Fruit Manufacturing

FRUIT MANUFACTURING Scientific Basis, Engineering Properties, and Deteriorative Reactions of Technological Importance ...

Author: Jorge E. Lozano

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FRUIT MANUFACTURING Scientific Basis, Engineering Properties, and Deteriorative Reactions of Technological Importance

FOOD ENGINEERING SERIES Series Editor Gustavo V. Barbosa-Ca´novas, Washington State University

Advisory Board Jose Miguel Aguilera, Pontifica Universidad Catolica de Chile Pedro Fito, Universidad Politecnica Richard W. Hartel, University of Wisconsin Jozef Kokini, Rutgers University Michael McCarthy, University of California at Davis Martin Okos, Purdue University Micha Peleg, University of Massachusetts Leo Pyle, University of Reading Shafiur Rahman, Hort Research M. Anandha Rao, Cornell University Yrjo¨— Roos, University College Cork Walter L. Spiess, Bundesforschungsanstalt Jorge Welti-Chanes, Universidad de las Ame´ricas-Puebla

Food Engineering Titles Jose M. Aguilera and David W. Stanley, Microstructural Principles of Food Processing and Engineering, Second Edition (1999) Stella M. Alzamora, Marı´a S. Tapia, and Aurelio Lo´pez-Malo, Minimally Processed Fruits and Vegetables: Fundamental Aspects and Applications (2000) Gustavo V. Barbosa-Ca´novas and Humberto Vega-Mercado, Dehydration of Foods (1996) Gustavo V. Barbosa-Ca´novas, Pedro Fito, and Enrique Ortega-Rodriguez, Food Engineering 2000 (1997) Gustavo V. Barbosa-Ca´novas, Enrique Ortega-Rivas, Pablo Juliano, and Hong Yan, Food Powders: Physical Properties, Processing, and Functionality (2005) P.J. Fryer, D.L. Pyle, and C.D. Reilly, Chemical Engineering for the Food Industry (1997) A.G. Abdul Ghani Al-Baali and Mohammed M. Farid, Sterilization of Food in Retort Pouches (2006) Richard W. Hartel, Crystallization in Foods (2001) Marc E.G. Hendrickx and Dietrich Knorr, Ultra High Pressure Treatments of Food (2002) S.D. Holdsworth, Thermal Processing of Packaged Foods (1997) Lothar Leistner and Grahame Gould, Hurdle Technologies: Combination Treatments for Food Stability, Safety, and Quality (2002) Michael J. Lewis and Neil J. Heppell, Continuous Thermal Processing of Foods: Pasteurization and UHT Sterilization (2000) Jorge E. Lozano, Fruit Manufacturing: Scientific basis, engineering properties, and deteriorative reactions of technological importance (2006) R.B. Miller, Electronic Irradiation of Foods: An Introduction to the Technology (2005) Rosana G. Moreira, M. Elena Castell-Perez, and Maria A. Barrufet, Deep-Fat Frying: Fundamentals and Applications (1999) Rosana G. Moreira, Automatic Control for Food Processing Systems (2001) M. Anandha Rao, Rheology of Fluid and Semisolid Foods: Principles and Applications (1999) Javier Raso-Pueyo and Volker Heinz, Pulsed Electric Field Technology for the Food Industry: Fundamentals and Applications (2006) George D. Saravacos and Athanasios E. Kostaropoulos, Handbook of Food Processing Equipment (2002)

FRUIT MANUFACTURING Scientific Basis, Engineering Properties, and Deteriorative Reactions of Technological Importance

Jorge E. Lozano PLAPIQUI (UNS-CONICET) Bahia Blanca, Argentina

Dr. Jorge E. Lozano PLAPIQUI (UNS-CONICET) Camino La Carrindanga KM.7 C.C.: 717 8000 Bahia Blanca Argentina [email protected]

Color illustration: Santiago Lozano, Amblagar Studio, www.amblagar.com

Library of Congress Control Number: 2005936521 ISBN-10: 0-387-30614-5 ISBN-13: 978-0387-30614-8

e-ISBN 0-387-30616-1

Printed on acid-free paper. ß 2006 Springer ScienceþBusiness Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer ScienceþBusiness Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed in the United States of America. 9 8 7 6 5 4 3 2 1 springer.com

PREFACE The fruit processing industry is one of the major businesses in the world. While basic principles of fruit processing have shown only minor changes over the last few years, major improvements are now continuously occurring, and more efficient equipment capable of converting huge quantities of fruits into pulp, juice, dehydrated, frozen, refrigerated products, etc. make possible the preservation of products for year-round consumption. The fruit processing and storage, even under the most industrially available ‘‘mild conditions,’’ involves physical and chemical changes that negatively modify the quality. These negative or deteriorative changes include enzymatic and nonenzymatic browning, off-flavor, discoloration, shrinking, case hardening, and some other chemical, thermophysical, and rheological alterations that modify the nutritive value and original taste, color, and appearance of fruits. The ability of the industry to provide a nutritious and healthy fruit product to the consumer is highly dependent on the knowledge of the quality modifications that occur during the processing. This book emphasizes the products rather than the processes, procedures, or plant operations. It presents the influence in fruit products’ quality of the different processing methods, from freezing to high temperature techniques. Origin of deterioration, kinetics of negative reactions, and methods for inhibition and control of the same are discussed. Probable changes in thermodynamical, thermophysical, and rheological properties and parameters during processing of fruits at a wide range of soluble solids, temperatures, and pressure are also summarized. This book is intended to provide professionals involved in development and operations of the fruit industry, with the necessary information for the understanding of the deteriorative effects on the fruit quality during processing.

v

CONTENTS 1.

2.

Overview of the Fruit Processing Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Classification of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 World Production and Commercial Applications of Fruits . . . . . . . . . . . . . . . . 1.4 History of Fruit Products’ Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Harvest of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Chemical Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Postharvest Handling of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.1 Postharvest Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.2 Cooling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Controlled Atmosphere Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Modified Atmosphere Packaging of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.1 Factors Affecting Fruit Respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.2 Factors Influencing the Exact Modified Atmosphere Within a Sealed Pack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Technology of Semiprocessed Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.1 Preservation of Semiprocessed Fruit Products . . . . . . . . . . . . . . . . . . . . .

1 1 1 1 2 6 7 8 9 9 12 13 14 16 17 17

Processing of Fruits: Ambient and Low Temperature Processing . . . . . . . . . . . . . . . . . 2.1 Fruit Products and Manufacturing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Fruit Juice and Pulp Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Front-End Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1.1 Reception Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1.2 Final Grading, and Inspection and Sorting . . . . . . . . . . . . . . . 2.2.2 Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2.1 Citrus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2.2 Pomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2.3 Pressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2.4 Other Extraction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Clarification and Fining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3.1 Partial Concentrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Use of Enzymes in the Fruit Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4.1 Other Enzymes in Juice Production . . . . . . . . . . . . . . . . . . . . . . 2.2.4.2 Pectinase Activity Determination . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4.3 pH Dependence on the Pectic Enzymes Activities . . . . . . . . . . 2.2.4.4 Enzymatic Hydrolysis of Starch in Fruit Juices . . . . . . . . . . . . 2.2.5 Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5.1 Pressure Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5.2 Filter Aid and Precoating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21 21 24 27 27 28 30 30 30 32 34 35 35 36 37 38 39 40 42 42 42 vii

viii

Contents 2.2.5.3 Types of Pressure Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5.4 Vacuum Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Membrane Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6.1 Stationary Permeate Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6.2 Permeate Flux as a Function of Time . . . . . . . . . . . . . . . . . . . . 2.2.6.3 Influence of VCR on the Permeate Flux . . . . . . . . . . . . . . . . .

43 45 45 49 50 51

Processing of Fruits: Elevated Temperature, Nonthermal and Miscellaneous Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Batch Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 HTST (Short Time) Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 UHT Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4 Nonthermal Pasteurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Sterilization of Food by High Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 High-Pressure Equipment and the System . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Concentration by Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Batch Pan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Rising Film Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Falling Film Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Scraped-Surface Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5 Multiple Effect Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5.1 Thermocompression (TC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5.2 Mechanical Vapor Recompression (MVR) . . . . . . . . . . . . . . . 3.4 Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Spray Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Powder Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Miscellaneous Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Size Enlargement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.1 Instantizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.2 Agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.3 Agglomeration Process and Equipment . . . . . . . . . . . . . . . . . . 3.5.1.4 Agglomeration Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.5 Selective Agglomeration (Spherical Agglomeration) . . . . . . . .

55 55 55 55 55 56 57 57 58 58 59 60 60 61 61 62 62 65 66 67 67 67 68 70 70 71

Thermodynamical, Thermophysical, and Rheological Properties of Fruits and Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Thermophysical Properties’ Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Fruits and Fruit Products’ Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Fruit and Fruit Products’ Properties During Freezing . . . . . . . . . . . . . . 4.3.2 Water Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Experimental Data and Prediction Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1.1 Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1.2 Density Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1.3 Empirical Equations and Theoretical Density Models . . . . . .

73 73 73 75 75 75 76 77 77 77 78

2.2.6

3.

4.

Contents

ix 4.4.2

4.4.3

4.4.4

4.4.5

4.4.6 5.

Specific Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2.1 Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2.2 Prediction Models and Empirical Equations . . . . . . . . . . . . . . Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3.1 Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3.2 Prediction Models and Empirical Equations . . . . . . . . . . . . . . Thermal Diffusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4.1 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4.2 Empirical Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5.1 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5.2 Newtonian Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5.3 Non-Newtonian Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5.4 Effect of Temperature and Pressure on the Viscosity of Foodstuffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boiling Point Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Color, Turbidity, and Other Sensorial and Structural Properties of Fruits and Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Measurement of Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Absorbance Spectrophotometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.1 Spectrophotometer Components . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.2 Improved Spectrophotometers . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.3 Turbidity and Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.4 Reflection Spectrophotometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.5 Tristimulus and Special Colorimeters . . . . . . . . . . . . . . . . . . . . 5.2.1.6 CIELAB Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.8 Measurement of Tristimulus Values . . . . . . . . . . . . . . . . . . . . . . 5.2.1.9 Application of Colorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Food Dispersions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Food Dispersion Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Particle Size, Shape, and Size Distribution . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3.1 Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3.2 Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3.3 Photon Correlation Technique . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 Cloudy Fruit Juice Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Fruit Aroma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Activity Coefficients of Fruit Juice Aroma . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3 Thermodynamic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3.1 Wilson Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3.2 NRTL Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3.3 UNIQUAC Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3.4 UNIFAC Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.4 Fruit Aroma Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

80 80 80 82 82 83 83 85 85 85 87 89 91 92 94

99 99 100 100 101 103 103 103 104 106 108 108 109 109 111 112 112 113 115 115 118 118 120 121 121 122 122 123 123

x

Contents 5.4.5 5.4.6

Fruit Shrinkage During Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 5.4.5.1 Shrinkage coefficient, sb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Structural Damage During Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

6.

Chemical Composition of Fruits and its Technological Importance . . . . . . . . . . . . . . . 6.1 Proximate Composition of Fruit and Fruit Products . . . . . . . . . . . . . . . . . . . . . 6.1.1 Proteins and Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Organic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3.1 Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3.2 Pectin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.4 Lipids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.5 Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.6 Vitamins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.7 Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.8 Aroma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.9 Color compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anthocyanidins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flavones and Flavonols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flavonones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catechins and Leucoanthocyanidins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Influence of Processing and Storage on the Composition of Fruits . . . . . . . . . 6.2.1 Vitamin Destruction During Processing and Storage . . . . . . . . . . . . . . . 6.2.2 Effect of Storage on Metal Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2.1 Influence of Storage on Fruit Juice Aroma . . . . . . . . . . . . . . . 6.2.3 Fruit Juice Change in Amino Acid Content During Storage . . . . . . . . 6.2.4 Effect of Storage on Fruit Sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.5 Effect of Processing and Storage on Fruit Pigments . . . . . . . . . . . . . . . 6.2.6 Changes in Organic Acid Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.7 Changes in Phenolic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

133 133 136 136 137 139 140 140 140 141 144 144 147 148 148 148 149 150 150 152 152 153 155 157 157 157

7.

Fruit Products, Deterioration by Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Different Mechanisms of Deterioration . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Enzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Phenolic Compounds and Oxidases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Kinetics of Enzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2.1 Effect of the Temperature in the Color Change . . . . . . . . . . . 7.3 Nonenzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Maillard Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1.1 Tristimulus Parameters and Absorbance as a Measurement of Browning in Fruit Juices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1.2 Kinetics of Nonenzymatic Browning (NEB) . . . . . . . . . . . . . . 7.3.1.3 Effect of Soluble Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1.4 Effect of Reducing to Total Sugars’ Ratio (R/T) . . . . . . . . . . 7.3.1.5 Effect of the Fructose to Glucose Ratio (F/G) . . . . . . . . . . . . 7.3.1.6 Effect of Amino Acids (AA) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

163 163 163 164 164 165 167 167 168 170 170 170 171 171 172

Contents

xi 7.3.1.7 Effect of the Content of Organic Acids . . . . . . . . . . . . . . . . . . . 7.3.1.8 Effect of Other Minor Components . . . . . . . . . . . . . . . . . . . . . . 7.3.1.9 Effect of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-HMF Formation During Storage and Processing of Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

173 173 173

Inhibition and Control of Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Inhibition and Control of Enzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Thermal Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1.1 Elevated Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1.2 Refrigeration Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Chemical Inhibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Effect of the Ascorbic Acid (AA) Content in Color Change . . . . . . . . . 8.2.4 Nonconventional Chemical Inhibition of EB . . . . . . . . . . . . . . . . . . . . . . 8.2.4.1 Honey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4.2 Aromatic Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4.3 Proteases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5 Miscellaneous Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5.1 Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5.2 Ultrafiltration (UF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5.3 High-pressure treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Inhibition and Control of Nonenzymatic Browning (NEB) . . . . . . . . . . . . . . . . 8.3.1 Preventive Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1.1 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1.2 Process Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1.3 Ion Exchange Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Restorative Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2.1 Effect of Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2.2 Use of PVPP (Polyvinyl Polypyrrolidone) . . . . . . . . . . . . . . . . . 8.3.3 Miscellaneous Methods for Inhibition and Control of Nonenzymatic Browning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3.1 Color Reduction by Combined Methods . . . . . . . . . . . . . . . . . 8.3.3.2 Use of Chemical Inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

183 183 183 184 184 188 189 192 193 194 195 195 195 196 196 197 197 198 198 200 203 205 206 209

7.3.2

8.

