transparency. A good barrier system should present certain requirements, such as dense morphology without cracks, pinholes or other defects, good adhesion to the substrate, low stress, uniform thickness and reproducibility [6]. Gas-barrier systems for food packaging are developed to prevent oxidative processes in order to assure long-term high-quality products. Typical permeation rates of uncoated to coated polymer of the order of 100 [7] are achieved with 15 nm thick aluminium coatings formed by resistive evaporation [8]. In food packaging, transparent inorganic layers several dozen nanometres thick deposited onto polymers can produce excellent results in terms of transparency and barrier properties to oxygen, water vapour, carbon dioxide and aromas, increasing the shelf-life of the packed product. These transparent oxide-type coatings present a commercial interest for those packaging applications that require microwaveability or product visibility. At present, silicon oxide thin films are competitive with Al-coated polymer films which, though they present adequate barrier characteristics, are not transparent or suitable for microwave ovens [9]. Silicon oxide thin films have been produced since the early 1980s by means of plasma enhanced chemical vapour deposition (PECVD) and physical vapour deposition (PVD) [10]. Although SiOx nanocoatings suffer from problems that include cracking and poor adhesion to polymer substrates [11], different studies are still being carried out in order to improve the quality of SiOx based thin films as well as to change the substrate used. For industry, polypropylene (PP) can be an interesting substitute for PET as packaging polymer since it has an inherent water vapour barrier, low density and extremely low cost.

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At the same time, SiOx substitutes are now being considered. The most common coatings for this specific application are silicon, aluminium and titanium based thin films in the form of nitride, oxide, oxynitride and carbonitride [12±20]. It is believed that by changing the chemical structure of the transparent oxide film through the introduction of different chemical species, the intrinsic defects of the nanocoating can be modified, resulting in better barrier films. It has already been proved that silicon nitride (SiN) presents better barrier quality at lower thicknesses relative to the widely used SiOx [21, 22]. Erlat and co-workers produced aluminium oxynitride (AlOxNy) films by reactive magnetron sputtering on PET substrates. They attained nanocoatings with good oxygen barrier properties as well as excellent water vapour resistance (comparable to high-quality SiOx and AlOx layers). The authors also observed that the produced AlOxNy films have approximately the same barrier performance as the sputtered AlOx. On this basis, they concluded that AlOxNy can be a promising barrier layer material [14]. Gas-barrier properties are also very important for beverage containers produced in PET. The carbon dioxide gas in beer can easily escape through PET bottles and the ingredients of orange juice can easily be oxidized by incoming oxygen through the bottle [23]. Although silicon, aluminium and titanium alloy thin films were reported to be good gas-barrier materials, amorphous hydrogenated-carbon (a-C:H) films or diamond-like carbon (DLC) [24] were also considered good materials for food packaging purposes, due to their flexibility, recyclability and biocompatibility [25]. DLC is known to be a very hard, very low friction material that possesses low wear rate, excellent tribological properties and excellent corrosion resistance. It consists of dense amorphous carbon or hydrocarbon with high electrical resistivity, high refractive index and chemical inertness. The mechanical properties of DLC thin films are between those of graphite and diamond [26]. Generally, DLC films are produced by PECVD methods [27]. Finch and co-workers [28] coated the interior of PET bottles with 10±100 nm thick a-C:H films, improving their gas barrier properties by 20±30 times compared with uncoated bottles. Enhancing gas barrier performance extends the shelf-life of bottles since they can preserve the taste and quality of stored food/ drink in better conditions for a longer time [29]. These coated bottles are already on the market as beer containers in North America, Europe and Korea. Recently, under the Portugal±Spain International Nanotechnology Laboratory Nanotechnology Projects Call (Subject: Nanotechnologies; Topics: Security and Food Quality Control), a project in which University of Minho participation was approved. Entitled NanoPackSafer: NANO-engineered PACKaging systems for improving quality, SAFEty and health characteristics of foods (www.nanopacksafer.com) has been launched. Within the framework of the NanoPackSafer project, novel nanomaterials for flexible packaging systems are being developed to increase the safety of foods while keeping their

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natural characteristics, among them active packaging, inorganic coatings and edible coatings. Edible coatings and films are a thin layer of a biopolymer that is deposited on the surface of a food and is co-consumed. Until now it has been used to improve handling properties (`M&M'sÕ melt in your mouth, not in your hand'), to prevent moisture loss (wax coatings on fruits and vegetables), for seasonings on snack foods (e.g. as adhesive of salt on dry roasted peanuts), as a glaze on baked goods (instead of egg-based coatings), etc. Besides edible films, which are outside the scope of this chapter, the University of Minho is responsible for applying nanotechnology approaches to the development of non-edible films, such as novel thin films, nanoreinforced biopolymers and nanosensors. The main task of this R&D project involves the production and characterization of advanced nanocomposite oxide-based sputtered thin coatings on plastic packaging materials. The coating deposition systems available at the University of Minho are being used to produce novel nanocomposite metal oxides for high-performance barriers, for catalysis, and with antimicrobial capabilities that can minimize biological attachment and biofilm formation in the functional food pack. Another research task is concerned with the incorporation of sensorial functions (e.g. smart labels for `smart packaging'), such as the development of nanoparticle-based sensors made by soft templating methods and sensorial thin film systems. The active surface of the resulting nanoparticles will be functionalized with appropriate molecules in order to give the desired response to O2, CO2, pH or temperature [30±32].

11.3

Thin film technologies for polymer coating using vacuum processes

11.3.1 Basics of thin film preparation Thin film deposition technologies using vacuum processes are usually classified into two main classes due to the nature of the processes themselves: physical processes and chemical processes. Several different combinations of any of these processes have been tried because in many cases a single process is not able to give rise to films with the required properties. The number of these socalled hybrid processes has increased significantly during the last decade and is already virtually countless. Many of the processes have more than one name designation and sometimes there is an overlap in process mechanisms. For instance, for physical vapour deposition (PVD) processes, although there have been several attempts to classify deposition processes, the best is thought to be by Bunshaw and Mattox [33], which dates from 1970, with addition of advanced techniques [34]. In this section we will focus on and describe solely the processes used for depositing transparent oxide barrier films applied to flexible food packaging

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applications, although they are also applied in encapsulation of flexible devices such as organic light emitting diodes (OLEDS) or solar cells. The process principles and materials used in the different film technologies are discussed. The different vacuum processes are described and the advantages and disadvantages of each process are summarized. Thin film materials exhibit unique material properties resulting from the atomistic growth process which is clearly dependent on the deposition process and chosen parameters. The synthesis of a film prepared using vacuum processes comprises three main steps: (1) creation of the appropriate atomic, molecular, or ionic depositing species; (2) transport of the species from the source to the substrate; and (3) condensation of the depositing species on the substrate directly or via chemical reaction with reactive constituents, forming a solid deposit. In atomistic processes, the solid film is formed by condensation of the atoms in the vapour phase onto a substrate and migration to nucleation and growth sites. The adsorbed atoms, so-called adatoms, require energy enough to occupy their lowest possible energy configurations avoiding structural imperfections. The microstructure and morphology of the growing film is also a result of the energy of the atoms which in turn is dependent on the deposition process and respective parameters. The deposition processes that are presently being utilized for the deposition of transparent inorganic barrier film will be hereinafter described, though the influence of the parameters in each process on the final properties of the films will not be addressed.

11.3.2 Chemical vapour deposition (CVD) processes In a CVD process the solid film is deposited from a vapour by a chemical reaction occurring on or in the vicinity of a normally heated substrate surface. The experimental parameters such as power, precursor compositions and flows, working pressure, temperature, substrate material, etc. will dictate the final properties of the film. CVD processes are generally classified and categorized according to the characteristics of the processing parameters such as temperature, pressure, wall/substrate temperature, precursor nature, depositing time, gas flow state and activation manner [35]. Herein, we will explore solely the most common CVD processes found in the literature concerning the production of thin films for gas-barrier application applied to food packaging. The most common CVD processes are plasma-enhanced chemical vapour deposition (PECVD), atmospheric-pressure chemical vapour deposition (APCVD) and catalytic chemical vapour deposition (Cat-CVD) also designated hot-wire chemical vapour deposition (HW-CVD). In PECVD the chemical process is activated by plasma generation, in APCVD the process, as stated by its designation, is conducted at atmospheric pressure, and in Cat-CVD source gases are decomposed by the catalytic cracking reaction with heated catalyser.

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Plasma enhanced CVD (PECVD) PECVD is a widely used technique to obtain device quality thin films at low substrate temperatures [36]. In PECVD, source gases are decomposed in plasma by the collisions between energetic electrons and gas molecules. For the deposition of SiOx films, the plasma promotes the decomposition of silicon source gases (silane (SiH4), tetramethoxysilane (TMOS) or hexamethyldisiloxane (HMDS)) to silicon radicals and allows them to react with oxygen radicals that issue from oxygen or nitrous oxide (N2O) source gases [37]. PECVD processes could be operated using microwave (2.45 GHz), AC (50 Hz), MF (kHz range) or RF (13.56 MHz) electrical power supplies, the latter being the most common. A schematic diagram of a RF PECVD system used to prepare SiOx films onto PET below 100ëC for food packaging products [18] is illustrated in Fig. 11.2. The system comprises a vacuum chamber where the deposition process takes place, and an evacuating system composed of a rotary mechanical pump for the primary vacuum level and to backup a turbomolecular pump which then takes the pressure down to 10ÿ4 Pa. The feeding of gases (tetramethoxysilane (TMOS) and oxygen) is controlled by a mass flow controller which dictates the partial pressures of the source gases. The initiation of the plasma and decomposition of the source gases are accomplished with a power supply operating at the RF (13.56 MHz) and require an impedance matching network. A low-pressure microwave plasma reactor system based on a plasmaline antenna, shown in Fig. 11.3, has been used for plasma deposition of SiOx

11.2 Schematic diagram of a RF PECVD system, from Ref. [18].

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11.3 Schematic illustration of the plasmaline antenna reactor system used for SiOx coating of PET bottles, from Ref. [39].

coatings in PET bottles [38, 39]. In this approach, liquid HMDS is evaporated as process gas for deposition of barrier coatings and is fed into the chamber as a mixture with oxygen. Microwave power is applied to the system by means of a modified plasmaline antenna operating at 2.45 GHz. This antenna consists of a copper tube, which is the inner conductor of the microwave antenna, with surrounding inner (also used to deliver the feeding gases) and outer quartz tube. The bottles are inserted upside-down, allowing the copper tube inside the bottle. The plasma is generated inside the PET bottle along with the source gases, which makes the deposition restricted to the bottle interior. Various kinds of inorganic films with gas barrier properties can be prepared by PECVD by simply varying the gas sources and their ratios. PECVD has been successfully used to prepare silicon suboxide (SiOx) films [18], silicon nitride (SiNx) films [40], silicon oxynitride SiOxNy coatings [41], and more recently ternary SiCxNy coatings [13] with improved barrier properties. SiOxCyHz films [42] have also been prepared by this method, although they revealed poor barrier performances. The nature of the power source generated plasma may also be an important factor in influencing the final properties of the films. As reported by Zhang et al. [43], the use of medium-frequency (MF) power generated plasma resulted in SiO2 coatings instead of the low purity polymer-like SiOxCyHz obtained in RF plasma. PECVD is a cold plasma deposition technique and is moderately useful for coating heat-sensitive materials such as polyimide (PI) [40], poly(ethylene 2,6-naphthalate) (PEN) [13], polyethylene terephthalate (PET) [44], and polyethersulfone (PES) [45]. However, it seems to be limited until now to silicon-based coatings and DLCs. Furthermore, the speed of deposition is still far below that of thermal evaporation.

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Atmospheric pressure CVD (APCVD) APCVD is a novel technique that has the great advantage of not needing vacuum for the deposition of films, therefore no vacuum chamber and vacuum equipment are required for the process. Operating at a power source frequency over 1 kHz, inserting dielectric plates between metal electrodes, and using helium as the dilution gas, it is possible to obtain an atmospheric-pressure glow (APG) plasma stabilized in air, argon, oxygen and nitrogen [46]. The APG plasma technique enabled development of a low-cost line-type production system. This technique has been used to deposit SiOx films [47]. These films, prepared from tetraethoxysilane (TEOS)/air, showed low porosity and high hardness and transparency [48]. APCVD has also been used to deposit DLC films with gasbarrier properties inside PET bottles [29]. The gas-barrier properties of AP-DLC films ~1 m thick were 5±10 times those of uncoated PET substrates [48]. Catalytic CVD (Cat-CVD) Cat-CVD, often called hot-wire CVD [36], is a recent low-temperature deposition technique working without any help from plasma. In Cat-CVD, source gases are decomposed by the catalytic cracking reaction with heated catalyser, usually a heated tungsten wire placed near substrates [36]. The decomposition mechanism, being different from that of a PECVD method, results in films with also different properties. SiNx and SiOxNy have been obtained by this technique for barrier purposes [17, 49]. SiNx films on polymeric substrates revealed high gas-barrier ability as well as high transparency and low stress. SiOxNy is a very recent achievement with this technique and along with SiNx in a multilayered architecture has low water vapour transmission rates [17]. The preparation of such films was limited to the use of SiH4 but now HMDS, which is safe and inexpensive, is found to be an effective alternative.

11.4

Physical vapour deposition (PVD) processes

The use of PVD processes has been increasing rapidly since current technology requires engineered materials with several distinctive properties, often incompatible, combined in the same product. Such processes are very flexible, allowing the deposition of practically every type of inorganic materials: metals, alloys, compounds and also some organic materials. This is perhaps the major advantage over any other deposition process. Moreover, these processes are appropriate to form multilayer coatings, graded composition deposits, very thick deposits and freestanding structures. In CVD processes, the three steps for the preparation of a film referred to in the previous section ± creation of the vapour phase, transport and condensation ± occur simultaneously at the substrate and cannot be independently controlled. Unlike in PVD processes, these steps can be independently controlled and

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therefore have a much greater degree of flexibility in controlling the structure and properties, and the deposition rate. There are several other advantages of PVD processes over CVD: versatility in composition of film; aptitude for the preparation of unusual microstructures and crystallographic modifications; films can have a very low contamination level; films have excellent adhesion to the substrate and excellent surface finish; the substrate temperature can go from subzero to high temperatures; and absence of pollutants in the process, which is a very important ecological factor. However, there are also some limitations, including the lack of ability to deposit most of the polymeric materials, the difficulty in coating complex substrate shapes, and the high initial cost of the processing equipment.

11.4.1 Evaporation In the evaporation process, the vapour phase is obtained from the source material being heated by direct resistance, radiation, eddy currents, e-beam, laser beam, or an arc discharge [34]. The vaporized material is transported in vacuum, typically at 10ÿ5±10ÿ6 mbar, in a straight line for a given distance (which depends on the gas pressure) prior to condensation on the substrate. As reported in a review paper on inorganic coatings on polymers, this deposition method was the first used for barrier purposes [37].

11.4.2 Electron beam evaporation (EBE) This process is particularly appropriate for the evaporation of materials with very high melting temperatures. Commercially available sources typically use magnetic deflection to direct an intense electron beam into a water-cooled crucible which contains the material to be evaporated. Scanning, if available, allows the beam to sweep over the charged surface, improving material usage and extending source lifetime [50]. The common problems associated with electron beam deposition are short filament (source of electrons) lifetime, deposition non-uniformity, and spatter. EBE systems involve similar costs to sputtering but the deposition rates are higher; however, the thickness control of the films is not as precise as in a sputtering system. A ten-fold reduction in the oxygen permeation rate has been observed in SiO films prepared by electron beam reactive evaporation in the presence of oxygen. In other studies, reductions of 60-fold in the oxygen permeation rate through 12 m PET were obtained with SiOx coatings deposited by electron-beam reactive evaporation of silicon monoxide in the presence of oxygen [7]. SiOx films can be also prepared directly by EBE from raw Si±O material without the presence of an oxygen atmosphere [19, 51]. It is also possible to prepare Al2O3 films by EBE from an Al2O3 source. However, the deposition rates obtained are low: 0.3±0.5 nm/min at ~6 kV.

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11.4.3 Sputter deposition Sputter deposition is a non-thermal vaporization process which uses a physical phenomenon to produce the microscopic spray effect [34]. When a fast ion strikes the surface of a material (target), atoms of that material are ejected by a momentum transfer process, as illustrated in Fig. 11.4. As with evaporation, the ejected atoms or molecules can be condensed on a substrate to form a surface coating. When the process involves a partial pressure of a reactive gas which reacts with the sputtered material to form a compound surface coating, the process is called reactive sputter deposition. This is, in fact, the case when depositing gas barrier layers such as SiOx or SiNxOy from a Si target, using O2 (or O2 + N2) as the reactive gas. Reactive magnetron sputtering In this process Ar gas is introduced as sputtering working gas which will be ionized and accelerated towards the target material. The planar sputtering source with no magnetic enhancement (diode sputtering) has a high loss of electrons and hence a poor ionization efficiency. The result of this is that the power supply to drive the source needs to deliver the required current at around 2 kV [52]. The use of a magnetron allows trapping of the electrons by the magnetic field lines close to the sputtering target in a well-defined region: see Fig. 11.5. Therefore, electrons stay within the plasma for a considerably longer time, increasing the probability of ionizing the working gas. This increase in ionizing efficiency results in a denser plasma capable of carrying a much higher current at a significantly low voltage. Magnetrons can be configured in several different forms such as planar in a variety of shapes or cylindrical. Moreover, they can operate in conjunction with other magnetrons or other vacuum deposition sources, simultaneously or alternately, for instance for deposition of composite or multilayer films from different material sources, respectively. Magnetron sputtering cathodes can be run using AC or DC power supplies. The most common AC power supplies

11.4 Schematics of the physical sputtering process: a collision of an accelerated ion with a target surface.

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11.5 Configuration of a magnetron cathode.

usually operate in the RF range at 13.56 MHz although medium frequency (MF) range is also used. DC power can be supplied to the cathode with a negative constant voltage, which is the conventional method, or pulsed in the medium frequency (kHz) range. The power pulsing can be unipolar, which allows only negative voltage pulses to the target, or bipolar, which alternately switches from negative to positive voltage during the process. This way the target is frequently alternating between sputtering and positive charge dissipation. This is of particular interest when dealing with conductive targets which form insulator films in a reactive atmosphere. RF power supplies are more expensive than DC ones and require a complex impedance matching system [50, 53]. Moreover, the deposition rate with RF power is very low, about half the rate for DC power, for an equivalent amount of power [53]. Figure 11.6 shows a typical planar magnetron system operated by DC power. A technique known as successive pulsed plasma anodization (SPPA) has been used to deposit AlOx gas barrier films on PET substrates [20]. This technique makes use of a coaxial dual magnetron system in which a thin metal layer is firstly deposited, in this case Al, and subsequently anodized through self-biased oxygen plasma. The cycle is repeated until the desired oxide thickness is achieved. SiOx thin films have been deposited on PET films by reactive sputtering using a DC magnetron sputtering from a Si target and in an atmosphere of Ar + O2 [54]. Results showed that low oxygen concentrations are preferable since less micro-defects are observed in the films and consequently the oxygen transmission rates are lower. SiON films on PET prepared in N2 + Ar atmosphere from Si target showed oxygen transmission rates even lower than those of SiOx films [15]. The films revealed a fine and amorphous structure without pinholes or cracks. Other studies [55] on SiOx gas barrier layers deposited on PC film by a roll-to-roll DC magnetron reactive sputtering method revealed an extremely smooth surface and excellent gas barrier performance against moisture and oxygen with a 40 nm thick film. Reactive pulsed DC sputtering in double-ring magnetron is also an effective method for SiO2, Si3N4 and SiOxNy single barrier layers [56]. Al2O3 films were reactively deposited

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11.6 Schematic illustration of a planar magnetron sputtering system using DC discharge current.

from Al targets onto PET by AC magnetron sputtering in a dual-magnetron rollto-roll process, for OLED devices [16]. With a layer thickness of 200 nm, the permeation barrier can be improved by nearly three orders of magnitude. AlOxNy are also a possibility as gas barrier films and, when produced by medium frequency (40 kHz) AC magnetron sputtering, utilizing a coaxial dual magnetron source with Al targets, seem to be a promising barrier layer material, demonstrating highly competitive gas permeation values for oxygen and water vapour [14]. TiNxOy films were deposited on PET substrates from pure Ti and N2 + Ar atmosphere by means of RF magnetron sputtering and applying substrate bias [57]. The deposited films exhibited an amorphous or a columnar structure with fine crystallites dependent on power density. The residual O2 plus the eventual O2 coming from the PET substrate are enough to form the oxynitride. Ion beam sputter deposition (IBD/IBAD) Ion beam sputtering uses an ion source to generate a relatively focused ion beam direct at the target to be sputtered. The ion source comprises both the cathode and the anode which are concentrically aligned. With application of a high

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voltage field of 2±10 kV an electrostatic field is created inside the ion source, confining electrons in the centre of the source. When argon gas is injected into the ion gun, the high electric field causes the gas to ionize, creating a plasma inside the source region. The ions are then accelerated from the anode region to the exit aperture (cathode), creating a collimated ion beam. The resulting ion beam impinges upon a target material and, via momentum transfer between the ion and the target, sputters the target material towards the substrate. A second ion gun may be also used to assist the deposition (ion beam assisted deposition ± IBAD) by bombarding the growing film. Dual ion beam sputtering utilizing ion beam assisted deposition conditions can improve the adhesion, density, control of the stoichiometry, and low optical absorption (at short wavelengths) in thin films [58]. Low-energy reactive ion beam bombardment can increase the rate of compound formation, control the stoichiometry and improve adhesion of the deposited thin film. SiOx films prepared by sputtering by 1 keV Ar ion beam from a Si target and assisted by a second Ar + O2 ion beam [58] revealed a dense amorphous nature when oxygen content is 20% and presented an oxygen transmission rate lower than that obtained by CVD and e-beam processes.

11.5

Inorganic thin film systems

Nowadays, high food quality is a requirement in most developed societies. Consumers acquire packaged goods that, generally, are produced thousands of kilometres away. This has been the most important driving force for research and improvement of packages characteristics. Polymeric materials play an important role in the food packaging industry and can be found in several forms such as films, bottles and other kinds of packages. The most important reasons for this fact are their low cost, high flexibility and relatively good chemical stability. Table 11.1 summarizes the most common polymers used for food packaging. From this table it is clear that these polymers present a poor barrier performance to oxygen. Table 11.1 Multilayered and composites systems and O2 permeability Polymer

Permeability, PO2 (1016 cm3 (STP).cm/cm2/s/Pa)

LDPE/EVOH blends PET/EVOH blends Polypropylene/polyamide 6 blends PET copolymer/talc 32 wt% composite Polyethylene/mica 10 wt% composite Polyamide 6/layered silicate nanocomposite Source: ref. [59].

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To improve the barrier properties of polymers to oxygen and water vapour, several technologies have been developed, the most common being the deposition of inorganic thin films. In the last 20 years some prominent research groups have been working in this field, with the specific objective of developing materials and techniques for depositing metals or oxides in polymeric substrates [37]. Inorganic oxide coatings present some advantages over aluminium, such as optical transparency and microwaveability. In 1996 Chatham [7] presented a review of several inorganic materials deposited by different techniques. Table 11.2 summarizes probably the best reported results since 1996 for different inorganic systems, deposition techTable 11.2 Substrate/thin film systems and respective deposition techniques, oxygen transmission rate (OTR) and water vapour transmission rate (WVTR) in works reported between 1996 and 2003 Ref.

51 20 60 14 9 13 15 54 61 62 63 45 29 58 64 65 49 42 57 11 66 67 68

Coating material

Deposition technique

EB evap. SiOx Al MS a-C:H CVD Al MS MS AlOxNy MS SiNx PECVD SiOx PECVD SiCxNy SiON MS MS SiOx DLC PECVD DLC PSII DLC (Si) PECVD PECVD SiOxNy a-C:H CVD DIBAD SiOx PECVD SiOx PECVD SiOx/ parylene CatCVD SiNx PECVD SiOxCyHz MS TiNxOy MMT clay LbL and CPAM EB evap. SiOx ALD Al2O3 Al2O3

MS

OTR WVTR (cm3.mÿ2. (g.mÿ2.dayÿ1) dayÿ1.atmÿ1) 5

Application field FP FP Packaging FP FP FP FP EE FP FP Packaging FP FP EE FP ± FP EE

± 0.012 0.02 1 0.6 0.6 ± ± 0.8 0.4 0.17 ± ± 5 2 0.27 ±

0.05 0.1 1.06 ± 0.2 0.4 ± ± ± 0.032 0.0235 ± ± ± 0.005

PET PET PET PET PET PET PET PEN PET PET PET PET PET PES PET PC PET PES

± ± 0.6 0.05

0.0045 0.6 0.98 ±

PET PET PET PET

EE Packaging EE Packaging

PET PEN PP PET PLA PET PES

EE FP

1.5 10 ±

± 0.17

Subs

± 0.6 10ÿ4

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niques and substrates. The authors related in their works important features like defects in mechanical properties, aspects of nucleation and growth of thin films and their influence in the final barrier properties. The majority of these authors focused their work on systems for food packaging (FP) applications as well as for electronic device encapsulation (EE). From a general overview of this table it is possible to conclude that silicon based materials are the most studied system. However, aluminium-based and a-C:H (DLC) compounds are becoming very promising materials for providing a gas barrier in food packaging applications. Regarding the specific values of OTR it is possible to observe that the best results were achieved with CVD by Vasquez-Borucki et al. [60]. In this work the authors successfully synthesized a-C:H in PET substrates. GruÈniger et al. [64] claimed 0.27 cm3.mÿ2.dayÿ1.atmÿ1 for OTR for silicon oxide thin films, deposited by PECVD. Promising results were reported by Erlat et al. [14] using magnetron sputtering, where the OTR for an aluminium metallization was 0.02 cm3.mÿ2.dayÿ1.atmÿ1. Concerning WVTR, the best results were obtained with Al2O3 thin film deposited by magnetron sputtering. The authors reported an OTR value of 0.001 g.mÿ2.dayÿ1 [68]. Different authors [40, 69, 70] claim that the barrier performance of the systems and consequently their success for food packaging applications are greatly influenced by the presence of defects on deposited films, their thickness and mechanical properties. It is also worth noting the increasing number of studies of barrier systems for electronic device encapsulation in comparison to food packaging. Considerable attention has been given in the last years to carbon compounds for applications as gas barrier coatings. Inhibition of CO2 loss in drink bottles is an attractive area for implementation of carbon compounds, where the transparency of the final product is less important than for other types of food packs [46]. The increasing market for flexible electronic devices has led to new needs for flexible packaging, and gas barrier thin films can be an important technology. Figure 11.7 shows the permeability requirements for the different application fields. From this figure it is possible to observe that the requirements for encapsulation of OLEDs and organic solar cells are much more exacting than for food packaging. For this reason, any technological progress in gas barrier properties for electronic encapsulation can also be applied to food packaging systems. Industrial demand for packaging is the key to a new challenge in the research community. The development of technologies for mass and continuous production is now one of the most important research fields. The development of, for example, roll-to-roll equipment [16] and hardware for thin film deposition in the internal walls of bottles [71] is now a big research and industrial challenge. Moreover, the increased interest in high flexible gas barrier solutions for packaging (for both electronic encapsulation and food containers) has led in

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11.7 Gas barrier requirements for different technological areas, adapted from Refs [16] and [52].

recent years to an expansion of the research in many different areas. In Fig. 11.8 it is possible to observe how the scientific community drives the research in this area. The present data were compiled from all the publications cited in this section and from the specific work area for each one. In this way it is possible to observe the preferred paths being followed by the different authors with relevant research work in this area. The development of flexible organic electronic devices represents a new challenge in terms of encapsulation. PECVD appears to have a preferential deposition technique. The lower concentration of defects in the films deposited by this technique can explain the preference, but no less important are the base materials used, silicon and carbon. However, sputtering techniques are always very good solutions because of the high quality of the films they produce, their simplicity, their upscaling facility and their high flexibility in terms of industrial setup. The recent developments of a new generation of pulsed power supplies and rotating targets for the case of magnetron sputtering make this technique a valid solution. Table 11.3 presents a good example of non-conventional sputtered materials for this type of applications, with promising results achieved by Fahlteich et al. [72]. Understanding these organic/inorganic systems for packaging is the key factor for their successes in this field. Barrier properties of inorganic systems have high importance in the final system behaviour. For this reason, great efforts are spent in achieving new materials with superior properties. Nevertheless, the need to apply the knowledge obtained from already existing commercial

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11.8 Percentage of total publications cited in this section for the different deposition techniques, materials systems, work objectives and application fields.

Table 11.3 Lowest reported oxygen transmission rates (OTR) for different sputtered barrier layers Material Thickness (nm) OTR (cm3/m2dbar)

ZTO

ZnO

SiO2

TiO2

Al2O3

>120 <0.05

150 0.08

200 0.14

50 0.38

120 0.1

products explains the large number of works aimed at developing new deposition techniques as well as upgrading existing ones.

11.6

Functional properties of diffusion barrier coated polymers

The functional properties of a system composed of an inorganic coating deposited in a polymer substrate are largely dependent on the nature and interaction of both materials and the thickness ratio between the two components. Different natures imply different thermal expansion coefficients, different

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11.9 Transport of gases through a film.

viscoelastic properties and different interactions of both materials. All these factors could play a decisive role in the final functional performance of the system (coating plus substrate). On this basis, depending on the required final properties of the system, an adequate combination of the polymeric substrate and the coating should be chosen. However, independently of the selected system and the deposition method, the polymer substrate will present a much lower working temperature regarding the inorganic coating. This represents a major limitation during the deposition of the inorganic coating. Two fundamental properties are required for this specific application: high performance gas diffusion barrier and flexibility. These are somewhat contradictory properties, since thick and high density coatings present higher diffusion barrier properties but lower flexibility. On this basis thin inorganic layers (<200 nm) are used as diffusion barrier coatings [73]. Gas transport through a film is often described by the solution±diffusion mechanism [74]. The feed gas at a high pressure (P2 , Fig. 11.9) dissolves into the feed-side surface of the film, due to a concentration gradient it diffuses through the film, and finally it desorbs from the permeate side of the film (P1 ). Assuming the absence of any chemical reaction between the gas and the polymer, the flux of gas through the film (NA) in the perpendicular direction of the polymeric film can be described by Fick's Law [75]: dCA ‡ !A …NA ‡ NP † 11:1 dx where D is the gas diffusion coefficient in the film, CA is the local concentration of dissolved gas and !A is the weight fraction of gas A in the film; NP is the flux of the film, which is assumed to be zero, reducing the above expression to [75]: NA ˆ ÿD

NA ˆ ÿ

D dCA 1 ÿ !A dx

11:2

The steady-state permeability of a gas (PA) through a thickness d can be expressed by:

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p

305 11:3

The effective permeability is an intrinsic property of a film. If the structure of the coating does not change with the film thickness, the permeability generally decreases with this parameter [76]. The use of inorganic coatings deposited in polymeric substrates for increasing gas barrier performance is related to the transport of small penetrants through bilayer films. In a first approximation this transport could be expressed following a parallel-type equation [77]: h hC hS ‡ ˆ P PC PS

11:4

where h, hC and hS represent the thicknesses of the coated polymer, coating and substrate, respectively, and P, PC and PS represent the gas permeabilities of the coated polymer, coating and substrate, respectively. Gas permeation through a polymer-coated system depends on several factors including thickness [73], composition [15], density [60], surface topography [14], and nano/micro defects of the inorganic layer [21]. In the following text we will present a brief reÂsume of the influence of most relevant parameters in the performance of gas diffusion barrier coatings. It is well known that the permeation of gases through inorganic coatings is highly dependent on the nanometre- to micron-scale defects. Several authors reported that gas permeation through SiOx/polymer films is significantly affected or even dominated by defects in the oxide [19]. Several authors have attempted to correlate the type, size and density of the nano/micron scale defects with gas barrier properties [78]. From these studies it is possible to conclude that the size of the defects ± micron scale, so-called pinholes and microcracks (mostly related to substrate micro-roughness, see Fig. 11.10a) ± presents a major role in the gas barrier performance of the coatings, whereas in the absence of micro-scale defects, the density, type and distribution of nanoscale defects (see Fig. 11.10b), play a critical role in the gas barrier performance of the coatings. Rougher polymeric

11.10 (a) Micron-scale defects related to substrate micro-roughness. (b) Nanoscale defects: space between columns and (c) boundaries between grain-like structures.

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substrate surfaces require thicker coatings for maintaining barrier properties, which consequently increase the cracking tendency [79]. GruÈniger and von Rohr [80], in their study on the influence of defects in SiOx thin films on barrier properties, concluded that if the total defect area fraction is given, small and compact defects are most effective in destroying the barrier performance. Rossi [81] claimed that many small holes in a barrier layer lead to much higher permeation than a few large holes of the same total area. Regarding the thickness effect on gas barrier performance, several authors report the decrease of water vapour transmission rate (WVTR) until reaching a critical thickness at which an effective barrier layer is achieved. This wellknown critical thickness is reported by several authors for different types of gas barrier coatings on polymer substrates [19]. This phenomenon is characterized by an initial decrease of permeation of the composite with the increase of coating thickness and a critical thickness value; beyond this thickness value, the gas transmission rate is not altered significantly. Several authors have already published their researches into the influence of composition on gas barrier performance. DLC films usually present high intrinsic and surface cracking, as has been previously observed for a-C:H [60] and ta-C films [82]. Abbas et al. [83] used silicon incorporation in a-C:H films to suppress the appearance of these cracks. In the case of silicon oxynitride coatings, the introduction of nitrogen induces an increase in the density of the coatings and higher connectivity of the nitridecontaining network, resulting in the formation of less defective film, reducing the oxygen transmission rate [15]. In contrast, opposite behaviour is observed in DLC coatings, where the increase of hydrogen content decreases the density of the coating, increasing the permeability coefficient [60]. This contrary result was explained by the authors to be due to the increase of density of microcracks between the interface a-C:H layer and polymer substrate [60]. In the case of SiOx coating produced by plasma enhanced chemical vapour deposition, the OTR values decreased markedly with increase of the oxygen content due to the decrease of carbon concentration [43]. Lamendola et al. [84] claimed that oxygen permeation was strongly influenced by the carbon content in the coating having a negative effect on the barrier properties. Gioti et al. [66] claimed that close stoichiometric SiOx films, deposited by an electron beam evaporation technique, present a barrier property similar to that of the uncoated PET substrate. The best barrier performance was achieved for sub-stoichiometric SiOx with x  1:4. Similar results were confirmed by Iwamori et al. [54]. GruÈniger et al. [64] claimed that the best barrier performance was achieved for sub-stoichiometric SiOx with x  1. In a recent publication Fahlteich et al. [72] claimed that doping ZnO with tin (ZTO) deposited by reactive sputtering decreases the permeation of oxygen and water vapour. From a comparison of all the reports in the literature establishing a correlation between the influencing characteristics (chemical composition, mor-

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phology, density and defects) on deposited coatings, it is possible to summarize some of the most relevant factors: · Defects of thin films on polymers: For the same total defect area, barrier effectiveness is higher when the thin film presents a low number of large defects. High roughness of the polymer surface leads to a necessary increase of film thickness, which promotes cracks. · Internal stress: In a general way, internal stress of thin films is a drawback for gas barrier organic/inorganic systems. Internal stress can be responsible for poor adhesion of ceramic films to a polymer surface, leading to debonding and crack formation. Controlled compression stress can improve barrier properties because it provides higher cohesion of thin film (higher mechanical properties) but also lower intrinsic permeability to gases. · Failure initiation and thin film strength: Metal oxide strength depends dramatically on microcrack behaviour. The brittle nature of ceramic thin films is easily affected by internal stress of these films, and the number and nature of defects are the major problem for microcrack development and consequent thin film failure. · Composition: The presence of metal±O±C covalent bonds on the interface is the key to achieving good performance in terms of adhesion. However, regarding OTR and WVTR performance, the best results are achieved by coatings with dopants that promote an increase in the density of the thin films. Although all these factors could play a very important role in OTR and WVTR performance, defect density is dominant. Flexible polymer films coated with brittle inorganic coatings present high susceptibility to cracking when stretched or bent [85]. Residual stresses develop when the film changes its dimensions relative to the substrate. The resulting induced stresses can be quite large and can lead to high residual stresses or to high deformation, cracking, or delamination of the film. The coating's residual stresses, together with chemical bonding, determine the adherence of the coating [86]. In contrast to hardness testing, physical quantities can be directly obtained from uniaxial tensile testing, which is therefore one of the most important techniques for investigating the mechanical properties of bulk materials. Yanaka et al. [87] report that the crack density, stress and crack spacing as a function of percentage strain are a good indication of the mechanical durability of these materials, and depend strongly on the coating thickness. As in standard tensile testing the following physical quantities can be determined: · The elastic modulus, obtained from the slope of the initial loading or subsequent loading±unloading cycles in the stress±strain curve

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11.11 The three main stages of coating fragmentation: A random cracking; B mid-point cracking and initiation of transverse buckling; C delamination.

· The yield strength, which for ductile films is usually defined as the stress at which a plastic deformation of 0.2% has been attained · The ultimate tensile strength and the corresponding elongation to failure · The critical tensile thin film stress, which for brittle films is usually defined as the film stress just before the crack onset. In a tensile fragmentation test multiple cracks, perpendicular to the load direction, are generated in the film. The phenomenology of coating fragmentation comprises three main stages as shown in Fig. 11.11. In the first stage, termed random cracking, crack interaction is negligible, therefore the rate of crack generation is governed solely by the coating strength distribution, and the crack location is determined by the defect distribution in the coating (stage A). The second fragmentation stage, mid-point cracking, corresponds to a crack spacing which becomes small with respect to the critical length (stage B). In the second stage, transverse buckling failure of coating fragments is often observed, due to lateral contraction of the substrate which results from Poisson's ratio effects. The third and final stage (large deformation) begins with the delamination of the coating, and the fragmentation rate virtually stops (stage C). As already referred to previously, two determining factors are responsible for the barrier performance of a system: thickness and defect density. However, increasing thickness and/or density could give rise to brittle coatings and formation of cracks that will substantially reduce the barrier performance of the system. As an example of multiple cracking in a thin film, Fig. 11.12 presents the nominal stress±strain derivative of the system inorganic coating/substrate, for

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11.12 Nominal stress±strain derivative of the system coating/substrate deposited by magnetron sputtering for different working pressures and different thicknesses of inorganic oxide coatings.

different working pressures and different thicknesses of the inorganic coatings. From this figure it is possible to observe that the system (coating/substrate) presents Young's modulus values lower than 4 GPa. This apparent low value results from the large difference between the substrate and inorganic coating thicknesses, reducing the influence of the ceramic coating on the final Young's modulus value. Regarding specific Young's modulus values, one may observe that an increase of thickness and a decrease of working pressure induce an increase of Young's modulus. This evolution is related to the increase of density with the decrease of working pressure, due to the higher bombardment of the coating, and the increasing influence of the coating with thickness in the determination of Young's modulus of the combined system. In conclusion,

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appropriate deposition parameters, deposition time and working pressure should be selected in order to ensure excellent functional properties, a high performance gas diffusion barrier and flexibility.

11.7

Future trends

It is predicted that nanotechnology production approaches will change about 25% of the food packaging business in the next decade, representing an annual market of over $30 billion. The major market trends include enhancing the performance of packaging materials, prolonging shelf-life, antimicrobial packaging and interactive/sensorial packaging. One of the main priorities in food packaging technology is to keep the original properties of the food. This goal is achieved by keeping all the nutrients in their original condition, by ensuring minimum interaction between the environment and the packaged food, and by reducing microbial growth. In recent years researchers have studied the ability of silver to kill harmful bacteria and have used this knowledge to create consumer products containing silver nanoparticles, including fabrics and deodorants containing silver nanoparticles as well as energy-efficient washing machines that disinfect clothes by generating the tiny particles. Silver-based antimicrobials capture much attention because of their novelty, being a long-lasting biocide with high temperature stability and low volatility [88]. Regarding the development of new packaging systems, transparent barrier films could be modified by deposition of silver nanoparticles with increasing antibacterial properties [88±90]. However, a progressive increase of silver nanoparticles in consumer products could represent a potential negative environmental impact with the release of silver nanoparticles into sewage lines, waste treatment facilities, etc. This could eventually contaminate streams and rivers. In order to improve scientific knowledge concerning the impact of silver nanoparticles on health and the environment, scientists have already started to study the potential of silver to cause health and ecotoxicity issues in a concentrationdependent manner. The growing demand by consumers for healthier and more ecologically friendly foods has driven researchers to develop new systems of packaging that prolong the useful life of products. In 1985, Matsunaga et al. [91] reported the antibacterial properties of TiO2 for the first time. Since then, titanium oxide as a photo-induced antibacterial agent has attracted increasing interest [92±94]. However, TiO2 only becomes active under irradiation with ultraviolet (UV) light whose energy exceeds the band gap of 3.2 eV in the anatase crystalline phase, and it is not expected that food be submitted to UV radiation. According to the literature, the two most effective ways to activate photocatalytic and/or antibacterial effects, while working within the visible light spectrum, are by doping TiO2 with transition elements to form a TiOx structure (x < 2) [95±98] or

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by changing the structure of the photocatalyst by the defection effect, thus achieving surface adsorption in the visible range [99]. Some techniques have been examined to increase efficiency in terms of improving the antibacterial and/or photocatalytic activity of TiO2 in the visiblelight region; however, these films require a real-time heating or post-annealing process [100, 101] or, due to the deposition process itself [94, 102] (wet processes), the coatings cannot be used as a diffusion barrier due to the low coating density. On this basis, deposition of TiO2 at low temperature to render it sensitive to visible light and at the same time to present good diffusion barrier properties should represent one major goal to enable the increased utility of TiO2 as an antibacterial and diffusion barrier coating. The ability to use thin films and nanoparticles with transparent properties in more flexible and transparent packaging materials will provide consumers with fresher, more customized packs in which the products can be readily observed. Carbon dioxide is commonly used in food packaging with a controlled atmosphere in order to increase food lifetime by reducing aerobic bacterial growth [103, 104]. According to Jeremiah et al. [105], the best food packaging system for low temperature (ÿ1.5ëC) meat conservation is packaging in a rich CO2 atmosphere. However, some authors [106] claimed that the extension of lifetime of packaged meat in such a controlled atmosphere could allow the growth of psychrotrophic pathogenic microorganisms to unacceptable levels without any sensorial detection. On this basis, precise temperature control is required to reduce the growth of pathogenic microorganisms. In the near future, irreversible thermochromic compounds should be used to inform the consumer that the optimal conservation temperature has been surpassed. Future trends will also include research and development on sensorial packaging which can monitor the food and transmit information on its quality. For instance, the ultimate pH of meat greatly affects its quality. Monitoring this parameter can give consumers information regarding the manner of transportation of animals from the farm to the abattoir, diet restrictions, mixing animals of different lots, and pathological conditions. With nanosensors embedded in the packaging surface materials, consumers will be able to check the quality of the food inside or even to track the history of the pack. Electronics built on thin film substrates could be used in future sensory packaging applications (examples include nanoRFIDs).

11.8

References

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Functional barriers against migration for food packaging C . J O H A N S S O N , Karlstad University, Sweden

Abstract: The function of packaging is to protect the packed food and to maintain its integrity and quality. The package should hinder gain or loss of moisture, prevent microbial contamination and act as a barrier against transfer of oxygen, carbon dioxide and aromatic compounds. Of utmost importance is that the packaging material itself does not promote deteriorative food quality changes or endanger the health of the consumer of the packed food as a consequence of uncontrolled migration of any chemical substances from packaging into food. Recent legislative issues concerning food safety related to migration are summarized. An overview of functional barriers on the market, the sources for and identity of potential migrating substances, and the mechanisms behind migration, as well as of modern analysis techniques, is given. Novel strategies for improvement of functional barriers are discussed. In recent years, increased environmental concern has set the focus on the use of recycled plastics and paper in food packaging, thus enhancing the need for effective functional barriers to prevent contaminants from migrating to food. Future trends in the food packaging market are increased use of biobased polymers, enhanced utilization of nanotechnology for performance improvement and increased use of active or intelligent packaging. Key words: functional barrier, migration, food safety, plastics, multilayer, nanotechnology.

12.1

Introduction

The function of a package is to protect the packed food and to maintain its integrity and quality. The package should hinder gain or loss of moisture, prevent microbial contamination and act as a barrier against transfer of oxygen, carbon dioxide and aromatic compounds. To form an effective packaging material fulfilling all these desired properties and requirements, a combination of materials of various chemical natures and physical structures is required. Possible interactions between packaging material constituents as well as between the packaging material and the food must be controlled. Of utmost importance is that the packaging material itself does not promote deteriorative food quality changes or endanger the health of the consumer of the packed food, as a consequence of the migration of any chemical substances.

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In this chapter food safety issues related to migration with special focus on recent legislation is presented. An overview of functional barriers on the market, the sources for and identity of potential migrating substances, and the mechanisms behind migration, as well as of modern migration test methods, is given. Novel strategies for improvement of functional barriers are discussed. At the end of the chapter a forecast of possible future trends appears. Some relevant literature on the subject is summarized and useful sources of further information are presented.

12.2

Food safety issues related to migration

European Council Directive 89/109/EEC on the approximation of the laws of the Member States relating to materials and articles intended to come into contact with foodstuffs lays down that `Materials and articles must be manufactured in compliance with good manufacturing practice so that, under their normal or foreseeable conditions of use, they do not transfer their constituents to foodstuffs in quantities which could: · endanger human health · bring about an unacceptable change in the composition of the foodstuffs or a deterioration in the organoleptic characteristics thereof.'

The European Commission Directive 2002/72/EC and its amendment 2008/39/ EC list authorized substances for plastic materials and articles intended to come in contact with foodstuffs. The directive establishes an overall migration limit of 60 mg/kg food or 10 mg/m2 for all substances migrating from these materials into foodstuffs. As from 1 January 2010, the list of substances works as a positive list to the exclusion of all other substances. The Community list specifies the identity and function of each substance, its reference number, any conditions of use, any restrictions and/or specifications of use and, finally, conditions of use of the material or article to which the substance or component is added or into which it is incorporated. European Commission Regulation (EC) No. 450/2009 on active and intelligent materials and articles intended to come into contact with food stipulates that `Intelligent systems may be positioned on the outer surface of the package and may be separated from the food by a functional barrier, which is a barrier within food contact materials or articles preventing the migration of substances from behind that barrier into the food. Behind a functional barrier, non-authorised substances may be used, provided they fulfill certain criteria and their migration remains below a given detection limit.'

Furthermore, the regulation states that

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Multifunctional and nanoreinforced polymers for food packaging `. . . a maximum level of 0.01 mg/kg in food should be established for the migration of a non-authorised substance through a functional barrier.'

Article 10 in the regulation clarifies that the limit of non-authorized substances `. . . shall always be expressed as a concentration in foods. It shall apply to a group of substances, if they are structurally and toxicologically related...'

The Council of Europe and the Public Health Committee have published a Partial Agreement policy statement concerning paper and board materials and articles intended to come into contact with foodstuff. A resolution is not legally binding, but acts merely as a recommendation. In Resolution ResAP (2002) 1 it is stated that `. . . any layer which is composed of paper and board must fulfill the requirements on this resolution, unless separated from the foodstuffs by a functional barrier to migration.'

A functional barrier is defined as `. . . any integral layer which under its normal or foreseeable conditions of use reduces all possible material transfers (permeation and migration) from any layer beyond the barrier into food to a toxicologically and organoleptically insignificant and to a technologically unavoidable level.'

Moreover, it is specified that paper and board `. . . should not transfer their constituents to foodstuffs in accordance with Directive 89/109/EEC.'

A resolution on packaging inks applied to the non-food contact surface of food packaging is also published: ResAP (2005) 2. It should be kept in mind that all existing natural and synthetic chemical substances are toxic at some exposure level, even those that are commonly considered as harmless. Any impact of chemical substances on the human body and/or the environment should be regarded with respect to the concentrations actually present during exposure, i.e. in food packaging and/or in air, water and soil respectively (Colvin 2003). In an ideal case, the constituents of each layer in a multilayer package, the polymer itself and all additives, including any secondary products originating from these, should be consistent with the legal limitations or, in other words, be part of the European Community positive list or comply with the American FDA (Food and Drug Administration) regulations. Since the primary concern is the original amount of potentially migrating substance in the packaging material, it is fundamental to design packages so as to minimize this initial level. Plastic materials and their constituents have been thoroughly identified and regulated. There is, however, still a great deal of work to be done to cover all substances present in multilayer paper or board packaging materials. One should also distinguish between non-intentionally added substances (chemical and technical

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impurities) and intentionally added substances (Franz and StoÈrmer 2008). A simplified case is to consider only the layer in contact with food and let this be compliant with the food contact regulation, whereas it is expected to act as a barrier for migration from any substances present in the layers behind.

12.3

Functional barriers

12.3.1 Migration of substances from packaging to food Let us first define some terms. The term migration is used to describe the mass transport of a substance present in the packaging material to the content of the package, which is in a liquid or a semi-liquid state. To describe the mass transport in the gas phase to some packed product in the solid state, the term adsorption is preferred (Franz and StoÈrmer 2008). The term extraction is sometimes used to describe the process of removal of additives from a packaging material under extreme conditions, e.g. at high temperatures (Crompton 2007), and should not be confused with the word migration which refers to the transition taking place under normal storage and handling conditions. During the migration process, the properties of the packaging itself are considered as practically unaffected. The undesired migration of molecular species and ions from the packaging into the food may, however, cause both quality changes to the food and health risks for the consumer (Robertson 1993, Brown and Williams 2003). Any quality changes refer to any alteration of organoleptic properties, i.e. in the taste, odour, flavour, colour or texture of the food product. A functional barrier essentially works as a physical layer between the migrating substance and the packed food, which effectively prevents or prolongs the time it takes for the substance to reach the packed food (Fig. 12.1). The ability of molecular species to migrate has been beneficially utilized in the

12.1 Mass transfer of molecules through a two-layer packaging in contact with foodstuff.

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formation of active and intelligent packaging, which is described in other sections of this book.

12.3.2 Mechanisms behind migration Migration of chemical substances from food packaging material to the packaged food takes place as a series of diffusion processes which are driven by both thermodynamic and kinetic forces. Migration is also controlled by the solubility of the migrating substance and is governed by concentration gradients. Lipophilic plastic constituents migrate more easily to food with high fat content, whereas hydrophilic substances are dissolved more easily in aqueous food, which is why also equilibrium partitioning coefficients should be taken into account. The mobility of the migrant through the polymer barrier is controlled by the elastic or thermoplastic properties of the polymer but also by the thickness of this layer. Once the migrant has reached the polymer/food interface, the viscosity of the liquid food affects the actual amount of mass transfer of the migrating substance into the food species. The migration process thus includes the steps diffusion and desorption. Mass transport can also take place in the adverse direction, e.g. adsorption of flavour substances onto the polymer surface, followed by diffusion (absorption) into the plastic barrier, a phenomenon commonly known as scalping (Fig. 12.1). The permeation of gases through barrier materials, i.e. mass transport into or out of the package, can take place in each direction and involves the three steps of adsorption, diffusion and desorption from the opposite side (Fig. 12.1). For a more thorough description of diffusion theories, see e.g. Comyn (1985) or Crank (1999).

12.3.3 Sources for and identity of migrating substances Plastics are very versatile materials and offer high performance with respect to ease of processing, flexibility, transparency and barrier properties, at relatively low cost. Plastic materials themselves are considered as chemically stable and do not react with solvents or strong acids or bases under normal circumstances. Conventional plastic films used in food packaging such as polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) are non-toxic and do not release substances that may cause off-taste or off-flavour reactions with packed food (RoÈper and Koch 1990). Species having the potential to migrate from the packaging material mainly consist of plasticizers that are added for increase of flexibility, or other formulation additives (Robertson 1993, Brown and Williams 2003). Besides polymer additives, impurities like monomer residuals (unreacted substances after completion of the polymerization process), oligomers (low molecular weight fractions of the polymer having sufficient mobility), polymerization catalysts, non-polymerizable contaminants in the raw material and solvents are examples

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of substances that may be transferred from the packaging to food. The size or more specifically the molecular weight of migrating substances is of fundamental importance when considering their toxicological aspects. The macromolecular species (polymers) themselves lack the capability of migrating due to their large size. As a rule of thumb, molecules with molecular weight higher than 1000 g/mol are not physically absorbed by the gastrointestinal tract (Franz and StoÈrmer 2008). Some conventional plastics used in food packaging, the typical additives required for their processing or end product performance and typical food packaging applications are given in Table 12.1. The data matrices displayed in Tables 12.1±12.3 are by no means complete but serve the purpose to represent a schematic overview of the most common features. The largest class of plastics are the polyolefins, with low density polyethylene (LDPE) as the workhorse, accompanied by high density polyethylene (HDPE) and polypropylene (PP). For more detailed information on the mechanical and barrier properties and other aspects relating to food packaging applications of these plastic materials, see e.g. Robertson (1993), Crompton (2007) or Brandsch and Piringer (2008). Plastic films can be used alone or be combined with other materials through co-extrusion, blending, lamination or coating to achieve high-performance packaging materials. Lamination of two or more plastic films sometimes requires an additional adhesive, which can be solvent-based or solvent-free. Coating can be done in a variety of processes, i.e. by extrusion, by solvent application or by vacuum deposit (Kirwan and Strawbridge 2003). Additives are virtually essential for the processing and for achievement of the desired chemical and mechanical properties of plastic films. Table 12.2 lists some common additives used in plastics for food packaging. The additives are classified either as processing aids (e.g. catalysts or initiators) or as compounds used to enhance the end product performance. A thorough description of different additives, their functions and chemical structure can be found in PospõÂsÏil and NesÏpuÊrek (2008). For a more complete list of potential chemical substances, see European Commission Directive 2002/72/EC. The presence of residues or processing aids is generally not narrated in commercial plastic products whereas the used stabilizers or plasticizers must be declared (PospõÂsÏil and NesÏpuÊrek 2008). To prevent migration of plasticizers from plastic into foodstuffs, high molecular weight polymers are sometimes used, e.g. polybutylene succinate (Crompton 2007), which are less prone to migrate due to their lower solubility. In general, plasticizer addition to plastics in food contact is kept below the 5% level (Crompton 2007). Polyvinyl chloride (PVC) forms very brittle materials upon processing and thus possesses a strong need for plasticizing substances. Hence, PVCs account for more than 80% of the plasticizers used industrially in plastic materials (PospõÂsÏil and NesÏpuÊrek 2008). Internal plasticization of PVC can, however, be achieved by

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Table 12.1 Conventional plastic materials, typical additives and food packaging applications ß Woodhead Publishing Limited, 2011

Food packaging applications Polymer

Typical additives

Thermoformed containers and moulded products

Films/bags

Coatings for paper and plastic films

Polyethylene (LDPE/ HDPE)

Pigments, slip additives, antistatic and anti-blocking agents, antioxidants

Food storage containers, sauces

Household bags, wrapping films

Paperboard laminates: cartons for confectionery, frozen food, liquid packaging cartons

Polypropylene (PP)

Copolymers

Screw caps, food storage containers; yoghurt, ice cream and butter pots

Biscuits, snacks, sugar, vegetables, metallized films

Plastic laminates; coatings with PVdC; paperboard coatings for frozen/chilled trays (microwave heated)

Polyvinyl chloride (PVC)

Plasticizers, heat stabilizers, copolymers (vinyl acetate, vinylidene chloride)

Tubs, trays, bottles for beverages and oils

Shrink wrapping films for meat, inserts in chocolate boxes, fruit, vegetables, cheese

Vinyl chloride-vinylidene chloride barrier coatings

Ethylene vinyl acetate (EVA)

Anti-blocking agents

Stretch wrapping films

Plastic laminates; hot melt adhesives

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Co-extruded with PP for sauce, ketchup and mayonnaise bottles and tubs

Laminate films for cheese, milk products, fresh meat, salad

Laminate sandwiched between polyolefin films; paperboard laminations with PE

Polyethylene terephthalate Adhesives (PET)

Drinking bottles, readymeal trays

Boil-in-the-bag packs, metallized snack bags

Plastic laminates, metallized films; paperboard coatings for microwave heated trays

Polystyrene (PS)

Copolymers (butadiene, acrylates)

Drinking cups, yoghurt, ice cream, jam and syrup containers

Wrapping films for sugar confectionery (SB copolymer)

Dispersion coating for barrier purposes (wrapping paper, frozen food cartons, disposables)

Regenerated cellulose (Cellophane)

Plasticizers, coatings

Fresh meat, cakes, chocolates, biscuits, sugar, bread, vacuum packaging of coffee, cheese

Coated with nitrocellulose or vinyl chloride copolymer; laminated with paper

Sources: Robertson (1993), Kirwan and Strawbridge (2003), Crompton (2007) and other references.

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Table 12.2 Common additives in plastic packaging, their role and the nature of the chemicals used Additive

Function

Chemical substances

Antioxidants

Prevent oxidation during processing

Phenols, organic sulfides

Heat stabilizers

Prevent thermal decomposition during processing

Metal salts, organic phosphites

Slip additives

Lower friction

Fatty acid amide

Antistatic agents

Reduce static charge to avoid sticking together of films

Glycol derivatives, quaternary ammonium salt derivatives, ethoxylated fatty amines

Anti-blocking agents

Prevent sticking of thin films

Silica

Lubricants

Reduce viscosity and/or friction during processing

Fatty alcohols, fatty acids, calcium stearate, paraffin wax

Plasticizers

Increase flexibility

Phthalic, adipic and phosphoric acid esters, phthalates, di- or tricarboxylic acids, polychlorinated hydrocarbons, ethylene oxide condensates

Bactericides

Prevent bacterial growth

Quaternary ammonium compounds

Brighteners or whitening agents

Prevent discoloration

Stilbenes, thiopen derivatives

Colorants

Decorative purposes; light screening

Titanium dioxide, carbon black, organometallic pigments, organic dyestuffs; binders, dispersants, solvents

Fillers and reinforcing agents

Improvement of mechanical strength and/or barrier properties

Calcium carbonate, talc, kaolin, silica, layered silicates, cellulose fibrils and whiskers, carbon nanotubes

Adhesives

Lamination of plastic films or plastic films/paper; closure of filled packages

Solvents, polyvinyl acetate, ethylene vinyl acetate, polysaccharides, polyurethanes

Sources: Robertson (1993), Kirwan and Strawbridge (2003), Crompton (2007) and other references.

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copolymerization with vinyl acetate, ethylene or methyl acrylate (Robertson 1993). Barrier polymer dispersion coatings consist of aqueous dispersions of spherical polymer particles commonly denoted as latex. The purpose of these coating dispersions is to provide paper or paperboard with barrier layers to prevent the migration of substances from the fibre-based packaging material to food (KimpimaÈki and SantamaÈki 1998). Dispersion coatings work effectively as odour or taste barriers if dried perfectly, but the presence of monomer residuals can cause strange off-flavours (KimpimaÈki and Savolainen 1997). Typical latexes are copolymers of styrene in combination with butadiene, acrylates, methacrylates or vinyl acetates. Typical additives are fillers (mineral pigments), stabilizers, thickeners, waxes, antifoamers, plasticizers, biocides, antioxidants, chelating agents and buffers. An advantage of dispersion coating of paper packaging material over lamination with plastic films is the simplified repulping process. Dispersion coated material is also compostable (KimpimaÈki and SantamaÈki 1998). The term migration is frequently used to describe the transport of additives (e.g. wax or surfactants) from the bulk of the dispersion coating layer to its surface. Enrichment of these species on the film surface has an adverse effect on many properties, e.g. by causing film defects (Andersson et al. 2002). Furthermore, such an enrichment of a non-polymeric substance on the film surface may enhance the probability of its further transfer to packed food. Table 12.3 lists some potential impurities originating from the emulsion polymerization of dispersion coatings with their potential use in food packaging applications. Other sources of migrating substances can be chemicals present in the paper in the case of cellulose-based packaging material and in adhesives used for sealing of the packaging which are not in direct contact but rest in close Table 12.3 Common sources of impurities in dispersion coatings for food packaging and their chemical nature Polymerization components

Chemical substances

Initiators Catalysts Monomers Emulsifiers, surfactants

Metal persulfates, peroxides Metals, halides, peroxides, organic acids Hydrocarbons, dimers and oligomers Alkyl sulfates, sulfosuccinates, sulfonates, disulfonates Sodium chloride, sodium sulfate, persulfates, polyacrylate salts Divinylbenzene, acrylate and methacrylate monomers Mercaptans, isopropanol, hydroxylamine, carbon tetrachloride

Electrolytes Crosslinking agents Chain transfer agents

Sources: Klein and Daniels (1997) and other references.

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proximity to the packaged food (Brown and Williams 2003). Recycled paper can bring about flavour changes due to the presence of aromatic compounds (Blaikstone 1999) or add toxicological substances to food originating from, e.g., bleaching chemicals or other components used in special papers not intended for food contact (Brown and Williams 2003). Recycled paper is banned in some food packaging applications due to the high demand on microorganism purity. However, a study summarized by HoÈuÈtmann (1999) carried out at three independent laboratories showed no significant differences in the transfer of bacteria, mould or yeast from paper made of either virgin or recycled paper to dry, moist or greasy foodstuff. Also, the initial concentrations of microorganisms in the food itself are generally much higher than in any packaging material. Constituents of printing inks, especially UV-curable inks, might be of concern due to the potential presence of residues in form of monomers, initiators and pigments. The UV-curing process also produces free radicals with the prospective risk of uncontrolled side reactions. Organic solvents used in printing are another source of migration (Brown and Williams 2003). Typical residual solvents in gravure or flexographic printing are acetone, ethanol, ethyl acetate and ketones. Heat-set offset print may contain mineral oil solvent residues, which do not evaporate during drying, and hence has the potential to taint packed food products (Aurela and RaÈisaÈnen 1993). Contamination of food can also occur by the phenomenon known as set-off, where the ink components are transferred from the printed surface to the non-printed surface by direct contact during manufacture, storage or use of the material (Johns et al. 2000, Aurela et al. 2001). Compounds present in the environment can also be sorbed by the packaging with subsequent migration into the food (Robertson 1993). Catalysts used in the production of plastics can contain low amounts of heavy metals. Lead and cadmium can be transferred via impurities from inorganic pigments and printing inks and from recycled fibres (Conti 2007).

12.3.4 Migration testing European Council Directive 82/711/EEC and its amendment 97/48/EC lay down the basic principles for migration testing. The European Council Directive 85/ 572/EEC established a list of simulants to be used in the migration tests with respect to various foodstuffs. After revision (97/48/EC) the list now covers water (neutral aqueous foodstuffs), 3% acetic acid in water (acidic aqueous foodstuffs), 10% ethanol in water (alcoholic beverages) and olive oil. The US FDA also lists 3% sodium bicarbonate, 3% sodium chloride and 20% sucrose as aqueous food simulants (Crompton 2007). Olive oil is the most common stimulant for fatty foods, sometimes replaced by triglyceride oils or sunflower oil. For testing the mass transfer to solid foodstuffs, modified polyphenylene oxide, a porous polymer with high absorption capacity known under the trade

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name TenaxÕ, is generally used. When evaluating migration tests, the highest value obtained with these different simulants should be compared to the legal limits. Hence, as a rule of thumb, it is sufficient to test lipophilic substances with an appropriate fatty food simulant, since the migration to all other simulants obviously will be substantially lower. The directives differentiate between the overall migration which includes all the different substances released per unit area from the packaging material under specified testing conditions, and the specific migration which refers to individual, identifiable compounds. Testing can be done by total immersion of the packaging material into the test liquid, but a more realistic route for multilayer packaging is a single-sided test where only the food contact side faces the food simulant. In the US, single- or double-sided testing is selected depending on the thickness of the material to be tested. Single-sided testing is applied in a US FDA test method when the material is less than 500 m thick (Crompton 2007). The test temperature should be chosen so as to mimic the conditions the package is subjected to under the normal and foreseeable fate of food processing and storage life. The FDA also lists different testing conditions for the intended use of the packaging, i.e. for containers which are heat sterilized or pasteurized at filling, or for food which does not undergo any thermal treatment at filling. The subsequent storage conditions, e.g. room temperature, refrigerated or frozen milieu, should be regarded as well. Ready meals commonly belong to the two latter classes, and in this case any migration taking place during reheating of the food at the time of consumption should be considered. Also the actual lifetime of the packed food should be deliberated. Migration can be a very slow process and the requirements set on, for example, take-away food wrappings (order of minutes) differ from the requirements connected with, e.g., groceries with a shelf-life of several months. Testing of migration in multilayer packaging material should consider migration kinetics, i.e. cannot be based solely on a one time point measurement (Franz and StoÈrmer 2008). The most stringent test conditions laid down by European Commission Directive 97/48/EC are 10 days at 40ëC for materials and articles intended for use at room temperature and below. In cases where no information about the expected conditions is given, contact with aqueous food simulants should be carried out for 4 hours at 100ëC or with fatty food simulants for 2 hours at 175ëC. Two hours at 70ëC is appropriate for testing of hot filling applications. Migration testing experiments are generally carried out under `worst case' assumptions and the real amount transferred under normal circumstances might be substantially lower. As a rule of thumb, the migration rate increases 10 times when the temperature rises by 20ëC. The occurrence of migration can be monitored by sensory assessment or by chemical analysis of the food or food simulant. Sensory assessment is an approach in which a panel is engaged to record any effects on sensory

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properties. Typical chemical analytical methods for identification of migrants in food simulants are atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES), inductively coupled plasma (ICP), gas chromatography (GC) or high performance liquid chromatography (HPLC) in combination with mass spectrometry (MS), ultraviolet spectroscopy (UV-VIS), infrared spectroscopy (IR), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) (Paez Soares and Saiki 2006, Belhaneche-Bensemra et al. 2002, Crompton 2007, Franz and StoÈrmer 2008). Paez Soares and Saiki (2006) also introduced a method based on irradiation of the samples with radioactive gamma rays followed by spectrometric analysis of the radioisotopes. Labelling with 14C is another widely adopted method. Testing using TenaxÕ commonly involves extraction of the adsorbent by diethyl ether, followed by evaporation of the solvent and gravimetric determination of the residual solid substance. Sanches Silva et al. (2006) present a useful overview of analytical methods and guidelines for analytical determination of some model migrants. Certain attention should be directed to analytical uncertainties in the migration testing and the possible measurement errors due to highly volatile migrants or possible chemical reactions between migrant and food simulant, to validate that the obtained values of overall or specific migration are within the legislation limits (Franz and StoÈrmer 2008).

12.3.5 Functional barriers in practice Plastic packaging components Belhaneche-Bensemra et al. (2002) studied the migration of additives from PVC plastic films typically used for packaging of fresh meat and cheese. They concluded that the migration of plasticizer was highest in a fatty food simulant (sunflower oil) but also stipulated that the migration of one species (the plasticizer) might be accompanied by the migration of other additives (e.g. stabilizers and lubricants). The amount of migration increased with the percentage ratio of plasticizer in the PVC sample. The transfer of food species, e.g. fatty acids, to the packaging material may also have a plasticizing effect on the latter, thus enhancing the mobility of migrating species (additives) and thereby enhancing the amount of substances transferred from the packaging into the foodstuff (Brown and Williams 2003). Belhaneche-Bensemra et al. (2002) observed weight increase of PVC samples immersed in aqueous food simulants, indicating that the food simulant was penetrating the plastic sample. On the contrary, weight loss of PVC samples occurred in fatty food simulant, which is indicative of migration taking place. Wang and Storm (2006) studied the migration of a 50:50 blend of paraffin oil and fatty acid ester mixed in polypropylene. The substances are effective as

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lubricants during processing of PP films but are also used as dispersing oils for pigments and colorants in the plastic material. They concluded that the fatty acid ester present near the surface of the plastic sample was more prone to migrate to the olive oil stimulant, whereas the paraffin oil seemed to be more resistant to migration. The samples containing higher amounts of paraffin oil/fatty acid ester absorbed more olive oil during the migration tests, which was attributed to a higher free volume (larger spherulite size). The total migration increased with increased oil blend concentration. At higher temperature, complete migration of the added oil together with other additives present in the virgin PP sample occurred. MuÈller et al. (2001) studied Vitamin E (-tocopherol) as a stabilizer for polyolefin films and found it to be effective for control of the melt flow properties even at lower levels of addition than are usually required with traditional synthetic additives. Vitamin E is reported to be an excellent polymer antioxidant; it has the ability to enhance food product shelf-life and is also advantageous from an organoleptic perspective. Furthermore, Vitamin E is attractive also because it is approved for food contact and exerts low migration tendency. Recycling of plastic packaging for manufacture of new packaging material has gained increased interest over the last decade, not least owing to the European Parliament and Council Directive on packaging and packaging waste (94/62/EC). However, recycled plastics may be severely contaminated, not only by deteriorated food but also from unknown substances originating from unconventional handling of empty packages (Katan 1996, Feigenbaum et al. 1997). Purification of collected packaging waste can be technically difficult, and also expensive. The reuse of plastic can be both safe and economically viable by the utility of a functional barrier between the packed food and the recycled plastic layer. Franz et al. (1996) studied the efficiency of virgin PET as a functional barrier in a three-layered structure with deliberately contaminated PET in the centre. A mixture of toluene, 1,1,1-trichloroethane, phenyldecane, benzophenone and copper(II)-acetylacetonate was used as contaminant. Bottles were produced and migration tests were carried out with 3% acetic acid for 10 days at 40ëC. It was found that the virgin PET layer limited the migration of the contaminants to <1 g/kg food simulant. It was also concluded that the normal washing and drying procedures effectively reduce the contaminants in recycled plastics prior to bottle manufacture. Linssen et al. (1998) reported increased migration of phenolic antioxidants from LLDPE films into ethanolic food simulants with increased temperature and increased ethanol concentration. Printing ink components Aurela et al. (2001) studied the migration of alkylbenzenes (typical solvents in offset printing ink) from a set of commercial packaging board samples. It was

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found that a grease barrier on the food contact side in the form of a varnish was able to reduce the migration of alkylbenzene by about 70% as compared to a sample free from varnish. It was, however, not clear whether the varnish layer hindered migration through the board or whether it reduced the set-off of alkylbenzene from the printed surface. The migration of benzophenone, a photoinitiator in UV-cured inks, and model ink components from cartonboard to food during freezing storage and subsequent microwave heating was reported by Johns et al. (2000). Benzophenone was found to migrate from the paperboard, even through a PE-coating, which was attributed to the high permeability of this plastic film to low molecular weight species. Microwave heating of the packed food resulted in accelerated migration of higher molecular weight components. The transfer of printing ink components, even if not in direct contact with the food, was considerable over the long time storage ± up to 12 months ± for the studied frozen food products. Pastorelli et al. (2008) studied the efficiency of multilayer films consisting of PP/EVOH/PP or PET/SiOx/PE in preventing the migration of benzophenone from printed paperboard packages to cakes. The substance has a legal specific migration limit of 0.6 mg/kg. The PET/SiOx/PE multilayer film proved to be a perfect barrier for benzophenone diffusion both over 10 days at 40ëC and over 2 hours at 70ëC. Also the PP/EVOH/PP combination proved to be much more efficient than a single layer of PP. The ethylene±vinyl alcohol (EVOH) copolymer offers a superior barrier to gases over PP and the silicone oxide (SiOx) layer has the same effect on the medium gas barrier PET. The migration testing of seven benzophenone-based photoinitiators across the gas phase into dry foodstuffs clearly showed that the transfer substantially increased into food with a high porosity like bread or cake over less porous foodstuff like pasta (RodrõÂguez-Bernaldo de QuiroÂs et al. 2009). Migration was also found to be higher into lipophilic foodstuffs. Benzophenone derivatives with higher vapour pressures were more prone to migrate. The study showed that a printed surface does not have to be in contact with the packed food for migration of unhealthy substances to take place. Arab-Tehrany et al. (2007) investigated the transfer of printing solvents to water, 3% acetic acid or ethanol of 10 or 95% concentration by means of the phase ratio variation method for headspace analysis by GC. They concluded that the molecular size of the migrants as well as the polarity and the solubility of the solvents and the food simulants strongly affected the amount of solvent components transferred. Also the hydrogen-bonding capacity of the migrant was found to have an influence on the partition coefficient. Fiselier et al. (2010) studied the migration of di(2-ethylhexyl) maleate from printed cardboard into a number of different food products. The substance originates from the emulsifier in printing varnish and is essentially a residual from non-reacted starting substance of the corresponding anionic sulfosuccinate

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surfactant. Substantial amounts of the substance were found in commercial printed cardboard boxes with concentrations potentially resulting in mass transfer above the recommended migration limit of 50 g/kg. Paper constituents and adhesives Overall migration, determined gravimetrically and by FTIR, and specific migration, determined by GC/MS, for transfer of compounds from ovenable paperboard trays coated with PET or PP plastics or alternatively SB or SA dispersion coatings, was reported by Aurela et al. (2000). The transfer of paperboard or coating components into TenaxÕ was studied at temperatures close to the upper critical usage temperature of the polymer coatings. The study showed that the overall migration was below 10 mg/dm2 in all cases, i.e. within the legislative limits. Components originating from sizing agents in the base board were recognized as the main sources of migrants. Two-sided coating with the SA dispersion effectively reduced the migration of substances from the base board. The authors concluded that no migrants originating from the PET coating could be identified and that the coat weight of this plastic layer did not affect the overall migration. Homoelle (1999) presented a study of the effectiveness of various hydrophobic coatings applied to recycled paper substrates to prevent impurities in the form of inorganic and microbiological contaminants to come in contact with food. The study showed a very effective inhibition of transfer of the lead acetate model contaminant in the kraft liner substrate to a 3% acetic acid food simulant already at low coat weight (5 g/m2). The hydrophobic coatings also showed efficiency in hindering the transfer of Gram-negative bacteria from contaminated substrate to a sterile aqueous food simulant. Gruner and Piringer (1999) investigated the migration of typical adhesives used for gluing of primary or secondary food packaging and concluded that a starch adhesive resulted in an overall migration 1 and 10 orders of magnitude lower than for an EVA-hotmelt or a PVAc dispersion adhesive, respectively. Novel approaches to functional barriers A quite novel technique to improve functionality of food packaging material is to incorporate bioactive components in the packaging. Migration of the substances is prevented by covalently binding the bioactive component to the polymer backbone or by entrapping it in the polymer matrix (Steven and Hotchkiss 2003). As for these materials, each component must comply with existing regulations and the absence of migration must be confirmed by testing. Typical examples of bioactive compounds are enzymes, peptides and proteins. These are used with the aim of inhibiting microbial growth, improving nutritional value or extending the shelf-life.

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Immobilization of bioactive compounds on an inert plastic like PE first requires the creation of reactive functional groups on the polymer backbone, e.g. by oxidation (chemical or physical) to form carbonyl functional groups which provides site for further attachment of the active components (Steven and Hotchkiss 2003). There also exist a number of inherently bioactive polymers. A typical example is chitosan with natural antimicrobial properties. Release of lactic acid from low molecular weight poly(lactic acid) has been suggested as a mechanism behind the observed antimicrobial activity of this food packaging polymer (Steven and Hotchkiss 2003). Active systems can be placed in contact with the food surface or be incorporated in the food but can also be located outside the primary packaging and thus separated from the food by a functional barrier. Active packaging covers both non-migratory as well as active release approaches, allowing controlled migration. Typical examples of non-migratory substances are moisture absorbers, oxygen or ethylene scavengers and substances having antimicrobial effects (Dainelli et al. 2008). Intelligent systems cover features to monitor the food product quality, e.g. time±temperature or leak indicators. A range of different techniques to attach antimicrobial substances to polymer films have been reviewed: solution spreading on the film surface, anchoring with binders, covalent attachment by crosslinking and even co-extrusion. The most frequently used antimicrobial substance is nisin, a small molecular-sized peptide that has FDA approval for use in combination with food and also is commercially available (Joerger 2007). Even non-migratory active packaging substances are known to exhibit some migration, thus affecting safety with regard to these applications (Dainelli et al. 2008). The same is valid for intelligent packaging approaches, e.g. when indicators are placed inside the packaging and chemical substances are unintentionally migrating into the food. Both active and intelligent substances should thus comply with the positive list and the migration regulations. Migration test methods also need to be adjusted to record mass transfer under the special circumstances connected with these interactive substances (Dainelli et al. 2008). The migration of active or intelligent substances, whether intentional or not, might bring about or provide a pathway for the migration of other substances from the packaging as well. An example is commercial products with nisin, in which the active peptide makes up only a small fraction in a matrix of polysaccharides, salts and proteins (Joerger 2007). Other functional materials include thin films of edible coatings which are consumed with the food. Edible films are applied by immersion coating or by wrapping, brushing or spraying a thin layer directly on the food surface. The function of an edible film can be to retard migration of moisture or fat, retard transportation of oxygen, carbon dioxide and solutes, and retain volatile flavour compounds. They can also be carriers of food additives, e.g. by enhancing

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nutritional value or by providing antimicrobial or antioxidative functionality (Robertson 1993). So, the main reason for their use is to improve food quality and extend shelf-life, acting as a complement rather than a substitute for synthetic packaging materials. These edible films or coatings can thus be considered as part of a multilayer food packaging, being the internal layer in direct contact with food. The main components used for the formation of edible films and coatings are natural polysaccharides, e.g. alginate, carrageenan, starch derivatives and cellulose derivatives. Other compounds are natural proteins and natural waxes (Robertson 1993). Care should be taken not to use components that may cause allergic reactions, like gluten and shellfish derivatives, as edible films. Lowering the migration through a polymeric edible film can be made by crosslinking, e.g. by calcium ascorbate or calcium acetate (Pavlath and Orts 2009).

12.3.6 Modelling of migration The migration of substances can be predicted with sophisticated mathematical models taking their starting point from the famous Fick's first and second laws of diffusion. If the migration is diffusion-controlled, the amount of migrated substance is proportional to the square root of time. Fundamental constants are the partition coefficient KP/F of the migrant between the plastic material and the food or food simulant and the diffusion coefficient DP of the migrant in the plastic material. The models also take into account the initial concentration of migrant in the polymer and its molecular weight. Crompton (2007) summarizes predictions based on various models with experimental data for different plastics/additive combinations. Well-established migration models might substitute experimental analysis of migration and provide advantages over timeconsuming measurements including analytical uncertainties. Analytical uncertainties are connected, e.g., with isolation of migrants from fatty food simulants, which is complicated due to the numerous non-volatile components present in oils (Arab-Tehrany et al. 2007). Aurela et al. (1997) also reported the difficulties connected with separation of the individual layers in a multi-layer structure when testing the efficiency of functional barriers. Migration models often assume that migration will start once contact between food and the package is established. However, in the case of volatile substances, diffusion can take place in the film and eventually lead to the release of the substance into the environment prior to food contact (Katan 1996). Such a process can start immediately after packaging manufacture. However, a time lag behaviour can delay migration through a functional barrier and should hence be taken into account in migration models concerning multilayer packaging (Feigenbaum et al. 1997). Franz et al. (1996) reported that long-term migration experiments taking the possible time lag into account did not result in any migration of model contaminants through a PET barrier, thus demonstrating the high efficiency of this material as a functional barrier.

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Tosa and Mercea (2008) give an overview of the solution of the diffusion equation for multilayer packaging which takes into consideration the diffusion coefficients of the migrant in each layer (Di , Dj ) and in the foodstuff (DF), respectively (Fig. 12.1). The model also comprises the partition coefficients between layer i and layer j (Ki=j ) and between the food contact layer j and the foodstuff (Kj/F). Other parameters included are the thickness and the density of each layer, and the initial concentration of migrant in layer i. A more effective functional barrier is ultimately achieved by decreasing the diffusion coefficient rather than by increasing the layer thickness. Han et al. (2003) applied a numerical approach ± the finite element method ± to develop a computer program for simulation of migration through a multilayer structure. Simulation predictability was compared to experimental data. A 300 m thick core layer of 90:10 HDPE:LDPE contaminated with 3,5-di-t-butyl4-hydroxytoluene was sandwiched between two layers of virgin HDPE with thickness varying between 150 and 450 m. Migration of the contaminant into ethanolic solutions was found to obey Fickian diffusion. Complete migration occurred in pure ethanol whereas a partitioning effect was found in a 50% aqueous ethanol solution. The rate of migration decreased with increased outer layer thickness and a lag time effect was observed in all cases, due to the migration of the contaminant through the outer layer. The experimental data fitted the simulated finite element-based computer model very well. The study also demonstrated that a rather poor barrier plastic like HDPE can act efficiently as a functional barrier to the migration of contaminants when co-extruded with an inner layer consisting of recycled plastic. The lag time to migration can ultimately be taken as a measure of the efficiency of a functional barrier. This approach was utilized for evaluation of the migration of model contaminants (surrogates) to simulate the behaviour of recycled plastic materials (Pennarun et al. 2005). Organic substances with varying volatility, polarity and reactivity were used as surrogates to investigate the migration kinetics in mono- and tri-layers of PET. Good correlation with Fickian behaviour was observed when using pure ethanol as food simulant since ethanol is a good solvent for all the tested surrogates. However, partitioning occurred in the case of migration testing with 3% acetic acid. Only the fastdiffusing and water-soluble molecules were found to migrate into the aqueous food simulant and a lag time of 6 months was observed for tri-layer PET bottles with the contaminated material in the core. The ability of ethanol to swell and plasticize PET was also suggested to have an impact on the higher degree of migration into this food simulant. An amorphous hydrogenated carbon coating applied by plasma-enhanced chemical vapour deposition on PET deliberately contaminated with chloroform, toluene, benzophenone and lindane surrogates was shown to be an effective functional barrier for recycled plastics (Cruz et al. 2010).

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Nanostrategies for functional barriers

Conventional strategies for improvement of the poor gas barrier properties of polyolefins are the formation of multilayer structures with high barrier plastics or by surface coatings (Dadbin et al. 2008). Early attempts were made to vacuum coat commodity plastic materials with thin layers in the nanometre range consisting of aluminium or metal oxides (Langowski 2008). However, more cost-effective procedures involve the incorporation of fillers in the polymer matrix, eventually leading to mechanically reinforced nanocomposites with superior barrier properties. Antimicrobial properties, oxygen scavenging and enzyme immobilization are examples of active or intelligent functionality of nanoparticles for food packaging applications (de Azeredo 2009). Nanotechnology refers to the use of nanosized materials, i.e. particles, fibres, nanotubes, whiskers, etc., having at least one dimension in the nanometre (1± 100 nm) range (Alexandre and Dubois 2000). The nanotechnology concept covers two different approaches: top-down strategies, which refer to size reduction of larger materials, e.g. by milling, grinding, etching, nanolithography or ultrasonic treatment (Sanguansri and Augustin 2006, de Azeredo 2009). Such strategies are generally not considered as giving rise to new, unnatural materials in relation to their larger, existing forms. Nevertheless, the properties of these smaller structures differ significantly from those of their macroscopic counterparts when it comes to chemical safety regulations (Chau et al. 2007). The second approach, the bottom-up strategy, however, produces completely new materials, since it relies on building functional nanostructural substances from atoms or molecules via the utilization of self-assembling properties (Sanguansri and Augustin 2006, de Azeredo 2009). Reinforcing filler materials typically consist of naturally occurring, or organically modified, layered silicates, SiO2 nanoparticles, cellulose nanofibres and nanowhiskers, carbon nanotubes, starch nanocrystals or chitosan nanoparticles (de Azeredo 2009). Nanoparticles providing active or intelligent functionality are based on, e.g., titanium dioxide or nanocomposites including silver ions, metal oxides or conducting polymers (de Azeredo 2009). It is beyond the scope of this chapter to give detailed information about the characteristics and functional properties of all currently available nanostructured materials. Nanotechnology offers the ability to strongly improve the feasibility of polymers with respect to reduced brittleness, enhanced mechanical properties and enhanced barrier against moisture and gases. Especially, biobased polymers may increase their competitiveness to non-renewable plastics owing to the performance improvement by the formation of bio-nanocomposites. Barrier properties of polymeric materials are improved by the creation of a tortuous path for diffusing molecules across the polymer, thus slowing down the rate of diffusion. The efficiency of a nanocomposite functional barrier thus relies on hindering or prolonging the migration of substances through the barrier layer.

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Dadbin et al. (2008) showed for example a 50% improvement in oxygen barrier by addition of 3 pph organoclay to LDPE and LLDPE composite films formed by melt compounding. ImpermÕ is a nanocomposite material based on nylon with the intended use in multilayer PET bottles, application fields of which are bottles for fruit juices, dairy foods or soft drinks, or in multilayer films for packaging of meats, cheese, confectionery and cereals (de Azeredo 2009). The barrier properties of biaxially oriented polypropylene (BOPP), PET and regenerated cellulose films were improved considerably by application of a thin SiOx coating by vacuum deposition (Amberg-Schwab et al. 1998). Application of a coating a few microns thick of a novel hybrid inorganic±organic polymer molecular composite on top of the polymer±SiOx coating showed synergistic effects with greatly improved barrier properties as a result (Amberg-Schwab et al. 1998). This type of molecular inorganic±organic composite coating was also shown to drastically reduce the permeation of a variety of flavour substances through a BOPP film. The efficiency of a SiOx layer as a functional barrier was demonstrated by Pastorelli et al. (2008) and a further improvement of its barrier properties against mass transfer of various substances by the formation of composites on a nano- or molecular scale is likely to increase its attractiveness as a functional barrier even more. A novel concept in the formation of barrier layers at the micro/nanoscale was reported by Mueller et al. (2000). PE and PP films were filled with CaCO3 particles and multiple layers were applied on polyethylene oxide (PEO) in a microlayer coextrusion process. Multilayering was achieved by continuously dividing the fed components in the melt stream by a layer-multiplying element placed before the exit die in the extruder, eventually leading to thousands of layers. PEO is a hydrophilic polymer with poor mechanical properties. By controlling the microlayer thickness and the composition of the film system, it was possible to create breathable films with excellent mechanical properties. Such novel film systems would be interesting candidates as functional barriers since they might be designed to effectively hinder the transport of specific molecules whereas others, e.g. active substances, can be allowed to migrate. High-aspect-ratio nanosized platelets of inorganic phosphate glass working as impermeable inclusions in a polypropylene film were discussed by Gupta et al. (2009). Composite films of 5±8 m thickness were formed by melt blending of phosphate glass and poly(propylene-g-maleic anhydride) followed by biaxial stretching giving elongated glass platelets ordered parallel to the film surface. The oxygen permeability of the stretched composite films was found to decrease by up to two orders of magnitude compared to the unoriented polypropylene film. Bilbao-SaÂinz et al. (2010) studied the barrier properties of edible films of hydroxypropyl methylcellulose. Addition of microcrystalline cellulose (MCC) nanoparticles effectively lowered the water vapour permeability and the water

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uptake of the films, effects which were further pronounced by enhancing the hydrophobicity of the MCC nanoparticles by lipid coating. Nanotechnology is a versatile approach to enhancing the properties of plastic films and coatings. Any method for enhancement of barrier properties, i.e. any strategy to hinder or prolong the mass transfer of substances through a film or the migration of substances from the film itself by creating tortuous pathways for migrating molecules, will likely improve the efficiency of the film as a functional barrier.

12.4.1 Food safety issues related to nanostrategies Nanotechnology is already used in food processing, for manufacture of so-called functional food, e.g. by incorporation of bioactive substances like omega-3 fatty acids, vitamins, etc. (Sanguansri and Augustin 2006). Encapsulation of these substances is often necessary for controlled release during consumption of the food but also for protecting the sensitive substances against, e.g., oxidation during processing and storage. The encapsulation material should be selected from ingredients that are considered as safe. There are already over 200 consumer food products on the market relying on nanotechnology, with the health and fitness field as the largest (Chau et al. 2007). In European Commission Regulation (EC) No. 450/2009 on active and intelligent materials and articles intended to come into contact with food, it is stated that `. . . nanoparticles should be assessed on a case-by-case basis as regards their risk until more information is known about such new technology.'

This should be of the highest relevance for engineered substances, i.e. nanoparticles manufactured and intended to impart active or intelligent functionality to food packaging, rather than alluding to inert nanoparticles used as reinforcing fillers for improvement of the functional barrier itself. Engineered nanoparticles are a very heterogeneous group of substances, which differ in size, shape, surface area, chemical composition and biopersistence, thus strongly affecting their potential impact on health (Hoet et al. 2004, Chau et al. 2007). The large surface-to-volume ratio renders nanosized particles more biologically active than larger particles of the same chemical origin. It is reasonable to believe that migration of nanosized material from food packaging into foodstuff may take place. There is, however, a lack of scientific data on the subject (de Azeredo 2009). An accurate analysis of possible toxicological properties of nanosized material, whether acting as passive reinforcement fillers or as providers of active or intelligent functionality should be undertaken prior to their incorporation in food packages.

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Future trends

Increased global environmental concern has put extensive efforts into the development of renewable, biobased and biodegradable polymer coatings and so-called bioplastic films for the replacement of synthetic, petroleum-derived plastic materials in food packaging. Incineration of plastic packages or paperbased packages laminated with plastic films causes emission of greenhouse gases and future packaging material will likely be selected on the basis of its carbon footprint. The conventional plastics used so far are essentially inert but several possible chemical and physical interactions may take place between biobased polymers and packaged food. Other types of residuals or impurities present in biobased polymer raw materials such as plant residues are inherently non-toxic, but may nevertheless adversely affect the quality or taste of the food. Biobased polymers may also act as sources for nutrition of microorganisms thus accelerating their growth, and this issue will likely enhance the efforts to introduce antimicrobial functionality in future food packaging. The importance of controlling the migration from these novel functional barriers will increase with their increasing use as replacements for plastics, and it may be necessary to develop new testing methods and protocols. The branch association European Bioplastics states that the annual global consumption of plastics was 250 million tonnes in 2009, which is the largest application field for crude oil outside the energy and transport sectors. In Europe, about 40% of all plastic materials manufactured are used in the packaging sector and about 50% of the food products on the market are packed into plastics (Kirwan and Strawbridge 2003). The market share of bioplastics is well under 1% but has strong potential for growth in the future, not least because of the development of nanotechnology. The rapid growth of nanotechnology has been compared to the biotechnology revolution, and government grants in this research field were estimated to reach $3 billion in 2003 (Paull et al. 2003). In addition to this, investments of venture capital have been growing annually. Industry analysts reported that in 2005, there were about 250 packaging products on the market incorporating nanosized substances and the forecast was that within 10 years the so-called nanopackaging should reach a commercial value of $30 billion (Cole and Bergeson 2006). New academic programmes are designed to meet the demand for knowledge in the field (Mazzola 2003). In order for nanostrategies to survive market introduction and further development, companies must consider the impact of public acceptance of these novel materials or functionalities introduced in food packaging. Much of the recent fear against nanotechnology has been due to the fact that the technology is too diverse and due to the lack of real data for a rational discussion on the topic (Mazzola 2003). This lack of technical data is a strong incitement for the research community to evaluate the effects on health

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and environment and to better define the actual risks connected with the use of nanotechnology in each specific field. It should be kept in mind that nowhere near all possible applications of nanotechnology are to be potentially suspected. The concept of bioactive polymers incorporated in packaging by immobilization of enzymes is likely to increase (Steven and Hotchkiss 2003). Enhanced environmental concern will likely suppress the amount of food spoilage by more efficient packaging material.

12.6

Sources of further information and advice

The book Plastic Packaging edited by Otto G. Piringer and Albert L. Baner covers the interactions between plastic packaging materials with food and pharmaceuticals. It contains a thorough description of plastic components and manufacture, polymer additives, models for diffusion in polymers and the prediction of diffusion coefficients, the permeation of gases, and modelling of migration of plastic constituents. It also gives a deep insight into the US and EU legislation on food contact materials. Additive Migration from Plastics into Foods: A Guide for the Analytical Chemist by Thomas R. Crompton contains an introductory text to plastics used in commodity packaging and their accompanying additives. The book also contains a comprehensive and practical guide to analytical testing of various substances and gives an overview of European and US regulation systems. The basic theories behind migration are also explained. Food Packaging: Principles and Practice by Gordon L. Robertson is a useful introduction to food packaging and covers all relevant aspects of the theme with special consideration to packaging requirements related to various types of food. Food Packaging Technology edited by Richard Coles, Derek McDowell and Mark J. Kirwan is a book that covers deterioration and preservation of food as well as product quality and shelf-life. Overviews of plastics and paper-based packaging materials are given. Novel Food Packaging Techniques edited by Raija Ahvenainen describes new approaches to improvement of food shelf-life or quality by active and intelligent packaging and the legislative issues related to these. A thorough description of the concept of modified atmosphere packaging is given. One chapter is devoted to the interactions between packaging and the flavour of the food. The book also covers aspects of recyclability. On the European Commission website, fresh information on food contact issues, legislation and standard methods for migration testing can be found: www.ec.europa.eu/food. In the US the Food and Drug Administration website covers the same function (www.fda.com). The Institute of Occupational Medicine (IOM) in the UK via their venture The Safenano Initiative (website www.safenano.org) provides relevant and continuously updated information on the potential risks of nanotechnology.

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AssuredNanoTM offers an accreditation scheme regarding safety, health and environment for producers and users of nanomaterials and nanotechnology (www.assurednano.com). The Association of Plastics Manufacturers in Europe, APME (www.plastics europe.org) and the British Plastic Federation (www.bpf.co.uk) websites provide useful information on plastic manufacture and use. The trade association for the US plastics industry, the American Plastics Council, can be found on www.americanchemistry.com. Information on recent developments in the bioplastics field is given on the European Bioplastics website, www.europeanbioplastics.org.

12.7

References and further reading

Alexandre, M. and Dubois, P. (2000). Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Materials Science and Engineering, 28, 1±63. Amberg-Schwab, S., Hoffman, M., Bader, H. and Gessler, M. (1998). Inorganic±organic polymers with barrier properties for water vapor, oxygen and flavors. Journal of Sol-Gel Science and Technology, 1/2, 141±146. Andersson, C., JaÈrnstroÈm, L. and Hellgren, A.-C. (2002). Effects of carboxylation of latex on polymer interdiffusion and water vapor permeability of latex films. Nordic Pulp and Paper Research Journal, 17 (1), 20±28. Arab-Tehrany, E., Mouawad, C. and Desobry, S. (2007). Determination of partition coefficient of migrants in food simulants by the PRV method. Food Chemistry, 105, 1571±1577. Aurela, B. and RaÈisaÈnen, T. (1993). Residual solvent content in heatset offset print. Journal of High Resolution Chromatography, 16, 422±424. Aurela, B., Tapanila, T., Osmonen, R.-M. and SoÈderhjelm, L. (1997). Development of methods for testing barriers in food packaging materials. Journal of High Resolution Chromatography, 20, 499±502. Aurela, B., Vuorimaa, M. and Lindell, H. (2000). Migration from ovenable boards at high temperatures. Nordic Pulp and Paper Research Journal, 15 (2), 150±154. Aurela, B., Ohra-aho, T. and SoÈderhjelm, L. (2001). Migration of alkylbenzenes from packaging into food and TenaxÕ. Packaging Technology and Science, 14, 71±77. Belhaneche-Bensemra, N., Zeddam, C. and Ouahmed, S. (2002). Study of the migration of additives from plasticized PVC. Macromolecular Symposia, 180, 101±201. Bilbao-SaÂinz, C., Avena-Bustillos, R.J., Wood, D.F., Williams, T.G. and McHugh, T.H. (2010). Composite edible films based on hydroxypropyl methylcellulose reinforced with microcrystalline cellulose nanoparticles. Journal of Agricultural Food Chemistry, 58, 3753±3760. Blaikstone, B. (1999). What suppliers need to know: a brief course in food chemistry, TAPPI 1999 Polymers, Laminations and Coatings Conference Proceedings, pp. 277±291. TAPPI Press, Atlanta, GA. Brandsch, J. and Piringer, O. (2008). Characteristics of plastic materials. In O.G. Piringer and A.L. Baner, eds, Plastic Packaging: Interaction with Food and Pharmaceuticals. Second, completely revised edition, pp. 15±62. Wiley-VCH Verlag, Weinheim, Germany.

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Brown, H. and Williams, J. (2003). Packaged product quality and shelf life. In R. Coles, D. McDowell and M.J. Kirwan, eds, Food Packaging Technology, pp. 65±94. Blackwell Publishing, CRC Press, Boca Raton, FL. Chau, C.-F., Wu, S.-H. and Yen, G.-C. (2007). The developments of regulations for food nanotechnology. Trends in Food Science & Technology, 18, 269±280. Cole, M.F. and Bergeson, L.L. (2006). FDA regulation of food packaging produced using nanotechnology. Food Safety Magazine, April/May 2006, www.foodsafety magazine.com (accessed 3 December 2009). Colvin, V.L. (2003). The environmental impact of engineered nanomaterials. Nature Biotechnology, 21 (10), 1166±1170. Comyn, J. (1985). Polymer Permeability. Elsevier Applied Science, London. Conti, M.E. (2007). Heavy metals in food packaging. In P. Szefer and J. Nriagu, eds, Mineral Components in Food, pp. 339±362. CRC Press, Boca Raton, FL. Crank, J. (1999). The Mathematics of Diffusion, 2nd edition. Oxford University Press, Oxford, UK. Crompton, T.R. (2007). Additive Migration from Plastics into Foods: a Guide for the Analytical Chemist. Smithers Rapra, Shrewsbury, UK. Cruz, S.A., Zanin, M., Nascente, P.A.P. and Bica de Moraes, M.A. (2010). Superficial modification in recycled PET by plasma etching for food packaging. Journal of Applied Polymer Science, 115, 2728±2733. Dadbin, S., Noferesti, M. and Frounchi, M. (2008). Oxygen barrier LDPE/LLDPE/ organoclay nano-composite films for food packaging. Macromolecular Symposia, 274, 22±27. Dainelli, D., Gontard, N., Spyropoulos, D., Zondervan-van den Beuken, E. and Tobback, P. (2008). Active and intelligent food packaging: legal aspects and safety concerns. Trends in Food Science & Technology, 19, S103±S112. de Azeredo, H.M.C. (2009). Nanocomposites for food packaging applications. Food Research International, 42, 1240±1253. European Bioplastics, www.european-bioplastics.org (accessed 11 January 2010). European Commission Directive 97/48/EC. Official Journal of the European Union, L 222, 12.8.1997, 10±15. European Commission Directive 2002/72/EC. Official Journal of the European Union, L 220, 15.8.2002, 18±58. European Commission Directive 2008/39/EC. Official Journal of the European Union, L 63, 6.3.2008, 6±13. European Commission Regulation (EC) No. 450/2009. Official Journal of the European Union, L 135, 29.5.2009, 3±11. European Council Directive 82/711/EEC. Official Journal of the European Union, L 297, 23.10.1982, 26±30. European Council Directive 85/572/EEC. Official Journal of the European Union, L 372, 31.12.1985, 14±21. European Council Directive 89/109/EEC. Official Journal of the European Union, L 40, 11.2.1989, 38±44. European Parliament and Council Directive 94/62/EC. Official Journal of the European Union, L 365, 31.12.1994, 10±23. Feigenbaum, A., Laoubi, S. and Vergnaud, J.M. (1997). Kinetics of diffusion of a pollutant from a recycled polymer through a functional barrier: recycling plastics for food packaging. Journal of Applied Polymer Science, 66, 597±607. Fiselier, K., Rutschmann, E., McCombie, G. and Grob, K. (2010). Migration of di(2ethylhexyl) maleate from cardboard boxes into foods. European Food and

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Research Technology, 230, 619±626. Franz, R. and StoÈrmer, A. (2008). Migration of plastic constituents. In O.G. Piringer and A.L. Baner, eds, Plastic Packaging: Interaction with Food and Pharmaceuticals, second, completely revised edition, pp. 349±415. Wiley-VCH Verlag, Weinheim, Germany. Franz, R., Huber, M., Piringer, O.-G., Damant, A.P., Jickells, S.M. and Castle, L. (1996). Study of functional barrier properties of multilayer recycled poly(ethylene terephthalate) bottles for soft drinks. Journal of Agricultural and Food Chemistry, 44, 892±897. Gruner, A. and Piringer, O. (1999). Component migration from adhesives used in paper and paperboard packaging for foodstuffs. Packaging Technology and Science, 12, 19±28. Gupta, M., Lin, Y., Deans, T., Crosby, A., Baer, E., Hiltner, A. and Schiraldi, D.A. (2009). Biaxially oriented poly(propylene-g-maleic anhydride)/phosphate glass composite films for high gas barrier applications. Polymer, 50, 598±604. Han, J.-K., Selke, S.E., Downes, T.W. and Harte, B.R. (2003). Application of a computer model to evaluate the ability of plastics to act as functional barriers. Packaging Technology and Science, 16, 107±118. Hoet, P.M., Nemmar, A. and Nemery, B. (2004). Health impact of nanomaterials? Nature Biotechnology, 22 (1), 19. Homoelle, J. (1999). The inhibition of migration of contaminants from paper into packaged foods. TAPPI Journal, 82 (8), 127±131. HoÈuÈtmann, U. (1999). Germ load on packaging paper and board: transfer of microorganisms. TAPPI Journal, 82 (1), 74±77. Joerger, R.D. (2007). Antimicrobial films for food applications: a quantitative analysis of their effectiveness. Packaging Technology and Science, 20, 231±273. Johns, S.M., Jickels, S.M., Read, W.A. and Castle, L. (2000). Studies on functional barriers to migration. 3. Migration of benzophenone and model ink components from cartonboard to food during frozen storage and microwave heating. Packaging Technology and Science, 13, 99±104. Katan, L.L. (1996). Do `functional' barriers function? Packaging Technology and Science, 9, 289±296. KimpimaÈki, T. and SantamaÈki, K. (1998). Barrier dispersion coating ± New feasibility for the packaging industry. Paperi ja Puu, 80 (4), 249±256. KimpimaÈki, T. and Savolainen, A.V. (1997). Barrier dispersion coating of paper and board. In J. Brander and I. Thorn, eds, Surface Application of Paper Chemicals, pp. 208±228. Blackie Academic & Professional, Chapman & Hall, London. Kirwan, M.J. and Strawbridge, J.W. (2003). Plastics in food packaging. In R. Coles, D. McDowell and M.J. Kirwan, eds, Food Packaging Technology, pp. 174±240. Blackwell Publishing, CRC Press, Boca Raton, FL. Klein, A. and Daniels, E.S. (1997). Formulation components. In P.A. Lovell and M.S. ElAasser, eds, Emulsion Polymerization and Emulsion Polymers, pp. 207±237. John Wiley & Sons, Chichester, UK. Langowski, H.-C. (2008). Permeation of gases and condensable substances through monolayer and multilayer structures. In O.G. Piringer and A.L. Baner, eds, Plastic Packaging: Interaction with Food and Pharmaceuticals, second, completely revised edition, pp. 297±347. Wiley-VCH Verlag, Weinheim, Germany. Linssen, J.P.H., Reitsma, J.C.E. and Cozijnsen, J.L. (1998). Research note ± migration of antioxidants from polyolefins into ethanolic simulants. Packaging Technology and Science, 11, 241±245.