174

209 209 210 210

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

CHAPTER 1

OVERVIEW OF THE FRUIT PROCESSING INDUSTRY 1.1. INTRODUCTION The Latin word fruor, meaning ‘‘I delight in,’’ is the source of the word fruit. Fruits are essential in the human diet as they contain compounds of nutritional importance, including vitamins that are not synthesized by the human body. Fruits are defined as the reproductive organs arising from the development of floral tissues with or without fertilization (Fig. 1.1).

1.2. CLASSIFICATION OF FRUITS It is common to classify fruits as temperate fruits, subtropical fruits, and tropical fruits depending on the region where they grow (Kader and Barret, 1996). A more specific classification is given in Table 1.1. Commonly the term fruit is often restricted to succulent (fleshy) edible fruits of woody plants like apples, melons, and such small fruits as strawberries and blueberries. As the ovary matures, its wall develops to form pericarp. Pericarp is divided into three layers: exocarp (outer), mesocarp (middle), and endocarp (inner). In fleshy fruits the pulpy layer is usually the mesocarp (as in peaches and grapes). The seed or seeds, which in some cases constitute the entire edible portion of the fruit, lie immediately within the pericarp (Koning, 1994). For example, the hard husk of coconut is the pericarp and the edible part inside is the seed. While in typical cases the fruit is confined to the ripened ovary, in apples it includes ovary plus receptacle. On the other hand, strawberry is an aggregation of small fruits and pineapple is a development of the entire inflorescence. Dehiscent and indehiscent dry fruits, and some other fruits like pumpkin and cucumber are classified as vegetables. Their processing is specifically not considered in this book. Cereals, sunflower, peanuts, and beans are also beyond the interest of this work. With this in mind, the simplified classification of fruits shown (Table 1.1) will be of interest when considering different processing methods.

1.3. WORLD PRODUCTION AND COMMERCIAL APPLICATIONS OF FRUITS Table 1.2 lists world production and major commercial applications of selected fruits. In 2003 world fruit production reached nearly 380 million metric tons (MT). World fruit production grew at an average of 0.86% per year for the period 2000–2003 (FAOSTAT, 2005) and rose 1.6% in 2004, according to the latest crop production data collated by the United Nations’ Food and Agriculture Organization (FAO). This marks the fourth consecutive annual increase in 1

2

Fruit Manufacturing Strawberry

Cashew apple

Mangosteen

Fig

Grape

el

Ari

dic

l

Receptacle

Pe le nc

du

Pe

Pomegranate seed

arp Peric Endodermal intralocular tissue

l e enta Plac lar tissu u c o l intra tum Sep

ssu

Mesoc

y ti sor ces Ac

Pineapple

arp

e

Ac c tis ess su or e y

Outer layer of the testa cle un d Pe

Apple

Orange

Tomato Peach

Figure 1.1. The origin of selected fruits from plant floral tissue. Reprinted, with permission, from the Annual Review of Plant Physiology, Vol. 27 (copyright) 1976 by Annual Reviews.

international fruit production levels. The FAO data reveal bananas as the most commonly produced fruit in the world, with output inching upward from 103,000 MT to 70.63 million MT in 2004. It was followed by grapes, with production rising from over 3 million MT to 65.5 million in 2004. Oranges are the third most widely grown fruit in the world, with output in 2004 rising roughly from 2.4 million MT to just over 63 million MT. Apple production rose from almost 270,000 MT to just over 59 million MT in 2004.

1.4. HISTORY OF FRUIT PRODUCTS’ DEVELOPMENT People have been trying to improve the quantity and quality of their food and drink for centuries. As soon as the first humans decided to settle in one place and grow their own food, they started to improve its quality and increase its quantity. The following are the remarkable milestones associated to fruit and fruit processing:

1

.

Overview of the Fruit Processing Industry

3

Table 1.1. Classification of edible fleshy fruits based on: (1) the structure of the flower where the fruit belongs; (2) the number of ovaries included; and (3) the number of carpels in each ovary.

One carpel One single ovary

Drupe

True berry Single More than one carpel

Hesperidium

Pepo

Aggregate of single fruits Multiple Derived from different parts of the flower

YEAR -4000 -2000

-1000 -300

False berries Peduncle and accessory tissues (inflorescence) Receptacle

Peach Plum Coconut Blueberry Gooseberry Grape Squash All citrus fruits Apple Squash Watermelon Raspberry Blackberry Pineapple

Fig Strawberry

MILESTONE The Egyptians master viticulture and the art of wine making. Egyptians and Sumerians learn fermentation. Although people had been eating naturally fermented foods since the Neolithic Age, the process was never understood. The Greeks develop grafting techniques, leading to the creation of orchards and groves. In the Roman Empire, drying process was used for preservation. In addition, honey was sometimes used as a preserving agent for fruit. Pliny the Elder in his Natural History describes 20 varieties of apples

0 1000 1276 The first whiskey distillery was established in Ireland. 1400 Modern candy is created in Europe when cooks dip fruits and berries into melted sugar. 1650 Fre`re Jean Oudart and Dom Pierre Pe´rignon, abbeys of Saint Pierre aux Monts de Chaˆlons and d’ Hautvillers became the fathers of naturally sparkling wine (Champagne) 1676 The Compagnie de Limonadiers of Paris was granted a monopoly for the sale of lemonade soft drinks 1850 Soft drinks are invented by mixing fruit juice with other ingredients such as sugar, carbonated water and citric acid.

4

Fruit Manufacturing Table 1.2. World production and commercial applications of selected fruits (based on Hui, 1991). 1998 world production (106 ton/year)

Fruit

Scientific name

Apples

Pyrus malus

56,060

Apricots

Prunus armeniaca

2,670

Avocados Bananas

2,325 58,618

Black berries

Persea americana M. Caveindeishii, M. Paradisiaca Rubus alleghaniensis

—

Boysen berries

Rubus

—

Breadfruits Cherries, sour

Artocarpus altilis Prunus cerasus

— 1,550

Cherries, sweet

Prunus avium

Coconuts

Livistona chinensis

47,695

Crabapples Cranberries

— 285

Currants

Malus pumila Vaccinium macrocarpum Ribes vulgare, rubrum

Elder berries

Sambuca nigra

—

Figs

Ficus carica

1,178

Goose berries

Ribes hirtellum

—

Grapefruits

5,072

Grapes

Citrus paradisi, pommelo V. rotundifolia V. vinifera Vitis labrusca

Guavas

Psidium guajava

2.1

654 —

57,397

Major commercial applications Brandy, cakes, cider, citric acid, cookies, confections, dried, essence, filling, jelly, juice, marmalade, pectin, preserves, sauce, and vinegar Brandy, cakes, citric acid, confections, dried, essence, filling, jelly, juice, marmalade, preserves, and pure´e Crushed and Pure´e Cookies, crushed, pure´e, dried, and frozen Brandy, cakes, canned, cocktail, cookies, crushed, essence, jam, jelly, juice, nectar, pie, pie filling, preserves, pure´e, snack bars, strained pure´e, syrup, and wine Brandy, cakes, canned, cereals, cocktail, confections, cookies, essence, jam, jelly, juice, nectar, pie, pie filling, preserves, strained pure´e, syrup, and wine Chunks and pure´e Brined, cakes, canned, citric acid, cookies, diet spread, frozen, jam, preserves, pure´e, and wine Brined, cakes, canned, cocktail, confections, cookies, frozen, nectar, pie, pie filling, preserves, and pure´e Brandy, cakes, canned, chunks, cocktail, confections, cookies, crushed, dried, frozen, jam, juice, nectar, pie, pure´e, snack bars, strained pure´e, and syrup Jelly, pectin, and pickles Canned, confections, crushed, glace´, jam, jelly, juice, pure´e, sauce, strained pure´e, and syrup Canned, diet spread, and pure´e. Canned, cookies, crushed, essence, glace´, jam, and pure´e Brandy, cocktail, frozen, glace´, jelly, juice, nectar, and pure´e Cakes, canned, cereals, cocktail, confections, cookies, crushed, diet spread, dried, frozen, Glico, leather, preserves, pure´e, and snack bars Canned, cocktail, confections, jelly, juice, nectar, pie, pie filling, preserves, and wine Canned, chunks, confections, crushed, rozen, Glico, juice, and marmalade Cocktail, jam, jelly, juice, sauce, Grapes, and wine Champagne, cocktail, crushed, diet spread, jam, juice, sauce, vinegar, and wine Crushed, diet spread, jam, jelly, leather, pure´e, and snack bars (continued )

1

.

Overview of the Fruit Processing Industry

5

Table 1.2. (Continued ) 1998 world production (106 ton/year)

Fruit

Scientific name

Honeydew melons Kiwifruits Kumquats Lemons

Cucumic melo

—

Actidia chinensis Fortunella margarita Citrus lemon

852

Limes

Citrus aurantifolia

Longans Loquats Lychees Mangoes Nectarines

Euphoria Eriobotrya japonica Litchi chinensis Mangifera indica Prunum nectarina

—

Olives Oranges

Olea europala Citrus quarantium, sinensis

13,757 66,212

Papayas Passion fruit

Carica papaya Passiflora

4,801 —

Peaches

Prunus persica

11,065

Pears

Pyrus cominunis

14,379

Persimmons

D. virginiana

1,960

Persimmons Pineapples

Diospyros kaki Ananas cormosus

— 12,100

Plums and Prunes Pomegranates

Prunus domestica

8,008

Punica granatum

—

Quinces

Cydonia vulgaris

334

Raspberries

Rubus idueus or R. stigosus

326

Sapotes Strawberries

Pouteria sapota Fragaria chiloensis

— 2,600

Tangerines

Citrus reticulata

6.0

9,335

23,455 —

Major commercial applications Crushed, diet spread, jam, jelly, leather, pure´e, and snack bars Canned, cocktail, crushed, and dried Jam Citric acid, cocktail, essence, frozen, Glico, juice, marmalade, and pectin Citric acid, cocktail, crushed, essence, frozen, juice, marmalade, and pectin Citric acid and crushed Cocktail and pure´e Canned and dried Crushed and pure´e Canned, cocktail, confections, cookies, crushed, diet spread, frozen, jam, and pure´e Brined and canned Cereals, champagne, confections, cookies, crushed, diet spread, essence, frozen, Glico, juice, marmalade, pectin, and wine Crushed, juice, leather, pure´e, and snack bars Crushed, juice, nectar, pure´e, and strained pure´e Brandy, brined, cakes, canned, cereals, chunks, cocktail, confections, cookies, crushed, diet spread, dried, essence, frozen, jam, juice, leather, pickles, pie, pie filling, preserves, pure´e, sauce, snack bars, strained pure´e, syrup, and wine Brandy, canned, cereals, chunks, cocktail, confections, cookies, crushed, diet spread, dried, and frozen Brandy, chunks, crushed, jam, juice, marmalade, pectin, pie, pie filling, preserves, and syrup Chunks and crushed Cakes, canned, cereals, chunks, cocktail, confections, cookies, crushed, diet spread, frozen, glace´, jam, leather, preserves, jelly, pectin, preserves, pure´e, vinegar, and wine Cereals, cocktail, confections, crushed, diet spread, juice, pure´e, and strained pure´e Jam, juice, leather, pure´e, snack bars, and strained pure´e Jam, jelly, juice, pickles, preserves, pure´e, strained pure´e, and wine Cakes, canned, cereals, confections, cookies, crushed, diet spread, essence, frozen, jam, jelly, uice, nectar, pickles, pie filling, preserves, snack bars, strained pure´e, and syrup Crushed, juice, and pure´e Brandy, cakes, cereals, confections, cookies, crushed, frozen, and juice Cocktail, frozen, glace´, juice, and pure´e

6

Fruit Manufacturing 1861

1870 1906 1913 1920 1940 1957 1958 1965 1970 1990 1992 2005

Louis Pasteur develops his technique of pasteurization, in which he protects food by heating it to kill dangerous microbes, removing the air and sealing it in a container. The Navel orange is introduced into the United States from Brazil. Modern freeze-drying techniques are mastered in France. Home refrigerators are invented in the United States. American Charles Birdseye invents the process of deep freezing foods. Microwave technology is developed, which leads to the invention of the microwave oven. The first aluminum cans were used. ‘‘Basic Four’’ food guide introduced by USDA: milk, meat, vegetable and fruit, and bread and cereal groups. Soft drinks in cans dispensed from vending machines. Plastic bottles are used for soft drinks. Irradiation approved by the FDA and USDA for use on selected foods, including papaya. USDA releases the new Food Guide Pyramid, USDA releases the new Food Guide Pyramid, providing a graphics based quantitative guide to food consumption.

1.5. HARVEST OF FRUITS Harvesting at the correct time is essential to the production of quality fruits (O’Brien et al., 1983). The correct time to pick fruit depends upon several factors, including variety, location, weather, ease of removal from the tree, and purpose to which the fruit will be put. Oranges change with respect to both sugar and acid as they ripen on the tree: sugar increases and acid decreases. The ratio of sugar to acid determines the taste and acceptability of the fruit and the juice. For this reason, in some countries there are laws that prohibit picking until a certain sugar–acid ratio has been reached. These and other measurements indicate when the fruit is ready for harvesting and subsequent processing. A large amount of the harvesting of most fruit crops is still done by hand; this labor may represent about half of the cost of growing the fruit. Mechanical harvesting is currently one of the most active fields of research for the agricultural engineer. For proper harvesting: . . .

the fruit should be picked by hand and placed carefully in the harvesting basket, in order to avoid any mechanical damage; the harvesting basket and the hands of the harvester should be clean; the fruit should be picked when it is ready to be processed into a quality product.