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Mazzola, L. (2003). Commercializing nanotechnology. Nature Biotechnology, 21 (10), 1137±1143. Mueller, C., Topolkaraev, V., Soerens, D., Hiltner, A. and Baer, E. (2000). Breathable polymer films produced by the microlayer coextrusion process. Journal of Applied Polymer Science, 78, 816±828. MuÈller, D., Voigt, W. and Ghosh, J. (2001). Vitamin E ± a new choice for polyolefin stabilization. Macromolecular Symposia, 176, 17±29. OberdoÈrster, G., OberdoÈrster, E. and OberdoÈrster, J. (2005). Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles. Environmental Health Perspectives, 113 (7), 823±839. Paez Soares, E. and Saiki, M. (2006). Study on element migration from plastic food packagings to simulating solutions. Macromolecular Symposia, 245±246, 129±131. Pastorelli, S., Sanches-Silva, A., Cruz, J.M., Simoneau, C. and Losada, P.P. (2008). Study of the migration of benzophenone from printed paperboard packages to cakes through different plastic films. European Food Research and Technology, 227, 1585±1590. Paull, R., Wolfe, J., HeÂbert, P. and Sinkula, M. (2003). Investing in nanotechnology. Nature Biotechnology, 21 (10), 1144±1147. Pavlath, A.E. and Orts, W. (2009). Edible films and coatings: why, what and how? In M.E. Embuscado and K.C. Huber, eds, Edible Films and Coatings for Food Applications, pp. 1±23. Springer Science+Business Media, New York. Pennarun, P.Y., Saillard, P., Feigenbaum, A. and Dole, P. (2005) Experimental direct evaluation of functional barriers in PET recycled bottles: comparison of migration behaviour of mono- and multilayers. Packaging Technology and Science, 18, 107± 123. PospõÂsÏil, J. and NesÏpuÊrek, S. (2008). Polymer additives. In O.G. Piringer and A.L. Baner, eds, Plastic Packaging: Interaction with Food and Pharmaceuticals, second, completely revised edition, pp. 63±88. Wiley-VCH Verlag, Weinheim, Germany. Resolution ResAP (2002). 1 on paper and board materials and articles intended to come into contact with foodstuffs, www.coe.int/soc-sp (accessed 14 April 2010). Resolution ResAP (2005). 2 on packaging inks applied to the non-food contact surface of food packaging materials and articles intended to come into contact with foodstuffs, www.coe.int/soc-sp (accessed 14 April 2010). Robertson, G. L. (1993). Food Packaging Principles and Practice, Marcel Dekker, New York. RodrõÂguez-Bernaldo de QuiroÂs, A., Paseiro-Cerrato, R., Pastorelli, S., Koivikko, R., SImoneau, C. and Paseiro-Losada, P. (2009). Migration of photoinitiators by gas phase into dry foods. Journal of Agricultural and Food Chemistry, 57, 10211± 10215. RoÈper, H. and Koch, H. (1990). The role of starch in biodegradable thermoplastic materials. Starch/StaÈrke, 42 (4), 123±130. Sanches Silva, A., SendoÂn GarcõÂa, R., Cooper, I., Franz, R. and Paseiro Losada, P. (2006). Compilation of analytical methods and guidelines for the determination of selected model migrants from plastic packaging. Trends in Food Science & Technology, 17, 535±546. Sanguansri, P. and Augustin, M.A. (2006). Nanoscale materials development ± a food industry perspective. Trends in Food Science & Technology, 17, 547±556. Steven, M.D. and Hotchkiss, J.H. (2003). Non-migratory bioactive polymers (NMBP) in food packaging. In R. Ahvenainen, ed., Novel Food Packaging Techniques, pp. 71± 102. Woodhead Publishing, Cambridge, UK.

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Tosa, V. and Mercea, P. (2008). Solution of the diffusion equation for multilayer packaging. In O.G. Piringer and A.L. Baner, eds, Plastic Packaging: Interaction with Food and Pharmaceuticals, second, completely revised edition, pp. 247±296. Wiley-VCH Verlag, Weinheim, Germany. Wang, Q. and Storm, B.K. (2006). Migration study of polypropylene (PP) oil blends in food simulants. Macromolecular Symposia, 242, 307±314.

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Silver-based antimicrobial polymers for food packaging A . M A R T IÂ N E Z - A B A D , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: Within the family of nanomaterials, the silver-based nanotechnology has the highest degree of commercialization in the medical and consumer product markets. This chapter first discusses historical and present applications for silver as an antimicrobial agent, explaining the molecular basis for its efficacy. Different techniques to fabricate nanoparticles and silver-based nanocomposites are reviewed, emphasizing all of the factors which govern their potential. Finally, application of silver technology to foodstuffs is discussed, taking into consideration regulatory issues and future perspectives. Key words: silver, nanoparticles, silver ions, nanocomposites, antimicrobial polymers, antimicrobial silver, silver zeolites.

13.1

Introduction

13.1.1 Historical use of silver as an antimicrobial agent The antimicrobial properties of silver have been recognized since ancient times. Even before the Neolithic era, it was known that cooking or storing water in silver pots would keep it safe (Vaidyanathan et al., 2009). The first recorded medicinal use of silver dates from the eighth century. Avicenna, for example, reported in 980 its use as a blood purifier. In the seventeenth and eighteenth centuries, the use of silver was quickly generalized in the treatment of venereal diseases, fistulae and abscesses. By the nineteenth century, pencils charged with a 0.5% silver nitrate solution formed part of the basic surgical equipment, as silver nitrate not only prevented wounds from becoming infected but also allowed epithelization and promoted crust formation of the wounds (Bhattacharya and Mukherjee, 2008; Klasen, 2000). However, with the discovery of penicillin in 1928 and the advent of antibiotics after World War II, the use of silver against microbes was pushed to the background and has been mostly limited to the treatment of burns and ulcers until the present time.

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13.1.2 Renewed interest in silver: nanosilver for limitless applications The interest in antibiotics has been waning in the last few years as the emergence of resistances manifests more rapidly every time a new antibiotic is introduced in the healthcare system. The urge to solve the increase of nosocomial infections (Hetrick and Schoenfisch, 2006) and a public opinion ever more concerned with safety have pushed the market towards the development of new technologies that help maintain the safety of medical and consumer products. In this scenario, nanotechnology is emerging as a rapidly growing field with its application in science and technology for the purpose of manufacturing new materials at the nanoscale level. Owing to extreme increase in their surface area to volume ratio, metal nanoparticles have unique mechanical, optical and chemical properties as compared to the bulk material (Morones et al., 2005). The combination of nanotechnology with the antimicrobial effect of silver has enabled the market to give rise to a new generation of materials with enhanced antimicrobial properties that has engulfed the healthcare sector. Out of all nanomaterials used in medicine, nanosilver has the highest degree of commercialization (Chen and Schluesener, 2008). Orthopedic implants, prostheses, vascular grafts and wound dressings are the most common applications of nanosilver reinforced polymers in the medical field, but nanosilver can also be found in creams, gels, contraceptive devices, recipient sites and all kinds of surgical instruments (Chen and Schluesener, 2008; Tolaymat et al., 2010; Gupta and Silver, 1998). The success and outstanding cost-effectiveness of these materials in the medical field is the driving force for their implantation in other consumer product markets. Even though regulatory issues for nanomaterials are still unclear, over 100 consumer products with nanosilver could already be found in the market in 2007, and the number might still be rising (Li et al., 2008). Nanosilver can be found in home appliances like inner liners in refrigerators or washing machines, in cosmetics or hygiene products (creams, lotions, soaps, deodorants, toothbrushes, toothpaste), in textiles (clothing, underwear, socks, upholstery), in toys, in air and water purification systems, etc. In the food industry, silver-zeolite coated stainless steel is used as a self-sterilizing surface in containers and cutlery (Chen and Schluesener, 2008; Tolaymat et al., 2010; Gupta and Silver, 1998). Even the ingestion of nanosilver is marketed as a nutrition supplement in form of a colloidal solution (www.mesosilver.com).

13.1.3 Antimicrobial mechanism of nanosilver: advantages and drawbacks In dealing with its antimicrobial efficacy, a difference must be made between silver at the macro- or microscale and silver nanoparticles. The biocidal properties of the bulk material, which itself has no antibacterial effect, rely on

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the sustained oxidation and release of very small quantities of silver cations (Ag+) to an aqueous or moistured environment. Silver cations strongly interact with thiol groups (±SH) in biomolecules (Liau et al., 1997). In bacteria, this causes the inactivation of enzymes in the outer membrane, which disturbs its permeability and the proton motive force leading to pits in the membrane and eventual cell death. Additionally, silver cations can enter the cell and bind DNA components, preventing replication (Sharma et al., 2009; Li et al., 2008; Nair and Laurencin, 2007). Concentrations as low as 0.1 M have shown antibacterial properties (Hwang et al., 2007). But the antibacterial concentration of silver cations, in form of silver salts, depends highly on their chemical environment, as they are rapidly inactivated by biomolecules and remain insoluble in the presence of chloride anions (Clÿ), which severely limits the range of possible applications (Shrivastava et al., 2007). The antibacterial efficacy of silver in its reduced state (Ag0) has been established only on the nanoscale. Under these circumstances, nanoparticles are a meta-stable high energy form of elemental silver which seems to interact with bacterial membrane constituents and penetrate the cell, unbalancing respiratory functions, leading to an increase of reactive oxygen species, and also intercalating between DNA bases, interfering replication (Nair and Laurencin, 2007, Sharma et al., 2009, Li et al., 2008). When their particle size is below 25 nm, silver nanoparticles exhibit similar antibacterial properties as silver cations (Li et al., 2008). Although the mechanism of nanosilver remains disputed, it is somewhat accepted that these effects could be related either to an increased reactivity of the particles due to high active surface or to the increase in released free silver cations or radicals when exposed to water (Nair and Laurencin, 2007; Li et al., 2008). In view of the above, manufacturers facing the design of a silver-based antimicrobial polymer should have to confront all issues affecting the stability of silver nanoparticles. These include the sizes, shapes and distribution of the particles, the specific material in which they are incorporated, and the chemical environment where the material has to exert its effect. The combination of these factors will govern the antibacterial efficacy on the surface of the material as well as the release to the environment of particles or free cations and their reactivity.

13.1.4 Effects on human health Silver cations are not a cause of concern to humans, as they are rapidly inactivated by biomolecules. However, if bigger quantities are ingested, deposits of silver can in the long term result in a brownish discoloration of the skin called argyria (Russell and Hugo, 1994). The effects of silver nanoparticles on human health are still unknown, though it has been reported that silver nanoparticles easily pass through the gastrointestinal and respiratory barriers, and increase oxidative mitochondrial activity (Chen and Schluesener, 2008). On the other hand, promising benefits to

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human health are expected to be gathered from the study of silver nanoparticles, as they seem to possess anti-inflammatory properties, and promote apoptosis, for which they could be used in the fight against cancer (Nair and Laurencin, 2007; Bhattacharya and Mukherjee, 2008).

13.2

Incorporation of silver into coatings and polymer matrices

13.2.1 Preparation of silver nanoparticles: mastering the size and shape of nanosilver One of the most important steps in the design of a nanosilver-based material is the procuration of silver particles on the nanoscale. This step is most critical, as it will govern the size, shape, size distribution and stability of the nanoparticles. This procedure can be approached in two different ways. The first one, the socalled top-down technique, consists of mechanically reducing the size of silver metal in its bulk form to the nanoscale (Tolaymat et al., 2010). This can be done with the help of specialized methodologies such as nanolithography or laser ablation. These techniques have lower running costs and are easy to automate and no solvents are used in the process. However, up to August 2008, less than 5% of all scientific articles related to silver nanoparticles focused on these methods (Tolaymat et al., 2010). This is mainly attributed to surface imperfection of the formed particles as well as to the difficulties in adapting experimental conditions to shift the shape and size of the particles. The second and predominantly used approach is the bottom-up or self-assembly technique, which involves the dissolution of a silver salt into a solvent and its subsequent reduction. As most studies deal with the latter approach, the rest of the section will refer to the particularities of chemical, physical and biological reduction of silver salts to form nanoparticles, their stabilization and the influence of this process on the size and shape of the nanoparticles. Nitrate, though considered a drinking water contaminant, is the most commonly used counteranion for silver in the preparation of nanoparticles from silver salts. This is due to its low cost, chemical stability and high solubility in water. It appears from studies with other counteranions like acetate, perchlorate or sulfate that its role is not crucial in the process (Tolaymat et al., 2010). Water is the most commonly used solvent, followed by ethanol, dimethylformamide (DMF) or ethyleneglycol (EG), among others, depending on the application (Tolaymat et al., 2010). The reduction of the salt is the critical step and can be approached by physical, chemical or biological means. Physical reduction The yield of silver nanoparticles by physical irradiation of a silver salt solution has been demonstrated with UV, gamma-ray, microwave and ultrasonic

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irradiation (Sharma et al., 2009; Nair and Laurencin, 2007). The size of the particles is dose-dependent so that higher intensities yield smaller particles (Nair and Laurencin, 2007). The main advantage over chemical or biological methods is easily controlled reduction rates without excess of reducing agent, oxidation by-products or other unwanted interfering compounds (Nair and Laurencin, 2007). On the other hand, these techniques are usually associated with low production rates and high expense. Among the most interesting studies, photoinduced conversion of silver nanospheres to nanoprisms with fluorescent light has proven the feasibility of tailoring the size and shape of nanoparticles with irradiation methods, which in turn determines the scattering properties, reactivity or antibacterial effectiveness of the nanoparticles. Chemical reduction Chemical reduction is one of the most prevalent methods to synthesize colloidal metal particles, because of its convenient operation and simple equipment needed. Among all chemical reducing agents, sodium borohydride, as a strong reductant, and citrate, as a weak reductant, are most commonly used (Tolaymat et al., 2010). Strong reducing agents yield smaller nanoparticles compared to weak reducing agents. However, due to excess surface energy and high thermodynamic instability, smaller particles rapidly undergo alternating nucleation and Ostwald ripening leading to agglomeration into oligomers, clusters and eventually precipitates (Nair and Laurencin, 2007). Therefore, different strategies have been developed to increase the stability of the silver colloids. One approach is a two-step process where a strong reductant, mostly NaBH 4, is used to initiate the reaction, forming small particles. These particles in a second step are enlarged with a weaker reductant (Sharma et al., 2009). Another approach involves using Tollens or modified Tollens methods. These imply the stabilization of the silver cations with ammonia. According to the basic Tollens method, the Ag(NH3)2 complex is afterwards reduced with an aldehyde. However, research has focused more deeply on so-called `green' synthesis, where the silver ammoniacal solution is reduced by environmentally friendlier reductants like saccharides, ascorbic acid or polyols (Sharma et al., 2009). When using weak reductants, the reaction rate can be intensified if the temperature is increased. Whatever the strategy, the stability of the nanoparticles is usually further supported with a stabilizer or `capping agent'. The capping agent introduces electrostatic or steric impediments preventing the coalescence of the particles, but it can also play an important role in the size, size distribution and biocompatibility of the nanomaterials (Nair and Laurencin, 2007). Among the different compounds used as capping agents we find mostly surfactants like cetyl trimethyl ammonium bromide (CTAB), sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and triton X-100, which allow the formation of micelles, vesicles or microemulsions, polymers like poly(vinyl pyrrolidone) (PVP),

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polyacrylates, polyacrylamides, poly(ethylene glycol) (PEG) or poly(vinyl alcohol) (PVA), and biomolecules like heparin or starch (Tolaymat et al., 2010; Wang et al., 2005; Nair and Laurencin, 2007). Many of these not only act as capping agents but also take an active part in the reduction mechanism of silver, like PVP. In combination with heating, some are able to reduce silver cations even in the absence of reducing agents (with a lower yield), like some microemulsions, starch or heparin. Biological reduction The interest in biological synthesis of silver nanoparticles versus physical or chemical methods has been growing in the last few years. Since early studies on biosorption of metals on bacteria inferred that bacterial cells could bind metals at the nanoscale (Vaidyanathan et al., 2009), researchers have put forth the capability of a great number of biological substrates to reduce silver. Most studies focus on bacteria, predominantly bacilli, and fungi, but there are also studies with plant extracts from alfalfa, lemongrass, geranium leaves or cinnamon (Vaidyanathan et al., 2009; Sharma et al., 2009). The particles are either deposited onto microbial constituents or secreted to the culture supernatant, their size varying from tenths to hundreds of nanometres depending on the substrate. The mechanism suggested for the process implies the enzymatic reduction of silver nitrate by nitrate reductase together with other electron shuttling compounds (Vaidyanathan et al., 2009). Biological methods present an interesting alternative over physical or chemical methods. They usually involve less capital, avoid the use of hazardous chemicals and provide a biomolecularly rich environment that naturally acts as a capping agent. Furthermore, the usually lower kinetics offer the possibility of better manipulation and control over crystal growth (Vaidyanathan et al., 2009; Sharma et al., 2009). In this sense, genetic engineering techniques could constitute a very useful tool in turning the production of nanosilver into a profitable cost-effective market.

13.2.2 Silver-based nanocomposites: from the medical field to everyday life Starting point: the medical field As commented in Section 13.1.2, though nanosilver is being marketed for countless applications, the driving force in the innovative search for new silverbased antimicrobial nanocomposites was and still is reliant mainly on the medical and healthcare sector. Half of all nosocomial infections in hospitals are caused as a consequence of the implantation of an indwelling device. Typical antibiotic therapies are ineffective against biofilm formation and promote fast development of resistances (Hetrick and Schoenfisch, 2006). The high demand

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for medical devices with antimicrobial properties and the high economic impact of these products promote research attention in this sense. Silver products used to treat infection can be classified in two categories depending on the presence of either silver ions (Ag+) or silver nanoparticles (Ag0). Initially, catheters and fixation pins were coated with metallic silver with little or no success, as silver ions were not sufficiently released from the materials (Hetrick and Schoenfisch, 2006). The incorporation of silver sulfadiazine (a silver nitrate and sulfamide combination historically used for the treatment of burns) in hydrogels, alginates or foam formulations has enabled the rise of different wound dressings (SilvercellÕ, UrgotulÕ, SilverexÕ, etc.) or catheters (e.g. Bardex IcÕ) releasing silver ions. In the second category, different dressings have been embedded or coated with nanocrystalline silver, e.g ActisorbÕ (charcoal), Aquacel-AgÕ (carboxymethyl cellulose), SilverIonÕ, ContreetÕ (polyurethane foam) and ActicoatÕ (PE) (Ip et al., 2006; Bhattacharya and Mukherjee, 2008; Lo et al., 2007). Nanocrystalline silver dressings prove to be generally more effective than silver ion release dressings (Ip et al., 2006). Furthermore, bone cement (PMMA) and hydroxyapatite doped with nanosilver have shown good antibacterial efficacy without compromising osteoblasts and epithelial cells (Zhao et al., 2009). Techniques and materials Due to its high thermal stability, silver can be introduced into polymers by many well-established techniques which assure homogeneous distribution of the nanoparticles on the surface. These include plasma immersion ion implantation (PIII), pulse filtered cathodic vacuum arc deposition, autocatalytic electroless chemical plating (AECP), magnetron sputtering or physical vapour deposition (PVD). The last method, used for example in the fabrication of ActicoatÕ, allows deposition of a 15 nm thick sheet of silver nanoparticles homogeneously distributed (Dunn and Edwards-Jones, 2004). AECP has been used to coat silver nanoparticles on silica spheres with adsorbed tin. When exposed to silver ions, tin oxidizes to simultaneously form and adsorb silver nanoparticles (Nair and Laurencin, 2007). Magnetron sputtering enables tuning of the dissolution kinetics of the particles but lacks adhesion to the substrate (Nair and Laurencin, 2007). But when facing applications with less economic impact other techniques must be investigated. Being a simple and relatively efficient process, the reduction of silver once inside the material, so called self-assembly, could be a more cost-effective solution in the fabrication of nanosilver products. While casting techniques experience aggregation and thus inhomogeneous size distribution (Sharma et al., 2009), physical techniques like UV or microwave irradiation, thermal annealing or sonochemical processes are easy to apply with good results in materials like polycarbonate (Nair and Laurencin, 2007), PVP or PVA (Sharma et al., 2009). Polyelectrolyte multilayers allow the

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immobilization of silver ions, silver reduction and alternative deposition of oppositely charged polymers where particle concentration and size can be varied with the processing conditions (Shi et al., 2004). Thermal annealing of silver nanoparticles has been demonstrated in polyacrylonitriles (PAN), polyacrylates and PVA. In PAN the reduction conveniently occurs during in-situ polymerization (Sharma et al., 2009). Chemical in-situ reduction of silver-based polymers has also been studied in a Tollens process (explained in Section 13.2.1), mostly on PVP. In these cases, size and size distribution can be tuned by varying the stoichiometry of silver and ammonia (Wang et al., 2005). These processes can also be combined with UV irradiation. The immersion of the polymer in a silver colloidal solution is followed by enlargement of these silver seeds with a weak reductant under UV light (Jia et al., 2006). In addition to all these strategies, the creation of functional groups in the polymer can promote or enhance adhesion of the nanoparticles. Polycarbonate was UV-etched and silanized to produce amino groups which allowed the deposition of nanoparticles on the surface (Aslan et al., 2006). Chromia surfaces have been modified with 3-mercaptopropyl trimethoxysilane. The polyols formed were able to reduce surface-adsorbed silver ions into nanoparticles (Kim et al., 2007). Hydroxyl groups in hydroxyapatite are able to bind silver by electron transfer to the ions. One promising field of interest is the combination of silver with TiO2. TiO2 in itself causes photocatalytic inactivation of bacteria. Recently, it was discovered that the antimicrobial activity of TiO2 is greatly enhanced when the material is reduced to the nanoscale, being even able to kill viruses (Li et al., 2008). Furthermore, it has been found that doping TiO2 surfaces with silver has a synergistic effect on their antimicrobial efficacies. Silver absorption of UVlight induces electron transfer to TiO2 resulting in charge separation and thus activation by visible light, improving the antibacterial effect of TiO2 (Li et al., 2008). In view of these findings, much interest has arisen in formulating silvertitania doped materials or coatings on either the micro- or the nanoscale.

13.2.3 Antimicrobial effectiveness: what to have in mind Apart from the fabrication of the material, there are other issues which have to be faced by manufacturers, legislators, researchers and consumers. As explained before (Section 13.1.3), the antimicrobial efficacy of the material relies on the leaching of silver nanoparticles or silver ions to the surface and surrounding environment. Clinical studies on collagen cuffs (Pascual, 2002) and silicon discs (Hetrick and Schoenfisch, 2006; Rai et al., 2009) incorporating nanosilver have demonstrated that the mere presence of nanoparticles does not ensure antimicrobial efficacy. Thus, it is crucial to elucidate the release kinetics of the material in question and evaluate the equilibrium between reactivity of the particles on bacteria and their stability in a specific environment.

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Size and shape The size is a very important aspect considering that thermodynamic properties are directly proportional to the diameter of the nanoparticles. Although antibacterial efficacy has been proven with bigger particles (Rai et al., 2009), reviews set the optimum range between 3 and 10 nm (Tolaymat et al., 2010). When the colloidal particles are smaller than the wavelength of visible light, the solutions acquire a yellow to brownish colour with an intense band at the 380± 420 nm range. This unique property, known as surface plasmon resonance, is attributed to collective excitation of the electron gas in the particles with a periodic change in electron density at the surface. The band shifts to longer wavelengths with increased particle size (Sharma et al., 2009), accompanied with a change in colour. Accordingly, monitoring the appearance, position and thickness of this band constitutes a very useful tool to roughly predict the yield, size and size distribution. Particle morphology as investigated by TEM, HRTEM or XRD is predominantly spherical, but other shapes like nanocubes or nanoprisms cannot be overlooked (Wang et al., 2005; Sharma et al., 2009; Wiley et al., 2005). Pal et al. (2007) produced truncated triangular nanoplates and nanorods by the solution-phase method and nanospheres by the seeded growth method. They observed different plasmon absorbances depending on the shape of the particles (418 nm for triangular, 420 nm for spherical, and 514 nm for rodshaped particles). Comparing their efficacy on E. coli, they found bactericidal concentrations of about 1, 12.5 and 50±100 ppm for triangular nanoplates, nanospheres and nanorods, respectively. Thus, they demonstrated that bacterial adhesion of silver is favoured by high-atom-density facets such as {111} as proposed by Morones et al. (2005). Release issues and inactivation Another key aspect influencing the efficacy of nanosilver-based materials is the release kinetics of silver ions or nanosilver to the surface and surrounding environment and their stability in it. These are governed by factors depending on the material and the environment in contact with it. Concerning the polymer, the release will depend mainly on its water-uptake capacity. Kumar and MuÈnstedt published several studies with silver±polyamide nanocomposites measuring silver release by anodic stripping voltammetry during up to three months. They found release kinetics could be modified based on the crystallinity of the polymer. Furthermore, introducing hygroscopic fillers in the matrix or in a multilayer coating greatly enhanced release kinetics of the nanocomposite and its biocidal efficacy (Kumar and MuÈnstedt, 2005a,b; Kumar et al., 2005; Damm and MuÈnstedt, 2008). Dowling et al. (2003) approached the challenge of tuning release kinetics by incorporating platinum into a coating on polyurethane. Since platinum has a higher redox potential, silver oxidation is enhanced when the two metals are in contact, increasing release of free silver ions from the polymer.

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The release from the material can also depend on the characteristics of the aqueous or moistured environment in contact with the surface of the material. As an example, tests on human plasma have revealed that a greater amount of ions is released in these conditions than in deionized water (Rai et al., 2009). But the role of the release environment in release kinetics is not so important compared to the enormous influence it can exert on the stability of silver ions or nanoparticles. If the material is to show antimicrobial efficacy, the released silver species must be reactive enough to be able to bind bacterial constituents. At the same time, however, they have to remain stable enough not to be inactivated by biomolecules or organic matter probably present in the release environment. Despite the widespread use of nanosilver-based materials, there is still much to learn about factors affecting silver stability. While studies focus on the characterization of the silver particles and the release rates from the different materials, bactericidal concentrations prove to be more strongly related to the growth conditions of the bacterial culture. This is easily evidenced when looking at released silver bactericidal concentrations among the different publications. These go from the ppb range when water or salt buffers are used (Kim et al., 1998; Hwang et al., 2007; Bjarnsholt et al., 2007) to hundreds of ppm when complex growth media like LB, TSB or MHB come into play (Sondi and Salopek-Sondi, 2007; Thomas et al., 2007; Ruparelia et al., 2008). These huge differences in antibacterial response (up to four orders of magnitude) put forth the need for standardizing biocidal tests if the potential of different materials throughout the literature is to be compared (Chopra, 2007). This pending task is difficult due to the complexity of silver bioavailability and speciation issues. It is well known that silver ions form very stable complexes with sulfur groups and chlorides. However, silver chloride colloids retain antimicrobial activity (Choi et al., 2008) and interactions between naturally organic matter (NOM) and human plasma remain still unclear.

13.3

Antimicrobial silver in food packaging

13.3.1 Introduction: active packaging and silver Active packaging is one of the innovative food packaging concepts that have been introduced in response to the continuous changes in current consumer demands and market trends. Among these active functions we find scavenging of oxygen, moisture or ethylene, emission of ethanol and flavours, and antimicrobial activity. In antimicrobial packaging, a substance with biocidal properties is included in the packaging system to extend shelf-life and reduce the risk of contamination by pathogens (Quintavalla and Vicini, 2002). This task is approached by different strategies, including: · Addition of sachets or pads containing volatile antimicrobial agents into packages

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· Incorporation of volatile and non-volatile antimicrobial agents directly into polymers · Coating or adsorbing antimicrobials onto polymer surfaces · Immobilization of antimicrobials to polymers by ion or covalent linkages · Use of polymers that are inherently antimicrobial. The most successful commercial application of active packaging has been sachets enclosed loose or attached to the interior of a package mostly containing moisture absorbers, oxygen scavengers or ethanol vapour generators (Appendini and Hotchkiss, 2002). But for antimicrobial packaging, this approach is only feasible with volatile compounds, which severely limits the range of available antimicrobials. The direct application of antibacterial substances onto foods has limited benefits because the active substances are neutralized by product constituents on contact or diffuse rapidly from the surface into the food mass. The incorporation of antimicrobials into polymers constitutes a solution, as it allows the biocidal substance to be released from the package during an extended period, prolonging its effect into the transport and storage phase of food distribution (Quintavalla and Vicini, 2002). Among the different antimicrobials which have been incorporated in polymers for food packaging applications we find organic acids, enzymes, bacteriocins, fungicides and other preservatives (Coma, 2008; Appendini and Hotchkiss, 2002). In previous sections, the extended use of silver in the medical field has been reviewed, showing that its outstanding cost-effectiveness and wide range of applicability are unmatched by other antimicrobial substances. Therefore, it is not surprising that silver has also colonized the active packaging sector, being the most widely used polymer additive in food applications (Quintavalla and Vicini, 2002; Appendini and Hotchkiss, 2002).

13.3.2 Current applications: ion-exchange from minerals Although it is the most commonly used additive for antimicrobial food packaging, one single form of silver-releasing systems predominates in the sector: ion-exchange from microporous minerals. In this technology, naturally occurring sodium ions in clays or other porous minerals are partially replaced by silver ions using ion-exchange methods (Rai et al., 2009). In contact with moisture, silver ions are again substituted by sodium ions present in the release environment and sustainably leach from the surface (Fig. 13.1). This is practical, as release of silver ions will depend on the amount of saline moisture, which is a crucial risk factor for the development of microbes on surfaces. The substituted minerals are incorporated or coated into a wide range of polymers and other surfaces. A combination of low migration rates and high melting points makes them able to withstand any kind of plastic processing or operating temperature in contrast to other antimicrobial natural substances (Simpson, 2003). Among the

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13.1 Silver ion exchange mechanism in silver zeolites upon contact with moisture.

minerals used, we find different clays like montmorillonites (MMT) (Praus et al., 2009) or tobermorites (Coleman et al., 2009), silver zirconium phosphates (Simpson, 2003) and silver zeolites (Cowan et al., 2003; Galeano et al., 2003; È lkuÈ, 2008). In the Matsumura et al., 2003; Nakane et al., 2006; Akdeniz and U food sector, these materials are usually manufactured as a 3±6 m thick layer with 1±5% silver content. This layer is then coated on polymeric or stainless steel surfaces of any food processing equipment: cutlery, cutting boards, counter tops, containers, etc. The silver zeolites are the minerals most widely represented commercially under brands like ZeomicÕ, ApaciderÕ (Sangi Co.), BactekillerÕ, NovaronÕ, BiomasterÕ (Addmaster), IrgaguardÕ (Ciba Specialty Chemicals), AgIonÕ or BiocoteÕ (Simpson, 2003; Quintavalla and Vicini, 2002). In most cases, the food sector is only one area of application among many others, such as in textiles, hygiene products, electronics, houseware, etc. (see customers listed in the Internet sites in Section 13.5.2). Silver zirconium phosphates with brands like AlphasanÕ (Milliken Chemical) or FosfargolÕ, also manufactured on the same principle of sodium±silver ion exchange, are oriented to the food and medical sectors, respectively. AlphasanÕ is applied in extrusion moulding for the production of conveyors, or as liners in ice-making machines. BactiblockÕ (Nanobiomatters Ltd) since 2008 has provided commercially available proprietary nanoclay-based antimicrobials. The MMT-based nanoclays have been found to reduce the discoloration problems usually associated with silver zeolites and offer at least the same prospects as the zeolites (Busolo et al., 2010; www.nanobiomatters.com).

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13.3.3 Regulatory issues Regulation of silver, silver ions or nanosilver in the food and food-contact area is confronted differently by the various legislative bodies in Europe, USA, Japan and Australia, becoming even contradictory in some cases. On the one hand, historical use of silver has proved its benefits and safety for society throughout the centuries during which people have been in contact with it. On the other hand, new techniques of silver ion release and the emergence of nanotechnology could pose a threat to the environment or to human health. The confrontation between past and present circumstances is most noticeable in European law. Under Directive 94/36/EC, still in force, silver is considered a colouring agent used in confectionery without any restriction limits. However, when looking at Directive 2002/72/CE for `materials and articles intended to come into contact with foodstuffs' (food-contact law), any form of silver is not yet considered. The European Food Safety Authority (EFSA), in charge of evaluating new petitions, included silver zeolites up to 5%, silver zirconium phosphate, and silvercontaining glasses up to 3% in its provisional list of additives in plastics (EFSA Journal, 2006) with a restriction of a maximum of 0.05 mg/kg food for the whole group. As from 1 January 2010, the list under Directive 2002/72/CE has become a positive list, excluding all other food-contact materials. Substances in the provisional list by the EFSA may continue to be used according to national legislation after this date until a decision on their inclusion or non-inclusion in the positive list is taken (EFSA provisional list of additives used in plastics, third update, 17/02/2009). In the US and especially in Japan, the use of silver zeolites is well established with several commercial brands approved for food packaging (Quintavalla and Vicini, 2002; Appendini and Hotchkiss, 2002; Internet sites). Substances leaching from food contact materials in these countries fall under food additive legislation. In this list, only silver nitrate is regulated, with a maximum limit of 0.017 mg/kg (FDA/CFSAN). As far as nanosilver is concerned, colloidal solutions are accepted in the US and commercialized as nutrition supplements (e.g. MesosilverÕ), claiming to have important benefits on human health (www.mesosilver.com). In Europe, however, the EFSA did not include silver hydrosols in the list of additives `because of lack of appropriate information about silver bioavailability' from them (EFSA Journal). Other countries, like Australia, have totally banned the use of silver in foodstuffs.

13.4

Future trends

When reviewing scientific literature concerning silver nanoparticles, two clear goals seem to predominate. First, there is the effort to discover the method to synthesize silver nanoparticles which best matches in size, stability, simplicity and cost-effectiveness. In this respect, chemical reduction methods still predominate in number, but green synthesis has newly emerged and attracted

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Table 13.1 Relative frequency of scientific articles dealing with silver-based nanocomposites according to the material chosen for incorporation. Only the three most recent references are cited in each case Materials

Prevalence

Polyacrylates/ polyacrylamides Polyamides

11.8%

Cellulose

8.1%

Cotton

5.9%

Chitosan

5.1%

PE/PEO

5.1%

Gelatines/agars/ alginates Glass

4.4%

Ceramics/ polyphosphates Clays

2.9%

PU

2.9%

PS

2.9%

PVA

2.9%

PLA PCL Others

1.5% 0.7% 29.7%

8.8%

4.4%

2.9%

References HÌntzschel et al., 2009; Mohan et al., 2009; Thatiparti et al., 2009 Park et al., 2009b; Dong et al., 2008; Rangari et al., 2008 Tankhiwale and Bajpai, 2009; Pinto et al., 2009; Zhu et al., 2009 Ilic et al., 2009; Khalil-Abad et al., 2009; Yazdanshenas et al., 2009 Yoksan and Chirachanchai, 2009; Wei et al., 2009; Vimala et al., 2009 An et al., 2009b; Sanchez-Valdes et al., 2009; Chen et al., 2008 Ayyad et al., 2009; Rattanaruengsrikul et al., 2009; Tofoleanu et al., 2008 Lv et al., 2008; Perkas et al., 2008; EstebanTejeda et al., 2009 Loher et al., 2008; Lv et al., 2009; Du et al., 2009 Maga·a et al., 2008; Carja et al., 2009; Park et al., 2009b Jeon et al., 2008; Thatiparti et al., 2009; Sheikh et al., 2009 An et al., 2009a; Dutra et al., 2008; Paula et al., 2009 Park et al., 2009a; Galya et al., 2008; Liu et al., 2009 Yu et al., 2007; Xu et al., 2008 Jeon et al., 2008 ±

much attention. Second, there is the search for new materials with nanosilver technology. Table 13.1 presents a classification of scientific articles according to the material chosen for incorporation of the nanoparticles. The table is based on an electronic search conducted on the Scopus database with the keywords `silver nanoparticles' and including `polymer, plastic or nanocomposite' in the abstract. According to the table, the most commonly used polymers are polyacrylamides and polyacrylates. The reason why these are mostly selected is probably the simplicity in the preparation of these silver hydrogels and the control over the reduction process. Cellulose and cotton are materials in which a silver ion solution can be easily absorbed to be subsequently reduced to silver nanoparticles, being third and fourth in predominance. The group of polyamides is second in importance probably because it includes polymers which could be used for prostheses or implants as well as for textile applications. Polymers

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typically used for food applications, like polyolefins, do not have great representation, and the studies do not specifically focus on the food sector. Among these publications, we also find combinations of silver with other additives like carbon nanotubes, genipin, antibiotics, activated carbon and others, but the most used combination is silver±TiO2 nanocomposites. The search for new materials not only implies the mastering of the nanoparticle procuration (size, size distribution and stability) but should also take into account release kinetics and possible inactivation of the silver in the specific environment. These last issues are now increasingly beginning to be taken into consideration, and constitute a future perspective, indispensable if the material is finally to find specific applications, such as, for example, in the food area. Up to the present, only slow silver ion releasing systems, like zeolites, nanoclays or zirconium phosphates, have hit the food industry. The high relative silver content (2±5%) of these materials could limit their expansion into food packaging. Biodegradable polymers, mostly chitosan due to its inherent antimicrobial properties, but also poly vinyl alcohol (PVA), poly(caprolactone) (PCL) and poly(lactic acid) (PLA), have also been objects of study in the incorporation of silver nanoparticles (Table 13.1). Due to their environmentally friendly nature, biopolymers show an upward trend in the food packaging sector. The combination of silver nanotechnology with these polymers could allow tuning of the release kinetics so that much smaller amounts of silver would be needed to be incorporated in the polymer, reducing the economic and environmental costs. The release of the silver nanoparticles would be activated once in contact with foodstuffs and extended over the desired shelf-life period until exhaustion. How legislators and public opinion will confront a possible introduction of antimicrobial food packaging systems based on nanosilver is uncertain. But nanosilver has already established itself in the healthcare and consumer product markets in an upward trend. Assuming that many existing products, like textiles in clothes or toys accepted in the American and Japanese markets, actively release silver which might well be ingested by the consumer, it would seem contradictory not to accept nanosilver in food-contact applications. Future findings will help ascertain whether this problem will result in an expansion or a limitation of nanosilver applications.

13.5

Sources of further information and advice

13.5.1 Legislation · European Commission Directive 2002/72/EC of 6 August 2002 relating to plastic materials and articles intended to come into contact with foodstuffs · European Parliament and Council Directive 94/36/EC of 30 June 1994 on colours for use in foodstuffs

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· The EFSA Journal (2006) 395 to 401, 1-21 Opinion of the Scientific Panel on food additives, flavourings, processing aids and materials in contact with food (AFC) on a request related to a 12th list of substances for food contact materials · The EFSA Journal (December) 2008 Issue 12 Scientific Statement of the Panel on Food Additives and Nutrient Sources added to Food (ANS) (Question no. EFSA-Q-2005-169) · US FDA/CFSAN listing of food additive status. Silver nitrate-172.167.

13.5.2 Internet sites www.stinkatnothing.com www.alphasan.com www.mesosilver.com www.sangi-co.com www.irgaguard.com www.agion-tech.com www.biocote.com www.crocsrx.eu www.astp.com www.ecopolynae.com www.silverphase.fi www.laboratorios-argenol.com www.nanobiomatters.com

13.6

References and further reading

È lkuÈ S. (2008) `Thermal stability of Ag-exchanged clinoptilolite rich Akdeniz Y, U mineral' J Therm Anal Calorim 3: 703±710 An J, Wang D, Luo Q, Yuan X (2009a) `Antimicrobial active silver nanoparticles and silver/polystyrene core±shell nanoparticles prepared in room-temperature ionic liquid' Mat Sci Eng C 29: 1984±1989 An J, Zhang H, Zhang J, Zhao Y, Yuan X (2009b) `Preparation and antibacterial activity of electrospun chitosan/poly(ethylene oxide) membranes containing silver nanoparticles' Colloid Polym Sci 287: 1425±1434 Appendini P, Hotchkiss JH (2002) `Review of antimicrobial food packaging' Innovative Food Sci Emerging Technol 3: 113±126 Aslan K, Holley P, Geddes CD (2006) `Metal-enhanced fluorescence from silver nanoparticle-deposited polycarbonate substrates' J Mater Chem 16: 2846 Ayyad O, MunÄoz-Rojas D, Oro-Sole J, Gomez-Romero P (2009) `From silver nanoparticles to nanostructures through matrix chemistry' J Nanopart Res doi 10.1007/s11051-009-9620-3 Bhattacharya R, Mukherjee P (2008) `Biological properties of ``naked'' metal nanoparticles' Adv Drug Delivery Rev 60: 1289±1306 Bjarnsholt T, Kirketerp-Moller K, Kristiansen S, Phipps R, Nielsen AK, éstruo P, Hoiby

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N, Givsko M (2007) `Silver against Pseudomonas aeruginosa biofilms' APMIS 115: 921±928 Busolo MA, Fernandez P, Ocio MJ, LagaroÂn JM (2010) `Novel silver-based nanoclay as an antimicrobial in polylactic acid food packaging coatings' Food Additives & Contaminants: A published 13 August 2010 (iFirst): doi 10.1080/ 19440049.2010.506601, URL: http://dx.doi.org/10.1080/19440049.2010.506601 Carja G, Kameshima Y, Nakajima A, Dranca C, Okada K (2009) `Nanosized silver± anionic clay matrix as nanostructured ensembles with antimicrobial activity' Int J Antimicrob Agents 34: 534±539 Chen Q, Yue L, Xie F, Zhou M, Fu Y, Zhang Y, Weng J (2008) `Preferential facet of nanocrystalline silver embedded in polyethylene oxide nanocomposite and its antibiotic behaviors' J Phys Chem C 112: 10004±10007 Chen X, Schluesener HJ (2008) `Nanosilver: a nanoproduct in medical application' Toxicol Lett 176: 1±12 Choi O, Deng KK, Kim NJ, Ross L, Surampallie RY, Hua Z (2008) `The inhibitory effects of silver nanoparticles, silver ions and silver chloride colloids on microbial growth' Water Res 42: 3066±3074 Chopra I (2007) `The increasing use of silver-based products as antimicrobial agents: a useful development or a cause for concern?' J Antimicrob Chemother 59: 587±590. Coleman NJ, Bishop AJ, Booth SE, Nicholson JW (2009) `Ag+- and Zn2+-exchange Ê tobermorites' J Eur Ceram Soc 29: kinetics and antimicrobial properties of 11A 1109±1117. Coma V (2008) `Bioactive packaging technologies for extended shelf life of meat-based products' Meat Sci 78: 90±103 Cowan MM, Abshire KZ, Houk SL, Evans SM (2003) `Antimicrobial efficacy of a silverzeolite matrix coating on stainless steel' J Ind Microbiol Biotechnol 30(2): 102±106 Damm C, MuÈnstedt H (2008) `Kinetic aspects of the silver ion release from antimicrobial polyamide/silver nanocomposites' Appl Phys A: Mater Sci Proc 91: 479±486 Dong H, Wang D, Sun G, Hinestroza JP (2008) `Assembly of metal nanoparticles on electrospun Nylon 6 nanofibers by control of interfacial hydrogen-bonding interactions' Chem Mater 20(21): 6627±6632 Dowling DP, Betts AJ, Pope C, McConnell ML, Eloy R, Arnaud MN (2003) `Antibacterial silver coatings exhibiting enhanced activity through the addition of platinum' Surf Coat Technol 163±164: 637. Du W, Niu S, Xu Y, Xu Z, Fan C (2009) `Antibacterial activity of chitosan tripolyphosphate nanoparticles loaded with various metal ions' Carbohydr Polym 75: 385±389 Dunn K, Edwards-Jones V (2004) `The role of ActicoatTM with nanocrystalline silver in the management of burns' Burns 30 Suppl. 1 SlLS9 Dutra R, Segala K, Oliveira E, de Souza EP, Rossi L, Matos J, Noda L, Paula M, Franco C (2008) `Preparation and characterization of the novel terpolymers of poly{trans[RuCl2(vpy)4]-styrenedivinylbenzene} and styrene-divinylbenzene-vinylpiridine impregnated with silver nanoparticles' Polym Bull 60: 809±819 Esteban-Tejeda L, Malpartida F, Esteban-Cubillo A, PecharromaÂn C, Moya JS (2009) `The antibacterial and antifungal activity of a soda-lime glass containing silver nanoparticles' Nanotechnology 20(8): 085103 Galeano B, Korff E, Nicholson WL (2003) `Inactivation of vegetative cells, but not spores, of Bacillus anthracis, B. cereus, and B. subtilis on stainless steel surfaces coated with an antimicrobial silver- and zinc-containing zeolite formulation' Appl Environ Microbiol 69(7): 4329±433.

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Galya T, Sedlark V, Kuritka I, Novotny R, Sedlarkova J, Saha P (2008) `Antibacterial poly(vinyl alcohol) film containing silver nanoparticles: Preparation and characterization' J Appl Polym Sci 110: 3178±3185 Goyal A, Kumar A, Patra PK, Mahendra S, Tabatabaei S, Alvarez PJ, John G, Ajayan PM (2009) `In situ synthesis of metal nanoparticle embedded free standing multifunctional PDMS films' Macromol Rapid Commun 30: 1116±1122 Gupta A, Silver S (1998) `Silver as a biocide: Will resistance become a problem?' Nat Biotechnol 6: 888 HaÈntzschel N, Hund RD, Hund H, Schrinner M, LuÈck C, Pich A (2009) `Hybrid microgels with antibacterial properties' Macromol Biosci 9, 444±449 Hetrick EM, Schoenfisch MH (2006) `Reducing implant-related infections: active release strategies' Chem Soc Rev 35: 780±789 Hwang MG, Katayama H, Ohgaki S (2007) `Inactivation of Legionella pneumophila and Pseudomonas aeruginosa: Evaluation of the bactericidal ability of silver cations' Water Res 41: 4097±4104 Ilic V, SÏaponjic Z, Vodnik V, Potkonjak B, Jovancic P, Nedeljkovic J, Radetic M (2009) `The influence of silver content on antimicrobial activity and color of cotton fabrics functionalized with Ag nanoparticles' Carbohydr Polym 78: 564±569 Ip M, Lai Lui S, Poon VM, Lung I, Burd A (2006) `Antimicrobial activities of silver dressings: an in vitro comparison' J Medical Microbiol 55: 59±63 Jeon HJ, Kim JS, Kim TG, Kim JH, Yu WR, Youk JH (2008) `Preparation of poly(ecaprolactone)-based polyurethane nanofibers containing silver nanoparticles' Appl Surf Sci 254: 5886±5890 Jia H, Zeng J, Song W, An J, Zhao B (2006) `Preparation of silver nanoparticles by photoreduction for surface-enhanced Raman scattering' Thin Solid Films 496: 281 Khalil-Abad MS, Yazdanshenas ME, Nateghi MR (2009) `Effect of cationization on adsorption of silver nanoparticles on cotton surfaces and its antibacterial activity' Cellulose 16: 1147±1157 Kim D, Lee J, Kang P, Kim YC, Oh S (2007) `Formation and immobilization of silver nanoparticles onto chromia surface by novel preparation route involving polyol process' Surf Coat Technol 201: 7663 Kim TM, Feng QL, Kim JO, Wu J, Wang H, Chen GC, Cui FZ (1998) `Antimicrobial effects of metal ions (Ag+, Cu2+, Zn2+) in hydroxyapatite' J Mater Sci: Mater Med 9: 129±134 Klasen HJ (2000) `A historical review of the use of silver in the treatment of burns. Part I. Early uses' Burns 30: 1±9 Kumar R, MuÈnstedt H (2005a) `Silver ion release from antimicrobial polyamide/silver composites' Biomaterials 26: 2081 Kumar R, MuÈnstedt H (2005b) `Polyamide/silver antimicrobials: effect of crystallinity on the silver ion release' Polym Int 54: 1180 Kumar R, Howdle S, MuÈnstedt H (2005) `Polyamide/silver antimicrobials: Effect of filler types on the silver ion release' J Biomed Mater Res 75: 311 Li Q, Mahendra S, Lyon DY, Brunet L, Liga MV, Li D, Alvarez PJ (2008) `Antimicrobial nanomaterials for water disinfection and microbial control: Potential applications and implications' Water Res 42: 4591±4602 Liau SY, Read DC, Pugh WJ, Furr JR, Russell AD (1997) `Interaction of silver nitrate with readily identifiable groups: relationship to the antibacterial action of silver ions' Lett Appl Microbiol 25: 279±283 Liu S, He J, Xue J, Ding W (2009) `Efficient fabrication of transparent antimicrobial poly(vinyl alcohol) thin films' J Nanopart Res 11: 553±560

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Lo S, Hayter M, Chang C, Hu W, Lee L (2007) `A systematic review of silver-releasing dressings in the management of infected chronic wounds' J Clin Nursing 17: 1973± 1985 Loher S, Schneider O, Maienfisch T, Bokorny S, Stark W (2008) `Micro-organismtriggered release of silver nanoparticles from biodegradable oxide carriers allows preparation of self-sterilizing polymer surfaces' Small 4(6): 824±832 Lv Y, Liu H, Wang Z, Hao L, Liu J, Wang Y, Du G, Liu D, Zhan J, Wang J (2008) `Antibiotic glass slide coated with silver nanoparticles and its antimicrobial capabilities' Polym Adv Technol 19: 1455±1460 Lv Y, Liu H, Wang Z, Liu S, Hao L, Sang Y, Liu D, Wang J, Boughton RI (2009) `Silver nanoparticle-decorated porous ceramic composite for water treatment' J Membr Sci 331: 50±56 MaganÄa SM, Quintana P, Aguilar DH, Toledo JA, Angeles-Chavez C, Cortes MA, Leon LA, Freile-Pelegrin Y, Lopez T, Torres-Sanchez RM (2008) `Antibacterial activity of montmorillonites modified with silver' J Mol Catal A: Chemical 281: 192±199 Matsumura Y, Yoshikata K, Kunisaki SI., Tsuchido TY (2003) `Mode of bactericidal action of silver zeolite and its comparison with that of silver nitrate' Appl Environ Microbiol 69(7): 4278±4281 Mohan YM, Vimala K, Thomas V, Varaprasad K, Sreedhar B, Bajpai SK, Mohana-Raju K (2009) `Controlling of silver nanoparticles structure by hydrogel networks' J Colloid Interface Sci doi 10.1016/j.jcis.2009.10.008 Morones JR, Elechiguerra JL, Camacho A, Ramirez JT (2005) `The bactericidal effect of silver nanoparticles' Nanotechnology 16: 2346±2353 Nair LS, Laurencin CT (2007) `Silver nanoparticles: Synthesis and therapeutic applications' J Biomed Nanotechnol 3: 301±316 Nakane T, Gomyo H, Sasaki I, Kimoto Y, Hanzawa N, Teshima Y, Namba T (2006) `New antiaxillary odour deodorant made with antimicrobial Ag-zeolite' Int J Cosmetic Sci 28(4): 299±309 Pal S, Tak YK, Song JM (2007) `Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle? A study of the gram-negative bacterium Escherichia coli' Appl Environ Microbiol 27(6): 1712±1720 Park JH, Karim MR, Kim IK, Cheong IW, Kim JW, Bae DG, Cho JW, Yeum JY (2009a) `Electrospinning fabrication and characterization of poly(vinylalcohol) montmorillonite silver hybrid nanofibers for antibacterial applications' Colloid Polym Sci doi 10.1007/s00396-009-2147-4 Park SW, Bae HS, Xing ZC, Kwon OH, Huh MW, Kang IK (2009b) `Preparation and properties of silver-containing Nylon 6 nanofibers formed by electrospinning' J Appl Polym Sci 112: 2320±2326 Pascual A (2002) `Pathogenesis of catheter-related infections: Lessons for new designs' Clin Microbiol Infect 8: 256±264 Paula MM da S, Franco CV, Baldin MC, Rodrigues L, Barichello T, Savi CD, Bellato LF, Fiori MA, da Silva L (2009) `Synthesis, characterization and antibacterial activity studies of poly-{styrene-acrylic acid} with silver nanoparticles' Mater Sci Eng C 29: 647±650 Perkas N, Amirian G, Applerot G, Efendiev E, Kaganovskii Y, Ghule AV, Chen BJ, Ling YC, Gedanken A (2008) `Depositing silver nanoparticles on/in a glass slide by the sonochemical method' Nanotechnology 19(43): 435604 Pinto R, Marques P, Pascoal-Neto C, Trindade T, Daina S, Sadocco P (2009) `Antibacterial activity of nanocomposites of silver and bacterial or vegetable cellulosic fibers' Acta Biomaterialia 5: 2279±2289

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Praus P, Malachova K, PavlõÂckova Z, Turicova M. (2009) `Activity of antibacterial compounds immobilised on montmorillonite' Appl Clay Sci 43: 364±368 Quintavalla S, Vicini L (2002) `Antimicrobial food packaging in meat industry' Meat Sci 62: 373±380 Rai M, Yadav A, Gade A (2009) `Silver nanoparticles as a new generation of antimicrobials' Biotechnol Adv 27: 76±83 Rangari VK, Mohammad GM, Jeelani S (2008) `Fabrication of nylon nanocomposite fibers for antimicrobial textile applications' in Advances in Heterogeneous Material Mechanics Conference: ICHMM-2008, Huangshan, China Rattanaruengsrikul V, Pimpha N, Supaphol P (2009) `Development of gelatin hydrogel pads as antibacterial wound dressings' Macromol Biosci 9: 1004±1015 Ruparelia JP, Chatterjee AK, Duttagupta SP, Mukherji S (2008) `Strain specificity in antimicrobial activity of silver and copper nanoparticles' Acta Biomaterialia 4: 707±716 Russell AD, Hugo WB (1994) `Anti-microbial activity and action of silver' Prog Med Chem 31: 351±70 Sanchez-Valdes S, Ortega-Ortiz H, Ramos-de Valle LF, Medellin-Rodriguez FJ, GuedeaMiranda R (2009) `Mechanical and antimicrobial properties of multilayer films with a polyethylene/silver nanocomposite layer' J Appl Polym Sci 111: 953±962 Sharma VK, Yngard RA, Lin Y (2009) `Silver nanoparticles: Green synthesis and their antimicrobial activities' Adv Colloid Interface Sci 145: 83±96 Sheikh FA, Barakat NAM, Kanjwal MA, Chaudhari AA, Jung IH, Lee JH, Kim HY (2009) `Electrospun antimicrobial polyurethane nanofibers containing silver nanoparticles for biotechnological applications' Macromol Res 17(9): 688±696 Shi X, Shen M, Mohwald H (2004) `Polyelectrolyte multilayer nanoreactors toward the synthesis of diverse nanostructured materials' Prog Polym Sci 29: 987±1019 Shrivastava S, Bera T, Roy A, Singh G, Ramachandrarao P, Dash B (2007) `Characterization of enhanced antibacterial effects of novel silver nanoparticles' Nanotechnology 18: 225103 Simpson K (2003) `Using Silver to fight microbial attack' Plastics Additives and Compounding 5(5): 32±35 Sondi I, Salopek-Sondi B (2007) `Silver nanoparticles as antimicrobial agent: a case study on E. coli as a model for gram-negative bacteria' J Colloid Interface Sci 275: 177±182 Tankhiwale R, Bajpai SK (2009) `Graft copolymerization onto cellulose-based filter paper and its further development as silver nanoparticles loaded antibacterial foodpackaging material' Colloids Surfaces B: Biointerfaces 69: 164±168 Thatiparti TR, Kano A, Maruyama A, Takahara A, (2009) `Novel silver-loaded semiinterpenetrating polymer network gel films with antibacterial activity' J Polym Sci: Part A: Polym Chem 47: 4950±4962 Thomas V, Yallapu MM, Sreedhar B, Bajpai SK (2007) `A versatile strategy to fabricate hydrogel±silver nanocomposites and investigation of their antimicrobial activity' J Colloid Interface Sci 315: 389±395 Tofoleanu, F, Balau Mindru T, Brinza, F, Sulitanu N, Sandu IG, Raileanu D, Floristean V, Hagiu BA, Ionescu C, Sandu I, Tura V (2008) `Electrospun gelatin nanofibers functionalized with silver nanoparticles' J Optoelectronics Adv Mater 10(12): 3512±3516 Tolaymat TM, El Badawy DM, Genaidy A, Scheckel KG, Luxton TP, Suidan M (2010) `An evidence-based environmental perspective of manufactured silver nanoparticle in syntheses and applications: A systematic review and critical appraisal of peer-

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reviewed scientific papers' Sci Total Environ 408: 999±1006 Vaidyanathan R, Kalishwaralal K, Gopalram S, Gurunathan S (2009) `Nanosilver ±the burgeoning therapeutic molecule and its green synthesis' Biotechnol Adv 27: 924± 937 Vimala K, Murali Mohan Y, Samba Sivudu K, Varaprasad K, Ravindra S, Narayana Reddy N, Padma Y, Sreedhar B, MohanaRaju K (2009) `Fabrication of porous chitosan films impregnated with silver nanoparticles: A facile approach for superior antibacterial application' Colloids Surf B: Biointerfaces doi 10.1016/ j.colsurfb.2009.10.044 Wang H, Qiao X, Chena J, Wang X, Ding S (2005) `Mechanisms of PVP in the preparation of silver nanoparticles' Mater Chem Phys 94: 449±453 Wei D, Sun W, Qian W, Ye Y, Ma X, (2009) `The synthesis of chitosan-based silver nanoparticles and their antibacterial activity' Carbohydr Res 344: 2375±2382 Wiley B, Sun Y, Mayers B, Xia Y (2005) `Shape-controlled synthesis of metal nanostructures: the case of silver' Chem Eur J 11: 454±463 Xu X, Yang Q, Bai J, Lu T, Li Y, Jing X (2008) `Fabrication of biodegradable electrospun poly(L-lactide-co-glycolide) fibers with antimicrobial nanosilver particles' J Nanosci Nanotechnol 8(10): 5066±5070 Yao Y, Ohko Y, Sekiguchi Y, Fujishima A, Kubota Y (2007) `Self-sterilization using silicone catheters coated with Ag and TiO2 nanocomposite thin film' J Biomed Mat Res Part B: App Biomater 85: 453±460 Yazdanshenas ME, Shateri Khalilabad M, Nateghi MR (2009) `A new method based on exhaustion for immobilization of silver nanoparticles on the surface of cotton fabrics' J Chem 21(7): 5741±5748 Yoksan Z, Chirachanchai S (2009) `Silver nanoparticles dispersing in chitosan solution: Preparation by -ray irradiation and their antimicrobial activities' Mater Chem Phys 115: 296±302 Yu D, Lin W, Yang M (2007) `Surface modification of poly(L-lactic acid) membrane via layer-by-layer assembly of silver nanoparticle-embedded polyelectrolyte multilayer' Bioconjugate Chem 18: 1521±1529 Zhao L, Chu PK, Zhang Y, Wu Z (2009) `Antibacterial coatings on titanium implants' J Biomed Mater Res Part B: Appl Biomater 470±480 Zhu C, Xue J, He J (2009) `Controlled in-situ synthesis of silver nanoparticles in natural cellulose fibers toward highly efficient antimicrobial materials' J Nanosci Nanotechnol 9(5): 3067±3074

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Incorporation of chemical antimicrobial agents into polymeric films for food packaging B A L D E V R A J , R . S . M A T C H E and R . S . J A G A D I S H , Central Food Technological Research Institute, India

Abstract: Antimicrobial packaging is a fast emerging area in food packaging. Antimicrobial agents can be incorporated into polymeric food packaging materials to enhance the shelf-life of packaged foods, by preserving the foods against microbial spoilage and hazardous food-borne microorganisms. This chapter reviews different classes of antimicrobials: chemically synthesized and biobased components such as organic acids and salts, inorganic compounds/gases and salts and biobased bacterions, enzymes, plant origin materials, etc., along with synthetic and biopolymer films as carriers of antimicrobial agents as packaging material. Special emphasis is given to the application of nanotechnologies in antimicrobial packaging and synergistic effects of different antimicrobial agents. Migration behaviour of antimicrobial agents from packaging materials is discussed to prolong the effect of antimicrobial agents. Antimicrobial activity and its method of evaluation are also discussed. Key words: antimicrobial agent, shelf-life, coating, bacteriocin, nano preservatives, organic acids, synergistic effects, pathogenic microbes, plant extracts, immobilization, biopolymers.

14.1

Introduction

The globalization of food trade has increased demand for fresh produce, minimally processed, easily prepared and ready-to-eat food products. Most food products are perishable, so the distribution of these foods poses major challenges for food safety and quality. Spoilage can be biological or chemical. Chemical spoilage of food ingredients can occur by oxidation processes and biological spoilage by autodegradation of tissues by enzymes, viral contamination, protozoan and parasitic contamination, microbial contamination, and loss by rodents and insects. The growth of microorganisms is the major problem of food spoilage leading to degradation of quality, shortened shelf-life, and changes in natural microflora that could induce pathogenic problems. Microbial spoilage of food products is caused by many bacteria, yeast and moulds. Their sensitivities to spoilage are dependent on nutrients, pH, water activity and presence of oxygen, increasing the risk of food-borne illness.

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Therefore, there are many different potential microorganisms that could contaminate food products and the various growth environments, presenting difficult problems in preventing spoilage. Food-borne microbial outbreaks are driving the search for innovative ways to inhibit microbial growth in foods while maintaining quality, freshness and safety. One option is to use packaging to provide an increased margin of safety and quality. These packaging technologies could play a role in extending shelf-life of foods and reduce the risk from pathogens. Antimicrobial polymers may find use in other food contact application as well (Paola and Joseph, 2002). Traditional methods of preserving foods from the effect of microbial growth include thermal processing, drying, freezing, refrigeration, irradiation, modified atmosphere packaging and adding antimicrobial agents or salts (Quintavalla and Vicini, 2002). Antimicrobial additives are mixed into initial food formulations to control microbial growth and extend shelf-life. Direct surface application onto foods has limited benefits because the active substances can be neutralized on contact with food or diffuse rapidly from the surface into the food mass. In addition, the antimicrobial compound directly added into the food cannot selectively target the food surface where spoilage reactions occur more intensively. Antimicrobial packaging is an alternative method to overcome these limitations, since these agents are slowly released from the film onto the food surface during storage and hence maintain the critical concentration necessary for inhibiting microbial growth by extending the lag period and reducing the growth rate or decrease live counts of microorganisms (Han, 2000). Such packaging possesses attributes beyond basic barrier properties, which are achieved by adding active ingredients to the packaging system and/or by using antimicrobial polymeric materials. The antimicrobial packaging system may not be suitable for some products that are not sensitive to microbial spoilage or contamination. The primary goals of an antimicrobial packaging system are (1) safety assurance, (2) quality maintenance, and (3) shelf-life extension. There is no single antimicrobial agent which can work effectively against all spoilage and pathogenic microorganisms. Different antimicrobial agents have different activities which affect microorganisms differently. The microorganisms may be classified based on oxygen requirement (aerobes and anaerobes), cell wall composition (Gram-positive and Gram-negative), growth stage (spores and vegetative cells), optimal growth temperature (thermophilic, mesophilic and psychrotrophic) and acid/osmosis resistance. It is important to understand the efficacy as well as the limits of the microbial activity. Some antimicrobial agents inhibit essential metabolic (or reproductive/ genetic) pathways of microorganisms while some others alter cell membrane/ wall structure. Antimicrobial polymeric materials show generally three types of mode: (1) release; (2) absorption and (3) immobilization. The release type allows the migration of antimicrobial agents into foods or headspace inside packages, and inhibits the growth of microorganisms. The absorption mode

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14.1 (a) Release of antimicrobial agent (A M) through multilayer film; (b) release of antimicrobial agent (A M) through single-layer film.

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removes essential factors of microbial growth from the food systems and inhibits the growth of microorganisms. The immobilization system does not release antimicrobial agents but suppresses the growth of microorganisms at the contact surface as shown in Fig. 14.1b. The immobilization systems may be less effective in the case of solid foods compared to liquid foods, because there is less possibility for contact between the antimicrobial package and the whole food products. The effectiveness of the antimicrobial release depends on the type of packaging materials and the food chemistry. Antimicrobial agents are incorporated into a multilayer structure for controlled antimicrobial release as shown in Fig. 14.1a. The advantage of the multilayer design is that the antimicrobials can be added in one thin layer, and their migration and release are controlled by the thickness of the contact layer or coating. In practice, a matrix of several layers is used to control the antimicrobial agent's rate of release. The application of antimicrobials to food packages can take several approaches. One is to put the antimicrobial into the film by adding it in the extruder when the film or coextrusion is produced. The disadvantage of doing this is that the high temperatures and shearing associated with the extrusion process can cause deterioration of the antimicrobial additives. An alternative to extrusion or extrusion coating application of antimicrobials is to apply the antimicrobial additives as a coating. This has the advantage of placing the specific antimicrobial additive in a controlled manner without subjecting it to high temperature or shearing forces. In addition, the coating can be applied at a later step, minimizing the exposure of the product to contamination (Han, 2005).

14.2

Antimicrobial agents

There is demand for ready-to-eat foods free of foodborne pathogens, to have a reasonably long shelf-life preferably without chemical preservatives. Due to globalization, foods produced in one area are often shipped to another area for processing and to several other areas for distribution. Some improvements have been made using packaging and processing systems to preserve foods without chemicals. Antimicrobial preservatives play a significant role in protecting the food supply (Davidson and Harrison, 2002). Preservatives are used to prevent or retard both chemical and biological deterioration of foods. These preservatives are used to prevent chemical deterioration, e.g. antioxidants to prevent autooxidation of pigments, flavours, lipids and vitamins, anti-browning compounds to prevent enzymatic and non-enzymatic browning; and anti-staling compounds, to prevent texture changes. Antimicrobial agents are those additives which are used to prevent biological deterioration. Substances used to preserve food by preventing growth of microorganisms and subsequent spoilage include fungistats, mould and spore inhibi-

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tors. The traditional function of food antimicrobials is to prolong shelf-life and preserve quality through inhibition of spoilage microorganisms. However, antimicrobials have been used increasingly as a primary intervention for inhibition or inactivation of pathogenic microorganisms in foods (Davidson and Zivanovic, 2003). Antimicrobials may be traditional or naturally occurring (Davidson and Taylor, 2001). Naturally occurring antimicrobials include compounds from plant, animal and microbial sources. Mostly they are used in foods. A few, such as nisin, natamycin, lactoferrin and lysozyme are approved for application to food by regulatory agencies in some countries. For selection of antimicrobial agent, the target pathogen or spoilage microorganisms must be identified followed by evaluation of the preservation systems, which might have a combination of chemical preservatives and other preservation methods (Leistner, 2000). To use antimicrobial agents in foods, the industry must follow the guidelines and regulations of the country in which they are going to use them. The new antimicrobial packaging materials may be developed using only agents which are approved by the authorization agencies, approved or notifiedto-use within the concentration limits for food safety enhancement or preservation. Various antimicrobial agents may be incorporated in the packaging system ± chemical antimicrobials, antioxidants, biotechnology products, antimicrobial polymers, natural antimicrobials and gas. Some of the important antimicrobial agents are shown in Table 14.1.

14.3

Chemical antimicrobial agents

14.3.1 Organic acids and salts Organic acids and salts are very efficient in inhibiting microbial growth. These molecules inhibit the outgrowth of both bacterial and fungal cells. Organic acids such as benzoic acids, parabens, sorbic acid, propionic acid, acetic acid, lactic acid, medium-size fatty acids, salts (sorbates, benzoates, propionates) and their mixture possess strong antimicrobial activity and have been used as food preservatives, food contact materials and sanitizers. Sorbic acid is reported to inhibit the germination and outgrowth of bacterial spores. Their effect, however, is strongly dependent on the pH value, and their use is not recommended if pH exceeds 6. The antimicrobial properties of acetic, lactic, citric and malic acid have been utilized by the food industry for food preservation. It is generally accepted that the undissociated molecule of the organic acid or ester is responsible for the antimicrobial activity. Many weak acids, in their undissociated form, can penetrate the cell membrane and accumulate in the cytoplasm and acidify its interior. Potassium sorbate (1%) was used with ice for cooling red hake and salmon packaged in modified atmosphere (Fey and Regenstein, 1982). Sorbates were used in fish and fish products in combination with other

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Table 14.1 Application of antimicrobial food packaging

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Class

Antimicrobial agent

Packaging material

Food/Target organism

References

Organic acid/salt

Benzoic acid Sorbic acid Propionic acid, acetic acid Lactic acid

PE-co-PE Edible films, EVA, LLDPE Chitosan

Culture media

LDPE LDPE MC/palmitic acid MC/HPMC/fatty acid MC/chitosan Starch/glycerol CMC/paper MC/chitosan PE PE

Cheese Culture media Culture media Culture media Culture media Chicken breast Bread Culture media Fish fillet Fungal growth, culture media

Weng et al. (1997) Baron and Sumner (1993) Quattara et al. (1999) Torres and Karel (1985), Devlieghere et al. (2000) Weng and Hotchkiss (1993) Han and Floros (1996) Han and Floros (1996) Rico-Pena and Torres (1991) Vojdani and Torres (1990) Chen et al. (1996) Baron and Sumner (1993) Ghosh et al. (1973, 1977) Chen et al. (1996) Huang et al. (1997) Weng and Chen (1997)

Malic acid Potassium sorbate

Calcium sorbate Sodium benzoate Benzoic acid anhydride Sorbic acid anhydride Parabens

Propylparaben Ethylparaben

Oxygen absorber/ antioxidant

Reduced iron complex Butylated hydroxyanisole (BHT) Butylated hydroxytoluene (BHA), tertiary butylhydroquinone (TBHO)

Water

Katz (1998) Dobia¨sí et al. (1998) Sachet (AgelessTM) Polyethylene

Bread Breakfast cereal

Smith et al. (1987) Hoojjat et al. (1987)

Polyethylene

Oxygen-sensitive foods

Hotchkiss (1997)

Table 14.1 Continued Class

Inorganic metallic ions/salts

Antimicrobial agent

Packaging material

Food/Target organism

References

Tocopherols Ascorbic acid

Polyethylene Polyethylene

Oxygen-sensitive foods Oxygen-sensitive foods

Smith et al. (1990) Smith et al. (1990)

Copper, silver, zinc

Various polyolefins

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Ishitani (1995), Luck and Jager (1997), An et al. (1998), Chung et al. (1998) Shinagawa and Shinanen (1992) Anon (1991)

Aluminium silicate Copper or silver hydroxyapatite Zeolite of silver or copper, manganese or nickel

LDPE

Magnesium oxide and zinc oxide Phosphoric acid

Culture media Bactericidal

Ishitani (1995), Abe (1990), Kunisaki et al. (1993), Shinanen and Chugoku (1989) Sawai et al. (1993) Hotchkiss (1997)

Fungicides

Benomyl Imazalil

Ionomer LDPE LDPE

Culture media Bell pepper Cheese

Halek and Garg (1989) Miller et al. (1984) Weng and Hotchkiss (1992)

Bacteriocin/ peptide

Nisin

Silicon coating SPI, corn zein films

Culture media Culture media Dairy and canned products Meat products

Daeschel et al. (1992) Padgett et al. (1998) Fowler and Gasson (1991), Chen and Hoover (2003) Nettles and Barefoot (1993), Barnby-Smith (1992), An et al. (2000), Scannell et al. (2000a)

Lacticin, pediocin, diolococcin, bavaricin, brevicin, carnocin, sakacin, subtilin, propionicins and mesenteocin

Enzyme

Lysozyme

Glucose oxidase Chitinase, ethanol oxidase, lactoperoxidase, myeloperoxidase Glucanase ß Woodhead Publishing Limited, 2011

Spices

Essential oils (plant extracts)

Alcohol/thiol

Gram-positive bacteria, cheese

SPI film, corn zein films Alginate

Culture media Fish

Cinnamon, rosemary, cloves, horseradish, mustard, thyme Caffeine

Nylon/PE, cellulose

Grapefruit seed extract, hinokitoil, bamboo powder, Rheum palmatum, Coptis chinensis extracts Basil

LDPE, cellulose

Ethanol

Silica gel sachet Silicon oxide sachet (EthicapTM) Cyclodextrin/plastic (SeiwaTM)

Hinokithiol Gas

PVOH, nylon, cellulose acetate

CO2 Sachet SO2 Chlorine dioxide

Beuchat and Golden (1989), Appendini and Hotchkiss (1996) Padgett et al. (1998) Field et al. (1986) Fuglsang et al. (1995)

Fungus

Conner (1993), Fuglsang et al. (1995), Selitrennikoff (2001)

Bacteria

Hoshino et al. (1998), Day (1998) Anon (1995), Nielsen and Rios (2000)

Calcium hydroxide Fruit/vegetable Sodium metabisulfite Various polyolefins

Lettuce, soybean sprouts

Lee et al. (1998), Imakura et al. (1992), Chung et al. (1998), Hong et al. (2000)

Bacteria, yeast and moulds

Suppakul (2003)

Culture media Bakery

Shapero et al. (1978) Smith et al. (1987)

±

Gontard (1997)

Coffee Sacharow (1988) Grape

Labuza (1990) Gontard (1997) Wellinghoff (1995)

Table 14.1 Continued

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Class

Antimicrobial agent

Chelating agents

Citrate Conalbumin EDTA

Packaging material

Food/Target organism

References

Lactoferrin Polyphosphate

Hotchkiss(1997) Conner (1993) Luck and Jager (1997), Rodrigues and Han (2000) Conner (1993) Shelef and Seiter (1993)

Fatty acid/ fatty acid ester

Lauric acid Palmitolic acid Monolaurin (lauricidin)

Ouattara et al. (1997, 2000) Ouattara et al. (1997) Luck and Jager (1997)

Natural phenols

Catechin p-Cresol Hydroquinones

Walker (1994) Hotchkiss (1997) Hotchkiss (1997)

Antibiotic

Natamycin

Luck and Jager (1997)

Probiotics

Lactic acid bacteria

Others

UV radiation

Nylons

Culture media

Paik and Kelley (1995), Hagelstein et al. (1995)

LDPE ˆ low-density polyethylene; MC ˆ methyl cellulose; HPMC ˆ hydroxypropyl MC; CMC ˆ carboxy MC; PE ˆ polyethylene; MA ˆ methacrylic acid; SPI ˆ soy protein isolate; PVOH ˆ polyvinyl alcohol; HDPE ˆ high-density PE.

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compounds or techniques as an effective preservative tool for extending the shelf-life of fish products (Thakur and Patel, 1994). Potassium sorbate was used as antimicrobial agent in biodegradable films prepared with sweet potato starch (Xiao et al., 2010).

14.3.2 Triclosan Triclosan, 5-chloro-2-(2,4-dichlorophenoxy) phenol, is a chlorinated phenoxy compound that inhibits the growth of a broad range of bacteria, moulds and fungi. It is a widely used antibacterial substance in cosmetics and toothpaste, etc. It can be incorporated in plastic articles for the kitchen and bathroom. It has also been introduced in applications such as conveyor belts in the food industry (Day, 1998).

14.3.3 Antioxidants An antioxidant is a molecule capable of slowing or preventing the oxidation of other molecules. Antioxidants restrict oxygen requirement and thereby act as antifungal agents. Antioxidants, both natural and synthetic, are used by the food industry as food additives to prolong the shelf-life and appearance of many foodstuffs. Food-grade antioxidants such as butylated hydroxy anisole (BHA), butylated hydroxy toluene (BHT), tert-butyl hydroquinone (TBHQ), tocopherols and ascorbic acid could be incorporated into packaging materials to create an anaerobic atmosphere inside packages, and eventually protect the food against aerobic spoilage (Smith et al., 1990).

14.3.4 Inorganic materials Some inorganic materials are capable of emitting infrared radiation with an antimicrobial effect; however, this has not been well established for antimicrobial activity. Radiation-emitting materials are reported to have been developed that emit long-wavelength infrared radiation, which is thought to be effective against microorganisms (Hotchkiss and Rooney, 1995). The active inorganic compounds consist of aluminium silicate and silver (Shinagawa and Shinanen, 1992), dry powder made by substituting antimicrobial copper or silver for calcium atoms in hydroxyapatite (Anon, 1991), synthetic zeolite and silver (Abe, 1990; Kunisaki et al., 1993) or copper, manganese or nickel and silver containing zeolite (Shinanen and Chugoku, 1989). Magnesium oxide and zinc oxide have been proved to be bactericidal and bacteriostatic agents respectively (Sawai et al., 1993). Metallic ions of silver, copper, zinc and others are safe antimicrobial agents. Ag-substituted zeolite is the most frequently used antimicrobial agent (Ishitani, 1995). A recent study has shown the use of a novel silver-based antimicrobial layered silicate additive in polylactic acid (PLA)

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biocomposites, by solvent casting, The silver-based nanoclay showed strong antimicrobial activity against Gram-negative Salmonella spp. with minimum inhibitory concentration and minimum bactericidal concentration below 1 mg per 10 ml for use in active food packaging applications. The additive was dispersed throughout the PLA matrix to a nanoscale, yielding nanobiocomposites. The films were highly transparent with enhanced water barrier and strong biocidal properties. Migration levels of silver, within the specific migration levels referenced by the European Food Safety Agency (EFSA), exhibit antimicrobial activity, supporting the potential application of this biocidal additive in active food-packaging applications to improve food quality and safety (Busolo et al. 2010). The purpose of the zeolite is to allow for slow release of antimicrobial metal ions into the surface of the food products. The antimicrobial activity of metals is due to minute quantities of ions formed. Ag-zeolite is manufactured from synthetic zeolite by replacing a portion of its natural sodium with silver ions. Ag-zeolite content of 1±3% can be laminated as a thin film on the food contact surface of the packaging material. Only the ions of the zeolite particles at the surface are active. Sulfates, hydrogen sulfide and certain kinds of sulfurcontaining amino acids and proteins are considered to influence the antimicrobial activity of Ag-zeolites. Silver nitrate forms silver ions in water solutions which have a strong antimicrobial activity. These ions are considered to have inhibitory activities on metabolic functions of respiratory and electronic transport systems of microbes, and mass transfer across cell membranes.

14.3.5 Fungicides The fungicide imazalil was chemically coupled to polyethylene to prevent mould growth on cheese surfaces (Weng and Hotchkiss, 1992). The antifungal agent benomyl was incorporated into ionomer film for inhibition of microbial growth in defined media (Halek and Garg, 1989).

14.3.6 Gases Gaseous antimicrobials have some benefit compared to the solid or solute types of chemical antimicrobial agents. These can be vaporized and penetrated into any air space inside packages that cannot be reached by non-gaseous antimicrobial agents. Alcohols The ethanol sachet is one example of a gaseous antimicrobial system. Headspace ethanol vapour can inhibit the growth of moulds and bacteria. Antimicrobial activity of ethanol (or common alcohol) is well known and it is used in medical

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and pharmaceutical applications. Ethanol has been shown to increase the shelflife of bread and other baked products when sprayed onto product surfaces prior to packaging. A novel method of generating ethanol vapour is through the use of an ethanol releasing system enclosed in a small sachet which is placed in a food package. Food-grade ethanol is absorbed onto a fine inert powder which is enclosed in a sachet that is permeable to water vapour. Moisture is absorbed from the food by the inert powder and ethanol vapour is released and permeates the sachet into the food package headspace. Chlorine dioxide Chlorine dioxide in aqueous and gaseous forms has emerged as a strong antimicrobial agent with various applications in food storage and processing. Scientists at Purdue University investigated the use of chlorine dioxide, mainly in its gaseous form, on various surfaces, produce and meat products. Studies using chlorine dioxide gas at 6 to 10 mg per litre were performed on epoxycoated surfaces used in the orange juice industry. Investigators obtained more than a 5-log reduction for selected yeasts, moulds and lactic acid bacteria on these surfaces. Similarly, when treatment at 15 mg per litre was performed on paper, plastic and wood surfaces, Bacillus spores experienced a greater than 5-log reduction. For nearly all fruit and vegetable surfaces, a more than 5-log reduction was obtained for E. coli, L. monocytogenes and Salmonella spp., using 0.6 to 5 mg per litre of chlorine dioxide at different contact times. The treatments had no impact on product quality. Processing parameters, including gas concentration, treatment time, relative humidity and temperature, all play a significant role in how effective the gas is in terms of antimicrobial activity. The treatment was less effective on lettuce. Chlorine dioxide yielded a 1.3log reduction in microbial levels. The gas also led to poor quality in terms of leaf colour. Treatment of poultry, beef and pork surfaces with chlorine dioxide gas at 1.5 to 13.5 mg per litre resulted in a 1- to 4-log reduction in bacterial levels. However, the treatment often altered the colour of the product. Chlorine dioxide acts as an oxidizing agent and reacts with several cellular constituents, including the cell membrane of microbes by stealing electrons from them (oxidation); it breaks their molecular bonds, resulting in the death of the organism by the break-up of the cell. Since chlorine dioxide alters the proteins involved in the structure of microorganisms, the enzymatic function is broken, causing very rapid bacterial kills. The potency of chlorine dioxide is attributable to the simultaneous oxidative attack on many proteins, thereby preventing the cells from mutating to a resistant form. Additionally, because of the lower reactivity of chlorine dioxide, its antimicrobial action is retained longer in the presence of organic matter (Singh, et al., 2002; Knapp and Battisti, 2000).

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Other gases Allylisothiocyanate, hinokithiol and ozone have been incorporated into packages and demonstrated to give effective antimicrobial activity. However, the use of these reactive gaseous agents has to be considered after careful studies of their reactivity and permeability through packaging materials.

14.4

Natural antimicrobial agents

14.4.1 Bacteriocins Various biologically active peptides are bacteriocins, produced by microorganisms, that inhibit the growth of pathogenic microorganisms. These fermentation products include nisin produced by Lactococcus lactis (Fowler and Gasson, 1991), lacticins, pediocin, diolococcin, and propionicins (Daeschul, 1989; Han, 2002). Other non-peptide fermentation products such as reuterin also demonstrate antimicrobial activity. Besides the above food-grade bacteriocins, other bacteriocins could be utilized for the development of antimicrobial packaging systems. Nisin was initially evaluated as a clinical antibiotic in the 1940s (Hirsch and Mattick 1949). Later, it was found to be suitable for food preservation due to its lethal activity against food-borne pathogens and spoilage microorganisms by inhibition of cell wall biosynthesis and spore outgrowth, and it is safe for use and approved by the US Food and Drug Administration for use in particular food products (Hurst, 1981; Montville et al., 1995). It is used in a crude extract containing up to 5% of nisin of the solid. One of the used commercial forms of nisin is NisaplinTM, which contains 2.5% of the active ingredient nisin, 77.5% sodium chloride (NaCl), and 12% non-fat dry milk (Chen and Hoover, 2003); it is permitted for use mostly in dairy products and canned goods. It is also used in baby foods, baked goods, mayonnaise and milk shakes. Nisin, however, displays several shortcomings: low solubility at physiological pH reduces its activity and limits its use in most cured meat products (Rayman et al., 1983). Pediocin showed the most promising results (Nielsen et al., 1990). The use of bacteriocins in food preservation presents serious limitations because of their relatively narrow activity spectra and moderate antibacterial effects. Combinations of agents Most antimicrobial agents have different antimicrobial mechanisms, so a mixture of antimicrobial agents can increase antimicrobial activity through synergistic mechanisms when they do not have any interference mechanisms. Therefore, optimization of the combination of various antimicrobials will extend the antimicrobial activity of the mixture and maximize the efficacy and the safety of the antimicrobial packaging system.

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The presence of NaCl enhanced the antimicrobial action of bacteriocins such as nisin, leucocin, enterocin and others. Sodium chloride also decreased the antilisterial activity of acidocin (at 1±2%), lactocin (at 5±7%), leucocins (at 2.5% NaCl), pediocin (at 6.5% NaCl), curvacin and Carnobacterium piscicola bacteriocin (at 2±4% NaCl). The protective effect of sodium chloride may be due to its interference with ionic interactions between bacteriocin molecules and charged groups involved in binding to target cells (Bhunia et al., 1991). Sodium chloride may also induce conformational changes of bacteriocins (Lee et al., 1993) or changes in the cell envelope of the target organisms (Jydegaard et al., 2000). Combinations of nisin and nitrite delayed botulinal toxin formation in meat systems and showed increased activity on clostridial endospore outgrowth and also on L. mesenteroides and L. monocytogenes. Addition of nitrite also increased the anti-listerial activity of bacteriocinogenic lactobacilli in meat and the activities of enterocin against L. monocytogenes, Bacillus coagulans, B. macroides and B. cereus. Organic acids and their salts can potentiate the activity of bacteriocins greatly, while acidification enhances the antibacterial activity of both organic acids and bacteriocins (Jack et al., 1995; Stiles, 1996). The increase in net charge of bacteriocins at low pH might facilitate translocation of bacteriocin molecules through the cell wall. The solubility of bacteriocins may also increase at lower pH, facilitating diffusion of bacteriocin molecules. The sensitivity of L. monocytogenes to nisin (400 IU/ml) increased in combination with lactate. Other studies have confirmed the increased antibacterial activity of nisin in combination with sodium lactate in several food systems. A nisin±sorbate combination showed increased activity against Listeria and B. licheniformis. In the production of ricotta-type cheeses, the combination of nisin with acetic acid and sorbate controlled L. monocytogenes contamination over a long storage period (70 days) at 6±8ëC. Lacticin activity also increased in combination with sodium lactate or sodium citrate, as well as did pediocin activity in combination with sodium diacetate and sodium lactate. Activity of enterocin against B. cereus in rice gruel was also potentiated by sodium lactate. Lactic acid, sodium lactate and peracetic acid (as well as several other chemical compounds) increased activity for decontamination of L. monocytogenes in sprouts. When nisin and pediocin were used in combination with chemical compounds to combat L. monocytogenes in fresh produce, the best results were obtained for nisin with phytic acid. Chelating agents permeate the outer membrane of Gram-negative bacteria by extracting calcium (Ca2+) and magnesium (Mg2+) cations that stabilize lipopolysaccharide of this structure, allowing bacteriocins to reach the cytoplasmic membrane. The enhanced effect of chelators such as ethylene diamine tetraacetic acid (EDTA), disodium pyrophosphate, trisodium phosphate, hexametaphosphate or citrate and bacteriocins against Gram-negative bacteria has been

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demonstrated for nisin both under laboratory conditions and in foods. Brochrocin and enterocin also showed increased antimicrobial activity on EDTA-treated Gram-negative bacteria. Sensitization of Gram-negative bacteria to bacteriocins by other chelators such as maltol or ethyl maltol has been reported. Sodium lactate or sodium citrate in combination with nisin showed increased antimicrobial activity against Arcobacter butzleri in chicken due to their chelating effect. Chelating agents can also enhance the activity of bacteriocins on Gram-positive bacteria. A combination of nisin and sodium polyphosphate showed an increased activity against L. monocytogenes. More recently, an increased activity of chrisin (a commercial nisin preparation) in combination with EDTA has been reported against several Gram-positive bacteria. The combination of sodium tripolyphosphate and enterocin showed increased activity against B. coagulans and B. macroides. Other antimicrobial compounds such as ethanol can act synergistically with nisin to reduce the survival of L. monocytogenes. Sub-lethal concentrations of nisin (30 IU/ml) and monolaurin (100 g/ml) in combination acted synergistically on B. licheniformis vegetative cells and spore outgrowth in milk. Synergism was also observed for the sucrose fatty acid esters sucrose palmitate and sucrose stearate and nisin against several strains of L. monocytogenes, B. cereus (cells and spores), L. plantarum and Staphylococcus aureus, but not against Gram-negative bacteria. Reuterin also showed a significant synergistic effect on L. monocytogenes and a slight additive effect on S. aureus after combination with nisin (100 IU/ml). Essential oils and their active components, the phenolic compounds, are also attractive natural preservatives (Burt, 2004). When used in combination with bacteriocins, the dose of added phenolic compounds could be lowered, thereby decreasing their impact on the food flavour and taste. Nisin acted synergistically with carvacrol, eugenol or thymol against B. cereus and/or L. monocytogenes. Combinations of nisin with carvacrol, eugenol or thymol resulted in synergistic action against B. subtilis and Listeria innocua, while nisin and cinnamic acid had synergistic activity against L. innocua but only additive activity against B. subtilis. Carvacrol (0.5mM) was used to enhance the synergy found between nisin and a pulsed electric field treatment (PEF) against vegetative cells of B. cereus in milk. The combination of nisin and cinnamon accelerates death of Salmonella typhimurium and Escherichia coli in apple juice. The natural variant nisin also acted synergistically with thymol against L. monocytogenes and B. subtilis. The antimicrobial activity of enterocin against S. aureus cells in vegetable sauces was potentiated significantly in combination with the phenolic compounds carvacrol, geraniol, eugenol, terpineol, caffeic acid, p-coumaric acid, citral and hydrocinnamic acid. Various other combinations of bacteriocins are also listed in Table 14.2. Combinations of bacteriocins have also been tested in order to increase their antimicrobial activities. The simultaneous use of nisin with pediocin or with

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Table 14.2 Combination of bacteriocins with other antimicrobial agents Sl. no

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Effect on activity

1. 2. 3.

Nisin + NaCl Leucocin + NaCl Enterocin + NaCl and others

Enhanced activity Enhanced activity Enhanced activity

4.

Acidocin (at 1±2%) + NaCl

5.

Lactocin (at 5±7%) + NaCl

6.

Leucocins + 2.5% NaCl

7.

Pediocin + at 6.5% NaCl

8.

Curvacin + NaCl

9.

Carnobacterium piscicola bacteriocin + at 2±4% NaCl Nisin + nitrite

Decreased antilisterial activity Decreased antilisterial activity Decreased antilisterial activity Decreased antilisterial activity Decreased antilisterial activity Decreased antilisterial activity Increased the antilisteria activity in meat

10.

11. 12.

Nisin + lysozyme + sodium benzoate Enterocin + nitrite

Enhanced activity Enhanced activity

Target microorganism

References

Harris et al. (1991) Thomas and Wimpenny (1996) Mazzotta and Montville (1997), Parente et al. (1998), Ananou et al. (2004) Chumchalova¨ et al. (1998) Vignolo et al. (1998) Hornb×k et al. (2006) Jydegaard et al. (2000) Verluyten et al. (2002) Himmelbloom et al. (2001) Clostridial endospores outgrowth, lactobacilli, Leuconostoc mesenteroides, Listeria monocytogenes Listeria micrococcus, Saccharomyces cerevisial Listeria monocytogenes, Bacillus coagulans and Bacillus macroides, Bacillus cereus

Rayman et al. (1981, 1983), Taylor et al. (1985), Gill and Holley (2003) Buonocore et al. (2003) Garc|¨ a et al. (2003, 2004a, 2004b), Abriouel et al. (2002)

Table 14.2 Continued

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Target microorganism

References

13. 14.

Nisin + lactate Nisin-sorbate

Enhanced activity Enhanced activity

15.

Nisin with acetic acid + sorbate

Buncic et al. (1995) Avery and Buncic (1997), Mansour et al. (1998) Davies et al. (1997)

16.

Lacticin + sodium lactate or sodium citrate Pediocin AcH activity in combination + sodium diacetate and sodium lactate Enterocin + sodium lactate Nisin and pediocin + phytic acid Lactic acid, sodium lactate and peracetic acid EDTA, disodium pyrophosphate, trisodium phosphate, hexametaphosphate or citrate + bacteriocins

Enhanced activity in Ricotta-type cheeses Enhanced activity

Listeria monocytogenes Listeria and Bacillus licheniformis Listeria monocytogenes

17. 18. 19. 20. 21.

22. 23. 24. 25.

EDTA + brochrocin C and enterocin Maltol or ethyl maltol + bacteriocins Sodium lactate or sodium citrate + nisin Sodium polyphosphate + nisin

Scannell et al. (2000a, 2000b) Uhart et al. (2004)

Rice gruel Fresh produce Sprouts

Bacillus cereus Listeria monocytogenes Listeria monocytogenes

Grande et al. (2006) Bari et al. (2005) Cobo Molinos et al. (2005)

Gram-negative bacteria

Gram-negative bacteria

Stevens et al. (1991), Cutter and Siragusa (1995a, 1995b), Carneiro De Melo et al. (1998), Boziaris and Adams (1999) and Fang and Tsai (2003) Abriouel et al. (1998), Gao et al. (1999), Ananou et al. (2005) Schved et al. (1996)

Arcobacter butzleri

Long and Phillips (2003)

Listeria monocytogenes

Buncic et al. (1995)

Gram-negative bacteria

Chicken

26. 27. 28. 29.

EDTA + chrisin (a commercial nisin preparation) Sodium tripolyphosphate + enterocin Ethanol + nisin Nisin (30 IU/ml) and monolaurin (100 g/ml)

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Sucrose fatty acid esters sucrose palmitate and sucrose stearate + nisin

31.

Reuterin with nisin (100 IU/ml),

32.

Carvacrol, eugenol or thymol + nisin

33. 34. 35.

Cinnamic acid + nisin Carvacrol (0.5 mM) + nisin and a pulsed electric field treatment (PEF) Cinnamon + nisin

36.

Thymol + nisin Z

37.

Phenolic compounds carvacrol, geraniol, eugenol, terpineol, caffeic acid, p-coumaric acid, citral and hydrocinnamic acid + enterocin AS-48

Milk

Milk Apple juice

Vegetable sauces

Gram-positive bacteria

Gill and Holley (2003)

Bacillus coagulans and Bacillus macroides Listeria monocytogenes Vegetative cells and spore outgrowth of Bacillus licheniformis Several strains of Listeria monocytogenes, Bacillus cereus (cells and spores), Lactobacillus plantarum and Staphylococcus aureus Listeria monocytogenes, Staphylococcus aureus Bacillus cereus and/or Listeria monocytogenes

Garc|¨ a et al. (2003, 2004a)

Listeria innocua Vegetative cells of Bacillus cereus Salmonella Typhimurium and Escherichia coli Listeria monocytogenes and Bacillus subtilis Staphylococcus aureus

Brewer et al. (2002) Mansour et al. (1999) Thomas et al. (1998)

Arque¨s et al. (2004) Pol and Smid (1999), Periago et al. (2001), Yamazaki et al. (2004) Olasupo et al. (2004) Pol et al. (2001a, 2001b) Yuste and Fung (2004) Ettayebi et al. (2000) Grande et al. (2007)

Table 14.2 Continued

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Nisin with pediocin AcH or with leucocin as well as lactacin B or lactacin F with nisin or pediocin AcH, and lactacin/pediocin AcH Nisin (50 IU/ml) + curvaticin 13 (160 AU/ml) Nisin and lysozyme

39. 40. 41. 42.

43. 44. 45. 46.

Nisin-lysozyme + chelating agents Nisin + lactoperoxidase system (LPS)

Nisin + lactoperoxidase system (LPS) Lactoferrin Nisin + lactoferrin Pleurocidin (an antimicrobial peptide from fish) + pediocin, sakacin P and curvacin A

Effect on activity

Target microorganism

References

Hanlin et al. (1993), Parente et al. (1998), Mulet-Powell et al. (1998)

Skim milk

Culture media

Listeria monocytogenes

Bouttefroy and Millie©re (2000)

Gram-positive bacteria, lactobacilli and Staphylcoccus aureus Gram-negative bacteria

Chung and Hancock (2000), Nattress and Baker (2003)

Listeria monocytogenes Ohio and (to a lesser extent) Listeria monocytogenes Scott A and Listeria monocytogenes ATCC 15313 Fish spoilage flora

Zapico et al. (1998) and Boussouel et al. (2000)

Escherichia coli

Gill and Holley (2000)

Elotmani and Assobhei (2004) Ellison (1994) Branen and Davidson (2004) LÏders et al. (2003)

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leucocin as well as the use of lactacin with nisin or pediocin provide a greater antibacterial activity than each bacteriocin separately. Simultaneous or sequential additions of nisin (50 IU/ml) and curvaticin 13 (160 AU/ml) also induced a greater inhibitory effect against L. monocytogenes than each bacteriocin individually. The simultaneous use of two or more bacteriocins could be useful not only to lower the added bacteriocin doses, but also to avoid regrowth of bacteriocin-resistant/adapted cells. Increased antibacterial activity can also be achieved by combining bacteriocins with other (non-bacteriocin) antimicrobial proteins or peptides. Nisin and lysozyme acted synergistically against Gram-positive bacteria, including spoilage lactobacilli and S. aureus. The spectrum of the nisin±lysozyme combination could be extended to Gram-negative bacteria by adding chelating agents. The combined addition of nisin and the lactoperoxidase system (LPS) had a strong antimicrobial effect against L. monocytogenes, to a lesser extent without affecting the harmless microbiota and also against fish spoilage flora. Lactoferrin (and its partial-hydrolysis derivative lactoferricin) is another natural protein with antimicrobial activity due to its iron-binding capacity and polycationic nature. The combination of nisin and lactoferrin showed increased antilisterial activity. The antimicrobial activity of pleurocidin (an antimicrobial peptide from fish) against E. coli was greatly enhanced by pediocin, sakacin and curvacin (GaÂlvez et al., 2007).

14.4.2 Enzymes Lysozyme hydrolyses 1,4-glucosidal linkages in the peptidoglycan layer of the bacterial cell walls. It is the most effective against Gram-positive bacteria, and is particularly useful against thermophilic spore formers (Beuchat and Golden, 1989). It can be used freely in ripened cheeses. Other groups of antimicrobial enzymes are glucanases that hydrolyse the glucan structure of fungal walls, and chitinases that cleave cell wall chitin. They can be isolated from tobacco, peas, grains, fruits and other plant sources upon fungal attack (Selitrennikoff, 2001). Other enzymes are the oxidoreductases, e.g. lactoperoxidase, glucoseoxidase and catalases, which catalyse reactions producing cytotoxic compounds and may also protect against oxidative food deterioration. Peptides with a great range of amino acid sequences, different sizes and structures, exert antimicrobial activity (Barra and Simmaco, 1995). Besides originating from `traditional' sources, peptides from insects, fish and amphibians have in recent years attracted much attention. Membrane disruption is thought to be centre for activity of these peptides (Brul and Coote, 1999). Peptides with iron-binding properties are found in egg white (ovotransferrin or conalbumin) and in blood (serum transferrin) where they act as the main carrier of iron (Gould, 1996). The antimicrobial effect apparently results from the binding of essential iron needed for growth of microorganisms (Conner,

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1993). Many food pathogens can be inhibited, such as Staphylococcus, Clostridium, Listeria, Salmonella, Pseudomonas, Yersinia, Vibrio and Aeromonas (Naidu, 2000). Peptide digestion of bovine lactoferrin has resulted in a very potent peptide, named lactoferricin. It has some similarity to protamines and may act on the cell surface to increase membrane permeability. The much higher activity of lactoferrin hydrolysate compared to the native lactoferrin indicates that ion-chelation is a weak inhibitory mechanism (Branen and Davidson, 2000). The antimicrobial peptides currently used as food preservatives are the products of lactic acid bacteria, nisin and related compounds like pediocin.

14.4.3 Plant origin Plants protect themselves against microorganisms and other predators by synthesizing a wide range of compounds. These compounds are also termed secondary metabolites, as they generally are not essential for the basic metabolic processes. Secondary metabolites represent a diverse array of chemical compounds mostly derived from the isoprenoid, phenyl propanoid, alkaloid or fatty acid/polyketide pathways (Dixon, 2001). Antimicrobial compounds from plants are preinfectional agents at the plant surface, agents bound in vacuoles and associated with hydrolytic enzyme activation systems and phytoalexins, which are compounds produced in response to invasion. Plant extracts The use of natural plant extracts is desirable for the development of new food products and nutraceuticals, as well as new active packaging systems. Some plant extracts such as grapefruit seed (GPFS), cinnamon, horseradish and clove have been added to packaging systems to demonstrate effective antimicrobial activity against spoilage and pathogenic bacteria. More use of natural extracts is expected because of the easier regulation process and consumer preference when compared to chemical antimicrobial agents. Naturally occurring preservatives that have been proposed and/or tested for antimicrobial activity in plastics include spice and herb extracts, e.g. from rosemary, cloves, horseradish, mustard, cinnamon and thyme (Day, 1998). Most frequently studied is horseradish-derived allyl isothiocyanate, and natural spices such as rosemary and its derivatives (Brody et al., 2001). Essential oils Essential oils are regarded as natural alternatives to chemical preservatives. Their practical application is limited due to flavour considerations, and their effectiveness is moderate due to their interaction with food ingredients and

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structures. The study showed that volatile compounds of oregano essential oil are capable of affecting both the growth and metabolic activity of the microbial association of meat stored at modified atmospheres (Skandamis et al., 2002). This inhibition was not as strong as that found in the contact of pure essential oil with microorganisms when added directly on the surface of meat (Skandamis et al., 2002; Skandamis and Nychas 2001). The volatile compounds of oregano essential oils improve shelf-life by (1) delaying the growth of specific spoilage organisms, (2) inhibiting or restricting metabolic activities that cause spoilage through the production of spoilage microbial metabolites, and (3) minimizing the flavour concentration. The oils consist of a mixture of esters, aldehydes, ketones, terpenes and phenolic compounds, and harbour the characteristic flavour and aroma of the particular spice or herb. The traditionally best-known antimicrobial spices and herbs are clove, cinnamon, chilli, garlic, thyme, oregano and rosemary, but also bay, basil, sage, anise, coriander, allspice, marjoram, nutmeg, cardamom, mint, parsley, lemongrass, celery, cumin, fennel and many others have been reported to have an inhibitory effect toward microorganisms (Deans and Ritchie, 1987; Hili et al., 1997; Hammer et al., 1999; Elgayyar et al., 2001). Basil (Ocimum basilicum L.) essential oils and their principal constituents were found to exhibit antimicrobial activity against a wide range of Gram-negative and Gram-positive bacteria, yeasts and moulds (Suppakul, 2003).