Moreover, the proximity of the processing plant to the source of supply for fresh raw materials presents several advantages, including the possibility to pick at the best suitable moment, reduce losses by handling/transportation, minimize raw material transport costs, and simplify methods for raw material transport. After harvesting, the organoleptic and nutritional properties of fruits deteriorate in different degrees. Causes of deterioration include the growth and activity of microorganisms, the activities of the natural food enzymes, the action of insects and rodents, changes in temperature and water content, and the effect of oxygen and light. Usual storage

1

Overview of the Fruit Processing Industry

.

7

life of fruits is between 1 and 7 days at 218C if proper measures are not taken (Kader and Barret, 1996). Many quality measurements can be made before a fruit crop is picked in order to determine if proper maturity or degree of ripeness has developed: . . .

.

Color can be checked with instruments (see Chapter 4) or by comparing the color of fruit on the tree with standard picture charts. Texture may be measured by compression by hand or by simple type of plungers. Percentage of soluble solids, which are largely sugars, is generally expressed in degrees Brix, which relates specific gravity of a solution to an equivalent concentration of pure sucrose. The concentration of soluble solids in the juice can be estimated with a refractometer or a hydrometer. The refractometer measures the ability of a solution to bend or refract a light beam, which is proportional to the solution’s concentration. A hydrometer is a weighted spindle with a graduated neck, which floats in the juice at a height related to the juice density. The acid content of fruit changes with maturity and affects flavor. Acid concentration can be measured by a simple chemical titration on the fruit juice. For many fruits the tartness and flavor are affected by the ratio of sugar to acid. In describing the taste of tartness of several fruits and fruit juices, the term sugar to acid ratio or Brix to acid ratio is commonly used. The higher the Brix the greater the sugar concentration in the juice, the higher the Brix to acid ratio the sweeter and less tart is the juice.

Once the fruit is harvested any natural resistance to microorganisms is lost. Fruits are living tissues and they continue to respire even after they have been harvested. In case of aerobic respiration, refrigeration is not enough to retard ripening and foods may not develop desired flavor/texture. Moreover, it may be harmful for tropical or subtropical fruits. To ensure maximum storage life, fruits should be harvested when mature, but not yet fully ripe or overripe (Claypool, 1983). Ripe fruit should be avoided because it will continue to ripen in storage. If harvested before they have matured, fruits will be more susceptible to storage disorders. Firmness and the level of soluble solids in the fruit are good indicators of maturity in determining picking time. Fruits are very susceptible to bruising and other forms of mechanical damage, and therefore should not be handled more than necessary. Fruits are normally transported and stored in bulk boxes (bins) kept in the orchard. Bins should not be allowed to sit for extended periods in direct sunlight, nor for more than a few hours before cooling is started (Hanson, 1976).

1.5.1. Chemical Treatments Harvested fruits are often treated with chemicals to inhibit storage disorders. Dip or spray treatments with calcium chloride plus a scald inhibitor mixed with a surfactant and fungicides are commonly used to prevent scald and a group of disorders such as bitter pit. If necessary, a surfactant is used to provide complete wetting of the fruit. Many chemicals destroy microorganisms or stop their growth but most of these are not permitted in foods; those that are permitted as food preservatives are listed in Table 1.3. Chemical food preservatives are those substances that are added in very low quantities

8

Fruit Manufacturing Table 1.3. Selected chemicals permitted as food preservatives. Agent Citric acid Acetic acid Sodium diacetate Sodium benzoate Sodium propionate Potassium sorbate Methyl paraben Sodium nitrite Sulfur dioxide*

Acceptable daily intake (mg/kg body weight)

Commonly used levels (%)

No limit No limit 15 5 10 25 10 0.2 0.7

No limit No limit 0.3–0.5 0.03–0.2 0.1–0.3 0.05–0.2 0.05–0.1 0.01–0.02 0.005–0.2

Source: Dauthy, 1995. *Sulfite containing additives have been used extensively as antibrowning agents to keep vegetables and fruits fresh looking. Because sulfites have been linked to allergic reactions, the Food and Drug Administration (FDA) prohibited the use of sulfite preservatives in fresh vegetables and fruits (Langdon, 1987).

(up to 0.2%) and do not alter the organoleptic and physicochemical properties of the foods or change only very little. Preservation of food products containing chemical food preservatives is usually based on the combined or synergistic activity of several additives, intrinsic product parameters (e.g., composition, acidity, water activity) and extrinsic factors (e.g., processing temperature, storage atmosphere, and temperature).

1.6. POSTHARVEST HANDLING OF FRUITS Fruits continue to live and respire even after they are picked (Biale and Young, 1981). A major economic loss occurs during transportation and/or storage of fresh fruits due to the effect of respiration. A conventional attempt to reduce such degradation has been to refrigerate the fruits, thereby reducing the rate of respiration. Even if fruits are to be stored for only a short period, it is still very important that the field heat be removed from them as soon as possible. The higher the holding temperature, the greater the softening and respiration rate, and the sooner the quality becomes unacceptable. Apples, for instance, respire and degrade twice as fast at 4.58C as at 08C. At 168C they will respire and degrade more than six times faster. The optimum storage temperature for fruits depends also on the variety. On the other hand, fruits require humidity to preserve, which may be reached by adding water vapor to the air in the storage room with one or more humidifiers. Maintaining the humidity within this range will also reduce weight loss. Humidity near the saturation point will promote the growth of bacteria and fungi. Table 1.4 lists the recommended storage temperature and relative humidity for selected fruits. If chilled fruits are suddenly transferred into warm air, water vapor in the air will condense on them. This ‘‘sweating’’ also occurs when the doors of a cold storage room are opened, allowing warm, moist air to enter. Sweating causes wetting, which facilitates the growth of microorganisms. If molds are found to be growing in the storage room, the interior surfaces, refrigeration coils, fans, and ducts must be disinfected.

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Table 1.4. Recommended storage temperature and relative humidity for selected fruits. Fruit Apples Apricots Bananas Berries (other than cranberries) Figs Grapefruits Grapes, vinifera Kiwis Oranges Passion fruit Peaches Pears Pineapples Plums and prunes

Temperature(8C)

Relative humidity(%)

Storage life (weeks)

1.0/4.0 0.5/0.0 13/14 2/4 0.5/0.0 10–15 1/0.5 0.5/0.0 3–9 7–10 0.5/0.0 1.5/0.5 7/13 0.5/0.0

90–95 90–95 90–95 90–95 85–90 85–90 90–95 90–95 85–90 85–90 90–95 90–95 85–90 90–95

5–52 1–3 — 1–5 1–2 6–8 4–31 14–25 8–12 3–5 2–4 2–7 2–5 2–4

Source: Hardenburg et al. (1986) and Hanson (1976).

1.6.1. Postharvest Cooling Proper postharvest cooling is advisable to: . . . .

suppress enzymatic degradation (softening) and respiratory activity; slow down or inhibit water loss (wilting); slow down or inhibit the growth of decay-producing microorganisms (molds and bacteria); reduce the production of ethylene (a ripening agent) or minimize the commodity’s reaction to ethylene.

1.6.2. Cooling Methods To reduce the cooling load fruits should be harvested as much as possible during the cool hours of the day. Allowing the fruits to sit outside overnight in bulk boxes will generally not lower their temperature. Moreover, bulk storage may cause the fruit temperature to increase. It is recommended to cool the fruits quickly and thoroughly. There are many methods of cooling fruit products before storage or shipment, including room cooling, forced-air cooling, vacuum cooling, hydrocooling, package icing, and top icing. One of the common and least expensive methods for cooling fruits is room cooling (Raghavan et al., 1996). .

Room cooling is accomplished by stacking bulk boxes, or bins, inside a refrigerated room where the heat is allowed to dissipate slowly. Cooling is achieved by moving room air around the containers. An airflow rate of 5---10 m3 min1 is necessary to cool fruits. Moreover a high relative humidity (90–95%) is necessary to avoid fruit dehydration. Although time of cooling may be too long with this method, it requires minimum handling and labor. The time of cooling may vary from several days to more than 2 weeks for the fruits to reach approximately the same temperature as the air in a cold storage room. After cooling is completed, the facility can be used for short-term storage. Bins should be spaced

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Fruit Manufacturing

between the containers and walls must be from 25 to 60 cm, and between the bins and ceiling, 45 to 60 cm. Normally, more refrigeration is required to cool down fruits than to maintain fruits at a cool temperature. Forced-air cooling. The rate of heat transfer from fruits in the middle of the box may be insufficient to overcome the temperature rise produced by natural respiration. In such a case, forced-air movement is necessary and adequate space for proper air circulation between rows of stacked bins should be allowed. Cooling is carried out by exposing the bulk boxes in a storage room to a higher air pressure on one side than the other, by means of fans that draw refrigerated air through the container vents (Fraser, 1991).

Pressure difference increases the cooling rate up to 4 times more than room cooling. Relative humidity needs to be checked to avoid substantial water loss and fruit shrinkage. Water loss increases with the cooling air velocity. As the key to forced-air cooling is the moving of cold air through the containers vents, location and size of vents need to be carefully calculated. While few or small vents slow the flow of cooling air, too many vents may produce container collapse. Some of the forced-air cooling alternatives are: (i) cold wall (where cold air is driven from a false wall, or air plenum, to cold room by fans); (ii) forced-air tunnel (an exhaust fan is placed at the end of the aisle of two rows of bins; the aisle’s top and ends are covered with plastic or canvas, creating a tunnel); and (iii) serpentine cooling (a serpentine system, which is a modification of the cold wall method, is designed for bulk bin cooling). Figure 1.2 shows a typical cold wall alternative for the forced-air cooling of fruit bins. Hydrocooling is one of the quickest methods for removing field heat from fruits. This process can be used on most commodities that are not sensitive to wetting and generally requires large volumes of chilled water to flood the fruit (Raghavan et al., 1996). Cold well or stream water may be used as a source of hydrocooling fluid, after it has been checked for purity. Fruits in boxes are placed on a conveyor that pushes the boxes through a cooling tunnel. Large quantities of chilled water are sprayed directly on the tops of the boxes. Water flows down through the product and is collected in a tank or sump underneath the tunnel.

Cooling units

Cold wall

Fans

Fruit bins

Figure 1.2. Cold wall alternative for the forced-air cooling of fruit bins.

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It may be possible to apply fungicides and scald inhibitors during hydrocooling. Water removes heat about 5 times faster than air, but is less energy efficient. Mechanical refrigeration is the most efficient method of cooling water; however, ice in water will also provide a source of coolant. If hydrocooling water is re-circulated, it should be chlorinated to minimize disease problems.

The temperature of water for hydrocooling must be kept as near to 08C as possible. To save time and energy, fruits are seldom hydrocooled to lower than 78C. Slower methods, such as forced-air or room cooling, are usually employed complementarily. The rate at which fruits may be hydrocooled depends on fruit size. Figure 1.3 shows a compact hydrocooling equipment using water as the refrigerant. It can be approximated that the size of the refrigeration systems needed for hydrocooling is 10 tons of refrigeration capacity for each ton of fruits cooled per hour. Hydrocooling may prevent recently harvested fruits from wilting, shrinking, and losing flavor. Top or liquid icing. This may be used on a variety of commodities and is particularly effective on dense and palletized packages that are difficult to cool with forced air. Because of its residual effect ice methods work well with high-respiration commodities such as sweet corn, and are not recommended for fruits. Alternative cooling methods. Alternatives to the above-mentioned cooling methods, particularly to smaller volumes of commodities, are: . . . . .

Harvest time: Harvest should be made during mornings or, when possible, night time, when commodities and air temperatures are usually coolest. Refrigeration with well water: Temperatures are usually lower than 158C. Altitude: If easily accessible, higher elevations can provide cooling. Cellars/caves: Generally maintain fairly constant, cooler-than-air temperatures. Shade: If refrigeration is not available, at least keep commodities from warming up.

Figure 1.3. Hydrocooling system (from Boyette et al. (1990). Published by North Carolina Cooperative Extension Service, with permission).

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Fruit Manufacturing

Temperature

Initial fruit temperature

Still air

Hydrocooling

Forced-air cooling

Room temperature

Time (relative) Figure 1.4. Comparison of relative cooling rate of different cooling systems.

Refrigerated trucks are not designed to cool fresh commodities. They can only maintain the temperature of previously cooled products. While Fig. 1.4 compares the relative cooling rate of different cooling systems, Table 1.5 lists recommended cooling methods for selected fruits.

1.7. CONTROLLED ATMOSPHERE STORAGE Controlled atmosphere (CA) storage prolongs fruit life by lowering the oxygen concentration and increasing the carbon dioxide concentration in the storage atmosphere. The effects of CA are based on the often-observed slowing of plant respiration in low O2 environments. There is about 21% O2 in air (Table 1.6). As the concentration of O2 falls below about 10%, fruit

Table 1.5. Recommended cooling methods for selected fruits. Commodity

Recommended cooling methods*

Apples Blueberries Peaches Strawberries Watermelons

Room cooling, forced-air cooling, hydrocooling Room cooling, forced-air cooling Forced air cooling, hydrocooling Room cooling, forced-air cooling Room cooling

Adapted from Wilson et al. (1999).

Normal storage life 1–12 months 2 weeks 2–4 weeks 5–7 days 2–3 weeks

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Table 1.6. Average air composition. Air components* Nitrogen (%) Oxygen (%) Argon (%) Carbon dioxide (ppm) Neon (ppm) Helium (ppm) Methane (ppm) Krypton (ppm) Hydrogen (ppm) Nitrous oxide (ppm) Xenon (ppm) Carbon monoxide (ppm) Ozone (ppm)

Symbol

Volume (dry air)

N2 O2 Ar CO2 Ne He CH4 Kr H2 N2 O Xe CO O3

78.08 20.95 0.93 350 18.2 5.24 2 1.1 0.5 0.3 0.08 0.05 – 0.2 0.02–0.03

*Dry atmosphere below 80 Km (ppm ¼ parts per million).

respiration starts to slow. This suppression of respiration continues until O2 reaches about 2–4% for most fruits. Depending on product and temperature, if O2 gets lower than 2–4%, fermentative metabolism replaces normal aerobic metabolism; and off-flavors, off-odors, and undesirable volatiles are produced. Similarly, as CO2 increases above the 0.03% found in air, a suppression of respiration results for some commodities. Reduced O2 and elevated CO2 together can reduce respiration more than either alone. These concentrations of oxygen and carbon dioxide also reduce the ability of the ethylene produced by ripening fruits to further accelerate fruit ripening (Kader, 1986). CA storage facilities are specially constructed, airtight cold storage rooms with auxiliary equipment to monitor and maintain specific gaseous atmospheres. Oxygen, carbon dioxide, and ethylene levels should be monitored daily and controlled within narrow limits. Recommendations for CA storage conditions change as a result of ongoing research. Optimum conditions depend on several factors, including variety and growing conditions. In general CA methods are much too expensive for applying to process fruit. Table 1.7 lists recommended oxygen and carbon dioxide storage condition for various fruits (Kader, 1985; Raghavan et al., 1996). It must be considered that higher storage temperatures lead to higher respiration rates, and gas concentrations recommended in Table 1.7 will not be successful.

1.8. MODIFIED ATMOSPHERE PACKAGING OF FRUITS Vegetables and fruits differ from other foodstuffs in that they continue to respire even when placed in a modified atmosphere. Due to the respiration, there is a danger that CO2 will increase to levels harmful to the packed commodities. On the other hand, respiration consumes oxygen and there is a danger of anaerobiosis. If fruits are packed in a sealed impermeable package, O2 is rapidly used up, CO2 builds up, anaerobic respiration will take place, and off-flavors and odors will develop. There is also the risk of the growth of anaerobic foodpoisoning organisms such as Clostridium botulinum. On the other hand, if the packaging film is completely permeable the fruit would not benefit from modified atmosphere. Furthermore,

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Fruit Manufacturing Table 1.7. O2 and CO2 condition for several fruits’ storage (optimal temperatures are listed in Table 1.4). Fruit

O2 concentration (%)

CO2 concentration (%)

2–3 2–3 2–5 5–10 5–10 3–10 2–5 2 5–10 1–2 2–3 5 1–2

1–2 1–2 2–5 15–20 15–20 5–10 1–3 5 0 –5 5 0 –1 10 0 –5

Apples Apricots Bananas Berries (other than cranberries) Figs Grapefruits Grapes, vinifera Kiwis Oranges Peaches Pears Pineapples Plums and prunes Adapted from del Valle and Palma (1997).

when fruits 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 that allows oxygen to diffuse into the interior cells more rapidly, thereby increasing metabolic activity of injured cells. Fruit packaging has progressed in the past several years. Appropriate packaging materials have been developed for most of the more common fresh-cut products. Technical challenges still exist in fruit packaging. A number of special packaging materials intended for vegetables and fruits have been developed such as smart films, microporous films, and microperforated films. 1.8.1. Factors Affecting Fruit Respiration The ability of modified atmosphere packaging (MAP) to extend the shelf life of foods has been recognized for many years. MAP may be defined as the packaging of a perishable product in an atmosphere, which has been modified so that its composition is different from that of air. In MAP of respiring foods, e.g., fresh fruits, once the atmosphere has been changed to the desired level, the respiration rate of the produce should equal the diffusion of gases across the packaging material in order to achieve an equilibrium atmosphere in the package. The potential advantages and disadvantages of MAP have been reviewed by Farber (1991). The main effects of MAP on fruits are to: . . .

lower the rate of fruit respiration (slow down ripening and senescence); slow down the rate of ethylene production, which is a natural plant hormone involved in the control of ripening; retard the growth of molds, extending the storage/shelf life of the fresh fruit.

If the packaging film is semipermeable O2 and CO2 can diffuse through it, and an equilibrium concentration of both gases is established when the rate of diffusion through the package is equal to the rate of respiration.

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The main disadvantages are: . . . . .

cost increase need of temperature control different gas formulations for each product type special equipment and personnel training product safety.

MAP technology is largely used for minimally processed fruits. MAP combined with low-temperature storage is a common method to improve the storage stability of ready-to-use products. The three main gases used commercially in MAP are oxygen, nitrogen, and carbon dioxide. The gases and their concentrations should be tailored for each individual product. The required combinations of temperature, oxygen, and carbon dioxide levels vary with fruit type, variety, origin, and season. Carbon dioxide is important because of its biostatic activity against many spoilage organisms that grow at refrigeration temperatures. Oxygen inhibits the growth of anaerobic pathogens, but in many cases does not directly extend shelf life. Nitrogen is used as a filler gas to prevent pack collapse, which may occur in high CO2 -containing atmospheres. Modified atmospheres may be produced naturally by respiration (passive MA) and by the application of gas flushing techniques (equilibrium MA) (Fig. 1.5). A sliced fruit is still alive and it continues respiring. Therefore it creates an MA within the pack with a reduced level of oxygen and an increased level of carbon dioxide. During passive MA gases’ equilibrium due to package film permeability will be reached at a certain relatively long time (a week or longer). When the estimated final equilibrium concentration of gases is

% CO2 or O2 (relative)

Active MAP CO2

Passive MAP

O2 Active MAP

Time after packaging Figure 1.5. CO2 and O2 concentration evolution during passive and active MAP of packaged fruit products (adapted from Zagory and Kader, 1988).

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Fruit Manufacturing

artificially created immediately after packaging (by air purge and replacing) respiration rate is more quickly controlled. For respiring products, the permeability characteristics of the film determine the equilibrium gas concentration achieved in the package to a large degree. The actual equilibrium MA attained within a package will also depend on factors such as the prepared form of the fruit studied, the rate of respiration at storage temperature, the pack volume and fill weight, and the surface areas for gas exchange. 1.8.2. Factors Influencing the Exact Modified Atmosphere Within a Sealed Pack Respiration depends not only on the variety of fruit but also on the stage of maturity of fruit when harvested. Storage temperature influences respiration rate of the product, affects the rate of diffusion of O2 and CO2 through the package film, and affects the rate of spoilage. The temperature must be kept both constant and low #48C. In the second place, the fitted atmosphere within the pack is influenced by gaseous environment inside the pack. The initial gas mix must be corrected and tailored to the individual products. As previously indicated, too high levels of CO2 will still reduce the respiration rate and inhibit the growth of bacteria, but physiological damage of the product may take place. Too low levels of O2 may still reduce the respiration rate, but if they are too low anaerobic respiration will take place. The package must be made from a suitable material. PVC and LDPE are the most commonly used films. Among other factors affecting MAP, antifogging agents added to film, weight of product, volume of gas, and area of film must be considered. Lipton (1975) proposed a helpful approach to selecting the required ratio of O2 to CO2 permeability of a polymeric film, which was expressed as: in Pout PCO2 O PO2 ¼ in 2 , P O2 PCO2 Pout CO2

(1:1)

in,out where PCO2 and PO2 are the permeability coefficients of CO2 and O2 respectively, and P CO2 in,out and P O2 are the partial pressure of gases inside and outside package, respectively. Table 1.8 gives permeability coefficients of different polymeric films to oxygen and carbon dioxide.

EXAMPLE Using Eq. (1.1) and Table 1.4 select the appropriate film to create an atmosphere containing 2% O2 and 5% CO2 . Partial pressure of atmospheric gases at normal conditions is Pout CO2 ¼ 0:001 atm and Pout O2 ¼ 0:21 atm, respectively. Then, from Eq. (1.1): PCO2 0:21 0:02 ¼ 3:8 ¼ 0:05 PO2 From Table 1.8, both HDPE and PP have permeability ratio to oxygen and carbon dioxide close to the required calculated value. It is worth noting that the actual ratio for a particular film is not constant, but depends on the temperature. In general, the ratio increases as the temperature decreases.

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Table 1.8. Permeability coefficient of selected polymer films. Permeability coefficient [mL (STP) cm cm2 s1 cmHg1 ] Film material

Low-density polyethylene (LDPE) High-density polyethylene (HDPE) Polypropylene (PP) Polystyrene Polyethylene terephthalate (PET)

O2

CO2

55 10.6 23 11 0.22

352 35 92 88 1.53

Adapted from Robertson, 1993.

In summary, handling of fruits requires care during harvesting, transportation and handling operation within the processing plant. Temperature and relative humidity need to be properly maintained. Refrigeration is required for prolonged storage or transportation for long distances.

1.9. TECHNOLOGY OF SEMIPROCESSED FRUIT PRODUCTS The semiprocessed fruit products are manufactured in order to be delivered to industry processing plant, to be manufactured in finished products such as jams, jellies, syrups, fruits in syrup, etc. The following categories of semiprocessed fruit are defined: .

.

.

Fruit pulps: Obtained by mechanical treatment (or, less often, by thermal treatment) of fruit followed by their preservation. Either whole fruit, halves, or big pieces are used, which enables easy identification of the species. Pulps can be classified as boiled or nonboiled. Fruit pure´es-marks: Obtained by thermal and mechanical treatment operations by which all nonedible parts (cores, peels, etc.) are removed. Pure´es-marks are also classified as boiled or nonboiled. Semiprocessed juices: Products obtained by cold pressure, or eventually by other treatments (diffusion, extraction, etc.) followed by preservation.

1.9.1. Preservation of Semiprocessed Fruit Products Preservation can be achieved by chemical means, freezing, or pasteurization. The choice of preservation process for each individual case depends on the semiprocessed product type and the shelf life needed. Chemical preservation may be carried out with sulfur dioxide, sodium benzoate, formic acid, and, on a small scale, with sorbic acid and sorbates. Preservation with sulfur dioxide is a common process because of its advantages: universal antiseptic action and very economic application. The preservation with sulfur dioxide, although linked to allergic reactions, is mainly used in pulps and pure´es. Sodium benzoate is also in use in pulps and pure´es-marks. Preservation with sodium benzoate does not firm up the texture and does not modify fruit color. The disadvantages are that it is not a universal antiseptic, and needs an acidie medium to act. Moreover, sodium benzoate is difficult to remove. Practical dosage level for 12 months’ preservation was

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Fruit Manufacturing

recommended in the range 0.18 – 0.20% sodium benzoate, depending on the product to be preserved. Sodium benzoate is used as a solution in warm water; the dissolution water level has to be at maximum 10% reported to that of semiprocessed product weight. Formic acid, an antiseptic effective against yeasts, may be used for semi-processed fruit juices at a dosage level of 0.2% pure formic acid (100%). Formic acid does not influence color and is easily removed by boiling. Because of a potential effect of pectic substance degradation, formic acid is less used in pulps and pure´es-marks preservation. Sorbic acid can be used for preservation of semiprocessed fruit products at a dosage level of 0.1% maximum. Advantages of sorbates are that they are completely harmless and without any influence on the organoleptic properties of semiprocessed fruit products. Heat treatment. As fruits have a low pH, preservation of semiprocessed fruit products by heat treatment step at maximum temperature of 1008C, can be done (pasteurization). This treatment results in a more hygienic process, thereby assuring long-term preservation. However, air-tight containers are needed and pectic substances could deteriorate if the thermal treatment is too long. Freezing. Freezing is applied to semiprocessed fruit products with a very high quality and cost. This can be done with or without sugar addition. The obvious advantages of this process are the absence of added substances, a very good preservation of quality of fruit constituents (pectic substances, vitamins, etc.), and good preservation of organoleptic properties. Freezing is done at about 20 to 308C and storage at 10 to 188C.

REFERENCES Biale, J.B. and Young, R.E. (1981). Respiration and ripening in fruits—retrospect and prospect. In Recent Advances in the Biochemistry of Fruits and Vegetables, Friend, J. and Rhodes, M.J.C. (eds.). Academic Press, NY, pp. 1–39. Boyette, M.D., Estes, E.A. and Rubin, A.R. (1990). Hydrocooling. Published by North Carolina Cooperative Extension Service 10/92.2M. TAH.220544 AG-414–4. www.bae.ncsu.edu/programs/extension/publicat/ postharv. Claypool, L.L. (1983). Biological and cultural aspects of production and marketing of fruits. In Principles and Practices for Harvesting and Handling Fruits and Nuts. O’Brien, M., Cargill, B.F. and Friedly, B.B. (eds.). AVI Publishing Company, Inc., Westport, CT, pp. 15–42. Coombe, B.G. (1976). The development of fleshy fruits. Ann. Rev. Plant Physiol. 27: 507. Dauthy, M.E. (1995). Fruit and Vegetable processing. FAO AGRICULTURAL SERVICES BULLETIN No. 119 Food and Agriculture Organization of the United Nations, Rome. In: http://www.fao.org/documents. del Valle, J.M. and Palma M.T. (1997). Preservacio´n II. Atmo´sferas controladas y modificadas. In Temas en Tecnologı´as de Alimentos. Vol. 1. J.M. Aguilera Ed. CYTED-IPN MEXICO. FAOSTAT Data (2005). FAO Statistical Databases. www.fas.usda.gov/http/Presentations./ Farber, J.M. (1991). Microbiological aspects of modified-atmosphere packaging technology—a review. J. Food Protection 9: 58–70. Fraser, H.W. (1991). Forced-air rapid cooling of fresh Ontario fruits and vegetables. Ministry of Agriculture and Food, Toronto, Ontario, AGDEX 202–736. Hanson, L.P. (1976). Commercial processing of fruits. Noyes Data Corporation, London, p. 302. Hardenburg, R.E., Watada, A.E. and Wang, C.Y. (1986). The commercial storage of fruits, vegetables, and florist and nursery stocks. USDA, Agric. Handbook, 66, p. 130. Hui, Y.H. (1991). Data sourcebook for Food Scientists and Technologists. VCH Publishers, New York. Kader, A.A. (1985). Modified atmospheres: an indexed reference list with emphasis on horticultural commodities, supplement no. 4. University of California, Davis, Postharvest Hort. Series No. 3, 31 pp. Kader, A.A. (1986). Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol. 40(5): 99–103.