14.5

Polymers (synthetic or natural)

Some synthetic or natural polymers also possess antimicrobial activity. Ultraviolet or excimer laser irradiation can excite the structure of nylon and create antimicrobial activity. Among natural polymers, chitosan (chitin derivative) exhibits antimicrobial activity. Short or medium-size chitosan possesses quite good antimicrobial activity, while long-chain chitosan is not effective. Chitosan has been approved as a food ingredient by the US Food and Drug Administration. Chitin is a biopolymer consisting of polysaccharide. Large amounts of chitin are obtained from shellfish and can be used to prepare chitosan by deacetylation.

14.5.1 Synthetic peptides Synthetic peptides have been tested in food products such as apple juice, 14residues of long peptide, composed of eight lysine and six leucine residues (Blondelle and Houghten, 1992; Haynie et al., 1995). The peptide showed a bactericidal effect against E. coli (Appendini and Hotchkiss, 1999). Synthetic peptides used as a preservative in food packaging materials were able to reduce E. coli population by 3.5-log units after 10 min incubation in citrate buffer (pH 3.5) at a concentration of 5 g/mL (Appendini and Hotchkiss, 2000). Testing the

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bactericidal effect of the peptide in apple juice at 25ëC (pH 3.7) revealed only 3.5-log unit reduction after a long incubation period of 8 h at the high peptide concentration of 100 g/mL. Unlike antimicrobial peptides (AMPs) that inhibit growth of mainly Gram-positive organisms, AMPs that are produced by animal cells often display activity against a much larger spectrum of microorganisms. The potential of several native AMPs as food preservatives is increasingly being reported. The peptide was proposed to penetrate the outer membrane of E. coli to gain entry into the cytoplasm, and thus affect bacterial viability by interfering with deoxyribonucleic acid (DNA) and/or protein synthesis (Boman et al., 1993; Cabiaux et al., 1994). Scanning electron microscopy studies of exposed bacterial cells indicated that the peptide does not lyse cells by pore-forming mechanisms (Shi et al., 1996). Its effectiveness against pathogenic strains of E. coli and L. monocytogenes was tested at different temperatures (Annamalai et al., 2001). Although after 24 h incubation the peptide decreased bacterial populations by 4and 5 log units at 24 and 37ëC, respectively, it had poor activity at the lower temperatures representing normal refrigeration.

14.6

Nano-antimicrobial agents

Nanotechnology is a method of controlling matter at near-atomic scales to produce unique or enhanced materials, products and devices. In the 1990s, research on use of nanocomposites for food packaging started using montmorillonite clay as the nanocomponent in a number of polymers like polyvinyl chloride, polyethylene, nylon and starch. An antimicrobial nanocomposite film has structural integrity and barrier properties imparted by the nanocomposite matrix, and the antimicrobial properties contributed by the natural antimicrobial agents impregnated within (Rhim and Ng, 2007). Materials in the nanoscale range have a higher surface-to-volume ratio when compared with their microscale counterparts. This allows nanomaterials to be able to attach more copies of biological molecules, which confers greater efficiency (Luo and Stutzenberger, 2008). Nanoscale materials have been investigated for antimicrobial activity so that they can be used as growth inhibitors (Cioffi et al., 2005), killing agents (Huang et al., 2005; Kumar and MuÈnstedt, 2005; Lin et al., 2005; Qi et al., 2004; Stoimenov et al., 2002) and/or antibiotic carriers (Gu et al., 2003; Busolo et al., 2010). Nano titanium dioxide (TiO2) is widely used as a photocatalytic disinfecting material for surface coatings (Fujishima et al., 2000). TiO2 photocatalysis, which promotes peroxidation of the polyunsaturated phospholipids of microbial cell membranes (Maness et al., 1999), has been used to inactivate several foodrelated pathogenic bacteria (Kim et al., 2003, 2005). TiO2 powder-coated packaging film was developed to reduce E. coli contamination on food surfaces for fresh-cut produce (Robertson et al., 2005; Chawengkijwanich and Hayata, 2008). The efficacy of TiO2-coated films exposed to sunlight was demonstrated

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to inactivate fecal coliforms in water (Gelover et al., 2006). Metal doping improves visible light absorbance of TiO2 (Anpo et al., 2001) and increases its photocatalytic activity under UV irradiation (Choi et al., 1994). It has been demonstrated that doping TiO2 with silver greatly improved photocatalytic bacterial inactivation (Page et al., 2007). This combination showed good antibacterial properties from TiO2/Ag+ nanoparticles in a nanocomposite with PVC (Cheng et al., 2006). The antimicrobial mechanism of nanoscale chitosan involves interactions between positively charged chitosan and negatively charged cell membranes, increasing membrane permeability and eventually causing rupture and leakage of intracellular material. The raw chitosan and engineered nanoparticles are ineffective at pH values above 6, which would be due to the absence of protonated amino groups (Qi et al., 2004). Other two antimicrobial mechanisms are chelation of trace metals by chitosan, inhibiting enzyme activities; and, in fungal cells, penetration through the cell wall and membranes to bind deoxyribonucleic acid (DNA) and inhibit ribonucleic acid (RNA) synthesis (Rabea et al., 2003). Carbon nanotubes (CNTs) have been reported to have antibacterial properties. Direct contact with aggregates of CNTs was fatal for E. coli, possibly because the long and thin CNTs puncture microbial cells, causing irreversible damage (Kang et al., 2007). Other studies suggested that CNTs are cytotoxic to human cells when in contact with skin (Monteiro-Riviere et al., 2005; Shvedova et al., 2003) and lungs (Warheit et al., 2004). The risk of ingestion of particles incorporated in a food packaging material must be taken into account because of the possibility of migration to food. Multilayer peptide nanofilms were produced by intercalating different peptides designed to be oppositely charged at neutral pH. Disulfide (S±S) crosslinking resulted in formation of a three-dimensional network which was much more stable than when the peptide film was stabilized only by electrostatic interactions (Li et al., 2006). Silver nanocomposites-based antimicrobial films for food packaging are known for their strong toxicity against a wide range of microorganisms (Liau et al., 1997), with high temperature stability and low volatility (Kumar and MuÈnstedt, 2005). Regarding some of the proposed mechanisms for the antimicrobial property, silver nanoparticles (Ag-NPs) adhere to the cell surface by degrading lipopolysaccharides and forming pits in the membranes, largely increasing permeability (Sondi and Salopek-Sondi, 2004); penetration inside the bacterial cells damage DNA (Li et al., 2008) and release antimicrobial Ag+ ions by Ag-NPs dissolution (Morones et al., 2005). The antimicrobial activity of silver-based polymers depends on release of Ag+, which binds to electron donor groups in biological molecules containing sulfur, oxygen or nitrogen (de Azeredo, 2009). Smaller Ag-NPs with a larger surface area result in better bactericidal effect than larger Ag particles (An et al., 2008; KvõÂtek et al., 2008). Polyamide 6/silver nano- and microcomposites with a low silver content

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presented a better increased efficacy against E. coli (Damm et al., 2008). Polyamide 6 filled with 2 wt % Ag-NPs was effective against E. coli, even after immersion in water for 100 days (Damm et al., 2007). Moreover, Ag-NPs absorb and decompose ethylene (Hu and Fu, 2003), which may contribute to their effects on extending shelf-life of fruits and vegetables. A nanocomposite polyethylene (PE) film with Ag-NPs retarded the senescence of jujube (Li et al., 2009). A coating containing Ag-NPs was effective in decreasing microbial growth and increasing shelf-life of asparagus (An et al., 2008). Nanostructured calcium silicate (NCS) was used to adsorb Ag+ from solution; NCS±Ag composite exhibited effective antimicrobial activity at desirably low levels of silver down to 10 mg/kg and could be incorporated into food packaging as an antimicrobial agent (Johnston et al., 2008). Based on the antimicrobial action of nanosilver, a number of active food contact materials (FCMs) have been developed that are claimed to preserve the food materials for a longer shelf-life by inhibiting the growth of microorganisms. Some of the commercial FCMs with nanosilver as antimicrobial agents are FresherLongerTM Miracle Food Storage Containers and Plastic Storage Bags from Sharper ImageÕ USA, Nano Silver Food Containers and cutting boards from A-DO Korea, and Nano Silver Baby Milk Bottle from Baby DreamÕ Co. Ltd of South Korea. Nanosilver has also been incorporated into the inner surface of domestic refrigerators (LG, Samsung and Daewoo) to prevent microbial growth and maintain a clean and hygienic environment in the fridge. It has been used in the development of antimicrobial active coatings, such as antibacterial kitchenware, tableware and pet products from Nano Care Technology Ltd, China. The nanoparticles of zinc oxide and magnesium oxide are expected to provide a more affordable and safe food packaging solution in the future compared to nanosilver (Chaudry et al., 2008). DuPont has produced a nano-titanium dioxide plastic additive which can reduce UV damage in foods in transparent packaging. Nano-packaging can also be designed to release antimicrobials, antioxidants, enzymes, flavours and nutraceuticals to keep the packaged food tasting `fresh' for a longer period. Some of the packaging materials with nano-antibacterial agents are summarized in Table 14.3. The main risk of consumer exposure to nanoparticles from food packaging is likely to be through potential migration of nanoparticles into food. A number of food contact materials containing nanomaterials are already in commercial use in some countries. A study that determined the migration of minerals (Fe, Mg, Si) from biodegradable starch/nanoclay nanocomposite films showed an insignificant trend in the levels of Fe and Mg in packaged vegetables, but a consistent increase in the amount of Si (Avella et al., 2005). The rapid use of nano-based packaging in a wide range of consumer products has also raised a number of safety, environmental, ethical, policy and regulatory issues. The main concerns stem from the lack of knowledge with regard to the interactions of

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Table 14.3 Packaging materials with nano-antibacterial agents Company/institution

Application

SongSing Nano Technology Co., Ltd Sharper Image

Cling wrap treated with nano zinc oxide Plastic storage bags treated with nano silver Storage containers treated with nano silver

BlueMoonGoods, A-DO Global, Quan Zhou Hu Zheng Nano Technology Co., Ltd and Sharper Image Daewoo, Samsung and LG Baby DreamÕ Co., Ltd A-DO Global SongSing Nano Technology Co. Nano Care Technology Ltd

Refrigerators treated with nano silver Baby cup treated with nano silver Chopping board treated with nano silver Tea pot treated with nano silver Kitchenware treated with nano silver

Source: Friends of the Earth, 2008.

nano-sized materials at the molecular or physiological levels and their potential effects and impacts on consumer health and the environment. The design of tailor-made packaging is a real challenge and implies the use of reverse engineering approaches based on food requirements and no longer just on the availability of packaging materials. Nanotechnologies are expected to play a major role, taking into account all additional safety considerations and fulfilling present packaging needs.

14.7

Antimicrobial films and coatings

Antimicrobial films contain an antimicrobial agent that migrates to the surface of the food. This would require a molecular structure large enough to retain activity on the microbial cell wall even though bound to the plastic. Such agents are likely to be limited to enzymes or other antimicrobial proteins, and those that are effective against surface growth of microorganisms without migration. Antimicrobial agents can be incorporated into the matrix of packaging materials to control microbial growth in a packaged food product. The packaging material could be a synthetic or natural polymer, edible films and/or coatings. Due to the increase in consumer demand for fresh produce, minimally processed and preservative-free products, antimicrobial packaging has been in focus. The preservative agents may be applied to the packaging system such that a low level of preservatives comes into contact with the food. New antimicrobial packaging materials are continually being exploited using natural agents to control common food-borne microorganisms. A greater emphasis on safety features associated with the addition of antimicrobial agents is the need for development in packaging technology (Dong and Manjeet, 2004).

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The film or coating technique is considered to be more effective, though more complicated to apply. Antimicrobial agents incorporated into edible films or coatings are released onto the surface of food to control microbial growth. Such coatings can also serve as a barrier to moisture and oxygen. Edible coatings have become popular in the food industry, because they offer protection and produce less waste after the package has been opened. Edible coatings were studied to enhance the shelf-life of fresh fruits and vegetables (Park, 1999). Antimicrobial coatings and films have a variety of advantages and constitute an innovation within the biodegradable active packaging concept. They have been developed in order to reduce and/or inhibit the growth of microorganisms on the surface of foods. The use of appropriate coatings can impart antimicrobial effectiveness. A polymer-based solution coating would be the most desirable method in terms of stability and adhesiveness of attaching a bacteriocin to a plastic film.

14.7.1 Requirements Basic requirements for the preparation of ideal antimicrobial (AM) packaging materials are as follows: · Synthesized easily · Inexpensive · Stable in long-term usage and storage at the temperature of its intended application · Insoluble in water for a water disinfection application · Does not decompose to and/or emit toxic products · Non-toxic or non-irritating to handle · Can be regenerated upon loss of activity · Biocidal to a broad spectrum of pathogenic microorganisms in brief times of contact. The direct incorporation of antimicrobial additives in packaging films is a convenient methodology by which antimicrobial activity can be achieved. Some of these additives may be effective as indirect food additives incorporated into food packaging materials. Several agents have been proposed and tested for antimicrobial packaging using this method. However, the use of such packaging materials is not meant to be a substitute for good sanitation practices, but it should enhance the safety of food as an additional hurdle for the growth of pathogenic microorganisms.

14.7.2 An antimicrobial packaging system Antimicrobial agents can be incorporated into the packaging materials by blending just before final extrusion, immobilization, dissolved into coating solvents or mixed into sizing/filling materials of paper and paperboards (Han

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and Floros, 1997; Nam et al., 2002; Rodrigues and Han, 2000; Rodrigues et al., 2002; Nadarajah et al., 2002). In the case of non-permitted agents, immobilization is done by the covalent binding of the agents into chemical structures of packaging materials (Appendini and Hotchkis, 1996, 1997; Halek and Garg, 1989; Miller et al., 1984). Immobilized agent cannot migrate while the blended antimicrobial agents can migrate from packaging materials into the foods. Migration can be through diffusion and/or evaporation. In diffusion, the antimicrobial agent is directly in contact with food or one or more layers in between for controlled release. In the case of evaporation the agent is released into the headspace. Antimicrobial coating on pre-packaged products or edible coating on the food itself can produce an extra physical barrier layer. An edible coating system has various benefits due to its edibility, biodegradability and simplicity (Krochta and de Mulder-Johnston, 1997). The dry coating can incorporate chemical and natural antimicrobials (Han, 2001, 2002). In the case of wet coating a wrapping is required to avoid loss of the wet coating (e.g. lactic acid). Wet coating can be useful for fresh products, meats and poultry (Gill, 2000).

14.7.3 Synthetic polymeric packaging materials Different types of synthetic polymeric packaging materials such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene (PP), polyvinyl chloride (PVC), ethylene±vinyl alcohol (EVOH), ethylene acrylic acid (EAA) and ethylene±vinyl acetate (EVA) are used to incorporate antimicrobial agents for food packaging applications. The selection of the antimicrobial agent depends on its compatibility with the packaging material or its heat stability during extrusion (Han and Floros, 1997; Weng and Hotchkiss, 1993). Low density polyethylene (LDPE) Low density polyethylene (LDPE) film was coated with nisin using methylcellulose (MC)/hydroxypropyl methylcellulose (HPMC) as a carrier (Cooksey, 2000). Nisin coated onto a LDPE film to inhibit Micrococcus luteus and the microbiota of raw milk during storage was pH and temperature dependent (Mauriello et al., 2005). Low-density polyethylene (LDPE) films coated with a mixture of polyamide resin in i-propanol/n-propanol and a bacteriocin solution showed an antimicrobial activity against Micrococcus flavus. The incorporation of 1.0% w/w potassium sorbate in low-density polyethylene films lowered the growth rate and maximum growth of yeast, and lengthened the lag period before mould growth (Han and Floros, 1997). Benzoic anhydride impregnated with low density polyethylene (LDPE) films completely suppressed the growth of Rhizopus stolonifer, Penicillium species and Aspergillus toxicarius on potato dextrose agar (PDA). Similarly, LDPE films that contained benzoic anhydride

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delayed mould growth on cheese (Weng and Hotchkiss, 1993). LDPE film containing benzoic anhydride demonstrated the efficiency against mould growth of packaged cheese and toasted bread (DobiaÂsÏ et al., 2000). LDPE films impregnated with either 1.0% w/w Rheum palmatum and Coptis chinensis extracts or silver-substituted inorganic zirconium retarded the growth of total aerobic bacteria, lactic acid bacteria and yeast on fresh strawberries (Chung et al., 1998). Imazalil concentration of 2000 mg/kg LDPE film delayed A. toxicarius growth on potato dextrose agar, while LDPE film containing 1000 mg/kg Imazalil substantially inhibited Penicillium sp. growth and the growth of both of these moulds on cheddar cheese (Weng and Hotchkiss, 1992). The incorporation of 1% w/w grapefruit seed extract in LDPE film used for packaging of curled lettuce reduced the growth rate of aerobic bacteria and yeast (Lee et al., 1998). LDPE films incorporated with clove showed a positive antimicrobial effect against L. plantarum and F. oxysporum. Linear low density polyethylene (LLDPE) LLDPE films containing 0.05% w/w linalool or methyl chavicol showed a positive activity against E. coli (Suppakul et al., 2002). The bactericidal action of carvacrol against E. coli in ground beef was eliminated or reduced with the addition of another compound with high antiradical properties, such as ascorbic acid (Chiasson et al., 2004). Nisin-coated polymeric films such as PVC, LLDPE and nylon showed inhibition of Salmonella typhimurium on fresh broiler drumstick skin (Natrajan and Sheldon, 2000a). Ethylene acrylic acid (EAA) Benzoyl chloride-modified EAA films showed positive effects on the inhibition of growth of Penicillium sp. and Aspergillus sp. (Matche et al., 2006). Polyethylene-co-methacrylic acid film was fabricated with antimicrobial properties by the incorporation of benzoic or sorbic acids (Weng et al., 1999). Polypropylene (PP) and polyethylene/ethylene±vinyl alcohol copolymer (PE/ EVOH) films incorporated with a nominal concentration of 4% (w/w) of cinnamon or oregano essential oil completely inhibited the growth of fungi; higher concentrations were required to inhibit the Gram-positive bacteria and higher concentrations were necessary to inhibit the Gram-negative bacteria (8 and 10%, respectively) (Lopez et al., 2007). Coatings Nisin/methylcellulose coatings were used for polyethylene, ethylene±vinyl acetate, polypropylene, polyamide, polyester, acrylics, and polyvinyl chloride films (Appendini and Hotchkiss, 2002). Nisin adsorbed onto silanized silica

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surfaces inhibited the growth of L. monocytogenes thereby extending the shelflife and enhancing the microbial safety of meats (Cutter et al., 2001). Several combinations using nisin±EDTA blended with PE and polyethylene oxide (PEO) were found effective for reducing growth of Brochothrix. Coating of pediocin onto cellulose casings and plastic bags has been found to completely inhibit growth of inoculated L. monocytogenes in meats and poultry. Coating of solutions containing nisin, citric acid and EDTA onto polyvinyl chloride, linear low density polyethylene and nylon films reduced the counts of Salmonella typhimurium in fresh broiler drumstick skin. The nisin-based treatments into calcium alginate/agar gels reduced the Salmonella typhimurium population (Natrajan and Sheldon, 2000a, 2000b). Cinnamon paraffin coating totally inhibited Candida albicans, Aspergillus flavus and Eurotium repens, and provided significant activity against both Penicillium nalgiovense and P. roqueforti. Cinnamon and oregano enriched active coatings showed inhibitory activity against Gram-negative bacteria with increased shelf-life of strawberries with no visible fungal contamination (Ben Arfa et al., 2007). Saran copolymer of vinylidene chloride coated with antimicrobial nisin, lactoferrin (derived from bovine lactoferrin in cow's milk), sodium diacetate, sorbic acid and potassium sorbate inhibited L. monocytogenes. Multilayer structures A coextruded multilayered polyethylene film with 1.0% w/w grapefruit seed extract showed antimicrobial activity against M. flavus, whereas a coated film with 1.0% w/w grapefruit seed extract showed activity against E. coli, S. aureus and B. subtilis. In the case of ground beef, coating resulted in a higher level of antimicrobial activity than when incorporated by coextrusion (Ha et al., 2001). The release of lysozyme, nisin and sodium benzoate was controlled by regulating the degree of crosslinking of polyvinyl acohol (PVOH) films or by using multilayer structures (Buonocore et al., 2003, 2004). Lysozyme and nisin are both antimicrobial proteins effective against Gram-positive bacteria. However, the use of these antimicrobials in combination with chelating agents such as EDTA displays increased effectiveness against Gram-negative bacteria (Padgett et al., 1998).

14.7.4 Biopolymer-based packaging Polysaccharide-based Starch-based coatings containing potassium sorbate extended the storage life of strawberries by reducing the microbial count (Garcia et al., 1998). Methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) mixed with lauric, palmitic, stearic and arachidic acid significantly lowered the potassium sorbate

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permeation rate relative to cellulose ether films (Rico-Pena and Torres, 1991). HPMC coating with ethanol was effective in inactivating Salmonella montevideo on the surface of tomatoes (Zhuang et al., 1996). Nisin containing cellophanebased coating reduced the growth of the total aerobic bacteria significantly in chopped meat under refrigeration temperatures (Ming et al., 1997). Paper Paper coated with the fungicides sodium o-phenylphenate (PreventolÕ) and ophenylphenol propionic acid used in fruit and vegetable packaging inhibited growth of fungi Botrytis cinerea, Penicillium expansum, Monilia fructigena, Trichothecium roseum and Rhizopus. Paper coated with a dispersed mixture of a silver, copper, lead or tin salt such as silver N-stearoyl-L-glutamate is used as paper bags for protection of fruits from pathogenic microorganisms. The paper is impregnated with salts of guanidine compounds containing lipophilic groups for fruit packaging bags for protection against the fungus Alternaria kikuchiana. Grease-proof paper was coated with an aqueous dispersion of sorbic acid and an antioxidant (namely, Embanox) in carboxymethyl cellulose solution for preservation of foods by wrapping and then enclosing them in a polyethylene bag. Carvacrol-coated papers could act as a reservoir, able to gradually or totally release the antimicrobial agent and to maintain a constant microbial inhibitory effect (Afef et al., 2007). Other antimicrobial agents such as an acid adjunct of 1,17-diaguanidino-9azaheptadecane (fungicide) and calcium carbonate or calcium phosphate treated with a silver, copper (as copper sulphate), or zinc (as zinc oxide) salt are incorporated into a sheet of paper to be used as the inner liner for corrugated fibreboard boxes (Yoshida et al., 1993). Organotin tributyltin oxide (TBTO) has been used in papers to inhibit the bacterial growth of Escherichia coli, Staphylococcus pyogenes var. aureus and Aspergillus niger (Bomar, 1968). Tectosilicate (composed of silicon dioxide, aluminium oxide and sodium oxide) mixed with an aqueous antimicrobial metal salt such as silver nitrate along with an organic antimicrobial compound such as cetylpyridinium chloride is added to paper for use in wrapping fresh food (Suzuki et al., 1991). A paper incorporated with radioactive mineral powder thorium oxide, silver-containing mineral powder and/or zinc oxide powder singly or in combination is used to wrap fresh food. Other antimicrobial agents such as the metal salt of an oxyquinoline derivative, humic acid salt, and a water-soluble multivalent aluminium sulfate were used as a mixture in paper sheets used for wrapping fruits. Alginates Sodium alginate film containing nisin, lysozyme, EDTA and GFSE inhibited Gram-positive and Gram-negative bacteria (Cha et al., 2002). Carrageenan can

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be used as edible protective coatings for extending the shelf-life of poultry (Meyer et al., 1959). Cheeses coated with hydrocolloid films based on carrageenan, alginate and gellan enhanced stability against the growth of surface microorganisms in an intermediate-moisture cheese (Kampf and Nussinovitch, 2000). The synergistic effect of antimicrobials (lysozyme, nisin and GFSE) and chelating agent (EDTA) in -carrageenan-based biopolymer films against food spoilage bacteria and pathogens showed inhibitory effects against all indicator microorganisms (Cha et al., 2002). Chitosan films A chitosan coating as antimicrobial agent extended the post-harvest life and maintained the quality of longan fruit (Jiang and Li, 2001). Chitosan films made in dilute acetic acid solutions showed inhibition of the growth of Rhodotorula rubra and Penicillium notatum by direct application. Chitosan lactate and chitosan glutamate have displayed antagonistic effects against Escherichia coli, Staphylococcus aureus and Saccharomyces cerevisiae (Chen et al., 1996). Chitosan has been used as a coating and appears to protect fresh vegetables and fruits from fungal degradation (Arvanitoyannis et al., 1998). Bacterial acrylic polymers made by copolymerizing acrylic protonated amine co-monomers have been proposed as packaging materials for increased fruit and vegetable shelf-life (Shih, 1994). Chitosan±HPMC films with stearic acid and citric acid as crosslinking agent inhibited the growth of Listeria monocytogenes (Gontard and Guilbert, 1994; Hershko and Nussinovitch, 1998; MoÈller et al., 2004). Garlic oil incorporated into chitosan films led to an increase in its antimicrobial efficiency against Escherichia coli, Staphylococcus aureus, Salmonella typhimurium, Listeria monocytogenes and Bacillus cereus. However, application of garlic oil into chitosan films depends on the type of food where flavour is not a problem (Pranoto et al., 2005). The development and characterization of the antimicrobial properties of novel renewable blends of chitosan with more water-resistant gliadin proteins isolated from wheat gluten, which presented significant antimicrobial activity, have been studied. Dissolution and release of the water-soluble chitosan fractions biocide glucosamine groups increased with the amount of chitosan present in the formulation with the antimicrobial properties (Fernandez-Saiz et al., 2008). Antimicrobial activity was shown by the release of protonated glucosamine fractions from the chitosan cast films into the microbial culture during antimicrobial performance of the biopolymer under the studied conditions against Staphylococcus aureus and in some experiments also against Salmonella spp. (Fernandez-Saiz et al., 2009). Antimicrobial chitosan-based blends with EVOH copolymers, when low molecular weight chitosan was used as the dispersed phase in the blend, exhibited optimum performance in terms of water resistance, enhanced water barrier and, therefore, excellent application outlook

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in antimicrobial applications. The study also showed that EVOH copolymers can also be made antimicrobial by a water sorption-induced release mechanism, if acetic acid is incorporated into the polymer formulation before casting from solution (Fernandez-Saiz et al., 2010). Protein-based films and coatings Lysozyme, nisin and EDTA incorporating corn zein and soy protein isolates (SPI) films exhibited inhibition zones against L. plantarum (Padgett et al., 1998). The film with nisin alone and in combination with lauric acid reduced the cell count, as well as increased the zone size in the assay (Padgett et al., 2000). The sorbic acid-incorporated peanut protein films extended the product shelf-life of intermediate-moisture foods (Jangchud et al., 1999). Milk protein based film containing oregano essentials oils controlled the growth of pathogenic bacteria Escherichia coli and Pseudomonas and increased the shelf-life of beef muscle (Oussalah et al., 2004). The retention of sorbic acid in casein films treated with tannic and lactic acids, respectively, and placed over an aqueous model food system showed 30% retention of sorbic acid (GutieÂrrez et al., 2009). Whey protein isolate (WPI) edible antimicrobial films incorporated with lysozyme, nisin and EDTA were effective in inhibiting Brochothrix thermosphacta. The combined efficacy of nisin and pediocin with sodium lactate, citric acid, phytic acid and potassium sorbate and EDTA was studied as a possible sanitizer for reducing the Listeria monocytogenes population of inoculated fresh-cut produce (Bari et al., 2005). Nisin±phitic acid and nisin±pediocin±phytic acid caused significant reductions of L. monocytogenes on cabbage and broccoli (Rodrigues and Han, 2000).

14.7.5 Commercial antimicrobial systems Some of the commercial antimicrobial materials manufacturers for packaging films are MicroGardTM by Rhone-Poulenc (USA) and Piatech by Daikoku Kasei Co. (Japan); for concentrates and extracts MicroFreeTM by DuPont (USA) and MicrobanÕ by Microban Products (USA); and for a wide range of products such as cutting boards and dish cloths which contain triclosan used in soaps, shampoos and toothbrushes, Ultra-FreshÕ by Thomson Research Associates (Canada), NovaronÕ by Milliken Co. (USA), and SanitizedÕ by Sanitized AG/ Clariant (Switzerland). Antimicrobials are extracted from natural seeds such as grapefruit seed extract CitrexTM by Quimica Natural Brasileira Ltd (Brazil) and from mustard seeds WasaOuroÕ by Green Cross Co. (Japan), NisaplinÕ (Nisin) by Integrated Ingredients (USA) (Brody et al., 2001; Vermeiren et al., 2002), BactiblockÕ by Nanobiomatters Ltd (Paterna, Spain). Some of the important commercialized antimicrobial materials are summarized in Table 14.4.

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Table 14.4 Commercialized antimicrobial materials

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Company

Trade name

Active compound

Applications

Sanitized AG, Switzerland/Clariant

SanitizedÕ ActigardÕ SaniprotÕ

Triclosan and others

Textiles, plastics, leather and paper Home textiles and PU-foams Films and in-can preservation

DuPont, USA

MicroFreeTM

Ag, copper oxide and zinc silicate

Textile and carpet fibres, paints, packaging films, etc.

Milliken Co., USA

NovaronÕ

Ag-substituted zirconium phosphate

Many

Microban Products, USA

MicrobanÕ

Triclosan

Many

Õ

Thomson Research Associates, Canada

Ultra-Fresh

Triclosan and others

Polymers, adhesives, latexes, plastics, foams, etc.

Surfacine Development Company, USA

SurfacineÕ

Ag-halide/polymer complex

Many

Ishizuka Glass Co., Japan

Ionpure

Ag/glass

Many

Õ

AgION Technologies LLC, USA

Agion

Ag

Food contact, plastics, textiles, medical devices and personal care/cosmetics

Sangi Co. Japan

Apacider-AÕ Cosme-UpÕ

Silver and zinc metals

Plastics, textiles and cosmetics Manufacturing

Shinanen New Ceramics Co. Japan

ZeomicÕ

Silver zeolite

Home electrical appliances, synthetic fibres, food packaging materials, household goods, toiletries, construction materials and stationery

Table 14.4 Commercialized antimicrobial materials ß Woodhead Publishing Limited, 2011

Company

Trade name

Active compound

Applications

Quimica Natural Brasileira Ltd (Brazil)

CitrexTM

Grapefruit seed (vitamin C, citric and acetic acids and glycerine)

Many applications in food processing industries

Integrated Ingredients, USA

NisaplinÕ NovasinTM

Nisin

Wide range of foods ± liquid or solid, canned or packaged

Green Cross Co. Japan

WasaOuroÕ

Allyl mustard oil

Quality maintenance and sanitary management for food products

Daikoku Kasei Co. Japan

Piatech

Silver oxide ions

Fresh green vegetables

Nimiko Company of Osaka, Japan

Silvi film

Silver and SiO2

Fresh green vegetables

Source: Vermeiren et al., 2002.

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14.8

403

Antimicrobial activity

Antimicrobial activity evaluation methods in food are as old as the existence of disinfectants and antibiotics. For example, determination of the antimicrobial effect of vapours of crushed garlic against Mycobacterium species, Escherichia coli, Serratia marcescens and B. subtilus was studied as early as 1936 (Walton et al., 1936). Crushed garlic was placed on the lid of a Petri dish and the bottom of the dish containing nutrient medium was inverted over the top. The vapours of the garlic were allowed to enter into agar with the test microorganism for varying periods of exposure and incubated to determine inhibition. Most methods for evaluating the activity of food antimicrobials have been adopted directly or with modifications. Most of the methods used for evaluation of the antimicrobial activity in foods can be classified as shown in Fig. 14.2. An in vitro or screening test is for the preliminary information about the antimicrobial activity in which the compound is not applied to a product under use conditions. The endpoint tests give qualitative information about effective concentration. In

14.2 Various methods of evaluation of antimicrobial activities (sources: Davidson and Parish, 1989; Lo¨pez-Malo et al., 2000).

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this method a microorganism is challenged for an arbitrary period and the results reflect the inhibitory power of a compound for the specified time. The descriptive screening methods give quantitative information about the growth dynamics, in which periodic sampling is carried out to determine changes in viable cell numbers over time. In applied tests, the antimicrobial is applied to the actual food and the efficacy of the antimicrobial is evaluated (Davidson and Parish, 1989; LoÂpez-Malo et al., 2000).

14.9

Future trends

Antimicrobial packaging is playing an important role in the inhibition of pathogenic contamination in foods, thereby extending the shelf-life of foods. With the potential in providing food quality and safety, antimicrobial packaging is gaining lot of interest in research and development. Antimicrobial agents, packaging materials, coating/binding materials compatible with polymers, functionalized surfaces for ionic and covalent links, and new printing methods combined with encapsulation, are some of the newer technologies that will play a role in the development of antimicrobial packaging. Methods such as electron beam, ion beam, plasma and laser treatment in modifying polymer surfaces are emerging technologies to be studied for functionalized inert surfaces for polymers such as PE/PP/PET, etc., that are already in use. Functionalization of polymers LDPE and HDPE has already been carried out through grafting with amino and carboxylic groups in order to immobilize proteins and enzymes. Biologically active derived antimicrobial compounds have great potential for binding with pathogens, with wide-spectrum activity and low toxicity for new antimicrobial agents. Research in the area of development of smart packaging to sense the presence of microorganisms in packaged food and to trigger an antimicrobial mechanism as a response in a controlled manner is in demand. The major potential food applications of antimicrobial films include some of the sensitive foods like bakery products, dairy products (cheese), fresh produce such as fruits and vegetables, and meat, fish and poultry products. Study of the synergistic effects of different combinations of naturally derived antimicrobial agents, biopreservatives and biodegradable packaging materials is required. Natural plant extracts have great potential in the research, specifically as they have specific odours which may not be desired for packaged foods. Regulations might require some amendments related to toxicology and testing of antimicrobial compounds for the newer materials as they might not be covered under the existing regulations.

14.10 References Abe Y (1990), `Active packaging: a Japanese perspective', in Proceedings, International Conference on Modified Atmosphere Packaging, Alveston, Avon, UK, Part 1, 15±17.

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Branen J and Davidson P M (2000), `Activity of hydrolysed lactoferrin against foodborne pathogenic bacteria in growth media: the effect of EDTA', Letters in Applied Microbiology, 30, 233±237. Branen J K and Davidson P M (2004), `Enhancement of nisin, lysozyme, and monolaurin antimicrobial activities by ethylenediaminetetraacetic acid and lactoferrin', International Journal of Food Microbiology, 90, 63±74. Brewer R, Adams M R and Park S F (2002), `Enhanced inactivation of Listeria monocytogenes by nisin in the presence of ethanol', Letters in Applied Microbiology, 34, 18±21. Brody A L, Strupinsky E R and Kline L R (2001), `Antimicrobial packaging', in Active Packaging for Food Applications, Technomic Publishing, Lancaster, PA, pp. 131± 194. Brul S and Coote P (1999), `Preservative agents in foods mode of action and microbial resistance mechanisms', International Journal of Food Microbiology, 50, 1±17. Buncic S, Fitzgerald S, Bell C M and Hudson R G (1995), `Individual and combined listericidal effects of sodium lactate, potassium sorbate, nisin and curing salts at refrigeration temperatures', Journal of Food Safety, 15, 247±264. Buonocore G C, Del Nobile M A, Panizza A, Corbo M R and Nicolais L (2003), `A general approach to describe the antimicrobial agent release from highly swellable films intended for food packaging applications', Journal of Control Release, 90, 97±107. Buonocore G C, Sinigaglia M, Corbo M R, Bevilacqua A, La Notte E and Del Nobile M A (2004), `Controlled release of antimicrobial compounds from highly swellable polymers', Journal of Food Protection, 67, 1190±1194. Burt S (2004), `Essential oils: their antibacterial properties and potential applications in foods-a review', International Journal of Food Microbiology, 94, 223±253. Busolo M A, Fernandez P, Ocio M J and LagaroÂn J M (2010), `Novel silver-based nanoclay as an antimicrobial in polylactic acid food packaging coatings', Food Additives and Contaminants, DOI: 10.1080/19440049.2010.506601. Cabiaux V, Agerberth B, Johansson J, Homble F, Goormaghtigh E and Ruysschaert M (1994), `Secondary structure and membrane interaction of PR-39, a Pro+Arg-rich antibacterial peptide', European Journal of Biochemistry, 224, 1019±1027. Carneiro De Melo A M S, Cassar C L and Miles R J (1998), `Trisodium phosphate increases sensitivity of Gram-negative bacteria to lysozyme and nisin', Journal of Food Protection, 61, 839±844. Cha D S, Choi J H, Chinnan M S and Park H J (2002), `Antimicrobial films based on Na alginate and -carrageenan', Lebensmittel-Wissenschaft und -Technologie, 35, 715± 719. Chaudhry Q, Scotter M, Blackburn J, Ross B, Boxall A, Castle L, Aitken R and Watkins R (2008), `Applications and implications of nanotechnologies for the food sector', Food Additives and Contaminants: Part A, 25, 241±258. Chawengkijwanich C and Hayata Y (2008), `Development of TiO2 powder-coated food packaging film and its ability to inactivate Escherichia coli in vitro and in actual tests', International Journal of Food Microbiology, 123, 288±292. Chen H and Hoover D G (2003), `Bacteriocins and their food applications', Food Science Safety, 2, 82±100. Chen M C, Yeh G H C and Chiang B H (1996), `Antimicrobial and physicochemical properties of methylcellulose and chitosan films containing a preservative', Journal of Food Processing and Preservation, 20, 379±390. Cheng Q, Li C, Pavlinek V, Saha P and Wang H (2006), `Surface-modified antibacterial

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compounds into biodegradable packaging films', Journal of Food Protection, 61, 1330±1335. Padgett T, Han I Y and Dawson P L (2000), `Effect of lauric acid addition on the antimicrobial efficacy and water permeability of corn zein films containing nisin', Journal of Food Processing and Preservation, 24, 423±432. Page K, Palgrave R G, Parkin I P, Wilson M, Savin S L P and Chadwick A V (2007), `Titania and silver±titania composite films on glass-potent antimicrobial coatings', Journal of Materials Chemistry, 17, 95±104. Paik J S and Kelley M J (1995), Annual Meeting of the Institute of Food Technologists, Chicago, IL. Paola A and Joseph H (2002), `Review of antimicrobial food packaging', Innovative Food Science and Emerging Technologies, 3, 113±126. Parente E, Giglio M A, Riccardi A and Clementi F (1998), `The combined effect of nisin, leucocin F10, pH, NaCl and EDTA on the survival of Listeria monocytogenes in broth', International Journal of Food Microbiology, 40, 65±75. Park H J (1999), `Development of advanced edible coatings for fruits', Trends in Food Science and Technology, 10, 254±260. Periago P M, Palop A and Fernandez P S (2001), `Combined effect of nisin, carvacrol and thymol on the viability of Bacillus cereus heat-treated vegetative cells', Food Science and Technology International, 7, 487±492. Pol I E and Smid E J (1999), `Combined action of nisin and carvacrol on Bacillus cereus and Listeria monocytogenes', Letters in Applied Microbiology, 29, 166±170. Pol I E, Mastwijk H C, Slump R A, Popa M E and Smith E J (2001a), `Influence of food matrix on inactivation of Bacillus cereus by combinations of nisin, pulsed electric field treatment, and carvacrol', Journal of Food Protection, 64, 1012±1018. Pol I E, van Arendonk W G, Mastwijk H C, Krommer J, Smid E and Moezelaar R (2001b), `Sensitivities of germinating spores and carvacrol adapted vegetative cells and spores of Bacillus cereus to nisin and pulsed electric-field treatment', Applied and Environmental Microbiology 67, 1693±1699. Pranoto Y, Rakshit S K and Salokhe V M (2005), `Enhancing antimicrobial activity of chitosan films by incorporating garlic oil, potassium sorbate and nisin', Lebensmittel-Wissenschaft und -Technologie, 38, 859-865. Qi L F, Xu Z R, Jiang X, Hu C and Zou X (2004), `Preparation and antibacterial activity of chitosan nanoparticles', Carbohydrate Research, 339, 2693±2700. Quattara B, Simard S, Piette G J P, Holley R A and Begin A (1999), Annual Meeting of the Institute of Food Technologists, Chicago, IL. Quintavalla S and Vicini L (2002), `Antimicrobial food packaging in meat industry', Meat Science, 62, 373±380. Rabea E I, Badawy M E, Stevens C V, Smagghe G and Steurbaut W (2003), `Chitosan as antimicrobial agent: applications and mode of action', Biomacromolecules, 4, 1457±1465. Rayman K, Aris B and Hurst A (1981), `Nisin: possible alternative or adjunct to nitrite in the preservation of meats', Applied and Environmental Microbiology, 41, 375±380. Rayman K, Malik N and Hurst A (1983), `Failure of nisin to inhibit outgrowth of Clostridium botulinum in a model cured meat system', Applied and Environmental Microbiology, 46, 1450±1452. Rhim J W and Ng P K W (2007), `Natural biopolymer-based nanocomposite films for packaging applications', Critical Reviews in Food Science and Nutrition, 47, 411± 433. Rico-Pena D C and Torres J A (1991), `Sorbic acid and potassium sorbate permeability of

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14.11 Appendix: Abbreviations Ag-NPs AM AMP BHA BHT CNT DNA EAA EDTA EVA EVOH FCM GFSE HPMC LDPE LLDPE LPS MC NCS PDA PE PEF PEO PP PVC RNA TBHQ TBTO WPI

Silver nanoparticles Antimicrobial Antimicrobial peptide Butylated hydroxy anisole Butylated hydroxy toluene Carbon nanotube Deoxyribonucleic acid Ethylene acrylic acid Ethylene diamine tetraacetic acid Ethylene vinyl acetate Ethylene vinyl alcohol Food contact material Grapefruit seed extracts Hydroxypropyl methylcellulose Low density polyethylene Linear low density polyethylene Lactoperoxidase system Methylcellulose Nanostructured calcium silicate Potato dextrose agar Polyethylene Pulsed electric field treatment Polyethylene oxide Polypropylene Polyvinyl chloride Ribonucleic acid Tertiary butyl hydroquinone Tributyltin oxide Whey protein isolate

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Natural extracts in plastic food packaging P . S U P P A K U L , Kasetsart University, Thailand

Abstract: Nowadays, natural extracts are of increasing interest. They have been applied to packaging applications as antimicrobial and/or antioxidant plastic packaging. This chapter first reviews antimicrobial and antioxidant activities of highlighted potential natural plant extracts. Secondly, possible techniques to design active plastic packaging systems related to natural extracts are discussed. Thirdly, the effectiveness of active packaging films based on natural extracts is mentioned. The chapter then provides information regarding factors to consider in designing active systems. In addition, future trends complete the chapter. Key words: natural plant extract, essential oil, plastic packaging, antimicrobial packaging, antioxidant packaging, active packaging, food packaging.

15.1

Introduction

The appearance of foods is one of the major determinants of its appeal to consumers and consequently, sales of the product. Microbial contamination and lipid oxidation are crucial factors that determine food quality loss and shelf-life reduction. Therefore, both preventing microbial contamination and delaying lipid oxidation are highly relevant to food processors. The growth of microorganisms in food products may cause spoilage or foodborne diseases. Oxidative processes in food products lead to the degradation of lipids and proteins. In turn, they have contributed to the deterioration in safety, flavour, texture and colour of the products (Decker et al., 1995). In response to the dynamic changes in current consumer demand for minimally processed foods, in retail and distribution practices associated with globalisation, new consumer product logistics, new distribution trends (e.g. Internet shopping), automatic handling systems at distribution centres, and the stricter requirements regarding consumer health and safety (Vermeiren et al., 1999; Sonneveld, 2000), the area of active packaging (AP) technologies is becoming increasingly significant. This is particularly important in the area of fresh and extended shelf-life foods as originally described by Labuza and Breene (1989) and followed by Guilbert et al. (1996). The mere mention of active plastic packaging brings about a discussion of different classes of antimicrobials (AM) and antioxidants (AO) (Suppakul et al.,

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2003a), in that packaging technologists utilise these active additives for active packaging applications. Although natural plant extracts are not highly considered in this application due to some drawbacks associated with volatility at high temperature or tainted flavour in some food applications, these aforementioned natural extracts can serve as an alternative source of AM and/or AO with many advantages in terms of volatile migration, in which this system can be effectively used for highly porous foods, powdered, shredded, irregularly shaped, and particulate foods (Han, 2005). This chapter will highlight potential natural plant extracts with their AM and/or AO activity. Possible techniques to design active plastic packaging systems related to natural extracts will be discussed. The effectiveness of active packaging films based on natural extracts will be mentioned. This chapter will also provide information regarding factors to consider in designing active systems. In addition, future trends will complete the chapter. The author suggests that the review of the referenced literature will be of value in this regard.

15.2

Natural plant extracts as antimicrobials and antioxidants

Prevention of pathogenic and spoilage microorganisms in foods is usually achieved by using chemical preservatives. These chemical preservatives act as AM compounds which inhibit the growth of undesirable microorganisms. However, the onset of increasing demand for minimally processed foods, the enhancement of shelf-life of foods, or the excessive use of chemical preservatives, some of which are suspect because of either post-contamination or potential toxicity, have resulted in a demand on food manufacturers to find alternative means (Conner, 1993; Nychas, 1995). There is a currently strong debate about the safety aspects of chemical preservatives since they are considered responsible for many carcinogenic and teratogenic attributes as well as residual toxicity. For these reasons, consumers tend to be suspicious of chemical additives and thus the exploration of naturally occurring antimicrobials for food preservation receives increasing attention due to consumer awareness of natural food products and a growing concern about microbial resistance towards conventional preservatives (Schuenzel and Harrison, 2002). Oxidative deterioration of fat components in food products is responsible for off-flavours and rancidity which decrease nutritional and sensory qualities. An addition of antioxidants is required to preserve product quality. Synthetic antioxidants (e.g. butylate hydroxytoluene, BHT; butylate hydroxyanisole, BHA; tert-butylhydroxyhydroquinone, TBHQ; and propyl gallate, PG) are widely used as antioxidants in the food industry. Their safety, however, has been questioned. BHA was revealed to be carcinogenic in animal experiments. At high doses, BHT may cause internal and external haemorrhaging, which leads to death in some strains of mice and guinea pigs (Ito et al., 1986). There is much interest among food manufacturers

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in natural antioxidants, to act as replacements for synthetic antioxidants currently used (Plumb et al., 1996). Consequently, extensive research on AM and AO effects of spices and herbs used as active agents in foods and beverages has been carried out. Food packages can be made AM/AO active by incorporation and immobilisation of AM/AO agents or by surface modification and surface coating. Present plans envisage the possible use of naturally derived AM/AO agents in packaging systems for a variety of processed meats, cheeses and other foods, especially those with relatively smooth product surfaces that come in contact with the inner surface of the package. This solution is becoming increasingly important, as it represents a perceived lower risk to the consumer (Nicholson, 1998). Nonetheless, the `natural' origins of the chemicals are likely to be a selling point, but this does not necessarily make them safer than artificial additives (Biever, 2003). Nowadays, natural plant extracts have been extensively applied to food, pharmaceutical and cosmetics industries due to a perceived higher risk of synthetic materials to the consumer. Plant extracts consist of phytochemicals which have biological activities. These phytochemicals may contain colour, odour and/or flavour which yield unique characteristics varied in qualitative and quantitative attributes of phytochemicals in accordance with species, parts of plant, plant age, and environment (Tragoolpua, 1996). Examples of herbs and spices are bay (Pimenta racemosa), betel (Piper betel), black mustard (Brassica nigra), brown mustard (Brassica juncea), cinnamon (Cinnamomum iners), clove (Syzygium aromaticum), curcumin (Curcuma longa), eucalyptus (Eucalyptus polybractea), fingerroot (Boesenbergia pandurata), galangal (Alpinia galangal L.), garlic (Allium sativum), ginger (Zingiberis rhizome), green tea (Camellia sinensis), holy basil (Ocimum sanctum), lemongrass (Cymbopogon citratus), oregano (Origanum vulgare), rosemary (Rosmarinus officinalis), sage (Salvia officinalis), sweet basil (Ocimum basilicum), thyme (Thymus vulgaris), wasabi (Wasabia japonica), etc. (see Table 15.1).

15.2.1 Betel oil Betel is a tropical plant closely related to the common pepper and belongs to the Piperaceae family. Atal et al. (1975) reported that betel oil contains chavicol, allylpyrocatechol, chavibetol, methyl chavicol, methyl eugenol, 1,8-cineole, eugenol, caryophyllene and cadinene. In addition, Rimando (1986) reported that betel oil also contains chavibetol acetate, allylpyrocatechol diacetate, carvacrol, campene, methyl chavibetol, eugenol, pinene, limonene, safrole, and allylpyrocatechol monoacetate. Recently, Bhattacharya et al. (2006) have found that betel ethanolic extract appears to be a promising formulation for further investigation as a new natural photo-protector. Several researchers have reported that betel extract and betel oil showed AM and AO activities in model systems (Salleh et al., 2002; Lei et al.,

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Table 15.1 Examples of AM and AO additives for potential use in food packaging materials Activity

Class

Examples

Antimicrobial

Plant extract

Bay, betel, black mustard, brown mustard, cinnamon, citronella, clove, eucalyptus, fingerroot, garlic, grapefruit seed, lemon, lemongrass, oregano, rosemary, sage, sweet basil, thyme, turmeric, wasabi Allicin, allyl isothiocyanate (AIT), carvacrol, chavibetol, cineole, cinnamaldehyde, citral, citronellal, p-cymene, estragole (methylchavicol), eucalyptol, eugenol, geraniol, hinokitiol ( -thujaplicin), hydrocinnamaldehyde, linalool, terpineol, thymol Betel, cinnamon, clove, green tea, rosemary, sage, sweet basil, thyme, turmeric Carnosol, carvacrol, catechin, chavibetol, cineole, cinnamaldehyde, estragole (methylchavicol), eugenol, geraniol, gingerol, hydrocinnamaldehyde, rosemanol, thymol

Plant volatile compound

Antioxidant

Plant extract Plant volatile compound

Sources: Isshiki et al. (1992); Kim et al. (1995); Hammer et al. (1999); Suppakul et al. (2003a; 2003b); Chrubasik et al. (2005); Han (2005); Holley and Patel (2005); Thongson et al. (2005); Bozin et al. (2006); Hazzit et al. (2006); Suppakul et al. (2006a); Bakkali et al. (2007); Becerril et al. (2007); Bozin et al. (2007); Sanla-Ead (2007); Hussain et al. (2008).

2003; Dilokkunanant et al., 2004; Suliantari et al., 2005; Bhattacharya et al., 2006). Essential oil (EO) of betel showed AM activity against nine pathogenic and spoilage bacteria including Aeromonas hydrophila, Bacillus cereus, Enterococcus faecalis, E. coli, E. coli O1587:H7, Listeria monocytogenes, Micrococcus luteus, Salmonella Enteridis and Staphylococcus aureus and three strains of yeast including Candida albicans, Saccharomyces cerevisiae and Zygosaccharomyces rouxii using an agar well diffusion assay. Using an agar dilution method ranging from 0.78 to 200 L mLÿ1, the minimum inhibitory concentrations (MICs) of betel oil in a range of 12.5±100 L mLÿ1 could inhibit the growth of all test microorganisms (Suppakul et al., 2006a). This oil also revealed AO activity against oxidative bleaching of -carotene using a carotene agar well diffusion assay. The minimum oxidative bleaching inhibitory concentration (MOBIC) of betel oil was 100 L mLÿ1 (Suppakul et al., 2006a).

15.2.2 Cinnamon oil Cinnamon is a member of the Lauraceae family and traditionally harvested in Asian countries. It is, perhaps, one of the oldest herbal medicines, having been mentioned in Chinese texts as long as 4000 years ago. Cinnamon oil has

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exhibited health beneficial properties, such as AM activity. Cinnamon oil contains cinnamaldehyde, ethyl cinnamate, eugenol, -caryophyllene, linalool and methyl chavicol (Hu et al., 1985; Chang et al., 2001). Cinnamon oil has exhibited antifungal, antiviral, antibacterial and larvicidal activities. Specifically, constituents in cinnamon are able to kill E. coli, C. albicans and S. aureus. In addition, its AM and antifungal properties have also drawn great attention from many researchers (Hili et al., 1997; Ouattara et al., 1997; Chang et al., 2001; Kim et al., 2004). Cinnamaldehyde, which was identified in the oil, is an effective inhibitor of the growth of yeasts, bacteria and moulds as well as toxin production by microorganisms. It can completely inhibit the growth of a number of bacteria such as Staphylococcus spp., Micrococcus spp., Bacillus spp. and Enterobacter spp. (Masuda et al.,1998). The MICs of cinnamon oil and cinnamaldehyde in a range of 6.25±25 and 0.78±12.5 L mLÿ1, respectively, could inhibit the growth of A. hydrophila, B. cereus, E. faecalis, E. coli, E. coli O1587:H7, L. monocytogenes, M. luteus, S. Enteridis, S. aureus, C. albicans, S. cerevisiae and Z. rouxii (Sanla-Ead et al., 2006). This oil revealed AO activity against oxidative bleaching of -carotene using a -carotene agar well diffusion assay (see Fig. 15.1) and radical scavenging activity against free radicals using a 2,2-diphenyl-1-picryhydrazyl (DPPH) assay (see Fig. 15.2). The MOBIC of cinnamon oil was 50 L mLÿ1. At a concentration of 0.39 L mLÿ1 solution in ethanol, cinnamon oil yielded the radical scavenging activity of 91.13% (Phoopuritham et al., 2006; Phoopuritham, 2007) with EC50 of 0.0198 L mLÿ1 (Phoopuritham, 2007).

15.2.3 Clove oil Clove belongs to the Myrtaceae family. Clove is indigenous to the Moluccas and widely cultivated in Madagascar, Sri Lanka, Indonesia and the south of China. Clove oil has biological activities, such as antibacterial, antifungal, insecticidal and AO properties. Clove oil consists of major phenolic compounds called eugenol, caryophyllene and eugenyl acetate (Pallado et al., 1997). Eugenol acts as both AO (Dorman et al., 2000) and AM (Farag et al.,1989; Blaszyk and Holley, 1998). Eugenol showed an inhibitory effect on the growth of L. monocytogenes, Campylobacter jejuni, S. Enteritidis, E. coli and S. aureus (Beuchat, 2000; Cressy et al., 2003). Dorman and Deans (2000) reported on the antibacterial activity of 21 plant volatile oil components (including eugenol and linalool) against 25 bacterial strains by the agar well diffusion technique. Eugenol exhibited the widest spectrum of activity against 24 out of 25 bacteria, except for Leuconostoc cremoris, followed by linalool (against 23 strains, except L. cremoris and Pseudomonas aeruginosa). The MOBIC of clove oil was 50 L mLÿ1. At a very low concentration of 0.39 L mLÿ1, its percentage of 93.39 indicates that clove oil exhibited very powerful radical scavenging activity in a similar fashion to the

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15.1 Unbleached zone of undiluted selected plant extracts using a -carotene agar well diffusion assay in comparison with synthetic references: (a) BHA; (b) cinnamon oil; (c) clove oil; (d) green tea extract.

synthetic antioxidants, e.g. BHA, BHT (see Fig. 15.2) (Phoopuritham et al., 2006; Phoopuritham, 2007). Shan et al. (2005) found that phenolic compounds as the principal active components in spice extracts paid a significant contribution to their AO capacity. The AO activity of phenolic compounds is mainly due to their redox properties, which allow them to act as reducing agents, hydrogen donators and singlet oxygen quenchers (Rice-Evans et al., 1995).

15.2 DPPH radical scavenging activity of 0.39 L mLÿ1 selected plant extracts compared to synthetic reference.

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15.2.4 Fingerroot oil Fingerroot belongs to the family Zingiberaceae. The major constituents of fingerroot oil from rhizomes are geraniol, camphor, boesenbergin A, boesenbergin B, cardamonin, chavicinic acid, and 1,8-cineole (Jantan et al., 2001). Jantan et al. (2001) evaluated the EO of fingerroot with the EO of other Zingiberaceae species against several genera of moulds and yeast. Fingerroot oil was the most effective antifungal agent against both yeasts and moulds including species of Aspergillus spp., Mucor spp., Candida spp., Saccharomyces spp., and Torulopsis spp. Thongson et al. (2005) determined the potential AM activity of extracts and essential oils of rhizome spices from Thailand against food-borne pathogenic bacteria. It was found that fingerroot oil inhibited against L. monocytogenes and Salmonella Typhimurium all strains at 0.4% (v/v). Norajit et al. (2007) reported that fingerroot oil showed AM activity against L. monocytogenes, E. coli, B. cereus and S. aureus. At a concentration of 50 L mLÿ1, fingerroot oil showed moderate AM activity against B. cereus, E. faecalis, L. monocytogenes and A. hydrophila, except M. luteus, S. aureus, E. coli, E. coli O157:H7, P. aeruginosa, S. Enteritidis, Vibrio parahaemolyticus, C. albicans, S. cerevisiae and Z. rouxii (Srinaovaratkul, 2009). At a very low concentration of 0.78 L mLÿ1 solution in ethanol, fingerroot oil and geraniol failed to scavenge DPPH radical (Srinaovaratkul, 2009).

15.2.5 Galangal oil Galangal also belongs to the family Zingiberaceae. The essential oil from the rhizome possesses a strong and spicy odour. The oil possesses antibacterial, antispasmodic, antitubercular and carminative properties. Galangal is also used as a medicine for curing stomach ache in China and Thailand (Yang and Eilerman, 1999). It has been shown that essential oils from both fresh and dried rhizomes of galangal have antimicrobial activities against bacteria, fungi, yeast and parasites (Farnsworth and Bunyapraphatsara, 1992). The main constituents identified in the rhizome of galangal oil were 1,8-cineole, -pinene (Raina et al., 2002) and acetoxychavicol acetate (Oonmetta-aree et al., 2006). Galangal rhizomes were tested against several bacteria such as Bacillus subtilis, E. coli, S. aureus, A. hydrophila and P. aeruginosa. The essential oil was found to be effective against many Gram-positive bacteria (Barik, 1987).

15.2.6 Oregano oil Oregano belongs to the Origanum genus and to the Lamiaceae (Labiatae) family. The principal constituents of oregano oil include carvacrol, thymol, pcymene or -terpinene, depending on its chemotype (Burt, 2004). Oregano oil showed antimicrobial activity against a wide spectrum of microorganisms

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including A. hydrophila, B. subtilis, B. cereus, E. coli, Salmonella Typhimurium, S. aureus, Brochotrix thermosphacta, Salmonella enterica, Serratia marcescens, Salmonella flexneri, Yersinia enterocolitica, Enterobacter aerogenes, Salmonella choleraesius, Salmonella sonnei, Klebsiella pneumoniae, L. monocytogenes, E. coli O157:H7, C. albicans and Aspergillus ochraceus (Burt, 2004; Souza et al., 2006; Bakkali et al., 2007). Veldhuizen et al. (2006) determined antimicrobial activity of carvacrol against S. aureus. It was found that an inhibitory effect on lag phase extension increases with increasing carvacrol concentrations. At a concentration of 50 L mLÿ1, carvacrol showed strong antimicrobial activity against B. cereus, E. faecalis, L. monocytogenes, M. luteus, S. aureus, A. hydrophila, E. coli, E. coli O157:H7, S. Enteritidis, Vibrio parahaemolyticus, C. albicans and Z. rouxii except P. aeruginosa and S. cerevisiae (Srinaovaratkul, 2009). At a very low concentration of 0.78 L mLÿ1 solution in ethanol, carvacrol yielded the highest DPPH radical scavenging activity of 64.30%, followed by oregano oil (63.50%) with EC50 of 0.4174 L mLÿ1 (Srinaovaratkul, 2009).

15.2.7 Rosemary oil Rosemary belongs to the Lamiaceae (Labiatae) family. Chemical components of rosemary oil are 1,8-cineole, -pinene, bornyl acetate, camphor and limonene (Burt, 2004; FernaÂndez-LoÂpez et al., 2005; Bozin et al., 2007). Phenolic compounds such as carnosol, carnosoic acid, rosemanol, rosmadial, epirosmanol, rosmadiphenol, rosmarinic acid, rosemariquinone, etc. (Madsen and Bertelsen, 1995; Bicchi et al., 2000; Yanishlieva et al., 2006; Wijeratne and Cuppett, 2007) are considered to have AO capacity. Bozin et al. (2007) reported that the most important antibacterial activity of rosemary oil was expressed on E. coli, Salmonella Typhimurium, S. Enteritidis and Shigella sonei. A significant rate of antifungal activity of this oil was also exhibited. Strong inhibition of lipid oxidation in two systems of induction, Fe2+/ascorbate and Fe2+/H2O2, was observed for rosemary oil. Surprisingly, undiluted rosemary oil could not inhibit the oxidative bleaching of -carotene agar. At a concentration of 0.39 L mLÿ1 solution in ethanol, rosemary oil yielded a radical scavenging activity of 12.11% (Phoopuritham et al., 2006; Phoopuritham, 2007) in agreement with the study of Bozin et al. (2007) which reported that 0.50 and 1.20 L mLÿ1 rosemary oil gave radical scavenging activity of 13.89 and 19.44%, respectively.

15.2.8 Sweet basil oil Sweet basil is one of the oldest spices belonging to the Ocimum genus and to the Lamiaceae (Labiatae) family. The chemical analysis of essential oils derived from O. basilicum L. has been the subject of many studies with varying results from country to country (Suppakul et al., 2003b). Major constituents of sweet

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basil oil are dependent upon its chemotype (such as `methyl chavicol', `linalool', `methyl cinnamate', `eugenol', `1,8-cineole', `methyl chavicol-linalool', `linalool-methyl chavicol', `linalool-eugenol', etc.). The variation in the chemical composition of basil essential oils is thought to be due to polymorphism in O. basilicum L., which in turn is caused by interspecific hybridisation (Paton and Putievsky, 1996). Dube et al. (1989) studied the antifungal activity of the essential oil of sweet basil by an agar dilution method. They showed that the essential oil of basil at a concentration of 1.5 mL Lÿ1 completely suppresses the mycelial growth of 22 species of fungi, including the mycotoxin-producing strains of Aspergillus flavus and Aspergillus paralyticus. Ozcan and Erkmen (2001) studied the antifungal activity of basil essential oil collected in Turkey. They found the oil to be ineffective on S. cerevisiae, Aspergillus niger and Rhizopus oryzae, contrary to the findings of Prasad et al. (1986), Dube et al. (1989) and Meena and Sethi (1994). This contradiction might be due to the different chemotype of sweet basil or due to different test methods (Suppakul et al., 2003b). Opalchenova and Obreshkova (2003) studied the activity of basil against multi-drug resistant clinical bacterial strains by using different test methods including MIC determination and time-kill kinetics. The experimental data obtained after the application of different methods of investigation demonstrated a strong inhibitory effect of sweet basil oil on multi-drug resistant clinical isolates of the genera Staphylococcus, Enterococcus and Pseudomonas. The chosen bacteria are widespread and pose serious therapeutic difficulties because of their high extent of resistance. For this reason, Opalchenova and Obreshkova (2003) considered that the results they obtained were encouraging. Pattnaik et al. (1997) studied the antibacterial properties of the aromatic constituents of essential oils. The results of the disc diffusion assays showed that linalool was the most effective compound and retarded 17 out of 18 bacterial strains (only VR-6, a Pseudomonas, was found to be resistant), followed by cineole, geraniol, menthol and citral. They also found that the MIC values of the essential oils were usually lower than those of their constituents. One possible reason for this result could be the synergistic action of the constituents in the oils. Mazzanti et al. (1998) found that linalool was the active compound that completely inhibited the growth of all yeasts (seven strains of Candida albicans, C. krusei and C. tropicalis), S. aureus and E. coli. Authentic pure linalool showed a similar antibacterial spectrum to that of basil essential oils. However, pure methylchavicol exhibited a much narrower antibacterial spectrum, with an activity against only eight out of the 24 strains of organism tested (Lachowicz et al., 1998). Friedman et al. (2002) screened a broad variety of naturally occurring and potentially food compatible plant-derived oils and oil compounds for their AM activities against an epidemiologically relevant class of four species of bacterial food-borne pathogens, C. jejuni, E. coli, L. monocytogenes and Salmonella enterica. It was found that eugenol and methylchavicol (estragole)

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are the most active agents showing AM activity against all of these pathogens. Variations in AM or AO activity of natural plant extracts are in association with (a) cultivar and chemotaxonomic classification, (b) compositional variation, (c) enantiomeric considerations, and (d) other factors affecting composition of natural plant extracts: (1) the harvest season and plant phenological stages, (2) the extraction method, (3) milling techniques, (4) drying techniques, (5) supplementary UV-B treatment of glasshouse-grown plants, (6) the wavelengths of light reflected from coloured mulches, and (7) field-grown vs greenhouse (Suppakul et al., 2003b).

15.3

Designing active plastic packaging systems from natural plant extracts

In order to design active plastic packaging systems from natural plant extracts, the processability of the polymers, the active additives and the other additives should be considered. This will enable the fabrication of active plastic packaging materials with sound morphological structures that will earn the designed properties at an economical cost (Matthews, 1982). In addition, it is essential to achieve not exclusively the required shapes (Han, 2005) but also a suitable extent of homogeneity in composition and properties (Kim and Kwon, 1996).

15.3.1 Coating of active additives Early developments in AM/AO packaging are derived from edible coatings, especially wax coating. In China, wax has been used to retard desiccation of citrus fruits since the twelfth and thirteenth centuries (Hardenburg, 1967). In the last three decades, application of fungicides including benomyl, imazalil and thiabendazole in wax coating has been studied primarily on citrus fruits (Brown, 1974; Eckert and Kolbezen, 1977; Brown et al., 1983). For coating film formation, there are many techniques (e.g. coacervation, solvent removal and solidification of melt), which have been developed for directly coating onto food surfaces, or as separate, self-supporting films (Donhowe and Fennema, 1994). Any of the coating film-forming techniques can be employed with any of the following application techniques: dipping, spraying, casting and other methods (e.g. brushing, falling-film enrobing, panning or rolling techniques) (Guilbert, 1986). Additionally, both tampography and serigraphy can be employed for coating application (Gardes et al., 2004). Naturally derived antimicrobials and antioxidants from plant extracts that cannot tolerate high temperatures used in polymer processing are often coated onto the material after fabrication. Appropriate coatings can sometimes impart AM/AO effectiveness. An et al. (2000) claimed that a polymer-based solution coating would be the most desirable method in terms of stability and adhesiveness of attaching an active agent to a plastic film.

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15.3.2 Incorporation of active additives The direct incorporation of bioactive agents into polymers has been commercially applied in drug and pesticide delivery, household goods, textiles, surgical implants and other biomedical devices. Few food-related applications have been commercialised (Appendini and Hotchkiss, 2002). Direct incorporation of AM/AO additives in packaging films is a convenient means by which AM/AO activity can be achieved. Han (2000) suggested an extrusion process for direct incorporation of AM/AO additives into a low density polyethylene (LDPE), as a polymeric matrix layer. A virgin plastic resin and active additives are compounded and pelletised to produce masterbatch resins. These expected amounts of the masterbatch pellets are then added to a plain plastic resin to homogeneously fabricate final active AM/AO packaging materials with a concentration distribution of active agents and a controlled release rate. Apart from compounding by an extrusion process, melt blending could be an alternative process. The first step consists of melt blending the polymeric matrix (such as low density polyethylene (LDPE), polylactic acid (PLA) and polycaprolactone (PCL)) with one of the natural plant extracts by using an internal mixer, controlled by a measuring drive. The mixing temperature is 155ëC, 140ëC and 80ëC for PLA, LDPE and PCL, respectively. In the second step a hot press is used to prepare slabs with a thickness of 1 mm. Materials were heated at the same temperature of mixing, pressed at 50 bar for 3 min and subsequently cooled to 30ëC under pressure. The slabs were cut into short pieces in order to obtain a material suitable for extrusion. Finally, the active AM/AO films are obtained by feeding the pellets into an extruder (Del Nobile et al., 2009). Due to the high loss of natural plant extracts during the higher temperature of the extrusion process, microencapsulation of these extracts via inclusion compounds (i.e. cyclodextrins) can minimise their loss. Cyclodextrins (CDs) are cyclic oligosaccharides that have been derived enzymatically from starch and have the ability to encapsulate other molecules within their ringed structure. Cyclodextrins are represented as shallow truncated cones composed of six, seven and eight glucose units and termed -, - and -CD, respectively. From the manufacturing point of view, the main product is -CD. Purification of the - and -CD considerably raises their production cost and puts them into the fine-chemical classification, i.e. very expensive. The primary polar hydroxyl groups project from one outer edge and the secondary polar hydroxyl groups project from the other end. While outer surfaces (tops and bottoms) are hydrophilic, an internal cavity has a relatively high electron density and is hydrophobic in nature, due to the hydrogens and glycosidic oxygens oriented to the interior of its cavity (Shahidi and Han, 1993). Because of the hydrophobic nature of the cavity, the molecules of suitable size, shape and hydrophobility enable non-covalent interaction with

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CDs to form stable complexes. For instance, -CD molecules are able to form inclusion complexes with volatile compounds of typical molecular mass ranging from 80 to 250 (Rusa et al., 2001). Microencapsulation is one of the most effective techniques for protecting them against oxidation, thermal degradation and evaporation (Szente and Szejtli, 1988; Reineccius, 1989; Hedges et al., 1995). This protection occurs because volatile molecules are held tightly within the molecular structure of -CD. The interaction between -CD (host) and volatile molecules (guests) may involve total inclusion or association with only the hydrophobic part of the molecule (Shahidi and Han, 1993). Goubet et al. (1998) stated that retention of volatile compounds is a complex phenomenon in which several factors take part. With particular regard to the volatile compound, chemical function, molecular weight, steric hindrance, polarity and relative volatility have been shown to be important. For example, the higher the molecular weight, the higher the retention (Reineccius and Risch, 1986; Goubet et al., 1998). Among the chemical groups reviewed, alcohols are usually the best retained compounds by carbohydrates including -CD. The same trend, namely a higher retention of alcohols than that of other compounds, has been observed also when encapsulating a mixture of 10 volatiles in -CD. Linalool was the most retained compound among mixtures including five esters, two aldehydes, -deca-lactone and butyric acid (Fleuriot, 1991). As far as polarity is concerned, the more polar the compound the less of it is retained (Voilley, 1995). According to Saravacos and Moyer (1968) and Bangs and Reineccius (1981), the higher the relative volatility of a compound, the lower is its retention. When the carrier is considered, it has been shown that retention is influenced by its chemical functions, its molecular weight and its state. Based on literature data, Goubet et al. (1998) summarised that the amorphous state of the carrier is the most efficient for retention of volatiles, the collapsed state results in a loss of volatiles and crystallisation leads to the largest release of the encapsulated compounds. -CD is widely used in the food, pharmaceutical, medical, chemical and textile industries. Huang et al. (1999) proposed using an inclusion compound (IC) between an antibiotic and -CD in biodegradable/bioabsorbable PCL film for medical applications. Antibiotic ICs have been incorporated into bandages, dressings and sutures. Later, Lu et al. (2001) reported a promising result for Irgasan DP300 (Triclosan)- -CD-IC embedded in PCL films. These are rendered resistant to the growth of E. coli. Japanese horseradish (wasabi) extract containing allyl isothiocyanate (AIT) as a major active AM component has been encapsulated in -CD to control its volatility. Allyl isothiocyanate becomes volatile when AIT- -CD-IC is exposed to a high humidity environment. This AIT- -CD-IC has been incorporated in LDPE film. After packaging of the food product, the evaporated AIT migrates to the food surface and then inhibits the growth of microorganisms (Koichiro, 1993).

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15.3.3 Smart blending of active additives Due to the lack of polymeric packaging materials that can provide the controlled release of active agents at a rate suitable for a wide range of food packaging applications, researchers from Rutgers University and Clemson University have collaborated to develop a concept of using smart blending to generate active polymeric packaging materials for controlled release packaging (CRP), which can be means to enhance food quality and safety and in turn to prolong the shelflife of foods (LaCoste et al., 2005). The mobility of active agent in the polymeric packaging material can be affected by the microstructure or morphology of its material. This is particularly true for a polymer blend material consisting for two or more immiscible phases, where a new approach is required to alter the blend morphology in order to provide controlled release of active compound for a wide range of food packaging applications, such as short-term or intermediate-term inhibition of microorganisms in fresh foods and long-term decrease in lipid oxidation in processed foods. The smart blending system consists of two extruders and a smart blender that is composed of a barrel and internal motor-operated stir rods. Two separate polymer melt flows from the extruders are combined at the smart blender. This is a structuring process rather than a mixing process. What makes the system `smart' is that the structuring is not random ± the same morphology is always created under a certain set of conditions through computer control, which permits the development of a predictable set of relationships. A single smart blending instrument can tailor a large variety of structures in active polymeric packaging materials that are often unattainable with traditional compounding. The operation of the smart blender is on the basis of the principle of chaotic advection, a relatively new field of fluid mechanics (Aref, 2002), in which two confocal elliptical cylinders rotate to set up a flow pattern characterised by repetitive stretching and folding of polymer melts so that intricate patterns emerge. Some instances of various film morphologies developed by Zumbrunnen and Inamdar (2001), Danescu and Zumbrunnen (2000), Zumbrunnen and Chhibber (2002) and Zumbrunnen et al. (2002) include multilayer morphology, selective permeable morphology, interpenetrating (sponge) morphology, fibrous morphology and networking morphology. Figure 15.3 outlines a systematic approach for developing controlled release packaging. A better understanding of these systematic relationships between the components will greatly enhance the successful development. Hence, smart blending is a promising technology for bridging this research gap and presents a new paradigm for designing novel active polymeric packaging materials (LaCoste et al., 2005).

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15.3 Systematic approach for smart blending of active additives. Adapted from LaCoste et al. (2005).

15.4

Packaging films based on natural extracts

Floros et al. (1997) reviewed the products and patents in the area of AP and identified AM packaging as one of the most promising versions of an AP system. Han (2000), Brody et al. (2001), Cooksey (2001), Appendini and Hotchkiss (2002), Quintavalla and Vicini (2002), Vermeiren et al. (2002), Suppakul et al. (2003a), LoÂpez-Rubio et al. (2004) and Coma (2008) published articles focused on AM systems with a detailed discussion of some of the principal AM concepts.

15.4.1 Antimicrobial packaging films Antimicrobial packaging is defined as a packaging system that acquires AM activity: the packaging system (or material) limits or prevents microbial growth by extending the lag phase and/or reducing the growth rate or decreases live counts of microorganisms. Korean researchers developed certain AM films impregnated with naturally derived AM agents (An et al., 1998; Chung et al., 1998; Lee et al., 1998; Hong et al., 2000; Ha et al., 2001) (see Table 15.2). These compounds are perceived to be safer and are claimed to alleviate safety concerns (Lee et al., 1998). Grapefruit seed extract (GFSE), which contains narigin, hesperidin and various organic acids such as ascorbic acid and citric acid (Sakamoto et al., 1996), exhibits a wide spectrum of microbial growth inhibition. It was reported that the incorporation of 1% w/w GFSE in LDPE film (30 m thick) used for packaging of curled lettuce reduced the growth rate of aerobic bacteria and yeast. In contrast, a level of 0.1% GFSE yielded no significant effect on the rate of microbial growth in packaged vegetables, except for lactic acid bacteria on soybean sprouts (Lee et al., 1998). Ha et al. (2001) studied GFSE incorporated (by co-extrusion or a solution-coating process) in

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Table 15.2 Antimicrobial food packaging materials based on natural plant extracts

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Natural plant extracts

Packaging materials

Foods

Microorganisms

References

Cinnamon oil

PP, PE/EVOH

Culture media

Lo¨pez et al. (2007)

Cinnamon oil fortified cinnamaldehyde Clove extract

PP, PE/EVOH

Culture media

LDPE

Culture media

Clove oil

PP, PE/EVOH

Culture media

Coptis chinensis extract

LDPE

Lettuce, cucumber Strawberries

Gram-positive bacteria, Gram-negative bacteria, yeast and mould Gram-positive bacteria, Gram-negative bacteria, yeast and mould Escherichia coli, Fusarium oxysporum, Lactobacillus plantarum, Saccharomyces cerevisiae Gram-positive bacteria, Gram-negative bacteria, yeast and mould Total aerobic bacteria, lactic acid bacteria, yeast Total aerobic bacteria, lactic acid bacteria, yeast Total aerobic bacteria Total aerobic bacteria, lactic acid bacteria, yeast Total aerobic bacteria, coliform bacteria Five strain cocktail of Pseudomonas spp. Gram-positive bacteria, Gram-negative bacteria, yeast and mould Gram-positive bacteria, Gram-negative bacteria, yeast and mould Total aerobic bacteria, lactic acid bacteria, yeast Total aerobic bacteria, lactic acid bacteria, yeast Listeria innocua, Escherichia coli Total aerobic bacteria

LDPE Garlic oil Grapefruit seed extract

SPI-OPP/PE LDPE

Lemon extract Oregano oil

LDPE, PA LDPE, PLA, PCL PP, PE/EVOH

Sprouts Curled lettuce Soybean sprouts Ground beef Culture media Culture media

PP

Culture media

LDPE LDPE

Lettuce, cucumber Strawberries

PA-LDPE SPI-OPP/PE

Culture media Sprouts

Rheum palmatum extract

Rosemary oleoresin

Lo¨pez et al. (2007) Hong et al. (2000) Lo¨pez et al. (2007) An et al. (1998) Chung et al. (1998) Gamage et al. (2009) Lee et al. (1998) Ha et al. (2001) Del Nobile et al. (2009) Lo¨pez et al. (2007) Gutie¨rrez et al. (2009a) An et al. (1998) Chung et al. (1998) Han et al. (2007) Gamage et al. (2009)

Table 15.2 Antimicrobial food packaging materials based on natural plant extracts

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Natural plant extracts

Packaging materials

Foods

Microorganisms

References

Allyl isothiocyanate

PE/EVOH/PET

Ground beef

Escherichia coli O157: H7

Carvacrol

SPI-OPP/PE MC/HPMC-LDPE

Sprouts Ground pork

LDPE/EVA LDPE/Nanomer

± Culture media

PA-LDPE PP

Culture media Culture media

PVA-starch PVA-starch MC/HPMC-LDPE

Culture media Fresh sliced beef Chicken fillet

MC/HPMC-LDPE

Vietnamese ham

Total aerobic bacteria Total aerobic bacteria, yeast and mould, Escherichia coli, Listeria monocytogenes, Pseudomonas spp., Salmonella spp., Staphylococcus aureus ± Brochothrix thermosphacta, Listeria innocua, Pseudomonas fragi, Carnobacterium spp. Listeria innocua, Escherichia coli Gram-positive bacteria, Gram-negative bacteria, yeast and mould Escherichia coli Yeast, Escherichia coli Total aerobic bacteria, lactic acid bacteria, yeast and mould, Escherichia coli, Salmonella spp., Listeria monocytogenes, Pseudomonas spp. Total aerobic bacteria, lactic acid bacteria, coliform bacteria, Clostridium perfringens, Escherichia coli, Listeria monocytogenes, Salmonella spp., Staphylococcus aureus, Pseudomonas spp.

Muthukumarasamy et al. (2003) Gamage et al. (2009) Srinaovaratkul (2009)

Catechins Cinnamaldehyde

Cran et al. (2010) Persico et al. (2009) Han et al. (2007) Gutie¨rrez et al. (2009a) Chen et al. (2003) Wu et al. (2010) Suppakul and Dejsuk (2010) Dejsuk (2008)

PA-LDPE PP

Culture media Culture media

SPI-OPP/PE EVA-LDPE

Sprouts Culture media

MC/HPMC-LDPE

Chicken fillet

MC/HPMC-LDPE

Vietnamese ham

Hydrocinnamaldehyde

PP

Culture media

Linalool

LDPE/EVA

Cheddar cheese

Methylchavicol

LDPE/EVA LDPE/EVA

± Cheddar cheese

Thymol

EVA-LDPE

Culture media

LDPE/EVA LDPE, PLA, PCL PA-LDPE PP

± Culture media Culture media Culture media

Eugenol

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Listeria innocua, Escherichia coli Gram-positive bacteria, Gram-negative bacteria, yeast and mould Total aerobic bacteria Gram-positive bacteria, Gram-negative bacteria Total aerobic bacteria, lactic acid bacteria, yeast and mould, Escherichia coli, Salmonella spp., Listeria monocytogenes, Pseudomonas spp. Total aerobic bacteria, lactic acid bacteria, coliform bacteria, Clostridium perfringens, Escherichia coli, Listeria monocytogenes, Salmonella spp., Staphylococcus aureus, Pseudomonas spp. Gram-positive bacteria, Gram-negative bacteria, yeast and mould Total aerobic bacteria, Escherichia coli, Listeria innocua ± Total aerobic bacteria, Escherichia coli, Listeria innocua Gram-positive bacteria, Gram-negative bacteria ± Five-strain cocktail of Pseudomonas spp. Listeria innocua, Escherichia coli Gram-positive bacteria, Gram-negative bacteria, yeast and mould

Han et al. (2007) Gutie¨rrez et al. (2009a) Gamage et al. (2009) Tippayatum et al. (2009) Suppakul and Dejsuk (2010) Dejsuk (2008)

Gutie¨rrez et al. (2009a) Suppakul et al. (2008) Cran et al. (2010) Suppakul et al. (2008) Tippayatum et al. (2009) Cran et al. (2010) Del Nobile et al. (2009) Han et al. (2007) Gutie¨rrez et al. (2009a)

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multi-layered polyethylene (PE) films and assessed the feasibility of their use for ground beef. It was found that coating the PE film with GFSE with the aid of a polyamide (PA) binder resulted in a more effective degree of AM activity on the agar plate medium that did its incorporation by a co-extrusion process. The film co-extruded with a 1.0% GFSE layer showed AM activity only against Micrococcus flavus, whereas the film coated with 1.0% GFSE showed activity against several microorganisms including E. coli, Staphylococcus aureus and Bacillus subtilis. Both types reduced the growth rates of bacteria on ground beef stored at 3ëC, as compared to plain PE film. The two investigated GFSE levels (0.5 and 1.0% w/w) did not differ significantly in the efficacy of the film in terms of its ability to preserve the quality of beef. Gamage et al. (2009) studied the effect of AM soy protein isolate (SPI) coated oriented polypropylene (OPP)/PE packaging on the prolonged shelf-life of fresh sprouts. Different concentrations (0.6±1.2% v/v) of antimicrobials (i.e. allyl isothiocyanate, trans-cinnamaldehyde, garlic oil or rosemary oleoresin) incorporated SPI were coated onto the OPP/PE film. Sprouts (alfalfa, broccoli and radish) packed in these AM pouches were stored at 10ëC for 5 days. Significant reduction of the total microbial count of sprouts was observed in treated samples. Allyl isothiocyanate was the most effective AM agent, followed by garlic oil and trans-cinnamaldehyde. Gas chromatography analysis of headspace gas composition revealed that the volume of AIT inside the pouches was the highest of the tested compounds due to its greatest diffusion efficiency. Garlic oil showed the least diffusion and showed a higher efficiency in controlling microbial growth on sprouts. Chung et al. (1998) found that LDPE films (48±55 m thick) impregnated with either 1.0% w/w Rheum palmatum and Coptis chinensis extracts or silversubstituted inorganic zirconium retarded the growth of total aerobic bacteria, lactic acid bacteria and yeast on fresh strawberries. However, the study of An et al. (1998) showed that LDPE films (48±55 m thick) containing 1.0% w/w R. palmatum and C. chinensis extracts or Ag-substituted inorganic zirconium did not exhibit any AM activity in a disc test (Davidson and Parish 1989) against E. coli, S. aureus, Leuconostoc mesenteroides, Saccharomyces cerevisiae, Aspergillus niger, Aspergillus oryzae and Penicillium chrysogenum. A film containing sorbic acid showed activity against E. coli, S. aureus and L. mesenteroides. The reasons for this unusual result are not clear. During diffusion assays, the AM agent is contained in a well or applied to a paper disc placed in the centre of an agar plate seeded with the test microorganism. This arrangement may not be appropriate for essential oils, as their components are partitioned through the agar due to their affinity for water (Davidson and Parish, 1989). Accordingly, broth and agar dilution methods are widely used to determine the AM effectiveness of essential oils (Davidson and Parish, 1989). Due to the low water solubility of essential oils or oil components, and the very low concentration of essential oils or oil components presented in the AM

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films, studies by Pranoto et al. (2005), Zivanovic et al. (2005), Rojas-GrauÈ et al. (2006), Seydim and Sarikus (2006), Du et al. (2008), Ponce et al. (2008) and Sivarooban et al. (2008) revealed a failure of AM effectiveness in films tested using an agar disc diffusion assay. For an agar disc diffusion assay, if there is no clear zone surrounding a colony, it will be assumed that the film does not show any inhibitory effect against test microorganisms. This assay, however, may lead to a misunderstanding of the properties of lipophilic AM film. Therefore, a vapour diffusion assay is more reliable in determining the AM effectiveness of lipophilic AM films (see Fig. 15.4). The number of colony-forming units is an important factor to consider. In this study, 0.1 mL of inoculum (5  102 colony-forming units (CFU) mLÿ1) was employed in order to attain about 25±40 colonies per plate after spreading and incubation. This number, randomly distributed on the agar surface, allows for ease of observation of differences between treated and control samples, and of measurement of colony diameter. In addition, to accurately compare the effectiveness of AM films with that of different plant volatile compounds at the same concentration, the thickness of the agar and the tested film must be standardised; this is because of the effect on volume of headspace and quantity of target test compound, respectively. With this technique, it is possible to quantitatively determine the AM activities of AM films containing essential oils or oil constituents, and to compare the AM activities of these films which differ in their hydrophilic/lipophilic properties (Sanla-Ead et al., 2006; Sanla-Ead, 2007). According to Hong et al. (2000), the AM activity of 5.0% w/w propolis extract, chitosan polymer and oligomer, or clove extract in LDPE films (0.030± 0.040 mm thick), against Lactobacillus plantarum, E. coli, S. cerevisiae and Fusarium oxysporum is best determined through viable cell counts. Overall, LDPE films with incorporated natural compounds show a positive AM effect against L. plantarum and F. oxysporum. Preliminarily studies by Suppakul et al. (2002) with linear low density polyethylene (LLDPE) films (45±50 m thick) containing 0.05% w/w linalool or methylchavicol showed a positive activity against E. coli. In a later investigation, Suppakul et al. (2003c) incorporated linalool or methylchavicol into LDPE±ethylene vinyl acetate (EVA) films of 45±50 m thickness to minimise the loss of active agent. A net loss of the active agent was observed during the extrusion process but this was significantly lower than the loss observed in a previous study (Suppakul et al., 2002). Cheddar cheese wrapped with AM incorporated LDPE-based films containing either 0.34% w/w linalool or methyl chavicol and stored at 4ëC revealed an inhibitory effect of these AM films against mesophilic aerobic bacteria and coliforms, as well as yeast and mould growths in naturally contaminated cheese. In addition, cheese samples inoculated with E. coli or Listeria innocua, wrapped with these AM films and stored at refrigerated (4ëC) or abuse (12ëC) temperatures, showed that the effect on suppression of E. coli and L. innocua growth was more pronounced at the abuse temperature. Methylchavicol±LDPE-based

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15.4 Growth inhibition of selected microorganisms by cellulose-ether coated LDPE film containing 1% (w/w) of either (1) cinnamaldehyde or (2) eugenol: (a) Staphylococcus aureus; (b) Escherichia coli; (c) Saccharomyces cerevisiae.

film exhibited a higher efficacy of inhibition than that of linalool±LDPE-based film. A sensory evaluation was performed with regards to possible taint in the flavour of the cheese. Taint in flavour as affected by linalool or methylchavicol was not significantly detectable by the panellists at the end of the storage period of 6 weeks (Suppakul et al., 2008).

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15.4 Continued

Srinaovaratkul (2009) investigated the AM activity of cellulose-ether films containing either oregano oil or carvacrol at different concentrations of 0.5% and 1.0% (w/w). All these films showed positive AM activity against all test strains including B. cereus, L. monocytogenes, S. aureus, A. hydrophila, E. coli, S. Enteritidis, V. parahaemolyticus, C. albicans, and Z. rouxii. In addition, minced pork wrapped with 0.5% or 1.0% (w/w) carvacrol-incorporated cellulose ethercoated LDPE film and stored at a refrigerated temperature of 4ëC and an abuse temperature of 10ëC revealed an inhibitory effect of these AM films against mesophilic aerobic bacteria, yeasts, moulds, S. aureus, E. coli, Pseudomonas spp., L. monocytogenes and Salmonella spp. growths in naturally contaminated minced pork. Sensory panellists did not perceive a difference between minced pork wrapped with AM LDPE film and with additive-free LDPE film. Persico et al. (2009) also investigated the AM activity of nanocomposite LDPE film containing 10% (w/w) carvacrol. It was found that this AM film showed a positive AM activity against B. thermosphacta, L. innocua and Carnobacterium spp., except Pseudomonas fragi. Recently, Tippayatum et al. (2009) studied the AM activity of EVA-coated LDPE film containing 2% or 4% (w/w) of thymol or eugenol or a combination of these AM compounds. The EVAflex150/LDPE incorporated with 4% thymol and eugenol was the most effective film against L. monocytogenes, B. cereus, S. aureus and E coli. The EVAflex40w/LDPE films containing thymol or eugenol, or a combination of these, showed limited inhibitory effects against all bacteria tested. These results indicated that the effectiveness of the antimicrobial substances depends on their interactions with the packaging materials.

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From a sensory quality viewpoint, GutieÂrrez et al. (2009b) reported that the use of AM coated PP film containing 2% (w/w) cinnamon oil has been proven to increase the shelf-life of a complex bakery product from 3 to 10 days with a minimal change in the packaging and no additional manipulation steps, as well as ensuring maximum quality and safety, as demonstrated by the sensory evaluation results. However, Ouattara et al. (2001) reported that low scores were obtained in sensory evaluation tests for odour and taste of pre-cooked shrimp (Penaeus spp.) coated with a protein-based solution containing 0.9 or 1.8 mL 100 gÿ1 of a mixture of thyme oil and trans-cinnamaldehyde. They claimed that these low scores resulted from the intrinsic sensory attributes of thyme oil and trans-cinnamaldehyde. Mejlholm and Dalgaard (2002) claimed that 0.05 mL 100 gÿ1 of oregano oil yielded a distinct, although pleasant, flavour to cod fillets, and the oil delayed spoilage reactions and extended the shelf-life of the fish.

15.4.2 Antioxidant packaging films Antioxidant packaging is defined as a packaging system that acquires AO activity: the packaging system (or material) limits or prevents lipid oxidation by donating electrons or hydrogen, quenching oxygen and/or scavenging free radicals. Phoopuritham (2007) determined the AO activity of cellulose-ether films containing 0.1% (w/w) cinnamon oil, clove oil or green tea extract. Clove± cellulose-ether film was found to be strong against -carotene bleaching and DPPH radical, followed by cinnamon± and green tea±cellulose-ether films (see Fig. 15.5 and Table 15.3). It is interesting to learn that all cellulose-ether films were more effective against DPPH radical in comparison with -carotene bleaching assay. Moreover, stressed AO-free soybean oil was packed with 1.0% (w/w) cinnamon-, clove- or green tea-incorporated cellulose-ether-coated LDPE pouch and stored at 45ëC for 8 weeks. They helped to retard oxidative deterioration of this aged soybean oil by significantly lowering the peroxide value (PV) and free fatty acid value (FFA) in comparison with additive-free LDPE pouch. NerõÂn et al. (2008) developed and evaluated AO-coated polypropylene (PP) film containing rosemary extract. It was found that 4.0% w/w rosemary extract-coated PP film inhibited the oxidation extent by up to 30.1% when compared to the control film at 15 days for iron(II) and by 42.2% in comparison with additive-free PP film at 20 days for fatty acids from flax seed oil. Ascorbic acid has proven to be not adequate for antioxidant quantitative purposes owing to its prooxidant properties. Srinaovaratkul (2009) investigated the AO activity of cellulose-ether films containing either oregano oil or carvacrol at different concentrations of 0.5% and 1.0% (w/w). All these films showed positive AO activity against both oxidative bleaching of -carotene and DPPH radical. Additionally, minced pork wrapped with 0.5% or 1.0% (w/w) carvacrol-incorporated cellulose-ether-coated

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Table 15.3 Antioxidant food packaging materials based on natural plant extracts ß Woodhead Publishing Limited, 2011

Natural plant extracts

Packaging materials

Foods

Oxidation

References

Cinnamon oil Clove oil Grapefruit seed extract Green tea extract Rosemary extract

MC-HPMC-LDPE MC-HPMC-LDPE LDPE, PA MC-HPMC-LDPE PP PP MC-HPMC-LDPE PVA-starch PVA-starch MC-HPMC-LDPE MC-HPMC-LDPE

Soybean oil Soybean oil Ground beef Soybean oil Oxidizable model ± Ground pork Oxidizable model Fresh sliced beef Vietnamese ham Vietnamese ham

Peroxide value, FFA value Peroxide value, FFA value TBA value Peroxide value, FFA value Ascorbic acid, iron (II), fatty acid ± Peroxide value, TBA value TBA value TBA value TBA value TBA value

Phoopuritham (2007) Phoopuritham (2007) Ha et al. (2001) Phoopuritham (2007) Ner|¨ n et al. (2008) Bentayeb et al. (2007) Srinaovaratkul (2009) Chen et al. (2003) Wu et al. (2010) Dejsuk (2008) Dejsuk (2008)

Carvacrol Catechins Cinnamaldehyde Eugenol

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15.5 DPPH radical scavenging activity of cellulose-ether films containing 0.1% (w/w) selected plant extracts: (a) control, DPPH solution; (b) treatments, clove cinnamon or green tea±cellulose-ether films in scavenged DPPH solution.

LDPE film and stored at a refrigerated temperature of 4ëC and an abuse temperature of 10ëC revealed an inhibitory effect of these AO films against a reduction in redness. They significantly showed a lower pH, peroxide value (PV) and thiobarbituric acid (TBA) value in minced pork samples during a storage period of 7 days. In summary, the major potential food applications of AM/AO packaging films generally include meats, seafoods, poultry, bakery goods, cheeses, fruits and vegetables, cereals, prepared meals or ready meals (Labuza and Breene, 1989; Suppakul et al., 2003a). Very low doses of active agents can possibly be used to exhibit AM and/or AO activities. These powerful active natural plant

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extracts and their principal constituents should be considered and selected for food applications in order to avoid the pitfall of research in relation to detrimental effect on flavour. Alternatively, flavour matching between additive and food product should be employed. For instance, wasabi extract-incorporated sheet is commercially used for bento (Japanese lunch box).

15.5

Factors to consider in designing active systems

It is clear that the selection of both the substrate and the active substance is important in developing an active packaging system. Furthermore, when an active agent is added to a packaging material, it may affect the inherent physicomechanical properties of the latter.

15.5.1 Process conditions and residual active activity The effectiveness of an active agent applied by impregnation may deteriorate during film fabrication, distribution and storage (Han, 2000). The chemical stability of an incorporated active substance is likely to be affected by the extrusion conditions, namely the high temperatures, shearing forces and pressures (Han and Floros, 1999). To minimise this problem, Han (2000) recommended using master batches of the active agent in the resin for preparation of active packages. Also, all operations such as lamination, printing and drying as well as the chemicals (adhesives and solvents) used in the process may affect the active activity of the package. In addition, some of the volatile active compounds may be lost during storage. All these parameters should be evaluated.

15.5.2 Characteristics of active additives and foods The mechanism and kinetics of growth inhibition are generally studied in order to permit mathematical modelling of microbial growth (Han, 2000). Foods with different biological and chemical characteristics are stored under different environmental conditions, which, in turn, may cause different patterns of microflora growth. Aerobic microorganisms can exploit headspace O2 for their growth. The pH of a product affects the growth rate of target microorganisms and changes the degree of ionisation of the most active chemicals, as well as the activity of the active agents (Han, 2000).

15.5.3 Chemical interaction of active additives with film matrix During incorporation of additives into a polymer, the polarity and molecular weight of the additive have to be taken into consideration. Since LDPE is nonpolar, additives with a high molecular weight and low polarity are more compatible with this polymer (Weng and Hotchkiss, 1993). Furthermore, the

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molecular weight, ionic charge and solubility of different additives affect their rates of diffusion in the polymer (Cooksey, 2000).

15.5.4 Storage temperature The storage temperature may also affect the activity of active packages. Several researchers found that the protective action of active films deteriorated at higher temperatures, due to high diffusion rates in the polymer (Vojdani and Torres, 1989; Wong et al., 1996). The diffusion rate of the AM agent and its concentration in the film must be sufficient to remain effective throughout the shelf-life of the product (Cooksey, 2000). Suppakul (2004) evaluated the storage of LDPE-based films containing either linalool or methylchavicol as antimicrobial (AM) additives. Rolls of LDPE-based film containing linalool or methylchavicol were stored at 25 and 35ëC. It was found that the amount of additive in the film decreased with time and the additive retention in all films tended to deviate from the theoretical first-order decay plot of ln(C) versus time (Mizrahi, 2000). These findings suggest that an amount of linalool or methylchavicol that is sufficient to maintain AM activity remained in the polymeric matrix after the storage period.

15.5.5 Mass transfer coefficients and modelling Mathematical modelling of the diffusion process could permit prediction of the AM agent release profile and the time during which the agent remains above the critical inhibiting concentration. With a higher diffusivity and much larger volume of the food component compared to the packaging material, a semiinfinite model in which the packaging component has a finite thickness and the food component has infinite volume could be practical. The initial and boundary conditions that could be used in mass transfer modelling have been identified (Han, 2000). Lim and Tung (1997) determined the vapour pressure of pure AIT and that of AIT above AIT±canola oil mixtures. Canola oil is effective in depressing the vapour pressure of AIT, and may be used as a controlling diluent for this purpose in modified atmosphere packaging (MAP) applications. It was found that the diffusion, solubility and permeability coefficients of AIT in polyvinylidene chloride (PVDC)/polyvinyl chloride (PVC) copolymer films are concentration and temperature dependent. At a fixed vapour activity, the diffusion and permeability coefficients increased, whereas the solubility coefficient decreased with an increase in temperature.

15.5.6 Properties of packaging materials Antimicrobial agents may affect the physical properties, processability or machinability of the packaging material. Han and Floros (1997) reported no significant differences in the tensile properties before and after the incorporation

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of potassium sorbate in LDPE films, but the transparency of the films deteriorated as the sorbate concentration increased. Weng and Hotchkiss (1993) reported no noticeable differences in clarity and strength of LDPE film containing 0.5 and 1.0% benzoic anhydride. Similar results were reported for naturally derived extracts such as propolis at 5.0% (Hong et al., 2000), clove at 5.0% (Hong et al., 2000), R. palmatum at 1.0% (An et al., 1998; Chung et al., 1998), C. chinensis at 1.0% (An et al., 1998; Chung et al., 1998) and basil at 0.34% (Suppakul et al., 2006b). On the other hand, LDPE film coated with MC/ HPMC containing nisin was difficult to heat-seal (Cooksey, 2000). Suppakul et al. (2006b) evaluated the properties of LDPE-based films containing either linalool or methylchavicol as AM additives. A slight decrease in transparency, water vapour and oxygen transmission rates was found in the extruded films containing 0.34% w/w of linalool or methylchavicol. The infrared (IR) spectra of the AM films were similar to that of the additive-free LDPE film. However, in the spectra of the AM films, the carbonyl peaks could also be observed. There was no significant difference in the degree of crystallinity and in the melting temperature range of the different films. Derivative thermogravimetry mass-loss curves showed that the thermal decomposition temperatures of the AM films were marginally lower than that of the LDPE film. Electron micrographs indicated that AM LDPE-based films exhibited no evidence of changes in the microstructures to suggest that linalool and methylchavicol were not evenly distributed in the film.

15.5.7 Cost There are no published data on the cost of films impregnated with AM and/or AO agents, but they can be expected to be more expensive than their basic counterparts. Commercialisation of such films could therefore become viable for high-value food products only (Cooksey, 2000). In order to expand these packaging systems to foodstuffs, reasonable cost recovery should be promised for the commercialisation (Meroni, 2000).

15.5.8 Food contact approval Some volatile compounds derived from plants have US Food and Drug Administration (FDA) approval as additives for certain foods (Suppakul et al., 2003a). Allyl isothiocyanate is not approved by the FDA for use in the USA (Brody et al., 2001) due to a safety concern that this synthetic compound may be contaminated with traces of the toxic allyl chloride used in the manufacturing process (Clark, 1992). In Japan, the use of AIT is allowed only when this compound is extracted from a natural source (Isshiki et al., 1992). Packages intended for food contact applications are required to belong to a positive list of approved compounds, and an overall migration limit from the material into the

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food or food simulant was set at 60 mg kgÿ1. This is incompatible with the aim of active packaging, especially when the system is designed to release active ingredients into the foods. Consequently, as was also stated by van Beest (2001), a new approach in food packaging regulations is needed. The current applications of AM/AO food packaging are rather limited, although promising. This is because of the legal status of the tested additives (Vermeiren et al., 2002). As a result of a final approval of active and intelligent materials regulation on 30 May 2009 by the European Parliament (Suppakul, 2009), globally, it will both directly and indirectly affect the science and technology, economics, society and environment.

15.6

Future trends

Antimicrobial/antioxidant packaging is a rapidly emerging technology. The need to package foods in a versatile manner for transportation and storage, along with the increasing consumer demand for fresh, convenient and safe food products, presages a bright future for AM/AO packaging. However, more information is required on the chemical, microbiological and physiological effects of these systems on the packaged food especially on the issues of nutritional quality and human safety (Floros et al., 1997). So far, research on AM/AO packaging has focused primarily on the development of various methods and model systems, whereas little attention has been paid to its preservation efficacy in actual foods (Han, 2000). Research is essential to identify the types of food that can benefit most from AM/AO packaging materials. It is likely that future research into a combination of naturally derived AM/AO agents, biopreservatives and biodegradable packaging materials will highlight the merits of AM/AO packaging in terms of food safety, shelf-life and environmental friendliness (Nicholson, 1998). The reported effectiveness of natural plant extracts suggests that further research is needed in order to evaluate their AM/AO activity and potential side-effects in packaged foods (Devlieghere et al., 2004). An additional challenge is in the area of odour/flavour transfer by natural plant extracts to packaged food products. Thus, research is needed to determine whether natural plant extracts could act both as an AM/AO agent and as an odour/flavour enhancer. Moreover, in order to secure safe food, amendments to regulations might require toxicological and other testing of compounds prior to their approval for use (Vermeiren et al., 2002). Lastly, natural plant extract-based AM/AO design and development can be created in various aspects. For research area expansion, it can be expanded to nutraceutical research (nutraceutical±-tocopherol-incorporated edible film (Park and Zhao, 2004)), aquaculture research (antianxiety±clove oil-incorporated edible-coated film (Pattanasiri et al., 2008; Pattanasiri, 2010)), dermatological research (antiacne±cassia oil-incorporated edible coating or gel (Charoenkul et al., 2004)), dental research (antiseptic±guava leaf extract-

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incorporated edible film as oral strip), neurological research (alerting±capsaicinincorporated edible film as secured-driving strip), etc. For bifunctionality, comprehensive assessment of bifunctional activity of natural plant extracts could be of great benefit to various types of foods. For instance, betel, cinnamon, clove and oregano oil showed bifunctional activities against both pathogenic and spoilage microorganisms and oxidative deterioration in foods (Suppakul et al., 2006a; Sanla-Ead et al., 2006; Phoopuritham et al., 2006; Srinaovaratkul, 2009). For barrier enhancement (LagaroÂn et al., 2005), nanoclay can be combined to fabricate AM/AO film such as AM-thymol-impregnated nanocomposite film (Sanchez-Garcia et al., 2008) and AM-carvacrol-incorporated nanocomposite film (Persico et al., 2009). For biodegradation, biodegradable polymers (i.e. PLA, PCL) can replace synthetic polymers as substrates of natural plant extractincorporation, such as AM-thymol-incorporated biodegradable film and AMlemon extract-incorporated biodegradable film (Del Nobile et al., 2009). For maximisation of natural plant extract effectiveness and minimisation of natural plant extract concentration, synergy of natural plant extracts should be studied as exemplified by Sukatta et al. (2008).

15.7

Sources of further information and advice

Blackburn C de W and McClure P J (eds) (2002), Foodborne Pathogens: Hazards, Risk Analysis and Control, Cambridge, UK, Woodhead Publishing. Burt S (2004), `Essential oils: their antibacterial properties and potential applications in foods ± a review', Int J Food Microbiol, 94, 223±253. Davidson P M and Branen A L (eds) (1993), Antimicrobials in Food, 2nd edn, New York, Marcel Dekker. Han J H (ed) (2005), Innovations in Food Packaging, San Diego, CA, Elsevier Academic Press. Joerger R D (2007), `Antimicrobial films for food applications: A quantitative analysis of their effectiveness', Packag Technol Sci, 20, 231±273. Kerry J and Butler P (eds) (2008), Smart packaging technologies, Chichester, UK, John Wiley & Sons. Kilcast D and Subramaniam P (eds) (2000), The Stability and Shelf-life of Food, Cambridge, UK, Woodhead Publishing. Krochta J M, Baldwin E A and Nisperos-Carriedo M O (eds) (1994), Edible Coatings and Films to Improve Food Quality, Lancaster, PA, Technomic Publishing. Peter K V (ed) (2001), Handbook of Herbs and Spices, Cambridge, UK, Woodhead Publishing. Pokorny J, Yanishlieva N and Gordon M (eds) (2001), Antioxidants in Food: Practical Applications, Cambridge, UK, Woodhead Publishing. Rosato D V (1998), Extruding Plastics: A Practical Processing Handbook, London, Chapman & Hall. Wilson C L (ed) (2007), Intelligent and Active Packaging for Fruits and Vegetables, Boca Raton, FL, CRC Press.

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References and further reading

An D S, Hwang Y I, Cho S H and Lee D S (1998), `Packaging of fresh curled lettuce and cucumber by using low density polyethylene films impregnated with antimicrobial agents', J Korean Soc Food Sci Nutr, 27, 675±681. An D S, Kim Y M, Lee S B, Paik H D and Lee D S (2000), `Antimicrobial low density polyethylene film coated with bacteriocins in binder medium', Food Sci Biotechnol, 9, 14±20. Appendini P and Hotchkiss J H (2002), `Review of antimicrobial food packaging', Innov Food Sci Emerg Technol, 3, 113±126. Aref H (2002), `The development of chaotic advection', Physics Fluids, 14, 1315±1325. Atal C K, Dhar K I and Singh J (1975), `The chemistry of Indian piper species', Lloydia, 38, 256±262. Bakkali F, Averbeck S, Averbeck D and Idaomar M (2007), `Biological effects of essential oils ± a review', Food Chem Toxicol, 46, 446±475. Bangs W E and Reineccius G A (1981), `Influence of dryer infeed matrices on the retention of volatile flavor compounds during spray drying', J Food Sci, 47, 254± 259. Barik B R, Kundu A B and Dey A K (1987), `Two phenolic constituents from Alpinia galanga rhizomes', Phytochemistry, 26, 2126±2127. Becerril R, GoÂmez-Lus R, GonÄi P, LoÂpez P and NerõÂn C (2007), `Combination of analytical and microbiological techniques to study the antimicrobial activity of a new active food packaging containing cinnamon or oregano against E. coli and S. aureus', Anal Bioanal Chem, 388, 1003±1011. Bentayeb K, Rubio C, Batlle R and NerõÂn C (2007), `Direct determination of carnosic acid in a new active packaging based on natural extract of rosemary', Anal Bioanal Chem, 389, 1989±1996. Beuchat L R (2000), `Control of foodborne pathogens and spoilage microorganisms by naturally occurring antimicrobials', in Wilson C L and Droby S, Microbial Food Contamination, Boca Raton, FL, CRC Press, 149±169. Bhattacharya S, Mula S, Gamre S, Kamat J P, Bandyopadhyay S K and Chattopadhyay S (2006), `Inhibitory property of Piper betel extract against photosensitizationinduced damages to lipids and proteins', Food Chem, 100, 1474±1480. Bicchi C, Binello A and Rubiolo P (2000), `Determination of phenolic diterpene antioxidants in rosemary (Rosmarinus officinalis L.) with different methods of extraction and analysis', Phytochem Anal, 11, 236±242. Biever C (2003), `Herb extracts wrap up lethal food bugs', New Scientist, 178, 26. Blaszyk M and Holley R A (1998), `Interaction of monolaurin, eugenol, and sodium citrate on growth of common meat spoilage and pathogenic organisms', Int J Food Microbiol, 39, 175±183. Bozin B, Mimica-Dukic N, Simin N and Anackov G (2006), `Characterization of the volatile composition of essential oils of some Lamiaceae spices and the antimicrobial and antioxidant activities of the entire oils', J Agric Food Chem, 54, 1822±1828. Bozin B, Mimica-Dukic N, Samojlik I and Jovin E (2007), `Antimicrobial and antioxidant properties of rosemary and sage (Rosmarinus officinalis L. and Salvia officinalis L., Lamiaceae) essential oils', J Agric Food Chem, 55, 7879±7885. Brody A L, Strupinsky E R and Kline L R (2001), Active Packaging for Food Applications, Lancaster, PA, Technomic Publishing. Brown G E (1974), `Benomyl residues in Valencia oranges from postharvest applications

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conataining emulsified oil', Phytopatho, 64, 539±542. Brown G E, Nagy S and Maraulia M (1983), `Residues from postharvest nonrecovery spray applications of imazalil to oranges and effects on green mold caused by Penicillium digitatum', Plant Disease, 67, 954±959. Burt S (2004), `Essential oils: their antibacterial properties and potential applications in foods ± a review', Int J Food Microbiol, 94, 223±253. Chang S T, Chen P F and Chang S C (2001), `Antibacterial activity of leaf essential oils and their constituents from Cinnamonum osmophloeum', J Ethnopharmacol, 77, 123±127. Charoenkul N, Rimkeeree H, Chompreeda P, Dilokkunanant U and Changchenkit C (2004), `Development of acne gel with cassia oil', Proc 42nd Kasetsart University Annual Conf, 3±6 February 2004, Bangkok, Thailand. Chen S, Wu J G and Chuo B Y (2003), `Antioxidant and antimicrobial activities in the catechins impregnated PVA-starch film', Book of Abstracts of IFT Annual Meeting, 15±20 July 2003, Chicago, IL. Chrubasik S, Pittler M H and Roufogalis B D (2005), `Zingiberis rhizome: a comprehensive review on the ginger effect and efficacy profiles', Phytomed, 12, 684±701. Chung D, Chikindas M L and Yam K L (2001a), `Inhibition of Saccharomyces cerevisiae by slow release of propyl paraben from a polymer coating', J Food Prot, 64, 1420± 1424. Chung D, Papadakis S E and Yam K L (2001b), `Release of propyl paraben from a polymer coating into water and food simulating solvents for antimicrobial packaging applications', J Food Process Preserv, 25, 71±87. Chung S K, Cho S H and Lee D S (1998), `Modified atmosphere packaging of fresh strawberries by antimicrobial plastic films', Korean J Food Sci Technol, 30, 1140± 1145. Clark G S (1992), `Allyl isothiocyanate', Perf Flav, 17, 107±109. Coma V (2008), `Bioactive packaging technologies for extending shelf life of meat-based products', Meat Sci, 78, 90±103. Conner D E (1993), `Naturally occurring compounds', in Davidson P M and Branen A L, Antimicrobials in Foods, 2nd edn, New York, Marcel Dekker, 441±468. Cooksey K (2000), `Utilization of antimicrobial packaging films for inhibition of selected microorganism', in Risch S J, Food Packaging: Testing Methods and Applications, Washington, DC, American Chemical Society, 17±25. Cran M J, Rupika L A S, Sonneveld K, Miltz J and Bigger S W (2010), `Release of naturally derived antimicrobial agents from LDPE films', J Food Sci, 75, E126± E133. Cressy H K, Jerrett A R, Osborne C M and Bremer P J (2003), `A novel method for the reduction of numbers of Listeria monocytogenes cells by freezing in combination with an essential oil in bacteriological media', J Food Prot, 66, 390±395. Danescu R I and Zumbrunnen D A (2000), `Production of electrically conducting plastic composites by three-dimensional chaotic mixing of melts and powder additives', J Vinyl Addit Technol, 6, 27±33. Davidson P M and Parish M E (1989), `Methods for testing the efficacy of food antimicrobials', Food Technol, 43, 148±155. Decker E A, Chan W K M, Livisay S A, Butterfield D A and Faustman C (1995), `Interactions between carnosine and the different redox states of myoglobin', J Food Sci, 60, 1201±1204. Dejsuk N (2008), `Release of cinnamaldehyde and eugenol from cellulose ether films and

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Bioactive food packaging strategies  P E Z - R U B I O , Novel Materials and Nanotechnology A. LO Group, IATA-CSIC, Spain

Abstract: This chapter reviews the latest developments in novel packaging concepts comprising active and bioactive packaging technologies. While active packages are developed to play an active role in food preservation, bioactive packaging is a novel concept of technologies intended to help in the production of functional foods, whose bioactive principles and actuators are devised to be contained within packaging or coating materials. Therefore, it gives rise to a novel conceptual approach to develop functional foods, while setting the roots of a new packaging technology, in which a food package or coating is given the unique role of enhancing food impact over the consumer's health. The technologies reviewed include novel integration technologies, micro- and nanoencapsulation and enzyme encapsulation and/or immobilization. Moreover, the role that novel nanocomposite structures can play in the controlled release of active and functional compounds from the polymeric or biopolymeric structures is described, as well as the available techniques for the characterization of the kinetics of release of the various compounds. Key words: active packaging, bioactive packaging, functional foods, biopolymers, controlled release.

16.1

Introduction

Traditionally, food packages have been defined as a passive barrier to delay the adverse effect of the environment over the packaged product. However, the current tendencies include the development of packaging materials that interact with the environment and with the food, even playing an active role in its preservation. These new food-packaging systems have been developed as a response to trends in consumer preferences towards mildly preserved, fresh, tasty and convenient food products with a prolonged shelf-life. In addition, changes in retail practices such as globalization of markets, resulting in longer distribution distances, present major challenges to the food packaging industry to develop packaging concepts that extend shelf-life while maintaining the safety and quality of the packaged food. Active packaging refers to those technologies intended to interact with the internal gas environment in order to improve the food preservation or sensory properties while maintaining the quality and safety of the packaged food. Such

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newly employed technologies continuously modify the gas environment (and may interact with the product surface) by removing gases from or adding gases to the headspace inside the package. The internal atmosphere may be controlled by substances that they absorb (scavenge) or release (emit). Polymers, due to their inherent capacity to allow the passage of low molecular weight compounds, are the materials of choice for these novel packaging concepts. Some examples of active-packaging systems are O2 scavengers, CO2 emitters, ethylene absorbers, moisture regulators, taint removal systems, ethanol emitters and antimicrobial-releasing systems (LoÂpez-Rubio et al., 2004). Bioactive packaging is an even newer concept of packaging which, based on the same principles as active packaging, seeks to have an impact on consumer health through the controlled incorporation of bioactive or functional substances, initially contained within the package walls or biopolymeric structures, to the packaged food products (LoÂpez-Rubio et al., 2006), as will be further explained below. The main difference between the well-known active packaging technologies and the bioactive packaging concept is that while active packaging primarily deals with maintaining or increasing the quality and safety of packaged foods, i.e. shelf-life of packaged food products, bioactive packaging has a direct impact on the health of the consumer by generating healthier packaged foods. In the present chapter, an overview of the state-of-the-art technologies of these novel packaging concepts with a special emphasis on bioactive packaging will be presented, together with the advantages of using nanotechnologies to tailor the release of the substances from the polymeric or biopolymeric matrices.

16.2

Definition and technologies

16.2.1 Definition of bioactive packaging The increasing consumer health consciousness and the growing demand for healthy foods are stimulating innovation and new product development in the food industry internationally, and are also responsible for the expanding worldwide interest in functional foods. Generally, a food product commercialized as functional contains added ingredients, technologically developed, that provide a specific benefit for human health (Alzamora et al., 2005). However, the development of novel functional foods is limited for a number of reasons, highlighting the activity loss of the functional substance during food processing, storage or commercialization, incompatibility of the bioactive ingredient with the food matrix, undesirable sensorial changes in the food product, etc. On the other hand, the physiological benefits of functional foods, as well as their effectiveness in reducing the risk of disease, depend on the bioavailability of the bioactive ingredients. This represents a challenge due to the inactivation of many functional additives during food processing (temperature, oxygen, light,

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etc.), storage and commercialization or in the gastrointestinal tract (pH, enzymes, presence of other nutrients, etc.). Currently, industrial demand for technologies ensuring the stability of bioactive compounds in foods remains strong. The bioactive packaging concept aims at the protection of the bioactive compounds using biomaterials, either through technologies of micro- and nanoencapsulation, or through the incorporation of functional ingredients within the packaging walls. The development of encapsulation techniques for food ingredients is still in the research phase and far from a final optimized commercial process. However, the knowledge needed for the development of new formulations can be gathered from the pharmaceutical and biomedical field, where the development of matrixes for controlled release of bioactive substances (drugs) is already a fact and it is an active research area in constant improvement. The functional, or more precisely bioactive packaging materials would thus be capable of withholding desired bioactive principles in optimum conditions until their eventual release into the food product either through controlled or fast release during storage, or just before consumption, taking into account the specific product/functional substance characteristics or requirements (LoÂpezRubio et al., 2006). The bioactive packaging concept embraces a series of technologies that can be used for the protection or stabilization of functional ingredients and can be grouped as follows: 1. Integration and controlled release of bioactive components or nanocomponents from biodegradable and/or sustainable packaging systems. 2. Micro- and nanoencapsulation of these active substances either in the packaging and/or within foods. 3. Packaging provided with enzymatic activity exerting a health-promoting benefit through transformation of specific food-borne components.

16.2.2 Existing technologies to improve the shelf-life or functionality of food (including active and bioactive technologies) The use of proper packaging materials and methods to minimize food deterioration and provide safe and wholesome food products has always been the focus of food packaging. As explained before, current trends in consumer preferences have stimulated research into novel packaging concepts that actively play a role in the preservation of foods, through either the adsorption or release of compounds, which is known as active packaging. It is not the aim of this chapter to review the existing active packaging technologies as they have been thoroughly described elsewhere (Labuza and Breene, 1989; LoÂpez-Rubio et al., 2004; Ozdemir and Floros, 2004). Instead, a few recent examples of promising

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active polymer-based packaging technologies will be given in the areas which are considered of more interest, mainly consisting of antimicrobial and antioxidant active packages. In a different area, bioactive packaging technologies go a step further and seek to provide added-value foods which, in some manner, have an impact on consumer health. This is a rather novel packaging area and there are only a few developments reported. The existing technologies will be commented on, as well as the potential contribution of bio-based packages for the development of functional foods. Recent developments in active packaging technologies Amongst the variety of existing active packaging technologies, antimicrobial packaging is probably the area where more research efforts have been recently focused. The development of packages able to inhibit or at least slow down the growth of spoiling or pathogenic microorganisms is of outstanding interest in food science as they would allow commercializing food products with milder preservation treatments but with the required safety standards and shelf-life. The antimicrobial action can be accomplished in several ways, mainly through: · Incorporation of antimicrobial substances within the polymeric structure or as a coating and subsequent controlled release · Covalently attached antimicrobial substances to the packaging wall requiring direct contact with the food product to exert the biocidal action · Inherently antimicrobial polymers. Regarding the first mechanism, i.e. controlled release of antimicrobial substances from the packaging structure, there is a trend towards the use of natural compounds instead of the traditionally used chemical preservatives (Burt, 2004). One of the most abundant groups of natural compounds with antimicrobial character is represented by the essential oils. Essential oils can extend the shelflife of unprocessed or processed foods by reducing microbial growth rate or viability (Beuchat and Golden, 1989). Essential oils and their constituents have been widely used as flavouring agents in foods, and it is well established that many of them have wide spectra of antimicrobial action (Alzoreky and Nakahara, 2002; Kim et al., 1995; Packiyasothy and Kyle, 2002). Amongst the wide variety of antimicrobial compounds in essential oils, thymol, carvacrol and cinnamaldehyde are some of the major components and are mainly responsible for the antimicrobial properties of most of the essential oils (Burt, 2004; Elgayyar et al., 2001; Oussalah et al., 2006). Several antimicrobial compounds from essential oils have been incorporated in polymeric and biopolymeric matrices of different natures such as polypropylene (PP), polyethylene (PE), blends of PE with ethylene±vinyl alcohol (EVOH) copolymers, sodium alginate, chitosan, zein and whey protein isolate, amongst others

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(Chalier et al., 2009; Chi et al., 2006; Del Nobile et al., 2008; Gamage et al., 2009; GutieÂrrez et al., 2009; Hosseini et al., 2009; Mayachiew et al., 2010; Oussalah et al., 2006; Pelissari et al., 2009; Raybaudi-Massilia et al., 2008; Salmieri and Lacroix, 2006; Seydim and Sarikus, 2006; Suppakul et al., 2008). The usefulness of these packaging alternatives has been demonstrated on a laboratory scale using in vitro conditions but, in general, more information is needed on their performance when working with commercial products. Probably, the main problem that can be encountered when developing these antimicrobial materials is an early release of the essential oil volatile compound during film formation, a fact that is not normally taken into account when studying these systems. For instance, Chi and co-workers (2006) observed that the concentration of oregano essential oil compounds sharply decreased during film preparation (from 757.7 ppm carvacrol in the film-forming solution to 2.1 ppm in dried films). Another issue to consider before the application of these active films to food products is the possibility of induced sensory changes. The taste and odour provided by the essential oils are not always acceptable and, thus, it would be interesting to find the right combination of the properties of essential oils and food aromas that provide the required active packaging material (GutieÂrrez et al., 2009). The enzymes represent another group of natural compounds with antimicrobial activity, which are normally covalently attached to the packaging structure and, thus, constitute the main example of the second mechanism described. In particular, lysozyme is a key enzyme that has been used in both the pharmaceutical and food industries. Lysozyme plays a particular role in extending meat shelf-life as well as in cheese ageing, through the reduction in butyric fermentation bacteria, which adversely affect cheese quality (Mastromatteo et al., 2010). Several active packaging structures have been designed containing lysozyme both covalently attached and for release purposes. Some of the polymers used in these new developments are poly(vinyl alcohol) (Conte et al., 2006, 2007), polyethylene (Conte et al., 2008), cellulose-based (Gemili et al., 2009; Mascheroni et al., 2010) and zein (Mecitoglu et al., 2006). Immobilized enzymes have been observed to retain the activity and great reductions in target microorganisms have been observed, although the results have to be analysed carefully as, in most of the studies, lysozyme was combined with other antimicrobial substances, like the bacteriocin nisin (Natrajan and Sheldon, 2000), or the target microorganism chosen for the study was very sensitive to cell lysis (Conte et al., 2007). Another interesting antimicrobial compound is chitosan. Chitosan is a biodegradable, biocompatible, non-toxic aminopolysaccharide obtained by deacetylation of chitin, which is one of the world's most abundant biopolymers. It is now widely produced commercially from crab and shrimp shell wastes, with different deacetylation grades and molecular weights and, hence, different functional properties, like emulsification ability, dye binding and gelation (No et

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al., 2007). This film-forming compound has inherently antimicrobial character, which is, however, dependent on the film-forming and storage conditions. Although the antimicrobial character of chitosan films has been widely reported, it was not until recently that the rationale behind this antimicrobial property was fully understood, being the presence of carboxylate groups in the polymeric structure necessary for biocidal properties (Fernandez-Saiz et al., 2006; LagaroÂn et al., 2007). The film-forming and storage conditions for optimum performance of chitosan films have been established (Fernandez-Saiz et al., 2009), and the antimicrobial materials were also tested in a real food product (fish soup), where it also proved to be biocidal (although the antimicrobial activity decreased probably due to interactions with food components) without affecting the sensorial properties of the commercial product (Fernandez-Saiz et al., 2010). Regarding the other big area of active packaging, i.e. antioxidant packages, they show a great potential not only to keep colour attributes in meat products, but also to protect against oxidative rancidity, degradation and enzymatic browning in fruits and vegetables. Several authors have studied the incorporation of several antioxidants, such as ascorbic acid, into edible films used to coat minimally processed fruits (Baldwin et al., 1996; Lee et al., 2003; Perez-Gago et al., 2006; Wong et al., 1994). Rojas-GrauÈ and co-workers (2007) applied alginate- and gellan-based coatings to fresh-cut apples and papayas, demonstrating that coatings were good carriers for antioxidant agents, including cysteine, glutathione, and ascorbic and citric acids. Most of these antioxidant agents, however, are hydrophilic compounds, and may increase water vapour transmission rate and induce water loss when incorporated into films and coatings (Ayranci and Tunc, 2004). Essential oils, apart from the already mentioned antimicrobial activity, can also provide antioxidant characteristics and, thus, are very interesting compounds for active packaging applications. Carvacrol, for instance, has been evaluated for the development of active antioxidant packages. Its phenolic chemical structure makes it a very promising natural additive for high-density polyethylene (HDPE) used in food packaging. The addition of carvacrol to HDPE could be interesting in terms of active packaging, as the release of this compound from the polymer matrix can be controlled and migration is greater in food containing fat, which is also more prone to oxidation (Peltzer et al., 2009). Yet another interesting natural antioxidant compound that has been investigated for active packaging use is -tocopherol (Heirlings et al., 2004). The polarity of the polymeric matrices has been observed to slightly influence the release of the antioxidant to the food simulants, but again, the release was mostly influenced by the chemical nature of the food product (Heirlings et al., 2004). Existing and potential bioactive packaging developments Functional foods contain a number of bioactive components that are considered to be beneficial to the health of their consumers. It is recognized that an

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important aspect of these functional foods is to provide an appropriate dose of these bioactive components in order to have a beneficial rather than a toxic effect on human health (Falk, 2004). Currently, the majority of commercial functional foods are presented with the bioactive components contained within compatible foods, an aspect which imposes to the food industry a number of limitations and complications during processing and manufacture. Some of the technical difficulties encountered by industries in the development of functional food products are (LoÂpez-Rubio et al., 2006): · A loss of product functionality during processing, storage and/or commercialization (Fogliano and Vitaglione, 2005). For instance, probiotic bacteria counts are known to substantially decrease as a consequence of processing, during product storage by for instance oxidation, and also during their passage through the gastrointestinal tract. · The functional substance is not often compatible with the food matrix. This is the case, for example, with fat-soluble vitamins in aqueous foods. · The added bioactive compound can induce the development of undesirable flavours or strange odours. Marine oils are, for instance, prone to undergo oxidization. · The modification of the food product texture as a consequence of the added compound has to be controlled in order to manufacture an acceptable product from a commercial point of view. · The need to adapt the production line to incorporate a new substance often implies a complete change of the parameters involved in the process. These necessary changes involve a considerable financial investment often only affordable by large corporations. · In the case of enzymes, due to their sensitivity to processing and to trace levels of substances that can actuate as inhibitors, the dissolved enzymes may have a short operational life or suffer inactivation. In this respect, the incorporation and controlled release for optimum bioactivity of the functional substances or the fixation/encapsulation (with or without partial release) of enzymes in the packaging structure, as well as the protective encapsulation of the bioactives either within foods or in food packaging technologies, can help solve many of the above-mentioned problems, increase bioactive performance and, furthermore, allow or at least facilitate the development of new and healthier food products. Thus, there are a number of product development constraints concerning the inclusion of bioactive components. Here is where the totally innovative approach of this technology in terms of creating a new multidisciplinary technological area, bioactive encapsulating and packaging technologies, can have significant interest for research bodies and the food industry. Nowadays, the most popular functional foods are those containing pro- and prebiotics but the range of application is limited to certain food products, mostly

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fermented milk products, because of the actual technological barriers concerning the processing of those bioactive substances. Probiotics are defined as living microorganisms which, when administered in adequate amounts, confer health benefits to the host, including the prevention and treatment of some pathologies (Havenaar and Huis in't Veld, 1992). These include reduction of gastrointestinal infections, improvement of lactose metabolism, reduction of serum cholesterol, and improvement of immune system defences (Kurmann and Rasic, 1991). However, surveys show that in the case of yoghurt preparations, there are large fluctuations and poor viability of probiotic bacteria, and especially of bifidobacteria during product storage or in the digestive tract after consumption (Shah et al., 1995; Schillinger, 1999). In recent years, microencapsulation has been found to be a useful tool for the stabilization of probiotic cells for functional food applications. Microencapsulation involves the incorporation of food ingredients, enzymes, cells or other materials in small capsules. Applications of this technique have increased in the food industry, since the encapsulated materials can be protected from moisture, heat or other extreme conditions, thus enhancing their stability and maintaining viability. Various techniques are employed to form the capsules, including spray drying, spray chilling or spray cooling, extrusion coating, fluidized bed coating, liposome entrapment, coacervation, inclusion complexation, centrifugal extrusion and rotational suspension separation (Gibbs et al., 1999). Microencapsulation can enhance the viability of probiotic bacteria during processing, storage and subsequent consumption and gastro-intestinal transit (Anal and Singh, 2007; Krasaekoopt et al., 2003). Recently, electrospinning has proven successful for the encapsulation of bifidobacteria, increasing their viability during storage at various temperatures (LoÂpez-Rubio et al., 2009). Electrospinning is a process that produces continuous polymer fibres with diameters in the sub-micron range through the action of an external electric field imposed on a polymer solution or melt. Recently, this technique has been receiving substantial attention, especially in the biomedical area, as the high surface area of the electrospun fibres mimics the extracellular matrix (Li et al., 2006) and can be used in a variety of applications such as scaffolds for tissue engineering (Martins et al., 2009; Smith et al., 2008), or drug delivery devices (Kenawy et al., 2009). In the food science area, this technique has only very recently been applied to encapsulate antioxidants (Fernandez et al., 2009) and to generate antimicrobial fibres (Torres-Giner et al., 2008), but it has a great potential as a nano- and microencapsulation technique for applications in the areas of active and bioactive packaging and to generate ingredients for functional food products. In the case of encapsulation systems for the release of vitamins or other functional ingredients in the lower gastrointestinal tract, an interface is needed with low permeability under the acidic conditions of the stomach and high permeability in the approximately neutral pH environment of the gastrointestinal

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tract (Sauer et al., 2001). Those microcapsules should have not only significant mechanical stability but also substantial stability against acidic or enzymatic digestion. Even more important, for applications in food products the capsules would obviously need to be food-grade and preferably inexpensive, putting substantial limitations on the materials that can be used. Besides that, the production method should involve available unit operations so that the process can easily be scaled up to produce large quantities of the capsules at low cost (Sagis et al., 2008). Apart from the encapsulation route, bioactive packages can be obtained through the direct incorporation of the functional substances in the package or coating material. Among the functional substances which are thought most suitable for their incorporation in the package wall we will highlight phytochemicals and vitamins due to their general sensitivity to food processing conditions. Phytochemicals are non-nutritive plant chemicals that contain protective, disease-preventing compounds. More than 900 different phytochemicals have been identified as components of foods, and many more phytochemicals continue to be discovered today (Liu, 2003). They are associated with the prevention and/or treatment of at least four of the leading causes of death in, for instance, the United States: cancer, diabetes, cardiovascular disease, and hypertension (Bloch and Thomson, 1995). They are involved in many processes including those that help prevent cell damage, prevent cancer cell replication, and decrease cholesterol levels. Many phytochemicals are polyphenolic compounds with antioxidant activity. The antioxidative effect of phenolics in functional foods is due to a direct free-radical scavenging activity and an indirect effect arising from chelation of prooxidant metal ions (Halliwell, 1996; Shahidi, 2000; Wettasinghe and Shahidi, 1999). Many phenolics are found in oilseeds, but during the processing steps of refining, bleaching and deodorization a large proportion of phytochemicals are removed. Several results have demonstrated that certain components of foods that are usually removed and discarded during bleaching, refining and processing are essential for health promotion and disease prevention. The other functional substances of interest to be incorporated in the packaging or coating materials, i.e. vitamins, are essential for good health. Food can supply all the vitamin requirements provided that the diet is adequate and well balanced. But the fact is that the fast pace of modern lifestyles and the increase in single-person households, one-parent families and working women have led to changes in food preparation and consumption habits which, in the end, result in unbalanced diets. Moreover, some vitamins are destroyed during processing. Most of the losses are due to heat generated during the canning and freezing steps (e.g., blanching, pasteurization and sterilization). In order to incorporate the previously described functional substances into the package wall, it is necessary to select the method of fabrication of the films, the

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optimal temperature/time conditions for mixing the biomaterials with the functional substances and, of course, the suitable material and/or engineering mechanism to attain the desired release rate. The election of the materials to be used either in the form of packages or as coatings is crucial because aside the main role of containing the functional substance they may also be required to exert their protecting function, maintaining and assuring the quality and safety of the packaged food. The process of film fabrication has to be adapted to the technological limitations of each bioactive compound, i.e. for those substances sensitive to high temperatures, such as certain vitamins, low process temperatures have to be used for casting or extruding the materials. The method of fabrication also determines the structure of the material and, therefore, the rate of bioactive substance release. However, the release of substances can be activated and controlled by several parameters such as humidity (many biopolymers strongly plasticize in the presence of humidity, which can come from the food) and pH (some smart polypeptides obtained from microorganisms can undergo conformational changes when submitted to different pH conditions). The greatest limitation of this technology is that, as the functional substances proposed for these packages are non-volatile compounds, direct contact between the package and the food surface is needed. Therefore, in the case of solid foodstuff, this technology could be applied in the form of edible coatings. Edible coatings have been applied to foods for centuries. They are intended to provide some level of protection, prevent the transfer of materials from one food component to another, enhance the appearance of fruits and vegetables, and frequently contain other compounds to retard insects, microorganisms, oxidation and other intruders that would spoil the product (Brody, 2005). More recently, they have been studied as carriers for antimicrobial substances with proven success like, for example, a sorbate-releasing plastic film for cheese packaging (Han, 2000) and, consequently, they have the potential to include functional ingredients. Moreover, as the edible films can be formed using polysaccharides (starch, chitosan, alginates, etc.), proteins (gelatin, soy protein, wheat gluten, etc.) and lipids (waxes, triglycerides, fatty acids, etc.), they can be tailored to contain any functional substance and to fit any food product. Finally, another very interesting bioactive packaging concept is the one containing covalently attached enzymes in the inner layer of the packaging structure, thus being able to `process' the food product inside the package. The objective of these materials is to catalyse a reaction which is considered beneficial from a nutritional point of view, i.e., by decreasing the concentration of a non-desired food constituent and/or producing a food substance attractive for the consumer. Many enzymes present direct application in food processing and some trials have been performed in synthetic plastic packages. Soares and Hotchkiss (1998) immobilized naringinase in a plastic package. The grapefruit juice contained reduced its bitterness by hydrolysis of naringin, a bitter principle of citrus juices. Another enzymatically bioactive packaging concept already

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commented elsewhere (Brody and Budny, 1995) consists of the binding of galactosidase and cholesterol reductase in the package walls for the hydrolysis of lactose and cholesterol, respectively. The use of this type of package would allow the production of a value-added product without modifying the manufacturing procedure. For example, UHT milk produced by a conventional process can be packaged in an -galactosidase bioactive package and, through storage, the product arrives at the market as a low-lactose or free-lactose product. This type of development was recently carried out by Goddard and co-workers (2007) in a polyethylene film, demonstrating that after covalently attaching the enzyme to the polymer, it retained significant lactose-breakdown activity. This processing plant inside the package appears to be a very promising technology. Regarding the packaging materials, although it is recognized that there is no universal support for all enzymes and their applications, a number of desirable characteristics should be common to any material considered for immobilizing enzymes. These include high affinity to proteins, availability of reactive functional groups for direct reactions with enzymes and for chemical modifications, hydrophilicity, mechanical stability and rigidity, regenerability, and ease of preparation in different geometrical configurations that provide the system with permeability and surface area suitable for a chosen biotransformation. Understandably, for food applications, non-toxicity and biocompatibility of the materials are also required. Furthermore, to respond to the growing public health and environmental awareness, the materials should be biodegradable, and to prove economically inexpensive (Krajewska, 2004). To save in time and costs it could be helpful to use food-grade chemicals, as well as cheap biopolymers generally recognized as safe (GRAS), as suitable carriers. Among the biomaterials already studied for the immobilization of enzymes and with potential use in functional bioactive packages, carrageenan, chitosan, gelatin, PLA, PGA and alginate are very promising materials.

16.3

Nanotechnologies

Hybrid or nanocomposite organic±inorganic materials that combine attractive qualities of dissimilar components have received immense interest for a wide range of applications, including active and bioactive packaging. This is mainly due to the ability of the nanoparticles to interact with the active or functional substances, thus acting as release-mediating agents. The control of the surface/ interface interaction between components appears thus of prime importance in the development of advanced materials having novel functionalities, out of those characteristic of the components (Sanchez et al., 2005). Biopolymers and clays, for instance, are common ingredients in pharmaceutical products. Their pharmaceutical applications are broad because of their distinct advantages: they are versatile (owing to the fact that they possess a wide range of mechanical, chemical and physical properties), usually considered inert

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and available at reasonable cost (Viseras et al., 2008). Pharmaceutical-grade clay minerals (including smectites, kaolinite and fibrous clay minerals) have been extensively applied for prolonged release of drugs because of the good intercalation capacity offered by the clay particles (Aguzzi et al., 2007). There are several mechanisms that may be involved in the interaction between clay minerals and organic molecules. The relevance of a specific mechanism depends on the clay mineral involved (Browne et al., 1980) as well as on the functional groups (Lagaly, 2001) and the physical±chemical properties of the organic compounds (Tolls, 2001). In particular, clay minerals mainly undergo ion exchange with basic compounds in solution, as they are naturally occurring inorganic cationic exchangers. On the basis of these interactions, and also because of their special swelling properties, clay minerals are effectively used to delay (extended release systems) the active or functional substance release. Microencapsulation of bioactive compounds can also take advantage of nanoparticle addition, as apart from slowing down the release of substances from the nanocomposite matrices, addition of nanoclays to biopolymeric encapsulates containing active or functional compounds has been observed to increase the encapsulation efficiency (Wang et al., 2008). Depending on the application, the demands on the properties of the shell of the microcapsules and the production method may vary widely. When used for the encapsulation of small components such as volatile flavours the interface should have a low permeability. Currently available encapsulation systems can retard the release of such components (Antipov and Sukhorukov, 2004; Sukhorukov et al., 2005) but for most applications, especially for products with an appreciable shelf-life, the release is still too fast. Through the adsorption and/or intercalation of the functional substances on the surface or into the interlayer of nanoclays, the release can substantially be slowed down. For instance, Li and co-workers (2009) demonstrated that the release of bovine serum albumin (as model drug) from hydrogels can be controlled by modulating the clay content, i.e. the temperature sensitivity of the hydrogels was weakened when increasing the clay content and, thus, the swelling/deswelling behaviour could be modified by varying the amount of clay incorporated. Nanoparticles, like nanoclays, can also provide extra protection to bioactive substances apart from the one provided by the polymeric matrix. For example, it was shown that the relative activities of immobilized amylases in a nanocomposite of chitosan and activated clay were higher than that of free enzymes over broader pH and temperature ranges. The kinetic parameters of the enzymatic activities were also found to be greater for immobilized enzymes than for those with free enzymes in most cases (Chang and Juang, 2005). Mineral supports within biopolymer materials, apart from improving their mechanical and barrier properties, have proved to be very efficient in enzyme binding, thus avoiding the need for using toxic chemicals, such as glutaraldehyde, for enzyme immobilization, as these are usually unsuited for food applications. Several

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research studies have been carried out concerning the attachment of enzymes in diverse clay mineral supports. The mechanisms by which enzymes can be immobilized on clay minerals include cation exchange, physical adsorption and ionic binding (Sarkar et al., 1989). In some cases the nanoparticles themselves are functional, as in the case of TiO2 nanoparticles or silver nanoparticles which have antimicrobial character. Nanocomposites of EVOH with TiO2 and silver have been developed, displaying antimicrobial properties against a range of Gram-positive and Gramnegative bacteria and yeast, and, moreover, showing outstanding resistance to biofilm formation (Kubacka et al., 2009). In the same line, a very clever concept was proposed by Loher and co-workers (2008) which can potentially lead to self-sterilizing polymer surfaces. The idea was to incorporate the silver nanoparticles in the surface of 20±50 nm carrier particles consisting of a phosphate-based, biodegradable ceramic which allowed the triggered release of silver in the presence of a growing microorganism. This effect is based on the organism's requirements for mineral uptake during growth creating a flux of calcium phosphate and other ions to the organism. The growing microorganism dissolves the carrier containing these nutrients and thereby releases the silver nanoparticles. The authors demonstrated the rapid self-sterilization of polymer surfaces containing silver on calcium phosphate nanoparticles using a series of human pathogens (Loher et al., 2008). These hybrid polymer/nanoparticle systems have demonstrated suitability in both active and bioactive packaging areas. In the active packaging area and, more specifically, in antimicrobial packaging, the inclusion of nanoclays into biopolyesters has been observed to prolong the antimicrobial effect of the package through slower kinetics of release (Sanchez-Garcia et al., 2008). Thymol, which was the antimicrobial agent used in the previous study, had a good affinity for the nanoclays, which resulted in greater uptake or solubility of the natural antimicrobial in the nanocomposites than in the neat polymer, probably due to retention over the surface of the apolar biocide agent. The thymol diffusion coefficient was seen to decrease with the addition of nanoclay, probably because of the larger tortuosity effect imposed on the diffusion of the biocide by the dispersed nanoclay. These results confirm that it is feasible to control the uptake and release of thymol by incorporation of laminar nanoclays in bioplastics such as polycaprolactone (Sanchez-Garcia et al., 2008). In the bioactive packaging area, nanoclays such as montmorillonites (MMT) have been found useful as carriers for vitamins. Joshi and co-workers (2009) showed that vitamin B6 can be successfully adsorbed on the surface of MMT and also intercalated into the interlayer of MMT. The release of this vitamin was monitored under in vitro conditions using simulated gastric fluid (pH 1.2) and simulated intestinal fluid (pH 7.4) at 37  0:5ëC and the release experiments revealed that vitamin B6 was steadily released from MMT, the release process being pH-dependent.

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Other fillers that can be added to biopolymers to improve their performance and to tailor the release of substances are micro- and nanosized fibres. Fibrereinforced microcapsules were prepared using common food proteins and polysaccharides with superior mechanical stability and with potential application as controlled release systems in foods (Sagis et al., 2008).

16.4

Controlled release of bioactives

16.4.1 Methods developed for the controlled release of bioactives Apart from the already mentioned nanocomposites strategy to tailor the release of active and functional substances from the biopolymeric systems, there are several methods that can be used to control the release of bioactives, based mainly on the conformational changes undergone by macromolecules upon changing conditions. It has been commented in the previous section that in hydrogels, for instance, the swelling and deswelling of the systems are highly dependent on temperature (Li et al., 2009). There are also some biopolymers that change conformation with the pH. Alginates, for instance, are very sensitive to pH and, at low pH (like the one present in the gastric environment), the polymer shrinks and thus release of macromolecules from alginate encapsulating structures in these conditions is greatly reduced (Chen et al., 2004). In the gastric fluid, the hydrated sodium alginate is converted into a porous, insoluble so-called alginic acid skin. Once passed into the higher pH of the intestinal tract, the alginic acid skin is converted to a soluble viscous layer. This pH-dependent behaviour of alginate can be exploited to customize release profiles (George and Abraham, 2006) and this kind of matricx would be very useful for the encapsulation of probiotic cultures, which need to be protected from the acid gastric environment. However, the rapid dissolution of alginate matrices in the higher pH ranges may result in burst release of bioactive compounds and, therefore, for certain applications, some modifications in the physicochemical properties are needed for the prolonged controlled release of the bioactive substances. Chitosan also exhibits a pH-sensitive behaviour as a weak poly-base due to the large quantities of amino groups on its chain. Chitosan dissolves easily at low pH while it is insoluble at higher pH ranges. The mechanism of pH-sensitive swelling involves the protonation of amine groups of chitosan under low pH conditions. This protonation leads to chain repulsion, diffusion of proton and counterions together with water inside the gel and dissociation of secondary interactions (Yao et al., 1994). This property has helped it to be used in the delivery of chemical drugs to the stomach and has been widely investigated as a delivery matrix (George and Abraham, 2006). Another very interesting biopolymer for controlled release applications is

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gelatin. Gelatin is a natural polymer that is derived from collagen, and is commonly used for pharmaceutical and medical applications because of its biodegradability (Ikada and Tabata, 1998) and biocompatibility in physiological environments (Yao et al., 2004). The isoelectric point of gelatin can be modified during the fabrication process to yield either a negatively charged acidic gelatin, or a positively charged basic gelatin at physiological pH. This theoretically allows electrostatic interactions to take place between a charged biomolecule and gelatin of the opposite charge, forming polyion complexes (Young et al., 2005). The key determinant of controlled release of bioactive compounds from gelatin matrices is normally the rate of gelatin degradation. Given that crosslinked gelatin does not undergo any appreciable degradation in aqueous solution, in vivo models have been used to study the effect of crosslinking density on the rate of gelatin degradation (Tabata et al., 1999). The degree of crosslinking can also be tailored to control the swelling of polymers for the release of bioactive substances. Poly(vinyl alcohol) (PVOH), for instance, is a very hydrophilic and thus highly swellable polymer. Through different degrees of crosslinking with glyoxal, Buonocore and co-workers (2004) were able to control the release of the antimicrobials lysozyme and nisin. In contrast, the degree of crosslinking did not influence the release kinetics of sodium benzoate, indicating that the molecular size of sodium benzoate is smaller than the mesh size of the investigated crosslinked PVOH films. Crosslinked hydrophilic polymers are often used as wall confinement materials in controlled-release structures, providing a temporary or permanent macromolecular network through which the diffusional release of active ingredients occurs (Bachtsi and Kiparissides, 1996).

16.4.2 Characterization methods for the controlled release of bioactives The characterization methods available to monitor and quantify the release of bioactives from the packaging, coating or encapsulating structures are mainly determined by the nature of the encapsulated substance, although as will be explained below, several methods can be used to characterize the release of the same bioactive substance. In the case of enzymes or bioactive peptides, a commonly used method is HPLC (Conte et al., 2006, 2007). Quantification of released compounds with HPLC requires constructing a calibration curve for peak area against substance concentration. Another relatively simple method for enzyme release determination is to quantify the total protein content using one of the available commercial colorimetric methods, like the Coomassie Plus total protein assay (Pitukmanorom et al., 2008). An indirect method that has also been used to follow the release kinetics of enzymes from biopolymeric structures into solutions is enzymatic activity (Mecitoglu et al., 2006; Nam et al., 2007).

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Spectrophotometry has also been used to characterize the release of encapsulated substances as a function of time (Bachtsi and Kiparissides, 1996; Gemili et al., 2009; Mascheroni et al., 2010; Portes et al., 2009). Normally, for quantification purposes a calibration curve is needed to correlate the absorbance at a fixed wavelength with the amount of substance released. As an example, spectrophotometric methods have been used to quantify the release of lysozyme by measuring the absorbance of the solutions in contact with the active polymeric structures at different times at 280 nm (Gemili et al., 2009; Mascheroni et al., 2010). The release of essential oils from packaging structures can be followed mainly using two different methodologies: gas chromatography (GC), often coupled to mass spectrometry (MS), or Fourier transform infrared spectroscopy (FT-IR). GC is a more tedious methodology, as it requires extraction of the remaining essential oil from the films or food-simulating fluid and posterior analysis using an internal standard (Chalier et al., 2009; Peltzer et al., 2009). Another option is to introduce the active packaging structure in an impermeable container of known volume and analyse the headspace as a function of time (Gamage et al., 2009). On the other hand, no special preparation is needed to follow the release of essential oils using FT-IR, which can be used for timeresolved experiments using the attenuated total reflection (ATR) accessory. This methodology was used to study the release kinetics and diffusion coefficients of thymol in polycaprolactone (Sanchez-Garcia et al., 2008). The biopolymeric solution containing the antimicrobial compound was directly cast on the surface of the ATR crystal to follow the kinetics of release of this substance. The release of thymol was followed by the decrease in the band at 807 cmÿ1 (assigned to ring vibrations of the thymol chemistry) over time (Sanchez-Garcia et al., 2008). HPLC has also been used to characterize the release of thymol from zein films in water, after filtering and diluting the samples in acetonitrile (Del Nobile et al., 2008). Release of vitamins or antioxidant pro-vitamins like vitamin B6 or alpha tocopherol has been characterized using gas±liquid chromatography after derivatization from a food stimulant (Heirlings et al., 2004) and UV/visible spectroscopy (Joshi et al., 2009), respectively.

16.5

Future trends

Packaging plays an increasingly important role in food quality and preservation and, according to food processing trends and changing consumer preferences and lifestyle, research into active and bioactive packaging technologies is foreseen to increase in the coming years. In view of the current status of active packaging technologies, there is a clear trend towards antimicrobial packages containing natural biocidal substances. However, the differences in the way antimicrobial assays have been carried out

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preclude a straightforward statistical analysis of the data to determine which types of antimicrobials and films have shown most promise for practical application. Not surprisingly, combinations of antimicrobials provided better efficacy, and it is likely that research will move in the direction of finding cost-effective and highly active combinations of antimicrobials. In the end, the acceptance of particular antimicrobial films by the food industry will probably depend on the regulatory climate and the balance between the cost of the antimicrobial film and the benefit of a second antimicrobial hurdle (Joerger, 2007). Regarding the bioactive packaging area, protection of probiotics, vitamins and sensitive antioxidant compounds with health benefits (like many polyphenols) is probably where research efforts will be focused. The great advantage in this novel food packaging sector is that similar examples can be extracted from the biomedical and pharmaceutical areas that can help in the creation and development of new packaging concepts for controlled release of functional substances. Finally, it is interesting to highlight the importance of nanotechnologies in these two areas, as inclusion of nanoparticles can help to control and even to tailor the release of the active and/or functional substances from the novel created structures.

16.6

References and further reading

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Bloch, A., Thomson, C.A. (1995). Position of the American Dietetic Association: Phytochemicals and functional foods. Journal of the American Dental Association, 95, 493±496. Brody, A.L. (2005). Edible packaging. Food Technology 56, 65±66. Brody, A.L., Budny, J.A. (1995). Enzymes as active packaging agents. In: Active Food Packaging, Rooney, M.L., ed.; Blackie Academic and Professional, Glasgow, pp. 174±192. Browne, J.E., Feldkamp, J.R., White, J.L., Hem, S.L. (1980). Characterization and adsorptive properties of pharmaceutical grade clays. Journal of Pharmaceutical Sciences 69, 816±823. Buonocore, G.G., Sinigaglia, M., Corbo, M.R., Bevilacqua, A., La Notte, E., Del Nobile, M.A. (2004). Controlled release of antimicrobial compounds from highly swellable polymers. Journal of Food Protection 67, 1190±1194. Burt, S. (2004). Essential oils: their antibacterial properties and potential applications in foods ± a review. International Journal of Food Microbiology 94, 223±253. Cerrada, M.L., Serrano, C., Sanchez-Chaves, M., Fernandez-Garcia, M., FernandezMartin, F., de AndreÂs, A., JimeÂnez RioboÂo, R.J., Kubacka, A., Ferrer, M., Fernandez-Garcia, M. (2008). Self-sterilized EVOH±TiO2 nanocomposites: interface effects on biocidal properties. Advanced Functional Materials 18, 1949±1960. Chalier, P., Arfa, A.B., Guillard, V., Gontard, N. (2009). Moisture and temperature triggered release of a volatile active agent from soy protein coated paper: Effect of glass transition phenomena on carvacrol diffusion coefficient. Journal of Agricultural and Food Chemistry 57, 658±665. Chang, M.-Y., Juang, R.-S. (2005). Activities, stabilities, and reaction kinetics of three free and chitosan±clay composite immobilized enzymes. Enzyme and Microbial Technology 36, 75±82. Chen, S.C., Wu, Y.C., Mi, F.L., Lin, Y.H., Yu, L.C., Sung, H.W. (2004). A novel pHsensitive hydrogel composed of N,O-carboxymethyl chitosan and alginate crosslinked by genipin for protein drug delivery. Journal of Controlled Release 96, 285± 300. Chi, S., Zivanovic, S., Penfield, M.P. (2006). Application of chitosan films enriched with oregano essential oil on bologna ± Active compounds and sensory attributes. Food Science and Technology International 12, 111±117. Conte, A., Buonocore, G.G., Bevilacqua, A., Sinigaglia, M., Del Nobile, M.A. (2006). Immobilization of lysozyme on polyvinylalcohol films for active packaging applications. Journal of Food Protection 69, 866±870. Conte, A., Buonocore, G.G., Sinigaglia, M., Del Nobile, M.A. (2007). Development of immobilized lysozyme based active film. Journal of Food Engineering 78, 741± 745. Conte, A., Buonocore, G.G., Sinigaglia, M., Lopez, L.C., Favia, P., D'Agostino, R., Del Nobile, M.A. (2008). Antimicrobial activity of immobilized lysozyme on plasmatreated polyethylene films. Journal of Food Protection 71, 119±125. Del Nobile, M.A., Conte, A., Incoronato, A.L., Panza, O. (2008). Antimicrobial efficacy and release kinetics of thymol from zein films. Journal of Food Engineering 89, 57±63. Elgayyar, M., Draughon, F.A., Golden, D.A., Mount, J.R. (2001). Antimicrobial activity of essential oils from plants against selected pathogenic and saprophytic microorganisms. Journal of Food Protection 64, 1019±1024. Falk, M. (2004). The impact of regulation on informing consumers about the health

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promoting properties of functional foods in the U.S.A. Journal of Food Science 69, R143±R145. Fernandez, A., Torres-Giner, S., LagaroÂn, J.M. (2009). Novel route to stabilization of bioactive antioxidants by encapsulation in electrospun fibers of zein prolamine. Food Hydrocolloids 23, 1427±1432. Fernandez-Saiz, P., Ocio, M.J., LagaroÂn, J.M. (2006). Film-forming process and biocide assessment of high-molecular-weight chitosan as determined by combined ATR± FTIR spectroscopy and antimicrobial assays. Biopolymers 83, 577±583. Fernandez-Saiz, P., LagaroÂn, J.M., Ocio, M.J. (2009). Optimization of the film-forming and storage conditions of chitosan as an antimicrobial agent. Journal of Agricultural and Food Chemistry 57, 3298±3307. Fernandez-Saiz, P., Soler, C., LagaroÂn, J.M., Ocio, M.J. (2010). Effects of chitosan films on the growth of Listeria monocytogenes, Staphylococcus aureus and Salmonella spp. in laboratory media and in fish soup. International Journal of Food Microbiology 137, 287±294. Fogliano, V., Vitaglione, P. (2005). Functional foods: Planning and development. Molecular Nutrition & Food Research 49, 256±262. Gamage, G.R., Park, H.-J., Kim, K.M. (2009). Effectiveness of antimicrobial coated oriented polypropylene/polyethylene films in sprout packaging. Food Research International 42, 832±839. Gemili, S., Yemenicioglu, A., Altinkaya, S.A. (2009). Development of cellulose acetate based antimicrobial food packaging materials for controlled release of lysozyme. Journal of Food Engineering 90, 453±462. George, M., Abraham, T.E. (2006). Polyionic hydrocolloids for the intestinal delivery of protein drugs: Alginate and chitosan ± a review. Journal of Controlled Release 114, 1±14. Gibbs, B.F., Kermasha, S., Alli, I., Mulligan, C.N. (1999). Encapsulation in the food industry: a review. International Journal of Food Sciences and Nutrition 50, 213±224. Goddard, J.M., Talbert, J.N., Hotchkiss, J.H. (2007). Covalent attachment of lactase to low-density polyethylene films. Journal of Food Science 72, E36±E41. GutieÂrrez, L., Escudero, A., Batlle, R., NerõÂn, C. (2009). Effect of mixed antimicrobial agents and flavors in active packaging films. Journal of Agricultural and Food Chemistry 57, 8564±8571. Halliwell, B. (1996). Antioxidants in human health and disease. Annual Review of Nutrition 16, 33±50. Han, J.H. (2000). Antimicrobial food packaging. Food Technology 54, 56±65. Havenaar, R., Huis in't Veld, J.H.J. (1992). In: The Lactic Acid Bacteria, Vol. 1. The Lactic Acid Bacteria in Health and Disease, Wood, B.J.B., ed., Chapman & Hall, London, pp. 209±224. Heirlings, L., SiroÂ, I., Devlieghere, F., Van Bavel, E., Cool, P., De Meulenaer, B., Vansant, E.F., Debevere, J. (2004). Influence of polymer matrix on adsorption onto silica materials on the migration of -tocopherol into 95% ethanol from active packaging. Food Additives and Contaminants 21, 1125±1136. Hosseini, M.H., Razavi, S.H., Mousavi, M.A. (2009). Antimicrobial, physical and mechanical properties of chitosan-based films incorporated with thymol, clove and cinnamon essential oils. Journal of Food Processing and Preservation 33, 727±743. Ikada, Y., Tabata, Y. (1998). Protein release from gelatin matrices. Advanced Drug Delivery Reviews 31, 287±301. Joerger, R.D. (2007). Antimicrobial films for food applications: a quantitative analysis of their effectiveness. Packaging Technology and Science 20, 231±273.

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Joshi, G.V., Patel, H.A., Bajaj, H.C., Jasra, R.V. (2009). Intercalation and controlled release of vitamin B6 from montmorillonites±vitamin B6 hybrid. Colloid Polymer Science 287, 1071±1076. Kenawy, E.R., Abdel-Hay, F.I., El-Newehy, M.H., Wnek, G.E. (2009). Processing of polymer nanofibers through electrospinning as drug delivery systems. Materials Chemistry and Physics 113, 296±302. Kim, J., Marshall, M.R., Wei, C. (1995). Antimicrobial activity of some essential oil components against five food borne pathogens. Journal of Agricultural and Food Chemistry 43, 2839±2845. Krajewska, B. (2004). Application of chitin- and chitosan-based materials for enzyme immobilizations: a review. Enzyme and Microbial Technology 35, 126±139. Krasaekoopt, W., Bhandari, B., Deeth, H. (2003). Evaluation of encapsulation techniques of probiotics for yoghurt. International Dairy Journal 13, 3±13. Kubacka, A., Cerrada, M.L., Serrano, C., Fernandez-Garcia, M., Ferrer, M., FernandezGarcia, M. (2009). Plasmonic nanoparticle/polymer nanocomposites with enhanced photocatalytic antimicrobial properties. Journal of Physical Chemistry 113, 9182± 9190. Kurmann, J.A., Rasic, J.L. (1991). In: Therapeutic Properties of Fermented Milks, Robinson, R.K., ed., Elsevier Applied Science Publishers, London, pp. 117±157. Labuza, T.P., Breene, W.M. (1989). Applications of active packaging for improvement of shelf-life and nutritional quality of fresh and extended shelf-life foods. Journal of Food Processing and Preservation 13, 1±69. Lagaly, G. (2001). Pesticide±clay interactions and formulations. Applied Clay Science 18, 205±209. LagaroÂn, J.M., Fernandez-Saiz, P., Ocio, M.J. (2007). Using ATR±FTIR spectroscopy to design active antimicrobial food packaging structures based on high molecular weight chitosan polysaccharide. Journal of Agricultural and Food Chemistry 55, 2554±2562. Lee, J.Y., Park, H.J., Lee, C.Y., Choi, W.Y. (2003). Extending shelf-life of minimally processed apples with edible coatings and antibrowning agents. LWT ± Food Science and Technology 36, 323±329. Li, B., Jiang, Y., Liu, Y., Wu, Y., Yu, H., Zhu, M. (2009). Novel poly(Nisopropylacrylamide)/clay/poly(acrylamide) IPN hydrogels with the response rate and drug release controlled by clay content. Journal of Polymer Science: Part B: Polymer Physics 47, 96±106. Li, J.X., He, A.H., Fang, D.F., Hsiao, B.S., Chu, B. (2006). Electrospinning of hyaluronic acid (HA) and HA/gelatin blends. Macromolecular Rapid Communications 27, 114±120. Liu, R.H. (2003). Health benefits of fruit and vegetables are from additive and synergistic combinations of phytochemicals. American Journal of Clinical Nutrition 78 (suppl), 517S±520S. Loher, S., Schneider, O.D., Maienfisch, T., Bokorny, S., Stark, W.J. (2008). Microorganism-triggered release of silver nanoparticles from biodegradable oxide carriers allows preparation of self-sterilizing polymer surfaces. Small 4, 824±832. LoÂpez-Rubio, A., Almenar, E., HernaÂndez-MunÄoz, P., LagaroÂn, J.M., Catala, R., Gavara, R. (2004). Overview of active polymer-based packaging technologies for food applications. Food Reviews International 20, 357±387. LoÂpez-Rubio, A., Gavara, R., LagaroÂn, J.M. (2006). Bioactive packaging: turning foods into healthier foods through biomaterials. Trends in Food Science and Technology 17, 567±575.

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LoÂpez-Rubio, A., Sanchez, E., Sanz, Y., LagaroÂn, J.M. (2009). Encapsulation of living bifidobacteria in ultrathin PVOH electrospun fibres. Biomacromolecules 10, 2823± 2829. Martins, A., Chung, S., Pedro, A.J., Sousa, R.A., Marques, A.P., Reis, R.L., Neves, N.M.J. (2009). Hierarchical starch-based fibrous scaffold for bone tissue engineering applications. Journal of Tissue Engineering and Regenerative Medicine 3, 37±42. Mascheroni, E., Capretti, G., Marengo, M., Iametti, S., Mora, L., Piergiovanni, L., Bonomi, F. (2010). Modification of cellulose-based packaging materials for enzyme immobilization. Packaging Technology and Science 23, 47±57. Mastromatteo, M., Conte, A., Del Nobile, M.A. (2010). Combined use of modified atmosphere packaging and natural compounds for food preservation. Food Engineering Reviews 2, 28±38. Mayachiew, P., Devahastin, S., Mackey, B.M., Niranjan, K. (2010). Effects of drying methods and conditions on antimicrobial activity of edible chitosan films enriched with galangal extract. Food Research International 43, 125±132. Mecitoglu, CË., Yemenicioglu, A., Arslanoglu, A., Elmaci, Z.S., Korel, F., CËetin, A.E. (2006). Incorporation of partially purified hen egg white lysozyme into zein films for antimicrobial food packaging. Food Research International 39, 12±21. Nam, S., Scnalon, M.G., Han, J.H., Izydorczyk, M.S. (2007). Extrusion of pea stach containing lysozyme and determination of antimicrobial activity. Journal of Food Science 72, E477±E484. Natrajan, N., Sheldon, B.W. (2000). Efficacy of nisin-coated polymer films to inactivate Salmonella Typhimurium on fresh broiler skin. Journal of Food Protection 63, 1189±1196. No, H.K., Meyers, S.P., Prinyawiwatkul, W., Xu, Z. (2007). Application of chitosan for improvement of quality and shelf life of foods ± a review. Journal of Food Science 72, 87±100. Oussalah, M., Caillet, S., Salmieri, S., Saucier, L., Lacroix, M. (2006). Antimicrobial effects of alginate-based film containing essential oils for the preservation of whole beef muscle. Journal of Food Protection 69, 2364±2369. Ozdemir, M., Floros, J.D. (2004). Active food packaging technologies. Critical Reviews in Food Science and Nutrition 44, 185±193. Packiyasothy, E.V., Kyle, S. (2002). Antimicrobial properties of some herb essential oils. Food Australia 54, 384±387. Pelissari, F.M., Grossmann, M.V.E., Yamashita, F., Pineda, E.A.G. (2009). Antimicrobial, mechanical and barrier properties of cassava starch±chitosan films incorporated with oregano essential oil. Journal of Agricultural and Food Chemistry 57, 7499±7504. Peltzer, M., Wagner, J., JimeÂnez, A. (2009). Migration study of carvacrol as a natural antioxidant in high-density polyethylene for active packaging. Food Additives and Contaminants 26, 938±946. Perez-Gago, M.B., Serra, M., del Rio, M.A. (2006). Colour change of fresh-cut apples coated with whey protein concentrate-based edible coatings. Postharvest Biology and Technology 39, 84±92. Pitukmanorom, P., Yong, T.-H., Ying, J.Y. (2008). Tunable release of proteins with polymer-inorganic nanocomposite microspheres. Advanced Materials 20, 3504± 3509. Portes, E., Gardrat, C., Castellan, A., Coma, V. (2009). Environmentally friendly films based on chitosan and tetrahydrocurcuminoid derivatives exhibiting antibacterial

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and antioxidative properties. Carbohydrate Polymers 76, 578±584. Raybaudi-Massilia, R., Mosqueda-Melgar, J., Martin-Belloso, O. (2008). Edible alginatebased coating as carrier of antimicrobials to improve shelf-life and safety of freshcut melon. International Journal of Food Microbiology 121, 313±327. Rojas-GrauÈ, M.A., Tapia, M.S., RodrõÂguez, F.J., Carmona, A.J., MartõÂn-Belloso, O. (2007). Alginate and gellan based edible coatings as support of antibrowning agents applied on fresh-cut Fuji apple. Food Hydrocolloids 21, 118±127. Sagis, L.M.C., de Ruiter, R., Rossier Miranda, F.J., de Ruiter, J., SchroeÈn, K., van Aelst, A.C., Kieft, H., Boom, R., van der Linden, E. (2008). Polymer microcapsules with a fiber-reinforced nanocomposite shell. Langmuir 24, 1608±1612. Salmieri, S., Lacroix, M. (2006). Physicochemical properties of alginate/ polycaprolactone-based films containing essential oils. Journal of Agricultural and Food Chemistry 54, 10205±10214. Sanchez, C., Julian, B., Belleville, P., Popall, M. (2005). Applications of hybrid organic± inorganic nanocomposites. Journal of Materials Chemistry 15, 3559±3592. Sanchez-Garcia, M.D., Ocio, M.J., Gimenez, E., LagaroÂn, J.M. (2008). Novel polycaprolactone nanocomposites containing thymol of interest in antimicrobial film and coating applications. Journal of Plastic Film and Sheeting 24, 239±251. Sarkar, J.M., Leonowicz, A., Bollag, J.-M. (1989). Immobilization of enzymes on clays and soils. Soil Biology and Biochemistry 21, 223±230. Sauer, M., Streich, D., Meier, W. (2001). pH sensitive nanocontainers. Advanced Materials 13, 1649±1651. Schillinger, U. (1999). Isolation and identification of lactobacilli from novel-type probiotic and mild yoghurts and their stability during refrigerated storage. International Journal of Food Microbiology 47, 79±87. Seydim, A.C., Sarikus, G. (2006). Antimicrobial activity of whey protein based edible films incorporated with oregano, rosemary and garlic essential oils. Food Research International 39, 639±644. Shah, N.P., Lankaputhra, W.E.V., Britz, M., Kyle, W.S.A. (1995). Survival of L. acidophilus and Bifidobacterium bifidum in commercial yoghurt during refrigerated storage. International Dairy Journal 5, 515±521. Shahidi, F. (2000). Antioxidants in food and food antioxidants. Nahrung 44, 158±163. Smith, L.A., Liu, X., Ma, P.X. (2008). Tissue engineering with nano-fibrous scaffolds. Soft Matter 4, 2144±2149. Soares, N.D.F.F., Hotchkiss, J.H. (1998). Naringinase immobilisation in packaging films for reducing naringin concentration in grapefruit juice. Journal of Food Science 63, 61±65. Sukhorukov, G., Fery, A., Mohwald, H. (2005). Intelligent micro- and nanocapsules. Progress in Polymer Science 30, 885±897. Suppakul, P., Sonneveld, K., Bigger, S.W., Miltz, J. (2008). Efficacy of polyethylenebased antimicrobial films containing principal constituents of basil. LWT ± Food Science and Technology 41, 779±788. Tabata, Y., Nagano, A., Ikada, Y. (1999). Biodegradation of hydrogel carrier incorporating fibroblast growth factor. Tissue Engineering 5, 127±138. Tolls, J. (2001). Sorption of veterinary pharmaceuticals in soils: A review. Environmental Science & Technology 35, 3397±3406. Torres-Giner, S., Ocio, M.J., LagaroÂn, J.M. (2008). Development of active antimicrobial fiber based chitosan polysaccharide nanostructures using electrospinning. Engineering in Life Sciences 8, 303±314. Viseras, C., Aguzzi, C., Cerezo, P., Bedmar, M.C. (2008). Biopolymer±clay

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nanocomposites for controlled drug delivery. Materials Science and Technology 24, 1020±1026. Wang, X., Du, Y., Luo, J. (2008). Biopolymer/montmorillonite nanocomposite: Preparation, drug controlled release property and cytotoxicity. Nanotechnology 19, 065707. Wettasinghe, M., Shahidi, F. (1999). Antioxidant and free radical scavenging properties of ethanolic extracts of deffated borage (Borago borealis L.) seeds. Food Chemistry 67, 399±414. Wong, W.S., Tillin, S.J., Hudson, J.S., Pavlath, A.E. (1994). Gas exchange in cut apples with bilayer coatings. Journal of Agricultural and Food Chemistry 42, 2278±2285. Yao, C.H., Liu, B.S., Hsu, S.H., Chen, Y.S., Tsai, C.C. (2004). Biocompatibility and biodegradation of a bone composite containing tricalcium phosphate and genipin crosslinked gelatin. Journal of Biomedical Materials Research 69A, 709±717. Yao, K.D., Peng, T., Feng, H.B., He, Y.Y. (1994). Swelling kinetics and release characteristics of crosslinked chitosan±polyether polymer network (semi-IPN) hydrogels. Journal of Polymer Science: Part A: Polymer Chemistry 32, 1213±1223. Young, S., Wong, M., Tabata, Y., Mikos, A.G. (2005). Gelatin as a delivery vehicle for the controlled release of bioactive molecules. Journal of Controlled Release 109, 256±274.

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Polylactic acid (PLA) nanocomposites for food packaging applications  N , Novel Materials and Nanotechnology Group, J.-M. LAGARO IATA-CSIC, Spain

Abstract: PLA is without doubt the most commercially used biopolyester in food packaging applications. In spite of this, the biopolymer has significant property shortages such as excessive rigidity, low thermal resistance and relatively low gas and vapour barrier properties compared to PET. In this chapter, the general properties of PLA are first discussed and then the latest literature regarding the development of PLA nanocomposites with enhanced performance of interest in food packaging applications is reviewed. Key words: polylactic acid (PLA), nanocomposites, packaging, high barrier.

17.1

Introduction and properties of polylactic acid (PLA)

Amongst biobased materials, three families are usually considered: polymers directly extracted from biomass, such as the polysaccharides chitosan, starch, carrageenan and cellulose; proteins such as gluten, soy and zein; and various lipids. A second family makes use of biomass-derived monomers but uses classical chemical synthetic routes to obtain the final biodegradable and/or renewable polymers, including thermoplastics and thermosets. In regard to thermoplastics, this is the case of polylactic acid (PLA) and the nonbiodegradable sugarcane ethanol-derived biopolyethylene (Haugaard et al., 2001; LagaroÂn et al., 2008). The third family makes use of polymers produced by natural or genetically modified microorganisms such as polyhydroxyalcanoates (PHA) and polypeptides (Reguera et al., 2003). Amongst nonbiobased materials, i.e. using either petroleum-based monomers or mixtures of biobased- and petroleum-based monomers, there are also a number of biodegradable resins such as polycaprolactones (PCL), polyvinyl-alcohol (PVOH) and its copolymers with ethylene (EVOH) and some biopolyesters. Nevertheless, it seems clear that although biodegradability can help reduce plastic waste, from a `greenhouse' perspective, biobased sustainable materials, the socalled bioplastics, are currently considered the way to go and may be the only alternative in the future as fossil resources become exhausted. From the above family of biopolymers, the ones that are now attracting more commercial interest are the biopolyesters, which can be processed by

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conventional processing equipment, are rather water insensitive and are already being used in a number of monolayer and also multilayer applications, particularly in the food packaging and biomedical fields. The most widely researched thermoplastic sustainable biopolymers for monolayer packaging applications are starch, PHA and PLA. From these, starch and PLA biopolymers are without doubt the most interesting families of biodegradable materials for packaging applications because they have become commercially available (by, for instance, companies such as Novamont and Natureworks, respectively) and are produced on a large industrial scale, and also because they present an interesting balance of properties. Of particular interest in food packaging is the case of PLA (currently commercially produced in excess of 150,000 tonnes/year) due to its excellent transparency, equivalent to its petroleum-based counterpart, and relatively good water resistance (see Table 17.1). Water permeability of PLA is, for instance, much lower than in proteins and polysaccharides but it is still higher than that of conventional polyolefins and PET. The stiffness of the material tends to be rather high and the thermal resistance in the form of the heat deflection temperature (HDT) is significantly lower than that of PET (the glass transition temperature of PLA is typically 59ëC and that for PET is ca. 70ëC) (Sanchez Garcia and LagaroÂn, 2010a), and hence the commercial grades are inherently difficult to process when formed and begin to soften just above room temperature. Its relatively high stiffness is usually reduced by addition of plasticizers such as PCL and others, but these also lead to a decrease in oxygen barrier and transparency. Thus, the main drawbacks of this polymer regarding performance are still associated with low thermal resistance, excessive brittleness and insufficient barrier to oxygen and water compared to, for instance, other benchmark packaging polymers such as PET and polyolefins. Nevertheless, whereas some more or less efficient solutions do exist for the mechanical properties, the thermal and barrier properties are more challenging. It is, therefore, of great industrial interest to enhance the thermal and barrier properties of this material while maintaining its inherently good properties such as transparency and biodegradability (Park et al., 2002; Tsuji and Yamada, 2003; Jacobsen et al., 1999; Jacobsen and Fritz, 1996; Chen et al., 2003).

17.2

Nanobiocomposites of polylactic acid (PLA) for monolayer packaging

17.2.1 Nanoclays Within the polymer/clay nanocomposite technologies, the case of the biocomposites containing biopolymers and clays is without doubt one of the most significant novel developments. Concerning PLA, two techniques are frequently used to produce nanocomposites of this material, namely solution-casting (Torres-Giner et al., 2008; Ogata et al., 1997) and melt mixing (Bandyopadhyay

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Table 17.1 Typical general properties measured in a 1.0 mil film of a commercial PLA grade (NatureWorksÕ IngeoTM 4032D Film Grade PLA) for packaging film extrusion applications as reported by Matweb (http://www.matweb.com) Properties

Metric

Physical properties Specific gravity Moisture vapour transmission Oxygen transmission Carbon dioxide transmission

English

1.24 g/cm3 1.24 g/cm3 3 2 8.30 cm -mm/m - 21.1 cm3-mil/100 in224h-atm 24 h-atm 14.0 cm3-mm/m2- 35.6 cm3-mil/100 in224 h-atm 24 h-atm 76.0 cm3-mm/m2- 193 cm3-mil/100 in224 h-atm 24 h-atm

Mechanical properties Film elongation at break, MD 180% Film elongation at break, TD 100% Flexural modulus 2.85 GPa Flexural strength 44.0 MPa Secant modulus, MD 3.44 GPa Secant modulus, TD 3.784 GPa Elmendorf tear strength, MD 0.669 g/micron Elmendorf tear strength, TD 0.551 g/micron Film tensile strength at break, MD 103.2 MPa Film tensile strength at break, TD 144.5 MPa

180% 100% 413 ksi 6380 ksi 499 kis 548.8 ksi 17.0 g/mil 14.0 g/mil 14970 psi 20960 psi

Thermal properties Melting point

160ëC

320ëF

Optical properties Haze Gloss

2.10% 90.0%

2.10% 90.0%

180ëC 200ëC 200ëC 202±218ëC 20.0±100 rpm

356ëF 392ëF 392ëF 396±424ëF 20.0±100 rpm

Processing properties Feed temperature Adapter temperature Die temperature Melt temperature Screw speed

Comments ASTM D1505 ASTM D1434 ASTM E96 ASTM D1434

ASTM D882 ASTM D882 ASTM D790 ASTM D790 ASTM D882 ASTM D882 ASTM D1922 ASTM D1922 ASTM D882 ASTM D882

ASTM D1003 20ë, ASTM D1003

et al., 1999; Sinha Ray et al., 2002a,b,c,d,e, 2003a,b,c; Yamada et al., 2002). The cited literature reports that, compared with the neat PLA, the PLA nanocomposites show improvements in material properties such as storage modulus, flexural modulus, flexural strength and heat distortion temperature, but also in gas barrier properties compared to neat PLA. Maiti et al. (2002) postulated that the barrier properties of non-interacting gases in nanocomposites primarily depend on two factors: the dispersed silicate particles aspect ratio and the extent of dispersion of these particles within the polymer matrix. When the degree of dispersion of the layered organoclay is maximum, an exfoliated morphology is attained and the barrier properties solely

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Table 17.2 Typical reductions (%) in oxygen and water vapour permeability reported for nanocomposites of PLA compared with the virgin resin Matrix

PLA1 PLA1 PLA1 PLA2 PLA2 PLA2 PLA2 PLA3 PLA4 PLA5 PLA6 PLA6 PLA6 PLA6 PLA6 PLA6 PLA6 PLA6 PLA7

Type of clay

Nanoclay content

Organically-modified-MMT Organically-modified-MMT Organically-modified-MMT MMT MMT-modified Saponite Synthetic fluorine mica MMT-layered silicate MMT-modified Bentonite Hexadecylamine-MMT Hexadecylamine-MMT Hexadecylamine-MMT Dodecyltrimethyl ammonium bromide-MMT Dodecyltrimethyl ammonium bromide-MMT Dodecyltrimethyl ammonium bromide-MMT Cloisite 25A (organicallymodified-MMT) Cloisite 25A (organicallymodified-MMT) O2BlockTM (organicallymodified-MMT)

(%)

Reduction in O2 permeability (%)

4 5 7 4 4 4 4 5 5 5 4 6 10 4

12 15 19 14 12 40 65 48 46 6 42 56 58 41

6

55

10

58

6

45

10

56

5

32

Reduction in H2O permeability (%)

50

54

Sources: 1 Sinha Ray et al. (2003a). 2 Sinha Ray et al. (2003b). 3 Thellen et al. (2005). 4 Lagaro¨n et al. (2005). 5 Petersson and Oksman (2006). 6 Chang et al. (2003). 7 Sanchez-Garcia et al. (2007).

depend on the particles' aspect ratio. Table 17.2 summarizes reported improvements in oxygen and water permeability of nanocomposites of PLA. Sinha Ray et al. (2002a) claimed reductions in oxygen permeability of ca. 65% for PLA + 4 wt% of synthetic fluorine mica prepared by melt compounding. Nanocomposites with similar clay contents (4±7 wt%) but with a different type of nanoclay showed less improvement in oxygen permeability, i.e. ranging from 6% to 56%. Moreover, some nanoclays, i.e. mica base systems, have been reported to be able to not only enhance the barrier but also block the passage of

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17.1 Typical TEM picture of a PLA nanocomposite containing a food contact compliant nanoclay UV blocking.

UV-visible radiation (Sanchez-GarcõÂa and LagaroÂn, 2010b) in PLA due to the inherently enhanced scattering and reflection phenomena and absorbing properties of the highly dispersed long-aspect-ratio clay nanolayers (see Fig. 17.1). The attained ratio of UV protection was very efficient at 5 wt% of the nanoclay addition in the damaging UV region of 280±320 nm. Thus, low nanoclay contents (1 and 5 wt%) added to the transparent PLA matrix led to significant reductions in the UV light transmission and enhanced gas and vapour barrier, while retaining transparency to a significant extent (Sanchez-Garcia and LagaroÂn, 2010b). Also of significant relevance is the novel development of PLA nanocomposites containing active nanoclays, i.e. oxygen scavenging and biocide nanoclays, based on the incorporation of metallic compounds and natural extracts (see Chapter 2). In summary, the barrier properties of PLA nanocomposites have been found to strongly depend upon clay type, processing route, organic modification of the clay, clay content, clay aspect ratio, clay interfacial adhesion and clay dispersion. Typically, solution casting has been reported to be more efficient in enhancing the barrier properties of PLA because, even when a better dispersion of the filler can be often achieved by melt blending, usually in solution casting there are no issues associated with filler-induced polymer hydrolysis. PLA hydrolysis, typically resulting in reductions in the barrier performance, can be enhanced during melt compounding in the presence of fillers. Nevertheless and in spite of all the work carried out, the clay-based nanocomposites of PLA, even when retaining great level of transparency (see Fig. 17.2), are not currently outperforming the barrier properties of its petroleum-based polyester counterpart

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17.2 Melt compounded films of ca. 120 microns of PLA (right-hand side) and of PLA + 5 wt% O2BlockTM nanoclay (left-hand side).

polyethylene terephthalate (PET) or of its nanoclay-based nanocomposites; and as a result further optimization work may still be required for the biocomposites of this polymer (Sanchez-Garcia et al., 2007). Regarding inherent nanoparticle hazard assessment, due to their small size, nanoparticles are generally much more reactive than their corresponding macrocounterparts. But it is also true that as a result of this, much smaller filler loadings are typically required, and hence added to the matrix, to achieve the desired properties. The large surface area of nanoparticles allows a greater contact with cellular membranes, as well as greater capacity for absorption and migration (Li and Huang, 2008). Therefore, assessment of the effects of nanoparticles in food biopackaging materials, such as migration to foods and potential bioaccumulation, needs to be considered in the expected dosages. Currently, data on toxicity and oral exposure of nanoparticles are extremely limited and controversial when it comes to the studied dosages. In addition, the small size of many nanoparticles causes them to take on unique chemical and physical properties that are different from those of their macroscale chemical counterparts. This implies that their toxicokinetic and toxicity profiles cannot be extrapolated from data on their equivalent non-nanoforms. Thus, the risk assessment of nanoparticles has to be performed on a case-by-case basis (Munro et al., 2009). However, it is also very important to differentiate between threedimensional nanoparticles (spherical or otherwise 3D nanoparticles such as nanometals), bidimensional nanoparticles (nanofibres, with only nanodimensions in the 2D cross-section) and the least concerned, one-dimensional nanoparticles (nanoclays with only one nanodimension in the thickness direction). Thus, nanoclays should be considered aside because in essence they are heatstable microparticles, which remain such all along the process of production and commercialization and to a significant extent also as two-dimensional microparticles within the biopolymer matrix during service. In any case, the general

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risk assessment of migration products resulting from packaging materials is and continues to pose a difficult challenge. As a general rule, nanocomposites within the European Union must comply with the EFSA total migration limit of 10 mg/dm2, with the functional barrier stringent migration level of 0.01 mg/kg of food or food simulant and/or with the specific migration levels for their constituents in case they comprise food contact components (Commission Directive 2007/19/CE modifying Directive 2002/72/CE). Some studies have shown that upon melting nanocomposite plastic films, some clay accumulation appears to occur at the surface of the materials generating a clay-containing barrier (Hao et al., 2006; Lewin and Tang, 2008). However, information about actual migration to food or food simulants is very scarce. In a recent study, Schmidt et al. (2009) characterized the migration and size of migrated particles from PLA nanocomposites in a fatty food stimulant (ethanol). The previous authors used an analytical platform consisting of asymmetrical flow field-flow fractionation (AF4) coupled with multi-angle light scattering (MALS) and inductively coupled plasma mass spectrometry (ICP-MS). Even though an increase in migration was observed for the nanocomposite in comparison with the neat PLA matrix, the migration levels were below the total migration limit and no traces of nanoclays were detected in the migrated substances. These results confirm the theoretical predictions by SÏimon et al. (2008), who concluded that considering a polymer matrix with low dynamic viscosity that did not interact with the nanoparticles, only very small particles with a diameter of about 1 nm were expected to migrate. In view of the results, it seems clear that the current evidence suggests that no specific relevant issues are to be expected with nanoclays in food contact but more research is needed in this area, not only investigating the migration and potential toxicity of nanoclays, but more importantly also of other nanoparticles used in food packaging structures.

17.2.2 Other fillers Most applications of nanocomposites in bioplastics have made use of laminar nanoclays as commodities and of carbon nanotubes as engineering materials. However, there are other types of reinforcing elements such as biodegradable fibres obtained by electrospinning, which are very promising in a number of application fields (Chronakis, 2005; Huang et al., 2003). The electrospinning method is a simple and versatile technology which can generate ultrafine morphologies, typically fibres, of many materials. The produced fibres can have very large surface to mass ratios (up to 103 higher than a microfibre), excellent mechanical strength, flexibility and lightness. The obtaining procedure is not mechanical but electrostatic and is applied to the polymer in solution or to polymer melts. As a result of the latter it is a very suitable technique for the generation of ultra-fine fibres of biodegradable materials, which are in general

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easy to dissolve. It has been reported that around 100 different polymers (including biopolymers) and polymer blends have been nanofabricated by electrospinning as either uniaxial or coaxial fibres, where complex multilayer structures can be produced. In spite of the fact that there is a significant body of scientific literature reporting on the characterization of nanofibres, less attention has been paid to the properties of nanocomposites of materials containing these fibres. We have recently developed a methodology to incorporate these nanostructured electrospun fibres with success into PLA to reinforce the gas barrier properties of PLA based packaging films (Busolo et al., 2009). Recently, there is also an increased interest in the nanofabrication of cellulose nanocrystals or nanowhiskers (CNW), the load-bearing constituents in the development of new and inexpensive biodegradable materials due to their reported high aspect ratio, good mechanical properties (Sturcova et al., 2005; Helbert et al., 1996) and the fully biodegradable and renewable character of their nanocomposites. Compared to other inorganic reinforcing fillers, CNW have many additional advantages, including low density, low energy consumption in manufacturing, ease of recycling by combustion, high sound attenuation, and comparatively easy processability due to their non-abrasive nature, which allows high filling levels, in turn resulting in significant cost savings (Podsiadlo et al., 2005; Azizi Samir et al., 2005). Cellulose whiskers or microcrystals are typically prepared by treating native microfibres of cellulose with sulfuric acid, where small amounts of sulfate ester groups are introduced to the surfaces (Marchessault et al., 1959). This treatment is, however, hydrolytic and thus results in dramatic decreases in both the yield and the fibril length down to 100±150 nm. The use of cellulose nanowhiskers as nanoreinforcements is a relatively new field in nanotechnology and as a result there are still many obstacles remaining to their use. Firstly, cellulose nanowhiskers are not standardized commercial products. Secondly, their production is time consuming and is still associated with low yields. Thirdly, they are difficult to use in systems that are not water based due to their strong hydrogen bonding. This affects the production of PLA based nanocomposites when using PLA which is not water soluble. This can be partly solved by appropriate solvent exchange of the whiskers. Some authors have studied the development of PLA nanocomposites based on CNW. Kvien et al. (2005) reported the study of the morphology by AFM, SEM and TEM for structure determination of cellulose whiskers (based on wood and prepared by acid hydrolysis of microcrystalline cellulose) and their nanocomposite with poly(lactic acid). Other authors have studied the processing technique for the preparation of cellulose based nanocomposites, using DMAc with 0.5 wt% LiCl as swelling/separation agent, and they reported an improvement in the mechanical properties (Oksman et al., 2006). Other researchers have reported on the treatment of CNW with either tert-butanol or a surfactant and have studied the thermal and mechanical properties of the CNW

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nanocomposites of PLA (Petersson et al., 2007). Lately, some authors have also tried to obtain nanowhiskers using different acids and even mixtures to generate different cellulosic nanowhisker grades (Dorgan et al., 2008). In the latter work, Dorgan et al. claimed to have increased to a significant extent the HDT of PLA by an in-situ polymerization incorporation route to facilitate dispersion. In spite of all the above, very little is known about the improvements in barrier properties of these PLA±CNW nanocomposites. Some authors studied the barrier properties of nanocomposites of PLA with MCC, but they obtained an increase in oxygen permeability (Petersson and Oksman, 2006). As opposed to this, a very recent work suggested that addition of cellulose nanowhiskers to PLA led to improvements in water permeability of ca. 82% compared with the pure PLA. This is one of the lowest water permeability values achieved in PLA nanobiocomposites, and is ascribed to the high nanodispersion of the nanowhiskers across the matrix, the high crystallinity and the good interfacial adhesion and dispersed morphology seen in these particular nanobiocomposites (Sanchez-Garcia and LagaroÂn, 2010a). In this context, our laboratory has also recently developed a methodology to incorporate proteins and polysaccharides into PLA by melt blending using thermoplastic polymers such as high vinyl content EVOH (Nordqvist, 2010a,b). In this work some improvements in barrier properties and enhanced biodegradability for the biopolymer were obtained by dispersion via melt compounding of amylopeptine in the PLA matrix. Other additives that have been very recently incorporated into biopolymers for tailoring their properties are carbon nanotubes (CNT) and/or carbon nanofibres (CNF). These types of fillers are typically added to biopolymers with the overall aim of increasing the biodegradation rate, enhancing the mechanical properties and increasing the thermal and electrical conductivity for different applications such as biomedical, automotive, packaging and electronics. A very recent work suggests that these nanofillers can also be used to enhance the barrier properties to gases and water vapour of biopolymers such as PCL and PHAs (Sanchez-Garcia et al., 2010) and hence of also PLA. These newly developed nanobiocomposites could have some potential applications in food packaging, for instance in microwavable packaging, in anti-static packaging or in intelligent packaging designs, due to their additional functionalities that include electrical and thermal conductivity. Nevertheless, the strong black colouring and the issues of non-intended migration and potential toxicity make the foreseeable use of these nanomaterials more adequate for very specific and niche applications (Sanchez-Garcia et al., 2010).

17.3

Future trends

Addition of natural micro- and nanoshields in the form of cellulose fibres, cellulose nanowhiskers, carbon nanotubes, carbon nanofibres and food-contact-

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complying nanoclays (especially non-MMTs) to PLA matrices for the development of novel nanobiocomposites is an upcoming area of potentially high interest in food packaging science. Using melt mixing, in-situ polymerization and casting processing techniques, dispersed morphologies were observed. These morphological features were correlated with significant improvements in properties such as HDT and in the barrier properties to gases (oxygen) and vapours (water and limonene) of significant interest in the wide implementation of this biopolymer to extend biopackaged food shelf-life. The optimum barrier performance was observed for loading percentages of 5 wt% and 3 wt% for nanoclays and nanowhiskers, respectively. Moreover, some clay-based nanobiocomposites have been demonstrated to have new additional functionalities, being useful to partially block damaging UV radiation and allowing the development of novel antimicrobial materials based on natural plant extracts and nanometals and of other releasable elements of great interest in novel food active and bioactive packaging applications. Thus, these naturally derived elements, i.e. clays and cellulosic materials, some of which are already commercially viable, present the greatest outlook for the development and widespread acceptability of more efficient PLA-based food biopackaging solutions that will enable the full implementation of multifunctional renewable-resources packaging material alternatives to the current petroleumderived packaging. Further improving the properties of melt-compoundable PLA to the required extent remains a challenge due to filler-induced polymer degradation issues. In any case, outperforming the current properties of commercial PLA bioresins and even of petroleum-based plastics becomes a necessary opportunity that could be widely realized in the short term with the aid of nanotechnology.

17.4

References and further reading

Azizi Samir, M.A.S., Alloin, F., Dufresne, A. (2005), Biomacromolecules 6, 612±626. Bandyopadhyay, S., Chen, R., Giannelis, E.P. (1999), Biodegradable organic±inorganic hybrids based on poly(L-lactide), Polym. Mater. Sci. Eng. 81, 159±160. Bastiolo, C., Bellotti, V., Del Tredici, G.F., Lombi, R., Montino, A., Ponti, R. (1992), Int. Pat. Appl. WO 92/19680. Busolo, M., Torres-Giner, S., LagaroÂn, J.M. (2009), Enhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers, ANTEC 2009 Conf. Proc. Chang, J.-H., An, Y.U., Sur, G.S. (2003), Poly(lactic acid) nanocomposites with various organoclays. I. Thermomechanical properties, morphology, and gas permeability. J. Polym. Sci. Part B: Polym. Phys. 41, 94±103. Chen, C.C., Chueh, J.Y., Tseng, H., Huang, H.M., Lee, S.Y. (2003), Preparation and characterization of biodegradable PLA polymeric blends, Biomaterials 24, 1167. Chronakis, I.S. (2005), Novel nanocomposites and nanoceramics based on polymer nanofibers using electrospinning process ± A review, J. Mater. Proc. Technol. 167 (2±3): 283±293.

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Dorgan, J.R., Sobkowicz, M., Patel, R. (2008), Ecobionanocomposites and bioplastic blends, International Symposium on Polymers and the Environment: Emerging Technology and Science (BEPS), Conference Proceedings Book, Abstract 3, Nashua (USA). Hao, J., Lewin, M., Wilkie, C.A., Wang, J. (2006), Additional evidence for the migration of clay upon heating of clay±polypropylene nanocomposites from X-ray photoelectron spectroscopy (XPS), Polym. Degrad. Stab., 91, 2482±2485. Haugaard, V.K., Udsen, A.M., Mortensen, G., Hoegh, L., Petersen, K., Monahan, F. (2001), Food biopackaging, in Biobased Packaging Materials for the Food Industry ± Status and Perspectives, ed. C.J. Weber, University of Copenhagen, Denmark. Helbert, W., Cavaille, J.Y., Dufresne, A. (1996), Polym. Compos. 17, 604±611. Huang, Z.M., Zhang, Y.Z., Kotaki, M., et al. (2003), A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Comp. Sci. Technol. 63(15), 2223±2253. Jacobsen, S., Fritz, H.G. (1996), Filling of poly(lactic acid) with native starch, Polym. Eng. Sci. 36, 2799. Jacobsen, S., DegeÂe, P.H., Fritz, H.G., Dubois, P.H., JeÂrome, R. (1999), Polylactide (PLA) ± A new way of production, Polym. Eng. Sci. 39, 1311. Koening, M.F., Huang, S.J. (1995), Biodegradable blends and composites of polycaprolactone and starch derivatives, Polymer 36, 1877. Kvien, I., Tanem, B.S., Oksman, K. (2005), Characterization of cellulose whiskers and their nanocomposites by atomic force and electron microscopy, Biomacromolecules 6, 3160±3165. LagaroÂn, J.M., Cabedo, L., Cava, D. et al. (2005), Improving packaged food quality and safety. Part 2. Nanocomposites. Food Addit. Contam. 22(10), 994±998. Lagaro n, J.M., Gimenez, E., Sanchez-Garcia, M.D. (2008), Thermoplastic nanobiocomposites for rigid and flexible food packaging applications, in Environmentally Compatible Food Packaging, ed. E. Chiellini, Woodhead Publishing, Cambridge, UK. Lewin, M., Tang, Y. (2008), Oxidation±migration cycle in polypropylene-based nanocomposites. Macromolecules, 41, 13±17. Li, S.D., Huang, L. (2008), Pharmacokinetics and biodistribution of nanoparticles, Molecular Pharmaceutics 5(4), 496±504. Maiti, P., Yamada, K., Okamoto, M., Ueda, K., Okamoto, K. (2002), New polylactide/layered silicatenanocomposites:Roleoforganoclays.Chem.Mater.14(11),4654±4661. Marchessault, R.H., Morehead, F.F., Walter, N.M. (1959), Nature 184, 632±633. Munro, I.C., Haighton, L.A., Lynch, B.S., Tafazoli, S. (2009), Technological challenges of addressing new and more complex migrating products from novel food packaging materials, Food Addit. Contam. 26(12), 1534±1546. Nordqvist, D., Sanchez-GarcõÂa, M.D., Hedenqvist, M.S., LagaroÂn, J.M. (2010a), Incorporating amylopectin in poly(lactic acid) by melt blending using poly(ethylene-co-vinyl alcohol) as a thermoplastic carrier. (I) Morphological characterization, J. Appl. Polym. Sci. 115(3), 1315±1324. Nordqvist, D., Sanchez-GarcõÂa, M.D., Hedenqvist, M.S., LagaroÂn, J.M. (2010b), Incorporating amylopectin in poly(lactic acid) by melt blending using poly(ethylene-co-vinyl alcohol) as a thermoplastic carrier. (II) Barrier properties, J. Appl. Polym. Sci., in press. Ogata, N., Jimenez, G., Kawai, H., Ogihara, T. (1997), Structure and thermal/mechanical properties of poly( L-lactide)±clay blend, J. Polym. Sci. Part B: Polym. Phys. 35, 389±396.

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Oksman, K., Mathew, A.P., Bondeson, D., Kvien, I. (2006), Manufacturing process of cellulose whiskers/polylactic acid nanocomposites. Comp. Sci. Technol. 66, 2776± 2784. Park, E.S., Kim, M.N., Yoon, J.S. (2002), Grafting of polycaprolactone onto poly(ethylene-co-vinyl alcohol) and application to polyethylene-based bioerodable blends, J. Polym. Sci.: Part B Polym. Phys. 40, 2561. Petersson, L., Oksman, K. (2006), Biopolymer based nanocomposites: Comparing layered silicates and microcrystalline cellulose as nanoreinforcement, Comp. Sci. Technol. 66, 2187±2196. Petersson, L., Kvien, I., Oksman, K. (2007), Structure and thermal properties of poly(lactic acid)/cellulose whiskers nanocomposite materials. Comp. Sci. Technol. 67, 2535±2544. Podsiadlo, P., Choi, S., Shim, B., Lee, J., Cussihy, M., Kotov, N. (2005), Biomacromolecules 6, 2914±2918. Reguera, J., LagaroÂn, J.M., Alonso, M., Reboto, V., Calvo, B., Rodriguez-Cabello, J.C. (2003), Thermal behavior and kinetic analysis of the chain unfolding and refolding and of the concomitant nonpolar solvation and desolvation of two elastin-like polymers, Macromolecules 36, 8470±8476. Sanchez-Garcia, M.D., LagaroÂn, J.M. (2010a), On the use of plant cellulose nanowhiskers to enhance the barrier properties of polylactic acid, Cellulose 17(5), 987±1004. Sanchez-Garcia, M.D., LagaroÂn, J.M. (2010b), Novel clay-based nanobiocomposites of biopolyesters with synergistic barrier to UV light, gas, and vapour, J. Appl. Polym. Sci. 118(1), 188±199. Sanchez-Garcia, M.D., Gimenez, E., LagaroÂn, J.M. (2007), Novel PET nanocomposites of interest in food packaging applications and comparative barrier performance with biopolyester nanocomposites, J. Plastic Film Sheeting 23(2), 133±148. Sanchez-Garcia, M.D., LagaroÂn, J.M., Hoa, S.V. (2010), Effect of addition of carbon, Comp. Sci. Technol. 70(7), 1095±1105. Schmidt, B., Petersen, J.H., Bender Koch, C., Plackett, D., Johansen, N.R., Katiyar, V., Larsen, E.H. (2009), Combining asymmetrical flow field-flow fractionation with light scattering and inductively coupled plasma mass spectrometric detection for characterization of nanoclay used in biopolymer nanocomposites, Food Addit. Contam., 26(12), 1619±1627. SÏimon, P., Chaudhry, Q., Bakos, D. (2008), Migration of engineered nanoparticles from polymer packaging to food ± a physicochemical view, J. Food Nutr. Res., 47(3), 105±113. Sinha Ray, S., Yamada, K., Okamoto, M., Ueda, K. (2002a), New polylactide/layered silicate nanocomposite: a novel biodegradable material, Nano Lett. 2, 1093±1096. Sinha Ray, S., Maiti, P., Okamoto, M., Yamada, K., Ueda, K. (2002b), New polylactide/ layered silicatenanocomposites. 1. Preparation, characterization and properties, Macromolecules 35, 3104±3110. Sinha Ray, S., Yamada, K., Ogami, A., Okamoto, M., Ueda, K. (2002c), New polylactide layered silicate nanocomposite: nanoscale control of multiple properties, Macromol. Rapid Commun. 23, 493±497. Sinha Ray, S., Okamoto, M., Yamada, K., Ueda, K. (2002d), New polylactide/layered silicate nanocomposites: concurrent improvement of materials properties and biodegradability. Proc. Nanocomposites 2002, San Diego, CA. Sinha Ray, S., Okamoto, M., Yamada, K., Ueda, K. (2002e), New polylactide/layered silicate nanocomposite: Nanoscale control over multiple properties, Macromol. Rapid Comm. 23(16), 943±947.

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Sinha Ray, S., Yamada, K., Okamoto, M., Ueda, K. (2003a), New polylactide/layered silicate nanocomposites. 2. Concurrent improvements of material properties, biodegradability and melt rheology, Polymer 44, 857±866. Sinha Ray, S., Yamada, K., Okamoto, M., Ogami, A., Ueda, K. (2003b), New polylactide/ layered silicate nanocomposites. 3. High performance biodegradable materials, Chem. Mater. 15, 1456±1465. Sinha Ray, S., Yamada, K., Okamoto, M., Fujimoto, Y., Ogami, A., Ueda, K. (2003c), New polylactide/layered silicate nanocomposites: 5. Designing of materials with desired properties, Polymer 44, 6633±6646. Sturcova, A., Davies, G.R., Eichhorn, S.J. (2005), Biomacromolecules 6, 1055±1061. Thellen, C., Orroth, C., Froio, D. et al. (2005), Influence of montmorillonite layered silicate on plasticized poly(L-lactide) blown films. Polymer 46(25), 11716±11727. Torres-Giner, T., Gimenez, E., LagaroÂn, J.M. (2008), Food Hydrocolloids 22(4), 601± 614. Tsuji, H., Yamada, T. (2003), Blends of aliphatic polyesters. VIII. Effects of poly(Llactide-co--caprolactone) on enzymatic hydrolysis of poly(L-lactide), poly(caprolactone), and their blend films, J. Appl. Polym. Sci. 87, 412. Yamada, K., Ueda, K., Sinha Ray, S., Okamoto, M. (2002), Preparation and properties of polylactide/layered silicate nanocomposites, Kobun Robun 59, 760±765.