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Kader and Barret (1996). Classification, composition of fruits, and postharvest maintenance of quality. In Processing Fruits: Science and Technology. V. 1 Biology, Principles, and Applications, Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomic Publishing Company, Inc., pp. 1–25. Koning, R.E. (1994). Plant Physiology Information Website. http://plantphys. info/index. html. Langdon, T.T. (1987). Prevention of browning in fresh prepared potatoes without the use of sulfiting agents. Food Technol. 41(5): 64–67. Lipton, W.J. (1975). Controlled atmospheres for fresh vegetables and fruits- why and when. In Postharvest Biology and Handling of Fruit and Vegetables, Haard, N.F. and Salunke, D.K. (eds.). AVI Publishing Company, Inc., Westport, CT, p. 130. Nagy, S., Chen C.S. and Shaw, P.E. (Eds.) (1993). Fruit Processing Technology. Agscience, Inc., Auburndale, FL. O’Brien, M., Cargill, B.F. and Fridley, R.B. (1983). Principles and Practices for Harvesting and Handling of Fruits and Nuts. AVI Publishing Company, Inc., Westport, CT, 636 pp. Raghavan, G.S.V., Alvo, P., Garie´py and Vigneault, C. (1996). Refrigerated and controlled modified atmosphere storage. In Processing Fruits: Science and Technology. V. 1 Biology, Principles, and Applications. Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomic Publishing Company, Inc., pp. 135–167. Robertson, G. (1993). Food Packaging. Principles and Practice. Marcel Dekker, Inc., NY, pp. 472–473. Wilson, L.G., Boyette, M.D. and Estes, E.A. (1999). Postharvest handling and cooling of fresh fruits, vegetables, and flowers for small farms. Part II. Cooling7/99 HIL-800 NC cooperative extension service publication AG-414–1, and USDA Agricultural Handbook No. 66. Zagory, D. and Kader, A.A. (1988). Modified atmosphere packaging of fresh produce. Food Technol. 42(9): 70–74, 76–77.

CHAPTER 2

PROCESSING OF FRUITS: AMBIENT AND LOW TEMPERATURE PROCESSING 2.1. FRUIT PRODUCTS AND MANUFACTURING PROCESSES World trade of fruit and vegetable juice averaged nearly US$4,000 million last decade (FAOSTAT, 2005). By far the largest volume of processed apples and oranges, the two most important fruit commodities, is in the form of juices, and a great part of the present chapter is devoted to describing the processing of these liquid foods. There are however many other products obtained from fruits, including canned, dried, and frozen fruit; pulps; pure´es; and marmalades. Table 2.1 lists final products and processes applied on selected fruits. In addition, developments in aseptic processing have brought new dimensions and markets to the juice industry. Juices are a product for direct consumption and are obtained by the extraction of cellular juice from fruits; this operation can be done by pressing or diffusion. Fruit juices are categorized as those without pulp (‘‘clarified’’ or ‘‘not clarified’’) and those with pulp (‘‘pulps,’’ ‘‘pure´es,’’ and ‘‘nectars’’). Other classifications include ‘‘natural juice’’ products obtained from one fruit, and ‘‘mixed juice’’ products obtained from the mixing of two or three juices of different fruit species or by adding sugar. Juices obtained by removal of a major part of their water content by vacuum evaporation or fractional freezing are defined as ‘‘concentrated juices.’’ Fruit composition is mainly water (75–90%), which is mainly found in vacuoles, giving turgor (textural rigidity) to the fruit tissue. Juice is the liquid extracted from the cells of mature fruits. Fruit cell wall is made of cellulose, hemicellulose, pectic substances, and proteins. The primary cell wall, composed of crystalline cellulose microfibrils, is made up of polymers of b-D-glucose linked by b-1-4-glycosidic linkages and cellulose embedded in an amorphous matrix of pectin and hemicelluloses. The definition of a mature fruit varies with each type. Typically, sugar and organic acid levels, and their ratio indicate maturity stage. The extracted liquid is composed of water, soluble solids (sugars and organic acids), aroma and flavor compounds, vitamins and minerals, pectic substances, pigments, and, to a very small degree, proteins and fats. The various sugars, such as fructose, glucose, and sucrose, combined with a large number of organic acids (most important being citric, malic, and tartaric), help give the fruit its characteristic sweetness and tartness. During ripening of fruits, a general decrease in acidity and starch as well as an increase in sugars is seen. Moreover, formation of odors, breakdown of chlorophyll, and hydrolysis of pectic substances also occur. It must be noted that plant tissues continue to ripen after harvest. Finally, senescence occurs, at a rate accelerated by the increase in ethylene. 21

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Fruit Manufacturing Table 2.1. Principal fruit products and manufacturing processes.

Product

Fruit

Process description

Canned

Apples

Canned apple is the product prepared from fresh apples of one variety, which are not overripe, and whose fruit is packed with or without any of the following ingredients: water, salt, spices, nutritive sweetening ingredients, and any other ingredients permissible under regulations. The product is then heat processed to ensure preservation in hermetically sealed containers Canned apple sauce is the product prepared from comminuted or chopped apples, which may or may not be peeled and cored, and to which may have been added thereto one or more of the optional ingredients specified by regulations. The product is heated and, in accordance with good manufacturing practices, bruised apple particles, peel, seed, core material, and other coarse, hard, or extraneous materials are removed. The product is processed by heat, either before or after sealing, so as to ensure preservation. The soluble solids’ content is $ 98Brix Canned cranberry sauce is the jellied or semijellied cranberry product prepared from clean, sound, matured cranberries, and contains sweetening ingredients and water. Pectin may be added to compensate for deficiency of the natural pectin content of the cranberries. The mixture is concentrated and sufficiently processed by heat to ensure preservation of the product. Final soluble solid is ﬃ 35–45% Canned fruits for salad consist of carefully selected apricots, cherries, yellow clingstone peaches, pears, pineapple, and grapes. The product is packed in a suitable liquid medium with or without the addition of sweetening ingredients, or other permissible ingredients. The product is heat processed and is processed to ensure preservation of the product in hermetically sealed containers Frozen apples are prepared from fresh apples of one variety, not overripe, which are peeled, cored, trimmed, sliced, sorted, washed, and properly drained before filling into containers. Sweetening ingredient and any other ingredient permissible under regulations may be used. The product is frozen in accordance with good commercial practice and maintained at temperatures necessary for the preservation of the product Frozen apricots are prepared from fresh fruit of one variety, which are not overripe, which are sorted, washed, and may be trimmed to ensure a clean and wholesome product. The apricots are properly drained of excess water before placing into containers. The addition of sweetening ingredients, including syrup containing pureed apricots, suitable antioxidant ingredients, and or any other ingredients permissible under regulations is allowed Frozen berries are prepared from properly ripened fresh fruit berries, are stemmed and cleaned, may be packed with or without packing media, and are frozen and stored at temperatures necessary for the preservation of the product. The same is applicable to frozen blueberries. Frozen cranberries do not need stemming before freezing Frozen sweet cherries are prepared from fresh fruit of one variety, which are not overripe, fruit of any commercial variety of sweet cherries, which are sorted, washed, and drained. The addition of nutritive sweetening ingredients is allowed. The product is frozen in accordance with good commercial practice and maintained at temperatures necessary for the preservation of the product Frozen grapefruit is prepared from fresh fruit of one variety, which are not overripe. After the fruit has been washed and peeled, and separated into segments by removing the core, seeds, and membrane it is packed with or without packing additives. The product is frozen and stored at temperatures necessary for the preservation of the product Frozen lemon concentrate is the product prepared from lemon juice (from fresh, sound, ripe, and thoroughly cleansed fruit) and lemonade ingredients (sweeteners; lemon oil, its extracts, or emulsions) and water in sufficient quantities to standardize the product. The product contains $48.08Brix (corrected for acidity). Such juices may be fresh or frozen, or fresh concentrated or frozen concentrated; processed in accordance with good commercial practice and is frozen and maintained at temperatures sufficient for the preservation of the product

Apple sauce

Cranberry sauce

Fruit salads

Frozen

Apples

Apricots

Berries

Frozen

Cherries

Grapefruit

Lemon

(continued )

2

.

Processing of Fruits

23 Table 2.1. (Continued )

Product

Fruit

Process description

Melon

Melon balls are spheres of melon flesh prepared from balls of suitable varieties of sound, fresh melons. The balls are prepared and washed in a manner to assure a clean and wholesome product. The product may be packed with the addition of a suitable fruit and or vegetable garnish; nutritive or non-nutritive sweetening ingredients, including syrup and any other ingredient permissible under regulations. It must be frozen in accordance with good commercial practice and maintained at temperatures necessary for preservation Frozen peaches are prepared from fresh peaches of one variety, which are not overripe, peaches are peeled, pitted, washed, cut, and trimmed to assure a clean and wholesome product. The peaches may be packed with the addition of a sweetening ingredient, including syrup and/or syrup containing pureed peaches and any other permissible ingredients. The product must be frozen in accordance with good commercial practice and maintained at temperatures necessary for the preservation Frozen pineapple is prepared from the properly ripened pineapple fruit, which is peeled, cored, trimmed, and washed; is packed with or without packing media; and is frozen and stored at temperatures necessary for the preservation of the product Frozen plums are prepared from clean, sound, fresh fruit of any commercial variety of plums, which are sorted, washed, drained, and pitted; which may be packed with or without the addition of a nutritive sweetening ingredient; and which are frozen in accordance with good commercial practice and maintained at temperatures necessary for the preservation of the product Dried apples are prepared from fresh apples of one variety, which are not overripe, by washing, sorting, trimming, peeling, coring, and cutting into segments. The prepared apple segments are properly dried to remove the greater portion of moisture to produce a semidry texture. The product may be sulfured sufficiently to retard discoloration. The sulfur dioxide content of the finished product should not exceed 1,000 parts per million. No other additives are allowed Dehydrated low-moisture apricots are prepared from fresh fruits of one variety, which are not overripe, which are cut, chopped, or otherwise prepared into various sizes and shapes; are prepared to assure a clean, sound, wholesome product; are processed by dehydration whereby practically all of the moisture is removed to produce a very dry texture; and are placed in a container, which has low moisture. The product is packaged to assure dryness retention and should be sulfured at a level sufficient to retain a characteristic color Dried figs are prepared from clean and sound fruits and are sorted and thoroughly cleaned to assure a clean, sound, wholesome product. The figs may or may not be sulfured, or otherwise bleached Dried peaches are the halved and pitted fruit from which most of moisture has been removed. The dried fruit is processed to cleanse and it may be sulfured sufficiently to retain color Dried pears are made with the halved fruit, which may or may not be cored, from which the external stems and calyx cups have been removed. Before packing, the dried fruit may be sulfured sufficiently to color Processed raisins are dried grapes of vinifera varieties, such raisins as Thompson Seedless Sultanian, Muscat of Alexandria, Muscatel Gordo Blanco, Sultana, or White Corinth. The processed raisins are from fresh fruit, which are not overripe. Grapes are properly stemmed and cap stemmed, seeded, sorted or cleaned, or both, and are washed in water to assure a wholesome product Dehydrated prunes are prepared from clean and sound prunes, which are pitted and prepared into various sizes and shapes, washed, and processed by dehydration to produce a very dry texture. The product is then packaged to assure retention of the dryness characteristic of the product. A safe preservative may be added

Peaches

Pineapple

Plums

Dried

Apples

Apricots

Figs

Peaches

Pears

Raisins

Prunes

(continued )

24

Fruit Manufacturing Table 2.1. (Continued )

Product

Fruit

Process description

Juices

Grape

Juices

Apple frozen concentrate

Frozen concentrated sweetened grape juice is prepared from concentrated unfermented single-strength grape juice from fresh fruit, which are not overripe, with or without aging, or grape juice depectinization, and is then concentrated. Single-strength grape juice or natural grape essence, or a combination of singlestrength grape juice and natural grape essence may be mixed to the concentrate and may or may not be packed with the addition of ingredients like sweeteners, edible fruit acid, and ascorbic acid. The product is then frozen in accordance with good commercial practice Frozen concentrated apple juice is prepared from the concentrated unfermented, liquid obtained from apple juice during the first pressing of properly prepared, clean, mature, fresh apples by good commercial processes. The juice is clarified and concentrated to at least 22.98Brix. The apple juice concentrate so prepared, with or without the addition of ingredients permissible under regulations, is packed and frozen in accordance with good commercial practice and maintained at temperatures necessary for the preservation Canned apple juice is the unfermented juice obtained from sound, ripe apples, with or without parts. No water may be added directly to the finished product. However, concentrated apple juice is allowed. Apple essence may be restored to a level that provides a natural apple juice flavor Canned grape juice is the unfermented liquid obtained from the juice of properly matured fresh grapes. Such grape juice is prepared without concentration, without dilution, is packed with or without the addition of sweetening ingredients, and is sufficiently processed by heat to assure preservation of the product in hermetically sealed containers Canned lemon juice is the undiluted, unconcentrated, unfermented juice obtained from sound, mature lemons of one or more of the high-acid varieties. The fruit is prepared by washing prior to extraction of the juice to assure a clean product. The product is sufficiently processed with heat to assure preservation in hermetically sealed containers The fruit is prepared by sorting and by washing prior to extraction of the juice. The concentrated lemon juice is prepared and concentrated in accordance with good commercial practice. It may or may not require processing by heat, subsequent refrigeration, or freezing to assure preservation of the product. The finished product may contain added pulp, lemon oil to standardize flavor, and or permissible chemical preservatives. Concentrated tangerine juice is the tangerine concentrated product obtained from sound, mature fruit. The fruit is prepared by sorting and by washing prior to extraction of the juice. The concentrated tangerine juice is processed in accordance with good commercial practice, and may or may not require processing by heat or subsequent refrigeration to assure preservation of the product. Cold-pressed oil to standardize flavor and permissible chemical preservatives may be added Orange marmalade is the semisolid or gel-like product prepared from orange fruit ingredients together with ingredients like sweeteners, food acids, food pectin, lemon juice, or lemon peel. Soluble solids of finished marmalade is $65%

Canned apple juice

Canned grape juice

Lemon singlestrength

Lemon concentrate

Tangerine

Others

Marmalade

Source: Hui (1991); Nagy et al. (1992); Somogyi et al. (1996).