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Polyhydroxyalkanoates (PHAs) for food packaging Â, D . P L A C K E T T and I . S I R O Technical University of Denmark, Denmark

Abstract: This chapter addresses the basic and applied characteristics of polyhydroxyalkanoates (PHAs) in the context of their potential future development for food packaging applications. The introduction provides a brief overview of the chemistry and synthesis of PHAs. This is followed by an outline of commercial developments to date and a main section in which the properties of PHAs and PHA-based nanocomposites of greatest relevance to food packaging are discussed. Past research on PHA foams and coatings is outlined and the chapter concludes with a summary as well as possible future trends and suggestions for further reading. Key words: polyhydroxyalkanoates (PHAs), nanocomposites, packaging.

18.1

Introduction

The term polyhydroxyalkanoates (PHAs) is used to describe a class of polyesters generated as a form of carbon and energy storage in bacterial cells. The occurrence of PHA particles was first observed by Beijerinck in 1888 (Chowdhury, 1963); however, it was not until 1925 that the composition of an unknown material produced by Bacillus megaterium was confirmed (Lemoigne, 1925, 1926). In a comprehensive review, Lenz and Marchessault (2005) pointed out that Lemoigne's publications occurred at about the same time that Hermann Staudinger was proposing the existence of high molecular weight molecules, which he termed `macromolecules'. Even though the material later known to be polyhydroxybutyrate (PHB), the simplest PHA, was subsequently mentioned in biochemistry textbooks, it was described as a lipid and not as a polyester and consequently remained relatively hidden from researchers interested in the then new field of polymer science. From 1923 to 1951, Lemoigne and colleagues published 27 papers, but it was only in the late 1950s that the role of PHB in the overall metabolism of bacterial cells was discovered and the significance of Lemoigne's work was fully realised. In 1958, Macrae and Wilkinson described the production of PHB as a function of the glucose:nitrogen ratio in the growth medium and noted that degradation was fast in the absence of any other energy source. The same authors suggested a pathway for PHB synthesis (Macrae and Wilkinson, 1958). For a long time poly-3-hydroxybutyrate was considered the

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18.1 Structures of poly-3-hydroxybutyrate (PH3B), polyhydroxyvalerate (PHV) and polyhydroxybutyrate-co-valerate (PHBV).

only PHA produced by bacteria as a reserve energy supply until poly-3hydroxyvalerate and poly-3-hydroxyhexanoate were discovered in chloroform extracts of activated sewage sludge. The possibility of preparing various polyesters as a function of the substrate was first reported by De Smet et al. (1983) when a polymer consisting mainly of 3-hydroxyoctanoate units was obtained after Pseudomonas oleovorans was cultivated in n-octane. The general structure of PHAs is illustrated in Fig. 18.1 and some 150 different monomers of this type have now been identified. As discussed later, from a commercial viewpoint there has also been interest in polyhydroxybutyrate-co-valerate or PHBV because it is more ductile and in principle more readily processable than PHB. The mechanism of bacterial synthesis of PHAs is now largely understood and processes can be modified to generate the desired product outcome. As shown in the simplified scheme in Fig. 18.2, microbial biosynthesis of PHB starts with the condensation of two molecules of acetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. The condensation reaction is catalysed by a -ketothiolase while the reduction of acetoacetyl-CoA involves an NADPH-dependent acetoacetyl-CoA reductase. Acetyl-CoA is a basic metabolite found in all PHA-producing organisms. Hydroxybutyryl-CoA then starts to polymerise to form PHB in which the chiral carbon centre is preserved. Propagation proceeds by bonding of each additional monomer unit to a free thiol active site. Depending upon the selected fermentation microorganisms and the cultivation conditions, homo- or co-polyesters of hydroxyalkanoic acids are obtained. PHA synthases, which are capable of utilising either short- or mediumchain length hydroxy acids, are the key enzymes involved in PHA synthesis and, for example, in the case of PHB, polymerisation of hydroxybutyryl-CoA occurs in the presence of the PHB synthase enzyme. Some bacteria are able to generate both short- and medium-chain length co-polyesters. Under ideal process con-

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18.2 Synthesis of polyhydroxybutyrate (PHB).

ditions, PHAs can grow to represent 80% of bacterial cell mass. It is now known that most bacteria can synthesise PHAs under nutrient-limited conditions when carbon is available in excess. Industrially, PHAs are solvent-extracted from the bacterial cell mass grown in the presence of sugars or other carbohydrates, but there has also been significant research activity aimed at the concept of gene transfer so that PHAs might be collected more efficiently from dedicated crops rather than bacteria. There are now some commercial developments in this direction; however, at the time of writing, commercialisation of PHA extraction from plant crops has yet to be achieved on a full industrial scale.

18.2

Commercial developments

The oil crisis of the 1970s provided a boost in the quest for alternative bioderived plastics and this has now been re-stimulated by recent fluctuations in global oil prices and environmental concerns. Several efforts to commercialise PHAs started in the early 1980s. In 1982, Imperial Chemical Industries Ltd (ICI) began production of a biodegradable polyester (BiopolÕ) which could be melt processed to produce films and fibres (Anderson and Dawes, 1990). The process was based on use of a glucose-utilising mutant of the bacterium Alcaligenes eutrophus (since renamed Ralstonia eutropha and more recently Wautersia eutropha) and the product was a PHBV co-polyester containing randomly arranged units of [R]-3-hydroxybutyrate and [R]-3-hydroxyvalerate (Holmes, 1988). The main PHA manufacturers and trade names are listed in Table 18.1. As one of the first pioneers, Chemie Linz (Austria) also produced PHB on a relatively large scale (20±100 tonnes/year) for a few years starting in the late 1980s.

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Table 18.1 Main suppliers of PHAs worldwide Company

Trade name

Type of PHA

Source

Biomer, Germany Metabolix/Telles/ ADM, USA Meredian, USA Copersucar/Biocycle, Brazil Bio-On, Italy Tianan Biologic Material, China Tianjin Green Bioscience, China

BiomerÕL MirelTM

PHB PHB

NodaxTM BiocycleÕ

Copolymers PHB

www.biomer.de www.metabolix.com www.mirelplastics.com www.meredianpha.com www.biocycle.com.br

Minerv-PHATM EnmatTM

Unknown PHBV

www.bio-on.it www.tianan-enmat.com

Greenbio-1

Unknown

www.bio-natural.com www.dsm.com

ICI was split up in 1993 and the Zeneca BioProducts branch of ICI started dealing with BiopolÕ. Zeneca sold the BiopolÕ technology to Monsanto in April 1996 and this was then acquired by Metabolix Inc. (US) in 1998 (Philip et al., 2007; Mooney, 2009). Recently, Metabolix Inc. has cooperated with ADM (Archer Daniels Midland Company, US) in order to commence large-scale production of PHAs under the trade name MirelTM from Telles, a joint sales company. MirelTM will be produced at a ~50,000 tonnes/year plant presently under construction in Clinton, Iowa (http://www.mirelplastics.com). Another company in the field of PHA production is Tianjin Green Bioscience Co., Ltd of China with an expected production of 10,000 tonnes/year. Recently, carbon sources other than glucose, including whey, organic acids, hydrolysed starch, methane and sewage, were tested by this company in order to reduce production costs. It has been suggested that the production of PHAs can serve as a beneficial step in sewage treatment if that is used as carbon source (http://www.bio-natural.com.hk). Mention should also be made here of the research conducted by Jian Yu and his group at the Hawaii Natural Energy Institute, which has included the use of food wastes as a feeding source for PHA-producing bacteria as well as the patenting of technology to produce biodegradable thermoplastics and elastomers from organic wastes (Du and Yu, 2002). In another commercial development, Procter & Gamble aimed to reduce the costs of PHA copolyesters to make them more competitive with established products; however, despite some success, the production of PHAs under the trade name NodaxTM stopped in 2006 (Philip et al., 2007). In 2007, Procter & Gamble sold the NodaxTM intellectual property to Meredian Inc. which announced plans to further develop and commercialise the technology. Other PHA suppliers include Ningbo Tianan Biologic Material Co., Ltd in China which produces different forms of PHBV under the tradename EnmatTM, Biomer (Germany) with its BiomerÕ P-series (PHB), Bio-On (Italy) producing MINERV-PHATM and PHB Industrial S.A., owner of the BiocycleÕ brand (PHB).

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Polyhydroxyalkanoates (PHAs) and their nanocomposite films

18.3.1 Overview PHA-based films have attracted interest for food packaging applications due to their renewability, biodegradability and potentially useful water vapour barrier properties. In particular, the properties of PHB can be compared with those of synthetic thermoplastics such as isotactic polypropylene (PP) (Choi et al., 2003; Thellen et al., 2008). However, the use of PHB has been limited by drawbacks such as costs of production, brittleness and low thermal stability in the molten state which results in a narrow melt processing window (Gregorova et al., 2009; Ohashi et al., 2009; Erceg et al., 2010). These limitations can be partly avoided by copolymerisation of PHB with other monomers, such as hydroxyvalerate or hydroxyhexanoate. For example, PHBV has a lower melting point than PHB homopolymer and therefore a larger melt processing window (Chen et al., 2004; Philip et al., 2007; Bordes et al., 2009a; Dagnon et al., 2009a, 2009b). However, PHBV suffers from a slow crystallisation rate, relatively difficult processing and low elongation at break, and therefore further improvements are needed (Choi et al., 2003; Chen et al., 2004; Wang et al., 2005). Blending with other biopolymers has been considered as a way of tailoring the properties of PHAs while maintaining biodegradability (Ohashi et al., 2009). Mixtures of PHB and PHBV with other biopolymers have been reported by numerous authors and the polymers mentioned in recent studies include starch (Godbole et al., 2003; Reis et al., 2008; Zhang and Thomas, 2010), thermoplastic starch (Parulekar and Mohanty, 2007), poly-(L-lactic acid) (PLLA) (Furukawa et al., 2006; Rychter et al., 2006), polycaprolactone (PCL) (Lovera et al., 2007; Sanchez-Garcia et al., 2008), and polyvinyl alcohol (PVOH) (Olkhov et al., 2003). In addition to blending, interest in enhancing material properties through fabrication of nanocomposites based on PHAs and organic or inorganic nanofillers has rapidly increased and, as recently described, there are developing opportunities for nanocomposites in food packaging (Arora and Padua, 2010). Studies have been dedicated to examining nanocomposites based on PHAs filled with layered silicates such as montmorillonite (MMT), layered double hydroxides (LDHs), cellulose nanowhiskers (CNWs), and multi-walled carbon nanotubes (MWCNTs) (Xu and Qiu, 2009). The rationale is that introduction of nanofillers into a polymer matrix can significantly modify morphology, crystallisation behaviour, thermal stability, mechanical and barrier properties, and biodegradation rate, all of which are relevant from the food packaging perspective. It is also widely accepted that property improvements increase as a function of nanofiller dispersion with many reports in which, for example, inorganic clay platelet exfoliation within a polymer matrix is the key target. The following sections discuss key properties of PHA and PHA nanocomposite films as reported in the literature and their significance in regards to food packaging.

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18.3.2 Mechanical properties Depending on the composition of the monomer units, PHAs can exhibit a wide variety of mechanical properties from those of hard crystalline polyesters (e.g., PHB) to more elastic materials such as poly(3-hydroxyoctanoate) or PHO (Khanna and Srivastava, 2005; Yun et al., 2008). The Young's modulus and tensile strength of PHB are comparable to those reported for PP but the elongation at break (5±10%) is significantly lower (McChalicher and Srienc, 2007; Sudesh and Iwata, 2008). This stiffness is attributed to cracks within the PHB spherulites that form under conditions of non-externally applied stress (Barham and Keller, 1986; Barham et al., 1992; El-Hadi et al., 2002a, 2002b). One way to reduce brittleness is to hasten uniform crystal formation, which may be done by adding nucleating agents to the polymer melt during processing (HaÈnggi, 1995). The mechanical properties of PHBVs are dependent on the molar ratio of HV (Philip et al., 2007). As a rule, the copolymer becomes tougher with increasing fraction of HV (i.e., increased impact strength) and more flexible (i.e., decreased Young's modulus), while the tensile strength gradually decreases (Khanna and Srivastava, 2005). Some authors have reported that PHBV becomes very soft when the HV composition is in the range of 30 to 60 mol% (Sudesh and Iwata, 2008). On the other hand, long-term analyses (i.e., up to 30 days) have shown that even though PHBV films can have elongation-at-break in excess of 500%, depending on the composition of the random copolymer, this value is relevant for only a few days after films are cast (McChalicher and Srienc, 2007) and the copolymer can become more brittle with time. The embrittlement of PHB and PHBV materials occurs during storage after initial crystallisation from the melt and it has been argued that secondary crystallisation results in the reorganisation of lamellar crystals formed during the initial crystallisation process, which then tightly constrains amorphous polymer chains between the crystals (Sudesh and Doi, 2000; Sudesh et al., 2000, El-Hadi et al., 2002a). In Table 18.2, the typical mechanical properties of some commercial plastics reported in the literature are compared with those of PHB and PHBV. There have been a number of recent studies dealing with the characterisation of PHB- and PHBV-based nanocomposites using MMTs and LDHs prepared either by solution casting (Lim et al., 2003; Bruzaud and Bourmaud, 2007; Hsu et al., 2007; Bruno et al., 2008; Wu et al., 2008; Erceg et al., 2009, 2010) or by melt intercalation (Maiti et al., 2007; Parulekar et al., 2007; Hablot et al., 2008; Sanchez-Garcia et al., 2008; Ohashi et al., 2009; Botana et al., 2010). The fabrication of PHB/MMT nanocomposites through solution casting was first described by Lim et al. (2003); however, the mechanical and rheological properties of the modified films were not reported at that time. Maiti et al. (2007) prepared PHB-based nanocomposites by melt extrusion and in this case PHB was reinforced using 2 wt% organo-modified fluoromica or up to 3.6 wt%

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Table 18.2 Mechanical properties of PHAs and some other polymers of interest for food packaging. Presented values are indicative only Polymer* Tensile strength (MPa) PHB PHBV PLA PCL PET LDPE PP PS PVC

40 30±38 28±50 16 56 10±15 35±40 12±50 10±60

Young's modulus (MPa) Strain at break (%) 1700±3500 700±2900 1200±2700 400 2200 200 1700 1600±3100 300±2400

3±6 20 7±9 120±800 70±100 300±500 150 3±4 12±32

* Values for individual films will depend on a number of factors including polymer molecular weight, crystallinity and film orientation as well as the mechanical testing conditions. Source: adapted from Sudesh et al., 2000; Khanna and Srivastava, 2005; Philip et al. 2007.

organo-modified montmorillonite (OMMT). The storage modulus of the nanocomposites was increased and better reinforcing was achieved with fluoromica than with OMMT. The authors explained this behaviour by the occurrence of greater polymer degradation in the presence of OMMT. This phenomenon could be due to the presence of aluminium Lewis acid sites in the inorganic clay layers which can act as sites for catalysing ester hydrolysis. In another study, Parulekar et al. (2007) modified MMT with neopentyl(diallyl)oxytri(dioctyl)pyrophosphato titanate which was then used as a reinforcement for PHB. Epoxidised natural rubber was included as an impact modifier and nanocomposites were prepared by extrusion followed by injection moulding. The diffraction patterns and TEM images from the resulting material suggested exfoliation of the organically modified clays, which was further validated by melt rheological analysis. Nanocomposites containing 5 wt% titanate-modified clay exhibited about 400% improvement in impact properties and 40% reduction in storage modulus when compared with unreinforced PHB. More recently, Botana et al. (2010) compared the reinforcing efficiency of two commercial MMTs, namely Na-MMT (CloisiteÕ Na+) and a methyl tallow bis-hydroxyethyl quaternary ammonium-modified MMT (CloisiteÕ 30B) in a PHB matrix. These authors concluded that although the moduli of the nanocomposites increased, the exfoliation/intercalation ratio was not high enough to significantly increase the tensile strength. Exfoliation was, however, more pronounced when using the OMMT, indicating better compatibility with the PHB matrix in that case. Studies on modifying the mechanical properties of PHBV by addition of nanoclays have been reported. Choi et al. (2003) prepared PHBV/CloisiteÕ 30B nanocomposites by melt intercalation. XRD and TEM clearly confirmed that intercalated nanostructures were obtained. The mechanical properties were modified by clay addition (up to 3 wt%) and, for example, the Young's modulus

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increased significantly from 481 to 795 MPa due to strong hydrogen bonding between PHBV and CloisiteÕ 30B. In contrast, strength hardly increased while elongation decreased from 8.5 to 5.6%. Changes in the original mechanical properties were also reported by Chen et al. (2004) when PHBV/OMMT nanocomposites were prepared by solution intercalation with 3 wt% filler content. However, due to clay aggregation, the tensile properties were much reduced when higher filler loading (10 wt%) was used. Bruzaud and Bourmaud (2007) reported a significant increase in tensile strength and Young's modulus in PHBV/CloisiteÕ 15A nanocomposites with 5 wt% filler content. These changes were attributed to the improved clay±PHBV interfacial area due to the presence of partially exfoliated clays. In this study, the corresponding strain-at-break decreased from 3.3 to 1.4% as a result of nanoclay addition. LDHs have also been used to prepare PHA-based nanocomposites (Hsu et al., 2007; Wu et al., 2008; Dagnon et al., 2009a, 2009b); however, the influence of such fillers on PHA mechanical properties has only been reported by Dagnon et al. (2009a). These researchers observed that the addition of stearic acidmodified Zn-AlNO3 LDH in PHBV (1±7 wt%) resulted in greater than a 10% increase in modulus with a corresponding decrease in strain-at-break. Although strength increased when up to 3 wt% LDH was added, it decreased with further nanofiller addition, probably as a result of aggregation. While there are numerous studies dealing with inorganic nanofillers, there have been few reports concerning the use of organic nanofillers in PHAs (e.g., nanocellulose). In a basic research investigation, Dufresne et al. (1999) discussed the use of hydrolysed tunicin cellulose whiskers to reinforce mediumchain length PHAs and obtained substantial improvement in mechanical properties, attributed to the formation of a transcrystalline network between the whiskers and the semi-crystalline matrix. Jiang et al. (2008) prepared cellulose nanowhisker/PHBV nanocomposites both by solution casting using N,N-dimethylformamide (DMF) as well as by extrusion blending and injection moulding of PHBV with freeze-dried nanowhiskers. These authors claimed that while a homogeneous dispersion of the whiskers was achieved and the composites showed improved tensile strength and modulus in the case of solvent-cast composites, whisker agglomerates formed during freeze drying which reduced the strength of the melt-processed materials. Dynamic mechanical analysis of PHB and PHBV has been performed by a number of researchers (Chen et al., 2002; Thellen et al., 2008; Gregorova et al., 2009) with the aim of characterising viscoelastic properties as a function of temperature. Storage modulus (E0 ), loss modulus (E00 ) and tan  are the main parameters used to describe such properties. The storage modulus of PHB has been reported to be in the range of 2500±3500 MPa at 20ëC while it is somewhat higher for PHBV (Maiti et al., 2007; Thellen et al., 2008; Gregorova et al., 2009). A decrease of storage modulus (E0 ) with temperature is generally observed. Several studies have demonstrated the increase of Tg and storage

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modulus as a result of nanofiller addition (Chen et al., 2002; Maiti et al., 2007; Jiang et al., 2008). The reinforcement effect of nanoclay additives becomes more prominent above the Tg, when the materials become soft, due to the restricted movement of the polymer chains (Ray and Okamoto, 2003). When mechanical properties of PHAs are considered in terms of food packaging uses, one can conclude that the lack of adequate flexibility presents the most significant challenge. Blending with other polymers can decrease brittleness to a certain extent. However, it has been shown that the addition of nanofillers to PHAs may result in a further decrease in polymer flexibility (i.e., higher Young's modulus), which is often undesirable. In order to be competitive with fossil fuel-based polymers used for food packaging (e.g. PP, PE, PS or PET) and which possess an elongation-at-break in the range of 400±7300%, there remains a significant need to improve the flexibility of PHAs by means that will not adversely compromise other properties.

18.3.3 Permeability A potentially positive characteristic of PHB and PHBV films with respect to food packaging applications is their water vapour permeability, which has been reported as similar to that of conventional thermoplastics such as PVC or PET (Miguel et al., 1997; Miguel and Iruin, 1999a, 1999b; Cyras et al., 2007, 2009). Furthermore, PHAs have an advantage over certain other biopolymers (e.g., starch, cellulose, gluten and chitosan) since they are non-swelling and have lower hydrophilicity (Miguel et al., 1997). On the other hand, diffusivity and solubility of water in PHAs is important, since degradation of these polymers can proceed via enzymatic or non-enzymatic hydrolysis (Yoon et al., 2000; Renard et al., 2004; CorreÃa et al., 2008). The water transport properties of PHB and PHBV films and their blends with other biodegradable polymers under different conditions were reported by a number of researchers (Iordanskii et al., 1996, 1998, 1999; Miguel et al., 1997; Miguel and Iruin 1999a, 1999b; Yoon et al., 2000; Olkhov et al., 2003; Renard et al., 2004; Thellen et al., 2008). Interestingly, PHAs have been used to reduce the water sensitivity of other biopolymers. For example, the water permeability of PVA films was decreased by adding 10±50 wt% PHB (Olkhov et al., 2003). PHAs also have good barrier properties against a number of organic solvents (Miguel et al., 1997; Miguel and Iruin, 1999a; Sanchez-Garcia et al., 2007) and exhibit low oxygen and CO2 permeability (Poirier et al., 1995; Kuusipalo, 2000a, 2000b). Miguel et al. (1997, 1999) studied the transport of some organic liquids and vapours, water and CO2 through PHB films. These authors demonstrated the relatively high permeability with respect to moderately polar solvents such as chloroform, acetone and toluene, while permeability was moderate to low for methanol, n-hexane, carbon tetrachloride and isopropyl ether. The same researchers also compared the water transport properties of PHA films and

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observed that water sorption as well as water vapour permeability in PHBV was virtually independent of the HV content in the range 0±24 wt%, which may be due to the similar crystallinity of the HB and HV segments (Miguel and Iruin, 1999b). It has been suggested elsewhere that PHBV exhibits lower water permeability when compared to PHB and that the water vapour barrier increases with increasing HV content (Thellen et al., 2008; Bordes et al., 2009a). A possible explanation offered by Poley et al. (2005) is that crystallinity will tend to decrease slightly as the hydroxyvalerate content increases. This may be a consequence of the greater difficulty of accommodating polymer chains in the crystalline phase due to the presence of the ethyl group in the HV monomer, although in some respects this argument appears counter-intuitive since barrier properties should decrease when crystallinity is reduced. Sanchez-Garcia et al. (2007, 2008) measured lower water vapour permeability values for PHB than for PHBV, and the importance of crystallinity in determining the permeability properties of PHAs has been discussed by a number of authors. For example, in a study by Yoon et al. (2000) the diffusion coefficient and equilibrium solubility of water molecules in PHB, polyglycolide (PGA), SkygreenÕ (SG, an aliphatic polyester of succinic acid/adipic acid-1,4-butanediol/ethylene glycol), PLLA, and PCL was explored. It was found that diffusion coefficients decreased in the order SG > PCL > PLLA > PHB > PGA, which was partly attributed to differences in the crystallinity of these polymers. The CO2 permeability of PHB is low and is comparable with that of polyvinylidene chloride (PVDC). A CO2 diffusion coefficient value of 1  10ÿ9 cm2 sÿ1 at 25ëC was reported for PHB by Poley et al. (2005), which is slightly higher than that measured earlier by Miguel et al. (1997) (4.4±4:7  10ÿ10 cm2 sÿ1 at 30ëC). The oxygen diffusion coefficient was  0:4  10ÿ9 cm2 sÿ1 at 25ëC for PHB and this increased slightly with increasing HV content (8±22 wt%), which was in this case attributed to decreasing crystallinity. The barrier properties of polymers can be enhanced by the addition of inorganic laminar nanofillers (e.g., clays). This well-known effect is associated with an increase in the tortuosity of the diffusion path as a result of introducing impermeable nanoplatelets (Sanchez-Garcia et al., 2008; de Azeredo, 2009). Articles in the literature report a decrease in the oxygen permeability of PLA (Ray et al., 2003; Chowdhury, 2008; Zenkiewicz and Richert, 2008; Sabet and Katbab, 2009), PCL (Sanchez-Garcia et al., 2007, 2008), PET (Frounchi and Dourbash, 2009) and PP (Mirzadeh and Kokabi, 2007; Villaluenga et al., 2007) when nanoclays are incorporated. However, the number of studies on the improvement of PHA barrier properties through addition of nanofillers is limited. As one of the few examples, Sanchez-Garcia et al. (2008) compared the thermal and barrier properties of organically modified kaolinite and OMMT in PHB-based nanocomposites prepared by melt blending with PCL added as a plasticiser. The result was a non-miscible but compatible interphase blend. Overall, the nanocomposites exhibited increased gas, aroma and water vapour

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barrier performance. For example, the oxygen permeability of PHB- and PHB/ PCL-based nanocomposites containing 4 wt% nanoclay measured at 24ëC and 0% RH decreased by up to 43%. The same researchers reported ~20 and 27% reduction in oxygen permeability of PHB and PHBV films, respectively when 5 wt% OMMT was added (Sanchez-Garcia et al., 2007). Although a decrease in permeability is generally expected as a result of nanocomposite formation using layered clay silicates, the coexistence of phases with different permeabilities can result in complex transport phenomena. On the one hand, an organophilic clay can give rise to superficial adsorption and specific interactions with the penetrants while, on the other hand, the polymer phase can be considered as a two-phase crystalline-amorphous system in which the crystalline regions are impermeable to penetrant molecules. At the same time, changes in matrix crystallinity and chain mobility, induced by the presence of the nanofiller, need to be considered (Osman and Atallah, 2004; Osman et al., 2004; Pavlidou and Papaspyrides, 2008). Table 18.3 presents a summary of representative permeability data derived from the literature in which PHB is compared with PHBV, PLA, PCL and a number of conventional synthetic plastics. As shown, this is not an easy task since the values reported for the different polymer types can cover a wide range. This observation is probably due to the use of different measuring techniques and equipment as well as variations in the specific properties of the tested polymers (e.g., crystallinity and molecular weight). Broadly speaking, the barrier properties of PHB and PHBV appear to be slightly better than those of PLA and potentially competitive with those of various synthetic plastics; Table 18.3 Permeability properties of PHAs and some other polymers of interest for food packaging. Presented values are indicative only Polymer*

PHB PHBV PLA PCL LDPE PET PP PS PVC

O2 permeability at 23ëC, 0±50% RH (ml mm mÿ2 dayÿ1 atmÿ1)

Water vapour permeability at 23ëC±38ëC, 50±90% RH (g mm mÿ2 dayÿ1)

CO2 permeability at 23ëC, 0±50% RH (ml mm mÿ2 dayÿ1 atmÿ1)

2±10 5±14 15±25 20±200 50±200 1±5 50±100 100±150 2±8

1±5 1±3 5±7 300 0.5±2 0.5±2 0.2±0.4 1±4 1±2

3 ± 35±70 ± 800±1000 15±20 200±400 250±500 10±15

* Values for individual films will depend upon a number of factors including polymer molecular weight and crystallinity as well as the permeability testing conditions. Source: adapted from Lange and Wyser, 2003; Miguel et al. 1997; Thellen et al. 2008; SanchezGarcia et al. 2007, 2008.

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however, given the spread of data reported in the literature and the various conditions under which materials were measured, caution is suggested and the evaluation of individual PHAs in terms of the barrier requirements for packaging of particular food types would be highly recommended.

18.3.4 Thermal stability The thermal instability of PHAs has been a limiting factor in the processing and application of these polymers (Chen et al., 2004; Erceg et al., 2009). Thermal degradation of PHB and PHBV and various approaches to improving thermal stability have been widely studied using techniques including TGA, DSC, timeresolved pyrolysis MS and pyrolysis GC/MS (Galego and Rozsa, 1999; He et al., 2001; Aoyagi et al., 2002; Carrasco et al., 2006). The thermal degradation of PHAs near the melting point occurs almost exclusively by a non-radical random chain-scission reaction and the depolymerisation of the macromolecular chains is the controlling step (Grassie et al., 1984; Spyros et al., 1997; Carrasco et al., 2006). Thermal degradation becomes particularly significant at temperatures above 200ëC (Galego and Rozsa, 1999). In the literature there is a consensus that increasing the HV content in a PHBV copolymer leads to a reduction in melting point. As a result the processing temperature window is increased and degradation rates are maintained within acceptable limits (Poirier et al., 1995; Kuusipalo, 2000a; He et al., 2001; Thellen et al., 2008; Bordes et al., 2009a, 2009b). Da Silva et al. (2005) and Poley et al. (2005) reported that the melting temperature (TM) of PHB decreased from 176ëC to 158ëC with 22 wt% HV content, while Bordes et al. (2009b) described the same reduction in TM for PHBV with only 8 wt% HV content. He et al. (2001) observed a 70ëC decrease in TM for PHBV containing 30 wt% HV. However, as Bordes et al. (2009b) pointed out, the initial molecular weight can have a more significant influence on thermal degradation than the HV content. The lower the Mw, the greater the degrading effect due to random chain-scission. The characteristic thermal properties of some biodegradable polymers of relevance to food packaging are presented in Table 18.4. Blending with other biopolymers as a method for increasing the thermal stability of PHAs has been the subject of many research studies (Chun and Kim, 2000; El-Hadi et al., 2002a; Godbole et al., 2003; Erceg et al., 2005; Ohashi et al., 2009; Zhang and Thomas, 2010). The thermal stability of PHAs can be improved by addition of inorganic nanofillers including MMTs and LDHs (Choi et al., 2003; Lim et al., 2003; Bruzaud and Bourmaud, 2007; Wu et al., 2008; Bordes et al., 2009a, 2009b; Dagnon et al., 2009a, 2009b; Erceg et al., 2009, 2010; Botana et al., 2010). Such improvements are usually attributed to the dispersed silicate layers acting as a barrier to oxygen and to the volatiles generated during thermal decomposition (Cho and Paul, 2001; Choi et al., 2003; Lim et al., 2003; Bruzaud and Bourmaud

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Multifunctional and nanoreinforced polymers for food packaging Table 18.4 Thermal properties of PHAs and polyolefins used in food packaging Polymer PHB PHBV LDPE PP

Tg (ëC)

Tm (ëC)

15 ÿ1 ÿ81 ÿ7 to ÿ35

175 136±162 105±110 160±168

Source: adapted from Kuusipalo, 2000a.

2007). For example, addition of 1±3 wt% CloisiteÕ 30B to PHBV increased the decomposition onset temperature from 252 to 263ëC (Choi et al., 2003). The temperature corresponding to 50% degradation of neat PHBV was found to increase by 30ëC with 5 wt% CloisiteÕ 15A nanoclay addition (Bruzaud and Bourmaud, 2007). The influence of nanofillers on the thermal stability of PHAs is complex and reports in the literature sometimes conflict. The degree of dispersion, in particular, significantly affects the thermal stability of nanocomposites, as agglomerates can cause local accumulation of heat and trigger more rapid thermal decomposition (Lim et al., 2003; Erceg et al., 2010). Dispersion of nanoclays within a polymer matrix depends on factors such as the amount and the nature of clay, the type and quantity of organomodifier, and processing conditions. The nanofiller content can also be crucial as agglomerates can form above a certain loading. As an example, Lim et al. (2003) demonstrated that although the decomposition onset temperature for PHB with 3 wt% OMMT was higher than that for unreinforced PHB, the thermal stability of the nanocomposites was reduced by further nanofiller addition. Similarly, Erceg et al. (2009) reported 5 wt% as an OMMT load limit for increasing the thermal stability of PHB. In contrast, Wang et al. (2005) found improved thermal stability at up to 10 wt% OMMT concentration in PHBV, despite the presence of agglomerates at the highest loading. It is suggested elsewhere that the presence of aluminium Lewis acid sites in the silicate layers enhances the thermal degradation of PHB by catalysing the hydrolysis of ester linkages and this phenomenon is more pronounced at higher loading levels (Erceg et al., 2009). The effect of nanofiller type on PHA thermal stability is exemplified in the research reported by Maiti et al. (2007). The addition of OMMT (1.2±3.6 wt%) increased the decomposition temperature but this was lowered when hydrophilic unmodified MMT (2.2 wt%) was used, probably as a result of poor dispersion. Other studies have shown that clay organomodifiers (e.g., quaternary ammonium salts) can have a strong catalytic influence on thermal degradation of PHB and PHBV (Xie et al., 2001; Hablot et al., 2008; Cabedo et al., 2009). The proposed degradation mechanism involves conversion of the quaternary ammonium surfactant into an amine through nucleophilic attack or Hofmann elimi-

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18.3 PHB random chain scission reaction.

nation (Bordes, 2009b). The released acidic proton and/or nucleophilic amine can act as catalyst for PHB random chain scission (Fig. 18.3). It has also been suggested that these surfactants can act in synergy with fermentation residues to degrade PHB; however, the corresponding mechanism is still unexplained (Hablot et al., 2008). The release of tightly bound water from nanoclay surfaces at elevated temperature may contribute to PHBV degradation during processing. Interestingly, in support of this suggestion, it has be shown that kaolinite-based nanofillers, which release water at much higher temperatures, do not catalyse the degradation of PHBV (Cabedo et al., 2009). Erceg et al. (2009, 2010) investigated the influence of two OMMTs, namely CloisiteÕ 30B and CloisiteÕ 25A, on the thermal stability of PHB and analysed the degradation kinetics. These authors concluded that the isothermal degradation of pure PHB and PHB/ CloisiteÕ 30B nanocomposites occurs in two distinct regions ± a first in which relatively low mass loss takes place, and a second, the main degradation mechanism, in which the greater mass loss takes place. Other reports indicate that a simple first-order kinetic model cannot be applied to describe the isothermal degradation behaviour of PHB and PHB-based nanocomposites due to the contribution of different mechanisms, including autocatalytic or chain reactions (Kopinke et al., 1996; Wu et al., 2008). Although some researchers have studied solution-cast PHA/clay nanocomposites in order to circumvent thermal degradation issues (Bruzaud and Bourmaud, 2007; Wu et al., 2008; Cabedo et al., 2009), melt processing remains a more industrially practicable method for fabrication of polymer nanocomposites and in this case high shear rates during extrusion, which are usually needed in order to achieve nanoclay exfoliation, can contribute to PHA degradation (Bordes et al., 2009a). The decrease in PHA molecular weight at high screw speeds in an extruder can lead to stickiness on the metal surface of the chill rolls or injection moulding tools and increased crystallisation times (ElHadi et al., 2002b). The extent of degradation in melt-processed PHBV is highly

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dependent on the type of mixing apparatus, the processing time and the processing atmosphere. For example, a reduction in PHBV degradation during nanocomposite compounding under nitrogen has been reported (Cabedo et al., 2009). There have been relatively few reports on LDH-based PHA nanocomposites, and investigations on thermal stability are also limited in number. In one of the few examples, Wu et al. (2008) explored the thermal degradation mechanism of PHB containing 2% and 5% poly(ethylene glycol) phosphonate (PEOPA)modified LDH (PMLDH). These authors found that the incorporation of organically modified LDH did not improve the thermal stability of the nanocomposites, with the decomposition onset decreasing from 263.6ëC in neat PHB to 240.2ëC in samples containing 5% PMLDH, suggesting that the organic modifier may catalyse the thermal degradation of PHB. To conclude, although improvement of the thermal stability of PHAs by addition of nanofillers can in principle be a viable option, the filler type and content, the organomodifier and the processing conditions have to be selected carefully in order to achieve the desired effect. In addition to thermal stability during melt processing, thermal stability of finished products at lower temperatures during storing or transportation of PHAbased packaging would clearly be important. In the case of PLA, it is known that stacked packaging trays can lose mechanical stability and collapse at temperatures above the Tg, which is typically in the range 50±59ëC (Huda et al., 2006, 2007). However, as noted (http://www.biomer.de), processed PHB can be highly crystalline and exhibit no such softening at temperatures likely to be encountered in storage and transport.

18.3.5 Migration For food packaging purposes, migration is a key issue, since monomers or additives used in PHA manufacturing processes may not be common in conventional food contact materials and might conceivably migrate into packaged food. However, to the authors' knowledge there has been no study yet which has monitored the migration of specific components from PHA packaging. The total migration from PHB films into different food simulants, including distilled water, 3% acetic acid, 15% ethanol and n-heptane has been investigated (Bucci et al., 2007). Tests were run at 40ëC for 10 days, with the exception of n-heptane where the tests were performed at 20ëC for 30 minutes. For all the simulants, total migration was below the recommended limit of 8.0 mg/dm2 or 50 mg/kg, suggesting that PHAs should be safe for packaging of various food products. Although in the European legislation both conventional and bio-based food contact materials are regulated in the same way, some special issues have to be considered in the latter case (Chowdhury, 2008). A challenge which the food packaging industry has to face in relation to the use of biodegradable polymers

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and which also complicates the migration issue is the durability of the packaging in relation to the product shelf-life. The environmental conditions that lead to degradation of packaging must clearly be avoided during the storage of the food product and should only exist after the packaging has been discarded (Petersen et al., 1999; Haugaard et al., 2001). While it is known that pure PHB and PHBV are non-toxic, more information is needed regarding the potential toxicity and migration behaviour of degradation products produced during either processing or biodegradation (HaÈnggi, 1995). Another issue to be considered in respect to future packaging applications is the potential migration of nanoparticles from PHA nanocomposite films into food products. Concerns may arise because nanoparticles are generally much more reactive than corresponding macroparticles. The large surface area of nanoparticles allows a greater contact with cellular membranes as well as a greater capacity for absorption and migration (Li and Huang, 2008). There is, however, limited scientific data about the migration of nanoparticles from packaging material into food or the eventual toxicological effects (de Azeredo, 2009). SÏimon et al. (2008) discussed the theory of particle migration from nanocomposites and concluded that only very small particles with a diameter of ~1 nm should migrate. Avella et al. (2005) determined the migration of certain minerals (Fe, Mg, Si) from biodegradable starch/nanoclay nanocomposite films; the results of this study showed an insignificant trend in the levels of Fe and Mg in packaged vegetables but a consistent increase in the amount of Si, one of the main elements present in MMT nanoclays. In a recent study (Schmidt et al., 2009), isotopes of Zr and Mg were selected for on-line detection of CloisiteÕ 30B in order to follow the potential migration of this nanoclay from PLA nanocomposite films. The technique involved particle separation using field flow fractionation and then multi-angle light scattering to determine particle sizes in combination with ICP±MS for chemical characterisation. Although nanoparticles in the range of 50±800 nm were detected, ICP±MS signals corresponding to clay minerals were absent. A more recent study (MauricioIglesias et al., 2010) suggests that the specific migration properties of nanoparticles should be monitored, rather than the migration of their constituent elements, although this can be challenging from an analytical viewpoint (Tiede et al., 2009). In summary, there are presently no data available concerning the migration of specific components including possible degradation products from PHA packaging materials, which is a critical issue in terms of food safety. Similarly, there are no studies reporting the migration of nanoparticles from PHA-based nanocomposite films, although it is reasonable to assume that migration may occur and hence, if these materials are to be developed in the future, there will be a continuing need for risk evaluation.

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18.3.6 Degradability The biodegradation of PHAs in both aerobic and anaerobic environments has been extensively studied (Abou-Zeid et al., 2001, 2004; Bucci et al., 2007). As a rule, PHAs are considered more readily biodegradable than PLA (Sudesh et al., 2000; Sudesh and Iwata, 2008). The biodegradation of PHAs involves biotic or abiotic hydrolysis followed by bioassimilation (CorreÃa et al., 2008). Various microorganisms can excrete extracellular PHA depolymerases which hydrolyse high molecular weight PHAs into water-soluble oligomers and monomers and subsequently utilise these products as nutrients (Khanna and Srivastava, 2005). The eventual metabolic products are water and carbon dioxide (Renard et al., 2004). Extracellular PHA depolymerase has been isolated and purified from several bacteria and fungi that are known to degrade PHAs. In this respect, the dominant genera among bacteria are Pseudomonas, Azotobacter, Bacillus and Streptomyces, and among fungi they are Penicillium, Cephalosporum, Paecilomyces and Trichoderma (Savenkova et al., 2000). The PHA depolymerase mechanism has been widely studied (Timmins et al., 1997; Iwata et al., 2002; Abe et al., 2005; Li et al., 2007). The rate of PHA biodegradation is influenced by (1) molar mass, copolymer composition, crystallinity, stereochemistry, hydrophilic/hydrophobic balance, and chain mobility; and (2) environmental factors including the microbial population, temperature, moisture, pH and nutrient supply (Khanna and Srivastava, 2005). Numerous studies have explored the factors determining the biodegradability of PHA materials in soil (Savenkova et al., 2000; Tsuji et al., 2003; dos Santos Rosa et al., 2004; CorreÃa et al., 2008), fresh water (Kasuya et al., 1998; Kusaka et al., 1999), marine environments (Tsuji and Suzuyoshi, 2002a, 2002b, 2003; Thellen et al., 2008), sewage environments (Briese et al., 1994; Bucci et al., 2007) and compost media (Yue et al., 1996; Maiti et al., 2007). In general, the higher the polymer crystallinity and melting point, the lower the degradation rate. In PHBV, an increased HV content is associated with faster degradation (Renard et al., 2004). Degradation mechanisms under aerobic conditions are different from those in anaerobic situations and reports indicate that PHBV degrades more rapidly than PHB under aerobic conditions (Mergaert et al., 1993; Yue et al., 1996; dos Santos Rosa et al., 2004; Li et al., 2007); however, the opposite effect has been reported by Abou-Zeid et al. (2001, 2004). Biodegradation of PHA-based nanocomposites has been investigated and a decrease in the rate of PHB or PHBV biodegradation with increasing nanoparticle content is often reported. For example, Wang et al. (2005) showed that the biodegradability of PHBV/OMMT in soil suspension decreased with increasing OMMT content. The reduced rate of biodegradation when high aspect ratio nanoclays are present in the matrix has been attributed to the formation of a tortuous path which can hinder penetration of microorganisms into the bulk of the material (Wang et al., 2005; Maiti et al., 2007). Reduced water permeability

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and an antimicrobial effect in some OMMTs may also play a role in lowering the rate of biodegradability (Bordes et al., 2009a). However, conflicting reports about the effect of nanoclays on polymer biodegradability can be found in the literature. As an example, titanate-modified MMT enhanced the biodegradation of toughened PHB several-fold. In the proposed mechanism, the terminal hydroxylated edge groups of the silicate clay layers can absorb moisture from compost and act as initiation sites for polyester hydrolysis (Ray et al., 2003). Typically, any factor which increases the hydrolytic tendency of PHAs will ultimately control the degradation (Pavlidou and Papaspyrides, 2008). In addition, there is evidence that well-dispersed clay particles cause the polymer chains to fragment more rapidly, resulting in increased degradation (Parulekar et al., 2007). In one study the biodegradation rate of PHB was enhanced significantly in the presence of 2 wt% organo-modified fluoromica with nearcomplete degradation observed in about seven weeks (Maiti et al., 2007). The authors also reported that at higher temperatures the rate of biodegradation drastically decreased for both neat PHB and PHB nanocomposites. This reduced biodegradation rate may be due to the suppression of microorganisms at and above 60ëC or to an increase in polymer crystallinity in these samples. The latter is considered important since the amorphous interspherulitic regions are prone to hydrolysis followed by microorganism attack. In summary, as indicated, the factors which influence the rate of PHA nanocomposite degradation remain under discussion and are the subject of continuing research. The use of PHAs for food packaging materials would ultimately require advance knowledge of performance in service under a variety of conditions and to date there have been few investigations in this direction. Kantola and HeleÂn (2001) analysed the performance of BiopolÕ-coated paperboard trays overwrapped with a Mater-BiÕ (Novamont) starch-based film when used to package organic tomatoes. The key finding was that the tomatoes stayed as fresh in the PHB-coated paperboard trays as those wrapped in perforated LDPE bags. Haugaard et al. (2003) explored packaging of an orange juice simulant and a dressing in PHB cups and concluded that the performance was as good as that of HDPE and superior when samples were stored under light. Hermida et al. (2008) discovered that there was no significant reduction in PHB properties when exposed to the levels of gamma radiation needed to sterilise food or packaging materials.

18.4

Polyhydroxyalkanoate (PHA) foams and paper coatings

As with other applications, cost of production is a significant problem in terms of introducing PHA to the price-sensitive foam packaging market. Kaneka Corporation, also known as Kanegafuchi, has been active in foaming of biodegradable polymers and in recent years has filed two patents on expandable

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PHA. In the first of these (Hirose et al., 2007), solid beads of a poly(3hydroxyalkanoate) are formed, suspended in a blowing agent in a sealed vessel, and then pressurised and heated to start the expansion. The blowing agent is preferably dimethyl ether, diethyl ether or methyl ethyl ether, all of which have low boiling points and will impregnate the polymer beads. Once the beads are sufficiently saturated with ether and heated to a temperature not far below their melting point, the vessel is opened and the beads complete their expansion. In a second patent (Miyagawa et al., 2007), benefits are claimed for the addition of an isocyanate chain extender to the polymer and a wider range of blowing agents is claimed. Current industrial applications of foamed PHAs have yet to develop because of manufacturing problems, largely as a result of thermal instability. Thermal degradation makes foaming difficult because of the low viscosity after addition of a foaming agent and the resulting cell collapse. In one of the few examples cited in the literature, a mixture of PHB, PVOH and starch with optimum viscosity was produced and azodicarbonamide used as a foaming agent. Due to a greater surface area, the resulting foam was found to have a faster rate of biodegradation than bulk materials of the same composition (Grosu et al., 2007). Although there have been studies reported and patents issued on the use of PHA foams for medical applications (e.g., tissue engineering), development for packaging uses has to the authors' knowledge not yet been reported. As reported elsewhere in this chapter, there have been a few studies on PHAs as coatings for paper and paperboard which might be suitable for food packaging (Cyras et al., 2007, 2009). Bourbonnais and Marchessault (2010) recently reported the effect of natural or artificially produced PHB and PHBV granules as paper sizing agents. Differences in the sizing effect were noticed when papers were impregnated and dried at ~110ëC and this was associated with differences in PHA particle morphology. Much improved sizing was noted when impregnated papers were pressed and heated at ~160ëC, under which conditions melted granules formed a thin film at the paper surface. The authors noted that the preparation technique might also open the way to new fibre-reinforced PHA films. As examples, PHB or PHBV coatings on paper or cardboard were successfully applied to reduce the moisture absorption and the water vapour permeability of these materials (Kuusipalo 2000a, 2000b; Cyras et al., 2007, 2009).

18.5

Conclusions

PHAs have been widely investigated in terms of their chemistry, biosynthesis, properties and potential applications, and the means of tailoring the specific polymers, as well as their direct extraction from bacterial cultures, have attracted commercial interest for some 50 years. As discussed in this chapter, commercial interest is now on the rise again as a result of industrial interest in `green'

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materials and concerns about the environment. Although a number of companies have developed and can supply PHB or PHBV raw materials, their conversion into readily processable bioplastics including, for example, nucleating agents and plasticisers has been the focus of fewer manufacturers. Considering the question of food packaging, wide uptake of PHAs is presently limited by cost factors and this appears unlikely to change in the short term. Putting the cost issue aside, by examining past literature, this chapter has addressed whether the properties of PHAs or PHA nanocomposites might generally match the needs for food packaging. Clearly, this is a complex question, since requirements vary according to food product and type of packaging; however, some overall indications can be gained. In terms of mechanical properties, increased flexibility appears to be a key issue and, although blending with other polymers offers this possibility, the introduction of nanofillers may in this respect have an undesirable effect. The literature on permeability of PHA films, although not always easy to interpret, suggests that this should not be a major limiting factor. Control of the thermal stability of PHAs during melt processing is a challenge but, as discussed and as also demonstrated industrially, is not an insurmountable problem. Also on the positive side, it seems that PHAs can be processed so as to be thermally stable at storage and transportation temperatures, the lack of which continues to be an issue in regards to wider adoption of PLA-based food packaging trays. The migration properties of PHAbased films for food packaging and their likely performance in service have received relatively little attention so far and will need further study if PHAbased materials are to enter this sector in the future.

18.6

Future trends

As indicated, a principal reason for the lack of PHA-based products in food packaging is the relatively high cost of PHAs. Typically, PHAs are produced using pure cultures of defined bacterial isolates and highly purified carbon sources such as glucose. This type of production yields an average cost of around ¨10/kg, which is considerably more than that of petroleum-based plastics such as PE (Mooney, 2009). However, production of PHA from mixed cultures using various waste streams has been proposed as an effective way to reduce these costs (Lemos et al., 2003; Reis et al., 2003; Khardenavis et al., 2005, 2007). Although PHAs are still expensive relative to petroleum-derived plastics, future increases in production volume combined with the possibility of higher oil prices could help to reduce the price gap. From a technical perspective, amongst other issues, processing technologies are needed to deal with low thermal stability, and optimised methods to increase the toughness of PHB are required. The use of PHAs in high-value medical applications is quite feasible but volumes are not sufficient to justify economical production, estimated to be at least 20,000 tons per annum.

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Considering biodegradable polymers as a whole in food packaging uses, market forces and government regulations are driving manufacturers in this direction and, if suitable processing parameters, properties and viable prices can be achieved, PHAs could play a significant role in future food packaging. As an example, partial substitution of PET bottles, where the price gap is not so significant, may be a future target for PHA products, providing that technological difficulties can be solved.

18.7

Sources of further information and advice

For further information on the subject of biodegradable polymers, including PHAs, a good starting point is http://www.biopolymer.net. This website provides a listing of commercial bioplastics by trade name, material type and application as well as links to relevant institutions and organisations such as the European Bioplastics Association (http://www.european-bioplastics.org). Excellent sources of information about biopolymers, including PHAs, are the textbooks edited by Doi and SteinbuÈchel (2002) and by Belgacem and Gandini (2008). The latter also covers the topic of composite materials based on renewable polymers. With regard to reviews on the topic of PHAs, mention should be made of the publication by Lenz and Marchessault (2005) and a more recent one on PHA origins, properties and applications by Chodak (2008). Philip et al. (2007) also produced a useful review on PHAs. A recent book by Sudesh and Abe (2010) entitled Practical Guide to Microbial Polyhydroxyalkanoates provides another general introduction. The websites of the key PHA manufacturers provide good introductions to PHAs (e.g., Metabolix, Biomer). Features available on the website http://www.makeitfrom.com provide basic property information about PHAs and also allow a quick and general comparison to be made between PHAs and other materials. For a good recent reference to detailed information on biogenesis and structure of PHAs, albeit largely with reference to nano-/micro-bead applications in biotechnology and medicine, the reader is referred to Grage et al. (2009).

18.8

References

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Starch-based polymers for food packaging

 L E Z and M . P . V I L L A N U E V A , R. M. GONZA Technological Institute of Plastic (AIMPLAS), Spain

Abstract: This chapter presents a review of the use of starch in food packaging. The chapter first reviews the use of starch during recent years and its particular properties. Then, it is focused on the latest developments in starch processing techniques for food packaging production. Finally, specific sections about the mechanical and barrier performance and the use of starchbased nanocomposites are included, due to their relevant importance in order to meet the often stringent food packaging requirements. Key words: starch structure, starch properties, processing, mechanical and barrier performance, starch nanocomposites.

19.1

Introduction

The use of natural polymers has received increased attention in recent years, having great potential as substitutes for conventional polymers in a broad range of applications. The development of more environmentally friendly thermoplastic materials has been the subject of a large number of studies and investigations. One of the most interesting applications for these materials is food packaging in which a short shelf-life of packages is required and there is an increasing demand for food products to be packed. The use of these new materials would allow new management of plastic residues and reduction of the dependence on petroleum. Among the natural polymers, there has been particular interest in the use of starch. Due to its nature, starch is inherently biodegradable. Only carbon dioxide and water are needed by plants to synthesize it by photosynthesis (Teramoto et al., 2003). On the other hand, its labile bonds can be hydrolysed into glucose by microorganisms or enzymes, and then metabolized into carbon dioxide and water (Primarini and Ohta, 2000). In addition, starch is a cheap material, abundant in the nature and renewable. In food packaging, starch films have been very interesting as they are excellent oxygen barriers due to their particular chemical structure (McHugh and Krochta, 1994; Nisperos-Carriedo, 1994). In this chapter, a review of the use of starch in food packaging is presented, starting with the evolution of the use of starch during recent years (Section 19.2) and its particular properties (Section 19.3). In the second stage, a summary of

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the latest developments in starch processing techniques for food packaging production is provided (Section 19.4). Finally, specific sections concerning the mechanical and barrier performance (Section 19.5) and the use of starch-based nanocomposites (Section 19.6) are included, due to their relevant importance in meeting food packaging requirements. To complete the chapter, a section about future trends in starch developments is included (Section 19.7).

19.2

Market for starch-based materials and potential applications

Starch production and its applications have undergone considerable evolution through the last few years. The following paragraphs explain the evolution of starch in the market, focused on the main manufacturers.

19.2.1 Evolution of starch in the plastic industry The introduction of starch in the plastic sector was motivated by its low cost and biodegradability. Starch was first used as a filler in commodity plastics to reduce the price and to increase the rate of biodegradation of synthetic polymers. Afterwards, the developments in starch plasticization and starch processing led to the use of different starch-based systems for different purposes. Currently, the tendency in the market is the production of starch-based materials with a high content of starch together with other biodegradable plastics (30±80%). These can be co-polymers derived from natural sources such as poly(lactic acid) (PLA), polyhydroxyalkanoate (PHA) and polyhydroxybutyrate (PHB), or derived from fossil fuels such as poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), poly(butylene adipate-coterephthalate (PBAT), polyvinyl alcohol (PVOH) and polycaprolactone (PCL) (Schwach and AveÂrous, 2009; Vroman and Tighzert, 2009). Some of them (PBS, PBSA, PBAT) can be potentially produced from bio-based succinic acid by fermentation. However, the fully bio-based blends are not yet commercially available. In 2003, the market for starch-based bioplastics accounted for about 25,000 tons/year (Shen et al., 2009). The market share of these products accounted for about 70% of the global market for bioplastics (Bastioli, 2005). The global consumption of starch-based biodegradable polymers increased up to 114,000 tons in 2007. However, the production capacity according to the latest data reported by the University of Utrecht shows a projected increase to 810,000 tons for 2020 (Fig. 19.1). The estimated production volumes of the main producers for 2009 and 2013 are given in Table 19.1. Figure 19.2 shows the global consumption of starch-based biodegradable polymers in the main sectors. As can be seen, loose-fill packaging represents the greatest consumption of starch (52%), followed by bags and sacks with 28% and

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19.1 Current and potential volume production of starch-based materials in Europe (obtained from Shen et al., 2009).

packaging with 14%. Other sectors (6%) include agricultural films, hygiene products and injected parts. The price of starch-based materials has been decreasing over recent years, allowing them to compete with traditional plastics in some limited areas. According to the composition of the blend the price may vary. An example of this is the price of Mater-BiTM which ranges at present from 1.5 to 4 ¨/kg, compared to prices of between 3 and 5 ¨/kg in 2003 (Bastioli, 2000). However, this is still a high price compared to commodity plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) (1±1.4 ¨/kg), which limits the use of starch plastics in some applications.

Table 19.1 Production capacity of starch-based polymers by the main producers in Europe Producer Novamont (MaterBiÕ) Biotec (BioplastÕ) Rodenburg (SolanylÕ) BIOP (BioParÕ)

Year 2009 (tons)

Year 2013 (tons)

60,000 60,000 40,000 4,000

100,000 60,000 40,000 24,000

Source: adapted from Shen et al., 2009.

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19.2 Global consumption of starch-based biodegradable polymers by end use sector, year 2005 (includes Western Europe, North America and Asia Pacific) (obtained from Platt, 2006).

19.2.2 Main applications and manufacturers One of the first applications of starch-based biodegradable materials in the packaging sector was introduced in the market with the production of foamed starch loose-fill packaging. The company National Starch Co. introduced two technologies for the production of this product: one from hydroxypropylated high-amylose starch and the second from unmodified starch. Currently, among the applications of starch-based blends, packaging is the dominant area. In 2003, Novamont devoted 75% of its total production to packaging, while BIOP devoted 80% in 2007. Starch-based plastics available on a large scale in the market are mainly to produce foams, films and mouldable products (De BragancËa and Fowler, 2004). · Foams: Starch has increased the production of loose-fill foam packaging in the last 10 years, mainly replacing polystyrene foams. The rate is increasing as the quality of starch-based materials is improving. For example in the US, around 25% of expanded polystyrene use has been replaced by starch-based foams. · Films and nets: Starch-based films may be applied to agriculture, e.g. mulch films, plastic shopping bags, the composting sector, e.g. bags and sacks, laminated paper and food containers. Such markets are developed in Italy, Germany and the Scandinavian countries. Applications as nets for fresh fruit, vegetables or seafood are also possible. · Moulded products: Compounded thermoplastic starch is mouldable in a similar way to traditional mouldable plastics like acrylonitrile butadiene styrene (ABS), polystyrene (PS) and low density polyethylene (LDPE).

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Commercially, the most important sector is the production of thermoformed products for fast-food packaging, for example trays for fresh food, plates, bowls and cups. In most of these products, starch is foamed. Injected parts are also possible to obtain hygiene products, sanitary products, etc. Nowadays, the two big worldwide producers of starch-based materials are Novamont and Biotec, but there are other relevant starch-based polymer producers as shown in Table 19.2.

19.3

Structure and properties of native and plasticized starch

Starch is a natural carbohydrate which acts as the main means of energy storage in a great variety of plants such as wheat, potato, corn, cassava, tapioca, rice or pea among others, being located in the roots, seeds and stems in granule form. The shape and size of these granules depend on the starch source, typical dimensions ranging between 0.5 and 175 microns (Donald, 2004). Chemically, starch is a polysaccharide consisting of a mixture of amylose, a linear polymer, and amylopectin, a highly branched polymer having the same backbone structure as amylose but with branched points. The polymer building block of both components is the monomer glucose. In the case of amylose, -1,4linkages take place to form a linear structure with a molecular weight of 105±106, whereas in the case of amylopectin, the linear chain based on -1,4 bonds has also -1,6-linkages forming the branches, the molecular weight being 107±109. Figure 19.3 shows schemes of (a) amylose and (b) amylopectin structures. The amylose/amylopectin ratio in starch granules varies with the source of the starch. The level of amylose in starch is usually between 20% and 30% in weight (Oxford et al., 1987; Parker and Ring, 1996; Ramesh et al., 1999), although in some cases it can be higher. AveÂrous and Halley (2009) reported that some mutant plant species present singular compositions, such as the case of amylose-rich starches, like amylomaize, where the amylose level is up to 80%, and some amylopectin-rich starches, like the waxy maize, with an amylopectin level of 99%. There is evidence of the great influence of the amylose/ amylopectin ratio in the physical and chemical properties of a particular starch, as well as subsequent mechanical processing (Fang and Fowler, 2003). Starch granules also contain small amounts of lipids and proteins. Physically, most native starches are semi-crystalline. Their crystallinity has been reported to be between 20% and 45% (Whistler et al., 1984). Amylose and amylopectin are arranged in the granules in complex structures consisting of crystalline and amorphous areas (French, 1984; Blanshard, 1987). The crystalline areas are formed as a consequence of the specific arrangement of the branches in the amylopectin chains (Manners, 1989). The short branches are believed to form double helices, which to a great extent are organized into

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Table 19.2 Summary of commercial biodegradable starch-based materials with applications in the packaging sector Commercial starch

Processing method

MaterBiÕ (Novamont)

Injection moulding, extruded articles, film, thermoforming

Applications in packaging

Bags, film for packaging, trays for fresh food, sheets, nets for fruit and vegetables, expanded trays, loose-fill packaging Injection moulding, sheet Packaging, film, trays, carrier BioplastÕ (Biotec GmbH & film extrusion, blown film, and refuse bags, net bags, Co.) thermoforming thermoformed products, single-use disposable fast food packaging BioparÕ Mono and co-extruded film Barrier packaging, food (BIOP Biopolymer blowing, bottle blowing, packaging, fruit and vegetable Technologies AG) cast film, injection packaging, shopping bags, moulding, thermoforming refuse and waste bags Injection moulding, Thermoformed trays and CereplastÕ thermoforming, extrusion packages, mugs (Cereplast, Inc.) coating, blow moulding, profile extrusion Injection moulding, film Foamed products: trays, EverCornTM (Japan Corn Starch blowing, sheet extrusion, packaging; multifilm, food Co. Ltd) thermoforming wrapping film, mouldings such as knife, fork, cup, etc. PlanticÕ Injection moulding, Packaging for chocolates, (Plantic Technologies thermoforming, sheet and flexible and rigid packaging GmbH) casting extrusion Injection moulding Protection corner for SolanylÕ (Rodenburg packaging Biopolymers B.V.) TerratekÕ Injection moulding Tableware (MGP Ingredients, Inc.) GraceBioÕ Film blowing Shopping bags, net bags (Grace Biotech Corporation) PSM Film blowing, injection, Flexible and rigid packaging (PSM North extrusion, foaming America) Film blowing Film for packaging fresh food, BiostarchÕ (Biostarch shopping bags Technology Pte. Ltd.) Film blowing, sheet Films, packaging, cosmetics, TerraloyTM (Teknor Apex extrusion, injection catering/housewares, Company) consumer products, food contact products Note: More information about these commercial grades can be found in the web pages of the companies specified in Section 19.8. Source: information obtained from the web pages of the manufacturers.

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19.3 Molecular structure of (a) amylose and (b) amylopectin.

crystallites. On the other hand, amylose is believed to form part of the amorphous regions, together with the long chains of amylopectin (AveÂrous and Halley, 2009). The amorphous and crystalline parts are arranged in alternate layers within the starch granules (AveÂrous and Halley, 2009; Perez and Imberty, 1996). As some authors reported (Wang et al., 1998; Hedley, 2001), according to the arrangement of the amylopectin double helices, their packing density and the amount of bound water within the crystal structure, two types of crystallites or polymorphs can be found in the starch granules, called A and B. The Apolymorph is a more dense packed structure and contains less water molecules than the B-polymorph (Sarko and Wu, 1978; Imberty and Perez, 1988). Starches can contain either A or B or both polymorph forms, classifying starches as A, B or C respectively. Whereas A-starches are present in cereals, B-starches can be found in tubers and C-starches in legumes (Manners, 1989; Oxford et al., 1987). Table 19.3 shows starch composition, granule diameter and degree of crystallinity of starches from different sources (AveÂrous, 2004). As can be seen, starches containing the less amount of amylopectin, in this case the amylomaize, show lower percentages of crystallinity. Among the other starches, all those with higher amounts of amylopectin, A-type (wheat, maize and waxy starch), showed a percentage of crystallinity between 36% and 39%, whereas in the case of Btype (potato), this is 25%. This difference may be associated with the differences in the packing density between the A- and B-polymorphs. Native starch is a powder insoluble in cold water or organic solvents (Radley, 1953). The melting temperature of pure dry starch has been reported to be

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Table 19.3 Composition and characteristics of different starches determined on a dry basis Starch source Wheat Maize Waxy starch Amylomaize Potato

Amylose Amylopectin Lipid content content content (%) (%) (%) 26±27 26±28 <1 50±80 20±25

72±73 71±73 99 20±50 79±74

0.63 0.63 0.23 1.11 0.03

Protein Granule content diameter (%) (microns) 0.30 0.30 0.10 0.50 0.05

25 15 15 10 40±100

Crystallinity (%) 36 39 39 19 25

Source: adapted from Ave¨rous and Halley, 2009.

between 220 and 240ëC, which overlaps with the starch decomposition temperature (Rusell, 1987). For this reason, native starch does not have a thermoplastic character, and it is necessary to modify it, either physically or chemically, in order for it to be processed by means of the conventional processing techniques for thermoplastic materials, like extrusion, injection moulding or thermoforming. The behaviour of native starch in the presence of heat and a plasticizer compatible with it has been widely studied by means of differential scanning calorimetry (DSC) techniques (Jones, 1979; Cooke and Gidley, 1992; Bogracheva et al., 1998, 2002; Donovan, 1979; Wang et al., 1998; Tan et al., 2004). Water has been found to be an excellent plasticizer for starch (Ching et al., 1993), probably due to its favourable interactions with the hydroxyl groups of starch. Heating starch in the presence of water results in the disruption of the ordered structures, which has been shown to be an endothermic process often called the order±disorder transition (Donovan, 1979; French, 1984; Blanshard, 1987; Zobel and Stephen, 1995) and is strongly dependent on the water content (Donovan, 1979; Elisasson, 1980; Colonna et al., 1987): in excess water, and heating up to a particular temperature for each starch, the starch granules swell. Beyond this temperature, hydrogen bonding is disrupted and water molecules attach to the hydroxyl groups of starch, leading to greater swelling (Lim et al., 2000). This behaviour is known as starch gelatinization. However, in low moisture conditions (less than 30% of water by weight) melting of the crystalline regions in the starch granules takes place (Donovan, 1979), as they become more mobile, leading to the usual viscoelastic behaviour of thermoplastic melts. The elimination of the native starch structure in the presence of a controlled amount of plasticizer, combined in a proper way with heat and shear, is known as starch destructurization, and the product obtained is a thermoplastic starch (TPS). When shear stresses are applied, gelatinization can be achieved at low moisture contents, as there is a faster transfer of water into the starch molecules (Burros et al., 1987), and at the same time, the shear forces provoke the disruption of molecular bonds (Wen et al., 1990).

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The combination of thermal and mechanical inputs can be obtained by extrusion. The energy provided to the starch during the extrusion process is known as the specific mechanical energy (SME), which is responsible for the fragmentation of starch molecules and gelatinization (Gomez and Aguilera, 1983, 1984; Gropper et al., 2002). Studies on the effect of SME on both the microstructure and the gelatinization process showed that the higher the SME the higher the degree of gelatinization, as well as the higher amylose content the less SME has to be provided (Funke et al., 1998). Fang and Fowler (2003) reported that during the extrusion process, three phenomena take place at different structural levels: starch granule fragmentation, hydrogen bond cleavage between starch molecules, and partial depolymerization of amylose and amylopectin. During extrusion with plasticizers, starch swells, forming a viscous paste with destruction of the inter-macromolecular hydrogen links. This leads to the reduction of the glass transition temperature (Tg) and the melting point temperature below the decomposition temperature, allowing starch to be processed like conventional polymers. The reduction of Tg and melt temperature achieved depend on different factors, like the water or plasticizer content and the destructurization conditions. Moreover, the brittleness of the plasticized starch obtained is strongly influenced by the moisture level. Extruded starch products obtained with water as a sole plasticizer become brittle with time at room temperature, which may not be desirable for some applications (Naced et al., 2003). In order to confer flexibility to starch materials, other plasticizers are used, the most common being the polyols, particularly glycerol and sorbitol. Some studies showed that other types of plasticizers can be effective as well, like ethylene glycol, diethylene glycol or polyethylene glycol, all of them affecting the sensitivity to humidity of the starch material (Lourdin et al., 1997). Other possibilities consist of the use of some gelatination-promoting additives, like urea, glycerol monostearate or soya lecithin, acting together with the main plasticizer. These additives have a great influence on the rheological properties of the plasticized starch, as they modify the starch viscosity (Anna et al., 2005). Compared with native starch, TPS shows lower crystallinity, and two types of crystallinity can be found: residual crystallinity from native starch and processing-induced crystallinity. The latter is influenced by parameters such as the extrusion residence time, the screw speed and the temperature, and is mainly caused by the fast recrystallization of amylose molecules into a singlehelical structure (AveÂrous, 2004). The changes in crystallinity are usually studied by X-ray diffraction techniques (Cooke and Gidley, 1992; Bogracheva et al., 1998, 2002; Barron et al., 2000; Pushpadass et al., 2009). As can be seen in Fig. 19.3, starch chains contain a great number of hydroxyl groups. These are responsible for both the hydrophilic character of starch and the strong intermolecular interactions via hydrogen bonding. The high hydrophilic character of starch is the main reason for the dependence of its glass

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transition temperature, dimensions, mechanical performance and barrier properties on moisture (Lu et al., 2009). Due to the plasticizing power of water, the relative humidity of the air has a great influence on the changes of starch, as sorption and desorption mechanisms take place (AveÂrous, 2002). On the other hand, it has been reported that the addition of plasticizers in starch facilitates polymer chain mobility, attributed to a reduction in the number of intermolecular forces between starch molecules, improving flexibility and extensibility of the films (Krochta and Sothornvit, 2001), as well as diffusion coefficients, leading to higher gas and water vapour permeabilities (McHugh and Krochta, 1994; Huang et al., 2006; Mali et al., 2004). The effect of the plasticizer content on the mechanical and barrier properties of plasticized starch and other starch-based systems has been widely studied (Gaudin et al., 2000; Sala and Tomka, 2004). Recent studies on plasticized cassava starch films obtained by the casting technique reported the effect of glycerol and sorbitol contents on the oxygen and water vapour transmission rates, as well on tensile strength and elongation (Boonsong et al., 2009; MuÈller et al., 2008). The reported results, summarized in Table 19.4, showed greater transmission properties of plasticized cassava starch when increasing the plasticizer content. For the mechanical properties, it was found that a decrease in film strength and an increase in the elongation properties occurred with the increase of plasticizer content. Comparing both plasticizers, sorbitol provided the best barrier properties, probably due to the greater hydrogen bonding between starch and sorbitol than between starch and glycerol, as sorbitol molecules have higher hydroxyl groups and reduce the motions needed for gas diffusion. Other studies (Godbillot et al., 2006; Gaudin et al., 2000) reported that the interactions between starch, plasticizer and water content vary according to the relative quantities of these components and revealed the influence of the relative Table 19.4 Mechanical and barrier properties of plasticized cassava starch films with different levels of glycerol and sorbitol Plasticizer

Plasticizer content (% w/w)

WVTR OTR (g mm/m2day) (cm3m/m2 day kPa)

Tensile strength (MPa)

Elongation (%)

Glycerol

15 20 25 30

1204 1231 1378 1577

n.a.* 24.78 n.a. 30.98

2.84 2.60 2.38 2.16

1.33 1.38 1.52 1.70

Sorbitol

15 20 25 30

595 638 666 785

n.a. 3.08 n.a. 3.69

3.88 2.61 1.43 1.38

2.71 7.22 29.42 30.07

* n.a.: not available. Source: adapted from Boonsong et al., 2009.

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humidity. Gaudin et al. reported the measures of oxygen permeability in starch± sorbitol films, showing that for sorbitol contents of 28% (w/w), oxygen permeability increased dramatically with the relative humidity, from 2:1  10ÿ16 (cm3 cm/cm cm2 s Pa) when relative humidity was 60% to 934:6  10ÿ16 (cm3 cm/cm cm2 s Pa) when relative humidity was 90%, whereas for sorbitol contents of 8.8% (w/w), the values measured were 0:24  10ÿ16 and 4:3  10ÿ16 (cm3 cm/cm cm2 s Pa) when relative humidity was 60% and 90% respectively. The study also showed that good barrier properties can be maintained up to 65% of relative humidity. From the evidence that the properties of TPS are highly dependent on the moisture and plasticizer contents, it has to be considered that in the particular case of food packaging applications, where gas permeability is a key factor for the product performance, it is necessary to select the correct type and amount of plasticizer in order to obtain a good balance between mechanical and barrier performance for each particular application. On the other hand, it has been reported that after processing, there is an evolution of the properties of TPS with time, leading to a more rigid material, even when moisture and temperature are controlled (Shogren, 1992; Van Soest and Knooren, 1997; Thirathumthavorn and Charoenrein, 2007). This behaviour is known as post-processing ageing, and according to Delville et al. (2004) is due to the movements of amylose and amylopectin. Different approaches have been widely studied in order to improve starch mechanical and barrier performance and reduce the hydrophilic character, like graft copolymerization (Athawale and Rathi, 1999; Athawale and Lele, 2000; Maliger et al., 2006), blending with thermoplastic polymers (Corti et al., 1992; Aburto et al., 1997; AveÂrous et al., 2000; Lai et al., 2005), chemical modification (Aburto et al., 1999; Fang et al., 2002; Miladinov and Hanna, 2000; Silva et al., 2006), the development of multilayer structures (AveÂrous, 2002; Dole et al., 2005; Wang et al., 2000; Martin et al., 2001), starch reinforcement with natural fibres (Wollerdorfer and Bader, 1998; Curvelo et al., 2001; Alvarez et al., 2004) or starch-based nanocomposites (McGlashan and Halley, 2003; Park et al., 2002; Dean et al., 2008; Zeppa et al., 2009). Other studies have been focused on modifying the ageing behaviour of TPS. For instance, Delville et al. (2002, 2003, 2004) produced crosslinked starch-based systems by UV radiation of starch films, showing the reduction of macromolecular motions that cause the ageing.

19.4

Processing in packaging

Although starch-based materials have been processed using the same techniques as for conventional polymers, the processing of starch is more complicated due to its high viscosity, the high influence of the components in the formulation and the processing parameters in starch properties, water evaporation or the low properties performance of the products obtained (Liu et al., 2009).

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The most common systems used to achieve the destructurization and obtain TPS, are intermeshing twin-screw extruders (Funke et al., 1998; Gropper et al., 2002), with a particular screw configuration which allows the plasticizer to interact with starch but controls the extent of molecular fragmentation which would reduce plasticized starch performance. Other studies reported the use of single-screw extruders for the same purpose. Thunwall et al. (2008) used a Buss co-kneading single-screw extruder to plasticize potato starch with glycerol. The starch destructurization can be achieved in one or two stages. In a one-stage process, native starch is fed into the extruder, and along the barrel, the plasticizer is incorporated by an injection inlet port. In a two-stage process, the first stage consists of the mixing of starch and plasticizer in a mixer to form a homogeneous dry-blend (AveÂrous, 2004; Dole et al., 2005; Pushpadass et al., 2009; Gonzalez and Roca, 2010). According to Janssen and Moscicki (2006), starch and glycerol have to be premixed and, if necessary, the moisture content must be adjusted. They reported that extra water improves the extrudability and decreases degradation, as starch viscosity is reduced, but too much water can prevent complete gelatinization. In a second stage, the mixture is fed into the extruder to obtain the TPS, which is usually pelletized. In this second stage, the processing conditions used for starch destructurization directly affect the quality of the TPS. Starch was processed at a barrel temperature of 40±90±160±160±130±90ëC, a mass flow rate of 30 kg/h and a screw speed of 180 min using an intermeshing twin-screw extruder (Funke et al., 1998). Another example of the processing conditions was given by Zullo and Iannace (2009) who used a twin-screw extruder with conical screws. In this case, TPS was prepared at a barrel temperature of 70±110±120±120ëC when the plasticizer was glycerol, although the barrel temperature was increased when formamide was used as plasticizer. In order to turn starch materials into the final starch-based products, the processing technologies commonly used for packaging production can be used, like film extrusion, injection moulding or thermoforming, using the already existing technology. Casting has been reported to be the most common method for producing starch films for experimental studies on starch plasticization and characterization tests. This method consists of solving native starch in the presence of water or other plasticizer by heating, and spraying the starch solution on a flat surface for the later evaporation of the solvent, leading to the formation of a thin film on it. However, the reported studies about starch film production using the conventional techniques for processing of polymers are in the minority, and investigations in this field are necessary for new developments for industrial applications. The most widely used technique for obtaining films for flexible packaging is film blowing. In this technique, good melt strength of the material processed is required, as longitudinal and transverse stretching of the melt is produced during

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the extrusion. In addition, low thickness and high production rates are demanded. In the case of extrusion of TPS, its poor melt tenacity and moisture sensitivity have been shown to be a limitation (Thunwall et al., 2006a; Janssen and Moscicki, 2006). In the case of the films, the effect of the moisture ambient is greater as the surface to volume ratio of the film during blowing is very large. Thunwall et al. (2008) studied the processing behaviour of thermoplastic potato starch and a hydroxypropylated and oxidized potato starch, using glycerol as plasticizer in both cases. Whereas TPS showed a sticky character, attributed to glycerol migration during processing, as well as low tenacity to be stretched, the modified starch containing 22 parts of glycerol (w/w) and a moisture content of 10% (w/w) was blown successfully, obtaining films 0.10±0.15 mm thick. However, higher contents of glycerol and moisture in the modified starch also produced a sticky extrudate, whereas lower contents of glycerol and moisture reduced the sticky character but led to a stiffer and brittle material. The better processing of the modified potato starch might be related to its lower hydrophilic character compared to TPS (Thunwall et al., 2006b). Janssen and Moscicki (2006) reported the use of a classical single-screw film blowing line with a modified screw design for the production of thermoplastic starch films. According to their study, the use of a mixing head at the end of the screw improves the process considerably, as better gelatinization and mixing of the components are achieved, stabilizing the film bubble and obtaining thinner films. Moscicki also observed that extrusion processing parameters, the presence of emulsifiers and water content exert an important influence on film strength and elongation. Whereas the use of a 2.0 compression ratio single screw equipped with an extra mixing section affected film strength more, a 1.4 compression ratio single screw influenced film elongation in a greater measure (Rejak and Moscicki, 2006). Zullo and Iannace (2009) studied the melt processability of different TPS by film blowing using a single-screw extruder with a length/diameter (L/D) ratio of 25 and a barrel temperature of 100±110±120±120ëC. They observed that plasticized starches containing urea/formamide formed more stable bubble films. They obtained films with a minimum thickness of 50 m, smooth-surfaced and not sticky. Using these plasticizers there was no migration of the plasticizer. The processing of starch blended with other biodegradable polymers, such as PVOH, PCL or different polyesters, has been proved to be an alternative to TPS processing, allowing thin films to be obtained by blow film extrusion (Halley et al., 2001; Matzinos et al., 2002). Different works reported that it is necessary to add a high content in polyester in order to obtain a material easy to process and with good balance in the final properties. McGlashan and Halley (2003) reported that among a series of starch blends, the blend with a starch/polyester ratio of 30/ 70 (w/w) was the easiest to process by blown film extrusion, as the addition of polyester led to an increase of the elongation at break of the starch. Moreover, they also reported that the addition of modified montmorillonite to the starch±

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polyester blends increased the tensile strength and caused several remarkable effects on the processing properties of the blend, decreasing the work required to melt and transport the polymer to the die, and making it possible to lower the die temperature. According to them, the exfoliated montmorillonite acted as a barrier for plasticizer migration and evaporation, and affirmed that different starch±polyester nanocomposite formulations have been used to produce blown films of 30 m on an industrial scale. Other reported studies deal with the addition of some high-amylose potato starch contents to normal potato starch (Thunwall et al., 2006b), in order to improve melt tenacity and higher strength to TPS. Due to the fact that highamylose starches have inherent higher melt viscosity than normal starches, their processing is more difficult and has to be adapted to the material. TPS with different glycerol contents were extruded in a Brabender compact extruder. The effect of three different screw configurations and different extrusion speeds in the extrudate was analysed. During the processing, pressure instabilities due to unstable flows were registered for glycerol contents of 30% (w/w), which were reduced by increasing the glycerol content to 45% (w/w). Melt tenacity and flow instabilities were also overcome by increasing extrusion speed or screw compression ratio, and a decrease in the die gap helped to obtain a more homogeneous extrudate. Multilayer systems are used in packaging in order to increase the lifetime of fresh products. The most common structures are formed by an internal layer of a polar polymer such as ethylene vinyl alcohol (EVOH) or polyamide (PA), and external layers made of polyolefin such as polyethylene. However, special attention has been focused on the use of starch-based materials as the central layer, due to their lower cost and good oxygen barrier properties (Van Tuil et al., 2000a). An example of the substitution of EVOH by TPS in conventional multilayer structures for packaging is the study reported by Dole et al. (2005), where plasticized wheat starch was combined with low density polyethylene (LDPE) to produce three-layered films by flat film co-extrusion. They also developed different methods for TPS and LDPE compatibilization, concluding that the use of LDPE grafted with maleic anhydride (PEg) produced the best adhesion. For the production of PEg/TPS/PEg films, a feed block attached to a wide flat and two single-screw extruders were used, the one for starch extrusion having a special screw design with a torpedo element. Although the TPS composition was tuned to obtain a similar viscosity to PEg and reduce interfacial instabilities, the compatibility of the materials combined was not enough, resulting in coextruded films with adhesion defects and layer thickness heterogeneities. Completely biodegradable multilayer films were produced by different authors (Martin et al., 2001; Gattin et al., 2002). Martin et al. (2001) used wheat starch and glycerol to prepare different TPS with glycerol contents of 10, 18 and 35% (w/w), and co-extruded them with several biodegradable polyesters as outer

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layers in a lab-scale co-extrusion line. The compatibility between starch and the polyesters was determined. The polyesters tested were aliphatic polyesteramide (PEA), polycaprolactone (PCL), poly(lactic acid) (PLA), polybutylene succinate adipate (PBSA) and poly(hydroxybutyrate-co-valerate) (PHBV). The three TPS were processed at temperatures between 100ëC and 130ëC. PEA showed the greatest affinity to TPS and PLA the lowest. It was also observed that the adhesion between layers was reduced when increasing the glycerol content. The results were in agreement with those from Wang et al. (2000), who reported the production of three-layered polyester/starch/polyester laminates, using a twinscrew extruder for the starch/water centre layer and a single-screw extruder for the outer layers, both feeding a coat-hanger type die. These studies demonstrated the feasibility of the co-extrusion technique for obtaining multilayer structures based on TPS. The main interest of multilayer structures is the reduction in oxygen permeability. US patent 6,242,102 B1 describes different methods for the improvement of barrier properties. Both surfaces of films made of starch or starch/ polyolefin blends can be coated with silicon oxide, aluminium or siloxane (Tomka, 2001). The blends of TPS with polyesters are not useful for thermoforming, due to the intrinsic softness of the polyester, and other starch-based systems have to be developed. AveÂrous et al. (2001a) studied the effectiveness of different types of cellulose fibres in plasticized wheat starch as reinforcing components. Fibre contents of 15 and 30% (w/w) were added. It was reported that the Young's modulus increased significantly with the addition of the fibres, whereas elongation at break decreased. They also prepared specific formulations suitable for thermoforming, which allowed them to obtain thermoformed trays in some continuous pilot machinery with a good ageing behaviour. Curvelo et al. (2001) reported similar research, based on the reinforcement of TPS with short cellulosic fibres by its direct mixing in an intensive batch mixer and the later hot pressing. Most of the reported experimental studies dealing with injection moulding of TPS are limited to the injection of test specimens for their mechanical characterization. Injection parameters such as the injection temperature have been shown to have an influence on the shape stability and characteristics of the injected parts. Janssen and Moscicki (2006) reported that the injection moulding at high temperatures of plasticized starch with glycerol resulted in good shape stability. TaÂbi and KovaÂcs (2007) proved that the holding pressure in the injection moulding of thermoplastic maize starch lowered the shrinkage of the product. The effect of injection moulding temperatures on the mechanical properties of starch/protein blends was clearly determined by Huang et al. (1999). They prepared injected specimens from 80ëC to 140ëC and observed that the tensile strength increased up to 130ëC. At the same time the water absorption and elongation at break were reduced. Higher temperatures gave worse results as a consequence of thermal degradation.

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19.5

Multifunctional and nanoreinforced polymers for food packaging

Mechanical and barrier performance of starchbased systems

In order to obtain a good quality material for food packaging, a good balance between barrier and mechanical properties should be achieved. The barriers to oxygen and water vapour are two essential properties to consider in starch-based materials because oxygen and water molecules can deteriorate food properties. Starch films have excellent oxygen barrier at low hydration levels and plasticizer content compared to commercial barrier plastics such as EVOH or PA. Forssell et al. (2002) corroborated this in starch films obtained by casting using glycerol and water as plasticizers. However, additions of a low content of plasticizer or water render starch unsuitable for packaging due to poor mechanical properties (Liu et al., 2009). The modification of starches by chemical reactions such as esterification and etherification is currently the most important process that starch-based polymer manufacturers are adopting for their commercial grades. By chemical modification the hydroxyl groups of starch are substituted by other chemical groups that show lower affinity for water molecules (Narayan et al., 2009). Compared to unmodified starch, the hydrogen bonding interactions are destroyed, reducing at the same time the capacity to absorb water. So the water uptake by the starch films, exposed under certain environmental conditions, becomes lower (Jansson and JaÈrnstroÈm, 2005). Low levels of hydroxyl substitution may have positive effects such as retarding retrogradation, an increase of water binding capacity, a drop of gelation temperature, and a reduction in clarity. As far as we are concerned, there are different modified starches that have been proved to be good substitutes for native starch to reduce water absorption. However, these materials showed higher permeability to gases such as oxygen and carbon dioxide, as Dole et al. observed in octanoated modified starch (Dole et al., 2004). The water vapour permeability (WVP) of different modified starches was analysed by different authors (Jansson and JaÈrnstroÈm, 2005; Zamudio-Flores et al., 2006). Jansson and JaÈrnstroÈm (2005) observed the best reduction in WVP for an oxidized and hydroxypropylated high-amylose potato starch. Meanwhile, Zamudio-Flores et al. (2006) compared the WVP between oxidized starch films and plasticized starch films with glycerol and sunflower oil. They observed lower WVP in the sample with oil because of the hydrophobic character of the oil. The variation of WVP after 60 storage days was negligible. However, after 90 days a significant reduction was observed, as a consequence of the increment in crystallinity. The modification of starch by crosslinking has also been observed to affect the permeability. Rioux et al. (2002) studied the effect of the crosslinking degree on WVP and oxygen permeability. Crosslinked starch presented a permeability increase for a low increase of crosslinking density, which could be explained by

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a reduction in the degree of crystallinity. Delville et al. (2004) obtained different crosslinked starch films by the addition of small amounts of derivatives of benzoic acid and by irradiation by UV light. They reported that the nonirradiated films disaggregate when immersed in water, whereas after irradiation, films were swollen only after six months of immersion. They also reported a slight improvement in the Young's modulus and ultimate strength for the photocrosslinked films. Regarding mechanical properties, it has been seen that tensile strength increased with the degree of starch modification, while the elongation at break decreased (Zamudio-Flores et al., 2006). This increment was due to the linkages produced between the carbonyl and carboxyl groups with the ±OH2 groups of amylose and amylopectin. An increment in the tensile strength after 90 days of storage time was also observed. Jansson and JaÈrnstroÈm (2005) also observed increments of tensile strength in hydroxypropylated and oxidized starches. However, the trend in the mechanical results was influenced by the glycerol content (over 30% w/w there was a reduction in the tensile strength). At present, many researchers are making an effort to improve starch performance by blending it with other biodegradable polymers. The Biotec Company describes in a patent that the addition of hydrophobic biodegradable polymers into starch supposes great advantages as the moisture resistance is enhanced and the tendency to become brittle is lowered. These polymers act as plasticizers or swelling agents in the preparation of thermoplastic starch, without producing volatile molecules or migration as happens with typical lowmolecular-weight plasticizers (Loercks et al., 2001). As an example, PCL showed good behaviour in starch blends by reducing water permeability (PeÂrez et al., 2008; MyllymaÈki et al., 1999). Furthermore, PCL contents between 0% and 20% gave an excellent oxygen barrier. Above this content, the oxygen barrier was reduced, whereas the water barrier was improved. As has been mentioned in the previous section, McGlashan and Halley (2003) reported how the addition of a high content of aliphatic polyester (30/70 w/w) drastically reduced water absorption and increased the tensile strength of plasticized starch, finding a positive effect in starch-blown films. The mechanical properties of starch blends have also been improved by the addition of different biodegradable polymers (PHB, PVOH, PCL, PLA). The mechanical properties of poly-3-hydroxybutyrate (PHB)/starch blends were developed by Godbole et al. (2003). They found that tensile strength, Young's modulus and elongation at break increased with PHB content. Tensile strength increased from 4.99 MPa to 10.06 MPa when starch changed from 70% to 50% (w/w). The incorporation of poly(vinyl alcohol) (PVOH) can enhance the mechanical properties of starch. However, the increase is limited due to the low interfacial adhesion between PVOH and starch. The addition of 10% (w/w) of PVOH resulted in an increase in the tensile strength from 1.8 MPa to 4.0 MPa

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Multifunctional and nanoreinforced polymers for food packaging

and in elongation at break from 113% to 150% (Rahmat et al., 2009). Ke and Sun (2003) also determined an increase in tensile strength with additions of PVOH into starch/PLA blends (50/50 w/w). Liu et al. (1999) suggested a mixture of glycerol and water in a 50/50 ratio as the optimum plasticizer for the blend. Nevertheless, the properties of these blends can deteriorate rapidly with time and they are not water resistant (Wang et al., 2003). Polycaprolactone (PCL) is another biodegradable polymer which has been demonstrated to improve the mechanical performance of native starch. An example of this is the commercial grades of Novamont under the trademark MaterBiÕ which are composed of thermoplastic starch and PCL (Bastioli et al., 1995). It has been reported that simple mixtures of starch and PCL prepared by extrusion do not exhibit interfacial adhesion. The modification of PCL and the use of compatibilizers can increase the interfacial adhesion at the same time as favouring the improvement of mechanical properties (Kim et al., 2001; Avella et al., 2000). PLA/starch blends have shown good adhesion and better mechanical properties when compatibilized with maleic anhydride as a coupling agent (Zhang and Sun, 2004). Special attention has been focused on the barrier of starch films with chitosan. Chitosan has shown great potential to be used in packaging for the preservation of a variety of food as it has antimicrobial activity against a wide variety of microorganisms and gives a good barrier to water vapour as well (Dutta et al., 2009). Therefore, starch/chitosan blends, obtained mainly by the solution-casting method, have been widely studied for the application of edible films or coatings in food packaging. The addition of chitosan does not reduce the water vapour permeability (WVP) because it is quite hydrophilic. However, it was reported that the addition of a fatty acid such as lauric acid or ferulic acid in a starch/chitosan blend reduces the WVP. The development of plasticized starch films with the addition of chitosan and 8% of lauric acid resulted in a material with good properties for antimicrobial packaging (Salleh et al., 2009). The oxygen transmission rate was reduced to 0.042 cm3/m2/day in a sample with a starch/chitosan ratio of 6/4 (w/w), which is 91% of the reduction with respect to pure chitosan. At this starch/chitosan ratio, intermolecular interactions between starch and chitosan molecules are supposed, decreasing the mobility of macromolecules and consequently the permeability. Mathew and Abraham (2008) also analysed the effect of chitosan and ferulic acid in the diminution of water vapour and oxygen permeability of starch, whereas VaÂsconez et al. (2009) also observed a decrease of 77% in water vapour permeability of starch/chitosan films. The low oxygen permeability indicated that the material could be used to protect food from oxidation and even for fruit and vegetable packaging, reducing the respiration rates and extending their shelf-life. One of the latest innovations in the development of starch blends to improve the resistance to water is the preparation of starch/protein blends. Corradini et al.

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(2006) prepared starch/zein blends by melt blending and observed that the addition of zein reduced the melt viscosity and decreased the water uptake at equilibrium. This was possible because zein is composed of amino acids, many of them with non-polar side groups, whereas starch has a more hydrophilic nature and can interact more strongly with water. Another strategy to protect starch from moisture and improve its mechanical performance deals with the development of multilayer structures where starch is combined with more hydrophobic and resistant polymers. The multilayer systems obtained by Dole et al. (2004) based on LDPE and starch with a sealant layer of maleic anhydride grafted polyethylene took more time to achieve the equilibrium in the water absorbed (15 days) compared to the typical multilayer EVOH/PE which were water-equilibrated in less than one day (Dole et al., 2005). The use of biodegradable aliphatic polyesters for the development of coextruded multilayers with an inner layer of TPS was reported some years ago (Martin et al., 2001; Wang et al., 2000; Van Tuil et al., 2000b). The study reported by Martin et al. (2001), where different biodegradable polyesters were combined with TPS, showed the effectiveness of the polyester layers in conferring moisture protection and improving the mechanical performance. PEA showed the greatest affinity to TPS but conferred the lowest moisture protection. On the other hand, PLA, which showed the lowest affinity to TPS, provided the greatest moisture resistance. As has been explained in the previous section, organic fillers from natural sources have been used as reinforcing agents in starch substituting glass fibres (Wollerdorfer and Bader, 1998; Curvelo et al., 2001). The addition of small contents of cellulose fibres to TPS has been reported as a good alternative for improvements in water resistance and tensile strength. According to Dufresne et al. (1998, 2000), the positive effects in moisture sensitivity are a consequence of the hydrophobic character of the fibres. In addition, the thermal stability has been shown to be increased and the resulting TPS matrix reinforced, which was in agreement with the results reported later by AveÂrous and Boquillon (2004). MuÈller et al. (2009) observed a decrease in water permeability with the increase in the fibre content. Permeability was decreased by 63.5% with 10% of cellulose fibres at a range of relative humidity between 2 and 33% and by 19.22% at relative humidities between 64 and 90%. Ma et al. (2008) incorporated carboxymethyl cellulose (CMC) and microcrystalline cellulose (MC) to reduce the water vapour permeability. They observed an optimum behaviour for 9% of filler and important differences depending on the type of filler. The lower permeability was obtained for MC, which was attributed to its more hydrophobic character in comparison to CMC; the reductions in the permeability were 49% and 31%, respectively. Svagan et al. (2009) reduced the moisture uptake to half the value with the addition of a cellulose nanofibre.

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Although cellulose fibres are the most used type of fibres, the literature shows examples of other types of fibre reinforcements. Alvarez et al. (2004) reported the starch reinforcement with short sisal fibres. Micro-winceyette fibres also decreased the water absorption of TPS, the absorption equilibrium being reached in 6 days in comparison to the 15 days for unfilled thermoplastic starch (Ma et al., 2005). Injection moulding has been one of the most widely used techniques to process starch-based systems for characterization purposes. The work reported by Janssen and Moscicki (2006) consisted of injecting test trips of TPS with different contents of glycerol, ranging from 20% to 30% (w/w). The tensile strength of the injected parts at different temperatures was measured. Values up to 20 MPa were achieved, which are comparable to those for commercially available polystyrene. The content of glycerol was shown to have a great influence on the mechanical properties, the tensile strength varying from 20 MPa to 4 MPa when glycerol was increased from 20% to 22%. Other authors reported the effect of the addition of fibres on the mechanical properties and water resistance of injection-moulded parts. Funke et al. (1998) injected different TPS reinforced with several fibre contents, showing considerable improvements with small amounts. As the strengthening effect supposed a decrease in elongation of the material, they considered levels up to 7%, doubling the tensile strength values. Following the same line, AveÂrous et al. (2001b) prepared different TPS biocomposites, based on wheat starch and cellulose fibres from leafwood. They observed, apart from the corresponding reinforcing effect, a drastic increase in dimensional stability of the injected parts, mainly for the composites containing the longest fibres. They also reported evidence for the interaction between starch and the fibres.

19.6

Nanocomposites

19.6.1 Introduction One of the main hazards in the use of a material in contact with food is the possible migration of certain components into the food. Addition of nanofillers to starch has great potential for enhancement of food quality, safety and stability as an innovative packaging and processing technology. The development of starch nanocomposites may be a solution to lower the migration of polymers and additives and to improve other physical properties of the neat matrix such as the mechanical and barrier performance (Zhao et al., 2008). According to the literature, the main types of nanofillers used for the production of starch nanocomposites are nanowhiskers, nanocrystals and layered silicates, all of them from different sources such as cellulose, chitin, starch or minerals, the nanoclays (layered silicates) being the type of nanofiller most researched.

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In this section, the most important methods used to prepare starch nanocomposites and the morphology obtained will be exposed before describing the main improvements in the physical and mechanical properties.

19.6.2 Methods of preparation Among all the existing techniques used for the preparation of nanocomposites (Alexandre and Dubois, 2000; Ray and Okamoto, 2003), the two methods used for the preparation of starch-based nanocomposites are the solution-casting method and the melt intercalation. The solution method is widely used for starch-layered silicate nanocomposites because starch is soluble in water. In this method, starch and clay are mixed together in a water solution (or with another solvent). Starch chains can penetrate into the interlayer of the silicate, and the film nanocomposite is obtained after solvent removal by evaporation (Cyras et al., 2008; Wilhelm et al., 2003; Mondragon et al., 2008; Pandey and Singh, 2005; Kampeerapappun et al., 2007). Some authors analysed the properties of starch film nanocomposites prepared by solution in different ways. For instance, Pandey and Singh (2005) found that the sequence of adding the plasticizer affects the level of clay dispersion and the mechanical properties. Better dispersion was achieved by adding the plasticizer after mixing the starch and the filler, and hence higher Young's modulus and lower water absorption. Kvien et al. (2007) reported that the order of addition of the plasticizer influences the elongation at break. However, the melt intercalation method is the most convenient technique from an industrial point of view. This is compatible with the current industrial processes such as extrusion and injection moulding and at the same time eliminates all the solvents that are used in the solution method. The method most commonly used to obtain the nanocomposites has been the addition of all components together into the feeding section of the extruder or in two roller mixers (Park et al., 2002). Following this procedure, it is important to mix the nanoclay and the starch in a high-speed mixer (HSM) prior to feeding them in the extruder. But the addition of clay previously dispersed in water before the mixing in the HSM has resulted in good improvements in clay dispersion and mechanical properties (Dean et al., 2007). The type of equipment used to melt-mix starch with the clay is crucial to obtain good exfoliation. Generally, the most efficient equipment is a co-rotating twin-screw extruder (Dean et al., 2007; Chiou et al., 2007; Tang et al., 2008). Dry starch can be mixed in a high-speed mixer with the nanoclay and the plasticizers, followed by the melt extrusion of the blend in a twin-screw extruder (Dean et al., 2007). Single-screw extruders are not suitable for correct mixing because they cannot generate enough shear to break the aggregates and disperse the clay. Huang et al. (2004) tried to disperse sodium montmorillonite in cornstarch with a single extruder but they obtained composites formed by big

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clay agglomerates. However, Wang et al. (2010) obtained intercalated nanocomposites with good improvement in mechanical and barrier properties by melt-mixing starch nanocomposites in a single-screw extruder, but they previously prepared a blend with the starch, clay and plasticizer using a highspeed mixer (3000 rpm, 2 min).

19.6.3 Morphology Most works about starch nanocomposites have been focused on the dispersion of montmorillonitic clays. The state of clay dispersion depends on the affinity between the clay and the matrix, but the type and content of plasticizer are also of great importance. Adhesion between starch and montmorillonite can be achieved by two different methods (Kalambur and Rizvi, 2004): 1. Introduction of functional groups in the polymer backbone of starch to favour the bonding with the nanoclay. Carboxyl, anhydride, epoxy, urethane and oxazoline groups are suitable functional groups that can be inserted by reactive extrusion and can react with hydroxyl and carboxyl groups in native or modified starches, respectively (Kalambur et al., 2004). 2. Cationic exchange modification of the clay. The inorganic cations present in the interlayer of the clay can be exchanged by organic ions which increase the d-spacing and facilitate the further intercalation of polymer molecules to achieve either an intercalated or an exfoliated structure (Alexandre and Dubois, 2000). Among all the different types of montmorillonites tested, the sodium montmorillonites with no organic modifications (MMT-Na+), are better exfoliated than the organomodified ones (Park et al., 2002, 2003; Tang et al., 2009). However, in most cases it is impossible to obtain a completely exfoliated structure and the morphology is a combination of different structures. Table 19.5 summarizes the types of morphologies obtained in nanocomposites with different clay modifications, i.e. different cations in the interlayer space, and the basal spacing of the clays. According to X-ray diffraction (XRD) results, the main structure observed in starch/MMT-Na+ nanocomposites plasticized with glycerol or sorbitol is the intercalation, but the exfoliation can be achieved using montmorillonite modified with cationic starch and glycerol as plasticizer (Chivrac et al., in press). However, TEM (transmission electron microscopy) images observed in nanocomposites with natural montmorillonite have shown partial exfoliation (Fig. 19.4) (Park et al., 2002; Chiou et al., 2007). Glycerol has been shown to be the most efficient plasticizer in most of the works reported until now, but the content added to obtain the TPS affects especially the morphology, mechanical and barrier properties of the nanocomposites. Films with 5% (w/w) of glycerol had better mechanical and barrier

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Table 19.5 Influence of the interlayer cation (modification) on the morphology (elaborated by AIMPLAS)

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Reference

Clay

Interlayer cation

Morphology d-spacing (nm)

Park et al., 2002; Schlemmer et al., in press; Chiou et al. 2007

MMT

Na+

Intercalation/exfoliation d ˆ 1:78 (5% clay) No peak (<5% clay)

Ave¨rous and Halley, 2009; Park et al., 2002

CloisiteÕ 6A

Dimethyl-dihydrogenated tallow ammonium Dimethyl-benzyl-hydrogenated tallow ammonium Methyl-tallow-bis-2hydroxyethyl ammonium

Aggregation d ˆ 2:85 (5% clay) Aggregation d ˆ 2:05 (5% clay) Intercalation d ˆ 2:05 (5% clay)

CloisiteÕ 10A CloisiteÕ 30B Huang and Yu, 2006

MMT

Ethanolamine

Intercalation, exfoliation No peak (8% clay)

Qiao et al., 2005

MMT

Trimethyldodecyl ammonium

Intercalation d ˆ 4:7

Chivrac et al., in press

MMT

Cationic starch

MMT

Chitosan

Exfoliation No peak (6% clay) Exfoliation

Hectorite

Ca2+

Intercalation, exfoliation

Wilhelm et al., 2003; Chen and Evans, 2005 Dean et al., 2007

Synthetic fluoromica

+

Na

Exfoliation No peak (2% clay) with ultrasonic treatment

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Multifunctional and nanoreinforced polymers for food packaging

19.4 TEM picture of starch nanocomposite with 29 wt% of moisture, 5 wt% of glycerol and 5 wt% of Na-MMT (obtained from Chiou et al., 2007).

performance compared with samples containing higher glycerol contents (Tang et al., 2008; Chiou et al., 2007). Regarding the clay content, there have been interesting findings concerning the effect of clay contents on the dispersion. Clay contents below 5% gave an exfoliated morphology according to the results observed by XRD. However, these results may not be representative of the final properties (Schlemmer et al., in press). Chung et al. (2010) observed no diffraction peak for nanocomposites with 5% and 7% (w/w) of MMT, but mechanical properties were better with 5% of clay. A further increase above this content supposed a decrease in the stiffness because clay dispersion decreased.

19.6.4 Mechanical properties Nanoclays, such as montmorillonite, are the nanofillers most used to improve the mechanical properties of starch-based biocomposites. These montmorillonite clays, having in their interlayer sodium cations, have been added into the polymer in their natural state or as organomodified clays containing organic cations exchanged by the sodium ones. The nature of the organic cation is the most important factor that affects the dispersion of the nanoclays determining the final properties of the starch nanocomposites. In general, unmodified sodium montmorillonites present better affinity for the structure of starch and therefore have better dispersion in the polymer. Table 19.6 represents a summary of the effect of the method of preparation, the type of clay modification and the clay content, on the structure and the mechanical tensile parameters. Regarding the method of preparation, the table compares nanocomposites obtained by single-screw extrusion (SSE), twin-screw extrusion (TSE) and casting. The table also makes a comparison of the mechanical properties depending on the organic modifier used to modify the montmorillonite: dimethyl-dihydrogenated tallow ammonium for CloisiteÕ 6A, dimethyl-benzyl-hydrogenated tallow ammonium for CloisiteÕ 10A and methyltallow-bis-2-hydroxyethyl ammonium for CloisiteÕ 30B.

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Table 19.6 Mechanical improvements in starch±montmorillonite nanocomposites: effect of method of preparation, effect of organic modification and effect of clay content (" represents an increase and # represents a decrease in the mechanical parameter) (elaborated by AIMPLAS)

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Reference

Clay

%

Structure

Huang et al., 2004

Na-MMT

10

Park et al., 2002

Na-MMT

Cyras et al., 2008

Na-MMT

Park et al., 2002

Dean et al., 2008

Change in modulus

Change in strength

Change in elongation at break

Method

Aggregation

"

"

##

SSE

5

Intercalation

±

"27.2%

"57.2%

TSE

5

Intercalation

"556%

"57.6%

#25.2%

Casting

Cloisite 30B CloisiteÕ 10A

5 5

± ±

"7.27% #18%

#5.3% #25.7%

TSE TSE

CloisiteÕ 6A

5

Little intercalation No intercalation (agglomerates) No intercalation (agglomerates)

±

#3.8%

#19.1%

TSE

Na-MMT

1

"21%

"16.6%

#13%

TSE

Na-MMT

2.5

"58.8%

"46.2%

#27.5%

TSE

Na-MMT

5

"81.6%

"50.9%

#29.3%

TSE

Õ

Intercalation Exfoliation Intercalation Exfoliation Intercalation Exfoliation

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Multifunctional and nanoreinforced polymers for food packaging

As can be appreciated from Table 19.6, Na-MMT has been reported to improve to a higher extent the tensile strength compared to organoclays such as CloisiteÕ 30B, CloisiteÕ 10A and CloisiteÕ 6A (Park et al., 2002; AveÂrous and Halley, 2009; Park et al., 2003). Tensile strength increased from 2.61 MPa to 3.32 MPa and elongation at break increased from 17% to 57.2% with 5% w/w of Na-MMT (Park et al., 2002). But the work of Majdzadeh-Ardakani et al. (2010) reported that nanocomposites with montmorillonite modified with citric acid had higher modulus compared to natural MMT and organoclays. Low clay contents (1% w/w) have been shown to be effective at improving the mechanical performance of starch. Higher clay contents increase mechanical properties (Dean et al., 2008; Chung et al., in press). Ren et al. (2009) reported the effect of the organomodified montmorillonite content on the mechanical properties. Tensile strength and modulus increased over the range of 2% to 8% (w/w) of clay content but elongation at break was reduced from 90% to 50%. Other studies have demonstrated that as the clay content increases, the clay dispersion decreases and as a consequence the capacity of reinforcement (Chung et al., in press). Majdzadeh-Ardakani et al. (2010) observed that the maximum mechanical strength was obtained with 6% of montmorillonite loading. Chung et al. (in press) observed that the maximum modulus was for the sample with 5% of MMT clay. Huang and Yu (2006) obtained 380% of increment in the Young's modulus, 267% in the tensile stress and 24% in the tensile strain, using 8% (w/ w) of montmorillonite activated with ethanolamine. Wang et al. (2010) observed an increase in the tensile strength and a decrease in the elongation at break over the whole nanoclay content range. Figure 19.5 represents the tendency of the mechanical parameters with the montmorillonite content.

19.5 Evolution of tensile strength and elongation at break with montmorillonite content (obtained from Wang et al., 2010).

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The type of extruder used for the preparation of starch nanocomposites is important to obtain a good dispersion. The most efficient equipment is the corotating twin-screw extruder (TSE). In this case, it was possible to increase the elongation at break using sodium montmorillonite thanks to the good dispersion and high aspect ratio achieved (Park et al., 2002). However, single-screw extruders (SSE) gave as a result an aggregate structure typical of a composite (Huang et al., 2004). The casting-evaporation process has also been shown to be a good method to disperse nanoclays in a starch solution (Cyras et al., 2008; Chung et al., in press). In this case, the order of addition of the plasticizer influences the elongation at break (Kvien et al., 2007). The mechanical and ultrasonic treatment applied in nanocomposites prepared by solution casting positively affected the clay dispersion (Majdzadeh-Ardakani et al., 2010). Although clays are used to increase the stiffness of the material, in general the nanocomposites have lower strain at break (Ning et al., 2009). In order to increase the flexibility of the material, some authors have incorporated biodegradable polyesters in the formulation (Avella et al., 2005). Although montmorillonitic clays are the type of nanofillers most used to reinforce starch, other nanofillers such as nanowhiskers, nanocrystals, layered double hydroxides, sepiolites, -zirconium phosphates and ZnO have been found to be efficient at increasing mechanical properties of starch (Chivrac et al., in press; Ma et al., 2009; Wu et al., 2009; Chung and Lai, in press; Chen et al., 2009; Cao et al., 2008a, 2008b). Cellulose nanowhiskers with a high aspect ratio dispersed in the polymeric matrix are able to increase the mechanical strength and stiffness of the starch, at the same time that the elongation at break increases (Chen et al., 2009; Teixeira et al., 2009; Alemdar and Sain, 2008). Moreover, these cellulose nanofibres are able to increase flexural strength (Takagi and Asano, 2008). A summary of the main improvements in these mechanical parameters is shown in Table 19.7. Hull nanowhiskers dispersed in pea starch gave higher tensile strength and elongation at break compared to hull fibres.

Table 19.7 Summary of mechanical properties of starch with cellulose nanowhiskers (" represents an increase) (elaborated by AIMPLAS) Reference

Material

% Increment Increment Change in filler in in elongation modulus strength (%) (%) (%)

Chen et al., 2009 Teixeira et al., 2009 Alemdar and Sain, 2008

Pea starch Cassava starch Modified potato starch

10% 10% 10%

± 53 145

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92 55 73

"106 "156 ±

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19.6 Effect of cellulose nanocrystals content in the modulus and tensile strength of pea starch (adapted from Cao et al., 2008b).

The mechanical performance achieved with nanowhiskers is affected also by their content and the type of plasticizer. Young's modulus and tensile strength showed a remarkable increase with the nanowhisker content (Mathew et al., 2008). The addition of a low molecular weight plasticizer was reported to have a low reinforcing effect due to the accumulation of the plasticizer in the interfacial zone. Because of that, Mathew et al. proposed the use of sorbitol instead of glycerol as plasticizer. They analysed the mechanical properties of nanocomposites with 33% of sorbitol and with whisker content between 0% and 25% (w/w). The relative humidity was also an important factor which affected directly the mechanical parameters (AngleÁs and Dufresne, 2001). Nanocrystals obtained by an acid treatment of starch, cellulose and chitin have been shown to be also good candidates to improve the modulus and tensile strength because of their similarity with the chemical structure of starch. However, the elongation at break can be reduced in a drastic way (Kristo and Biliaderis, 2007; Angellier et al., 2006). An example of the variation of these properties with nanocrystal content is represented in Fig. 19.6. The increases in modulus and tensile strength were correlated with the strong bonding between the cellulose nanocrystals and the starch molecules. However, the elongation at break decreased from 68.2% to 7.5% when the cellulose nanocrystals increased to 30 wt% (Cao et al., 2008b). Other studies reported that the best balance in mechanical properties was obtained with 35 wt% of sorbitol and 15 wt% of starch nanocrystals (Viguie et al., 2007). Meanwhile, chitin nanoparticles gave good mechanical properties under 5% (w/w) of filler. Higher loadings caused aggregated structures and inferior properties.

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19.6.5 Thermal properties The most important thermal properties of starch in food packaging are the glass transition temperature (Tg), the melting temperature (Tm), the heat distortion temperature (HDT) and the softening point (Ts) or Vicat temperature. Glass transition temperatures of starch-based materials are sometimes difficult to observe, so they are often determined by dynamic mechanical thermal analysis (DMTA). The change in Tg may be correlated most of the time with the level of clay dispersion. The addition of nanoclays has been shown to increase Tg when the clay is well dispersed in the matrix. Increments of the main thermal relaxations (tan  peaks) from ÿ64ëC to ÿ32ëC and from 7ëC to 10±20ëC have been detected in samples with 10% of Cloisite Na+ (w/w) (Park et al., 2003). However, in the case of hybrids with worse dispersion, for example with Cloisite 30B, these transitions were shifted to lower temperatures compared to unfilled starch. This last behaviour was due to the repulsion between hydrophilic TPS and this hydrophobic clay. Slight increments of Tg in samples with flax cellulose nanocrystals have been attributed to the restriction in mobility of starch chains thanks to the interactions between starch and cellulose (Cao et al., 2008b). Differential scanning calorimetry has been used to evaluate thermal transitions such as melting and crystallization temperatures. In general, no changes in the melting temperatures of starch-blend nanocomposites with montmorillonite have been found, but low additions of clay have caused small increases in the crystallization temperature as a result of their nucleating effect (McGlashan and Halley, 2003). Some authors have observed increments between 20ëC and 26ëC in Tm, using cellulose nanowhiskers and synthetic hectorite as nanofillers. It seems that these fillers increase the size of the crystals formed (Kvien et al., 2007). The softening point (Ts) of nanocomposites, measured by thermal penetration tests carried out in TMA equipment, increased by 15ëC in nanocomposites with only 1% of MMT and by 21ëC with 5% of MMT (Schlemmer et al., in press). The addition of clay in thermoplastic starch has also improved the thermal stability (Ma et al., 2007; Chiou et al., 2007). The improvement in the thermal stability is related to the clay dispersion, so the samples containing sodium montmorillonite give better results than the ones with modified montmorillonites. Increments in the degradation temperature up to 30ëC have been reported (Park et al., 2003).

19.6.6 Barrier properties The addition of small amounts of nanofillers, with dimensions in the range of nanometres, positively influences the barrier performance of plasticized starchbased polymers, reducing the permeability to gases and also water absorption. However, research concerning the barrier properties of starch nanocomposites has been focused mainly on the effect on water vapour permeability.

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It has been reported that the addition of 2.5 wt% of starch nanocrystals in cassava starch decreased the water vapour permeability by 40%. This improvement was attributed to the good dispersion of the nanocrystals which increased the tortuosity of the path that water molecules need to cross through the film starch (GarcõÂa et al., 2009). The addition of cellulose nanocrystals (CN) improved also the water resistance of starch. The reduction in the water uptake was between 32 and 37% depending on the final content of CN (5±20% w/w), in plasticized starch with 30 wt% of glycerol and 20 wt% of water. Chitosan nanoparticles reduced the water vapour permeability when the nanoparticles were well dispersed in the matrix. The highest effect of the nanoparticles was observed between 1 and 4% (w/w). Loads higher than 8% resulted in the aggregation of the filler (Chang et al., 2010a). Chitin nanoparticles reduced the WVP by 39% with respect to plasticized starch films with only 3% (w/w) of nanoparticles (Chang et al., in press). The WVP was also noticeably reduced when 1±2% of cellulose nanoparticles were added to glycerol plasticized starch. When more than 3% was added the nanoparticles tended to agglomerate and the permeability was not further decreased (Chang et al., 2010b). Regarding the effect of nanoclays, it is obvious that the addition of clay reduces the water absorption, although this improvement depends on the type of modification applied to the clay. Organoclays reduce water absorption of starch nanocomposites to a lower extent in comparison with sodium montmorillonites. Huang and Yu (2006) observed a reduction of 3.7% in the water absorption of starch nanocomposites formed by organoclays, then Ning et al. (2009) achieved a reduction of 60% using sodium montmorillonite. The nanoclay content also appeared to have an important influence on the reduction of water vapour permeability (WVP) as can be appreciated from Fig. 19.7 (Wang et al., 2010). The effect of the addition of different plasticizers and clays on the water sorption and oxygen barrier properties of native potato starch has been reported. The addition of glycerol or a urea/ethanolamine mixture as plasticizer increased the oxygen permeability. However, the further incorporation of 6% of nanoclay (unmodified sodium montmorillonite or organomodified with hydroxyethyl quaternary ammonium) decreased the oxygen permeability (Zeppa et al., 2009). In this study, there were also found to be important differences depending on the type of clay. The reduction was only 14% for the modified clay but 44% for the unmodified clay. These differences were correlated with the level of clay dispersion: the better the dispersion observed by TEM, the better was the decrease in permeability. The higher reduction in permeability in Na+-MMT nanocomposites was explained by the interactions established between hydroxyl groups of starch and clay and also by the higher increase in the tortuosity factor. The resistance to water of starch nanocomposites has been checked by filling blown films with water and observing the behaviour after one week (Willemse and De Vlieger, 2002). These experiments demonstrated that the film had good resistance after one week in water.

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19.7 Effect of montmorillonite content on WVP of starch/montmorillonite nanocomposites; UMMT ˆ urea modified montmorillonite (obtained from Wang et al., 2010).

19.7

Future trends

Currently, the main efforts are focused on the development of a complete costeffective, biodegradable and compostable starch-based material. The processes used at present, such as blending and esterification, give a rather expensive product. New concepts are required to solve the intrinsic problem of the hydrophilicity and mechanical instability of starch-based bioplastics without too much added cost (Fakirov and Bhattacharyya, 2007). Regarding food packaging applications, starch foams and injection-moulded products such as take-away containers are potential applications. Foamed polystyrene is currently being substituted by starch foams in loose-fill packaging and foam tray applications. With respect to this field there are some recent researches concerning the foaming process and properties of starch foams with small contents of PLA, improving the physical and mechanical properties of starch foams (Lee and Hanna, 2008; Lee et al., 2008). Many efforts are being made to find new substances which can be used as foaming agents. It has been reported that organomodified clays can act as nucleating agents and foaming agents at the same time (Chen et al., 2005). Food packaging systems have shown a constant evolution from the simplest ones, such as for vacuum-packed products, to those with a modified atmosphere or to active packages, the latest being the most interesting, as they allow controlled deterioration or alteration of food quality. In this sense, the development

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Table 19.8 Potential for technical substitution of synthetic plastics by starch plastics (++ full substitution; ‡ partial substitution; ÿ no substitution)

Novamont Biotec BIOP

PVC

HDPE

LDPE

PP

PS

PMMA

PA

PET

PBT

PC

POM

PUR

ABS

‡ ‡ ‡‡

‡ ‡ ‡‡

‡ ‡ ‡‡

‡ ‡ ‡

‡ ‡ ‡

ÿ ÿ ‡

‡ ÿ ÿ

‡ ÿ ‡

ÿ ‡ ‡

ÿ ÿ ÿ

ÿ ÿ ÿ

‡ ÿ ‡‡

ÿ ‡ ÿ

Source: adapted from Shen et al., 2009.

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of packages with a specific additive acting as an antioxidant or antimicrobial incorporated into the polymeric matrix of the package has been one of the most promising research lines in the last few years. The ultimate goal of these studies would be the development of bioactive and biodegradable packaging materials. Recent studies in which a biopolymer is used to obtain the active packaging are focused on PLA, PCL or pea starch (Nam et al., 2007), showing the efficiency of the active additives. Edible starch-based films have also a great potential in food packaging. Transparent edible films can replace conventional packaging in some applications, acting as edible coatings covering the food item and as a barrier to humidity and oxygen (Lu et al., 2009). Food companies are looking to edible films and coatings to add value to their products, increase shelf-life and reduce packaging, having at the same time benefits for the environment. However, the use of edible films is still under research and is not extended industrially due to their high cost. Currently, their use is limited to products of high added value (Pagella et al., 2002; Bertuzzi et al., 2007; VaÂsconez et al., 2009). The development of edible antimicrobial starch films can be one of the future trends of starch as active packaging. Durango et al. (2006) observed that starch/ chitosan coatings on carrots can control microbiological growth. Although this chapter is focused on starch-based plastics for packaging, it is important to stress that starch is under research for its use in other fields such as agriculture, to substitute for LDPE in greenhouse and mulching films, in medicine, e.g. as implants or dressings, and in the automotive sector. In addition, due to the potential of substitution of many synthetic polymers, the tendency in the future is to find new market applications for starch-based polymers (see Table 19.8).

19.8

Sources of further information and advice

Further excellent reviews about starch-based plastics, their properties and applications can be found in the following list. These contain at the same time a wide list of references to other articles. · Janssen L and Moscicki L (2009), Thermoplastic Starch: a Green Material for Various Industries, Weinheim, Germany: Wiley-VCH. · Halley P J (2005), `Thermoplastic starch biodegradable polymers', in Biodegradable Polymers for Industrial Applications, edited by R Smith, Queen Mary University, London. · De BragancËa R M and Fowler P (2004), Industrial Markets for Starch, The Biocomposites Centre, University of Wales, Bangor, Gwynedd, UK. · Shen L, Haufe J and Patel M K (2009), Product Overview and Market Projection of Emerging Bio-based Plastics, Utrecht University, The Netherlands.

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· Platt D K (2006), `The starch-based biodegradable polymer market', in Biodegradable Polymers, Market Report, Rapra Technology, Shawbury, Shrewsbury, UK. · AveÂrous L (2004), `Biodegradable multiphase systems based on plasticized starch: a review', Journal of Macromolecular Science, Part C Polymer Reviews, 44(3), 231±274. · Vilpoux O and AveÂrous L (2004), `Starch-based plastics', in Technology, Use and Potentialities of Latin American Starchy Tubers, edited by M P Cereda and O Vilpoux, NGO RaõÂzes and Cargill Foundation, SaÄo Paolo, Brazil. There are some interesting websites that are focused on thermoplastic starch or starch nanocomposites: · http://www.biodeg.net/biomaterial.html. Web page of Professor Luc AveÂrous. An excellent website with links to lots of references about starch blends, starch nanocomposites and starch processing. · http://www.biopolymer.net. Web page for biodegradable polymers. Information about commercial starches or starch-based blends can be found in the following websites: · · · · · · · · · · · ·

http://www.novamont.com/ http://www.biotec.de/engl/index_engl.htm http://www.biop.eu/index.php?english http://www.cereplast.com/homepage.php http://www.japan-cornstarch.com/h_13.html http://www.plantic.com.au http://www.biopolymers.nl/en http://www.starchtech.com http://www.midwestgrain.com/04_bio_based_solutions.htm http://www.grace-biotech.es http://www.psmna.com http://www.biostarch.com

19.9

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Chitosan polysaccharide in food packaging applications P . F E R N A N D E Z - S A I Z , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: Chitosan is a biodegradable, biocompatible, non-toxic aminopolysaccharide obtained by deacetylation of chitin. This biopolymer shows excellent film and coating forming properties as well as an inherent antimicrobial character that has been widely demonstrated. However, its high water sensitivity leads to a reduction in barrier properties or even the complete solubilization into foods, restricting its industrial application for packaging purposes. Blending this polysaccharide with other more waterresistant polymers has become a feasible strategy to overcome the solubility issue. Furthermore, it has already been demonstrated that reinforcing this biopolymer with nanoclays can lead to novel composites with enhanced physical properties, such as water resistance, without loss in biodegradability. Key words: chitosan, biopolymer, antimicrobial properties, active packaging, coatings, nanocomposites.

20.1

Introduction

Although there have been more than 22,600 publications related to chitin and chitosan since 1907, there is still great controversy regarding the phenomenology and mechanisms of the biocide properties of this natural component. The present chapter aims to summarize the mechanism of action and the main factors affecting the antimicrobial properties of chitosan films (bacteria type, pH of the food substrate, molecular weight of chitosan, temperature of incubation, film forming and storage conditions, etc.) for their subsequent optimal application in the area of food preservation. Moreover, since chitosonium salts present a strong hydrophilic character, their solubility is high and therefore they present poor water-resistance properties as packaging material. The dissolution of the biopolymer could compromise the packaging structure, physical integrity and overall properties and also organoleptic or microbiological food quality aspects and, therefore, may act as a restriction for its direct application as a food packaging or coating material. Blending chitosan with a more water-resistant polymer could be a means. Another possible approach to modify chitosan film properties is to make hybrid films with organic polymers and nanosized clay

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minerals such as layered silicates. This chapter reviews the published findings and the works performed on these topics.

20.2

Structure and properties

Chitin, a polymer of N-acetylglucosamine ( -1,4-linked 2-acetamido-Dglucose), is a cellulose-like biopolymer present in the exoskeleton of crustaceans and in cell walls of fungi, insects and yeast. Like cellulose in plants, it acts as supportive and protective material for biological systems. Chitin may be produced at approximately 109 metric tons annually and is the second most abundant natural biopolymer in the world. Chitosan is derived from chitin by deacetylation, to different degrees, in the presence of alkali. Therefore, chitosan is a copolymer consisting of -(1!4)-2-acetamido-D-glucose and -(1!4)-2amino-D-glucose units with the latter usually exceeding 80% (Arvanitoyannis et al., 1998) (see Fig. 20.1). Chitin is a by-product or a waste from the crab, shrimp and crawfish processing industries. However, isolation and preparation of chitin from various crustacean shells have taken place (Bough, 1976; No et al., 1989; Knorr, 1991; Shahidi and Synowiecki, 1991, 1992; Shahidi, 1995). Chitin and chitosan offer a wide range of applications, including clarification and purification of water and beverages, applications in pharmaceuticals and cosmetics, as well as agricultural, food and biotechnological uses (Knorr, 1991). Recent efforts for the use of chitin and chitosan have intensified since efficient utilization of marine biomass resources has become an environmental priority. Early applications of chitin and chitosan included the treatment of wastewater and agents for heavy metal adsorption in industry, immobilization of enzymes

20.1 Chitin and chitosan chemical structure.

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and cells, resin for chromatography, functional membranes in biotechnology, seed coatings and animal feeds in agriculture, artificial skin, absorbable surgical sutures, and wound healing accelerators in the medical field. However, chitin and chitosan have recently been developed as new physiological materials, since they possess anti-tumour activity by immune-enhancement, antibacterial activity, hypocholesterolemic activity, and anti-hypertensive action (Jeon et al., 2000). Chitosan shows excellent film and coating forming properties when cast from organic acidic water solutions (Yingyuad et al., 2006; Caner et al., 1998; Kim et al., 2006; LagaroÂn et al., 2007; Fernandez-Saiz et al., 2006, 2008, 2009a, 2009b, 2010a, 2010b). Moreover, it presents an inherent antimicrobial character against the growth of pathogen microorganisms, which has already been widely demonstrated in a general range of foods such as bread, strawberries, juices, mayonnaise, milk, rice cakes, etc. (El Gaouth et al., 1991a; Lee et al., 2000, 2002; Roller and Covill, 1999, 2000; Ha and Lee, 2001).

20.3

Processing in packaging

Chitin is easily obtained from crab or shrimp shells and fungal mycelia. In the first case, chitin production is associated with food industries such as shrimp canning. In the second case, the production of chitosan±glucan complexes is associated with fermentation processes similar to those for the production of citric acid from Aspergillus niger, Mucor rouxii and Streptomyces, which involves alkali treatment yielding chitosan±glucan complexes. The alkali removes the protein and deacetylates chitin simultaneously. The processing of crustacean shells mainly involves the removal of proteins and the dissolution of calcium carbonate, which is present in crab shells in high concentrations. The resulting chitin is deacetylated in 40% sodium hydroxide at 120ëC for 1±3 h. This treatment produces 70% deacetylated chitosan (Kumar, 2000). Chitosan films and coatings are prepared by dissolving chitosan in dilute acid and spreading on a level surface and air-drying at room temperature (the casting process). Films are also prepared by drying at 60ëC in an oven by spreading the solution on glass plates covered with biaxially oriented polypropylene film (BOPP) (Butler, 1996). However, these processes are time-consuming. Chitosan films could also be prepared by wet casting followed by infrared (IR) drying (Tharanathan et al., 2002), which is faster than the conventional method; no significant differences were observed in their mechanical and barrier properties in comparison with those prepared under conventional methods of drying (Srinivasa et al., 2004). The mechanical properties, permeability, thermal decomposition points, solvent stability, biocide capacity, etc., are parameters considered vital for selection of the right film for specific applications (Collins et al., 1973). Polymer blending is an effective method for providing new desirable polymeric materials for a variety of applications (see later). Furthermore, plastizicing agents are important ingredients generally used to overcome the excessive

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brittleness of the biopolymer films. Brittleness is an inherent quality attributed to the complex/branched primary structure and weak intermolecular forces of natural biopolymers. Plasticizers soften the rigidity of the film structure, increase the mobility of the biopolymeric chains, and reduce the intermolecular forces, thus improving the mechanical properties (elongation and tensile strength). Both processes, blending and plasticizer agent addition, will also be mentioned in Section 20.4 concerning chitosan films and their barrier performance.

20.4

Antimicrobial chitosan

Although the exact mechanism by which chitosan exerts its antimicrobial activity is currently unknown, it has been suggested that the polycationic nature of this biopolymer that forms from acidic solutions below pH 6.5 is a crucial factor. Thus, it has been proposed that the positively charged amino groups of the glucosamine units interact with negatively charged components in microbial cell membranes, altering their barrier properties and thereby preventing the entry of nutrients or causing the leakage of intracellular contents (Ralston et al., 1964; Helander et al., 2001; Liu et al., 2004; Mayachiew et al., 2010; Ganan et al., 2009). Another reported mechanism involves the penetration of low molecular weight chitosan in the cell, the binding to DNA and the subsequent inhibition of RNA and protein synthesis (Hadwiger et al., 1985). Chitosan has also been shown to activate several defence processes in plant tissues and inhibits the production of toxins and microbial growth due to its ability to chelate metal ions (El Gaouth et al., 1991a,b; Cuero et al., 1991). While there is no doubt that chitosan has shown a tremendous potential as antimicrobial component in a number of cases, a clear understanding between the phenomenology of the chitosan biopolymer as a biocide agent and its molecular structure, functional chemistry and physical state for optimum performance is of paramount importance from an applied viewpoint. ATR-FTIR spectroscopy has become one of the most versatile techniques for the quick identification and characterization of substances. The mechanism of antimicrobial action of chitosan has been studied by this technique through the intensity of the carboxylate biocide groups (±NH3+ ±OOCH) at 1546 and 1405 cmÿ1 in various chitosan samples (Fernandez-Saiz et al., 2006; LagaroÂn et al., 2007). The authors demonstrated that high molecular weight chitosan appears to act only in its acetate (salt) form and as carrier of the activated protonated species, which by passing from the film to the microbial solution lead to the inactivation of certain microorganisms. Moreover, a novel methodology based on the use of a normalized infrared band centred at 1405 cmÿ1 (see Fig. 20.2) was put forward. In relation to the feasible mechanisms by which chitosonium acetate decreases the number of `active' protonated groups, LagaroÂn et al. (2007) argued two feasible mechanisms. Firstly, the biocide carboxylate groups that form when

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20.2 ATR-FTIR spectra of a just-formed chitosonium acetate film (± ±), a stored chitosonium acetate film (48 h ambient conditions) before microbial testing (), the stored film after microbial testing (ÐÐ), a neutralized chitosonium acetate film (- - -) and chitosan flakes as received (ÐÐ) (Fernandez-Saiz et al., 2006).

chitosan is cast from acetic acid solutions are being continuously evaporated from the formed film in the form of acetic acid (mechanism I) in the presence of environmental humidity, rendering weak biocide film systems. Furthermore, upon direct contact of the cast chitosonium acetate film with liquid water, water solutions or the high moisture TSA hydrogel, a positive rapid migration, with diffusion coefficient faster than 3:7  10ÿ12 m2/s, of protonated glucosamine water-soluble molecular fractions (mechanism II) takes place from the film into the liquid phase, yielding strong antimicrobial performance and leaving in the remaining cast film only the non-water soluble chitosan fractions. In a more recent work, it has been shown that only dissolved polysaccharide molecules from the gliadin±chitosonium acetate blends were able to act as the antimicrobial agent (Fernandez-Saiz et al., 2008). This finding is in accordance with the work performed by Zhai et al. (2004), who evaluated the antimicrobial capacity of starch±chitosan blends. In this particular case, films presented significant biocide properties even with a low content of chitosan, but uniquely when formed under the action of irradiation. Degradation of chitosan by the irradiation treatment improved its dissolution into the nutrient broth, enabling a detectable bactericidal effect. These results appear, however, to contradict a previous study performed by Vartiainen et al. (2005) on the antimicrobial capacity of chitosan immobilized in a bioriented polypropylene (BOPP) matrix, where chitosan was reported to preserve strong bactericidal activity without any notable leaching. This finding was probably obtained because bacteria were

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directly exposed over the film, and therefore no migration was required for the microbes to be exposed. Tanabe et al. (2002) published their finding that the antimicrobial capacity of chitosan was probably due to the irreversible adsorption of bacteria into the film. In spite of the above arguments, there is a large body of experimental evidence that suggests that chitosan acts more efficiently from its solution form by precipitation onto, or even by penetration through, the microbial membrane (Liu et al., 2004; Chung et al., 2004; Fernandez-Saiz et al., 2006; Qin et al., 2006).

20.4.1 Optimization of the biocide properties of chitosan Individual researchers use chitosan grades with varying physicochemical properties and from various sources. Thus, the question arises as to how to globally produce chitosan with consistent properties. Each batch of chitosan, even produced by the same manufacturer, may differ in its quality. For proper quality control in chitosan production, there is a critical need to establish less expensive and more reliable analytical methods, especially for the evaluation of molecular weight and degree of deacetylation (No et al., 2007). Moreover, the physicochemical characteristics of chitosan can be variously affected by production methods and crustacean species. Any modification of these methods could affect chitosan's physicochemical and functional characteristics (that is, molecular weight, viscosity, degree of deacetylation, hydrophilicity, water and fat absorption capacities, and so on), which, consequently, influence the effectiveness of chitosan as antimicrobial agent (No et al., 2007). To date, there have been several works regarding the influence of the molecular weight of chitosan on its antimicrobial properties (Uchida et al., 1989; Jeon et al., 2001; Liu et al., 2001, 2006; No et al., 2002; Gerasimenko et al., 2004; Zivanovic et al., 2004). Some of them have demonstrated that chito-oligosaccharides (COS), which are soluble in water, were the least effective in terms of biocide properties (Uchida et al., 1989; No et al., 2002; Zivanovic et al., 2004). Moreover, in more recent work carried out by Qin et al. (2006) on the evaluation of chitosan solutions against the growth of C. albicans, E. coli and S. aureus, it has been shown that only water insoluble chitosan in organic acidic solutions, i.e. chitosonium salts, exhibit efficient biocide properties. On the other hand, research performed by Fernandes et al. (2008) showed that the growth of E. coli was markedly inhibited by COS, and this inhibition decreased slightly as molecular weight increased. In another work performed by Fernandez-Saiz et al. (2009a) changes in molecular weight of the chitosan materials tested, i.e. 310±375 kDa and 50±190 kDa, did not lead to significant variations in biocide properties. Therefore, in spite of the great number of reports on the topic, the reported conclusions are divergent probably since the methodology employed for the susceptibility tests varies from work to work.

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Thus, Jeon et al. (2001) and Liu et al. (2001) showed that the biocide effects increased with molecular weight whereas Liu et al. (2006) found the contrary trend. In these works the bacterial growth was evaluated by optical density. By this method, important differences in turbidity do not necessarily correspond to significant differences in bacterial counts and real effects may be masked. Besides, published works are not all comparable, as individual researchers used chitosans with varying physicochemical properties (affected by production methods) and perhaps from various sources. Concerning degree of deacetylation (DD), there are several works which consider this feature and there is no doubt that the antimicrobial properties of chitosan increase with this variable, which is related with an increase in the positive charge of the polymer (Chen et al., 2002; Tsai et al., 2002; Takahashi et al., 2008; Hongpattarakere and Riyaphan, 2008). Although several studies report the influence of the organic solvent employed to obtain chitosan films on their physical or antimicrobial properties (Park et al., 2002; Caner, 2005; Kim et al., 2006), the effect of the solvent concentration to obtain chitosan dispersions has been scarcely studied in the literature. A recent work compared the biocide properties of films of chitosonium acetate obtained from 0.5 wt% and 1 wt% of acetic acid in solution (Fernandez-Saiz et al., 2009b). Although no statistical differences were obtained between samples, a certain trend could also be observed in which chitosan dispersions prepared from an acid±water solution with 1% of acetic acid was demonstrated to lead to matrices with superior antimicrobial properties. A feasible explanation may be the fact that solubilization of chitosan is influenced by acetic acid concentration in the solution. In this sense, previous work performed by Rinaudo et al. (1999) demonstrated through potentiometric assays that complete solubilization of chitosan in acidic water caused by a maximum degree of protonation occurs with a stochiometry [AcOH]/[Chit-NH2] ˆ 0.6. Such a ratio corresponds to the one used in the work mentioned, i.e. for chitosan dispersions of 1.5 wt% of polymer and 1 wt% of acetic acid. The antimicrobial performance of chitosan films after a neutralization process has also been studied by some authors. For this purpose, chitosan films were, for instance, washed in a 0.1M NaOH solution for 30 min, subsequently rinsed thoroughly with distilled water and re-dried at 37ëC prior to the susceptibility test. It has been demonstrated by ATR-FTIR that a considerable drop occurs in the intensity of the active antimicrobial bands after the neutralization treatment (Fernandez-Saiz et al., 2006). Indeed, the molecular structure of the neutralized film was similar to that of chitosan as-received. Moreover, neutralization results in the loss of biopolymer solubility in aqueous media. As expected, when the biocide properties of a neutralized chitosan film were studied no antimicrobial activity was observed, indicating that amine groups are no longer activated by titration with an alkaline component (Fernandez-Saiz et al., 2006). These findings are also in conformity with the results obtained by

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Ouattara et al. (2000), who showed the lack of biocide properties of neutralized chitosan films when applied onto the surface of processed meat. In apparent opposition to these results, Tang et al. (2003) obtained a bactericidal effect towards S. aureus and E. coli when they measured biocide properties for suspensions of both neutralized chitosan films and neutralized crosslinked chitosan films. Among some feasible reasons for this behaviour could be either the incomplete neutralization process of the tested films or a protonation reaction during the preparation of the studied film suspensions, since the solvent composition to obtain the suspensions was not specified in the work. For chitosan matrices designed to be applied as food contact materials, in many cases the humid heat sterilization process becomes necessary. Even though the sterilization of the chitosan solution showed no significant impact on the biocide properties of the cast material, the antimicrobial activity of justformed autoclaved films (with and without steam contact) demonstrated that the sterilized films lost their biocide properties by this treatment (Fernandez-Saiz et al., 2009a). Both dry and humid heat sterilization processes of a cast film did cause in the resultant biopolymer an increase in yellowness or even an important browning effect, respectively, probably due to the formation of double bonds or coloured complexes (Zotkin et al., 2004; Marreco et al., 2005). These chemical changes are thought to be responsible for the insoluble character of the annealed samples. Fernandez-Saiz et al. (2009a) analysed the influence of the thickness of the chitosan matrix in its biocide properties and showed no significant differences in films ranging from 25 to 100 m. When the ATR-FTIR spectra were obtained for these particular samples, only minor differences were obtained between the different samples in terms of the intensity of the active carboxylate bands. These results suggest that the amount of protonated groups in the final film does not seem to depend on thickness for the range evaluated in this particular work. Thus, even in thicker chitosan films, due to their hydrophilic character, water diffuses very rapidly into the film and an immediate release of a significant fraction of the carboxylate groups is produced. Another important issue is the lack of proper standardized methods to determine the effectiveness of antimicrobial polymers. According to the standard method described by the National Committee for Clinical Laboratory Standards (NCCLS) (1999), minimal inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that inhibits visible growth, after a predetermined incubation period (usually 18 to 24 h). When Fernandez-Saiz et al. (2009a) studied the biocide properties of chitosonium acetate solutions or films, some turbidity was detected in the test tubes containing the nutrient broth even before bacterial inoculation. This additional turbidity causes an overestimation of bacterial final concentrations when calculated by optical density. Furthermore, important differences in turbidity do not necessarily correspond to significant differences in bacterial counts. For that reason, some previous published

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works on the antimicrobial capacity of chitosan films showed alterations or even a lack of performance when evaluated by optical density (Liu et al., 2001, 2006; Jeon et al., 2001; Fernandes et al., 2008). In other published works, the biocide properties of chitosan films (Pranoto et al., 2005; Zivanovic et al., 2005) and of chitosan solutions (Coma et al., 2002) were studied by the agar diffusion method with no satisfactory results, probably since no dissolution of chitosan took place through the agar. On the other hand, several reports studying this biomaterial showed considerable antimicrobial capacity when tested by means of other techniques, such as the macrodilution method and agar plate count or microcalorimetry (No et al., 2002; Qin et al., 2006; Fernandez-Saiz et al., 2006). When the effect of the cell age on the antibacterial properties of chitosan was evaluated, it was shown that the sensitivity of S. aureus seemed to be higher when bacteria were inoculated in the mid-log phase (Fernandez-Saiz et al., 2009a). Liu et al. (2006) obtained comparable results when testing chitosonium acetate solutions at different growth stages of E. coli. However, not all of the published works agree with the mentioned examples. For instance, the above results do not agree with those obtained by Chen and Chou (2005), who studied the effect of water-soluble lactose chitosan derivative against S. aureus growth in different physiological states, obtaining a greater susceptibility when late-log phase cells were tested. According to Tsai and Su (1999) the susceptibility of E. coli cells to chitosan may depend on the cells' surface electronegativity, which progressively increases from the early exponential phase to the late exponential phase. At the beginning of the stationary phase this parameter starts to decrease. Nevertheless, another study performed by the same authors (Tsai et al., 2006) concerning the action of low-molecular-weight chitosan against the growth of B. cereus showed that cells in the stationary phase were less sensitive, suggesting that changes in surface electronegativity seem to be different depending on bacteria type. When the influence of bacteria type was studied, the published results demonstrate that Gram-positive bacteria are generally more sensitive to the polymer than Gram-negative. It is worth noting that whereas some antimicrobial agents (for instance, bacteriocins) appear not to be able to act against Gramnegative bacteria due to the resistance conferred by the outer membrane (Rodriguez et al., 2005), chitosan has been demonstrated to have a significant antimicrobial effect against the growth of Gram-negative bacteria such as Salmonella spp. These results are in accordance with the hypothesis of an electrostatic interaction between chitosan and the cell wall. In this sense, Grampositive microorganisms should be more susceptible than Gram-negative since their wall is composed of a thick peptidoglycan layer and polymers called teichoic acids. This teichoic acid backbone is highly charged by phosphate groups with negative charge, which could establish electrostatic interactions with cationic antimicrobial compounds such as the chitosan salts. On the other hand, Chung et al. (2004) supported further in their work the major susceptibility of the

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Gram-negative bacteria to chitosan on the reasoning that Gram-negative bacteria possess higher negative charge values on the cell surface. The pH of the starting culture has also been demonstrated to have a large effect on the antimicrobial effectiveness of chitosan (Fernandez-Saiz et al., 2009a; No et al., 2002). Thus, the tests carried out at lower pHs (6.2) showed less bacterial counts than that performed at pH 7.4, indicating a stronger biocide effect. This finding is related to the particular pKa of this biopolymer (that is, 6.4). At this pH, the amount of positively charged amino groups (active groups) is close to 75% while at pH 7.4 this quantity drops to approximately 10% (Igarashi and Nakano, 2003). Additionally, it has been published that at lower pH values the physiology of the cell is more prone to suffer damage. Thus, at lower pH conditions the carboxyl and phosphate groups of the bacterial surface are anionic and offer potential sites for electrostatic binding of chitosan (Helander et al., 2001). The influence of the incubation temperature has also been investigated to a large extent. Thus, in recent research on the antimicrobial properties of mediummolecular-weight chitosan against Salmonella enterica, the antimicrobial agent presented strong antimicrobial properties at 10ëC while the inhibition was almost non-existent at 20ëC (Marques et al., 2008). In the same way, the addition of low-molecular-weight chitosan in cooked rice samples inhibited or retarded increases in total aerobic counts and Bacillus cereus, this effect being more significant at a low temperature (i.e. 18ëC) (Tsai et al., 2006). Thus, at low temperatures, even if bacterial cells presented optimal growth, they could be somehow injured and such damage hampered their repair mechanisms in the presence of chitosan, showing a higher susceptibility to the polymer in this condition. Nevertheless, the mentioned findings do not agree with a study performed by Chen and Chou (2005) who observed that as the temperature was increased from 5ëC to 37ëC, the population of S. aureus in the chitosan derivative-containing deionized water decreased. Furthermore, It has been observed that L. monocytogenes showed more resistance to the biocide action of chitosan at 12ëC than at other studied temperatures (4 and 37ëC) (FernandezSaiz et al., 2010a). The reason for such behaviour could be that this microorganism is known to adapt to lower temperatures by producing phospholipids with shorter and more branched fatty acids. Also, the high susceptibility at 37ëC could be attributed to exhaustion caused by efforts to multiply at a growthpromoting temperature (Simpson et al., 2008). Similarly, Briandet et al. (1999) found that L. monocytogenes grown at 8ëC had significantly less surface electronegativity than cells grown at 15, 20 and 37ëC. They speculated that the reduced charge at 8ëC was the result of synthesis of acclimation proteins and fewer carboxylic groups in the cell wall composition. In view of the mentioned results, the temperature of incubation seems to be a very important factor which could define the antimicrobial performance of chitosan, since bacterial cells are more or less susceptible to the polymer depending on this factor.

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From a practical point of view, the antimicrobial effectiveness of chitosanbased packaging in terms of microbiological safety will depend on the temperature of storage of the specific food product (that is, 4ëC for refrigerated foods or 12±25ëC for non-refrigerated foods). Thus, whereas chitosan could not be effective at 37ëC, it should present excellent biocide properties at 4ëC.

20.4.2 Optimization of the film-forming and storage conditions From an application viewpoint, the effectiveness of an antimicrobial agent when applied in or coated over foods or packaging materials may deteriorate during film forming, distribution and storage. Hence, the chemical stability of an incorporated substance is likely to be affected by the forming process or the storage conditions until its use. Consequently, in order to obtain highly efficient biocides, all these parameters should be assessed and optimized for the application. The properties of chitosan films obtained under specific forming and storage conditions in terms of mechanical and barrier properties have been investigated to some extent (Kam et al., 1999; Butler et al., 1996; Ritthidej et al., 2002; Zotkin et al., 2004; Srinivasa et al., 2004). Moreover, several published works have used infrared spectroscopy to follow the chemical changes of chitosan salts during film forming and even under different storage conditions (Demager-Andre and Domard, 1994; Kam et al., 1999; Osman and Arof, 2003; Zotkin et al., 2004; Fernandez-Saiz, 2006; LagaroÂn et al., 2007). The optimal procedure to preserve the full biocide capacity of chitosan-based films with minimum losses of the activated antimicrobial species in the materials has been thoroughly investigated (Fernandez-Saiz et al., 2009b). Thus, it has been demonstrated that just-formed low-molecular-weight chitosonium acetate films presented significant biocide properties against S. aureus and Salmonella when cast at 37 or 80ëC, while this capacity was reduced when cast at 120ëC. In addition, the films preserved significant biocide properties when maintained at low temperatures and in dry conditions (4, 23ëC, 0%RH) during the storage periods studied. On the other hand, when the same films were maintained in high relative humidity conditions (i.e. 75%) or at higher temperatures (i.e. 37ëC), the samples presented a progressive yellow coloration and a gradual loss of their antimicrobial capacity, due to water resistance induced by chemical and/or physical alterations in the films. Larena and Caceres (2004) also studied the colour changes of chitosan films after a storage period and obtained a decrease in the membrane transmittance with temperature, suggesting nonenzymatic browning reactions, which led to the presence of conjugated double bonds in the structure of the polymer. Similar findings were reported by Srinivasa et al. (2004) and by Kam et al. (1999) after certain thermal treatments of the chitosan matrices. In good agreement with this behaviour, it has been observed by ATR-FTIR spectroscopy that chitosonium acetate films cast at 120ëC or stored at 75% RH preserved a significant fraction of the biocide

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carboxylate chemistry after contact with water due to a strong reduction in cast film solubility (Fernandez-Saiz et al., 2009b).

20.5

Barrier performance

The control of gas exchanges, particularly of oxygen, allows better control of the ripening of fruits or a significant reduction in the oxidation of oxygen-sensitive foods and the rancidity of polyunsaturated fats. Organic vapour transfers have to be minimized in order to retain aroma compounds in the products during storage or to prevent solvent penetration in foods, which induces toxicity or offflavouring (Dutta, 2009). In this sense, due to its ability to form semipermeable films, chitosan can be expected to modify the internal atmosphere as well as to decrease transpiration loss and delay the ripening of fruits (Kittur et al., 1998). Thus, the use of edible coatings of chitosan to extend the shelf-life and improve the quality of peaches, Japanese pears and kiwifruits has been documented (Du et al., 1997). Similarly, cucumbers and bell peppers (El Gaouth, 1991a), strawberries (El Gaouth, 1991b) and tomatoes (El Gaouth, 1992a) could be stored for long periods after coating with chitosan. These results may be attributed to decreased respiration rates, inhibition of the fungal development and delaying of ripening due to the reduction of ethylene and carbon dioxide evolution (El Gaouth et al., 1991a,b, 1992b; Du et al., 1997). Nevertheless, although chitosan films tend to exhibit fat and oil resistance and selective permeability to gases, they present a noticeable lack of resistance to water vapour transmission and in many cases exhibit solubility in liquid water. This is due to the strongly hydrophilic character of this biopolymer, leading to a strong interaction with water molecules. The properties of chitosan films obtained under specific forming and storage conditions in terms of mechanical and barrier properties have been investigated to some extent. Thus, it has been demonstrated that water vapour permeability and elongation at break of low-molecular-weight chitosan films decreased with storage time in ambient conditions (23ëC and 50% RH) (Butler et al., 1996). Also, it has been shown that a thermal treatment (120ëC, 3 hours) of chitosan films led to a significant strengthening of the films and reduced their solubility in aqueous media (Zotkin et al., 2004). This last effect was also observed by other authors after storage of chitosan films in different conditions of temperature and relative humidity (Kam et al., 1999; Ritthidej et al., 2002). With regard to film-forming conditions, it has been demonstrated that infrared illumination drying was shown to be faster and superior in preserving desirable physical characteristics such as water or oxygen barrier properties than those prepared by oven drying or roomtemperature drying (Srinivasa et al., 2004). Furthermore, as the pH of the filmforming solution increased (from 3 to 5), water vapour permeability and total soluble matter of chitosan films rose (Kim et al., 2006). In the same way, the water permeability coefficient of chitosan films greatly increased with the

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relative pressure, and the water plasticization effect led to the loss of the gas barrier properties under wet conditions (Despond et al., 2001). Regarding molecular weight of chitosan, Chen and Hwa (1996) showed lower permeability values in membranes prepared from high molecular weight chitosans than in those of low molecular weight. Suyatma et al. (2005) demonstrated, by contact angle measurements, that plasticization with hydrophilic compounds (glycerol, ethylene glycol, polyethylene glycol and propylene glycol) increased chitosan film hydrophilicity. In the design of an antimicrobial chitosan-based packaging system for food applications, the presence of the high levels of humidity typically found in many foods has to be taken into account. In this sense, it has been found that the development of a chitosan active formulation in the food contact layer as a coating leads to both a very rapid release of the biocide groups onto the food surface and the film's partial or total dissolution. By means of physical or chemical treatments, film disintegration in water could be prevented, but, at the same time, dissolution of the protonated glucosamine groups in water would be blocked or deactivated, hence reducing the biocide character of the formulation. In this sense, the incorporation of crosslinking agents that form covalent bonds between the chains of chitosan preventing polysaccharide±water interactions could be a means to holding the film structure. Previous studies in relation to this matter have already been performed by the incorporation of compounds such as glutaraldehyde, glyoxal or epichlorohydrin (Suto and Ui, 1996; Tual et al., 2000; Zheng et al., 2000; Tang et al., 2003). Of additional concern is, of course, the inherent toxicity of these crosslinking agents, which could restrict their use. Polymer surface modifications, such as plasma treatments, have also been exploited to improve the adhesion between chitosan and other matrices like PP (Hu et al., 2002; Yang et al., 2003; Vartiainen et al., 2005; Elsabee et al., 2008; Abdou et al., 2008). Another strategy to overcome the drawback of film dissolution is to blend chitosan with a more moisture-resistant polymer in order to obtain water-resistant chitosan-based antimicrobial films. Some researchers have included chitosan into other biopolymer matrices such as cellulose, starch or Konjac glucomannan in order to improve the mechanical and water-swelling properties of antimicrobial chitosan films (Park et al., 2004; Moller et al., 2004; Wu et al., 2004; Zhai et al., 2004; Li et al., 2006; Sebti et al., 2007; Pelissari et al., 2009; Shen et al., 2010). Nevertheless, many of the published studies on this matter generated biocomposite materials with optimum antimicrobial activity but poor water barrier properties. A recent work performed by Fernandez-Saiz et al. (2008) showed an inhibitory effect of the growth of S. aureus when a gliadin/chitosonium acetate composite matrix (60/40% w/w) was tested, maintaining the film integrity in the nutrient broth. Nevertheless, although film disintegration was prevented, some turbidity in the nutrient medium was detected. Indeed, the gliadins protein network must exert a blocking effect against the dissolution of biocide chito-

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Table 20.1 Water permeability decrease (%) in the samples studied. Permeability decrease for each blend refers to the corresponding pure chitosan film obtained under similar conditions Sample

Casting temperature (ëC)

EVOH29/HMW 80/20a EVOH32/LMW 80/20a EVOH29/LMW 80/20a EVOH29/LMW 50/50 EVOH29/LMW 20/80 EVOH29/LMW 50/50a EVOH29/LMW 20/80a EVOH29/LMW 50/50 EVOH29/LMW 20/80 EVOH29/LMW 50/50 EVOH29/LMW 20/80

80 80 80 80 80 80 80 37 37 120 120

Water permeability decrease (%)b 69.1c 77.2c 86.1c 36.3c ÿ22.0c 38.6c 1.6 27.1 ÿ45.6c 63.3c 38.8

a

Glacial acetic acid was added to the EVOH solution. Water permeability decrease (%): 100 ÿ (water permeability of the sample  100/water permeability of chitosan matrix formed at the same temperature). c Statistical differences of water with respect to the matrix of reference in each case. Source: Fernandez-Saiz et al., 2010b. b

sonium acetate chains, albeit some of the latter chains do still migrate to exert their required antimicrobial role. Similar findings were observed by Tanabe et al. (2002) who studied keratin/chitosan films and observed a significant reduction of the swelling ratio as well as a significant reduction of E. coli (61%) for a keratin:chitosan ratio of 5:1. Other published work studied the water barrier and the antimicrobial activity of high and low molecular weight chitosonium acetate-based solvent-cast blends with ethylene±vinyl alcohol (EVOH) copolymers against S. aureus and Salmonella spp (Fernandez-Saiz et al., 2010b). These samples showed excellent antimicrobial activity as well as enhanced water barrier when low molecular weight chitosan was used as the dispersed phase in the blend, specifically 80/ 20% (w/w) EVOH/chitosan (see Table 20.1). In view of the results of the mentioned works, the most appropriate ratio between chitosan and the water-resistant polymer will depend on their compatibility. Also, it could be concluded that chitosan-based blends are shown to be a very suitable means to solve the excessively hydrophilic character of chitosan while retaining its biocide performance.

20.6

Nanocomposites

A possible approach to modifying biopolymer film properties is to make hybrid films with organic polymers and nanosized clay minerals such as layered

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silicates, which are known as nanocomposite films (Giannelis, 1996; Lagaly, 1999; Fischer et al., 2000; Alexandre and Dubois, 2000). Nanocomposite films consisting of inorganic nanolayers of layered silicate, such as montmorillonite (MMT) clay and organic polymers, have recently evoked intense research interest in the material and polymer science areas (Alexandre and Dubois, 2000; Sinha and Okamoto, 2003; Pandey et al., 2005). Usually, polymer/clay nanocomposites comprise an organic/inorganic hybrid polymer matrix containing platelet-shaped clay particles that have sizes in the order of a few nanometres thick and several hundred nanometres long. Partly because of their high aspect ratios and high surface area, the clay particles, if properly dispersed in the polymer matrix at a loading level of 1±5 wt%, impart unique combinations of physical and chemical properties that make these nanocomposites attractive for making films and coatings for a variety of industrial applications, such as food packaging. Examples of such property enhancements include decreased permeability to gases and liquids, better resistance to solvents, increased thermal stability, and improved mechanical properties (Alexandre and Dubois, 2000; Sinha and Okamoto, 2003). Moreover, biodegradability is retained; that is, after final degradation, only inorganic, natural minerals (actually soil) will be left (LagaroÂn and Fendler, 2009). Since chitosan is a polycation in acid conditions, it can be easily adsorbed on the MMT-Na surface. This property has been extensively used to elaborate chitosan/MMT-Na hybrid materials by the solvent route. Solutions were prepared by chitosan addition into 1 or 2% (v/v) of acetic acid solution. MMT-Na was dispersed into water to obtain a 2 wt% clay suspension. To avoid any structural alteration of the phyllosilicate structure, the polysaccharide solution was adjusted with NaOH to pH 4.9 and then slowly added to the clay suspension at ambient temperature. This mixture was stirred and finally washed with purified water to remove acetate and then cast (Darder et al., 2003, 2005; Wang et al., 2005; Xu et al., 2006; GuÈnister et al., 2007). Darder et al. (2003) demonstrated chitosan intercalation thanks to a shift of the MMT-Na diffraction peak to lower angles. Moreover, a broadening and intensity decrease in the diffraction peak was observed, indicating a disordered intercalated/exfoliated structure (Darder et al., 2005; Wang et al., 2005). The inter-layer chitosan structure was studied by Darder et al. (2003, 2005) evidencing the adsorption of two chitosan layers on the clay surface and even inside the inter-layer spacing. The first chitosan layer was mainly adsorbed thanks to electrostatic interactions between the chitosan ±NH3+ groups and the MMT negative charges. The second-layer adsorption was promoted by hydrogen bonds established between the chitosan amino and ±OH groups and the clay substrate. At low MMT content, several authors have even shown the formation of an exfoliated nanostructure (Wang et al., 2005, 2006; Lin et al., 2005; Xu et al., 2006). At higher MMT content, namely more than 5 wt%, the formation of intercalated/ flocculated structure was observed (Wang et al., 2005).

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The thermal transitions of these nano-biocomposites were investigated and related to the material dispersion state. GuÈnister et al. (2007) have measured the effect of the nanofiller addition on the Tg (by DSC) and observed an increase directly linked to the ionic interactions established between the chitosan and the nanofiller, which reduced chain mobility. Also, increases in the tensile strength correlated to a small decrease in the strain at break were observed in the different chitosan nano-biocomposites (Xu et al., 2006). These increases were induced by the nanofiller/chitosan interactions, which enhance the stress transfer at the interface. The decrease of strain at break was related to the morphology of the chitosan/MMT hybrid materials which displayed, in the best case, an intercalated/exfoliated structure. Such a stiffness increase is already well reported in the literature and is correlated to the clay rigidity and dispersion state (Luo and Daniel, 2003). This property has been extensively used to elaborate chitosan/MMT-Na hybrid materials by the solvent route (Darder et al., 2003, 2005; Wang et al., 2005; Xu et al., 2006; GuÈnister et al., 2007). In a work performed by Rhim et al. (2006), four different types of chitosanbased nanocomposite films were developed, and their film properties and antimicrobial functions were tested. By compositing with nanoparticles (such as unmodified and organically modified montmorillonites, nano-silver, and silver zeolite), mechanical and water vapour barrier properties of chitosan films were increased significantly (P < 0:05) compared with those of control chitosan films, and various degrees of antimicrobial activity were observed depending on the nanoparticles used. In another work performed by LagaroÂn and Fendler (2009), two commercially available clay additives were introduced in methyl cellulose and chitosan matrices by a solution casting process. In this study, the water barrier properties of the nanobiocomposites were found to be enhanced to a significant extent as compared to the pure matrix materials, without losing biodegradability as mentioned above. The nanoclay was also found to significantly reduce the water solubility in the polymer hence resulting in increased water resistance.

20.7

Future trends

Antimicrobial packaging is a very promising system for the future improvement of food quality and preservation during processing and storage. Antimicrobial packaging can also be helpful in extending the food shelf-life. Chitosan has offered itself as a versatile and promising biodegradable polymer for food packaging. In addition, it possesses immense potential as an antimicrobial packaging material owing to its antimicrobial activity and non-toxicity. However, individual researchers have used chitosan with varying physicochemical properties and perhaps from various sources. Thus, the question arises as to how to globally produce chitosan standard materials with consistent and

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reproducible properties. For proper quality control in chitosan production, there is a critical need to establish less expensive and reliable analytical methods, especially for the evaluation of molecular weight and degree of deacetylation (No et al., 2007). One major drawback of chitosan film is its high sensitivity to humidity, and thus it may not be appropriate for use when it is in direct contact with foods and/or for direct handling unless blending or other water-resistant strategies are used. In this sense, several works have been successful in improving the functional properties of chitosan films by blending this material with other, more water-resistant, film-forming materials. Another possible approach is to make hybrid films with chitosan and nanosized clay minerals such as layered silicates. By making composites with nanoparticles, mechanical and water barrier properties of chitosan were seen to increase significantly, without altering their antimicrobial properties and biodegradability. In any case, the numerous research works mentioned in this chapter have been carried out on a laboratory scale. As a consequence, further research on quality and shelf-life extension of foods coated or packaged with chitosan-based materials should be conducted on a pilot plant and industrial scale to ascertain the commercial value and the scaling-up difficulties of the use of this interesting biopolymer or of its nanobiocomposites. In any case, in Europe chitosan is still not permitted as a food contact material, although this may change in the near future. In spite of this, chitosan does surely have significant opportunities for implementation as a natural antimicrobial packaging material under the new EU regulation for active packaging (Commission Regulation (EC) No. 450/2009).

20.8

References

Abdou ES, Elkholy SS, Elsabee MZ and Mohamed E (2008), `Improved antimicrobial activity of polypropylene and cotton nonwoven fabrics by surface treatment and modification with chitosan', J Appl Polym Sci, 108, 2290±2296. Alexandre M and Dubois P (2000), `Polymer-layered silicate nanocomposites: preparation, properties and use of a new class of materials', Mater Sci Eng, 28, 1±63. Arvanitoyannis S, Nakayama A and Aiba S (1998), `Chitosan and gelatine based edible films: state diagrams, mechanical and permeation properties', Carbohyd Polym, 37, 371±382. Bough W A (1976), `Chitosan ± a polymer from seafood waste, for use in treatment of food processing wastes and activated sludge', Proc Biochem, 11, 13. Briandet R, Meylheuc T, Maher C and Bellon-Fontaine M N (1999), `Listeria monocytogenes Scott A: cell surface charge, hydrophobicity, and electron donor and acceptor characteristics under different environmental growth conditions', Appl Environ Microbiol, 65, 5328±5333. Butler B L, Vergano P L, Testin R F, Bunn J M and Wiles J L (1996), `Mechanical and barrier properties of edible chitosan film as affected by composition and storage', J Food Sci, 61, 953±955. Caner C (2005), `The effect of edible eggshell coatings on egg quality and consumer perception', J Sci Food Agric, 85, 1897±1902.

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and their applications in physiological functional foods', Food Rev Int, 16(2), 159± 176. Jeon Y J, Park P J and Kim S K (2001), `Antimicrobial effect of chitooligosaccharides produced by bioreactor', Carbohyd Polym, 44, 71±76. Kam H M, Khor E and Lim L Y (1999), `Storage of partially deacetylated chitosan films', J Biomed Mater Res, 48(6), 881±888. Kim K M, Son J H, Kim S K, Weller C l and Hanna M A (2006), `Properties of chitosan films as a function of pH and solvent type', J Food Sci, 71(3), E119±E124. Kittur F S, Kumer K R and Tharanathan R N (1998), `Functional packaging properties of chitosan films', Eur Food Res Technol, 44, 206±208. Knorr D (1991), `Recovery and utilization of chitin and chitosan in food processing waste management', Food Technol, 45(1), 114±122. Kumar M N V R (2000), `A review of chitin and chitosan applications', Reac Func Polym, 46, 1±27. Lagaly G (1999), `Introduction: from clay mineral±polymer interactions to clay mineralpolymer nanocomposites', Appl Clay Sci, 15, 1±9. LagaroÂn J M and Fendler A (2009), `High water barrier nanobiocomposites of methyl cellulose and chitosan for film and coating applications', J Plast Film Sheet, 25 (1), 47±59. LagaroÂn J M, Fernandez-Saiz P and Ocio M J (2007), `Using ATR-FTIR spectroscopy to design active antimicrobial food packaging structures based on high molecular weight chitosan polysaccharide', J Agric Food Chem, 55, 2554±2562. Larena A and Caceres D A (2004), `Variability between chitosan membrane surface characteristics as function of its composition and environmental conditions', Appl Surf Sci, 238, 273±277. Lee H Y, Kim S M, Kim J Y, Youn S K, Choi J S, Park S M and Ahn D H (2002), `Effect of addition of chitosan on improvement for shelf-life of bread', J Kor Soc Food Sci Nutr, 31(3), 445±450. Lee J W, Lee H H and Rhim J W (2000), `Shelf life extension of white rice cake and wet noodle by the treatment with chitosan', Kor J Food Sci Technol, 32(4), 828±833. Li B, Peng J, Yie X and Xie B (2006), `Enhancing physical properties and antimicrobial activity of konjac glucomannan edible films by incorporating chitosan and nisin', J Food Sci, 71, 174±178. Lin K F, Hsu C Y, Huang T S, Chiu W Y, Lee Y H and Young T H (2005), `A novel method to prepare chitosan/montmorillonite nanocomposites', J Appl Polym Sci, 98, 2042±2047. Liu H, Du Y, Wang X and Sun L (2004), `Chitosan kills bacteria through cell membrane damage', Int J Food Microbiol, 95, 147±155. Liu N, Chen X G, Park H J, Liu C G, Liu C S, Meng X H and Yu L J (2006), `Effect of MW and concentration of chitosan on antibacterial activity of Escherichia coli', Carbohyd Polym, 64, 60±65. Liu X F, Guan Y L, Yang D Z, Li Z and Yao K D (2001), `Antibacterial action of chitosan and carboxymethylated chitosan', J Appl Polym Sci, 79, 1324±1335. Luo J J and Daniel I M (2003), `Characterization and modeling of mechanical behavior of polymer/clay nanocomposites', Compos Sci Technol, 63, 1607±1616. Marques A, EncarnacËao S, Pedro S and Nunes M L (2008), `In vitro antimicrobial activity of garlic, oregano and chitosan against Salmonella enterica', World J Microbiol Biotechnol, 24, 2375±2360. Marreco P R, Moreira P L, Genari S C and Moraes A (2005), `Effects of different sterilization methods on the morphology, mechanical properties, and cytotoxicity of

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Carrageenan polysaccharides for food packaging M . D . S A N C H E Z - G A R C I A , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: This chapter discusses the applications of carrageenan materials for food packaging and the effect of the addition of nanoreinforcement vectors in the barrier properties to gases and vapours and to impart additional functionalities to biopackaging plastics. The chapter first discusses the carrageenan structure and properties and then describes the applications in food packaging and their barrier properties. Finally, a case study of nanocomposites of carrageenan based on nanoclays or cellulose nanowhiskers showing enhanced gas, vapour and UV barrier of interest in food biopackaging applications is shown. Key words: carrageenan, biopackaging, barrier properties, nanocomposites.

21.1

Introduction

Petrochemical-based plastics such as polyolefins, polyesters, polyamides, etc. have been increasingly used as packaging materials, because of their availability in large quantities at low cost and favourable functionality, characteristics such as good tensile and tear strength, good barrier properties to oxygen and aroma compounds and heat sealability. However, they are non-biodegradable, and therefore lead to environmental concerns, which pose serious ecological problems. Hence, their use in any form or shape has to be restricted and may even be gradually abandoned to avoid problems concerning waste disposal. Edible coatings from polysaccharides, proteins and lipids can extend the shelf-life of foods by functioning as solute, gas and vapour barriers. Factors contributing to a renewed interest in development of edible coatings include consumer demand for high quality foods, environmental concerns over disposal of non-renewable food packaging materials and opportunities for creating new market outlets for film-forming ingredients derived from under-utilized agricultural resources. The search for new renewable resources for the production of edible and biodegradable materials has steadily increased in recent years. In particular, nonconventional sources of carbohydrates have been extensively studied. There are various unique carbohydrates that are found in marine organisms that represent a largely unexplored source of valuable materials. These non-conventional and

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underexploited renewable materials, such as carrageenan, can be used as an interesting alternative to produce edible films and coatings.

21.2

Structure and properties of carrageenan

Carrageenan is a polysaccharide directly extracted from the biomass from seaweeds, red algae. Marine algae exist in an incredible variety of life-forms, from unicellular species to giant kelp which may extend in length up to 40 m. Algae are generally classified into four main groups, largely upon the basis of pigmentation: green algae (the Chlorophyceae), blue-green algae (Cyanophyceae), red algae (Rhodophyceae) and brown algae (Phaeophyceae). Green and blue-green algae, while present in salt water, are more commonly associated with fresh water. Red and brown algae, on the other hand, are found almost exclusively in marine environments. Brown algae are the most familiar, most conspicuous, largest and most abundant of the seaweeds, but in number and diversity are exceeded by the group of red algae of which there are some 4000 different species. The former are particularly abundant in cold northern waters and few species are found in tropical regions; red algae are present at all latitudes (Naylor, 1976). The cell walls of seaweeds contain long-chain polysaccharides, which give flexibility to the algae and allow them to adapt to the variety of water movements in which they grow. These polysaccharides are referred to as hydrocolloids because they disperse in water to give a solution with colloidal properties. When hydrocolloids are dispersed in water they increase its viscosity and so find many applications as thickening agents. Under some conditions they will also form gels and this property is useful for other applications. The hydrocolloids of commercial importance extracted from seaweeds are alginate, agar and carrageenan. Different species of Rhodophyceae (red seaweeds) contain naturally occurring polysaccharides which fill the voids within the cellulose structure of the plant. This family of polysaccharides includes carrageenan, furcellaran and agar. These polymers have a backbone of galactose but differ in the proportion and location of ester sulfate groups and the proportion of 3,6-anhydrogalactose. The differences in composition and conformation produce a wide range of rheological properties which are useful for a large number of foods (Imeson, 2000). Carrageenan is usually classified in three industrially relevant types: carrageenan, which is a highly sulfated galactan with viscosity enhancing properties; -carrageenan, which forms thermoreversible soft gels; and carrageenan, which gives strong and brittle gels with water syneresis. But in reality, carrageenan biopolymers possess a complex hybrid chemical structure comprising -, - or -carrageenan monomers together with non-gelling biological precursors monomers such as - or -carrageenan monomers (Lahaye, 2001). The chemical structures of the monomers corresponding to -, - and carrageenans are presented in Fig. 21.1, together with the chemical structures of

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21.1 Molecular structure of carrageenan monomers (Hilliou, 2006).

- or -monomers which are the biological precursors of - and -monomers, respectively. Carrageenan extraction from red seaweeds consists of alkali treatment and the extraction procedure. Alkali is used because it causes a chemical change that leads to increased gel strength in the final product. In chemical terms, it removes some of the sulfate groups from the molecules and increases the formation of 3,6anhydrogalactose: the more of the latter, the better the gel strength (McHugh, 2003). Figure 21.2 shows the flow diagram used for carrageenan extraction.

21.2 Flow diagram for carrageenan extraction (adapted from Bixler, 1996).

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Processing in packaging

Polysaccharides were the earliest and most extensively studied materials for biopackaging. A variety of polysaccharides and their derivatives have been tested for potential use as biodegradable/edible films, including alginate, pectin, carrageenan, konjac, chitosan, pullulan, cellulose and starch along with their derivatives. Carrageenan films are simply formed by cooling their hot neutral or alkaline aqueous solutions to form a gel, followed by drying. Carrageenan extracted from seaweeds possesses good film-forming properties (Park et al., 2001; Xiao et al., 2001; Cha et al., 2002; Briones et al., 2004) and are good carriers for antimicrobial agents (Kester and Fennema, 1986). These polysaccharide films are water soluble and their mechanical strength is generally weaker than that of other polysaccharide films. Due to the hydrophilic characteristic of these films, only minimal moisture barrier properties are expected. However, they are good oxygen and lipid barriers that can retard lipid oxidation in homogeneous foods and lipid migration in heterogeneous foods. The use of carrageenan as edible films and coatings already covers various fields of the food industry (mainly dairy products) to pharmaceutics (Van de Velde and de Ruiter, 2002). The dairy sector accounts for a large part of the carrageenan applications in food products, such as frozen desserts, chocolate milk, cottage cheese and whipped cream. In addition to this, carrageenans are used in various non-dairy food products, such as fresh and frozen meat, poultry and fish to prevent superficial dehydration (Shaw et al., 1980), ham or sausage casings (Macquarrie, 2002), granulation-coated powders, dry solid foods, oily foods (Ninomiya et al., 1997), etc., as protective coatings for fruits, vegetables, cheese and meat products, aiming to retard water loss by acting as sacrificing agents and to carry antimicrobial and antioxidant agents (Kester and Fennema, 1986), but also in manufacturing soft capsules (Tanner et al., 2002; Bartkowiak and Hunkeler, 2001) and especially non-gelatin capsules (Fonkwe et al., 2003), and non-food products such as pharmaceutical formulations, cosmetics and oilwell drilling fluid (Imeson, 2000; van de Velde and de Ruiter, 2002). Except starch, carrageenan is, with pectin, the main natural gelling polysaccharide extracted from plants or seaweeds and used as a high-value functional ingredient in foods, cosmetics and pharmaceuticals (De Ruiter and Rudolph, 1997). Indeed, this protective barrier can also be used in the food domain in order to prevent the transfer of moisture, gases, flavours or lipids and thus to maintain or improve food quality and to increase food product shelf-life (Krochta and De MulderJohnston, 1997). Another promising emerging technology that has been applied to various biopolymers, including carrageenan-based coating, is their use as antimicrobial agent carriers in active packaging systems (Hong et al., 2005; Choi et al., 2005).

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Barrier performance

Hydrocolloid films are characterized by their good gas barrier properties but poor water vapour permeability (Baldwin et al., 1997; Debeaufort and Voilley, 1997; Krochta and De Mulder-Johnston, 1997). Table 21.1 presents typical permeability data to water vapour for conventional and novel biodegradable plastics used in packaging. Compared with other films based on proteins and polysaccharides reported in the literature, films based on carrageenan (commercial and domestic) show similar WVP values, and in some cases lower, even an order of magnitude below (Table 21.1). With regard to synthetic polymers, carrageenan films have similar values to those of cellophane. Compared with thermoplastic biopolymers (poly(lactic acid) (PLA), polyhydroxyalkanoates (PHBV) and polycaprolactones (PCL)) the water permeability shows the same order of magnitude of the WVP of carrageenan films. However, they are higher than low density polyethylene (LDPE), the most common polymer used in the food packaging industry, as well as only one order or magnitude higher than polyethylene terephthalate (PET), also a commercial polymer for food packaging applications (see Table 21.1). Oxygen permeability is the next most commonly studied transport property of edible films after water vapour permeability (Miller and Krochta, 1997). Oxygen Table 21.1 Comparison of WVP values of biodegradable and synthetic films Film formulation

WVP References (kg m/m s Pa) 380 eÿ14 18 eÿ14 7.6 eÿ14 7.5 eÿ14 6.7 eÿ14

Whey protein Corn starch Methylcellulose -carrageenan /-hybrid carrageenan Chitosan Cellophane Poly(lactic acid) (PLA) Polyhydroxybutyrate (PHBV) Polycaprolactone (PCL) Polyvinyl alcohol (PVOH) Ethylene±vinyl alcohol (EVOH) Polypropylene (PP) Poly(ethyleneterephthalate) (PET) Low density polyethylene (LDPE) High density polyethylene (HDPE) Poly(vinylidene chloride) (PVC)

4.5 eÿ14 8.4 eÿ14 2.30 eÿ14 1.27 eÿ14 3.39 eÿ14 48.50 eÿ14 1.7 eÿ14 0.0726 eÿ14 0.3 eÿ14

Anker et al., 2002 Garcia et al., 2006 Garcia et al., 2004 Larotonda et al., 2007 Larotonda et al., 2007 Sanchez-Garc|¨ a et al., 2010 Garcia et al., 2006 Shellhammer and Krochta, 1997 Sanchez-Garc|¨ a and Lagaro¨n, 2010 Sanchez-Garc|¨ a and Lagaro¨n, 2010 Sanchez-Garc|¨ a and Lagaro¨n, 2010 Lagaro¨n et al., 2008 Lagaro¨n et al., 2008 Lagaro¨n et al., 2008 Sanchez-Garc|¨ a et al., 2007

0.091 eÿ14 Smith, 1986 0.023 eÿ14 Smith, 1986 0.044 eÿ14 Parris et al., 1995

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is involved in many degradation reactions in foods, such as fat and oil rancidity, microorganism growth, enzymatic browning and vitamin loss. Thus, many packaging strategies seek to exclude oxygen to protect the food product (Gontard et al., 1996). On the other hand, permeability to oxygen and carbon dioxide is essential for respiration of living tissues such as fresh fruits and vegetables. So, moderate barrier coatings are more appropriate. If a coating with the appropriate permeability is chosen, a controlled respiratory exchange can be established and thus the preservation of fresh fruits and vegetables can be prolonged (Ayranci and Tunc, 2003). Most edible films are water sensitive, but their hydrophilic nature makes them excellent barriers against non-polar substances such as aromas and oxygen (Khwaldia et al., 2004). Dry hydrocolloid films have good oxygen barrier properties. In the presence of moisture, the macromolecule chains become more mobile, leading to a substantial increase in O2 permeability (Kumins, 1965). Table 21.2 summarizes typical permeability data to oxygen vapour for conventional and novel biodegradable plastics like polysaccharides and protein films used in packaging with comparative purposes. Compared with other films based on proteins and polysaccharides reported in the literature, films based on carrageenan (commercial and domestic) show similar oxygen permeability values, in some cases an order of magnitude below (Table 21.2). With regard to synthetic polymers, carrageenan films have similar values to those of cellophane and high density polyethylene (HDPE). Also, carrageenan oxygen permeability shows similar values as for thermoplastic biopolymers (PLA, PCL and PHBV). However, they are lower than for low density polyethylene (LDPE), the most common polymer used in the food packaging industry (Table 21.2). Addition of plasticizers to carrageenan films yields more flexible films for industrial application; however, this produces a big impact on the barrier properties. A plasticizer is a very important component affecting the physicochemical properties of biopolymeric films. In principle, addition of plasticizers results in a decrease in intermolecular forces along the polymer chains which consequently improves flexibility, extensibility, toughness and tear resistance of the film. On the other hand, for good barrier properties of the film, the polymer should have a high crosslink density. So, for that reason, an increase in plasticizer concentration normally causes an increase in the vapour permeability of hygroscopic or hydrophilic films due to a reorganization of the protein network and the consequent increase in the free volume. There are some works which report that with the addition of plasticizers a marked increase in permeability and diffusion coefficients of gas or water vapour occurs (Tihminlioglu et al., 2010; Guilbert et al., 1996; Larotonda et al., 2007). The plasticizer content of the film also affects water vapour permeability. A decrease in the water permeability for low additions of glycerol (between 5 and 10 wt%) has been reported, but increasing the glycerol content results in increased

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Table 21.2 Comparison of oxygen permeability coefficients of biodegradable and synthetic films

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Film

Test conditions

Oxygen permeability (m3m/m2s Pa)

Carrageenan Corn starch Pectin Chitosan Wheat gluten Whey protein Cellophane Poly(lactic acid) (PLA) Polyhydroxybutyrate (PHBV) Polycaprolactone (PCL) Polyvinyl alcohol (PVOH) Ethylene±vinyl alcohol (EVOH) Polypropylene (PP) Poly(ethyleneterephthalate) (PET) Low density polyethylene (LDPE) High density polyethylene (HDPE)

23ëC, 50% RH 24ëC 25ëC, 96% RH 25ëC, 93% RH 25ëC, 91% RH 23ëC, 75% RH 23ëC, 95% RH 24ëC, 80% RH 24ëC, 80% RH 24ëC, 80% RH 24ëC, 75% RH 24ëC, 75% RH 24ëC, 0% RH 24ëC, 80% RH 23ëC, 50% RH 23ëC, 50% RH

5.21 eÿ18 0.15 eÿ18 29.56 eÿ18 10.44 eÿ18 21.70 eÿ18 1.68 eÿ18 2.92 eÿ18 2.77 eÿ18 1.44 eÿ18 7.06 eÿ18 0.9 eÿ18 0.091 eÿ18 6.7 eÿ18 0.42 eÿ18 21.64 eÿ18 4.94 eÿ18

References Larotonda, 2007 Allen et al., 1963 Guilbert et al., 1996 Guilbert et al., 1996 Guilbert et al., 1996 Taylor, 1986 Taylor, 1986 Sanchez-Garc|¨ a and Lagaro¨n, 2010 Sanchez-Garc|¨ a and Lagaro¨n, 2010 Sanchez-Garc|¨ a and Lagaro¨n, 2010 Lagaro¨n et al., 2008 Lagaro¨n et al., 2008 Lagaro¨n et al., 2008 Sanchez-Garc|¨ a et al., 2007 Salame, 1986 Salame, 1986

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Table 21.3 Effect of glycerol on water permeability and water % uptake at 11%, 54% and 75% RH for carrageenan films Pwater (kg m/s m2 Pa) Carrageenan 6:86  0:041 eÿ14 Carrageenan + 4:65  0:542 eÿ14 10% glycerol

Water uptake (%) 11% RH 5.12 3:62  0:32

54% RH

75% RH

10:90  0:29 17:02  0:34 12.41 26:03  0:62

water permeability (Talja et al., 2007; Schou et al., 2005). As an example, Sanchez-GarcõÂa et al. reported an increase in the barrier properties of carrageenan films and of their nanobiocomposites at 10 wt% of glycerol content (SanchezGarcõÂa et al., 2010). Talja et al. (2007) reported that the water permeability was higher for potato starch-based films without plasticizer compared to starch-based films plasticized with 20% of glycerol at all RH gradients, though the film plasticized with 30 and 40 wt% of glycerol exhibited increased water. Visible cracks during evaluation of the water permeability of samples of carrageenan without plasticizer were not found. However, the increase of the water permeability of carrageenan without glycerol could be caused by microcracks in the film. Also, Guo et al. (1993) reported that cellulose acetate films at plasticizer contents of 5% to 10% (w/w, solids) had lower water permeability than films without plasticizer because of decreased molecular mobility of cellulose acetate. The water uptake of carrageenan films with and without glycerol is summarized in Table 21.3 at different humidities. A general observation is that water uptake for all films increased with increasing RH, as expected (Talja et al., 2007). From Table 21.3, the water uptake decreased with the addition of glycerol at low humidity and increased at high humidity. The same behaviour was reported by Zeppa et al. (2009). So, the main effect of the glycerol is to decrease the water uptake at low activity and increase it at high activity. In particular, a water-clustering phenomenon is thought to begin at lower activity for the plasticized films. This phenomenon occurs because in plasticized films there are interactions between hydroxyl groups of carrageenan and hydroxyl groups of glycerol, which result in less sorption sites for water and hence in lower hydroxyl groups that can be accessible for water binding at low water activities (Talja et al., 2007; Zeppa et al., 2009).

21.5

Nanocomposites

In general, polysaccharide and protein film materials are characterized by high moisture permeability, low oxygen and lipid permeability at lower relative humidities, and compromised barrier and mechanical properties at high relative humidities (Brody, 2005).

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In order to tailor the properties and improve the water resistance of these biopolymers, it is often desirable to combine with other bipolymers more resistant to water or with the addition of nanoclays using nanotechonology routes. In the latter case these layers are impermeable to water, and thus water can only migrate through the polysaccharide matrix following a more tortuous path. As a consequence, the nanocomposite carbohydrate film has substantially reduced water-vapour permeability, solving one of the long-standing problems in the production of biopolymer films. Moreover, introduction of the dispersed clay layers into the biopolymer matrix structure has been shown to greatly improve the overall mechanical strength of the film, making the use of these films industrially practicable (Weiss et al., 2006). Very little is known about the development and characterization of carrageenan nanocomposites. Daniel-Da-Silva et al. (2007) reported the production of polysaccharide -carrageenan used in the production of macroporous composites containing nanosized hydroxyapatite, with application in bone tissue engineering. Gan and Feng (2006) developed a new injectable biomaterial carrageenan/nano-hydroxyapatite/collagen for bone surgery. However, to our knowledge, the addition of nanoclays to carrageenan and the study of the barrier properties of these nanocomposites have only been reported in the work of Sanchez-GarcõÂa et al., who showed the characterization of novel nanocomposites of carrageenan and their blends with zein prepared by solvent casting with enhanced mechanical and barrier properties to water and oxygen based on a new specifically developed food contact-complying mica nanoclay. Additionally, these nanocomposites are able to block the light against UVvisible, which is of interest in, for instance, packaging and membrane applications (see later) (Sanchez-GarcõÂa et al., 2010). Additionally, Sanchez-GarcõÂa et al. (2010) developed new nanocomposites of carrageenan with cellulose nanowhiskers (CNW) which proved to be able to enhance water and oxygen barrier properties as well. The CNW, prepared by acid hydrolysis of purified cellulose microfibres, consisted of nanofibres with lengths of around 25±50 nm and thickness of ca. 5 nm. The morphology of the developed nanocomposites of carrageenan by the solvent casting method is usually analysed using transmission electron microscopy (TEM). TEM allows for the characterization of the dispersion degree of the reinforcing agents (nanoclays or CNW) into the carrageenan matrix (see Fig. 21.3). Figure 21.3a indicates a highly dispersed irregular morphology consisting of intercalated thin tactoids with different sizes and some completely exfoliated (see arrows) layered clay particles of varying size and platelet orientation (Sanchez-GarcõÂa et al., 2010). Figure 21.3b shows a film of carrageenan containing 1 wt% of cellulose nanowhiskers (CNW) and exhibiting good dispersion of the nanowhiskers in the matrix; however, increasing the nanowhisker content led to agglomeration due to the natural tendency of the cellulose whiskers to self-associate via hydrogen bonding. The size of the cellulose nanowhiskers as

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21.3 Transmission electron micrographs of casting of carrageenan: (a) with 5 wt% clay (scale marker is 200 nm); (b) with 1 wt% CNW (scale marker is 200 nm).

determined by TEM is seen to be around 25±50 nm in length and around 5 nm in cross-section. So, in comparison with the size of the original purified cellulose microfibres of ca. 50±100 m in length and 10±25 m in cross-section, the size of the CNW attained by acid hydrolysis of the microfibres is much smaller (Sanchez-GarcõÂa et al., 2010). Figure 21.4 displays the vapour barrier performance to water of various polymers and of their nanobiocomposites. Nanocomposites of solvent-cast PLA containing 3 wt% of CNW showed similar water barrier values to PET. The

21.4 Permeability to water of various nanobiocomposites prepared by casting techniques in comparison with the unfilled materials and with the reference material PET prepared by melt mixing.

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nanocomposites of carrageenan with 1±5 wt% of nanoclay and glycerol had the lowest water permeability values in carrageenan nanocomposites, which were surprisingly close to those for PET. From Fig. 21.4, the addition of nanoclays to the carrageenan matrices with glycerol resulted in reductions of ca. 86%, 83%, 61% and 61% in the water permeability for the films of carrageenan with 1 wt%, 5 wt%, 10 wt% and 20 wt% of clay, respectively, compared with the unfilled material. So, the addition of the nanoclays in the carrageenan films produced a considerable improvement in the water barrier properties, and more significantly at low contents (between 1 and 5 wt%) of nanoclay, due to the favourable morphology. The addition of low contents of CNW to the carrageenan matrices also resulted in significant reductions in water permeability with and without glycerol, but particularly without glycerol. The lowest water permeability observed for carrageenan was with the addition of a low content (between 1 and 5 wt%) of nanoclay, probably due to the high nanodispersion of the clay across the matrix and the good interfacial adhesion and dispersed morphology seen in these nanobiocomposites. Figure 21.5 shows the water uptake, related to the solubility coefficient, of the carrageenan films and of their nanobiocomposites with nanoclays and CNW at different humidities. A general observation is that the water uptake of all films increased with increasing RH, as expected (Talja et al., 2007). Figure 21.5 shows that films of carrageenan with 1 wt%, 5 wt%, 10 wt% and 20 wt% of clay

21.5 Water uptake (%) at 11%, 54% and 75% RH for the films of carrageenan and their nanocomposites with different contents of clay and CNW.

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presented a decrease in the water uptake at 75% RH between 56 and 65% compared with the unfilled material. In the case of water uptake at 54% RH, a reduction of ca. 85±90% was observed for the films of carrageenan containing nanoclays. However, the water uptake measured at 11% RH presented a lower effect or even an increase in the water uptake with the addition of the nanoclay. Figure 21.5 shows that films of carrageenan with 10 wt% of glycerol and with 1 wt%, 3 wt% and 5 wt% of CNW present a decrease in the water uptake at 11% RH of ca. 60%, 76% and 47%, respectively, compared with the pure material with glycerol. In the case of water uptake at 54% RH, a reduction of 50%, 61% and 59% for the films of carrageenan with 1 wt%, 3 wt% and 5 wt% of CNW can be observed. The water uptake measured at 75% RH also presents a decrease in the water uptake of ca. 65±75%. As a result, the addition of CNW to carrageenan films also led to reduced water uptake. Apart from the above-mentioned improvement in mass transport properties for the carrageenan films, the incorporation of mica-based nanoclays was observed to exert certain UV/visible light blocking action, providing the materials with an additional functionality of interest in certain applications like food packaging. The UV blocking action was studied as a function of loading rate and, thus, biocomposites were prepared with clay contents ranging from 1 to 20 wt% (Sanchez-GarcõÂa and LagaroÂn, 2010; Sanchez-GarcõÂa et al., 2010). Figure 21.6 shows, as an example, the transmittance for films of carrageenan

21.6 UV±vis spectra of the castings of 30 microns of carrageenan nanocomposites: percentage transmission spectra of castings of carrageenan and their nanocomposites with 1, 5, 10 and 20 wt% clay.

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with thickness of 30 microns. The UV region is classified in three zones: UVC (100±280 nm), UVB (280±320 nm) and UVA (320±400 nm). From these results, the transmittance of the pure carrageenan in the region of UVC is around 30% of transmittance. A reduction of this transmittance can be observed until the minimum value of transmittance (around 0% transmission in UVC) is reached with 10 and 20 wt% clay content in the carrageenan film. Films of carrageenan + 20 wt% clay produce a reduction in the transmission of light in the UVB-UVA around 95±100%. This content of clay (20 wt% clay) in the carrageenan matrix blocks UV light. In the case of the visible region, pure carrageenan has a transmittance of 85%, whereas the film of carrageenan + 20 wt% clay has a transmittance of 5%. Higher clay loading in the carrageenan matrix produces a significant decrease of the transmission of UV/vis light. It is clear that 20 wt% filler content is perhaps too high a loading to apply, since it negatively affects transmission in the visible range and will be unlikely to yield optimum property balance. Nevertheless, and as with the barrier performance, the ratio of protection is still very efficient at 5 wt% of nanoclay addition, but with 10 wt% the differentiating benefits are not so strong. Thus, low nanoclay contents (5 wt%) in the carrageenan matrix lead to significant reductions in the UV light transmission, while retaining transparency to a significant extent due to a higher nanoclay dispersion in the matrix.

21.6

References and further reading

Allen, L., Nelson, A.I., Steinberg, M.P. (1963). Edible corn carbohydrate food coatings. I. Development and physical testing of a starch±algin coating. Food Technology, 17, 1437±1442. Anker, M., Berntsen, J., Hermansson, A.M., Stading, M. (2002). Improved water vapor barrier of whey protein films by addition of an acetylated monoglyceride. Innovative Food Science and Emerging Technologies, 3, 81±92. Ayranci, E., Tunc, S. (2003). A method for the measurement of the oxygen permeability and the development of edible films to reduce the rate of oxidative reactions in fresh foods. Food Chemistry, 80, 423±431. Baldwin, E.A., Nisperos-Carriedo, M.O., Hagenmaier, R.D., Baker, R.A. (1997). Use of lipids in coatings for food products. Food Technology, 51, 56±64. Bartkowiak, A., Hunkeler, D. (2001). Colloids Surf. B: Biointerfaces, 21, 285±298 Bixler, H.J. (1996). Recent developments in manufacturing and marketing carrageenan. Hydrobiologia, 326/327, 35±57. Briones, A.V., Ambal, W.O., Estrella, R.R., Pangilinan, R., De Vera, C.J., Pacis, R.L., Rodriguez, N., Villanueva, M.A. (2004). Tensile and tear strength of carrageenan film from Philippine Euchema species. Marine Biotechnology, 6, 148±151. Brody, A.L. (2005). Food Technology, 59, 65. Cha, D.S., Choi, J.H., Chinnan, M.S., Park, H.J. (2002). Antimicrobial films based on Naalginate and -carrageenan. Lebensmittel-Wissenschaft und -Technologie, 35, 715± 719. Choi, J.H., Choi, W.Y., Cha, D.S., Chinnan, M.J., Park, H.J. (2005). Diffusivity of potassium sorbate in -carrageenan based antimicrobial film. LWT ± Food Science

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and Technology, 38, 417±423. Daniel-Da-Silva, A.L., Lopes, A.B., Gil, A.M., Correia, R.N. (2007). Synthesis and characterization of porous -carrageenan/calcium phosphate nanocomposite scaffolds. Journal of Materials Science, 42, 8581±8591. Debeaufort, F., Voilley, A. (1997). Methylcellulose-based edible films and coatings: 2. Mechanical and thermal properties as a function of plasticizer content. Journal of Agricultural and Food Chemistry, 45, 685±689. De Ruiter, G.A., Rudolph, B. (1997). Carrageenan biotechnology. Trends in Food Science and Technology, 8, 389±395. Fonkwe, L., Archibald, D., Gennadios, A. (2003). Nongelatin capsule shell formulation. US Patent 0138482 A1. Gan, S.-L., Feng, Q.-L. (2006). Preparation and characterization of a new injectable bone substitute-carrageenan/nano-hydroxyapatite/collagen. Acta Academiae Medicinae Sinicae, 28(5), 710±713. Garcia, M.A., Pinotti, A., Martino, M.N., Zaritzky, N.E. (2004). Characterization of composite hydrocolloid films. Carbohydrate Polymers, 56, 339±345. Garcia, M.A., Pinotti, A., Zaritzky, N.E. (2006). Physicochemical, water vapor barrier and mechanical properties of corn starch and chitosan composite films. Starch/ StaÈrke, 58, 453±463. Gontard, N., Thibault, R., Cuq, B., Guilbert, S. (1996). Influence of relative humidity and film composition on oxygen and carbon dioxide permeabilities of edible films. Journal of Agricultural and Food Chemistry, 44, 1064±1069. Guilbert, S., Gontard, N., Morris, L.G.M. (1996). Prolongation of the shelflife of perishable food products using biodegradable films and coatings. LebensmittelWissenschaft und -Technologie, 29, 10±17. Guo, J.H. (1993). Effects of plasticizers on water permeation and mechanical properties of cellulose acetate: antiplasticization in slightly plasticized polymer film. Drug Development and Industrial Pharmacy, 19(13), 1541±1555. Hilliou, L., Larotonda, F.D.S., Abreu, P., Ramos, A.M., Sereno, A.M., GoncËalves, M.P. (2006). Effect of extraction parameters on the chemical structure and gel properties of /-hybrid carrageenans obtained from Mastocarpus stellatus. Biomolecular Engineering, 23, 201±208. Hong, S.-I., Lee, J.-W., Son, S.-M. (2005). Properties of polysaccharide coated polypropylene films as affected by biopolymer and plasticizer types. Packaging Technology and Science, 18, 1±9. Imeson, A.P. (2000). Carrageenan. In: G.O. Phillips and P.A. Williams (eds), Handbook of Hydrocolloids, Woodhead Publishing, Cambridge, UK. Kester, J.J., Fennema, O.R. (1986). Edible films and coatings: a review. Food Technology, 40, 47±59. Khwaldia, K., Perez, C., Banon, S., Desobry, S., Hardy, J. (2004). Milk proteins for edible films and coatings. Critical Reviews in Food Science and Nutrition, 44, 239±251. Krochta, J.M., De Mulder-Johnston, C. (1997). Edible and biodegradable polymer films: Challenges and opportunities. Food Technology, 51(2), 61±74. Kumins, C.A. (1965). Transport through polymer films. Journal of Polymer Science. Part C: Polymer Symposia, 10, 1±9. LagaroÂn, J.M., Gimenez, E., Sanchez-GarcõÂa, M.D. (2008). Property enhanced thermoplastic nanobiocomposites for rigid and flexible food packaging applications. In Environmentally Compatible Food Packaging, Woodhead Publishing, Cambridge, UK, pp. 63±83. Lahaye, M. (2001). Developments on gelling algal galactans, their structure and physico-

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chemistry. Journal of Applied Phycology, 13, 163±184. Larotonda, F.D.S. (2007). Biodegradable films and coatings obtained from carrageenan from Mastocarpus stellatus and starch from Quercus sube. Thesis. Larotonda, F.D.S., Hilliou, L., GoncËalves, M.P., Sereno, A.M. (2007). Film properties of /-hybrid carrageenan natural polymer. Polymer Processing Society 23rd Annual Meeting. Macquarrie, R. (2002). Edible film formulation. US Patent 0155200 A1. McHugh, D.J. (2003). A guide to the seaweed industry. FAO Fisheries Technical Papers T441. Miller, K.S., Krochta, J.M. (1997). Oxygen and aroma barrier properties of edible films: a review. Trends in Food Science and Technology, 8, 228±237. Naylor, J. (1976). Production, trade and utilization of seaweeds and seaweed products. FAO Fisheries Technical Paper 159. Ninomiya, H., Suzuki, S., Ishii, K. (1997). Edible film and method of making same. US Patent 5,620,757. Park, S.Y., Lee, B.I., Jung, S.T., Park, H.J. (2001). Biopolymer composite films based on -carrageenan and chitosan. Materials Research Bulletin, 36, 511±519. Parris, N., Coffin, D.R., Joubran, R.F., Pessen, H. (1995). Composition factors affecting the water vapor permeability and tensile properties of hydrophilic films. Journal of Agricultural and Food Chemistry, 43, 1432±1435. Salame, M. (1986). Barrier polymers. In: M. Bakker (ed.), The Wiley Encyclopedia of Packaging Technology, John Wiley, New York, pp. 48±53. Sanchez-GarcõÂa, M.D., LagaroÂn, J.M. (2010). Novel clay based nanobiocomposites of biopolyesters with synergistic barrier to UV light. Journal of Applied Polymer Science, accepted 2010. Sanchez-GarcõÂa, M.D., Gimenez, E., LagaroÂn, J.M. (2007). Comparative barrier performance of novel PET nanocomposites with biopolyester nanocomposites of interest in packaging food applications. Journal of Plastic Film and Sheeting, 23, 133±148. Sanchez-GarcõÂa, M.D., Hilliou, L., LagaroÂn, J.M. (2010). Nanobiocomposites of carrageenan, zein and mica of interest in food packaging and coating applications. Journal of Agricultural and Food Chemistry, accepted 2010. Schou, M., Longares, A., Montesinos-Herrero, C., Monahan, F.J., O'Riordan, D., O'Sullivan, M. (2005). Properties of edible sodium caseinate films and their application as food wrapping. Lebensmittel-Wissenschaft und -Technologie, 38, 605±610. Shaw, C., Secrist, J., Tuomy, J. (1980). Method of extending the storage life in the frozen state of precooked foods and product produced. US Patent 4,196,219. Shellhammer, T.H., Krochta, J.M. (1997). Whey protein emulsion film performance as affected by lipid type amount. Journal of Food Science, 62, 390±394. Smith, S.A. (1986). Polyethylene, low density. In: M. Bakker (ed.), The Wiley Encyclopedia of Packaging Technology, Wiley, New York, pp. 514±523. Talja, R.A., HeleÂn, H., Roos, Y.H., Jouppila, K. (2007). Effect of various polyols and polyol contents on physical and mechanical properties of potato starch-based films. Carbohydrate Polymers, 67, 288±295. Tanner, K., Getz, J., Burnett, S., Youngblood, E., Draper, P. (2002). Film forming compositions comprising modified starches and iota-carrageenan and methods for manufacturing soft capsules using same. US Patent 0081331 A1. Taylor, C.C. (1986). Cellophane. In: M. Bakker (ed.), The Wiley Encyclopedia of Packaging Technology, Wiley, New York, pp. 159±163.

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È zen, B. (2010). Water vapor and oxygen-barrier Tihminlioglu, F., Atik, I.D., O performance of corn±zein coated polypropylene films. Journal of Food Engineering, 96, 342±347. Tingaut, P., Zimmermann, T., Lopez-Suevos, F. (2010). Biomacromolecules, 11, 454± 464. Van de Velde, F., de Ruiter, G.A. (2002). Carrageenan. In: E.J. Vandamme, S. De Baets and A. SteinbuÈchel (eds), Biopolymers, Polysaccharides from Eukaryotes, Vol. 6, Wiley, Weinheim, Germany, pp. 245±274. Weiss, J., Takhistov, P., McClements D.J. (2006). Functional materials in food nanotechnology. Journal of Food Science, 71(9), R107±R116. Xiao, C., Liu, H., Lu, Y., Zhang, L. (2001). Blend films from sodium alginate and gelatin solutions. Journal of Macromolecular Science Part A ± Pure and Applied Chemistry, 38, 317±328. Zeppa, C., Gouanve, F., Espuche, E. (2009). Effect of a plasticizer on the structure of biodegradable starch/clay nanocomposites: Thermal, water-sorption, and oxygenbarrier properties. Journal of Applied Polymer Science, 112, 2044±2056.

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Protein-based resins for food packaging A . A . V I C E N T E , M . A . C E R Q U E I R A and L . H I L L I O U , University of Minho, Portugal and C . M . R . R O C H A , University of Porto, Portugal

Abstract: Currently, mainly non-biodegradable petroleum-based synthetic polymers are used as packaging materials for foods, because of their availability, low cost and functionality. However, biodegradable/edible films can be made from polysaccharides, proteins and lipids without the environmental issues of petroleum-based polymers and with the additional advantage of being available from renewable sources or as by-products or waste-products from the food and agriculture industries. Although these sources are not enough, by far, to replace the synthetic polymers/films, they can help in reducing packaging wastes and can replace them in specific applications. Proteins have the extra advantage of allowing a wider range of functional properties than polysaccharides and lipids. This is conferred by their composition based on combinations of 20 different amino acids, which usually allows high intermolecular binding. This chapter starts by describing the main protein sources used in packaging materials, followed by a section dealing with materials processing and characterization. The chapter ends with an overview of current and foreseen applications. Nanotechnology applications through the utilization of materials at the nano-scale size in protein-based films are a valuable alternative for the improvement of the properties of packaging materials (water vapour transmission, mechanical, thermal). In particular, nanofilms and multinanolayer films constitute promising solutions for food applications using protein-based materials. Key words: films, coatings, food, transport properties, mechanical properties, processing.

22.1

Materials (sources, extraction, structure and properties)

Two types of proteins are used for film formation: fibrous proteins (e.g. keratin), and proteins that under certain environmental conditions (e.g. temperature, pH, ionic strength) form aggregates. This last group can be divided into lowstructured proteins (e.g. casein) and globular proteins (e.g. ovalbumin, whey proteins, soy proteins, wheat gluten, corn zein). Fibrous proteins are generally

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associated in parallel structures through hydrogen bonding. They are water insoluble. Globular proteins have generally a spherical structure held by hydrogen, ionic, hydrophobic and disulphide bonds. The production of cohesive films is affected by the protein/protein interactions. Relevant issues are the amino acid composition and sequence. The sequence of amino acids will determine the distribution of polar, hydrophobic and thiol groups in the polypeptide chain, the possibility of hydrogen bonding, the presence of S±S bonds (intra- or intermolecular) and conditions affecting the formation of ionic crosslinks between amino and carboxyl groups (PeÂrez-Gago and Krochta, 2002; Bourtoom, 2008; Verbeek and van den Berg, 2010). Globular proteins are usually required to partially unfold and realign before film formation. The degree of this partial denaturation will be determinant in the production of cohesive films and can be achieved by thermal or chemical means (e.g. through the addition of a solvent or an electrolyte or change of the pH). The partially denatured peptide chains can associate and bind through hydrogen, disulphide hydrophobic or ionic bonding to form the film matrix. Many proteins have been tested for film formation, such as whey proteins, casein, collagen and gelatin, corn zein, soy protein, wheat gluten, egg white protein, fish myofibrillar proteins, cottonseed proteins and keratins. One important handicap of the use of proteins is their allerginicity. In fact, there are consumers who are allergic or intolerant to some of the proteins that have been used, such as milk, soy and egg proteins and wheat gluten.