2.2. FRUIT JUICE AND PULP PROCESSING Fruit processing plants can vary from a simple facility for single juice extraction and canning, to a complex manufacturing facility, which has ultrafiltration and reverse osmosis equipment, cold storage, and waste treatment plant. A simplified characteristic flow diagram of a juice

2

Processing of Fruits

.

25

processing line is shown in Fig. 2.1. Processed products can be either single strength or bulk concentrate, and are available either as clarified or cloudy juice. Production of fruit juices can be divided into four basic principal stages: . .

Front-end operation Juice extraction

Fruit Washing (brushing, spraying, etc.)

Grape, berries

Steming, destoning, peeling

Blanching

If necessary

Milling, chopping, crushing

Extraction

Seeds´removing (berries) Enzymatic treatment (maceration, liquefaction)

Puree

Centrifugation Pressing

Turbid juice

Enzymatic treatment

Heat treatment

Flocculation Clarification filtration

UF

Clear juice

Centrifugation

Cloudy juice Concentration step

Final product Figure 2.1. Typical fruit juice (clear or cloudy) and pure´e-processing line steps.

26

Fruit Manufacturing . .

Juice clarification and refining Juice pasteurization and concentration.

Figure 2.2a and b shows descriptive sketches of alternative processing steps for cloudy and/or clarified apple juice concentrate elaboration. Reception line (from silos)

Pressing

Milling/pulping

Enzymatic mashing Screening

Aroma stripping Cloudy

(a) (Sd)

UF

Supernatant (Sp) Sediment (Sd) Clarification Vacuum filtration Centrifugation

(Sp)

(UF) Ultrafiltration (UF)

Fining

Storage

Other

(b)

Concentration

Figure 2.2. Typical apple juice processing plant. (a) From fruit to cloudy juice; (b) From cloudy juice to concentrate.

2

Processing of Fruits

.

27

2.2.1. Front-End Operations This stage includes those operations related with the reception and classification of fruits in the manufacturing plant: 2.2.1.1. Reception Line . .

.

.

Weighing of incoming fruit: Origin and variety are usually recorded in this step. Unloading of fruits into silos system: Harvesting containers known as bins are commonly used worldwide for transportation of fruits from the orchard to the processing plant. Standard bins are 1:21 1:21 1:0 m in size. Up to 30 or more bins may be placed in a single truck. Once in the plant, bin dumping–unloading can be performed at least in three different ways, depending on the fruit (Fig. 2.3). Sampling and laboratory testing: Table 2.2 lists the recommended fruit controls at the reception in processing plant, including assay of soluble solids, yield, Brix-acid ratio, etc. Other special tests are Magnus–Taylor pressure tester for pears and apples, and background color for peaches. Washing of fruit: The harvested fruit is washed to remove soil, microorganisms, and pesticide residues. Spoiled fruits should be discarded before washing in order to avoid contaminating the washing tools and/or equipment and the contamination of other fruits during washing. Washing efficiency can be estimated by the total number of microorganisms present on fruit surface before and after washing.

Apples require heavy spray applications and rotary brush wash to remove any rot. Many fruits such as mechanically harvested berries are air cleaned on mesh conveyors or vibrators

• Hinged sides: Good for cherries, apricots, and peaches

Tilt when partly empty

• Bin tippers: Good for apples, pears, grapes, etc.

Bins •

Flotation unloaders: Good for fruits with density < 1 (apples)

Flotation Bins

Figure 2.3. Unloading of fruits into silos systems. Reprinted from the Encyclopedia of Food Science and Nutrition. Lozano J.E., Separation and Clarification, pp. 5187–5196 (copyright) 2003, with permission from Elsevier.

28

Fruit Manufacturing Table 2.2. Recommended fruit controls at reception. Checks per lot

Checks for every 10 lots

Once during harvest season

Color Taste Texture Flavor Soluble solids ( Brix) Variety Sanitary evaluation

Density Water content Total sugars, reducing sugars Total acidity

Ascorbic acid Mineral substances Tannic substances Pectic substances

passing over an air jet. Washers are conveyor belts or roller conveyors with water sprays, reel (cylinder) type with internal spray (Fig. 2.4), brushes and/or rubber rolls with or without studs. Vibratory-type washers are very effective for berries and small fruits. Brushes are effective in eliminating rotten portions of fruits, thus preventing problems with micotoxins (patulin in apples). Some usual practices in fruit washing are: . . .

Addition of detergents or 1.5%-HCl solution in washing water to remove traces of insecticides and fungicides; Use of warm water (about 508C) in the prewashing phase; Higher water pressure in spray/shower washers.

Washing must be done before the fruit is cut in order to avoid losing high-nutritive value soluble substances (vitamins, minerals, sugars, etc.). 2.2.1.2. Final Grading, and Inspection and Sorting Fruit sorting covers two main separate processing operations: (1) Removal of damaged fruit and any foreign substance; and (2) Qualitative sorting based on organoleptic criteria and maturity stage. The most important initial sorting is performed for variety and maturity. However, for some fruits and in special processing technologies, it is advisable to carry out a manual dimensional

Fruit in

Water in

Water spray Figure 2.4. Sketch of a reel washer with internal spray. Reprinted from the Encyclopedia of Food Science and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196 (copyright) 2003, with permission from Elsevier.

2

Processing of Fruits

.

29 Table 2.3. Fruit sorting methods.

Sorting method

Description

By size By weight By texture firmness By color

Rollers (cherries), diverging belts, reels with holes Apples and citrus sorters. Sort into 20 or more weight grades Bounce system (cranberries) Citrus color sorter measures green to yellow ratio

sorting (grading). Sorting may be performed by different ways, such as those listed in Table 2.3 (Fellows, 1988): . .

. . . .

Aligning: Feeding into some processes (peeling, trimming, etc.) needs the fruit to be placed in a single line. This may be performed with accelerating belts or water flumes. Peeling (skin removal): Although manual peeling is still used for certain large vegetables, the method is very expensive. When required, fruits are usually peeled with one of the methods (Woodroof, 1986; Fellows, 1988) listed in Table 2.4. In general, loss increases with surface to volume ratio and decreases with fruit size. Mechanical methods are the worst, with up to 30% loss, while chemical (caustic) methods reduce loss to #10%. Trimming: This is usually a manual operation that precedes cutting, in order to eliminate few defective pieces. Cutting: Many special cutters are available, including sector cutters for apples, berry slicers, dicers, etc. Pitting and coring: This operation usually occurs after sorting and peeling. In peaches, pitters cut away some flesh. Automatic cherry pitters have also been developed. Belt conveyor: Transport fruits to juice extractors (citrus), crusher and mills (pomes), or stem and seed remover (grapes and berries).

Table 2.4. Peeling methods. Method

Description

Mechanical peeling

.

. . .

Steam peeling

.

Chemical peeling

.

Hot gas peeling

.

Freeze–thaw peeling

.

By abrasion: It is used in batch with rotating abrasive base and water wash. This method is inefficient, with excessive losses. Abrasive roll peelers: This is a continuous method that combines rolls and brushes. Blade type: The fruit rotates and mechanized knives separate the peel. Live knife: Incorporates hydraulic control of the knife pressure. Good for apples and pears. Pressure steam peeling make the peel blow off with pressure drop coming out of peeling chamber. May be combined with dry caustic peeling system. Caustic peeling is extremely common. The simplest type involves immersion on a pocketed paddle wheel, with hot NaOH (20%), followed by scrubbing and washing. Tomatoes, peaches, and apples are peeled by this method. KOH is preferred because or its tissue penetration and disposal properties. When hot gas contacts a vegetable on the belt or roller conveyor, the skin is blown off by the steam formed. It is generally not used in fruits. Fruit is frozen in a low temperature medium (408C) for few seconds and then warmed in water (408C). As a result of freezing the immediate subpeel cells are disrupted, releasing pectinases, which free the peel. Peeling loss is reduced to a minimum.

30

Fruit Manufacturing

2.2.2. Extraction The method of separating most of water and soluble solids (juicing) depends on the fruit variety. 2.2.2.1. Citrus .

There are three main types of extractors manufactured by different companies (Ramaswamy and Abbatemarco, 1996):

(1) The FMC citrus juice extractor, in which juice is extracted from the whole fruit without first cutting the fruit into half. Outlet streams carry juice peel, center part, and oil emulsion. (2) The Brown extractor, in which the fruit is cut into half. Outlet streams are juice of high yield and quality, and rag and peel. (3) The Rotary press, in which the fruit is cut in half and the juice extracted in rotary cylinders. More than 75% of the world’s processors use FMC technology,this process is described in more detail here. When the upper and lower cups start to come closer to each other, the upper and lower cutters cut two holes in the fruit (Fig. 2.5a). As the upper and lower cups continue to come together, the peel is separated from the fruit (Fig. 2.5b). The peeled fruit moves into the strainer tube where the juice is instantaneously separated from the seeds and the rest of the fruit (Fig. 2.5c). 2.2.2.2. Pomes There are few problems in reducing the size of fresh ‘‘hard’’ apples or pears. After washing, pome fruits are milled. The fruit to be milled is continuously fed into the milling device. For the disintegration fixed positioned or rotating grinding knives may be used. Depending on the product quality different types of knives need to be selected. The types of fruit mills generally used are:

Figure 2.5. FMC citrus juice extractor (with permission).

2

.

Processing of Fruits

31

Figure 2.6. Fruit grinding mill.

.

. .

Fruit grinding mill: The milling tool is a rotating disk with radially arranged grinding knives. The speed of the disk is variable, permitting to produce the required particle size (Fig. 2.6). Rasp or grater mill: It consists of a revolving metal cylinder with adjustable toothed blades, which rotate toward a set of parallel metal knifes or plates. Fixed blade hammer mills: The rotor with fixed blades rotates within a perforated screen. Hammer mills may be horizontal, sloping, or vertical shaft mounting (Fig. 2.7).

Mills must not produce too much fines as these will contribute to pressing and later high pulp content in the juice. The particles should all be about the same size. Grater mills are found to be more efficient with firm fruits, while hammer mills are more suited for mature or softer fruits, provided speed is properly adjusted.

Fruit

Rotating hammer

Mesh

Pulp

Figure 2.7. Hammer mill.

32

Fruit Manufacturing

2.2.2.3. Pressing Most systems for extracting juices from apples and similar fruit pulps use some method of pressing juice through cloth of various thicknesses, in which pomace is retained. These systems, called filter presses, include (Lozano, 2003): (i) rack and cloth press, (ii) horizontal pack press, (iii) continuous belt press, and (iv) screw press. (1) In a rack and cloth press the milled fruit pulp is placed in a nylon, Dacron, or polypropylene cloth to form a ‘‘cheese,’’ with the help of a cheese form. Layers of up to 10-cm thick pulp cheeses, separated by racks made of hardwood or plastic, are stacked up to 1 m or more in height depending on maturity of the fruit and size of racks (Fig. 2.8). Rack and cloth presses are efficient but very labor intensive as they require operation, cleaning, and repairing. Maximum yield may be obtained by use of a series of two to three pressure heads located around a central pivot, using pressures up to 200 atm. (2) Cage presses are horizontal presses with enclosed cages of several cubic meters in which pressing takes place. Pomace is pumped into the cage without contact with air, thereby reducing oxidation (Fig. 2.9a). The cage is filled with a complex filter systems consisting of grooved flexible rods filled with sleeves of press cloth material. During the pressing step (Fig. 2.9b), the juice passes from the pulp, through press cloth sleeves, along grooves in the flexible rods, and out to collecting channels at the ends of the cage and the piston. The drum may be rotated, thereby breaking up the pulp and adding more water. This permits a second pressing with more juice extraction. The whole process may be automated. Although some cleaning labor is saved, rods and sleeves require a considerable amount of

Press

Pulp cheese

Racks

Expressed juice Figure 2.8. Sketch of a rack and cloth press. Reprinted from the Encyclopedia of Food Science and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196 (copyright) 2003, with permission from Elsevier.

2

.

Processing of Fruits

33

Pulp in

Juice out

Figure 2.9. Hydraulic press: (a) loading, (b) pressing.

maintenance. These presses may slow down the operating cycle for production of stable cloudy nonoxidized juice. (3) Continuous belt press: Based on the Ensink design for paper pulp pressing this type of presses offers a truly continuous operation (Fig. 2.10). In belt presses, a layer of mash (pulp) is pumped onto the belt entering the machine. The press aid may be added for improved yield. (4) Screw presses: A typical screw press consists of a stainless steel cylindrical screen, enclosing a large bore screw with narrow clearance between screw and cylinder. Adjustable back-pressure is usually provided at the end of the chamber. Breaker bars must be incorporated to disrupt the compressing mash. Capacity for screw presses of 41-cm diameter is up to 15,000 kg/h (Bump, 1989). Pulp Mesh

Bagasse

Juice Figure 2.10. Sketch of a typical fruit belt press.

34

Fruit Manufacturing

2.2.2.4. Other Extraction Methods .

.

Centrifugation: Both cone and basket centrifuges have been used in producing fruit juice. Both systems have resulted in high levels of suspended solids and a high investment cost for a given yield. Horizontal decanters are presently used for juice clarification. Diffusion extraction: This was adapted from the method used for the extraction of sugar from sugar beet. Extraction is a typical countercurrent-type process. It is desirable to retain the same driving force DC (concentration of soluble components in solids versus concentration of soluble components in liquids). In order to maintain a constant DC throughout the extraction process, it is necessary to carry out a continuous weighing of ingoing apple slice and control the water flow to the counterflow extractor, by means of a relatively simple control loop (Fig. 2.11).

The diffusion extraction process is influenced by a number of variables, including temperature, thickness, water, and fruit variety. Slices from extractors pass through a conventional press system, and the very dilute juice is returned to the extractors. It is seen that the extra juice yield from diffusion extraction compensates the extra energy cost involved for concentration. .

Addition of press aids: Hydraulic pressing does not usually require addition of press aids, unless exceptionally overmature fruit is used. For continuous screw presses however it is usually necessary to add 1% (w/w) or more of cellulose. Mixing of cellulose and fruit occurs in the mill and subsequent pumping to press. Pumping is commonly performed with a Moyno-type moving cavity food pump.

Warm water inlet

Fruit pulp out

Fruit pulp in

Juice Figure 2.11. Sketch of fruit juice diffusion extraction process.

2

.

Processing of Fruits

35

2.2.3. Clarification and Fining The conventional route to concentration is to strip aroma, then depectinize juice with enzymes, centrifuge to remove heavy sediments and filter through pressure precoat filters and polish filters (Figure 2.2a). The juice is then usually concentrated through a multistage vacuum concentrator. This process involves a slight decrease in concentration of juice during the stripping step (usually up to 10% volume is removed). Stripping usually precedes depectinization, as pectin methyl esterase releases significant quantities of methanol, which spoils the essence. The use of enzymes for clarification is described later in this chapter. When a more concentrated juice is clarified (ﬃ20 8Brix) the volume to handle is reduced practically in a half. However, viscosity increased with concentration, which may slow flocculation and filtration. If a cloudy product is required, the juice is pasteurized immediately after pressing to denature any residual enzymes. Centrifugation then removes large pieces of debris, leaving most of the small particles in suspension. 2.2.3.1. Partial Concentrates Fruit juices, both clarified or opalescent, may be concentrated up to 4 fold (ﬃ50 8Brix) with natural pectin gelling with little effort. At this point in the concentration process little heat damage is detected. This concentrate can be canned and frozen. For clear juice these suspended particles have to be removed (McLellan, 1996). It may seem simple merely to filter them out, but unfortunately some soluble pectin remains in the juice, making it too viscous to filter quickly. A dose of commercial enzyme is the accepted way of removing unwanted pectin. Depectinization has two effects: it degrades the viscous soluble pectins and it also causes the aggregation of cloudy particles. Pectin forms a protective coat around proteins in suspension. In an acidic environment (apple juice typically has a pH of 3.5) pectin molecules carry a negative charge. This causes them to repel one another. Pectinolytic enzymes degrade pectin and expose part of the positively charged protein beneath. As the electrostatic repulsion between cloudy particles is reduced, they clump together. These larger particles will eventually settle, but to improve the process flocculating agents (fining) such as gelatin, tannin, or bentonite (a type of clay) can be added. Some fining agents adsorb the enzyme onto their surface, so it is important not to add them before the enzyme has done its job. Fining agents (Table 2.5) work either by sticking to particles, thereby making them heavy enough to sink; or by using charged ions to cause particles to stick to each other, thereby making them settle to the bottom. Although this method of conventional clarification was widely used in the clarified juice industry, this technology has been practically replaced by mechanical processes such as ultrafiltration and centrifugal decanters. Yeasts and other microbes, which may have contaminated the juice, may also be precipitated by fining. What is left is a transparent, but by no means, clear juice. A second centrifugation and a subsequent filtration are needed to produce the clear juice that many consumers prefer. Another potential contributor to the haziness of juice is starch. This is particularly so if unripe apples have been used. Unripe apples may contain up to 15% starch. Although the first centrifugation—before the juice reaches the clarification tank—removes most of the starch, about 5% usually remains. This can be broken down using an amylase (amyloglucosidase) active at the pH of apple juice, added at the same time as the pectinase.

36

Fruit Manufacturing Table 2.5. Fruit juice clarification agents.

Name

Description

Sparkolloid

Sparkolloid is a natural albuminous protein extracted from kelp and sold as a very fine powder It is in general a mixture of gelatins and silicon dioxide, with animal collagen being the active ingredient Kieselsol is a liquid in which small silica particles have been suspended. It is usually used in tandem with gelatin. The dosage is 1 ml/g of gelatins. This fining aids in pulling proteins out of suspension Bentonite is sold as a powder and as course granules. It is refined clay. A better way is to add the same amount to a liter of hot water, stir well, and let stand for 36–48 h. In this time the clay swells and becomes almost a gelatin Produced from sturgeon swim bladders, isinglass is sold either as a fine white powder or as dry hard fragments. It is a protein extracted from the bladders of these fish. This product is also available as a prepared liquid called ‘‘super-clear’’ With a fine porosity pad, filters are very effective in removing particles (yeast cells, proteins, etc.) Almost all fruits contain pectin, some more than others. When added as directed, it eliminates pectin haze. There is no other way to prevent this condition, and if it is in a juice, the haze will never clear on its own

Gelatin Kieselsol

Bentonite

Isinglass

Filters or Polishers Pectic Enzymes

For juice processing both depectinization and destarching are essential. This is because most apple juice is concentrated by evaporating up to 75% of the water content before storage. This makes the juice easier to transport and store, and the concentrate’s high sugar content acts as a natural preservative. Unfortunately, heat treatment also drives off the juice’s pleasant aroma, so it is necessary to gently heat the juice and collect the volatile smell and flavor compounds, so that they can be put back again when the juice is reconstituted. Heating can cause residual pectin or starch in the juice to gel or form a haze, hence the necessity of enzyme treatment. Increased haze formation occurs when fining with gelatin and bentonite is not performed. Optimization of fining and ultrafiltration steps can help retard or prevent postbottling haze development. 2.2.4. Use of Enzymes in the Fruit Industry Commercial pectic enzymes (pectinases) and other enzymes are now an integral part of fruit juice technology (Grampp, 1976). They are used to help extract, clarify, and modify juices from many fruits, including berries, stone and citrus fruits, grapes, apples, pears, and even vegetables. When a cloudy juice or nectar is preferred (for example, with oranges, pineapples, or apricots) there is no need to clarify the liquid, and enzymes are used to enhance extraction or perform other modifications. The available commercial pectinase preparations used in fruit processing generally contain a mixture of pectinesterase (PE), polygalacturonase (PG), and pectinlyase (PL) enzymes (Dietrich et al., 1991). Enzymatic juice extraction from apples was introduced 25 years ago, and now some 3–5 million tons of apples are processed into juice annually throughout the world. The methods employed for apple juice are generally the same as those for other fruits (Table 2.6). As previously mentioned, after fruits like apples have been washed and sorted, they are crushed in a mill. Peels and cores from apple slice or sauce production may also be used

2

Processing of Fruits

.

37

Table 2.6. Application of pectolytic enzymes to fruit and vegetable processing. Enzymatic process

Examples of application

Clarification of fruit juices

Apple juice, depectinized juices can also be concentrated without gelling and developing turbidity Soft fruits, red grapes, citrus, and apples; for better release of juice (and colored material); enzyme treatment of pulp of olives, palm fruit, and coconut flesh to increase oil yield Used to obtain nectar bases and in baby foods

Enzyme treatment of pulp

Maceration of fruit and vegetables (disintegration by cell separation) Liquefaction of fruit and vegetables Special applications to citrus fruits

Used to obtain products with increased soluble solids’ content (pectinases and cellulases combined) Used for the preparation of clouding agents from citrus peel, cleaning of peels before use in candy and marmalade production, recovery of oil from citrus peel, depectinization of citrus pulp wash

(Source: Rombouts and Pilnik, 1978).

together with whole apples. Although pectinases are often added at this stage, better results are achieved if the apple pulp is first stirred in a holding tank for 15–20 minutes so that enzyme inhibitors (polyphenols) are oxidized (by naturally occurring polyphenols oxidized in the fruit). The pulp is then heated to an appropriate temperature before enzyme treatment. For apples 308C is the optimal temperature, whereas stone fruits and berries generally require higher temperatures—around 508C. This compares with 60–658C required if pectinase is not used (here the juice is liberated by plasmolysis of the plant cells). Prepress treatment with pectinases takes anything from 15 min to 2 h depending upon the exact nature of the enzyme and how much is used, the reaction temperature, and the variety of apple chosen. Some varieties such as Golden Delicious are notoriously difficult to breakdown. During incubation the pectinase degrades soluble pectin in the pulp, making the juice flow more freely. The enzyme also helps to breakdown insoluble pectin, which impede juice extraction. Enzyme treatment is considered to be complete once the viscosity of the juice has returned to its original level or less. It is important that the pulp is not broken down too much as it would then be difficult to press. Pressing is done using the previously described equipment. Juice yields can be increased by up to 20%, depending upon the age and variety of fruit used and whether preoxidation was employed. Enzyme treatment is particularly effective with mature apples and those from cold storage. 2.2.4.1. Other Enzymes in Juice Production .

.

Cellulases: The addition of cellulases during extraction at 508C improves the release of color compounds from the skins of fruit. This is particularly useful for treating blackcurrants and red grapes. Increasingly cellulases are being used at the time of the initial pectinase addition to totally liquefy the plant tissue. This makes it possible to filter juice straight from the pulp without any need for pressing. Arabanase: The polysaccharide araban (a polymer of the pentose arabinose) may appear as a haze in fruit juice a few weeks after it has been concentrated. Although commercial pectinase preparations often contain arabanase, certain fruits (like pears)

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.

are rich in araban and may require the addition of extra arabanase to the clarification tank. Glucose oxidase (from the fungi Aspergillus niger or Penicillium spp.): Catalyzes the breakdown of glucose to produce gluconic acid and hydrogen peroxide. This reaction utilizes molecular oxygen. Glucose oxidase (coupled with catalase to remove the hydrogen peroxide) is therefore used to remove the oxygen from the head space above bottled drinks, thereby reducing the nonenzymatic browning due to oxidation, which might otherwise occur.

2.2.4.2. Pectinase Activity Determination Complete pectin breakdown in apple juices can be ensured only if all the three types of pectinolytic enzymes (PG, PE, PL) are present in the correct proportion. Apple juice processor in general lacks reliable methods for checking the different enzyme activities. Application and success of a pectinase product also depend on the substrate where they act. The problems in evaluation of pectinolytic activities are caused by the difficulty in standardizing fruit substrate. Acidity, pH, and the presence of inhibitors or promoters of the enzymatic reaction depend upon the variety of apple processed. Figure 2.12 shows the residual polygalacturonase activity of two commercial enzymes after 30 min of heating at different temperatures. They start to become inactivated at temperatures higher than 508C, which is a very well defined breaking point, if the enzyme

1 PG-PECTINOL A1 2 PG-R HAPECT D5S 3 PL-R HAPECT D5S

100

Relative Residual Activities (%)

80

60

2

1

3

40

20

0 40

50

60

70

Temperature (⬚C) Figure 2.12. Enzymatic residual activities after thermal treatment 30 min at different temperatures) of enzyme solutions in 0.1 M citrate/0.2 M phosphate buffers at optimum pH). Reprinted from Food Chem. 31(½), Ceci, L. and Lozano, J.E. Determination of enzymatic activities of commercial pectinases, 237–241. (copyright) 2003, with permission from Elsevier.

2

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39

were to rapidly loses its activity. The rate of PG activity decrease could be divided into two periods (Fig. 2.12). The first period was characterized as a thermolabile fraction. The second can be defined as the thermoresistant fraction of the enzyme. Sakai et al. (1993) and Liu and Luh (1978) reported that the optimal temperature for PG activity was in the range 30–508C. Both authors indicated that for temperatures greater than 508C inactivation was notable after a short period of heating. Moreover, the optimal temperature is also a function of the type of substrate to be treated (Ben-Shalom et al., 1986). Inactivation curve of lyase activity (PL) is also shown in Fig. 2.12. As in the case of PG activities, 508C can be easily identified as the breaking point where PL rapidly inactivates. Alkorta et al. (1996) found that PL from Penicillum italicum was active after 1 h at 508C but resulted in complete inactivation for the same period at 608C. The commercial enzymes proved to be more heat tolerant than purified fractions (Liu and Luh 1978). This phenomenon was attributable to the heat protective action of impurities. 2.2.4.3. pH Dependence on the Pectic Enzymes Activities Figure 2.13 shows the behavior of PG and PE activities of RHD5 enzyme versus pH. The resulting optimum pH was approximately 4.6. However, the curve for lyase activity as a

RÖHAPECT D5S 100

RELATIVE ACTIVITIES (%)

80

60 PL

PG 40 PE

20

0 4.0

6.0 pH

Figure 2.13. Effects of pH on the enzymatic activities of Ro¨hapect D5S (Ceci and Lozano, 1998). Reprinted from Food Chem. 31(½), Ceci, L. and Lozano, J.E. Determination of enzymatic activities of commercial pectinases, 237–241. (copyright) 2003, with permission from Elsevier.

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function of pH was much broader, and it was difficult to identify a single optimal value. In this case, an optimal range of pH 5–6 may be defined. As a result, as much as 40% of PG and PE inactivation can be expected during the enzymatic clarification of the relatively acidic Granny Smith juice. It was found that a shift in the optimal pH toward the acid zone (Spagna et al., 1993) or a broadening of the optimal activities’ range (Ates and Pekyardimci, 1995) can be obtained after enzyme immobilization on appropriate supports. In general pectinolytic enzymes (PG and PE) show a rapid decrease in activity at about pH 5 and become practically inactivated near neutrality. However, this problem becomes irrelevant because pH values of fruit juices are lower than pH 5. It was found that a shift in the optimal pH toward the acid zone (Spagna et al., 1993) or a broadening of the optimal activities’ range (Ates and Pekyardimci, 1995) can be obtained after enzyme immobilization on appropriate supports As fruit juice clarification is usually done at 45–50C, special care must be taken to avoid excessive inactivation when lyase activity is considered important. It is known (Dietrich et al., 1991) that commercial enzyme preparations cause a certain degree of side activities other than those required (Ceci and Lozano, 1998). 2.2.4.4. Enzymatic Hydrolysis of Starch in Fruit Juices Starch can be a problem for juice processors. Polymeric carbohydrates like starch and arabans can be difficult to filter and cause postprocess cloudiness. In the case of a positive starch test, the following problems may occur: slow filtration, membrane fouling, gelling after concentration, and postconcentration haze. Apple juice is one of the juices that can contain considerable amounts of starch, particularly at the beginning of the season. Unripe apples contain as much as 15% starch (Reed, 1975). As the apple matures on the tree, the starch hydrolyzes into sugars. The starch content of apple juice may be high in years when there were relatively low temperatures during the growing season. Besides the generalized application of commercial amylase enzymes in the juice industry, there is a lack of information on apple starch characteristics and extent of gelatinization during juice pasteurization. Starch must be degraded by adding starch-splitting enzymes, together with the pectinase during depectinization of the juice. First starch must be gelatinized, by heating the juice to 778C. When an aqueous suspension of starch is heated the hydrogen bonds weaken; water is absorbed; and the starch granules swell, rupture, and gelatinize (Zobel, 1984). The juice must then be cooled to <508C to avoid enzyme inactivation. Starch is generally insoluble in water at room temperature. Because of this, it is stored in cells as small granules. Starch granules (Fig. 2.14) are quite resistant to penetration by both water and hydrolytic enzymes due to the formation of hydrogen bonds within the same molecule and with other neighboring molecules. When the starch granule is not broken down completely, a short-chained dextrin is left. This can lead to a condition known as retrograding. When starch retrogrades, the short-chained dextrin recrystallizes into a form that is no longer susceptible to enzyme attack, regardless of heating. Figure 2.15 shows a SEM photomicrograph of haze sediment obtained from a pasteurized apple juice sample.