22.1.1 Milk proteins Milk proteins can be divided into two main groups: caseins that precipitate at pH 4.6, 20ëC, and whey proteins that remain soluble under these conditions (Fox, 2001). Caseins represent the major group of bovine milk proteins (80%). They are small phosphorylated proteins (20±25 kDa) with a rheomorphic structure, which is open and quite flexible, that facilitates the film formation through hydrogen and electrostatic bonding and hydrophobic interactions (Sawyer et al., 2002; Chen, 2002). They have low levels of secondary and tertiary structures and are thus stable to denaturating agents. They have no typical globular behaviour, but they do not fall in the fibrous protein category either. This group of milk proteins has four main proteins: s1, s2, and . Only s2 and  caseins contain cysteine and form intermolecular disulphide bonds. The phosphate groups strongly bind to polyvalent cations. - and -caseins have a strong amphipathic nature resulting in highly surface-active molecules (Chen, 1995; Fox, 2001). Usually, caseins form films through hydrogen bonding and electrostatic interaction, though hydrophobic bonding may also be important. Commercial caseinates are produced by precipitating skim milk at pH 4.6 and 20ëC. The caseins are then resolubilized at pH 6.7, pasteurized and spray-dried. Rennet casein can also be commercially used (Chen, 1995).

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Whey is the milk serum (yellow-green liquid) that separates from the curd during casein coagulation. It can be produced either by acid precipitation (pH < 5) of caseins (acid or sour whey) or by rennet curdling (rennet or sweet whey). The main products of industrial separation of the protein fraction from whey are whey protein concentrate (WPC) and whey protein isolate (WPI). WPCs are usually defined as whey protein products having a protein content between 34 and 85% (de Wit, 2001; Huffman and Harper, 1999) while WPIs have at least 90% protein (de Wit and Moulin, 2001). They are usually produced through ultrafiltration and diafiltration for the removal of lactose and minerals or by ionic exchange or electrodialysis after lactose crystallization (Chen, 1995; Durham et al., 1997; de Wit, 2001; de Wit and Moulin, 2001). The major whey proteins in cow's milk are -lactoglobulin ( -Lg; 50%), lactalbumin (-La; 12%), immunoglobulins (10%) and bovine serum albumin (BSA; 5%). Acid and rennet wheys also contain casein-derived peptides; both contain proteose-peptones (that do not exist in human milk), produced by plasmin, mainly from -casein, and the latter also contains glycomacropeptides produced by rennets from -casein (Fox and McSweeney, 1998). In its native form, bovine -Lg is a globular protein with a monomer molecular weight of 18.3 kDa, with two disulphide bonds and one free thiol group, which exhibits an increased reactivity above pH 7 (Caessens et al., 1997). It has five cysteine residues and four of them are involved in the two disulphide bonds (66±160 and 106±119 or 106±121) that sustain the protein tertiary structure. The secondary structure of -Lg contains 43% -sheet, 10% -helix, and 47% unordered structure, including -turns (Papiz et al., 1986). Native -Lg is a predominantly -sheet protein consisting of a -barrel of eight continuous antiparallel -strands folded into two antiparallel -sheets shaped into a flattened cone or calyx and an additional -strand, one major helix and four short helices attached to this calyx (Kuwata et al., 1999). One side of sheet 1 is hydrophobic and the other side is hydrophilic. Sheet 2 is also hydrophobic on one side that faces the hydrophobic side of sheet 1, thus creating a very hydrophobic cavity, which is nevertheless filled with water. Small hydrophobic molecules may bind to this central cavity (the -barrel). There is also another hydrophobic region on the side of sheet 2, where a three-turn helix lies above. This -helix covers the CysH residue, providing that it remains packed against the exterior of the calyx (Considine et al., 2007). -Lg associated form changes with pH, temperature, ionic strength and protein concentration. Between pH 5.2 (the isoelectric point) and 7.5, native Lg occurs as a dimer in solution. Between pH 3.5 and 5.2 -Lg reversibly forms tetramers/octamers, whereas below 3.5 and above 7.5 it dissociates into monomers due to electrostatic repulsions. At temperatures higher than 30ëC the dimeric form of -Lg dissociates to monomers and at temperatures higher than 55ëC unfolding of the molecule starts to occur, which results in an increased activity and oxidation of the thiol group (Caessens et al., 1997).

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-Lg's sulphydryl group is buried within the molecule in the native protein but becomes exposed and active on denaturation of the protein by various agents (including heat, pressure and urea) and can then undergo sulphydryl±disulphide interactions with itself or other proteins (Fox, 2001). Thus, a slight denaturation of the globular -Lg molecule can have a great impact on its surface-active behaviour (Caessens et al., 1997). This property is responsible for many technological features of whey proteins. The second most abundant protein in cow's milk is -lactalbumin. -La is a globular protein with a molecular weight of 14.2 kDa and it is remarkably rich in tryptophan. The eight cysteine residues form four disulphide bridges (6±120, 26±111, 61±77 and 73±91) that stabilize its tertiary structure. The protein has an ellipsoid shape with two distinct lobes divided by a gap; one lobe comprises four helices and the other lobe comprises two -strands with a loop-like chain. At pH 4.0, -La unfolds and is susceptible to digestion by pepsin in the stomach (de Wit, 1998). One of the most interesting features of -La is its ability to bind metal cations. It has a strong calcium binding site and also several zinc binding sites. The binding of Ca2+ to -La causes pronounced changes in its tertiary structure and function and can increase its stability. Zinc or other cation binding might induce -La aggregation to forms that have anticancer activity and perform various transport functions with apolar, lipophilic vitamins and metabolites (Permyakov and Berliner, 2000). Bovine serum albumin is very similar to the human blood serum albumin. It has 582 amino acids and a molecular weight of ca. 69 kDa. Seventeen disulphide bridges stabilize its tertiary structure and it has one remaining free sulphydryl group. Glycomacropeptide (GMP) or caseinomacropeptide corresponds to a heterogeneous group of peptides having the same peptide chain but variable carbohydrate and phosphorus contents (Elsalam et al., 1996). The GMP peptide chain is composed of the 64 C-terminal amino acids of -casein, released by chymosin (or pepsin) cleavage of -casein during the manufacture of cheese (Thoma-Worringer et al., 2006), and has an average molecular weight of 8000 Da. Although it constitutes 15±20% of the total renneted cheese whey proteins, it is probably the least well known of its components. Possible reasons for this can be the absence of aromatic amino acids which makes it invisible at 280 nm (the common protein detection wavelength), its negative charge, even at pH 3 (it is not collected on cation exchangers, nor does it move with the rest of the proteins in native polyacrylamide gel electrophoresis) and its low molecular weight that makes it difficult to visualize with Coomassie Blue stain in sodium dodecyl sulphate (SDS)-PAGE (Brody, 2000). It is resistant to the enzymatic action of several rennets (including chymosin) and to pepsin. Native whey proteins have good film-forming properties, mainly due to hydrogen bonding (PeÂrez-Gago and Krochta, 2002). However, most applications

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of whey proteins to the formation of films involve protein heat denaturation, to expose ±SH and hydrophobic groups. When temperature rises above the proteins' denaturation point (typically 50±80ëC), there is a thermally induced unfolding of the native protein, possibly after some degree of dissociation if a multisubunit is involved, and a change in conformation occurs. The protein secondary and tertiary structures modify on heating and the protein molecule becomes more reactive as internal hydrophobic groups of the protein become more exposed. The degree of protein modification is, of course, dependent on the temperature profile and the time during which the protein was subjected to that temperature profile. In spite of this, the size and shape of the macromolecule suffer little change (<20%) and the resulting protein is often still globular and is able to bind to other similarly unfolded species (Tobitani and Ross-Murphy, 1997; Clark et al., 2001). This behaviour is common to many globular proteins. In order to reduce the exposure of the hydrophobic groups to the aqueous environment, aggregation of protein globules arises. Depending on the balance between attractive and repulsive forces among denatured molecules, two types of aggregates can appear: when intermolecular electrostatic repulsion is dominant, nanometre-thick strands are formed (Ikeda and Li-Chan, 2004); much coarser disordered particulate aggregates appear when the electrostatic repulsion is lower, that is when pH value is close to pI or when ionic strength is increased (Ikeda and Li-Chan, 2004; Foegeding, 2006). At neutral pH values, this aggregation can take place through sulphydryl±disulphide (SH/S-S) interchange reactions (Tobitani and Ross-Murphy, 1997). At these pH values the protein is highly charged (pH far away from pI) and if the ionic strength is low this aggregation step is limited in extent and the formed aggregates are predominantly linear (fibrils or small strands). Although much of the secondary structure of the protein remains, there is often an increased level of antiparallel -sheet (Clark et al., 2001).

22.1.2 Soy protein Soy proteins are mainly globular and have a molecular weight ranging from 8 to 600 kDa (Wolf, 1970). The most common way to classify them is based on their sedimentation rate in fractional ultracentrifugation. A larger Svedberg number (S) indicates a larger protein (Hernandez-Izquierdo and Krochta, 2008). Two major protein fractions are usually referred to: 7S which represents 35%, and 11S representing 52% (Weber, 2000). The 7S fraction is mainly constituted by conglycinin (>50%) and the 11S fraction consists mostly of glycinin. Both have complex quaternary structures and have the ability to undergo association± dissociation reactions important for film formation (Wolf, 1970; Hermansson, 1986). Conglycinin has a molecular weight of 180±210 kDa and is rich in asparagine, glutamine, leucine and arginine residues. It often exists as a dimer,

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depending on the pH and the ionic strength. It has low sulphur content and the formation of disulphide bonds is limited (Hernandez-Izquierdo and Krochta, 2008). Glycinin consists of alternately arranged acidic and basic subunits and has a molecular weight of 350 kDa. Intermolecular ±S±S-bond(s) exist between pairs of acidic and basic subunits (Hermansson, 1986). Soy proteins are commercially classified as soy flour, soy concentrate or soy isolate, depending on their protein content. Soy flour contains 40±60% protein, depending on the fat content, and is obtained by grinding soybean flakes (Wolf, 1970). Soy protein concentrate contains 65±72% protein and is obtained by aqueous liquid extraction or an acid leaching process (Park et al., 2002). Soy protein isolate (SPI) has a minimum protein content of 90% on a dry weight basis and is traditionally prepared by an isoelectric precipitation process (Hernandez-Izquierdo and Krochta, 2008).

22.1.3 Wheat gluten protein Gluten is the main storage protein in wheat and corn. Wheat gluten is an industrial by-product of wheat starch production via wet milling (Guilbert et al., 2002). It consists of the residue that is left after starch is washed away from wheat flour dough and has 70±80% of protein on a dry basis (Guilbert et al., 2002). Wheat gluten is then a mixture of the water-insoluble globular proteins. Two fractions can be distinguished: gliadins, alcohol-soluble proteins with a molecular weight of 20±50 kDa, and glutenins, (partially) soluble in dilute acid or alkali solutions and with an average molecular weight of 250 kDa (Bourtoom, 2008; Hernandez-Izquierdo and Krochta, 2008). Together, they represent up to 85% of the total wheat flour protein. Gliadin is also known as prolamin (due to a high content of proline and glutamine). It is poorly ionized over the entire pH range but can interact with other proteins or with itself through hydrophobic interactions and hydrogen bonds (Guilbert et al., 2002). Glutenin has a similar amino acid composition but a slightly lower content of hydrophobic amino acids than gliadin (Guilbert et al., 2002). Its high molecular weight can be attributed to the presence of intermolecular disulphide bonds, joining individual protein chains and resulting in a larger polymer (HernandezIzquierdo and Krochta, 2008). Cleavage of native disulphide bonds during heating of film-forming solutions and then formation of new disulphide bonds during film drying are believed to be important to the formation of wheat gluten film structure, along with hydrogen and hydrophobic bonds (Bourtoom, 2008).

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22.1.4 Corn zein Zein is the major group of proteins in corn (ca. 50%). It consists of the alcohol soluble protein fraction (prolamines) in corn endosperm, which has a protein content above 90% (Shih, 1998). It is soluble in water at high concentrations of urea, at pH 11 and above, or in the presence of anionic detergents (Turner et al., 1965). Zein is rich in glutamic acid, leucine, proline and alanine. Its hydrophobic nature and poor solubility in water are mainly due to the high proportion of non-polar amino acids (leucine, proline and alanine) and deficiency in basic and acid amino acids (Shukla and Cheryan, 2001). In whole corn, zein contains disulphide-linked molecular aggregates with ca. 44 kDa (Turner et al., 1965). The cleavage of these aggregates can be done using reducing or oxidizing agents. As the processing of the corn generally involves the use of reducing agents, commercial zein is a mixture of proteins with different molecular sizes, solubilities and charges with different properties from the `native' corn zein (Turner et al., 1965; Boundy et al., 1967). A large number of protein fractions have been identified by various techniques and by several researchers, thus leading to different systems of classification. Two main zein fractions have been described by McKinney (1958). The -zein fraction is soluble in 95% ethanol and represents ca. 50% of the total prolamine. The -zein fraction is insoluble in 95% ethanol but soluble in 60% ethanol and is relatively instable and not present in commercial preparations. Esen (1986, 1987) proposed a classification based on differences in solubility, amino acid composition and sequence, electrophoretic, chromatographic, and immunological properties. He used differential solubility into aqueous isopropyl alcohol solutions, with different pH values and urea and salt concentrations. As a result, zein was divided into three different classes: , and . The -zein fraction is soluble in 40±95% isopropyl alcohol and insoluble in 30% isopropyl alcohol solutions with sodium acetate 30 mM. It constitutes 75±85% of the total zein. The major components of -zein are polypeptides with a molecular weight of 20±24 kDa. A minor component of 10 kDa is also present. -Zein represents 10±15% of the total zein and includes exclusively methionine-rich polypeptides with a molecular weight of 17±18 kDa. It is soluble in 30±80% isopropyl alcohol under reducing conditions. The -zein fraction has a 27 kDa proline-rich polypeptide that constitutes 5±10% of the total zein. It is soluble in 0±20% of isopropyl alcohol under reducing conditions. Zein has a globular structure in non-aqueous solutions. It has nine repeating helical units arranged in an anti-parallel form and stabilized by hydrogen bonds (Shukla and Cheryan, 2001). Commercial zein is a by-product of the corn milling industry. During the wet-milling process, it is separated from steeped maize after germ and fibre removal (Dickey et al., 2001). It is then extracted by vigorous mixing in heated ethanol solutions (Dickey et al., 2002). Zein has excellent film-forming properties and is one of the few proteins that are used

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commercially (Krochta and De Mulder-Johnson, 1997). The film formation involves hydrophobic, hydrogen and limited disulphide bonds between zein chains (Bourtoom, 2008).

22.1.5 Collagen and gelatine Collagen is a fibrous, structural protein in animal tissues, found, e.g., in skin, tendons and bones, and represents more than 30% of the total proteins in animals. Although it is not thermoplastic and it is water insoluble, it is one of the most used proteins for commercial film formation (Krochta and De Mulder-Johnson, 1997). Its amino acid composition is unusual: it is rich in glycine, proline, hydroxyproline and alanine (Johnston-Banks, 1990). The amino acid sequence is unique: it has a glycine residue in every third position of the peptide chain. The fundamental unit of collagen structure is a triple helix with three polypeptide chains, each one with more than 1000 amino acids. Collagen has distinct hydrophobic and hydrophilic regions that lead to its tensioactive properties. In most tissues, collagen chains associate in fibres and fibres associate in beams. This complex helical and fibrous structure of collagen is responsible for its water insolubility and immunity to most enzymes (Johnston-Banks, 1990). Gelatine is obtained by partial controlled hydrolysis of collagen, which allows the partial denaturation and disruption of the fibrous structure and the production of water-soluble fragments (Weber, 2000). The source, age, and type of collagen as well as the denaturation process used will influence the properties and structure of the resulting gelatine. Two types of gelatine are described: type A results from an acid hydrolysis while type B is obtained through an alkaline hydrolysis. The isoelectric point of type A gelatine is 6.5 to 9.0 and for type B gelatine it is 4.8 to 5.0 (Johnston-Banks, 1990). The molecular weight of the resulting polypeptide chains is also highly heterogeneous. Unlike the described behaviour for the globular proteins that form films, gelatine forms a physical, thermoreversible gel on cooling, with a partial return to the ordered sequences in triple helix (Ross-Murphy, 1992).

22.1.6 Other proteins Other proteins have been used for film formation. These include myofibrillar proteins, keratins, egg white proteins and cottonseed protein. Peanut, rice (highly hypoallergenic), pea, lupin, gellan, silk film and sericin have also been mentioned in more specific works. Myofibrillar proteins are the main component of muscles in animals, representing more than 50% of total muscle weight. They include myosin and actin that are involved in the contraction of the muscles (Cuq et al., 1998). Fish has been the main source of myofibrillar proteins for edible film formation. These proteins are obtained after removing other components such as blood, lipids,

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myoglobin and collagen through a series of washing treatments (HernandezIzquierdo and Krochta, 2008). They can also be extracted at either acidic or alkaline pH and then precipitated at the isoelectric point. Keratins are by far the cheapest proteins, but they are also very difficult to process. They are fibrous proteins that can be found in birds, reptiles and mammals (e.g. in nails, hair or feathers). Their high stability and low solubility are due to disulphide linking between cysteine amino acid residues (Zoccola et al., 2009). There are two forms of keratin:  and . In mammals, only -keratin can be found, but birds and reptiles can produce both forms. However, -keratin can be converted to -keratin by stretching. It is the only protein that forms films that are not very sensitive to relative humidity (Weber, 2000). Cottonseed proteins make up 30±40% (w/w) of the cottonseed kernel. They are mainly globulins (60%) and albumins (30%). Globulins have a high content of ionizable amino acids and a low content of sulphur-containing amino acids. They are insoluble in water at pH 6.8. Albumins have low molecular weight (10±25 kDa), are soluble in water at pH 6.8 and rich in lysine and sulphurcontaining amino acids (Marquie and Guibert, 2002). Egg white represents ca. 60% of the total egg weight, of which 85% (dry basis) are proteins. Similar to whey proteins, egg white consists of a mixture of several proteins. Ovalbumin represents ca. 54% and is the only fraction with free sulphydryl groups. Ovotransferrin (12%), ovomucoid (11%) and lyzozyme contain disulphide bonds, important for film formation (Lim et al., 2002).

22.2

Structure and properties

22.2.1 Materials processing Formulation of edible and/or biodegradable packages must include at least one biopolymer able to form a suitably cohesive and continuous matrix (Guilbert and Gontard, 1997). The films can be produced by two general mechanisms (e.g. Guilbert and Gontard, 1997; Hernandez-Izquierdo and Krochta, 1997): one involves biopolymer dispersion or solubilization in a film-forming solution (solution-casting) followed by evaporation of the solvent (`wet process') and the other is based on the thermoplastic properties of the biopolymers at low moisture conditions (`dry process'). Solution casting Casting, also called the `solvent process' is based on the drying of the filmforming solution after its dispersion in a solvent (Guilbert and Gontard, 2005). For synthetic commercial plastics many technologies exist for film formation; however, for biodegradable polymer materials film casting is the foremost used method (Krochta, 2002; Kumar et al., 2008; Olabarrieta et al., 2006; Rossman,

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2009; Seydim and Sarikus, 2006). This method is mainly used for film production; however, when coatings are applied directly on food products, evaporation is also necessary in order to convert the aqueous solutions into a protective film layer (the so-called `coating'). Being based on biodegradable polymers, protein-based films are mainly formed by solvent evaporation (Khwaldia et al., 2004; Krochta, 2002; Zhou et al., 2009). Such a solvent may be water and/or ethanol, if the proteins are only soluble in ethanol (e.g. corn zein, wheat gluten, keratin). Film and/or coating formation is a step-by-step process where (1) the protein is dissolved in a solvent; (2) heating and/or pH adjustment of the solution takes place; and (3) if necessary other compounds are added. In some cases a degassing process (e.g. under vacuum) is required for the elimination of some bubbles formed during the process (Khwaldia et al., 2004; Krochta, 2002; Zhou et al., 2009). For filmforming solutions that will be used as coating, the film-forming solution is applied on the desired surface (e.g. fruits, vegetables) and then left to dry. If film formation is desired, the film-forming solution is applied in a solid surface, e.g. Petri dishes (Olabarrieta et al., 2006) or Teflon-coated glass plates (Sothornvit et al., 2009), and then left to dry. The configuration of the oven or drying chamber, temperature, relative humidity and time used for solvent evaporation are some of the most important factors during protein film formation (Rossman, 2009). In most cases, casting is performed at room temperature, in either the presence or absence of forced air convection (Denavi et al., 2009; Kristo et al., 2008; Sothornvit et al., 2009; Zhou et al., 2009). The high values of water vapour permeability of some protein films often lead to the introduction of lipids in the film formulation in order to decrease the water affinity. With this purpose, lipids can be dispersed during the film-forming solution preparation, if necessary with the addition of a surfactant, being heated above their melting point and homogenized to obtain an emulsion (Krochta, 2002; Morillon et al., 2002). However, the utilization of bilayer or multilayer films (obtained through the dipping of the film in a molten lipid dispersion or with the deposition of a lipid layer on protein films acting as support) has been shown to be more efficient as a water vapour barrier (Morillon et al., 2002; Weller et al., 1998). The protein used and the compounds added must combine in order to have the necessary characteristics for the purpose to which the film will be used. Independently of whether the result is a film or a coating, it must be adapted to each specific food product. Thermoplastic processing Conventional plastics are transformed at high volume and low cost into food packages using processing techniques such as injection moulding, film blowing

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or extrusion. Co-rotating extrusion is presently the technique of choice for compounding novel systems such as polymer/nanoclay composites and reactive blends. The formulation of the compound can be performed during the extrusion process, thus enabling tailoring the material in order to obtain targeted product properties. Besides mixing, the extrusion process essentially consists of melting the material in order to obtain a product with low viscosity but enough strength to sustain the large extension-dominated flows which are used to shape the molten material into tubes and sheets (in these cases, material flows in a shaping die and is later blown to form a pouch, or laminated to achieve a film), or trays and bottles (in these cases the material is injected in a mould). The processing window, namely temperature and pressure in the extrusion barrel and die and the speed of the extrusion screw (which defines the material output at the die exit), strongly depends on the thermal and rheological characteristics of the material to be transformed, as shaping is only achieved for temperatures above the glass transition temperature (for amorphous materials) or the melting point (for crystalline and semi-crystalline polymers). In addition, economic aspects come into play, as power consumption is to be kept at a minimum whereas optimum production rate remains the key objective. Proteins are very sensitive to thermal treatment and do not show attractive rheological properties at high temperature. Therefore, they present a very small processing window and thus cannot be used in conventional extrusion or moulding machines. As such, commercially available protein-based packages are still scarce, as specific processing techniques, additives or specific formulations must be designed to obtain a protein-based melt matching the demanding properties of plastics transformation processes (Verbeek and van den Berg, 2010). When designing such processes or formulations, physical and chemical mechanisms that develop during protein thermoforming need to be considered, along with the structural properties of the protein itself. Mechanisms such as crosslinking due to covalent sulphur±sulphur cystine bonds between neighbouring proteins, or intra- or intermolecular hydrogen bonding, occur during melting of a processable protein-based material. In addition, hydrophobic and hydrophilic interactions, mechanically induced orientation of protein crystals ( sheets and larger structural units), mechanical disruption of such units, thermally induced transitions and degradation are process-induced during the shaping of the material (Barone et al., 2006a). The control of all these mechanisms is achieved through optimization of processing parameters such as extrusion temperature, residence time in the barrel and die, pressure and flow rate, and addition of chemicals and water to aid the process, just to mention the most important ones. Processing aids: water and other plasticizers Heating of proteins results in unfolding of their globular conformation. Amino acids are thus exposed, which promote the formation of disulphide bonds with

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inherent aggregating and crosslinking effects. These effects impede the processability of proteins as high melt viscosity and glass transition temperature result from heating, and thermosets might even form which impede any material shaping. Breaking of intermolecular bonds by chemical or physical rupturing agents is thus required. Plasticizers are generally used as chemical additives, either to limit covalent crosslinking or to change the pH of the system and directly act on the hydrophilic and hydrophobic interactions between proteins. The efficiency of plasticizers is the result of a complex interplay between all these interactions, and as such it highly depends on the type of protein. A wide range of plasticizers ranging from hydrophilic (water, glycerol, sorbitol) to hydrophobic (phthalates) have been successfully tested (Hernandez-Izquierdo and Krochta, 2008). But amphiphilic additives seem to present the best plasticizing efficiency. After the screening of 23 additives, Pommet et al. (2005) reported that lactic acid had the best plasticizing effect on thermoformed wheat gluten materials, as hydroxyl and carboxyl groups favour both hydrophilic and hydrophobic interactions with proteins. In addition, acidic conditions preventing excessive aggregation (through disulphide bonding) were also obtained. Evidently, the amount of plasticizer to incorporate is a pertinent parameter to study, in addition to the chemistry. However, optimal formulation depends on the type of processed proteins and remaining processing parameters. It is worth mentioning that exudation of plasticizer during product storage may occur at high volume fraction (Mo and Sun, 2003). Protein modification Much like synthetic polymers, protein viscosity and processability do not depend exclusively on chemical structure but also on molecule size, namely the molecular mass. Smaller proteins are in principle easier to extrude, but also harder to shape as the formation of new intermolecular bonds and interactions is needed to stabilize the three-dimensional network building up the final product. Decreasing the molecular size of protein aggregates was successfully achieved by monitoring the feed rate, screw speed and barrel temperature of a co-rotating twin-screw extruder. The control of important processing parameters such as die pressure, product temperature and residence time resulted in the production of defect-free (smooth surface) extrudates (Redl et al., 1999). Enzymatic mediated modification of the secondary and ternary structures of wheat gluten protein was recently proposed as a new route to produce a glutamine-rich peptide matrix incorporating micron-sized fibres and nanometre-sized fibrils. The trypsin hydrolysis was carried out in situ, thus allowing the natural self-assembly of wheat gluten proteins into such structures (Athamneh and Barone, 2009). Protein-based composite materials could therefore be manufactured in the near future, using reactive extrusion concepts (Covas and Machado, 2004), where chemical reaction between polymers and additives mixed during extru-

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sion takes place to obtain specific structures and materials. The engineering strategy here is to combine reactive extrusion with the increased extrusioninduced generation of -sheets (Prudencio-Ferreira and Areas, 1993; Barone et al., 2006b), which is the route to the production of protein-based materials exhibiting higher crystallinity when compared to their native counterparts. However, the modification of secondary, ternary and quaternary structures of proteins by extrusion processes is still poorly documented (Verbeek and van den Berg, 2010).

22.3

Packaging materials characterization (barrier performance, mechanical properties)

Edible films of protein sources have attracted a lot of attention for potential use in food protection and preservation; they can bring a great number of advantages over synthetic films, including biodegradable and environmental characteristics (Rayas et al., 1997). Protein-based films are effective as gas barriers (O2 and CO2) but their water vapour permeability values are high (Miller and Krochta, 1997; Khwaldia et al., 2004). Their mechanical properties are less interesting when compared with the synthetic polymers, and in some cases they show values less interesting than polysaccharide films (Lacroix and Cooksey, 2005). The hydrophilic character of protein-based films led to a great number of works where the improvement of the barrier and mechanical properties was studied. Protein modifications through enzymatic (transglutaminase) or physical treatments (heating, irradiation) were tested (Ghoparde et al., 1995; Gennadios et al., 1996; Brault et al., 1997; Sabato et al., 2001; Tang et al., 2005). Lipid addition is also an effective way to decrease the hydrophilicity of protein films. Lipids can be added to film formulations by the formation of a bilayer or through lipid emulsions (Lacroix and Cooksey, 2005). It is normal procedure to add plasticizers to the films in order to improve their physical properties (Bergo and Sobral, 2007). They help to decrease brittleness and improve flexibility, through reducing the intermolecular forces and increasing the mobility of polymeric chains (Sothornvit and Krochta, 2001; Rivero et al., 2010). The most usual plasticizers are polyols, mono-, di-, and oligosaccharides. Polyols are shown to be particularity effective in plasticizing hydrophilic hydrocolloids. Glycerol is the most used, but also sorbitol, ethylene glycol and sucrose were tested as plasticizers (Cherian et al., 1995; Cuq et al., 1997; Hernandez-MunÄoz et al., 2004; Bergo and Sobral, 2007). Because of its importance, the effect of plasticizers on protein film properties will be discussed in detail below. However, the use of edible films is limited by their poor performance related to their water sensitivity and limited mechanical properties (Azeredo, 2009). The addition of rigid particles, fibres or composite materials, as nanocomposites, has been shown to be a possibility to improve the properties of edible films (Huang

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and Netravali, 2007; Sorrentino et al., 2007). However, the properties of the composite depend not only on the properties of the components and their relative proportions, but also on the shape, size and distribution of the reinforcement materials (Riande et al., 2000; Sperling, 2006; Sorrentino et al., 2007). The incorporation of active substances as antimicrobials, antifungals and antioxidants is one of the emerging trends in food packaging films. This incorporation often leads to changes in the physicochemical properties of the films (GoÂmez-Estaca et al., 2009; Gemili et al., 2010).

22.3.1 Transport properties The main interest in films and coatings is generally based on their potential to provide some combination of moisture, oxygen, carbon dioxide, flavour, aroma, colour or oil barrier for a food or drug, with a resulting increase in the material quality. Permeability is a steady-state property that describes the extent to which a permeating substance dissolves and then the rate at which the permeant diffuses through a film, with a driving force related to the difference in concentration of the permeate between the two sides of the film (Krochta, 2002). Polymer permeability is influenced by the crystallinity, polarity, density, orientation, molecular weight, free volume and crosslinking of the molecules during film formation (Miller and Krochta, 1997). Water vapour permeability (WVP) The water vapour mass transfer of protein films has been extensively studied, not only because of the importance of control of moisture content in keeping food quality but also because these results can be useful in understanding possible mass transfer mechanisms. Protein films have high permeability to polar substances, such as water vapour. They present high WVP values when compared with wax films, and values two- to four-fold greater than those of lowdensity polyethylene film (Krochta, 2002). Protein-based films without the addition of plasticizers are very brittle due to the strong cohesive energy density of the polymer. Plasticizers such as glycerol are required to increase film flexibility and processability. However, plasticizers generally affect the ability of the system to attract water, leading to the increase of film permeability (McHugh and Krochta, 1994a; Cuq et al., 1997; Lacroix and Cooksey, 2005; Pereda et al., 2009), and several works have shown how the addition of a plasticizer can change water vapour permeability of protein films (Ozdemir and Floros, 2008; Kokoszka et al., 2010a, 2010b). As an example, WVP values of sodium caseinate films were shown to increase exponentially for higher values of glycerol (Pereda et al., 2009). However, some works have supported the theory that the incorporation of low plasticizer concentrations can lead to the decrease of WVP when compared

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with the values of films without plasticizer. These films are very brittle and susceptible to the formation of cracks (Arvanitoyannis et al., 1998; Lacroix and Cooksey, 2005), in which case WVP values are higher once the flow of water molecules through the cracks is obviously improved. Gelatine films with glycerol concentrations between 10 and 40 g per 100 g of gelatine lead to a decrease of the WVP when compared with the unplasticized films. However, when these values are higher than 40 g of glycerol per 100 g of gelatine the presence of plasticizer leads to an increase of WVP values (Rivero et al., 2010). The utilization of different plasticizers can also lead to different properties. Three plasticizers (glycerol, sorbitol and triethanolamine) were used in glutenin films, showing that sorbitol is the most effective plasticizer in order to achieve lower values of WVP (Hernandez-MunÄoz et al., 2004). The same authors showed that WVP values of films plasticized with sorbitol and triethanolamine remained constant during 16 weeks, while for glycerol the WVP values decreased during that period. This phenomenon was explained by the loss of glycerol during storage and was associated with the reduction of water content (Hernandez-MunÄoz et al., 2004). The incorporation of sucrose instead of glycerol was used to decrease WVP values of gluten films while at the same time acting as plasticizer (Cherian et al., 1995). Several works showed that the hydrophilicity of protein films could be improved by the chemical modification of the protein. Crosslinking with aldehydes decreases WVP values of SPI films (Ghoparde et al., 1995) and the crosslinking of calcium caseinate film was successfully used to decrease WVP (Avena-Bustillos and Krochta, 1993). The same authors have shown that the utilization of pH 4.6 during crosslinking leads to lower values of WVP. The utilization of different pH values after the alkali treatment of proteins can also lead to a decrease of WVP values (Brandenburg et al., 1993). Physical treatments such as the heat-curing of protein films showed that this is another method that can be used to decrease the WVP values (Gennadios et al., 1996; Miller et al., 1997; Kim et al., 2002). Also the treatment of calcium caseinate/WPI films with gamma radiation has showed to decrease WVP values, where the continuous increase of gamma-radiation dose leads to lower WVP values (Ciesla et al., 2006). The incorporation/blending of other compounds in protein films has also been studied, especially lipids, polysaccharides and different types of proteins. The synergistic effects between the components are used to improve the properties of protein-based films (Krochta, 2002). Lipids, due to their hydrophobic character, seem to be the most efficient compounds to decrease WVP of protein films. However, depending on the technique used in the production of the films, their properties can be different, even if the same components were used in their preparation. The most used techniques are the deposition of a lipid layer previously molten or solubilized on the protein film used as a support to either the obtained bilayer or the multilayer film; or cast and dry a film forming emulsion

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in which lipids are dispersed in the protein solution, obtaining emulsified edible films (Morillon et al., 2002; PeÂrez-Gago and Krochta, 2005). Oleic acid± beeswax mixtures were dispersed on two different proteins, soy protein isolate and sodium caseinate, resulting in the reduction of the WVP values of those protein-based films (Fabra et al., 2008; Monedero et al., 2009). WVP values of sodium caseinate films were decreased by the addition of saturated fatty acids, subjected to different temperatures and relative humidity values (Fabra et al., 2009). Waxes were described as the most efficient lipids to reduce WVP due to their high content of long-chain fatty alcohols and alkanes with long chains (Morillon et al., 2002). The incorporation of beeswax into sodium caseinate films has been shown to be more effective in reducing WVP than that of stearic acid and acetylated monoglyceride (Avena-Bustillos and Krochta, 1993). Zein film bilayers with medium chain length triglyceride oil (MCTO), sorghum wax and MCTO, or carnauba wax and MCTO were prepared; it was demonstrated that the application of the lipid layer can reduce WVP values by up to 98.7% (Weller et al., 1998). Bilayer films with wheat gluten as the structural layer and beeswax and paraffin as the lipid layers were shown to have WVP values lower than those obtained for low density polyethylene (Gontard et al., 1995). The addition of polysaccharides can also be used to improve the transport properties of protein films. Despite the fact that their hydrophilic character does not allow them to be so efficient as lipids in reducing the water vapour permeability, generally the blending of polysaccharides with proteins is easier to process. The addition of pullulan to WPI was successfully used to decrease WVP values of protein films (Gounga et al., 2007). The addition of potato starch and sodium alginate to WPI improved the transport properties of this kind of protein film (Ciesla et al., 2006). The addition of cellulose to SPI films was also tested, and results showed that the increase of cellulose concentrations led to a decrease of WVP values (Wu et al., 2009). Films of soybean-protein isolate (SPI) with different concentrations of cod gelatine were studied by Denavi et al. (2009). The results showed that films with a 1:1 ratio of SPI and gelatine present the lowest WVP values. The incorporation of micro/nanoparticles and fibres can be used to improve the transport properties of films. WPI films with different amounts of the cloisite 30B organo-clay (0, 5, 10, and 20 g/100 g WPI) were prepared and it was demonstrated that the transport properties of WPI films were changed due to clay incorporation, leading to a decrease of WVP values (Sothornvit et al., 2010). WPI films were blended with biodegradable titanium dioxide (TiO2) and the results showed that the incorporation of TiO2 leads to an improvement of the films' barrier properties (Zhou et al., 2009). Montmorillonite clay particles were added to wheat gluten protein and the resultant films were characterized. The results showed that the clay incorporation could be efficient at decreasing the WVP of those protein films (Olabarrieta et al., 2006; Guilherme et al., 2010). Table 22.1 shows the WVP values obtained for various protein-based films.

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Table 22.1 Comparison of the water vapour permeability (WVP) of edible protein films Test WVP 10ÿ10 References conditions (g mÿ1sÿ1Paÿ1)

Film type Sodium caseinate:glycerol (1:0.15) Sodium caseinate:glycerol (1:0.5) Myofibrillar Myofibrillar:glycerol (40 g/100 g) Wheat gluten:glycerol (15:6) Wheat gluten:glycerol: sucrose (15:3:3) Glutenin:glycerol (3:1)

23ëC, 64.5% RH

1.3

Pereda et al., 2009

23ëC, 64.5% RH

4.2

Pereda et al., 2009

21ëC, 59% RH 21ëC, 59% RH

0.38 0.74

Cuq et al., 1997 Cuq et al., 1997

25ëC, 50% RH 25ëC, 50% RH

14.1 9.5

Cherian, 1995 Cherian, 1995

23ëC, 50% RH

4.3

Glutenin:sorbitol (3:1)

23ëC, 50% RH

0.67

WPI

25ëC, 50% RH

6.5

WPI:Cloisite 30B organoclay (20 g/100 g WPI) SPI: cellulose (9:1) SPI: cellulose (1:9) Corn zein Corn zein:MCT:carnauba wax (1:7:4) (bilayer)

25ëC, 50% RH

4.0

25ëC, 50% RH 25ëC, 50% RH 25ëC, 50% RH 25ëC, 50% RH

1.72 0.8 25.2 0.32

Hernandez-Mu·oz et al., 2004 Hernandez-Mu·oz et al., 2004 Sothornvit et al., 2010 Sothornvit et al., 2010 Wu et al., 2009 Wu et al., 2009 Weller et al., 1998 Weller et al., 1998

Thermoforming parameters in compression moulding do not seem to significantly affect the WVP properties of protein-based films, which still show WVP values orders of magnitude higher than those of polyethylene films. Sothornvit et al. (2003) varied both pressure and temperature during the thermoforming of whey protein isolate (WPI) films plasticized with glycerol or water, but without affecting the WVP properties of thermoformed films. The films obtained were much thicker than the corresponding films produced by solution casting, and displayed much higher WVP values. In contrast, compression-moulding of various layers prepared by solvent casting seems an attractive route to improve WVP properties. In particular, the sandwiching of hydrophilic soy protein films with two layers of hydrophobic corn zein can improve the WVP by 50% (Pol et al., 2002). O2 and CO2 permeability The hydrophilic character of proteins determines the barrier properties of protein films, providing a low permeability to non-polar substances, such as oxygen, aromas and oils. Film permeability can be decreased by the crystallinity, density,

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orientation, molecular weight and crosslinking of the molecules during film formation (Kester and Fennema, 1986; Krochta, 2002; Khwaldia et al., 2004). Since a certain degree of oxygen and carbon dioxide permeability is needed for respiration of living tissues, such as those of fresh and fresh-cut fruits and vegetables, moderate barriers that allow a controlled respiratory exchange (avoiding anaerobic metabolism) can in some cases be more appropriate (Kester and Fennema, 1986; Ayranci and Tunc, 2003). Oxygen (O2) permeability of protein films has been shown to be highly influenced by temperature. Temperature increase leads to higher values of O2 permeability of corn zein, wheat gluten and wheat gluten/soy protein isolate films. This behaviour was explained by the increase of the polymer segments and also by the increase of the energy level of O2 molecules. The WG/SPI film showed the lowest permeability values, ranging between 19.2 and 121:6  10ÿ19 g mÿ1 sÿ1 Pa±1 (Gennadios et al., 1993). The effect of relative humidity (RH) on O2 and CO2 permeabilities of protein-based films was also studied. The results showed that RH had an exponential effect on O2 and CO2 permeabilities of wheat gluten films, the temperature effects being less pronounced. CO2 and O2 permeabilities ranged from 3.872 to 2446 and from 2.464 to 63:04  10ÿ16 g mÿ1 sÿ1 Paÿ1, respectively (Mujica-Paz and Gontard, 1997). The O2 permeability of WPI films was also highly influenced by RH, and results showed that the increase of O2 permeability was greater for films plasticized with glycerol, in comparison with sorbitol (McHugh and Krochta, 1994b). The effects of the drying temperature and pH of film-forming solutions were tested in peanut protein films. The lowest values of O2 permeability were obtained for high values of pH (pH 9) and drying temperature (90ëC), while the highest values of O2 permeability were obtained for a pH of 6 and a drying temperature of 70ëC (Jangchud and Chinnan, 1999). The addition of pullulan to WPI was shown to increase the film permeability to O2 (Gounga et al., 2007).

22.3.2 Water sensitivity Water sensitivity is one of the major problems of protein films, and has been studied by storing films under different values of RH and monitoring moisture content, water activity and water sorption (Fabra et al., 2010). Solubility of films in water may also provide insight into the behaviour of a film in an aqueous environment and is a measure of its water resistance. This is also an important factor that determines the biodegradability of films when used as packaging materials (Gnanasambadam et al., 1997). Films of low solubility are required during storage if they are intended for preservation of intermediate or high moisture foods. Also when, for example, antimicrobial compounds are incorporated in films with poor water resistance, they will dissolve quickly, causing the film to lose its antimicrobial agent; this will increase the diffusion of that

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substance from the surface to the bulk of the food, resulting in a low concentration at the food surface where it should be at its maximum (Ozdemir and Floros, 2008). Myofibrillar protein films showed low solubility in water (2%); however, when the plasticizer content increased the films were more soluble, as expected from the hydrophilic character of the plasticizers used (Cuq et al. 1997). Conversely, gelatine films showed high solubility in water and the incorporation of sunflower oil was successfully used to decrease it from 88% to 80% (PeÂrezMateos et al., 2009). Films with different ratios of SPI and cod gelatine were also studied and exhibited high water solubility, ranging between 87.7% and 81.4%. However, with increasing SPI concentrations, the film water solubility and the soluble protein contents decreased (Denavi et al., 2009). Whey protein based films have been shown to be influenced by protein concentration and by the addition of sorbitol and beeswax: while the addition of protein and beeswax produces films with high water resistance, the increase of sorbitol leads to higher values of water solubility (Ozdemir and Floros, 2008). Also for zein-based films the addition of oleic and linoleic acids decreased the water absorption of the film; linoleic acid was shown to be more effective in decreasing water absorption than oleic acid, which was attributed to the higher polymerization of linoleic acid within the film matrix (Santosa and Padua, 1999). The influence of glycerol and beeswax and/or oleic acid on water sorption of sodium caseinate films was studied under different values of RH. Results showed that the incorporation of glycerol on films promotes the hygroscopic character of the films when aw is higher than 0.43. On the other hand, the incorporation of oil reduces water sorption, the effect of which was especially evident with beeswax incorporation (Fabra et al., 2010). Also for zein films, storage at higher RH values led to high water contents, showing that zein films are greatly influenced by storing conditions (Gillgren et al., 2009). The incorporation of cellulose nanofibrous material into soybean protein isolate (SPI) led to a decrease of the swelling ratio of the corresponding films, which changed from 106% to 22% when cellulose nanofibrous material content increased from 0 to 20% in weight (Chen and Liu, 2008). Other works showed that incorporation of cellulose on SPI films decreased water absorption (Wu et al., 2009). In a different work (Kumar et al., 2008), films were prepared by incorporation of different volume fractions of alkali-treated and untreated banana fibres into soy protein isolate (SPI) with different amounts of glycerol as plasticizer. In this case, the water resistance of the composites increased significantly with the addition of glutaraldehyde that acts as a crosslinking agent; the same happened when fibres were added. The properties of wheat gluten films were tested for wheat gluten alone and upon addition of montmorillonite (MMT) particles. Water uptake and water vapour sorption measurements showed that the presence of MMT led to a significant reduction of the water

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sensitivity of wheat gluten-based films. This effect was attributed to a different protein network in the presence of MMT (Tunc et al., 2007). Extruding protein-based materials at higher temperatures will promote crosslinking, therefore resulting in enhanced water resistance (Pommet et al., 2005). However, this usually comes at the expense of mechanical properties as tougher and more brittle packaging materials are obtained. One way to overcome the depressed mechanical properties is to add a chemical that will enhance water resistance while keeping the protein-based material processable. For that purpose, ZnSO4 was added to soy protein and the resulting mixture showed a 30% decrease in water absorption while keeping good extrusion properties into sheets (Zhang et al., 2001).

22.3.3 Thermal and mechanical properties Protein-based films generally exhibit poor mechanical properties when applications such as wrapping or soft packaging are under consideration. When compared to polyethylene, the targeted mechanical property to reach is a strain at break in the order of 500 to 700%, along with high toughness. Thus, most of the research effort has been aimed at improving flexibility while keeping high toughness figures and good barrier properties. Plasticizers are the common additive used to match the mechanical requirements, therefore many researchers have studied thermal and mechanical properties of edible films as a function of plasticizing. Generally the increase of plasticizer concentration leads to an increase of the polymer mobility (lower glass transition temperature (Tg)) and consequently a decrease of tensile strength (TS) and an increase of the elongation-at-break (EB) (Cherian et al., 1995; Sobral et al., 2002; Bergo and Sobral, 2007; Pereda et al., 2008). Also water has an important role in the thermal and mechanical behaviour of protein films. Modifications at both structural and molecular levels induced by the hydration were tested in gelatine films. They showed three hydration stages: (1) water bound by high-energy sorption centres, (2) structural level and (3) an outermost layer of water that covers the triple helix structure. The increase of the water content (above 14%) leads to the decrease of both Tg and Young's modulus (Yakimets et al., 2005). Tg was evaluated in films of zein, WPI and blends of the two proteins. Results showed that zein films had higher Tg than the whey protein and zein±whey composite films. The lower Tg of the whey protein films in comparison with zein films may be due to the more hydrophilic nature of the former together with the presence of lactose in whey powder. When glycerol and olive oil were added to these films a decrease of Tg values was observed. However, the films containing olive oil showed higher Tg than those containing glycerol (Ghanbarzadeh and Oromiehie, 2008). The incorporation of antimicrobial agents (such as sodium lactate and potassium sorbate) in caseinate films can be used to alter the thermomechanical properties of the films. They act as plasticizers, increasing

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the extensibility of sodium caseinate films but decreasing the Young's modulus and TS (Kristo et al., 2008). One of the successful techniques for the improvement of mechanical properties of films from protein is the (bio)chemical modification of film-forming solutions, which can improve film properties, leading to values comparable to commercial non-biodegradable films (Pavlath and Robertson, 1999). SPI was treated with microbial transglutaminase (MTGase), the effect of the treatment being reflected in the mechanical properties of the films. Results showed that treatment by four units of MTGase per SPI increased the TS values (Tang et al., 2005). The utilization of transglutaminase (TGase) allowed the modification of films made with a combination of soy flour proteins and pectin. The TGase presence increased TS and reduced EB (Mariniello et al., 2003). The effect of a crosslinking agent was tested in properties of gelatine films. Results have shown that glutaraldehyde (in amounts above 1%) can increase EB and TS values of gelatine films (Chiellini et al., 2001). Also pH was shown to influence TS values of protein films. Acidic and alkaline pH values during film formation improved the tensile strength of muscle protein films due to the presence of solubilized muscle protein forms in the film-forming solutions (Hamaguchi et al., 2007). Films composed by different ratios of SPI and cod gelatine were evaluated; those containing 25% SPI and 75% gelatine had the maximum TS, which was 1.8-fold and 2.8-fold greater than those of 100% of gelatine and 100% of SPI, respectively (Denavi et al., 2009). The incorporation of polysaccharides in the protein matrix is also one of the most used methods to change the properties of protein films. Blends of WPI with mesquite gum resulted in reduced TS and increased EB values, improving film manageability (OseÂs et al., 2009). The incorporation of chitosan on sodium caseinate films showed a different behaviour. In this case the incorporation of chitosan led to an increase of TS and a decrease of EB values when compared with the sodium caseinate films. The interactions developed between the cationic polymer (chitosan) and the carboxyl groups of sodium caseinate were proposed to be the reason for the increase of TS (Pereda et al., 2008). Gellan gum (Phytagel) and nanoclay particles were used to improve the mechanical and thermal properties of films from soy protein concentrate (SPC). SPC and Phytagel were mixed together to form a crosslinked structure. The films of Phytagel and SPC (PH-SPC) showed improved TS values and thermal stability as compared to the unmodified SPC. The PH-SPC (40% Phytagel) films with 7% of clay nanoparticles (CPH-SPC) registered an increase of TS and also a higher thermal stability when compared to the unmodified SPC (Huang and Netravali, 2006). The same authors blended SPC with nanoclay particles, and then crosslinked the structure using glutaraldehyde. The modified SPC showed significantly improved mechanical properties (Huang and Netravali, 2007). Films of cellulose and soy protein isolate (SPI) were prepared for different

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ratios of SPI/cellulose. With the increase of cellulose content in SPI films, TS, EB, and thermal stability of the SPI/cellulose blended films increased (Wu et al., 2009). Also, cellulose nanofibrous mats were used to reinforce SPI films. The incorporation of 20% of cellulose nanofibres in the SPI matrix resulted in a great improvement of TS and Young's modulus by respectively 13 and six times more than SPI films alone (Chen and Liu, 2008). In another experiment, mechanical tests revealed the antiplasticizing effect of TiO2 particles in WPI films. Small amounts (<1%) of TiO2 particles increased significantly the tensile properties of those films, while the addition of higher amounts (>1%) of TiO2 decreased their tensile properties (Zhou et al., 2009). The influence of pH was tested on wheat gluten films with the incorporation of montmorillonite clay (MMT). Films prepared at pH 11 with combination of MMT were the strongest, the stiffest and the most brittle (Olabarrieta et al., 2006). In a different work SPC was blended with nanoclay particles, and then crosslinked with glutaraldehyde. The increase of clay concentrations combined with the crosslinking reactions led to the increase of TS and EB values (Huang and Netravali, 2007). SPI films were fabricated with the addition of ramie fibres. The results showed that the increase in fibre length (>5 mm) and fibre weight content (>10%) led to an increase of TS and Young's modulus and a decrease of EB (Lodha and Netravali, 2002). The same type of protein was reinforced by the incorporation of different volume fractions of alkali-treated and untreated banana fibres, with different amounts of glycerol as plasticizer. Results showed that TS and Young's modulus of SPI reinforced with the alkali-treated fibre increased by 82% and 963%, respectively, compared to soy protein film without fibres (Kumar et al., 2008). Caseinate films were reinforced with two lignocellulosic fibres: wood pulp fibre and flax bast fibre. Young's modulus increased from 200 MPa to 1200 MPa when 20% of fibre was added to the film. Simultaneously, TS increased from 6 to 30 MPa, for films with 0 and 20% of fibre, respectively (Fossen et al., 2000). Pressure, temperature and holding time used for thermoforming whey protein sheets do not significantly affect the mechanical properties (Sothernvit et al., 2007). However, Table 22.2 shows that for nearly similar formulations, the process used to produce films dramatically changes the mechanical properties. A systematic comparison between solution casting and compression moulding suggested that improved mechanical properties are obtained in terms of enhanced tensile strength and elongation at break (Sothernvit et al., 2007). Extrusion seems to perform even better if elongation at break is the film's mechanical property to optimize. Extruded whey protein isolate films plasticized with glycerol (Hernandez-Izquierdo et al., 2008) show doubled elongation at break when compared to film obtained from cast solution (Sothornvit et al., 2009). Film blowing is the most popular cost-effective process to produce thin films and pouches, and low density polyethylene grades specifically designed for this

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Table 22.2 Comparison of the tensile strength (TS), elongation-at-break (EB) and Young's modulus (YM) of protein films Film type

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Test conditions

TS (MPa)

EB (%)

Soy protein concentrate Soy protein concentrate:Phytagel (40%) Soy protein concentrate:Phytagel (40%):clay(7%) Soy protein isolate (5%):glycerol (2.5%) Soy protein isolate:glycerol (10%) (extrusion) Soy protein isolate:glycerol (40%) (extrusion) Soy protein isolate:glycerol (40%) (compression moulding) Caseinate:glycerol (17%) Caseinate:glycerol (17%):wood pulp fibre (20%) Caseinate:glycerol (17%):flax bast fibre (20%) Soy protein isolate: cellulose (9:1) Soy protein isolate: cellulose (1:9) Whey protein isolate: glycerol (2:1) Whey protein isolate: glycerol (2:1) (extrusion)

21ëC, 65% RH 21ëC, 65% RH 21ëC, 65% RH 25ëC 25ëC, 50% RH 25ëC, 50% RH 25ëC

14.7 50.1 74.5 1 41 9 2.6

20ëC, 60% RH 20ëC, 60% RH 20ëC, 60% RH 20ëC, 70% RH 20ëC, 70% RH 25ëC, 50% RH

5 30 33 10.7 38.1 3.4 3.5

48.6 108.1 50.9 121

Whey protein isolate: glycerol (2:1) (compression moulding) Whey protein isolate: glycerol:Cloisite 20A (2:1.5%) Whey protein isolate: glycerol:Cloisite Na+ (2:1.5%) Soy flour/pectin Soy flour/pectin/TGase Wheat gluten:glycerol (15:6) Wheat gluten:glycerol: sucrose (15:3:3) Zein:polyethylene-glycol (75:25) (film blowing)

23ëC, 50% RH

4

94

25ëC, 50% RH

1.55

29.1

115.5

Sothornvit et al., 2009

25ëC, 50% RH

2.98

42.4

109.3

Sothornvit et al., 2009

23ëC, 50% RH 23ëC, 50% RH 25ëC, 50% RH 25ëC, 50% RH

6.8 12.4 4.2 3.8 0.04

25.7 14.8 9.5 275 3 159 74.5

89 11 270

YM (MPa) 201 717 30 1226 176 190 1050 600 171.8 37 60

11.61 7.21 5.7

Reference Huang and Netravali, 2006 Huang and Netravali, 2006 Huang and Netravali, 2006 Chen and Liu, 2008 Zhang et al., 2001 Zhang et al., 2001 Cunningham et al., 2000 Fossen et al., 2000 Fossen et al., 2000 Fossen et al., 2000 Wu et al., 2009 Wu et al., 2009 Sothornvit et al., 2009 Hernandez-Izquierdo et al., 2008 Sothornvit et al., 2007

Mariniello et al., 2003 Mariniello et al., 2003 Cherian et al., 1995 Cherian et al., 1995 Oliviero et al., 2010

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plastic transformation are marketed. Common strains at break are of the order of 500±700% and are achieved through a complex tailoring of process parameters and macromolecular architecture (namely, long-chain branching which promotes enhanced strain hardening of the melt). Recently, thermoplastic zein was successfully processed into extruded tubes to be further blown into films. The mechanical properties of the film are still below those of conventional plastics (see Table 22.2), but the film-blowing ability of such zein-based materials plasticized with polyethylene glycol, directly in the extruder, will turn this biodegradable protein-based material into a commercially attractive alternative to petroleum-based packages. Furthermore, Oliviero et al. (2010) studied the structural properties of zein batches which showed best performance in film blowing. They concluded that the optimum strain-hardening behaviour of the plasticized paste was correlated with a large content of -helices. This means that besides the enhanced mechanical properties achieved though the addition of plasticizers, the secondary structural properties of proteins and process-induced crystalline structures play an important role in the toughness and stretching of the final film. The storage time is one of the problems of protein films. Some works showed that the ageing of films leads to deterioration and breakdown of their properties due to the loss of plasticizers. Wheat gluten and corn-zein films with glycerol and poly(ethylene glycol) as plasticizers after 20 days of storage presented changes in mechanical properties (Park et al., 1994a). Also the mechanical properties of whey protein films plasticized with glycerol were changed after four months of storage; however, when glycerol was replaced by sorbitol they remained stable (Anker et al., 2001). Table 22.2 shows values for mechanical parameters of protein films. Despite the problems still encountered in protein-based films when mechanical properties are considered, if biodegradable protein films with satisfactory mechanical properties and good appearance are obtained they can become potential and ecological alternatives for replacing synthetic packaging materials in food and pharmaceutical applications.

22.3.4 Opacity and colour parameters Opacity provides an indication of how much light passes through a film; this may be crucial in cases where it is important to control the incidence of light on the food products. Also, the film colour can be an important factor in terms of consumer acceptance. In the L* a* b* colour system, L* represents the lightness, and a* and b* are colour coordinates, where ‡a* is in the red direction, ÿa* is in the green direction, +b* is in the yellow direction, ÿb* is in the blue direction, low L* is dark, and high L* is light. Also the whiteness index, the yellowness index and the total colour difference (
E) are often used to characterize the colour of films.

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The addition of oil decreased the transparency of caseinate films. Oleic acid presented higher values of transparency than fatty acids and beeswax, possibly due to the heterogeneity of the latter two components when present in the film's structure (Fabra et al., 2009). The same behaviour was observed for gelatine films when sunflower oil was incorporated: the films without oil were transparent, but when oil was added the transparency decreased by approximately 10% (PeÂrez-Mateos et al., 2009). The colour evaluation of WPI films showed that the incorporation of mesquite gum decreased lightness (L*) and increased a* and b*, leading to more reddish and yellowish films (OseÂs et al., 2009). SPI films reinforced with cellulose nanofibrous material showed high visible light transmittance with values over 75% at 700 nm (Chen and Liu, 2008). The incorporation of clay composite appears to influence colour film properties. WPI/Cloisite 30B organoclay films with different amounts of clay resulted in films with an opaque appearance, which depended on the amount of clay added (Sothornvit et al., 2010). The drying temperature and pH of film-forming solutions can also influence the colour parameters of protein films. Peanut protein films were obtained at different pH values and dried under different temperatures. Results showed that colour parameters L* and a* decreased with increasing pH, thus influencing the darkness of the film, while b* increased with increasing temperature, leading to more yellowish films (Jangchud and Chinnan, 1999).

22.4

Applications

Edible films can be useful as barriers to gases (water vapour, oxygen, carbon dioxide, aromas). Nevertheless, when possessing the appropriate mechanical properties, they can also be useful for food protection, reducing bruising and breakage, for example, and thus improving food integrity. Recently, edible films have also been used as a vehicle for antioxidants and/or antimicrobials, enhancing their functional properties (Han and Gennadios, 2005; GoÂmez-Estaca et al., 2009; Guillard et al., 2009; Lu et al., 2009).

22.4.1 Incorporation of functional compounds Antimicrobial compounds Antimicrobial packaging materials may be classified in two types: those that contain antimicrobial agents that migrate to the surface of the packaging material and thus can contact the food; and those that are effective against food surface microbiological growth without the migration of the active agent to the food surface (Brody et al., 2001). This means that different packaging strategies can be adopted. The bioactive compound may be (1) incorporated directly into the polymer; (2) adsorbed onto polymer surfaces; (3) immobilized in the

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polymers through ion or covalent linkages; or (4) not used, when the polymers making part of the packaging material are already inherently antimicrobial (Appendini and Hotchkiss, 2002). Protein-based films can be used as a solid thin layer that is subsequently applied on the food. Their utilization as coatings presents great advantages. They are applied in the liquid form on a food by dipping, spraying or brushing (Krochta, 2002; Zhao and McDaniel, 2005). They can also serve as carriers for a wide range of food additives, including colourants, flavours, antibrowning agents, spices, nutrients and various antimicrobials that can extend product shelf-life and reduce the risk of pathogen growth on food surfaces (Baldwin et al., 1996; Han, 2000; Lee, 2005). Incorporating antimicrobial compounds into edible films or coatings provides a way to improve the safety and shelf-life of ready-to-eat foods (Cagri et al., 2004). Various antimicrobial agents may be incorporated in the packaging system, such as chemical antimicrobials, natural antimicrobials, antioxidants and antimicrobial polymers (Appendini and Hotchkiss, 2002), among others. Several works have tested these functional compounds against some of the most problematic microorganisms in the food industry, often showing good results. WPI films with particles of Cloisite 30B showed a bacteriostatic effect against Listeria monocytogenes (Sothornvit et al., 2009). SPI films combined with grape seed extract, nisin and EDTA showed the greatest inhibitory activity against L. monocytogenes, Escherichia coli O157:H7 and Salmonella typhimurium, with reductions of 2.9, 1.8 and 0.6 log CFU/mL, respectively (Sivarooban et al., 2008). The antimicrobial properties of WPI films containing oregano, rosemary and garlic essential oils were tested against E. coli O157:H7, Staphylococcus aureus, S. enteritidis, L. monocytogenes and Lactobacillus plantarum. All of the essential oils are shown to be effective against those bacteria; however, oregano essential oil was the most effective when used at a 2% level (Seydim and Sarikus, 2006). Film blends of casein and starch were used to incorporate neem (Melia azadirachta) extract, which was shown to be effective in inhibiting the growth of E. coli, S. aureus, B. cereus, L. monocytogenes and Pseudomonas spp. (Jagannath et al., 2006). Antioxidants Oxidation can seriously limit food preservation, and is involved in one of the most important degradation reactions in foodstuffs (NerõÂn et al., 2008). The use of antioxidants is therefore very important in the food industry in order to decrease food oxidation and thus increase shelf-life. Protein-based films can provide a good vehicle for antioxidant application, incorporated in a film or in a coating (Zhao and McDaniel, 2005). Gelatine (tuna-fish) films were successfully enriched with murta extracts and showed ability to act as antioxidant carriers (GoÂmez-Estaca et al., 2009). Films

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of SPI with ferulic acid showed antioxidant activity, presenting an optimal concentration of 100 mg of ferulic acid per 100 g of SPI in the film-forming solution (Ou et al., 2005). Calcium caseinate and WPI films were prepared with a mixture of calcium lactate and gluconate and -tocopherol acetate, showing capabilities of carrying high concentrations of these antioxidant compounds (Mei and Zhao, 2003). Other compounds Application to agriculture can also be foreseen for biodegradable plastics as long as water absorbance and controlled release of fertilizers are inherent properties of the covering layers. Wheat gluten was mixed with glycerol and water in order to obtain a dough-like product to be compressed and moulded at high temperature (GoÂmez-MartõÂnez et al., 2009). KCl and citric acid were also added to the mixture during compounding in order to play the role of the released agent and plasticizer, respectively. Compression moulding of the optimal formulations in terms of rheological properties led to the successful production of sheets that were screened for their water absorption and KCl release properties. It was found that citric acid slowed down the release of KCl and significantly increased the water absorption of the sheets. From the mechanical testing of sheets, the authors hypothesized that the addition of citric acid actually modified the microstructure of the bioplastic. However, the exact mechanism of this additive on the slow release and improved water uptake still needs to be identified.

22.4.2 Applications to foods Protein-based films and coatings were tested in different food products in order to extend their shelf-life. The following is a summary. · Casein protein coatings were successfully used to reduce water loss in zucchini (Avena-Bustillos et al., 1994). · WPI was used to coat dry roasted peanuts and was shown to decrease their oxidative deterioration (Mate et al., 1996). Also WPI used with acetylated monoglyceride was shown to reduce peroxide and moisture loss values in stored frozen king salmon (Stuche and Krochta, 1995). · Corn-zein films delayed ripening and colour change in tomatoes during storage (Park et al., 1994b). · Edible films composed of soybean protein, stearic acid and pullulan were used to preserve kiwifruit; results showed that the use of edible film retarded the senescence process, the softening rates of coated and uncoated kiwifruit being 29% and 100%, respectively, after 37 days' storage (corresponding to a three-fold extension of the shelf-life of the product) (Xu et al., 2001). · Films of wheat gluten and a bilayer of lipids have shown a significant effect

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on the retention of firmness, and reduced the weight loss of strawberries. However, the appearance and taste of bilayer-coated fruits were unacceptable (Tanada-Palmu and Grosso, 2005). Films of collagen and galactomannan have been used to decrease the gas transfer rate of fruits (apple and mango), giving good indications for fruit preservation (Lima et al., 2010). Edible coatings of WPC decreased the initial respiration rate of minimally processed apples at 25ëC. Later, WPI coatings with an antibrowning (CaCl2) agent were shown to effectively increase the shelf-life of minimally processed apples by 2 weeks at 3ëC (Lee et al., 2003). Milk protein-based edible films containing oregano, paprika or mixed oregano±paprika essential oils were applied on beef muscle slices to control the growth of pathogenic bacteria and increase the shelf-life during storage at 4ëC. The film containing oregano was the most effective against bacteria (Pseudomonas spp. and E. coli O157:H7), whereas the film containing paprika oil seemed to be the least effective against those two bacteria. Films containing oregano extract showed 0.95 and 1.12 log reductions against Pseudomonas spp. and E. coli O157:H7, respectively, when compared to samples without film (Oussalah et al., 2004). Muscle protein films with a combination of palm oil and chitosan were used to cover dried fish powder. Samples showed lower thiobarbituric acid reactive substances and lower yellowness than other samples during extended storage up to 21 days (Artharn et al., 2009). WPI films incorporating different levels of oregano oil were used to increase the shelf-life of fresh beef (5ëC, 12 days). Wrapping of beef cuts with the antimicrobial films resulted in a decrease of colour changes, and while the maximum growth rate and total flora of Pseudomonas spp. were reduced, the growth of lactic acid bacteria was completely inhibited (Zinoviadou et al., 2009). The same authors used sodium lactate and 3-polylysine in WPI films against the spoilage in fresh beef. They showed that the total flora was reduced with antimicrobial films containing 3-polylysine, while for films with sodium lactate a decrease of growth of the total flora and Pseudomonas spp. was observed (Zinoviadou et al., 2010).

Protein-based films and coatings can be successfully used in enhancing and maintaining the quality of food products through increasing shelf-life and improving safety. The main problems associated with these materials are their mechanical and transport properties, together with their water solubility: though significant progress has been made, those properties are still not all at a satisfactory level when compared to, e.g., petroleum-based materials. It is expected, however, that the advantages mentioned above combined with the environmental friendliness of using biodegradable materials will certainly foster further research and application of protein-based resins for packaging.

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Future trends

In recent years great progress has been made in developing edible films from protein sources. Protein-based films present a good barrier to gases (oxygen and carbon dioxide), are promising vehicles for a great number of functional compounds, and have been shown to be usable in different food products in order to extend their shelf-life. Protein-based film production through solution casting is the method most often used; however, the `dry process' using equipment already used in commercial plastics has been tested and shown to be a promising method (within limits) for protein-based film production. The main drawbacks of protein-based films are their poor water vapour barrier, mechanical resistance and thermal properties. As already reported above, nanotechnology can be a feasible way to improve these properties. Particles on the nanoscale can affect film properties: they can be used to improve mechanical strength, increase heat resistance and decrease permeability to gases. Also the utilization of nanofilms and multi-nanolayer films and coatings holds promise for food applications, mainly as part of food preservation and safety strategies. The use of this technology is envisaged, e.g., in systems aimed at integrating the sensing, localization, reporting and remote control of food products and for the encapsulation of functional food ingredients (e.g. multi-nanolayer films could include various functional agents such as antioxidants, antimicrobials, flavours, enzymes, etc.). A more significant research effort should be undertaken in order to understand how the utilization of materials on the nanoscale can change the properties of protein-based film and show how such materials can affect, e.g., thermoplastic processing steps in order to make this an alternative to their production by conventional casting. In conclusion, the utilization of multinanolayered protein-based films/coatings is essentially unexplored and is one of the future trends in applying nanotechnology to protein-based packaging materials.

22.6

References

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Wheat gluten (WG)-based materials for food packaging H. ANGELLIER-COUSSY, V. GUILLARD, C . G U I L L A U M E and N . G O N T A R D , University of Montpellier II, France

Abstract: This chapter discusses the use of wheat gluten (WG) based materials for food packaging. It presents the two technological processes to prepare WG-based materials and reviews the ways to modulate mechanical and mass transfer properties, with a specific section devoted to WG-based nanocomposites. The chapter also includes a case study of using WG-based materials (paper coated by wheat gluten) as modified atmosphere packaging for fruits and vegetables. Key words: wheat gluten, food packaging, nanocomposites, barrier properties.

23.1

Introduction

Wheat gluten (WG) is a by-product of the wheat starch industry which is extensively used for both food and non-food applications. The use of wheat gluten for non-food applications is part of a trend to produce biodegradable materials with a large range of functional properties. WG has been widely investigated as a protein source because it is annually renewable and readily available at a reasonable cost (between 1 and 1.3 ¨/kg). With respect to its unique viscoelastic and film-forming properties, WG is an interesting raw material that can be used as a food packaging material. It is fully biodegradable without releasing toxic products (Domenek et al., 2004a). WG based-films present an attractive combination of strength and flexibility (Angellier-Coussy et al., 2008a; Cuq et al., 2000), a high gas (oxygen and carbon dioxide) permeability in dry condition and a significant gas perm-selectivity at high relative humidity (Gontard et al., 1996; MujicaPaz and Gontard, 1997; Guillaume et al., 2010), and good grease (Guillaume et al., 2010) and aroma barrier properties (Chalier et al., 2007a) which are key functional properties for food quality preservation. They also are translucent (Angellier-Coussy et al., 2008a) and can be heat-sealed (Cho et al., 2007). A downside to the use of WGbased materials is their per se reactivity and thus lower inertia than most

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conventional petrochemical-based plastics. WG-based films are sensitive towards water due to the hydrophilic nature of many amino acids constituting the protein chains and to the substantial and necessary amount of added hydrophilic plasticizer (glycerol). As a consequence, their mechanical properties and water vapour barrier properties in high moisture conditions are relatively poor as compared to synthetic films such as low-density polyethylene (PDL Handbook Series, 1995). Improving moisture resistance and mechanical properties are two of the critical issues in the development of WG-based materials for sustainable food packaging applications. Many studies have already been devoted to exploring the ways to improve the mechanical properties of WG-based materials, but only a few papers deal with the modulation of mass transfer properties which are key properties for food packaging purposes. This chapter will first give an overview on the main technological approaches in preparing WG-based materials. The mechanical and mass transfer properties of WG-based materials are discussed. The case of WG-based nanocomposites is considered in the following section. Finally, an example of integrated approach for the development of adequate food packaging will be presented. It will deal with the use of paper coated by wheat gluten as material for modified atmopshere packaging of fruits and vegetables like parsley having a high respiration activity.

23.2

Preparation of wheat gluten-based materials

Wheat gluten proteins consist of monomeric gliadins with a molecular weight ranging from 15,000 to 85,000 and a mixture of glutenin polymers with a molecular weight between about 80,000 to several million. Although proteins themselves are already heteropolymers, with -amino acids being their monomer units, the terms monomeric and polymeric refer in this case to the quaternary structure of the proteins. Gliadins represent a heterogeneous mixture of single-chained or monomeric gluten proteins, while glutenins consist of peptide chains associated through interchain disulfide bonds. While gliadins are readily extractable in aqueous alcohols, glutenins are partly insoluble in most common solvents due to their large size. However, their subunit building blocks have solubilities comparable to those of gliadins. The glutenin subunits can be obtained by treatment of glutenin with a disulfide reactive agent. WG proteins can undergo disulfide interchange upon heating, which leads to the formation of a covalent three-dimensional macromolecular network. For glutenins, crosslinking reactions occur above 60±70ëC, whereas for gliadins the reactive zone is evidenced around 90ëC (Domenek et al., 2002; Schofield et al., 1983). The reactivity of the WG proteins towards chemicals and temperatures permits the preparation of WG-based materials by using two technological approaches: either a solvent-based process, also called casting, or a common thermo-

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23.1 Schematic representation of the two technological processes used to prepare wheat gluten-based materials.

mechanical process based on the thermoplastic properties of WG. In both processes, the formation of a macromolecular network from proteins requires three steps (Fig. 23.1). The first one consists in disrupting intermolecular bonds that stabilize polymers in the native state. This first step enables the second one, which consists in rearranging and reorienting the polymer chains (shaping), leading to the formation of a three-dimensional network stabilized by new interactions and bonds after the agent that ruptures intermolecular bonds is removed (Cuq et al., 1998).

23.2.1 Solvent-based process The solvents used to prepare protein film-forming solutions are generally based on water and ethanol (Gontard et al., 1992, 1993). Dispersing proteins in solvents may require the addition of disruptive agents, pH adjustment by the addition of acids or bases, or ionic strength control by electrolyte addition. The functional properties of protein-based films prepared by casting depend on protein concentration in solution, pH, additives, solvent polarity, drying rate and temperature (Cuq et al., 1998). This process involves the use and elimination of large amounts of solvent, which could not be in line with the promotion of low cost and low environmental impact process, except for specific applications such as coatings.

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23.2.2 Thermo-mechanical process Thermal processing of wheat proteins has become a challenge for polymer material scientists as this technique can avoid the usage of solvents. Furthermore, as this technique enables the use of common processes like extrusion, it ensures subsequent take-up in existing converting thermo-mechanical processes at plant scale. Wheat proteins could be considered as partially thermoplastic polymers that could be changed in a reversible way from a rigid state to a soft state through a temperature increase and the plasticization through the addition of small polar molecules. In that regard, WG-based materials can be shaped by existing plastics processing machinery including extrusion (Hochstetter et al., 2006; Redl et al., 1999), lamination, roller milling or thermomolding (AngellierCoussy et al., 2008a; Gallstedt et al., 2004; Sun et al., 2008), which are all available at the industrial scale. The difficulty in extruding these materials is due to the strong self-association among protein chains through inter/intramolecular interactions and cross-linking via disulfide bonds. In addition the properties of wheat proteins are significantly modified on heating before melting. Consequently, a large amount of plasticizer is always required in thermal processing to reduce the strong intra/intermolecular interactions among wheat protein chains. Water is the most ubiquitous plasticizer of WG because of its high capability to modify the mobility of proteins, whereas glycerol is the other common residual plasticizing agent having a high retention in the WG-based material due to its high boiling point and strong hydrogen bonding with proteins (Zhang et al., 2005).

23.3

Mechanical and barrier properties of wheat gluten-based materials

23.3.1 Barrier properties of WG-based materials The particular interest of WG-based films compared to usual plastic films is their gas permeability and selectivity which are sensitive to temperature but more particularly to relative humidity (MujicaPaz and Gontard, 1997). At low relative humidity (RH), WG-based films display impressive gas barrier properties, especially towards O2. The increasing RH effect was attributed to a modification of the wheat gluten network structure and polymeric chain mobility, related to a change from a glassy to a rubbery state (Gontard and Ring, 1996). The increase of CO2 permeability with RH was much more pronounced than that of O2 permeability (Fig. 23.2). This phenomenon was explained by a selective sorption of CO2 due to the specific interactions setting up between carbon dioxide and the water-plasticized protein matrix, especially high content amide groups of wheat gluten protein (Pochat-Bohatier et al., 2006). At high RH value, adsorption of water should provide a better accessibility to active sites of CO2 sorption located on the mobile polymeric protein chains. Consequently,

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ß Woodhead Publishing Limited, 2011 23.2 Effect of temperature and relative humidity on carbon dioxide permeability, oxygen permeability, selectivity (CO2/O2) and ethylene permeability of a wheat gluten film (adapted from MujicaPaz and Gontard, 1997, and Paz et al., 2005).

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WG-based films exhibit a large range of selectivity values (from 3 to 28) as a function of moisture content, contrary to the selectivity of most synthetic films which is usually varying between 4 and 6 (MujicaPaz and Gontard, 1997). The high selectivity value of WG-based films (28 at 24ëC and 100% RH) and moreover a high ethylene permeability (Paz et al., 2005) could be very interesting for fresh or minimally processed fruits and vegetables preservation under modified atmosphere (Fig. 23.2). However, because of their hydrophilic nature, WG-based materials display a poor water resistance, which is revealed by an important swelling when they are immersed in liquid water (Domenek et al., 2004b; Tunc et al., 2007) and a high water adsorption in high moisture conditions. Their water vapour barrier properties (water vapour permeability (WVP) for a 0±100% RH difference ranging from 5  10ÿ12 mol.mÿ1.sÿ1.Paÿ1 to 6:2  10ÿ11 mol.mÿ1.sÿ1.Paÿ1 depending on the film preparation process; Gontard et al., 1993; Pommet et al., 2003) are also relatively poor compared to those of synthetic films such as lowdensity polyethylene (WVP ˆ 0:05  10ÿ12 mol.mÿ1.sÿ1.Paÿ1; PDL Handbook Series, 1995). Furthermore, their mechanical properties and water barrier properties are strongly affected by the presence of water or other plasticizers (Gontard et al., 1993), which restrict their utilization to a targeted range of applications. Decreasing water vapour permeability is one of the main critical issues in the development of biopolymers for extending potential food packaging applications.

23.3.2 How to modulate the functional properties of WGbased materials To widen the end-uses of wheat gluten based-films, their functional properties (especially mechanical and water barrier properties) must be improved. It is known that mechanical properties of WG-based materials mainly depend on the type and density of intra- and intermolecular interactions, but also from interactions with other constituents (Guilbert et al., 2002). Globally, when covalent bonds, such as disulfur bonds, stabilize the network or when the density of bond energy is high, WG-based films are resistant and relatively elastic (Guilbert et al., 2002). Thus, the most common studied way to alter gluten functionality, especially mechanical properties, is to modulate the degree of protein crosslinking. Crosslinking reactions can be induced by chemical (vapours of formaldehyde) (Micard et al., 2000), thermal (Ali et al., 1997; Angellier-Coussy et al., 2008a; Cuq et al., 2000; Micard et al., 2000; Song et al., 2008), enzymatic (Lai and Chiang, 2006; Larre et al., 2000; Wang et al., 2005) and radiation (Micard et al., 2000) treatments. These treatments were applied either as `pre-treatment' (Ali et al., 1997; Lai and Chiang, 2006), meaning that changes occurred in the film-forming solution, as `posttreatments', i.e. applied on the final films (Micard et al., 2000), or during the

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treatment itself as in the case of the thermo-mechanical process (AngellierCoussy et al., 2008a). Heat treatment, which is often combined with pressure and shear, is the most efficient and common way to modify the structure of WG-based materials as it avoids the use of solvents and chemical compounds. For example, increasing thermoforming temperature (from 60 to 120ëC) led to a rise in both the resistance and the deformability of WG-based materials due to the establishment of a covalent network. The increase in deformability was attributed to the thermo-mechanical treatment in a two-blade counter-rotating batch mixer inducing shearing that may favour the parallel orientation of protein chains, thus leading to an increase in the mean length of the polymer (Angellier-Coussy et al., 2008a). This result was also observed by Sun et al. (2008). Concerning the casting process, some authors have also tried to optimize the conditions. Gontard et al. (1992) showed that WG films made at low pH (pH 4 with acetic acid) from an ethanol solution were stronger than films obtained from alkaline conditions. Under those high pH conditions, the reduction of disulfide bonds to sulfydryl groups is favoured, thus allowing the film to stretch further. Later, Kayserilioglu et al. (2001) and Zhang et al. (2006) confirmed this result. They showed that alkaline conditions (pH 11 with NaOH) caused some level of deamidation of WG that modified the chemical and aggregation structure, enhancing intermolecular interactions between water and all components in WG (proteins, starch, lipids), and thus resulting in a more stable crosslinked WG network with strong intermolecular interactions. Figure 23.3 highlights that all the possible treatments previously listed, which influence the crosslinking degree of proteins, allow one to achieve a quite large window of mechanical properties and an attractive combination of strength and deformability as compared to polystyrene (PS) and biodegradable polyesters such as polylactic acid (PLA) and polycaprolactone (PCL). However, the mechanical properties of WG-based materials remain low compared to those of conventional plastics like polyolefins (Fig. 23.3). If the mechanical properties of WG-based films can be greatly improved by increasing the crosslinking degree of proteins, their permeability to water vapour, gases and organic volatile compounds as well as their solubility in water are usually less affected. Correlated to the mechanical changes observed, the wheat gluten solubility in water decreases with the increase of temperature (Angellier-Coussy et al., 2008a). Water vapour permeability is also reduced, but in a limited range, with more severe heat treatment (Ali et al., 1997) or thermoforming temperature (Angellier-Coussy et al., in press). Changes in water vapour transport properties (permeability, sorption and diffusion) were directly related to changes in the structure of the films. It was shown that an increase in temperature from 80ëC to 120ëC led to a decrease in both swelling and WVP values. This was related to an increase in the crosslinking degree of WG-based films resulting in a more locked structure less prone to chain mobility and

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23.3 Mechanical properties of wheat gluten-based materials (u) as compared to conventional plastics (ú) and biodegradable polyesters (s) (adapted from Angellier-Coussy et al., 2008b).

rearrangement in contact with water. The decrease in WVP was only related to a decrease in the apparent diffusivity of water, since the thermoforming temperature had no significant effect on the moisture sorption isotherms (AngellierCoussy et al., in press). The development of bilayer or multilayer films associating wheat gluten with lipids or another polymeric layer (synthetic polymer or biopolymer) which affords water resistance and/or mechanical resistance is a third strategy to extend the functional properties of wheat gluten films. For example, in order to tentatively increase the functional properties and notably the moisture resistance of WG-based films, edible composite films comprising wheat gluten as the structural matrix and various concentrations of different lipids as the moisture barrier component were tested (Gontard et al., 1994). Among the 10 lipids tested, beeswax was the most effective lipid for improving moisture barrier properties of the films (WVP was reduced by 74% as the beeswax concentration was increased from 0 to 36.8 g.100 gÿ1 dry matter). But these films were opaque, weak and disintegrated easily in water. Combining wheat gluten proteins with diacetyl tartaric ester of monoglycerides (20 g.100 gÿ1 dry matter) reduced WVP about 50%, increased strength and maintained transparency. Solid lipids such as beeswax or paraffin wax could be also deposited in a molten state onto the base film in a thin layer to form bilayer films. These films were proved to be more efficient than composite film of the same formulation (Table 23.2). For instance, a film consisting of wheat gluten, glycerol and diacetyl tartaric ester of monoglyceride as one layer, and beeswax as the other, yielded a water vapour permeability of 2:32  10ÿ14 mol.mÿ1.sÿ1.Paÿ1, which was less than that obtained with low density polyethylene (Gontard et al., 1995). Another example of such association is the realization of a bilayer film made of a layer of functionalized polyethylene (for example, ethylene/acrylic ester/

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maleic anhydride terpolymer or ethylene/glycidyl methacrylate copolymer) and a layer of pure wheat gluten with the aim of obtaining a bilayer film combining the low water vapour permeability of the polyethylene layer with the high gas permeability of the wheat gluten layer (Irissin-Mangata et al., 1999). The use of functionalized polyethylene had no influence on film opacity, and was effective in reducing dispersion in water and water vapour permeability of pure wheat gluten films. However, even if the high selectivity ratio of gluten films was preserved, the O2 and CO2 permeability values of the gluten/terpolymer bilayer were lower than those of pure gluten films, which reduce the window of application of the bilayer. Tables 23.1 and 23.2 show the gas and water vapour permeabilities of various WG-based films. A last example of association with the aim of improving mechanical properties of WG consists in reinforcing the protein matrix by associating WG to paper, exploiting their naturally occurring compatibility due to the hydrophilic nature of both constituents. WG proteins can be used as functional coatings for paper, the latter acting as a mechanical support packaging (Chalier et al., 2007a; Gastaldi et al., 2007; Guillaume et al., 2010). Moreover, due to the biodegradable feature of proteins, both the recyclability and biodegradability attributes of the paper would be maintained, unlike most of the synthetic coatings currently used for cellulosic substrates. Using wheat gluten as coating for paper (considered as porous) by using either a thermomoulding process (Gallstedt et al., 2005) or casting (Guillaume et al., 2010) underlined the necessity to have a continuous and thick enough coating to significantly improve Table 23.1 Oxygen and carbon dioxide permeabilities (10ÿ18 mol.mÿ1.sÿ1.Paÿ1) of wheat gluten-based films O2 perme- CO2 perme- T RH ability ability (ëC) (%) Paper/wheat gluten

8328

16927

25

Wheat gluten (casting)

1970

55580

24 100

Low density polyethylene 1078 Wheat gluten/DATEM* 790 Wheat gluten/beeswax 687 Wheat gluten (casting) 152

4134 11142 6614 545

20 100 25 93 25 91 24 50

Wheat gluten/DATEM* and beeswax (bilayer film) Wheat gluten/beeswax and beeswax (bilayer film) Polyamide 6 EVOH

11

76

25

56

Guillaume et al., 2010 MujicaPaz and Gontard, 1997 Charles et al., 2003 Gontard et al., 1996 Gontard et al., 1996 MujicaPaz and Gontard, 1997 Gontard et al., 1996

10

61

25

56

Gontard et al., 1996

10 6

(±) (±)

80

Reference

23 100 23 95

*DATEM is diacetyl tartaric ester of monoglyceride.

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Table 23.2 Water vapour permeability (WVP, 10ÿ12 mol.mÿ1.sÿ1.Paÿ1) of wheat gluten-based films WVP


RH (%)

T (ëC)

Wheat gluten (casting) Wheat gluten/refined paraffin Wheat gluten/carnauba wax Wheat gluten (casting) Wheat gluten/soy protein Wheat gluten/DATEM Wheat gluten/mineral oil Wheat gluten/beeswax Low density polyethylene

5.31 4.10 3.86 3.11 2.84 2.32 2.28 1.83 0.048

100±0 100±0 100±0 11±0 11±0 100±0 11±0 100±0 95±0

30 30 30 23 23 30 23 30 38

Wheat gluten and beeswax (bilayer film)

0.023

100±0

30

Reference Gontard et al., 1994 Gontard et al., 1994 Gontard et al., 1994 Gennadios et al., 1993a Gennadios et al., 1993b Gontard et al., 1994 Gennadios et al., 1993a Gontard et al., 1994 PDL Handbook Series, 1995 Gontard et al., 1995

the gas barrier properties of the paper and to manage to come near the oxygen permeability of the pure wheat gluten film. On the contrary, when the wheat gluten coating (deposited by casting) penetrated deeply into the paper, the resulting composite structure displayed transfer properties (water vapour, O2 and CO2) equivalent to those of pure wheat gluten film (Guillaume et al., 2010). The composite paper/wheat gluten structure thus appears more efficient than a strictly bilayer structure.

23.4

Wheat gluten-based nanocomposites

The development of food packaging materials with tailored barrier properties has necessitated study of the structure of such materials at the nanometric scale. In this context, many studies have been devoted to the use of hybrid organic± inorganic systems and, in particular, to those in which layered silicates are dispersed at a nanometric level in a polymeric matrix (Giannelis, 1996). The nanoscale plate morphology of layered silicates and other fillers often promotes improved physical properties of biopolymers, including improved mechanical properties, thermal stability and gas barrier properties (Alexandre and Dubois, 2000). This strategy has been recently applied to WG-based materials to modulate their mechanical and barrier properties (Angellier-Coussy et al., 2008a; Guilherme et al., 2010; Olabarrieta et al., 2006; Tunc et al., 2007; Zhang et al., 2007) as well as to create antimicrobial delivery systems (Mascheroni et al., 2010). All the studies devoted to WG-based nanocomposites involve the use of non-modified or organo-modified montmorillonites. The properties of these nanocomposite materials are directly related to their structure and, in this respect, the state of dispersion (intercalation/exfoliation) of the inorganic phase seems to be a key parameter (Ray and Bousmina, 2005).

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23.4.1 Preparation and structure of WG-based nanocomposites Several methods have been considered to prepare polymer/layered silicate nanocomposites with an optimal dispersion state of nanofillers (Alexandre and Dubois, 2000). Two of them are compatible with the production of WG-based materials: (1) the melt-intercalation technique, which is compatible with the preparation of protein-based materials via a thermo-mechanical process based on the thermoplastic properties of wheat gluten, and (2) the intercalation of proteins from solution, which is compatible with the preparation of WG-based materials via casting. So far the latter technique has been preferred due to a higher facility to exfoliate MMT nanoparticles due to the capacity of layered silicates to swell in an appropriate solvent. In both cases, layered silicates are mixed to wheat gluten in the first step of the WG-based materials preparation (Fig. 23.1). The state of dispersion of nanoparticles within the WG matrix is evaluated by wide-angle X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM), which are the two tools most frequently used. Other techniques such as FTIR are not used because of the difficulty of interpreting data related to the chemical complexity of the WG. It is shown that intercalated/ exfoliated systems could be achieved using either the solvent or the meltintercalation technique. However, the level of exfoliation depends greatly on the nature of the montmorillonites. Almost fully exfoliated structures were obtained in the case of non-modified sodium MMT (Nanofil EXM 757 from SuÈd-Chemie (from Tunc et al., 2007) or Na+ Cloisite from Southern Clay Products, Inc. (from Olabarrieta et al., 2006)) whereas the presence of both stacks on TEM pictures and a d001 peak characteristic of an intercalated structure on XRD patterns evidenced that organo-modified montmorillonites (Cloisite 10A (Olabarrieta et al., 2006) and Cloisite 30B (Zhang et al., 2007) from Southern Clay Products, Inc.) were not completely exfoliated. This highlighted the fact that the affinity between wheat proteins and nanoclays is a key parameter governing the state of dispersion of the nanoclays. The use of non-modified MMTs which are hydrophilic seems to be more appropriate.

23.4.2 Properties of WG-based nanocomposites All studies demonstrated that the introduction of MMT nanoparticles resulted in a significant improvement in mechanical strength and resistance accompanied by a decrease in deformability. For example, the addition of 5 wt% of nonmodified MMT in WG led to an almost threefold increase of the Young's modulus and the stress at break (Tunc et al., 2007). It appeared that an optimal filler content of 5 wt% was required to achieve the greatest improvement of mechanical properties to allow layered silicates to form a connected threedimensional network. In the case of organo-modified MMT, the quaternary

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alkylammonium could act as a plasticizer in the WG matrix, which might play a negative role in enhancing the strength of the nanocomposites (Zhang et al., 2007). Combinations of wheat gluten proteins with non-modified MMT have been demonstrated to efficiently reduce the moisture sensitivity of WG (Olabarrieta et al., 2006; Tunc et al., 2007). For example, the liquid water uptake in cast WGbased nanocomposite films was halved when 10 wt% of MMT was added in the matrix (Tunc et al., 2007). A decrease in WVP from 1:6  10ÿ11 to 0:6  10ÿ11 mol.mÿ1.sÿ1.Paÿ1 between the neat matrix and the nanocomposite (from 5 to 10 wt% of MMT) may be related to the establishment of hydrophilic interactions between gluten proteins and nanoclays, which resulted in a lower availability of the hydrophilic sites for water vapour. As regards O2 and CO2 permeability (evaluated at 25ëC and 80 and 90% RH), the addition of MMT into WG had no significant effect on the permeability of the films towards gases (O2 and CO2) over the range of studied MMT contents (up to 10 wt%) (Tunc et al., 2007). It was pointed out that O2 and CO2 permeabilities of nanocomposite materials increased with increasing RH. This phenomenon was also observed in pure wheat gluten films (Gontard and Ring, 1996) suggesting that the neat matrix of wheat gluten was the key element in the gas barrier properties of the studied materials. Increased barrier properties towards 2-heptanone were also observed for MMT contents 5 wt%. This decrease in permeability was attributed to a tortuosity effect liable for a significant decrease in diffusivity with increasing MMT contents. WG-based nanocomposite systems were also used for the controlled release of active agents. It has been shown that the release of a volatile compound, carvacrol, from paper coated with wheat gluten is RH-dependent (Mascheroni et al., 2010). Such behaviour is very interesting both for limiting volatile active agent losses before using the material as food packaging, and for triggering the active agent release in the presence of the food. In this study (Mascheroni et al., 2010), the release of carvacrol was assessed at 25ëC as a function of time and using a two-step gradient of relative humidity. It was shown that WG/paper materials lost more than 70% of carvacrol within 20 days of storage at 60% RH. This means that only 30% of the active agent would be available for release towards the food during its storage within the packaging. Once placed at 100% RH, this 30% was entirely released within 8 days. When MMT nanoparticles were introduced in the WG coating layer, only 20% of the carvacrol was released during the 20 days of storage at 60% RH. Consequently 80% of the volatile active agent remained available for release during the period in which food is packaged. Once placed at 100% RH, this 80% was entirely released within 13 days (from day 22 to day 35) (Mascheroni et al., 2010). However, if these novel materials are used in commercial applications, how should such materials be proven to comply with regulations? An important issue is the potential release of engineered nanomaterials (ENM) from nanocomposite

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materials. In Mauricio-Iglesias et al. (2010), it is found that the content of silicon increased for the three aqueous food simulating liquids (FSL) in contact with WG/MMT. In the case of 3% acetic acid, the levels of aluminium also increased. In this study as in previous ones (Avella et al., 2005), the results were obtained by elementary analysis. Therefore, it is impossible to know whether the `whole ENM' has migrated, or only a part, or how the ENM migrated. The toxicity of ENM depends on a large number of factors including their structure, surface area, particle number, charge, chemistry, size and size distribution, state of aggregation, shape and elemental composition (Oberdorster et al., 2005). Such a characterization of ENM in food or FSL represents a real challenge for the current analytical methodologies (although recently Tiede et al. (2009) reported that wet scanning electron microscopy was suitable for characterizing ENM in liquid environment).

23.5

Example of integrated approach for the packaging of fresh fruits and vegetables

Preservation of fresh or minimally processed fruits and vegetables still constitutes one of the most challenging applications for food packaging materials: this living produce is sensitive to over-maturation and physiological disorders due to the surrounding atmospheric composition. The use of modified atmosphere packaging (MAP) can reduce these losses by creating a favourable surrounding atmosphere able to slow down the respiration metabolism. Atmospheric composition within the package is the result of both respiration of the commodity and diffusion/permeation of gases through the film until a steady modified atmosphere is reached (Floros and Matsos, 2005). This steady atmosphere should be close to an optimal gas concentration specific to each produce. Hence, gas transfer properties of the packaging must correctly match physiological requirements of the commodity and could thus be called `passive MAP'. A favourable MAP can also be created actively by using packaging material able to absorb, for example, oxygen (Charles et al., 2003) or to release volatile active agent, e.g. antimicrobial agents (Matan et al., 2006; Valero et al., 2006; Valverde et al., 2005), for creating a so-called active MAP. In passive MAP, most conventional plastic films exhibit too low gas permeability, leading to a rapid and sharp drop in O2 followed by detrimental anaerobic conditions (Zagory and Kader, 1988). Therefore, they need to be perforated (with macro- or micro-perforations) to allow sufficient gas exchange. In that case, CO2 diffuses through the film at a similar rate to O2 and therefore the gas permselectivity value (ratio of CO2 permeability to O2 permeability) of perforated materials is close to 1. As an alternative to conventional plastics, the development of WG-based materials appears to be of great interest because of their high gas permeability and permselectivity values that are able to create a unique low oxygen and low carbon dioxide atmosphere, adapted to the

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preservation of fresh fruits and vegetables, especially CO2-sensitive commodities (Barron et al., 2002; Gontard et al., 1996). In active MAP, WG-based materials have also been demonstrated to make efficient antimicrobial packaging for their ability to control, in a relevant way, the release of volatile extracts of various essential oils (allylisothiocyanate, carvacrol, cinnamaldehyde or eugenol) (Ben Arfa et al., 2006, 2007; Chalier et al., 2007b). We present below an illustration of combining wheat gluten proteins with paper and MMT. Nanocomposite and composite materials were manufactured by coating a support paper with a wheat gluten layer containing MMT (nanocomposite) or not (composite). Characteristics and permeability of these materials were studied and passive MAP experiments were performed for assessing the effect on the quality of a real food: parsley (Gontard et al., 2008). O2 and CO2 permeabilities of WG/paper-based materials were evaluated at 25ëC and at 80% and 90% RH, in comparison with control uncoated paper, and results are presented in Table 23.3. The continuous layer formed by WG proteins onto the support paper greatly reduced the gas permeability of the paper. The coated paper could thus no longer be considered porous. O2 and CO2 permeability values were not significantly affected by the presence of MMT in the wheat gluten network, whatever the RH. Since permeability is known to be governed by two mechanisms, diffusion and sorption, it was assumed that introduction of MMT did not change solubility nor diffusivity of O2 and CO2, as observed in a previous study (Tunc et al., 2007). It should be pointed out that, in Table 23.3, O2 and CO2 permeability of both composite and nanocomposite materials increased with increasing RH. This phenomenon was also observed on pure wheat gluten films (Gontard et al., 1996), suggesting that the wheat gluten-based coating layer is the key element in the gas barrier properties of the studied materials. The increasing RH effect was attributed to a modification of the wheat gluten network structure and polymeric chain mobility, related to a change from Table 23.3 Gas permeability and permselectivity ( ) of control paper, composite and nanocomposite paper-based materials at 25ëC as a function of relative humidity (RH %) RH (%)

PO2*

PCO2*



Control

80 90

> 5  109 > 5  109

> 5  109 > 5  109

Composite

80 90

5210 8512

10130 67790

1.9 7.9

Nanocomposite

80 90

6400 9000

9870 68050

1.5 7.5

1 1

* O2 and CO2 permeability (PO2 and PCO2) expressed in amol.mÿ1.sÿ1.Paÿ1. Mean standard deviation is 300 and 400 amol.mÿ1.sÿ1.Paÿ1 for PO2 and PCO2 respectively.

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a glassy to a rubbery state. The increase of CO2 permeability was more pronounced than that of O2 permeability. This phenomenon was explained by a selective sorption of CO2 due to the specific interactions setting up between carbon dioxide and the water-plasticized protein matrix, especially high content amide groups of wheat gluten protein (Pochat-Bohatier et al., 2006). At high RH value, adsorption of water should provide better accessibility to active sites of CO2 sorption located on the mobile polymeric protein chains. As a consequence, gas permselectivity was greatly affected by RH and rose from 1.9 to 7.9 and from 1.5 to 7.5 for composite and nanocomposite materials, respectively. These results show that the unique gas permselectivity properties are preserved when combined with paper or MMT. Passive MAP experiments were conducted on parsley with the uncoated control paper and the nanocomposite material at 20ëC. As expected for a highly porous material, O2 and CO2 partial pressures at the steady state obtained when using control paper were close to those in the composition of air (21 and 0 kPa respectively). Such an atmosphere was detrimental to the quality of the product. After only four days of storage, more than 50% of ascorbic acid and chlorophyll were lost, and leaves were fully yellow (data not shown). In comparison, nanocomposite material generated a headspace atmosphere containing lower O2 (11 kPa) and higher CO2 (4 kPa) content. This steady atmosphere was in agreement with the atmosphere recommended by UC Davis1 and clearly improved the quality attributes of parsley during storage, by maintaining a high chlorophyll content (directly linked to the green colour of the herb) and ascorbic acid during eight days of storage. For consumers, the shelf-life of parsley is mainly determined through its discoloration. Sensory analysis results were perfectly correlated to chlorophyll content. A critical level of 1.5 mg/g of fresh parsley (corresponding to 70% of the initial content) was reached after only two days of storage when it was packed at 20ëC using uncoated paper. If a nanocomposite material was used, discoloration was delayed to more than eight days. As regards ascorbic acid, one of the major components of parsley, preserving at least 60% of initial vitamin C content could be considered as a critical level. It was reached after only three days of storage with uncoated paper against more than eight days for composite material. The use of nanocomposite materials for MAP of parsley led to an equilibrium atmosphere favourable for maintaining the quality of the parsley, by slowing down the oxidation reactions and physiological reactions that are responsible for product degradation.

1. University of California Davis, Postharvest Technology Research and Information Center, http:// [email protected].

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23.6

Multifunctional and nanoreinforced polymers for food packaging

Future trends

It can be concluded from this chapter on wheat gluten-based materials for food packaging that there is still an important need for improved knowledge on how `food and agropolymer sciences' can facilitate the development of efficient WGbased packaging for food. To make certain that the developed packaging materials will optimally fulfil the requirements of the food industry, compounds producers, packaging converters, food retailers, waste management and legislative bodies and consumers, future projects should adopt a holistic approach to preparing the ground for addressing the long-term development of WG-based materials for food packaging. It can be reasonably expected that the next studies will include the identification of targeted food products and the quantification of their needs in terms of packaging properties (mechanical and gas transfer properties, moisture resistance, stability towards pH, temperature, light, etc.) to increase their shelflife. An essential step in the application of WG-based packaging by the food industry is thus to develop integrated studies of the process±structure±properties relationships of WG-based materials based on the latest innovative developments for the characterization of complex polymeric materials. The next projects will also be focused on developing mathematical modelling tools to calculate how the structure±function relations at different scales will determine the end properties.

23.7

References

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atmosphere conditions', International Journal of Food Microbiology, 107, 180± 185. Mauricio-Iglesias M, Peyron S, Guillard V and Gontard N (2010), `Wheat gluten nanocomposite films as food-contact materials: Migration tests and impact of a novel food stabilization technology (high pressure)', Journal of Applied Polymer Science, 116, 2526±2535. Micard V, Belamri R, Morel M H and Guilbert S (2000), `Properties of chemically and physically treated wheat gluten films', Journal of Agricultural and Food Chemistry, 48, 2948±2953. MujicaPaz H and Gontard N (1997), `Oxygen and carbon dioxide permeability of wheat gluten film: Effect of relative humidity and temperature', Journal of Agricultural and Food Chemistry, 45, 4101±4105. Oberdorster G, Maynard A, Donaldson K, Castranova V, Fitzpatrick J, Ausman K, Carter J, Karn B, Kreyling W, Lai D, Olin S, Monteiro-Riviere N, Warheit D, Yang H et al. (2005), `Principles for characterizing the potential human health effects from exposure to nanomaterials: Elements of a screening strategy', Particle Fibre Toxicology, 2, 8. Olabarrieta I, Gallstedt M, Ispizua I, Sarasua J R and Hedenqvist M S (2006), `Properties of aged montmorillonite±wheat gluten composite films', Journal of Agricultural and Food Chemistry, 54, 1283±1288. Paz H M, Guillard V, Reynes M and Gontard N (2005), `Ethylene permeability of wheat gluten film as a function of temperature and relative humidity', Journal of Membrane Science, 256, 108±115. PDL Handbook Series (1995), Permeability and Other Film Properties of Plastics and Elastomers, Plastics Design Library (PLD), Norwich, NY. Pochat-Bohatier C, Sanchez J and Gontard N (2006), `Influence of relative humidity on carbon dioxide sorption in wheat gluten films', Journal of Food Engineering, 77, 983±991. Pommet M, Redl A, Morel M H and Guilbert S (2003), `Study of wheat gluten plasticization with fatty acids', Polymer, 44, 115±122. Ray S S and Bousmina M (2005), `Biodegradable polymers and their layered silicate nano composites: In greening the 21st century materials world', Progress in Materials Science, 50, 962±1079. Redl A, Morel M H, Bonicel J, Vergnes B and Guilbert S (1999), `Extrusion of wheat gluten plasticized with glycerol: Influence of process conditions on flow behavior, rheological properties, and molecular size distribution', Cereal Chemistry, 76, 361± 370. Salame M (1986), `Barrier polymers', in The Wiley Encyclopedia of Packaging Technology, Bakker M (ed.), Wiley, New York, 48±54. Schofield J D, Bottomley R C, Timms M F and Booth M R (1983), `The effect of heat on wheat gluten and the involvement of sulfhydryl-disulfide interchange reactions', Journal of Cereal Science, 1, 241±253. Song Y H, Zheng Q and Lai Z Z (2008), `Properties of thermo-molded gluten/glycerol/ silica composites', Chinese Journal of Polymer Science, 26, 631±638. Sun S M, Song Y H and Zheng Q (2008), `Thermo-molded wheat gluten plastics plasticized with glycerol: Effect of molding temperature', Food Hydrocolloids, 22, 1006±1013. Tiede K, Tear S P, David H and Boxall A B A (2009), `Imaging of engineered nanoparticles and their aggregates under fully liquid conditions in environmental matrices', Water Research, 43, 3335±3343.

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Tunc S, Angellier H, Cahyana Y, Chalier P, Gontard N and Gastaldi E (2007), `Functional properties of wheat gluten/montmorillonite nanocomposite films processed by casting', Journal of Membrane Science, 289, 159±168. Valero D, Valverde J M, Martinez-Romero D, Guillen F, Castillo S and Serrano M (2006), `The combination of modified atmosphere packaging with eugenol or thymol to maintain quality, safety and functional properties of table grapes', Postharvest Biology and Technology, 41, 317±327. Valverde J M, Guillen F, Martinez-Romero D, Castillo S, Serrano M and Valero D (2005), `Improvement of table grapes quality and safety by the combination of modified atmosphere packaging (MAP) and eugenol, menthol or thymol', Journal of Agricultural and Food Chemistry, 53, 7458±7464. Wang J S, Zeng Y W and Zhao M M (2005), `Development and physical properties of film of wheat gluten cross-linked by transglutaminase', Journal of Wuhan University of Technology ± Materials Science Edition, 20, 78±82. Zagory D and Kader A A (1988), `Modified atmosphere packaging of fresh produce', Food Technology, 42, 70±77. Zhang X Q, Burgar I, Do M D and Lourbakos E (2005), `Intermolecular interactions and phase structures of plasticized wheat proteins materials', Biomacromolecules, 6, 1661±1671. Zhang X Q, Hoobin P, Burgar I and Do M D (2006), `pH effect on the mechanical performance and phase mobility of thermally processed wheat gluten-based natural polymer materials', Biomacromolecules, 7, 3466±3473. Zhang X Q, Do M D, Dean K, Hoobin P and Burgar I M (2007), `Wheat-gluten-based natural polymer nanoparticle composites', Biomacromolecules, 8, 345±353.

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Safety and regulatory aspects of plastics as food packaging materials B A L D E V R A J and R . S . M A T C H E , Central Food Technological Research Institute, India

Abstract: Polymeric materials are used extensively in food packaging. In addition to the basic polymers, plastics also contain additives added in small quantities to alter the properties of the polymers in the desired way and simplify their processing. These additives along with low-molecular-weight non-polymeric components, which may remain in plastic packaging materials, possess high mobility. It is likely that some transfer of these lowmolecular-weight non-polymeric components will occur from the plastic packaging material into the packaged content, thereby contaminating the product with the risk of toxic hazard to the consumer. This chapter reviews guidelines for proper use of plastics for food packaging applications and discusses the specific migration of some of the toxic additives like acetaldehyde, terephthalic acid, methyl ethyl glycol and bisphenol-A. Nanocomposites are also used in food packaging materials. There are many safety concerns about nanomaterials, as their size may allow them to penetrate into cells and eventually remain in the system. Manufacturers have to follow good manufacturing practice using only the additives listed in the positive list. Prior to categorizing such plastics as toxic, evidence regarding degree of migration of their constituents has to be ascertained. In general, migration and extraction studies need to be simultaneously conducted on actual foodstuffs under conditions that are slightly more stringent than those encountered in normal usage. Hence, for good measure, the overall migration of all the migrants put together is considered for safe use, unless they are especially toxic and their specific limits are fixed by the regulatory authorities such as: Bureau of Indian Standards, the European Commission Directives, and the Code of Federal Regulations of the US Food and Drug Administration. Key words: food contact materials (FCMs), indirect additives, antimicrobial agent, migration, safety nanocomposites, legislation, food stimulants, toxic additives, GRAS.

24.1

Introduction

The global retail market is flourishing day by day with different innovative and designed polymeric materials and items. Today, plastic has almost replaced metal, wood, glass and paper in the field of packaging but there is no substitute for plastics. Plastic is one of the greatest inventions of the last millennium. There has been enormous development in the field of food packaging with plastics.

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Food requires protection against various environmental factors from the time of its production till it is consumed. Hence, packaging is required to protect the food. The shelf-life of packaged foods may vary from a few days to more than a year. Thus, the properties of packaging material must have sufficient permanence to assure that shelf-life is not compromised (Matche and Baldev, 2005/06; Vijayalakshmi and Baldev, 2010). In addition to the basic polymers, plastics also contain additional chemical components, called additives, which are added in small amounts to alter the properties of the polymers in the desired way and/or to simplify their processing. Only fillers and softeners (plasticizers) are used at high concentration to increase volume and/or weight to improve softening flexibility, elasticity, malleability and processability. Other additives are mostly low-molecular-weight components like stabilizers, antioxidants, antistatic agents, light stabilizers (UV absorbers), lubricants (slip agents), optical brighteners, etc. Polymer packaging materials may also contain small quantities of monomers, oligomers as well as polymerization catalysts and regulators, crosslinking agents, emulsifying agents, etc. These additives along with lowmolecular-weight non-polymeric components, which may remain in plastic packaging materials, possess high mobility. It is likely that some transfer of these low-molecular-weight non-polymeric components will occur from the plastic packaging material into the packaged content, thereby contaminating the product with the risk of toxic hazard to the consumer. However, it is to be remembered that useful properties of the plastics are not manifested without the addition of these additives. Therefore, guidelines for proper use of plastics for food packaging applications have been realized all over the world, which are necessary to safeguard the health of consumers (Baldev, 2001).

24.2

Indirect food additives

Concern over the safety-in-use of plastics as food packaging materials arises principally from the possible toxicity of other low-molecular-weight constituents that may be present in the plastics and hence may be leached into the foodstuff during storage. As stated above, such constituents arise from two sources. Polymerization residues include monomers, oligomers (with a molecular weight less than 200), catalysts (mainly metallic salts and organic peroxides), solvents, emulsifiers, wetting agents, raw material impurities, plant contaminants, inhibitors, decomposition and side-reaction products. The more volatile gaseous monomers, e.g. ethylene, propylene and vinyl chloride, usually fall in concentration with time, but very low levels may persist in the finished product almost indefinitely. Styrene and acrylonitrile residues are more difficult to remove. Processing aids such as antioxidants, antiblock agents, antistatic agents, heat and light stabilizers, plasticizers, lubricants and slip agents, pigments, fillers, mould release agents and fungicides are added to assist production processes or to enhance the properties and stability of the final product. They may be present in

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amounts varying from a few parts per million up to several percent (Crosby, 1981, Robertson, 2005; Jenkins and Harrington, 1991). Since compounds of the first group are present inadvertently, there is not a lot that can be done to remove them. However, the efforts made by the industries to reduce vinyl chloride monomer levels, in particular, illustrate the advantages of optimum manufacturing processes on the purity of the final product. Chemicals added deliberately during formulation to alter the processing, mechanical or other properties of the polymer are likely to be present in greater amounts than polymerization residues and should be subjected to strict quality control. They are normally restricted to compounds appearing on an approved list for food contact use. A brief mention of the function of some major additives is presented below.

24.2.1 Antiblock agents These agents are added to roughen the surface of thin films and, hence, prevent them sticking together during machine processing. Silica is most commonly used because its poor solubility in most polymers helps to increase the surface concentration and so introduces irregularity. Similarly, slip additives such as fatty acid amides are used to reduce mobility.

24.2.2 Antioxidants These additives prevent degradation of the polymer by reacting with atmospheric oxygen during moulding operations at high temperatures or when used in contact with hot foods, and to prevent deterioration during storage. Derivatives of phenols and organic sulphides are most frequently used as antioxidants. Some of these compounds are classified as heat stabilizers.

24.2.3 Antistatic agents Since all plastics are good electrical insulators and are in fact used on a large scale for this purpose, they will retain electrostatic charges produced by friction from contact with processing machinery. Accumulation of static electricity can cause problems through the pick-up of dust, adhesion between layers or particles of plastics, sparking, electrical shock and possibly fire hazards. Most antistatic agents are glycol derivatives or quaternary ammonium compounds, which increase the electrical conductivity and plate-out onto the surface of plastic.

24.2.4 Lubricants These are added to reduce frictional forces and are usually low to medium molecular weight hydrocarbons. They should possess good solubility in the plastic, low volatility and be relatively stable compounds.

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24.2.5 Plasticizers Plasticizers are added to make the product more flexible and less brittle. They are usually high molecular weight esters. The plasticizer also gives the material the limp and tacky qualities found in `cling' films. PVC containts about 20±30% of plasticizers. Typically phthalic esters such as dioctyl phthalate (DOP), also known as di-2-ethylhexyladipate (DEHA), are used as plasticizers.

24.2.6 Ultraviolet stabilizers UV stabilizers are needed to protect the product from deterioration by sunlight or even supermarket lighting. It is not only the finished packaging material but food products containing nutrients such as vitamin C that are also susceptible to this form of deterioration. Different UV stabilizers are utilized depending upon the substrate, intended functional life, and sensitivity to UV degradation. UV stabilizers, such as benzophenones, work by absorbing the UV radiation and preventing the formation of free radicals. Depending upon substitution, the UV absorption spectrum is changed to match the application. Concentrations normally range from 0.05% to 2%, with some applications going up to 5%.

24.2.7 Optical property modifiers The optical properties of a material from a technological aspect are normally described in terms of their ability to transmit light, to exhibit colour and reflect light from the surface (i.e. gloss). The majority of virgin food packaging films are unpigmented but some are coloured by the addition of colourants. The principal pigments for use as colourants in packaging materials are carbon black, white titanium dioxide, red iron oxide, yellow cadmium sulfide, molybdate orange, ultramarine blue, blue ferric ammonium ferrocyanide, chrome green, and blue and green copper phthalocyanins.

24.2.8 Foaming agents There are two types of foaming or blowing agents: physical and chemical, which are used for the production of cellular products. In the physical process gas is generated to produce the cells; this takes place through a physical transition, i.e. evaporation or sublimation. In a chemical process, decomposition reactions take place which result in evolution of gases. In food packaging applications, physical blowing agents are normally used. In expanded and extruded polystyrene foams fluorocarbon or light aliphatic hydrocarbon such as pentane is used as a blowing agent.

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24.2.9 Antimicrobial agents Antimicrobial packaging is playing an important role in the inhibition of pathogenic contamination in foods, thereby extending the shelf-life of foods. With the potential in providing food quality and safety, antimicrobial packaging is gaining a lot of interest in research and development. The major potential food applications of antimicrobial films include some for sensitive foods like bakery products, dairy products (cheese), fresh produce such as fruits and vegetables, and meat, fish and poultry products. Antimicrobials such as algicides, bactericides and fungicides can be added to polymers to prevent the growth of microorganisms inside the package. However, their use in food packaging is rare because of the possibility of migration into the food itself. Regulations might require some amendments related to toxicology and testing of antimicrobial compounds for the newer materials, as they might not be covered under the regulations.

24.3

Nanotechnology in food contact materials

Food contact materials (FCMs) and articles including packaging materials, cutlery, dishes, processing machines, containers, etc., intended to come into contact with foodstuffs, are based on metal/metal oxide nanoparticles and nanoclays. Nanotechnology in food contact materials is a newer area to be developed for the future. At present there is some hesitation to incorporate nanomaterials because of the uncertainty of future regulations and standards and for fear of negative consumer reactions (Lyndhurst, 2009). The report also indicates that attitudes to novel food technologies in the USA and Asia seem to be generally more positive than in Europe. Nevertheless, there is a possibility that the general public's attitude to nanotechnologies in food packaging might be less negative than to nanotechnologies incorporated into food itself. The use of nanotechnologies in food packaging in Europe is in principle sufficiently regulated by Commission Directive 2004/1935/EC which covers all materials coming in contact with foodstuffs. According to this Commission Directive, individual Member States may ask the European Food Safety Authority (EFSA) to conduct a safety evaluation of food contact materials. Food contact plastics are subject to additional measures regulated by Commission Directive 2008/282/ EC on recycled plastic materials and articles, and by Commission Directive 2009/450/EC which sets down additional requirements to Commission Directive 2004/1935/EC for active and intelligent materials and articles. Finely dispersed nanosilver particles permanently embedded in plastic containers significantly reduce bacterial growth by 99% and ensure safer, fresher and tastier food. Nanotitanium particles are used as antibacterial agents in ultrafine filters which can capture and eliminate bacteria and odours from up to 99% of the particles and ensure that fresh and purified air is circulated through the fridge compartments (for instance, Hitachi's Advanced Multi Flow system).

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Aluminium foil, widely used in flexible packaging for food with surface properties improved by anti-adhesive coating or black coating by nanotechnology, is used as baking foil which does not reflect heat in an oven. Moreover, ZnO nanoparticles do not discolour, nor do they require ultraviolet to get activated. These properties make nano-ZnO a superior non-organic antibacterial agent (Observatory Nano, 2009). With respect to the use of nanoparticles of the additive in the polymer matrix, there is no reason to believe that `adequately' modified nanocomposites making use of substances in positive lists can impose any immediate risk for food-contact applications; however, studies concerning potential migration issues and life-cycle analysis have to be undertaken to corroborate the fact (LagaroÂn et al., 2005). In silver-based nanoclay polylactic acid film, migration levels of silver, within the specific migration levels referenced by the European Food Safety Authority (EFSA), exhibit antimicrobial activity, supporting the potential application of this biocidal additive in active foodpackaging applications to improve food quality and safety (Busolo et al., 2010).

24.4

Migration of additives

The ingredients in plastic packaging materials may cause toxicity as a result of their migration to the foodstuffs that are packed in them. Therefore positive lists of constituents (additives) permitted in the respective plastics used in contact with foodstuffs, pharmaceuticals and drinking water have been specified by the Bureau of Indian Standards (BIS) (Table 24.1). Manufacturers have to follow good manufacturing practice (GMP), using only such additives as are listed in the positive list. Prior to categorizing such plastics as toxic, evidence regarding the degree of migration of their constituents has to be ascertained. In general, migration and extraction studies need to be simultaneously conducted on actual foodstuffs under conditions that are slightly more stringent than those encountered in normal usage. It is, however, not always possible to analyse actual foodstuffs for the nature and quantity of migrants from the plastics. In order to simplify such assessment, food simulants/extractants have to be substituted for the actual foodstuffs. Further, it is also very difficult to estimate all the migrants individually. Hence, for good measure, the overall migration of all the migrants put together is considered for safe use, unless they are especially toxic and their specific limits are fixed (Bureau of Indian Standards (BIS) IS:9845-1998; Commission Directive 2002/72/EC; US Food and Drug Administration (US FDA) Code of Federal Regulations (CFR) 21,176.170).

24.4.1 Migration model The extent of migration of a substance depends on its concentration in the material, the degree to which it is bound or mobile within the matrix of the material, the thickness of the packaging material, the nature of the food with

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Table 24.1 Indian standards for plastics in contact with foodstuffs, pharmaceuticals and drinking water IS no.

Title

10171:1999

Guide on suitability of plastics for food packaging (second revision) List of pigments and colorants for use in plastics in contact with foodstuffs, pharmaceuticals and drinking water Determination of overall migration of constituents of plastics materials and articles intended to come in contact with foodstuffs ± method of analysis (second revision) Polyethylene for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polyethylene in contact with foodstuffs, pharmaceuticals and drinking water (first revision) Positive list of constituents of polypropylene and its copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water (first revision) Polypropylene and its copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polystyrene (crystal and high impact) in contact with foodstuffs, pharmaceuticals and drinking water Polystyrene (crystal and high impact) for its safe use in contact with foodstuffs, pharmaceuticals and drinking water (first revision) Positive list of constituents of polyvinyl chloride and its copolymers for safe use in contact with foodstuffs, pharmaceuticals and drinking water Polyvinyl chloride (PVC) and its copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ionomer resins for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Ionomer resins for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ethylene/acrylic acid (EAA) copolymers for their safe use in contact with foodstuffs, pharmaceuticals and drinking water Ethylene/acrylic acid (EAA) copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polyalkylene terephthalates (PET & PBT) for their safe use in contact with foodstuffs, pharmaceuticals and drinking water Polyalkylene terephthalates (PET & PBT) for its safe use in contact with foodstuffs, pharmaceuticals and drinking water

9833:1981 9845:1998 10146:1982 10141:1982 10909:1984 10910:1984 10149:1982 10142:1999 10148:1982 10151:1982 11435:1985 11434:1985 11705:1986 11704:1986 12229:1987 12252:1987

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Table 24.1 Continued IS no.

Title

12248:1998

Positive list of constituents of Nylon-6 polymer for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Nylon-6 polymer for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ethylene vinyl acetate (EVA) for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Ethylene vinyl acetate (EVA) copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ethylene methacrylic (EMMA) copolymer and terpolymers in contact with foodstuffs, pharmaceuticals and drinking water Ethylene methacrylic and (EMMA) copolymer and terpolymers for their safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polycarbonate resins in contact with foodstuffs, pharmaceuticals and drinking water Polycarbonate resins for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of melamine-formaldehyde resins in moulded articles in contact with foodstuffs, pharmaceuticals and drinking water Melamine-formaldehyde resins in moulded articles in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of modified poly(phenylene oxide) (PPO) in contact with foodstuffs, pharmaceuticals and drinking water Modified poly(phenylene oxide) (PPO) resins in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of unsaturated polyester resins in contact with foodstuffs, pharmaceuticals and drinking water

12247:1998 13449:1992 13601:1993 13557:1992 13576:1992 Doc: PCD 12(1328) Doc: PCD12 (1329) Doc: PCD 12(1331) Doc: PCD 12(1332) Doc: PCD 12(1375) Doc: PCD 12(1375) Doc: PCD 12(1516)

which the material is in contact (dry, aqueous, fatty, acidic or/and alcoholic in nature), the solubility of the substance in the food, the duration of contact, and the temperature. In a polymer/food system as presented in Fig. 24.1, there is food on the left that can migrate into the polymer layers on the right side, along with an intermediate layer of swollen polymer with a profile of the migrating food component. On the other hand, we have a concentration gradient of the considered additives, where certain diffusion in the undisturbed polymer layer and a much improved mobility of the additive in the swollen layer and concentration jump at the interfaces are assumed.

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24.1 Migration model for the polymer/food system (adapted from Baldev Raj, 2001).

The following general formula relates the migration of an additive. In a system where a cut piece of the plastic P into a food F at a certain time t is kept at constant temperature, the model predicts direct proportionality of migration of the concentration CAP of the considered additive in the polymer and to the square root of time (t): p MAF …T† ˆ  CAP t where MAF …T† is the migration of additive A into test food F at a temperature T (Crosby, 1981).

24.5

Indian Standards for overall migration (IS:9845-1998)

Central Food Technological Research Institute, Mysore, India, has drafted IS:9845-1998 for `Determination of overall migration of constituents of plastics materials and articles intended to come in contact with foodstuffs ± method of analysis (second revision)' which is now implemented to be followed for overall migration of plastics constituents for their food grade quality, in the country. This standard is the result of R&D work in the laboratory on the study of various factors affecting the migration of additives in food simulants, and is at par with other international standards like US FDA, European Commission Directives, etc. A collection of data regarding the main composition and overall extractable amount of plastic constituents can help with the estimation of migration. This

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Multifunctional and nanoreinforced polymers for food packaging Table 24.2 Migration tests of plastic materials and articles for certain types of food Sl no.

Food type

Simulant

1 2 3 4 5 6 7

Aqueous foods All aqueous and acidic foods Alcoholic foods Fats/oils and fatty foods Alcoholic and acidic foods Fatty and aqueous foods All fatty and acidic foods

A B C D C and D D and A D and B

A: Distilled water B: 3% acetic acid C: 8%, 10%, 50% ethanol D: n-heptane or substitute of olive oil (isooctane and 95% ethanol).

can be a considerable asset both to the producers of such articles and for quality control laboratories. Much time and money may also be saved if studies are made in the evaluation of laminates containing layers of recycling material with unknown impurities which can migrate through the virgin plastic layer (functional barrier) in contact with food. A BIS list of all the specifications on different polymeric materials coming in contact with food application is given in Table 24.1. The choice of simulating solvents and test conditions (time±temperature) depends on the type of foods and conditions of use of food products. Food products have now been classified into seven major groups as shown in Table 24.2. This table has been prepared on the lines of the accepted classification of foodstuffs for such a purpose. The table also gives suitable simulants to be used for different types of foods.

24.5.1 Selection of samples Test samples representing the lot/batch have to be conducted in triplicate. Samples in each replicate shall consist of a number of containers (preformed or converted products) with nearest exposed area of 1000 cm2. In the case of heatsealable films a representative sample shall be of sufficient size to convert into two pouches with an exposed surface area of 1000 cm2 (size of each pouch 12.5 cm in width and 20 cm in length) and non-heat-sealable homogeneous films of size 50 cm  10 cm to be exposed over both sides with 1000 cm2 surface area coming in contact. In the case of lids/wads, 10 pieces are to be sealed to glass bottles only in the smallest size in actual use, to be placed reverted in position with simulant inside during the test period. The samples in the form of containers/pouches/film/lids used shall be carefully rinsed with water (25±30ëC) to remove extraneous materials prior to the actual migration test.

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24.5.2 Procedure Sample containers/pouches in each replicate are filled to their capacity with preheated simulant and closed/sealed. Non-heat-sealable film samples are exposed on both sides with preheated simulant at the test temperature (at least 1 ml/cm2 of contact area). The test samples exposed to the simulant are maintained at a specified temperature in an oven/water bath/autoclave for the specified duration. After completion of exposure time the extracted simulant is transferred into a clean Pyrex glass beaker/container along with three washings of the specimen with a small quantity of the fresh simulant.

24.5.3 Determination of amount of extractive The extracted simulant is evaporated/distilled in a Pyrex beaker/round-bottom flask to about 50±60 ml and transferred into a clean tared stainless steel dish along with three washings with a small quantity of fresh simulant. Further, the concentrate is evaporated in the dish to dryness in an oven at 100  5ëC. The dish with extractive is cooled in a desiccators for 30 minutes and weighed to the nearest 0.1 mg till a constant weight of residue is obtained. The extractives are calculated as mg/dm2 and mg/kg or ml/l or ppm of the foodstuff with respect to the capacity of the container/pouch to be used. A blank shall also be carried out without the sample for adjustment, if necessary. Then: Amount of extractive (Ex) ˆ ˆ

M  100 mg/dm2 A M  1000 mg/kg or mg/l or ppm V

where M ˆ mass of residue in mg minus blank value, A ˆ total surface area in cm2 exposed in each replicate, and V ˆ total volume in ml of simulant used in each replicate or filled capacity of containers. The simulants and test conditions (time±temperature) for extractability studies to be carried out as per different national and international standards depending on the type of food and conditions of use are given in Table 24.3.

24.5.4 Migration limits The test material shall comply with the overall migration limit when tested by the method prescribed in IS:9845-1998. In the case of liquid foodstuffs or of simulants, the upper limit shall be 60 mg/l or ppm. However, for the value of the overall migration the upper limit shall be 10 mg/dm2 of the surface of the material or article. In case of lids/wads the results can be expressed only as mg/kg, with 60 mg/l or ppm as the upper threshold limit.

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Table 24.3 Time±temperature test conditions using food simulants for overall migration in plastics SI no.

Condition of contact

H2O

3% acetic acid

Ethanol

(gen)

(BIS, EEC)

A

B

8% (US FDA) 10% (BIS, EEC) 50% (gen) C

n-Heptane* (BIS, US FDA)

Fat simulants D Substitute for olive oil (EEC) Isooctane 95% ethanol

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High temperature heat sterilized (retorting) (BIS + EEC + US FDA ÿ gen)

121ëC, 2 h

121ëC, 2 h

±

66ëC, 2 h

60ëC, 2.5 h

60ëC, 4.5 h

2

Hot filled or pasteurized above 66ëC, below 100ëC (gen)

100ëC, 2 h 100ëC, 0.5 h

100ëC, 2 h

±

49ëC, 0.5 h

60ëC, 1.5 h

60ëC, 3.5 h

3

Hot filled or pasteurized below 66ëC (gen)

70ëC, 2 h 66ëC, 2 h

70ëC, 2 h

70ëC, 2 h 66ëC, 2 h

38ëC, 0.5 h

40ëC, 0.5 h

60ëC, 2 h

4

Room temperature filled and stored and also in refrigerated and frozen condition (no thermal treatment in container) (gen)

40ëC, 10 d 49ëC, 1 d

40ëC, 10 d

40ëC, 10 d

38ëC, 0.5 h

20ëC, 2 d

40ëC, 10 d

5

Refrigerated storage (no thermal treatment in container) (US FDA)

21ëC, 2 d

±

21ëC, 2 d

21ëC, 0.5 h

±

±

6

Frozen storage (no thermal treatment in container) US FDA

21ëC, 1 d

±

21ëC, 0.5 h

±

±

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24.6

681

US Food and Drug Administration (US FDA) Code of Federal Regulations (CFR)

In United States of America all the packaging materials are evaluated for food contact application as per the US FDA, CFR 21, Parts 170 to 199, revised as of 1 April 2009.

24.6.1 Indirect food additives: general Regulations prescribing conditions under which food additive substances may be safely used predicate usage under conditions of good manufacturing practice. The quantity of any food additive substances that may be added to food as a result of use in articles that contact food shall not exceed, where no limits are specified, that which results from use of the substance in an amount not more than reasonably required to accomplish the intended physical or technical effect in the food-contact article; shall not exceed any prescribed limitations; and shall not be intended to accomplish any physical or technical effect in the food itself, except as such may be permitted by the regulations. Any substance used as a component of articles that contact food shall be of purity suitable for its intended use. The existence of a regulation prescribing safe conditions for the use of a substance as an article or component of articles that contact food shall not be constructed as implying that such substance may be safely used as a direct additive in food. Substances that under conditions of good manufacturing practice may be safely used as components of articles that contact food include the following subjects to any prescribed limitations: · Substances generally recognized as safe in or on food · Substances generally recognized as safe for their intended use in food packaging · Substances used in accordance with a prior sanction or approval · Substances permitted for use by regulations as such and parts.

24.6.2 Threshold of the regulations and migration limits Substances used in food-contact articles (e.g., food-packaging or foodprocessing equipment) that migrate, or may be expected to migrate, into food at negligible levels may be reviewed under the regulation US FDA, CFR 21, Parts 170 to 199. In the finished form in which it is to contact food, when extracted with the solvent or solvents characterizing the type of food, and under conditions of time and temperature characterizing the conditions of its intended use as determined from Tables 24.2 and 24.3, the extractives shall not exceed 0.5 mg per square inch (7.75 mg/dm2) of food-contact surface, nor exceed 50 parts per million of the water capacity of the container in general or

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other limits specified for a specific material when tested as per the prescribed method.

24.7

European Commission Directives on plastic containers for foods

At the European level Framework Directive 1989/109/EC defines comparable general requirements for plastic containers. In the early 1980s corresponding separate directives in the field of plastic utensils were adopted at the European level, which also included procedures for carrying out such migration tests. European regulations have been harmonized to a large extent, at least with regard to admissible monomers and starting substances (positive lists) as well as to maximum admissible migration of ingredients of plastics utensils: this also applies to overall migration limitations, maximum admissible residual content of certain monomers and starting substances in plastic containers (so-called QM(A) limits), and maximum admissible migration limits of defined specific substances (so-called SML(T)) (Commission Directive 2004/19/EC). Commission Directives (Table 24.4) have laid down procedures for selecting food simulants and also requirements for testing migration based on actual conditions of use (time/temperature combinations). On the other hand, the existing European Directives mentioned above partly cover the use of plastic additives and at present provide no regulations at all with regard to aids to polymerization and colouring materials in plastics. In practice, the evaluation of plastic containers Table 24.4 European Framework Directives on separate materials in contact with food Directive no.

Subject

Directive 2002/72/EEC Directive 90/128/EEC Directive 82/711/EEC Directive 85/572/EEC Directive 80/766/EEC Directive 81/432/EEC

Plastic materials and articles Plastic monomers Basic rules for migration tests List of simulants/foodstuffs VC in PVC Method of analysis for vinyl chloride released into foodstuffs Limits of vinyl chloride monomer Determining symbols Regenerated cellulose film (RCF) Ceramic articles First amendment to 83/229/ECC Amendment to 83/229/ECC First amendment to 82/711/ECC Nitrosamines in elastomers and rubber Second amendment to 82/711/ECC Epoxy derivatives

Directive 78/142/EEC Directive 80/590/EEC Directive 83/329/EEC Directive 84/500/EEC Directive 86/388/EEC Directive 92/15/EEC Directive 93/8/EEC Directive 93/11/EEC Directive 97/48/EEC Directive 2001/61/EEC

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(in particular packages) with regard to compliance with food regulation is a twostep procedure in most cases: first the ingredients of the recipe are examined so as to ensure that the materials used are admissible in principle. This examination is based on existing European Directives. If, in this first step, all components of the recipe turn out to be admissible in principle, migration tests are carried out. In the next step, individual components of the plastic container in question (e.g., additives, colouring materials, monomers, etc.) are not transmitted to the filling material (foodstuff) to an inadmissibly great extent. The corresponding tests are preferably carried out directly on the respective containers or on a test specimen taken from it, with specific attention paid to the requirement that the overall migration limit and any specific migration limits be met (Franz et al., 1992; Till et al., 1987).

24.7.1 Active and intelligent food contact materials As per Commission Directive 2004/1935/EC, active and intelligent food contact materials and articles designed to actively maintain or improve and monitor the condition of the food are not inert by their nature. It is therefore necessary, for reasons of clarity and legal certainty, to be included in the scope of the Regulation. Further requirements should be stated in specific measures, to include positive lists of authorized substances and/or materials and articles, which should be adopted as soon as possible. Active food contact materials and articles are designed to deliberately incorporate `active' components intended to be released into the food or to absorb substances from the food. They should be distinguished from materials and articles which are traditionally used to release their natural ingredients into specific types of food during the process of their manufacture. Active food contact materials and articles may change the composition or the organoleptic properties of the food only if the changes comply with the Community provisions applicable to food, such as the provisions of Commission Directive 1989/ 107/EC(4) on food additives. In particular, substances such as food additives deliberately incorporated into certain active food contact materials and articles for release into packaged foods or the environment surrounding such foods, should be authorized under the relevant Community provisions applicable to food and also be subject to other rules which will be established in a specific measure. As per amendments of Regulation (EC) No. 2004/1935/EC described in Regulation (EC) No. 2009/596/EC, active and intelligent food contact materials and articles should not change the composition or the organoleptic properties of a food or give information about the condition of the food that could mislead consumers. For example, active food contact materials and articles should not release or absorb substances such as aldehydes or amines in order to mask an incipient spoilage of the food. Such changes, which could manipulate signs of

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spoilage, could mislead the consumer and should therefore not be allowed. Similarly, active food contact materials and articles which produce colour changes to the food, giving the wrong information concerning the condition of the food, could mislead the consumer and therefore should also not be allowed. In addition, adequate labelling or information should support users in the safe and correct use of active materials and articles in compliance with the food legislation, including the provisions on food labelling. On grounds of efficiency, the normal time limits for the regulatory procedure with scrutiny should be curtailed for the adoption of a list of substances authorized for use in the manufacture of active or intelligent food contact materials and articles. When necessary, special conditions of use, purity standards and specific limits on the migration into or on to food are to be used. For substances exempt from specific migration limits or other restrictions, a generic specific migration limit of 60 mg/ kg or 10 mg/dm2, according to the case, is applied. However, the sum of all the specific migrations should not exceed the overall migration limits.

24.8

Specific migration of toxic additives

In addition to creating safety and health problems during production, many chemical additives that give plastic products desirable packaging qualities also have negative environmental and human effects. These effects include direct toxicity as in the case of lead, cadmium and mercury. Most of the colourful plastic containers, which are manufactured by recycling, would have these toxic additives. Plastic containers can contaminate food because some chemicals diffuse from the packaging polymer of which they are made to the foods they contain. Migration potential exists for traces of monomers, oligomers, additives, stabilizers, plasticizers and lubricants. Such substances may be toxic. A report of the Berkeley (US) Plastics Task Force published in 1996 found that styrene from polystyrene, plasticizers from polyvinyl chloride (PVC), antioxidants from polyethylene and acetaldehyde from polyethylene terephthalate (PET) have the potential to contaminate food (Stover et al., 1996/Berkeley Report).

24.8.1 Vinyl chloride As per mutagenicity and metabolism of vinylchloride monomer (VCM), a wide range of toxic effects has been reported in human case studies. The principal effects observed include lesions of the bones in the terminal joints of the fingers and toes (acro-osteolysis) as well as changes in the liver and spleen. Long-term exposure gives rise to a rare form of liver cancer (angiosarcoma) and the association with exposure to VCM has been reported amongst plant operatives in several countries. In recent years, however, exposure to VCM at production and polymerization plants has been markedly reduced. It is well known that vinyl chloride causes angiosarcomas for the liver as well as tumours of the brain,

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lung and haematolymphopoietic systems in humans. As per European Commission Directive, the limits for the level of vinyl chloride in materials and articles and for the level of vinyl chloride released by materials and articles to foodstuffs shall be 1 mg/kg of PVC material and 0.01 mg/kg of food (Commission Directive 1978/142/EC). As per Indian Standard, the vinyl chloride monomer content of PVC suspension resin used for manufacture shall not exceed 5 ppm, and in PVC containers/ film used for food packaging shall not exceed 1 ppm. The residual migration of VCM into foodstuffs being packed shall not exceed 10 ppb. The method developed at Central Food Technological Research Institute (CFTRI), Mysore, is suitable for estimation of residual VCM content in PVC material and foods packed in them up to 0.01 ppm levels (Ravi et al., 2000).

24.8.2 Vinylidene chloride (VDC) Less is known of the toxicology of VDC, both in animals and in humans. The LD value for rats is around 1500 mg/kg body weight, while in mice the value is 200 mg/kg body weight. VDC affects the activity of several rat liver enzymes and decreases the store of glutathione. Some tumours have been observed after prolonged exposure but no teratogenic effects were seen in rats or rabbits. The main pathway of excretion is via the lungs, with other metabolites being discharged by the kidneys.

24.8.3 Acrylonitrile (AN) Acrylonitrile is considerably more toxic than the chlorinated monomers and has a lethal dose (LD) value of 80±90 mg/kg body weight in rats and 27 mg/kg body weight in mice. It has also been shown to be mutagenic after metabolic activation with liver enzymes. In animals AN is metabolized to cyanide, which is converted to thiocyanate and excreted in the urine. There is also some evidence of carcinogenicity in animals and possibly humans too. As per US FDA, styrene±maleic anhydride copolymers shall not contain residual acrylonitrile monomer more than 0.1 wt%. In nitrile rubber modified acrylonitrile±methyl acrylate copolymers, the residual acrylonitrile monomer content is not more than 11 parts per million (US FDA, CFR 21, 177-1020).

24.8.4 Styrene The LD value of styrene for rats is 5 g/kg body weight. It is metabolized to styrene and its oxide, which is a potent mutagen in a number of test systems. Both styrene and its oxide have been found to produce chromosomal aberrations under certain conditions. Toxic effects of styrene in humans have been reviewed by the International Agency for Research on Cancer (IARC). The most fre-

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quently observed changes were of a neurological and psychological nature. The total residual monomers, when present, shall not exceed 0.2% by mass of the polymer, as per Indian Standard. As per US FDA styrene±maleic anhydride copolymers shall not contain residual styrene monomer more than 0.3 wt%. Polystyrene basic polymers shall not contain more than 1 wt% of total residual styrene monomer (US FDA, CFR 21, 177-1020).

24.8.5 Colourants in plastics Plastics are increasingly coloured to enhance the attractiveness of packaging, to protect the contents from the adverse effects of light or to differentiate between products. Depending on the type of packaging, the contents and the storage conditions, it is possible that components in the packaging, including colourants, could migrate to the food. It must therefore be ensured that the packaging components, including colourants, do not pose a health hazard for the consumer. This is also the aim of the relevant Directives, laws and regulations. The colouration of plastics that come into contact with food is an important application for the colourants industry. The following basic criteria are decisive for the safe use of colourants for the colouration of food contact articles and packaging: · Purity criteria of the colourants · Its fastness to migration · The tested toxicological properties. Colourant manufacturers guarantee that the colourants have been toxicologically tested and that the purity criteria are met. The manufacturers of the food contact article or packaging material are responsible for the migration testing. Consumer safety is the joint responsibility of manufacturers, processors and authorities. For the consumer the safe use of coloured plastic food contact articles is already provided for by the current regulations in combination with a responsible approach by the pigment manufacturers and processors. Nevertheless, new knowledge must always be taken into account. Basically the problem of colour migration is found in vegetable oils, when they are packed into coloured polythene containers. CFTRI has developed simple methods to detect the colour migration qualitatively from plastics. Migration of the colour can be observed by exposing coloured plastic pieces in decolourized coconut oil when compared with blank. However, quantitative estimation can also be done using spectroscopic analysis (Baldev et al., 2007). As per US FDA, CFR 21, 178.3297 Colourants and polymers, the substances may be safely used as colourants in the manufacture of articles or components of articles intended for use in producing, manufacturing, processing, preparing, treating, packaging, transporting or holding food. The term colourant means a

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dye pigment, or other substance that is used to impart colour to, or to alter the colour of, a food-contact material, but that does not migrate to food in amounts that will contribute to that food any colour apparent to the naked eye. As per Indian Standards, colour migrated to food simulant or decolourised coconut oil or packed food shall not be apparent to the naked eye. If the colour migrated is clearly visible, such materials are not suitable for food contact applications, even though the extractive value is within the limit (IS: 98331981).

24.9

Recent problems in specific migration

In recent years, there has been considerable demand by the food industries for information concerning the specific migration of some additives and their estimation ± acetaldehyde, terephthalic acid and methyl ethyl glycol in PET containers (Ewender et al., 2003), bisphenol-A (BPA) content in epoxy coatings and polycarbonate (Yoshiki et al., 2005). European Directives used for plastic materials and articles in contact with food regulations have fixed upper limits for the specific migration of hundreds of additives. The methodology of such specific migration of additives is not available. Work has been reported only on the few additives in plastics. There is a need to set up a facility to standardize the methodology for estimating such specific additives to help the industries to evaluate their packaging materials for safety and to prevent any health hazard to the consumer. Due to the estrogenic activity of BPA used as monomer in polycarbonate feeding bottles and epoxy-coated cans, there is a need for newer methods in order to have reliable tools for risk assessment and control of human exposure to BPA (Ballesteros-GoÂmez et al., 2009). Chemical degradation of epoxy resin into monomer using solvent has been reported (Sato et al., 2001; Braun et al., 2001). Formation of monomer (BPA) by recycling of polycarbonate resin was reported by Oku and co-workers (Oku et al., 2000; Hata et al., 2002; Kawai et al., 2005).

24.10 Future trends In recent years nanotechnology has entered the field of food packaging technology. Nanocomposites are used in food contact materials (FCMs), since the addition of nanoreinforcements can not only passively protect the food against environmental factors, but also incorporate properties to the packaging material related to improvements in overall performance by enhancing their mechanical, thermal and barrier properties, usually even at very low contents. Moreover, several nanoparticles can provide active and/or `smart' properties to food packaging materials, such as antimicrobial properties, oxygen scavenging ability, enzyme immobilization, or indication of the degree of exposure to some degradation-related factor.

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Current legislation pertaining to food ingredients, food additives and FCMs does not differentiate between substances produced routinely by `standard' manufacturing methods and those developed by nanotechnology. There is currently no size limitation on particle size for food additives except for E460 cellulose (microcrystalline). There are many safety concerns about nanomaterials, as their size may allow them to penetrate into cells and eventually remain in the system. There is no consensus about categorizing nanomaterials as new (or unnatural) materials. On the one hand, the properties and safety of materials in their bulk form are usually well known, but their nano-sized counterparts frequently exhibit different properties from those found at the macro-scale. There is limited scientific data about migration of most types of nanoparticles (NPs) from the packaging material into food, as well as their eventual toxicological effects. It is reasonable to assume that their migration may occur into foods, hence the need for accurate information on the effects of NPs on human health following chronic exposure is imperative (de Azeredo, 2009). There may not be the need to develop a new approach to risk assessment of nanomaterials, but there is a clear need to provide hazard identification data on the widest possible range of nanomaterials. In the absence of such data, it is not possible to derive conclusions about the spectrum of toxicological effects that might be associated with nanomaterials. There is a need for rules on substances and materials that are problematic and not dealt with elsewhere in the legislation. If and until such legislation is completed and adopted, the products of nanotechnology will continue to be dealt with by a combination of general food law and more specific controls on particular materials and articles. Specific legislation dealing with nanocomponents in food and FCMs is only likely to be made if there is sound scientific evidence to show that such materials present a higher risk than their macro equivalents. In the absence of detailed toxicological data but in view of the potential of some nanoparticles to cause harm, it may also be appropriate to consider application of the precautionary principle (PP) for certain applications of nanotechnology in the food sector. The PP is a wellaccepted tenet of international law and is an attempt to legally codify the maxim `better safe than sorry'. Although evidence is emerging to suggest that certain engineered nanoparticles have the potential to cause harm to human health, it is not clear at present whether there is enough scientific basis to invoke the PP in all applications of nanotechnology for food contact materials. There is a need for research on any significant risk of indirect contamination of food through migration of nanoparticles from food packaging or active surfaces used in food processing. Interdisciplinary research is vital to address the current uncertainties and much can be learnt from parallel areas. Like any other new technology, public confidence, trust and acceptance are likely to be the key factors determining the success or failure of nanotechnology applications for FCMs (Chaudry et al., 2008).

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24.11 References and further reading Baldev Raj (2001), `Food and packaging interaction ± Migration concepts and regulations', Indian Food Industries, 20, 67±74. Baldev Raj, Vijayalakshmi N S, Ravi P and Srinivas P (2007), `Migration behaviour and estimation of colourants from coloured plastics to edible oils', Deutsche Lebensmittel-Rundschau, 1, 15±20. Ballesteros-GoÂmez A, Rubio S and PeÂrez-Bendito D (2009), `Analytical methods for the determination of bisphenol A in food', Journal of Chromatography: A, 1216(3), 449±469. BIS, IS:9833-1981, List of pigments and colourants for use in plastics in contact with foodstuffs, pharmaceuticals and drinking water (reaffirmed 2003). BIS, IS:9845-1998, Determination of overall migration of constituents of plastics materials and articles intended to come in contact with foodstuffs ± method of analysis. BIS, IS:10146-1982, Specification for polyethylene for its safe use in contact with foodstuffs, pharmaceuticals and drinking water. Braun D, von Gentzkow W and Rudolf A P (2001), `Hydrogenolytic degradation of thermosets', Polymer Degradation and Stability, 74, 25±32. Busolo M A, Fernandez P, Ocio M J and LagaroÂn J M (2010), `Novel silver-based nanoclay as an antimicrobial in polylactic acid food packaging coatings', Food Additives and Contaminants: Part A, DOI: 10.1080/19440049.2010.506601. Chaudry Q, Scotter M, Blackburn J, Ross B, Boxall A, Castle L, Aitken R and Watkins R (2008), `Review: Applications and implications of nanotechnologies for the food sector', Food Additives and Contaminants, 25, 241±258. Commission Directive 1978/142/EC relating to limits of vinyl chloride monomer. Commission Directive 1989/107/EC of 21 December 1988 relating to food additives authorized for use in foodstuffs intended for human consumption. Commission Directive 1989/109/EC of 21 December 1988 relating to materials and articles intended to come into contact with foodstuffs. Commission Directive 2002/72/EC of 6 August 2002 relating to plastic materials and articles intended to come into contact with foodstuffs. Commission Directive 2004/19/EC amending Directive 2002/72/EC relating to plastic materials and articles intended to come into contact with foodstuffs. Commission Directive 2004/1935/EC of 27 October 2004 relating to materials and articles intended to come into contact with food and repealing Directives 80/590/ EEC and 89/109/EEC. Commission Directive 2008/282/EC of 17 March 2008 relating to recycled plastic materials and articles intended to come into contact with foods and amending Decision 2007/76/EC. Commission Directive 2009/450/EC of 29 May 2009 relating to active and intelligent materials and articles intended to come into contact with food. Commission Directive 2009/596/EC of 18 June 2009 relating to a number of instruments subject to the procedure referred to in Article 251 of the Treaty to Council Decision 1999/468/EC with regard to the regulatory procedure with scrutiny. Crosby N T (1981), `Food packaging materials: Aspects of analysis and migration of contaminants', in Food Packaging Materials, London, Applied Science Publishers. de Azeredo H M C (2009), `Review ± Nanocomposites for food packaging applications', Food Research International, 42, 1240±1253. Ewender J F R, Mauer A and Welle F (2003), `Determination of the migration of

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acetaldehyde from PET bottles into non-carbonated and carbonated mineral water', Deutsche Lebensmittel-Rundschau, 99, 215±221. Franz R, Lee K T, Knezevic G, Wolff E and Piringer O (1992), `Measuring and evaluation of the global migration from food contact materials into food: a comparison between official EC-techniques and alternative methods', Internationale Zeitschrift fuÈr Lebensmittel-Technik, Marketing, Verpackung und Analytik, 43, 291±296. Hata S, Goto H, Yamada E and Oku A (2002), `Chemical conversion of polycarbonate to 1,3-dimethyl-2-imidazolidinone (DMI) and bisphenol A', Polymer, 43, 2109±2116. Jenkins W A and Harrington J P (1991), Packaging Foods with Plastics, Lancaster, PA, Technomic Publishing Co. Kawai N, Tsujita K, Kamo T and Sato Y (2005), `Chemical recovery of bisphenol-A from polycarbonate resin and waste CDs', Polymer Degradation and Stability, 89, 317± 326. LagaroÂn J M, Cabedo L, Cava D, Feijoo J L, Gavara R and Gimenez E (2005), `Improving packaged food quality and safety. Part 2: Nanocomposites', Food Additives and Contaminants: Part A, 22, 994±998. Lyndhurst B (2009), An Evidence Review of Public Attitudes to Emerging Food Technologies, Social Science Research Unit, Food Standards Agency, March 2009. Matche R S and Baldev Raj (2005/06), `Applications of plastics in food packaging', Packaging India, Dec.±Jan., 38, 33±48. Observatory Nano (2009), 2 Agrifood market report ± content, 2.5.4 Food contact materials (FCMs) based on metal/metal oxide nanoparticles. Oku A, Tanaka S and Hata S (2000), `Chemical conversion of polycarbonate to bis(hydroxyethyl) ether of bisphenol A ± An approach to the chemical recycling of plastic wastes as monomers', Polymer, 41, 6749±6753. Paine F A and Paine H Y (1983), A Handbook of Food Packaging, Council of The Institute of Packaging, London, Leonard Hill. Proceedings of the Second International Symposium on Feedstock Recycling of Plastics and Other Innovative Recycling Technology, 27. Ravi P, Baldev Raj, Vijayalakshmi N S and Srinivas P (2000), `Estimation of vinyl chloride monomer in PVC and food materials under publication', Packaging India, 32, 33±37. Robertson G L (2005), Food Packaging: Principles and Practice, New York, Marcel Dekker. Sato Y, Tsujita K and Kawai N (2001), `Recovery of bisphenol-A from polycarbonate and epoxy resins by liquid-phase chemical recycling', Proceedings of the Polymer Degradation Discussion Group, 24th Meeting, C-8. Schwope A D and Reid R C (1988), `Migration to dry foods', Food Additives and Contaminants, 5, 445±454. Stover R L, Evans K and Pickett K (1996), Report of the Berkeley Plastics Task Force, 1± 48. Till D, Schwope A D, Ehntholt D J, Sidman K R, Whelan R H, Schwartz P S and Reid R C (1987), `Indirect food additive migration from polymeric food packaging', CRC Critical Reviews in Toxicology, 18, 215±243. US FDA (2009), CFR 21, 177.1020, Acrylonitrile/butadiene/styrene copolymer, revised as of 1 April 2009. US FDA (2009), CFR 21, Parts 170 to 199, revised as of 1 April 2009. US FDA (2009), CFR 21, 178.3297, Colorants for polymer. US FDA (2009), CFR 21, 176.170, Components of paper and paperboard in contact with

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aqueous and fatty foods. Vijayalakshmi N S and Baldev Raj (2010), `Suitability of plastic containers for drinking/ potable water and regulations', Indian Food Packer, 64, 66±73. Yoshiki S, Yasuhiko K, Koji T and Noboru K (2005), `Degradation behaviour and recovery of bisphenol-A from epoxy resin and polycarbonate resin by liquid-phase chemical recycling', Polymer Degradation and Stability, 89, 317±326.

24.12 Appendix: Abbreviations AN BIS BPA CFR CFTRI DOP DEHA EC EFSA FCMs GMP IARC IS LD NPs PET PP PVC QM(A) SML(T) US FDA VCM VDC

acrylonitrile Bureau of Indian Standards bisphenol-A Code of Federal Regulations Central Food Technological Research Institute dioctyl phthalate di-2-ethylhexyladipate European Commission European Food Safety Authority food contact materials good manufacturing practice International Agency for Research on Cancer Indian Standards lethal dose nanoparticles polyethylene terephthalate precautionary principle polyvinyl chloride quantity in material or article specific migration limit (test) US Food and Drug Administration vinylchloride monomer vinylidene chloride

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Index

-galactosidase, 470 -lactalbumin, 613 -tocopherol, 465 -zein, 616 Acetyl-CoA, 499 acrylonitrile, 685 Acticoat, 353 Actisorb, 353 active packaging, 3, 4, 31, 356±7, 460±1, 462 additives, 670 additives migration, 674±7 Indian standards for plastics in contact with foodstuffs, pharmaceuticals and drinking water, 675±6 migration model, 674, 676±7 polymer/food system, 677 A-DO Korea, 392 adsorption, 319 advanced single-site polyolefins and ethylene±norbornene copolymers, 152±61 macromolecular structure, 155±6 future trends, 160±1 macromolecular structure, 154±5 crystallinity and composition continuum for ethylene±propylene polymers, 155 single-site polyethylenes mechanical properties, 154 nanocomposite preparation, 156±60 blend preparation, 157 BUR film properties, 158 material characteristics, 157 non-polypropylene blends with compatibiliser, 160 synthesis and molecular structure, 153±4 isotactic ethylene/butylene copolymer, 154 metallocene structure, 153 agar diffusion method, 579 Ageless, 38

AgIon, 358 AIT see allyl isothiocyanate ALD see atomic layer deposition alginates, 473 alipathic polyketones, 265±6 allerginicity, 611 allyl isothiocyanate, 380, 432, 446, 447 alpha-olefins, 153 Alphasan, 358 aluminium silicate, 377 amino acids, 61±4 indicated intercalation compounds composition and basal spacing, 61 MgAl-DL-Phe structural model, 62 amorphous polyamides, 275±6 amorphous vinyl alcohol resins, 16 amylomaize, 533 amylopectin, 531, 533 amylose, 531, 533 anionic clays, 43, 45 antibiotic drugs, 55±61 HTIc±CFS computer-generated representation, 60 intercalated HTIc composition, interlayer distance and drug loading, 60 structural formulae and acronyms, 56 antiblock agents, 671 antimicrobial activity, 403±4, 574±82 antimicrobial agents, 64±6, 371±2, 373±6, 673 antimicrobial activity, 403±4 evaluation methods, 403 Bz and p-Bz-OH anions computergenerated models, 65 chemical antimicrobial agents, 372, 377±80 antioxidants, 377 fungicides, 378 gases, 378±80 inorganic materials, 377±8 organic acids and salts, 372, 377 triclosan, 377

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Index classification, 372 films and coatings, 393±402 antimicrobial packaging system, 394±5 biopolymer-based packaging, 397±400 commercial antimicrobial systems, 400±2 requirements, 394 synthetic polymeric packaging materials, 395±7 future trends, 404 incorporation into polymeric films for food packaging, 368±420 indicated intercalation compounds composition and basal spacing, 65 nano-antimicrobial agents, 390±3 packaging materials with nanoantibacterial agents, 393 natural antimicrobial agents, 380±9 bacteriocins, 380±7 enzymes, 387±8 plant origin, 388±9 polymers, 389±90 release through multilayer film and through single-layer film, 370 antimicrobial compounds, 634±5 antimicrobial films and coatings, 393±402 antimicrobial packaging system, 394±5 biopolymer-based packaging, 397±400 alginates, 398±9 chitosan films, 399±400 paper, 398 polysaccharide-based, 397±8 protein-based films and coatings, 400 commercial antimicrobial systems, 400±2 materials, 401±2 requirements, 394 synthetic polymeric packaging materials, 395±7 coatings, 396±7 ethylene acrylic acid, 396 linear low density polyethylene, 396 low density polyethylene, 395±6 multilayer structures, 397 antimicrobial nanoclays, 33±7 active nanoclays functioning, 34 antimicrobial packaging, 369, 586±7 applications, 373±6 future trends, 404 primary goals, 369 antimicrobial packaging films, 434±42 antioxidant packaging films, 442±5 antioxidants, 64±6, 377, 635±6, 671 hydroxycinnamic acid arrangement computer-generated models, 66 indicated intercalation compounds composition and basal spacing, 66 antistatic agents, 671 aPA see amorphous polyamides

693

Apacider, 358 Aquacel-Ag, 353 Archer Daniels Midland Company, 501 Arrhenius equation, 212, 214 Arrhenius law, 13 ascorbic acid, 377 atmospheric pressure chemical vapour deposition, 294 atomic layer deposition, 21 atom transfer radical polymerisation, 99 ATR see attenuated total reflection ATR-FTIR spectroscopy see attenuated total reflection Fourier transformed infrared spectroscopy ATRP see atom transfer radical polymerisation attenuated total reflection, 475 attenuated total reflection Fourier transformed infrared spectroscopy, 34, 574 AVOH see amorphous vinyl alcohol resins -lactoglobulin, 612±13 -zein, 616 Baby Dream Co. Ltd, 392 Bactekiller, 358 bacteriocins, 380±7 combination of agents, 380±2 combination with other antimicrobial agents, 383±6 Bactiblock, 36, 358, 400 dosages in plastic materials, 37 PLA-Bactiblock viable cell counts before and after incubation, 37 TEM pictures, 36 Bardex, 353 Barix, 145 benomyl, 378 benzophenone, 330 betel oil, 423±4 BHA see butylate hydroxy anisole BHT see butylate hydroxy toluene Bind-Ox, 247 bioactive food packaging, 460±76 controlled release of bioactives, 473±5 characterisation methods, 474±5 developed methods, 473±4 definition, 461±2 existing technologies to improve shelf-life or food functionality, 462±70 existing and potential bioactive packaging developments, 465±70 recent developments in active packaging technologies, 463±5 future trends, 475±6 nanotechnologies, 470±3 bioactive packaging, 461, 462 strategies, 4

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694

Index

biocide effects, 576±81 Biocote, 358 Biocycle, 501 biodegradable polymers, 204 biological reduction, 352 Biomaster, 358 Biomer, 501 Biomer P-series, 501 Bio-On, 501 BIOP, 530 bioplastic films, 338 bioplastics, 204 Biopol, 500, 501, 515 biopolymers, 4, 113, 114, 360, 470±1 bioriented polypropylene matrix, 573, 575 Biotec, 531 Biotec Company, 543 Boltzmann's constant, 132 Bondi's group contribution method, 131 bottom-up technique, 335, 350 bovine serum albumin, 613 brittleness, 574 butylate hydroxy anisole, 377 butylate hydroxy toluene, 377 CA see controlled atmosphere canola oil, 445 carbon dioxide, 311 carbon dioxide permeability, 626±7 carbon dioxide transmission rate, 248 carbon nanofibres, 493 carbon nanotubes, 391, 493 carboxymethyl cellulose, 545 carrageenan polysaccharides barrier performance, 598±601 biodegradable vs synthetic films WVP values, 598 glycerol effect on water permeability and water % uptake, 601 oxygen permeability coefficients of biodegradable vs synthetic films, 600 food packaging, 594±606 nanocomposites, 601±6 TEM of casting carrageenan, 603 UV±vis spectra of the castings, 605 water permeability, 603 water uptake at 11%, 4% and 75% RH, 604 processing in packaging, 597 structure and properties, 595±6 flow diagram for extraction, 596 monomers molecular structure, 596 carvacrol, 465 casein, 611 caseinmacropeptide see glycomacropeptide casting, 539, 573, 651 catalytic chemical vapour deposition, 294 cation exchange capacity, 35

CEC see cation exchange capacity cell age, 579 cellophane, 204 cellulose, 360, 630±1 cellulose-based plastics, 204 cellulose nanocrystals, 556 cellulose nanofiller cellulose-reinforced nanocomposites preparation, 99±101 extraction and refining, 91±5 CNFs (white) suspension, 92 extraction by chemical analysis, 91±2 extraction by mechanical force, 92±5 functional valve and microfluidiser with interaction chamber, 94 oxidised CNF from tunicates and wood, 95 food packaging, 86±102 future trends and applications, 101±2 mechanical properties, 95±6 morphological and structural characteristics, 87±90 FE-SEM micrograph displaying CNF from bacterial cellulose, 89 poly- (1,4)-D-glucopyranoside chain molecular structure, 90 wood pulp micrograph, 88 surface modification, 96±9 cellulose nanowhiskers, 492±3, 553, 602 chain immobilisation factor, 11 chelating agents, 381±2 chemical antimicrobial agents, 372, 377±80 antioxidants, 377 fungicides, 378 gases, 378±80 alcohols, 378±9 chlorine dioxide, 379 other gases, 380 inorganic materials, 377±8 organic acids and salts, 372, 377 triclosan, 377 chemical reduction, 351±2 chemical vapour deposition, 291±4 Chemie Linz, 500 chitin, 389, 572 chitin nanoparticles, 556 chito-oligosaccharides, 576 chitosan, 115, 389, 391, 464±5, 473, 544 chitosan films, 581±2 chitosan nanoparticles, 556 chitosan polysaccharide antimicrobial activity, 574±82 ATR-FTIR spectra, 575 biocide properties optimisation, 576±81 film-forming and storage condition optimisation, 581±2 barrier performance, 582±4 water permeability, 584

ß Woodhead Publishing Limited, 2011

Index food packaging applications, 571±87 processing in packaging, 573±4 structure and properties, 572±3 chitin and chitosan chemical structure, 572 chlorine dioxide, 379 cholesterol reductase, 470 cinnamon oil, 424±5, 426 antioxidant activity against oxidative bleaching of -carotene, 426 DPPH radical scavenging activity, 426 Citrex, 400 clays, 470±1 Cloisite 6A, 550, 552 Cloisite 10A, 258, 550, 552, 659 Cloisite 15 A, 504±5 Cloisite 30A, 550, 552 Cloisite 30B, 504±5, 513, 659 Cloisite Na+, 504 clove oil, 425±6 CMC see carboxymethyl cellulose CNF see carbon nanofibres; cellulose nanofiller CNT see carbon nanotubes CNW see cellulose nanowhiskers coating, 205, 619 coating fragmentation, 308 coaxial electrospinning, 111±12, 118±19 co-electrospinning see coaxial electrospinning coextrusion, 205±6, 216±17 cohesive energy density, 8 collagen, 617 collagen-chitosan complex, 115 colour parameters, 633±4 Commission Directive 2002/72/CE, 491 Commission Directive 2007/19/CE, 491 Commission Directive 1978/142/EC, 685 Commission Directive 1989/107/EC, 683 Commission Directive 2002/72/EC, 674 Commission Directive 2004/19/EC, 682 Commission Directive 2004/1935/EC, 683 Commission Directive 2009/450/EC, 673 composites, 119 conglycinin, 614±15 Contreet, 353 controlled atmosphere, 165 controlled-release packaging, 32, 433 Coomassie Plus total protein assay, 474 corn zein, 616±17 co-rotating extrusion, 620 cotton, 360 cottonseed proteins, 618 crosslinking, 474 CRP see controlled-release packaging crystals, 143 cup method, 147 cyclodextrins, 431±2

695

deacetylation, 577 delamination, 308 di(2-ethylhexyl) maleate, 330±1 differential scanning calorimetry, 534, 555 2,2-diphenyl-1-picryhydrazyl assay, 426 Directive 2002/72/CE, 359 Directive 1994/36/EC, 359 DMTA see dynamic mechanical thermal analysis DPPH assay see 2,2-diphenyl-1picryhydrazyl assay dual-mode sorption model, 6 DuPont, 392 dynamic mechanical thermal analysis, 555 EAA see ethylene acrylic acid ECM see extracellular matrix egg white, 618 electromagnet, 147±8 electron beam evaporation, 295 electrospinning, 108±13, 119±21, 122, 467, 491 chitosan porous nanofibres, 112 electrospun zein networks SEM images, 111 packaging applications, 119±21, 122 electrospun nanocomposite in novel food packaging materials design, 120 nanocomposite reinforcements based on electrospun fibres, 120 PLA-zein nanocomposite micrographs, 122 set-up, 109 electrospraying, 110 electrospun biopolymer-based ultrathin fibres, 113 electrospun nanofibres electrospinning, 108±13, 119±21, 122 chitosan porous nanofibres, 112 electrospun nanocomposite in novel food packaging materials design, 120 electrospun zein networks SEM images, 111 nanocomposite reinforcements based on electrospun fibres, 120 packaging applications, 119±21, 122 PLA±zein nanocomposite micrographs, 122 set-up, 109 functional nanofibres, 113±15 functional polymers electrospinning properties, 114 future trends, 121±3 nanoencapsulation, 116±19 electrospun zein/ -carotene nanofibres fluorescence image, 118 promising properties using electrospinning, 117

ß Woodhead Publishing Limited, 2011

696

Index

packaging applications, 108±23 encapsulation, 467±8 engineered nanomaterials, 660±1 Enmat, 501 enterocin, 381 enzymes, 387±8 EP 1529635, 255 essential oils, 382, 388±9 ethanol, 378±9 ethylene, 187 ethylene acrylic acid, 396 ethylene±norbornene copolymers and advanced single-site polyolefins, 152±61 macromolecular structure, 154±5 nanocomposite preparation, 156±60 synthesis and molecular structure, 153±4 future trends, 160±1 macromolecular structure, 155±6 norbornene and polymerisation modes, 156 ethylene/propylene/CO terpolymers, 266 ethylene±vinyl acetate, 203 ethylene±vinyl alcohol copolymers, 9, 202, 261±81, 584 future trends, 280±1 improving retorting, 271±6 alternatives for retortable packages, 275±6 blending with other materials, 273±4 FTIR absorbance, 273 FTIR spectra, 272 resistance to treatment, 271±3 synchrotron WAXS traces vs temperature, 275 WAXS patterns of retorting stages, 274 and poly(vinyl) alcohol nanocomposites, 276±80 EVOH nanocomposite, 278 processing in packaging, 266±71 EVOH32 before and after retorting, 268 novel food preservation technologies, 269±71 oxygen transmission rate of multilayer structures, 270 retorting, 266±9 WAXS patterns, 269 structure and general properties, 262±5 chemical structure, 262 oxygen transmission rate vs relative humidity, 264 vs alipathic polyketones, 265±6 polyketone terpolymers chemical structure, 265 eugenol, 425

European Commission Directive 2002/72/ EC, 321 European Commission Directives, 682±4 active and intelligent food contact materials, 683±4 European Framework Directives, 682 European Commission Regulation No. 450/ 2009, 337 European Council Directive 97/48/EC, 326, 327 European Council Directive 82/711/EEC, 326 European Council Directive 85/572/EEC, 326 European Council Directive 89/109/EEC, 317±18 EVAflex150/LDPE, 441 extracellular matrix, 71 extraction, 319 fibrous protein, 610±11 Fick's law, 5, 129, 130, 304 Fick's law diffusion coefficient, 206 Fick's law of diffusion, 207 film blowing, 539±40 fingerroot oil, 427 foaming agents, 672 food contact multifunctional nanoclays, 31±9 antimicrobial nanoclays, 33±7 future trends, 39 oxygen-scavenging nanoclays, 37±9 food contact materials, 673±4 food packages, 460 food packaging carrageenan polysaccharides, 594±606 barrier performance, 598±601 nanocomposites, 601±6 processing in packaging, 597 structure and properties, 595±6 cellulose nanofiller, 86±102 cellulose-reinforced nanocomposites preparation, 99±101 extraction and refining, 91±5 future trends and applications, 101±2 mechanical properties, 95±6 morphological and structural characteristics, 87±90 surface modification, 96±9 chemical antimicrobial agents incorporation into polymeric films, 368±420 antimicrobial activity, 403±4 antimicrobial agents, 371±2, 373±6 antimicrobial films and coatings, 393±402 chemical antimicrobial agents, 372, 377±80

ß Woodhead Publishing Limited, 2011

Index future trends, 404 nano-antimicrobial agents, 390±3 natural antimicrobial agents, 380±9 polymers, 389±90 chemical vapour deposition processes, 291±4 atmospheric pressure CVD, 294 catalytic CVD, 294 plasma enhanced CVD, 292±3 plasmaline antenna reactor system, 293 RF PEVCD system schematic diagram, 292 chitosan polysaccharide, 571±87 antimicrobial activity, 574±82 barrier performance, 582±4 future trends, 586±7 nanocomposites, 584±6 processing in packaging, 573±4 structure and properties, 572±3 diffusion barrier coated polymers functional properties, 303±10 coating fragmentation, 308 gas transport through a film, 304 micron-scale and nanoscale defects, 305 system coating/substrate nominal stress-strain derivative, 309 electrospun nanofibres, 108±23 electrospinning, 108±13 electrospinning in packaging applications, 119±21, 122 functional nanofibres, 113±15 future trends, 121±3 nanoencapsulation, 116±19 functional barriers against migration, 316±40 food safety issues related to migration, 317±19 functional barriers, 319±34 future trends, 338±9 nanostrategies for functional barriers, 335±7 functional packaging, 2±5 high barrier concept, 1±2, 3 plastics water and oxygen permeability, 3 high barrier plastics using nanoscale inorganic films, 285±311 future trends, 310±11 inorganic thin film systems, 299±303 deposition techniques and substrates, 300 deposition techniques publications, 303 gas barrier requirements, 302 lowest reported oxygen transmission rates, 303 multilayered and composites systems and O2 permeability, 299

697

mass transport and high barrier properties of polymers, 129±49 barrier, 143±6 characterisation techniques, 146±9 diffusivity, 130±1 mass transport basics, 129±30 solubility, 131±42 multifunctional and nanoreinforced polymers, 1±25 future trends, 25 nanocomposites, 21±4 novel polymers and blends, 15±21 structural factors governing barrier properties, 7±15 transport phenomenology in polymer, 5±6 nanostructured materials, 287 nanotechnologies of thin films, 287±90 natural extracts in plastic food packaging, 421±49 designing active plastic packaging systems, 430±4 factors to consider in designing active systems, 445±8 future trends, 448±9 packaging films, 434±45 plant extracts as antimicrobials and antioxidants, 422±30 Nylon-MXD6 resins, 243±59 applications, 253±5 future trends, 258±9 gas barrier properties, 246±50 nanocomposites, 255±8 other properties, 250±3 processing, 244±6 structure and general overview, 243±4 physical vapour deposition processes, 294±9 electron beam evaporation, 295 evaporation, 295 polyhydroxyalkanoates, 498±518 commercial developments, 500±1 foams and paper coatings, 515±16 future trends, 517±18 PHAs and their nanocomposite films, 502±15 polylactic acid nanocomposites, 485±94 future trends, 493±4 nanobiocomposites for monolayer packaging, 486±93 properties of polylactic acid, 485±6 protein-based resins, 610±38 applications, 634±7 future trends, 638 packaging materials characterisation, 622±34 sources, extraction, structure and properties, 610±18

ß Woodhead Publishing Limited, 2011

698

Index

structure and properties, 618±22 safety and regulatory aspects of plastics, 669±91 additives migration, 674±7 European Commission Directives on plastic containers for foods, 682±4 future trends, 687±8 Indian Standards for overall migration, 677±80 indirect food additives, 670±3 nanotechnology in food contact materials, 673±4 problems in specific migration, 687 specific migration of toxic additives, 684±7 US Food and Drug Administration Code of Federal Regulations, 681±2 silver-based antimicrobial polymers, 347±62 antimicrobial silver, 356±9 effects on human health, 349±50 future trends, 359±61 historical use of silver as antimicrobial agent, 347 incorporation of silver into coatings and polymer matrices, 350±6 nanosilver antimicrobial mechanism, 348±9 nanosilver for limitless applications, 348 sputter deposition, 296±9 ion beam sputter deposition, 298±9 magnetron cathode configuration, 297 planar magnetron sputtering system, 298 reactive magnetron sputtering, 296±8 schematics, 296 starch-based polymers, 527±60 future trends, 557±9 market of starch-based materials and potential applications, 528±31 mechanical and barrier performance of starch-based systems, 542±6 nanocomposites, 546±57 processing in packaging, 537±41 structure and properties of native and plasticised starch, 531±7 thin film technologies for polymer coating, 290±4 preparation, 290±1 wheat gluten-based materials, 649±64 future trends, 664 integrated approach for fresh fruit and vegetable packaging, 661±3 material preparation, 650±2 mechanical and barrier properties, 652±8 nanocomposites, 658±61

food simulating liquid, 661 Fosfargol, 358 Fourier transform infrared spectroscopy, 50, 264, 475 fractional free volume, 8, 9 FresherLonger Miracle Food Storage Containers, 392 FTIR see Fourier transform infrared spectroscopy functional barriers, 319±34 additives in plastic packaging, 324 impurities in dispersion coatings, 325 mass transfer of molecules, 319 mechanisms behind migration, 320 migrating substances sources and identity, 320±6 against migration, 316±40 food safety issues related to migration, 317±19 future trends, 338±9 against migration for food packaging, 316±40 migration modelling, 333±4 migration of substances from packaging to food, 319±20 migration testing, 326±8 nanostrategies, 335±7 related food safety issues, 337 plastic materials, typical additives and applications, 322±3 in practice, 328±33 novel approaches, 331±3 paper constituents and adhesives, 331 plastic packaging components, 328±9 printing ink components, 329±31 functional packaging, 2±5 fungicides, 378 galangal oil, 427 gas chromatography, 475 gases, 378±80 gas permeation, 208±11 gas-phase process, 98 gas transmission, 208±11 gas transmission rate, 208 gelatine, 474, 617 gellan gum see Phytagel GFSE see grapefruit seed extract Gibbs free energy, 132, 134 gliadin, 615, 650 globular protein, 610±11 glue effect, 277 glutenin, 615, 650 glycerol, 536, 548, 650 glycinin, 615 glycomacropeptide, 613 good manufacturing practice, 674 grapefruit seed extract, 434

ß Woodhead Publishing Limited, 2011

Index `green' synthesis, 351 GTR see gas transmission rate Guggenheim solution, 132 Hawaii Natural Energy Institute, 501 HCIc see hydrotalcite-like compounds HDPE see high-density polyethylene heat treatment, 655 HEBM see high energy ball milling Helmholtz free energy, 136 Henry's law, 5, 129 Henry's law solubility coefficient, 206±7 high barrier, 1 high barrier plastics, 285±311 high-density polyethylene, 200±1, 599 high energy ball milling, 69 high pressure processing, 269 hinokithiol, 380 HPP see high pressure processing hydrocolloids, 75, 595 hydrotalcite-like compounds, 45±52 hydrotalcites composition, interlayer distance and drug loading antibiotic-intercalated HTIc, 60 NSAID-intercalated HTIc, 57 computer-generated models Bz and p-Bz-OH anions, 65 HTIc±CFS, 60 HTIc±TIAP, 58 hydroxycinnamic acid arrangement, 66 future trends, 75±6 HTIc hybrids, 55±66 amino acids and proteins, 61±4 anti-inflammatory and antibiotic drugs, 55±61 antimicrobial and antioxidant species, 64±6 diclofenac release in phosphate buffer, 59 MgAl±DL±Phe structural model, 62 NSAID and antibiotics structural formulae and acronyms, 56 hydrotalcite-like compounds basic chemistry, 45±52 brucite sheet and MgAl±HTIc representation, 46 composition and structural aspects, 45±8 [Mg0.67Al0.33(OH)2] (CO3)0.1650.42H2O TG-DTA curves, 51 physical±chemical characterisation, 49±52 preparation methods, 48±9 steps in preparation by double waterin-oil microemulsions technique, 50 structural parameters, 48 ZnAl±HTIc micrographs, 52

699

indicated intercalation compounds composition and basal spacing amino acids, 61 antimicrobials, 65 antioxidants, 66 [Mg0.67Al0.33(OH)2] (CO3)0.1650.48H2O crystallographic data, 47 rietveld plot, 47 modified and biodegradable polymeric matrices nanocomposites, 67±75 poly(-caprolactone) case, 68 poly(hydroxyalkanoates) and hydrocolloids case, 75 procedures to obtain films, membranes and PCL-HTIc composites fibres, 68±70 nanobiocomposites, 43±76 organically modified biocompatible HTIc, 52±66 experimental routes to obtain HTIc intercalation compounds, 55 MgAl±NO3 anion exchange isotherms and MgAl±HTIc X-ray diffraction, 54 synthetic routes with molecular anions with biological activity, 53±5 PCL nanobiocomposites, 70±5 DPPH absorbance percentage reduction, 74 HTIc-Cfs/PCL film composites in vitro release tests, 71 modified drug release, 70±2 potential food packaging application, 72±5 hydroxycinnamic acid, 65 imazalil, 378 Imperm, 256 incubation temperature, 580 Indian Standards for overall migration, 677±80 extractive amount determination, 679 migration limits, 679 procedure, 679 selection of samples, 678 time-temperature test conditions, 680 indirect food additives, 670±3, 681 antiblock agents, 671 antimicrobial agents, 673 antioxidants, 671 antistatic agents, 671 foaming agents, 672 lubricants, 671 optical property modifiers, 672 plasticizers, 672 ultraviolet stabilisers, 672 inelastic X-ray scattering, 96 injection moulding, 541, 546

ß Woodhead Publishing Limited, 2011

700

Index

inorganic materials, 377±8 International Agency for Research on Cancer, 685±6 ion beam sputter deposition, 298±9 ionomers, 203 Irgaguard, 358 iron, 37±8 iron-based scavenging systems, 33 IXS see inelastic X-ray scattering JP7276582, 251 Kanegafuchi, 515 Kaneka Corporation, 515 kaolinite, 257, 276 keratins, 618 lacticin, 381 lactoferrin, 387 lamination, 205 laser ablation, 350 layered double hydroxides, 45 layer multiplying coextrusion technique, 20 LDPE see low-density polyethylene linalool, 33, 432 linear low-density polyethylene, 200, 396 liquid crystal polymers, 12 LLDPE see linear low-density polyethylene low-density polyethylene, 200, 395±6, 598 lubricants, 671 lysozyme, 387, 464 MA see modified atmosphere `macromolecules', 498 magnesium aluminium hydroxycarbonate, 43 magnesium oxide, 377 MAP see modified atmosphere packaging mass spectrometry, 475 mass transport barrier, 143±6 generated spherulite, 143 methanol permeability, 145 reciprocal of tortuosity, 144 basics, 129±30 plate subjected to steady-state gas transport, 130 characterisation techniques, 146±9 cup measurement with liquid water in the cup, 148 diffusivity, 130±1 from permeability and diffusivity data, 131 and high barrier properties of food packaging polymers, 129±49 non-equilibrium lattice fluid model, 139±41 solute (CO2)±polymer mass ratio, 141

non-equilibrium perturbed hard-sphere chain model, 141±2 CO2 solubility, 142 Sanchez±Lacombe equation-of-state model, 132±6 fluid lattice, 133 Henry's law solubility constants in Pluracol, 136 solubility, 131±42 statistical associated fluid theory models, 136±9 experimental n-pentane mass concentration in polyethylene, 139 experimental saturated liquid and vapour densities for toluene, 137 n-Pentane-polyetylene and CO2polyamide 11 data, 138 Mater-Bi, 515 medium chain length triglyceride oil, 625 melt-compounding method, 67 melt-intercalation technique, 659 melt mixing, 486 Mesosilver, 359 Metabolix Inc, 501 metalisation, 206 metallocene, 153±4 general structure, 153 metallocene technology, 217 Microban, 400 microcrystalline cellulose, 336±7, 545 microencapsulation, 467, 471 MicroFree, 400 MicroGard, 400 microperforated films, 218 microperforated polymeric films, 214 microporous films, 218±19 micro-winceyette fibres, 546 midpoint cracking, 308 migration, 319 functional barriers for food packaging, 316±40 functional barriers, 319±34 future trends, 338±9 nanostrategies for functional barriers, 335±7 mechanisms, 320 modelling, 333±4 related food safety issues, 317±19 substances from packaging to food, 319±20 testing, 326±8 milk proteins, 611±14 see also casein; whey MINERV-PHA, 501 minimal inhibitory concentration, 578±9 Mirel, 501 MMT see montmorillonites modified atmosphere, 165, 190

ß Woodhead Publishing Limited, 2011

Index modified atmosphere packaging, 165±7, 661±3 advanced technology, 215±20 antifog properties, 220 coextrusion, 216±17 continuous films, 216 customisable packaging materials, 220 film laminates tailoring, 216 interactive package, 219 metallocene technology, 217 microperforated films, 218 microporous films, 218±19 perforated films, 217 tailored oxygen transmission rate, 217 tray/lidstock compatibility, 219 advances in polymeric materials, 163±228 biodegradable polymers, 204 cellulose-based plastics, 204 future trends, 226±8 gas permeation or gas transmission, 208±11 mathematical modelling of gaseous exchange, 222±3 package management, 220 packaging systems, 214±15 polymeric films application for fruits and vegetables, 223±6 post-harvest pathology of fruits and vegetables, 188±9 advantages and disadvantages, 172±3 applications, 171±2 definition, 168 design, 221±2 methodology, 222 effect, 170 factors affecting respiration rate, 181±6 atmospheric composition, 183±4 climacteric pattern of respiration in ripening fruit, 185 fruits according to respiratory behaviour during ripening, 186 physical stress, 184 stage of development/maturity stage of the commodity, 184±6 temperature, 181±3 temperature on rate of deterioration, 182 variation of temperature quotient for respiration, 182 fresh produce response, 189±97 CO2% limits above occurrence of injury, 195 favourable and injurious effects, 191±5 MA/CA benefit for fresh fruits, 191 MA/CA benefit for fresh vegetables, 192 MA/CA optimum conditions, 193

701

O2 % limits below occurrence of injury, 194 physiological and biochemical effects, 196±7 required characteristics of plastic films, 197 tolerance limits, 195±6 gases, 172 history, 167±8 measurement of gas permeability, 209±11 concentration-increase method/equal pressure principle, 210±11 pressure-increase method/differential pressure principle, 209±10 multilayer plastic films, 205±8 barriers and permeation, 206±7 coating, 205 coextrusion, 205±6 concept and theoretical approach, 207±8 lamination, 205 metalisation, 206 objective/goal, 169 physiological factors affecting shelf-life of fresh produce, 173±88 biological structure, 186 compositional changes, 187±8 developmental processes, 188 ethylene production and sensitivity, 187 physiological breakdown, 188 respiration rate, 174±9 respiratory quotient, 180±1 transpiration, 187 polymeric films for application, 197±204 absorbers for extending shelf-life, 198 ethylene±vinyl acetate, 203 ethylene±vinyl alcohol, 202 high-density polyethylene, 200±1 ionomers, 203 linear low-density polyethylene, 200 low-density polyethylene, 200 permeability, 199 polyamide, 202 polycarbonate films, 203 polychlorotrifluoroethylene, 203 polyesters, 201 polyethylene terephthalate, 201±2 polyolefins, 200 polypropylene, 201 polystyrene, 203±4 polyvinyl alcohol, 203 polyvinyl chloride, 201 polyvinylidene chloride, 202 principles, 168±9 respiration and ethylene production rates fruits, 176±7 horticultural commodities, 175

ß Woodhead Publishing Limited, 2011

702

Index

vegetables, 178±9 utility, 170±1 water vapour permeability, 211±14 permeability coefficient of multiplayer films, 213 polymer structure and morphology on permeability, 214 subzero temperature on permeability, 213 temperature on permeability, 212±13 temperature quotient for permeability, 213 Monoxbar, 247 montmorillonites, 257, 276, 472, 548, 550, 585, 628±9 3M Scotchpak, 146 multifunctional nanoclays antimicrobial nanoclays, 33±7 active nanoclays functioning, 34 Bactiblock dosages in plastic materials, 37 PLA-Bactiblock viable cell counts before and after incubation, 37 TEM pictures, 36 food contact applications, 31±9 future trends, 39 oxygen-scavenging nanoclays, 37±9 headspace reduction, 38 LDPE-O2Block oxygen-scavenging capacity, 39 multilayer plastic films, 205±8 myofibrillar proteins, 617±18 Na+ Cloisite, 659 nano-antimicrobial agents, 390±3 nanobiocomposites, 119, 286 hydrotalcites, 43±76 future trends, 75±6 hydrotalcite-like compounds basic chemistry, 45±52 modified and biodegradable polymeric matrices nanocomposites, 67±75 organically modified biocompatible HTIc, 52±66 Nanobiomatters, 159 Nanobioter 202 A1.41, 159 Nanobioter 202 A1.49, 159 Nanobioter 404 C1.33, 159 Nanobioter 434 C1.33, 159 Nano Care Technology Ltd, China, 392 nanoclays, 471, 556 see also specific nanoclays nanocomposites, 119 carrageenan polysaccharides, 601±6 chitosan polysaccharide, 584±6 wheat gluten-based materials, 658±61 preparation and structure, 659 properties, 659±61

nanoencapsulation, 116±19 Nanofil EXM 757, 659 nanolithography, 350 nanopackaging systems high barrier plastics using nanoscale inorganic films, 285±311 diffusion barrier coated polymers, 303±10 inorganic thin film systems, 299±303 nanotechnologies of thin films, 287±90 physical vapour deposition, 294±9 thin films technologies, 290±4 nanoparticles, 471±2 nanosilver, 392 antimicrobial mechanism, 348±9 applications, 348 Nano Silver Baby Milk Bottle, 392 Nano Silver Food Containers, 392 nanotechnology, 21, 335±7 food contact materials, 673±4 nano titanium dioxide, 390±1, 392 naringinase, 469 National Committee for Clinical Laboratory Standards, 578 National Starch Co., 530 native starch, 533±4, 535 natural antimicrobial agents, 380±9 bacteriocins, 380±7 enzymes, 387±8 plant origin, 388±9 essential oils, 388±9 plant extracts, 388 natural extracts antimicrobials and antioxidant additives for food packaging materials, 424 designing active plastic packaging systems, 430±4 coating of active additives, 430 incorporation of active additives, 431±2 smart blending of active additives, 433 systematic approach for smart blending of active additives, 434 packaging films based on natural extracts, 434±45 plant extracts as antimicrobials and antioxidants, 422±30 betel oil, 423±4 cinnamon oil, 424±5, 426 clove oil, 425±6 fingerroot oil, 427 galangal oil, 427 oregano oil, 427±8 rosemary oil, 428 sweet basil oil, 428±30 plastic food packaging, 421±49 neutralisation, 577±8

ß Woodhead Publishing Limited, 2011

Index Ningbo Tianan Biologic Material Co., Ltd, 501 Nisaplin, 380, 400 nisin, 380, 464 nitrate, 350 nitrite, 381 Nodax, 501 non-equilibrium lattice fluid model, 139±41 solute (CO2)±polymer mass ratio, 141 non-equilibrium pertubed hard-sphere chain model, 141±2 CO2 solubility, 142 non-gravimetric methods, 146 non-steroidal anti-inflammatory drugs, 55±61 diclofenac release in phosphate buffer, 59 HTIc-TIAP computer-generated representation, 58 intercalated HTIc composition, interlayer distance and drug loading, 57 structural formulae and acronyms, 56 norbornene, 155 polymerisation modes, 156 Novamont, 530, 531 Novaron, 358, 400 NSAIDs see non-steroidal anti-inflammatory drugs nylon, 202 Nylon-MXD6 applications, 253±5 polymer blends, 253 aroma-staining and odour-blocking properties, 248, 250 barrier films, 250 food packaging, 243±59 future trends, 258±9 gas barrier properties, 246±50 carbonation retention, 249 carbon dioxide transmission rate, 248, 249 mechanical properties, 251±3 physical properties, 252 multilayer products, 254±5 multilayer bottle preform, 254 multilayer bottle with Oxbar oxygen scavenging additive, 255 nanocomposites, 255±8 carbon dioxide retention, 256 Imperm, 256 MXD6-kaolinite and MXD6montmorillonite, 257 other properties, 250±3 oxygen transmission rate, 246±8 humidity dependence of oxygen permeability, 247 oxygen scavenging systems, 246±8 polymer films, 246 processing, 244±6

703

biaxially drawn films, 245±6 drying and handling, 244 extrusion, 245 grades, 245 injection moulding, 245 retortability, 250±1 laminated containers, 251 structure and general overview, 243±4 chemical structure, 243 thermal properties, 250, 252 cumulative oxygen transmission coefficient, 252 injection-moulded specimens, 251 O2Block, 38 OMMT see organo-modified montmorillonite opacity, 633±4 optical density, 577, 579 optical property modifiers, 672 order±disorder transition, 534 oregano oil, 427±8 organic acids, 381 organoclays, 556 organo-modified montmorillonite, 504 OTR see oxygen transmission rate overall migration, 327 Oxbar, 247, 248, 254±5 oxygen permeability, 598±9, 626±7 oxygen-scavenging nanoclays, 37±9 headspace reduction, 38 LDPE-O2Block oxygen-scavenging capacity, 39 oxygen transmission rate, 213, 217, 246±8 ozone, 380 packaging films see antioxidant packaging films PCL see poly(-caprolactone) PC-SAFT see perturbed chain SAFT model PCTFE see polychlorotrifluoroethylene pediocin, 380, 381 PEN see polyethylene naphthalate PEO see polyethylene oxide peptides, 387±8 perforated polymeric films, 214 perforation-mediated packaging, 215 permachor values, 214 permeability, 623 permeability coefficients, 206±7, 212, 213 permeants, 8, 14±15 permeation, 206±7 pertubed hard-sphere-chain theory, 141 perturbed chain SAFT model, 136 PET see polyethylene terephthalate pH, 580 PHA see polyhydroxyalkanoates PHA synthases, 499

ß Woodhead Publishing Limited, 2011

704

Index

PHB see polyhydroxybutyrate PH3B see poly-3-hydroxybutyrate PHB Industrial S.A., 501 PHBV see polyhydroxybutyrate-co-valerate PHV see polyhydroxyvalerate physical reduction, 350±1 physical vapour deposition, 294±9, 353 Phytagel, 630 phytochemicals, 468 plant extracts, 388 plasma enhanced chemical vapour deposition, 292±3 plastic colourants, 686±7 plastic food packaging designing from natural plant extracts, 430±4 coating of active additives, 430 incorporation of active additives, 431±2 smart blending of active additives, 433 systematic approach for smart blending of active additives, 434 factors to consider in designing active systems, 445±9 characteristics of active additives and foods, 445 chemical interaction of active additives with film matrix, 445±6 cost, 447 food contact approval, 447±8 mass transfer coefficients and modelling, 446 packaging materials properties, 446±7 process conditions and residual active activity, 445 storage temperature, 446 future trends, 448±9 natural extracts, 421±49 packaging films based on natural extracts, 434±45 antimicrobial food packaging materials based on natural plant extracts, 435±7 antimicrobial packaging films, 434±42 antioxidant food packaging materials based on natural plant extracts, 443 antioxidant packaging films, 442±5 DDPH radical scavenging activity of cellulose-ether films, 444 growth inhibition of selected microorganisms by cellulose-ether coated LDPE film, 440, 441 plasticisers, 574, 599±600, 620±1, 672 plastics safety and regulatory aspects of food packaging materials, 669±91 additives migration, 674±7

European Commission Directives on plastic containers for foods, 682±4 future trends, 687±8 Indian Standards for overall migration, 677±80 indirect food additives, 670±3 nanotechnology in food contact materials, 673±4 problems in specific migration, 687 specific migration of toxic additives, 684±7 US Food and Drug Administration Code of Federal Regulations, 681±2 Plastic Storage Bags, 392 polyacrylamides, 360 polyacrylates, 360 polyamide, 202 poly(butylene succinate), 75 polycaprolactone, 472, 544, 598, 655 polycarbonate films, 203 polychlorotrifluoroethylene, 203 poly(-caprolactone), 68, 99 for modified drug release, 70±2 HTIc-Cfs/PCL film composites in vitro release tests, 71 for potential food packaging application, 72±5 DPPH absorbance percentage reduction, 74 polyesters, 201 polyethylene naphthalate, 201 polyethylene oxide, 336 polyethylene terephthalate, 201±2 polyhydroxyalkanoates, 498±518, 598 commercial developments, 500±1 foams and paper coatings, 515±16 future trends, 517±18 main suppliers world-wide, 501 PHAs and their nanocomposite films, 502±15 commercial plastics vs PHB and PHBV mechanical properties, 504 degradability, 514±15 mechanical properties, 503±6 migration, 512±13 permeability, 506±9 permeability data of PHB vs PLA and conventional synthetic plastics, 508 PHAs and polyolefins thermal properties used in food packaging, 510 PHB random chain scission reaction, 511 thermal stability, 509±12 PH3B, PHV and PHBV structures, 499 synthesis of polyhydroxybutyrate, 500 polyhydroxybutyrate, 498 poly-3-hydroxybutyrate, 75, 499

ß Woodhead Publishing Limited, 2011

Index polyhydroxybutyrate-co-valerate, 499, 500 polyhydroxyvalerate, 499 polyketones, 266 polyketone terpolymers, 265 polylactic acid, 598, 655 future trends, 493±4 general properties of commercial PLA grade, 487 melt compounded films, 490 nanobiocomposites for monolayer packaging, 486±93 nanoclays, 486±91 other fillers, 491±3 nanocomposite containing a food contact compliant nanoclay UV blocking, 489 nanocomposites for food packaging applications, 485±94 properties, 485±6 reductions in oxygen and water vapour permeability, 488 polylactides, 204 polymer blending, 573±4 polymer blends, 253 polymer chain rigidity, 9 polymer chains, 7 polymer grafting, 98±9 grafting-from technique, 99 grafting-to technique, 98 polymers, 389±90 chemistry, 7±10 polymer materials relative oxygen permeability, 7 PO2 vs fractional free volume/cohesive energy density ratio, 10 functional packaging, 2±5 future trends, 25 high barrier concept, 1±2, 3 plastics water and oxygen permeability, 3 molecular architecture, 12 morphology, 10±12 multifunctional and nanoreinforced for food packaging, 1±25 nanocomposites, 21±4 extruded films PO2 of EVOH29, 24 food retorting resistance experiments, 24 permeability reductions, 22 novel polymers and blends, 15±21 oxygen permeability modelling for EVOH/aPA blend components, 18 polymers OTR/WVTR vs properties claimed for PGA, 16 plasticisation, 12±13 structural factors governing barrier properties, 7±15 permeant, 14±15

705

temperature, 13±14 transport phenomenology, 5±6 see also specific polymers polymer-sorbate interactions, 15 polymer surface modification, 583 polyolefins, 200 polypropylene, 201 polysaccharides, 114, 115 polystyrene, 203±4, 655 polyvinyl alcohol, 13, 203, 474, 543±4 polyvinyl alcohol nanocomposites, 276±80 polyvinyl chloride, 201 polyvinylidene chloride, 202 positronium annihilation spectroscopy, 9 post-harvest pathology, 188±9 post-processing ageing, 537 potassium sorbate, 377 pressure decay method, 148 Preventol, 398 Printpack, 159 probiotics, 467 Procter & Gamble, 501 prolamin see gliadin protein-based resins applications, 634±7 applications to foods, 636±7 incorporation of functional compounds, 634±6 food packaging, 610±38 future trends, 638 packaging materials characterisation, 622±34 opacity and colour parameters, 633±4 tensile strength, elongation-at-break and Young's modulus, 632 thermal and mechanical properties, 629±33 transport properties, 623±7 water sensitivity, 627±9 sources, extraction, structure and properties, 610±18 collagen and gelatine, 617 corn zein, 616±17 milk proteins, 611±14 other proteins, 617±18 soy protein, 614±15 wheat gluten protein, 615 structure and properties, 618±22 processing aids, 620±1 protein modification, 621±2 solution casting, 618±19 thermoplastic processing, 619±20 protein modification, 621±2 proteins, 114 quartz crystal microbalance, 146 random cracking, 308

ß Woodhead Publishing Limited, 2011

706

Index

reactive magnetron sputtering, 296±8 relative humidity, 627, 652 respiration rate, 174±9, 181±6 respiratory quotient, 180±1 retortability, 250±1 retorting, 271±6 retort packaging, 250±1 Rhetech, 159 ring-opening polymerisation, 99 ROP see ring-opening polymerisation rosemary oil, 428 rubbery polymers, 6 Sanchez±Lacombe equation-of-state model, 132±6, 140 fluid lattice, 133 Henry's law solubility constants in Pluracol, 136 Sanitized, 400 SANS see small angle neutron scattering scanning electron microscopy, 51, 72 seaweeds, 595±6 self-assembly technique, 350 Sharper Image, 392 shelf-life, 173 silk fibroin, 115 silver, 34, 35, 36, 356±7, 377 antimicrobial effectiveness, 354±6 related issues and inactivation, 355±6 size and shape, 355 antimicrobial polymers for food packaging, 347±62 future trends, 359±61 scientific articles dealing with silverbased nanocomposites, 360 antimicrobial silver for food packaging, 356±9 active packaging and silver, 356±7 ion-exchange from minerals, 357±9 regulatory issues, 359 silver ion exchange mechanism, 358 effects on human health, 349±50 future trends, 359±61 historical use as antimicrobial agent, 347 incorporation into coatings and polymer matrices, 350±6 nanoparticles preparation, 350±2 biological reduction, 352 chemical reduction, 351±2 physical reduction, 350±1 nanosilver antimicrobial mechanism, 348±9 nanosilver for limitless applications, 348 silver-based nanocomposites, 352±4 medical field, 352±3 techniques and materials, 353±4 Silvercell, 353 Silverex, 353

silver hydrogels, 360 SilverIon, 353 silver ion-exchange, 357±9 silver nanoparticles, 391±2 silver sulfadiazine, 353 silylation, 97, 98 single-screw extruder, 550, 553 single-site polyolefins, 153 sisal fibres, 546 Skygreen, 507 small angle neutron scattering, 97 SME see specific mechanical energy sodium chloride, 381 sol-gel technique, 49 solubility coefficient, 604 solution blending method, 67 solution casting, 486, 618±19 solvent-based process, 651 solvent-casting technique, 101 solvent process see solution casting sorbitol, 536 soy flour, 615 soy protein, 614±15 soy protein concentrate, 615, 630 soy protein isolate, 615, 628, 630±1 specific mechanical energy, 535 specific migration, 327 split electrodes, 112 SPPA see successive pulsed plasma anodisation sputter deposition, 296±9 SSE see single-screw extruder starch-based polymers, 527±60 future trends, 557±9 technical substitution of synthetic plastics by starch plastics, 558 main applications and manufacturers, 530±1 films and nets, 530 foams, 530 moulded products, 530±1 market of starch-based materials and potential applications, 528±31, 532 current and potential volume production of starch-based materials in Europe, 529 global consumption of starch-based biodegradable polymers, 530 production capacity of starch-based polymers, 529 starch-based polymer producers, 532 starch evolution in plastic industry, 528±30 mechanical and barrier performance, 542±6 plasticised cassava starch films, 536 nanocomposites, 546±57 barrier properties, 555±7

ß Woodhead Publishing Limited, 2011

Index cellulose nanocrystals content effect on pea starch modulus and tensile strength, 554 evolution of tensile strength and elongation break with montmorillonite content, 552 influence of interlayer cation on the morphology, 549 mechanical improvements in starch± montmorillonite nanocomposites, 551 mechanical properties, 550±4 mechanical properties of starch with cellulose nanowhiskers, 553 methods of preparation, 547±8 montmorillonite effect on WVP of starch/montmorillonite nanocomposites, 557 morphology, 548±50 TEM picture showing partial exfoliation, 550 thermal properties, 555 processing in packaging, 537±41 structure and properties of native and plasticised starch, 531, 533±7 amylose and amylopectin molecular structures, 533 composition and characteristics of different starches determined on dry basis, 534 starch destructurisation, 534 starch gelatinisation, 534 statistical associated fluid theory models, 136±9 experimental n-pentane mass concentration in polyethylene, 139 experimental saturated liquid and vapour densities for toluene, 137 n-Pentane-polyetylene and CO2polyamide 11 data, 138 stereoisomerism, 12 styrene, 685±6 successive pulsed plasma anodisation, 297 surface acetylation, 98 surface-initiated single electron living radical polymerisation, 99 surface plasmon resonance, 355 surfactant, 96±7 Surshield, 254 Surshot, 254 sweet basil oil, 428±30 synthetic peptides, 389±90 TBHQ see tert-butyl hydroquinone Techbarrier, 146 teichoic acids, 579 TEMPO see tetramethypiperidine-1-oxyl radical Tenax, 327, 328

707

tert-butyl hydroquinone, 377 tetramethypiperidine-1-oxyl radical, 17 TG-DTA see thermogravimetric±differential thermal analysis thermal processing, 652 thermogravimetric±differential thermal analysis, 50 thermoplastic processing, 619±20 thermoplastic starch, 534, 535, 537, 541, 545 thymol, 33, 472 titanium dioxide, 354 tocopherols, 377 Topas, 156, 159 top-down technique, 335, 350 toxic additives, 684±7 acrylonitrile, 685 plastic colourants, 686±7 styrene, 685±6 vinyl chloride, 684±5 vinylidene chloride, 685 TPS see thermoplastic starch tranexamic acid, 61 transglutaminase, 630 transmission electron microscopy, 51, 602±3, 659 transpiration, 187 triclosan, 377 TSE see twin-screw extruder twin-screw extruder, 550, 553 Ultra-Fresh, 400 ultraviolet stabilisers, 672 Urgotul, 353 US Food and Drug Administration Code of Federal Regulations, 681±2 indirect food additives, 681 regulation and migration limits threshold, 681±2 UV light, 605±6 van der Waals, 15, 131, 142 Van't Hoof's law, 13 variable-range statistical associating fluid theory, 135, 136 vinyl chloride, 684±5 vinylidene chloride, 685 Vitamin E, 329 VR-SAFT see variable-range statistical associating fluid theory WasaOuro, 400 water barrier, 582±4 decreased permeability, 584 water clustering, 601 water sensitivity, 627±9 water vapour permeability, 211±14, 542, 544, 599, 601, 623±6

ß Woodhead Publishing Limited, 2011

708

Index

edible protein films, 626 water vapour transmission rate, 158 wheat gluten-based materials food packaging, 649±64 future trends, 664 integrated approach for fresh fruit and vegetable packaging, 661±3 material preparation, 650±2 solvent-based process, 651 technological processes, 651 thermo-mechanical process, 652 mechanical and barrier properties, 652±8 barrier properties, 652±4 functional properties modulation, 654±8 oxygen and carbon dioxide permeabilities, 657 temperature and relative humidity effect on CO2, 653 vs conventional plastics and biodegradable polyesters, 656 water vapour permeability, 658

nanocomposites, 658±61 preparation and structure, 659 properties, 659±61 wheat gluten protein, 615 whey, 611, 613±14 whey protein concentrate, 612 whey protein isolate, 612 wide-angle X-ray diffraction analysis, 659 wool keratose, 115 WVP see water vapour permeability WVTR see water vapour transmission rate XPRD see X-ray powder diffraction X-ray diffraction, 72, 73±4, 95, 96, 156 X-ray powder diffraction, 49±50 zein, 115, 633 Zeneca BioProducts, 501 zeolite, 378 Zeomic, 358 Ziegler±Natta catalysts, 153 zinc oxide, 377

ß Woodhead Publishing Limited, 2011