2

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Figure 2.14. Scanning electron photomicrograph of an isolated apple starch granule (5 kV 4,400).

The scanning electron micrograph shows how the apple starch granules collapsed after heat treatment, while dispersion of gel-like starch fragments among the other components of turbidity (pectin, cellular wall, etc.) can be observed. Similar behavior was found when wheat starch was gelatinized by heat in excess water (Lineback and Wongsrikasen, 1980).

Figure 2.15. Scanning electron photomicrograph of cloudiness precipitated from a pasteurized (5 min at 908 + 1 C) apple juice (5 kV 3,600).

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2.2.5. Filtration Filtration is also a mechanical process designed for clarification by removing insoluble solids from a high-value liquid food, by the passage of most of the fluid through a porous barrier, which retains most of the solid particulates contained in the food. Filtration is performed using a filter medium, which can be a screen, cloth, paper, or bed of solids. Filter acts as a barrier that lets the liquid pass while most of the solids are retained. The liquid that passes through the filter medium is called the filtrate. There are several filtration methods and filters (Lozano, 2003) including: .

.

.

Driving force: The filtrate is induced to flow through the filter medium by: (a) hydrostatic head (gravity), (b) pressure (upstream filter medium), (c) vacuum (downstream filter medium), and (d) centrifugal force across the medium. Filtration mechanism: (a) cake filtration (solids are retained at the surface of a filter medium and pile upon one another). (b) depth or clarifying filtration (solids are trapped within the pores or body of the filter medium). Operating cycle: (a) Intermittent (batch) and (b) Continuous rate.

These methods of classification are not mutually exclusive. Thus filters usually are divided first into the two groups of cake and clarifying equipment, then into groups of machines using the same kind of driving force, then further into batch and continuous classes. Some filtering devices usually employed in the food industry are described here. 2.2.5.1. Pressure Filters The main advantages of pressure filtration compared to other filtering methods are: Cakes are obtained with very low moisture content, clean filtrates may be produced by recirculating the filtrate or by precoating, and the solution can be polished (finished) to a high degree of clarity. Among disadvantages it must be noted that cloth washing is difficult, and if precoat is required, the operator cannot see the forming cake and is unable to carry out an inspection while the filter is in operation, and the internals are difficult to clean, which can be a problem with food-grade applications. With the exception of the rotary drum pressure filter, this type of filters has a semicontinuous machine in which wash and cake discharge are performed at the end of the filtration cycle. Since the operation is in batches, intermediary tanks are required. The collection of filtrate depends on the operating mode of the filter, which can be at constant flow rate, constant pressure, or both, with pressure rising and flow rate reducing during filtration. The filtration rate is mainly influenced by the properties of food (particle size and distribution, presence of gelatinous solids like pectin, liquid viscosity, etc.) Although continuous pressure filters are available, they are mechanically complex and expensive, so they are not common in the food industry. 2.2.5.2. Filter Aid and Precoating Filter aid and precoating are often used in pressing and in connection with pressure filtration. Filter aid is used when the pulp or turbid liquid food is low in solids’ content with fine and muddy particles that are difficult to filter. To enhance filtration coarse solids with large surface area that capture and trap the slow-filtering particles from the suspension in its interstices and produces a porous cake matrix are used.

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43

On the other hand, precoating is the formation of a defined thick medium of a known permeability on the filter plates. Precoating prior to filtration serves when the particles that are to be separated are gelatinous and sticky, forming a barrier that avoids cloth blinding. The most common filter aids and precoating materials employed in the food industry are: diatomaceous earth (silicaceous skeletal remains of aquatic unicellular plants), perlite (glassy crushed and heat-expanded volcanic rock), cellulose (fibrous light weight and ashless paper), and special groundwood. 2.2.5.3. Types of Pressure Filters Pressure filters usually found in the food industry are: .

.

Filterpress, also called Plate and Frame, consists of a head and a follower that contain in between a pack of vertical rectangular plates that are supported by side or overhead beams (Fig. 2.16). The head serves as a fixed end to which the feed and filtrate pipes are connected, while the follower moves along the beams and presses the plates together during the filtration cycle by a hydraulic or mechanical mechanism. Each plate is covered with filter cloth on both sides and, once pressed together, they form a series of chambers that depend on the number of plates. The plates have generally a centered feed port that passes through the entire length of the filter press, so that all chambers of the plate pack are connected together. Vertical and horizontal pressure leaf filters consist of a vessel that is fitted with a stack of vertical (Fig. 2.17), or horizontal leaves that serve as filter elements. The leaf is constructed with ribs on both sides to allow free flow of filtrate toward the neck and is covered with coarse mesh screens that support the finer woven metal screens or filter cloth that retain the cake. The space between the leaves varies from 30 to 100 mm depending on the cake formation properties and the ability of the vacuum to hold a thick and heavy cake to the leaf surface. The filters can be used for polishing fruit juices

Filtrate out Feed Filtrate

Feed

Filtrate Figure 2.16. Filterpress operation sketch and details of filtering plate. Reprinted from the Encyclopedia of Food Science and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196. (copyright) 2003, with permission from Elsevier.

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Feed in Plate Filtrate

Figure 2.17. Horizontal plate filter. Reprinted from the Encyclopedia of Food Science and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196. (copyright) 2003, with permission from Elsevier.

.

with very low solids or for cake filtration with a solids’ concentration of <20–25%. The cloth mesh screens that cover the leaves can be more easily accessed on horizontal tanks than on vertical tanks. Candle filters are used in applications that require efficient low moisture cake filtration or high degree of polishing (finishing). Candle filters can contain more than 250 filtering elements. They consist basically of three major components (Fig. 2.18): (1) The vessel; (2) The filtering elements (candles) and (3) The cake discharge outlet.

The vessel configuration has a conical bottom for cake filtration and polishing, or has a dished bottom for slurry thickening, though it is scarcely used in the food industry. The filtering element generally consists of the filtrate core and the filtering medium. The core helps in filtrate passage and supports the filter medium. The core is a bundle of

Filtrate

Bunch of candles

Vessel

Figure 2.18. Candle filter sketch showing major components. Reprinted from the Encyclopedia of Food Science and Nutrition, Lozano J.E., Separation and Clarification, pp. 5187–5196. (copyright) 2003, with permission from Elsevier.

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45

perforated stainless steel tubes. The filter medium can be a porous ceramic, woven mesh screen, sintered metal tube, or synthetic filter cloth. The advantages of a candle filter are the excellent cake discharge capacity and its mechanical simplicity. 2.2.5.4. Vacuum Filters Vacuum filters are simple and reliable machines widely used in the fruit industry. Among the different types of vacuum filter (drum, disk, horizontal belt, tilting pan, and table filters) drum filters are most commonly utilized in the food industry. The advantages and disadvantages of vacuum filtration compared to other separation methods are: . .

Advantages: Continuous operation, very effective polishing (finishing) of solutions (on a precoat filter), and easy control of operating parameters such as cake thickness. Disadvantages: Higher residual moisture in the cake and difficulty in cleaning (as required mainly for food-grade applications).

Precoat filters are used when liquid foods (e.g., clarified apple juice) require a very high degree of clarity. To polish the solution the drum deck is precoated with an appropriate medium (See Filter aids and precoating in this chapter). A scraper blade also called ‘‘Doctor Blade,’’ moves slowly toward the drum and shaves off a thin layer of the separated solids and precoating material. This movement exposes continuously a fresh layer of the precoat surface, so that when the drum submerges into the tank it is ready to polish the solution. Precoat filters are used to recover juice from the sediments originating in clarification tanks. In precoat filters the entire drum deck is subjected to vacuum (Fig. 2.19). 2.2.6. Membrane Filtration Filtration of coarse particles down to several microns is done by conventional dead-end filtration, where all influent passes through a filter medium that removes contaminants to produce higher-quality clarified juices. Rough screens, sand filters, multimedia filters, bag filters, and cartridge filters are examples of filtration products that remove 0.1-micron size particles or larger. Once the medium becomes loaded, it can be backwashed as with multimedia filters, or discarded and replaced as with cartridge filters. The method of obtaining clean filtration medium is based on economic and disposal concerns. Particles retained by the filter in dead-end filtration build up with time as a cake layer, which results in increased resistance to filtration. This requires frequent cleaning or replacement of filters.

Doctor blade

Vacuum pump Receiver

Figure 2.19. Vacuum drum precoat filter.

Feed

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Commercially available coarse filtration devices are effective in separating particles down to about 20 mm. On the other hand, membrane technology involves the separation of particles below this range, extending down to dissolved solutes that are as small as several Angstroms. Membranes are manufactured with a wide variety of materials, including sintered metals, ceramics, and polymers (Zeman and Zydney, 1996). In order to reject substances smaller than 0:1 mm using polymeric membrane is by far the most popular filtration method in the fruit processing industry. Millions of small pores per unit area of membrane allow water and lowmolecular weight substances to pass through it while undesired substances are retained on the influent side. The problem is solved by operating polymeric membranes in the crossflow mode. In crossflow membrane filtration, two effluent streams are produced, the permeate and the concentrate. The permeate is the purified fluid that has passed through the semipermeable membrane. The remaining fluid is the concentrate, which has become enriched with organics and salts that could not permeate the membrane. By doing so, rejected contaminants are continuously carried away from the membrane surface, thereby minimizing contaminant build up, leaving it free to reject incoming material and allow free flow of purified liquid. The size of the polymer’s pores categorizes the membrane into one of the following groups: reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. Reverse osmosis (RO) membranes have the smallest pore size ranging from 5 to 15 angstroms, nanofiltration (NF) covers separations in the 15–30 angstrom size, ultrafiltration (UF) removes organics in the 0:002---0:2 mm range, while microfiltration (MF) effects separation typically in the range of 0:1---10 mm. Figure 2.20 schematically shows the filtering capacity of these ‘‘crossflow’’ membrane systems. RO is both a mechanical and chemical filtration procedure by which the membrane’s surface sieves organic substance and actually repels ions. The dielectric repulsion of ions from the membrane is influenced by the ion’s charge density. Unlike RO membranes that have salt retentions of 80–99%, NF membranes reject 30–60% salts. UF and MF membranes have even larger pores and therefore pass most of the salts. Although membrane cleaning is periodically

Suspended particles

Macromolecules

Water

Ions (multivalent)

U F

N R Ions F (monovalent) O

Figure 2.20. Crossflow-type membrane classification by ‘‘rejection’’ capacity.

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required, the self-cleaning nature of crossflow filtration lengthens membrane life enough to make it economically attractive. The manufacturing processes result in a number of different membrane structures such as microporous, asymmetric, composite, etc. Membranes are assembled as modules that are easily integrated into systems containing hydraulic components. The module allows to accommodate large filtration areas in a small volume and resist the pressures required in filtration. Tubular, hollow fiber, spiral, and flat plate are the common modules (Cheryan, 1986). .

.

Tubular module consists of tubular membranes held inside individual perforated support tubes, assembled onto common headers and permeate into the container to form a module. When several channels are formed in a porous block of material, the tubular system is called ‘‘monolithic.’’ Hollow fiber module consists of bundles of hollow fibers (0.5–3 mm internal diameter) sealed into plastic headers and assembled in permeate casings (Fig. 2.21). Hollow fibers

Figure 2.21. Hollow fiber membrane configuration. (a) Manifold with HF cartridge; (b) SEM micrography of a single fiber (internal diameter 1 mm); (c) SEM magnification of a single fiber, showing filtration surface and support.

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.

.

are used in low-pressure applications only. These can accommodate moderate levels of suspended particles. Spiral modules are made by placing a woven plastic mesh, which acts as the permeate channel between two membrane layers and seals three sides. The fourth side of this sandwich is attached to the permeate tube. Another plastic mesh that acts as the feed channel is laid over it and the assembly is wrapped around the central permeate tube. Flat-platemodules use multiple flat sheet membranes in a sandwich arrangement consisting of a support plate, membrane, and channel separator. The membranes are sealed to the plates using gaskets and hydraulically clamped to form a tight fit. Several of these membranes are stacked together and clamped to form a complete module. The main advantages of the flat-plate module design are that they have high membranepacking densities and low hold-up volumes. This is due to the small channel height of the flat-plate modules. The main application for the flat-plate module is in recovering biological products. The advantages and disadvantages of the different UF configurations (Cheryan, 1986) are listed in Table 2.7.

Over time, the physical backwash will not remove some membrane fouling. Most membrane systems allow the feed pressure to gradually increase over time to around 30 psi and then perform a clean-in-place (CIP). CIP frequency might vary from around 10 days to several months. Another approach CIP practice is to use a chemically enhanced backwash (CEB), where on a frequent basis (typically every 1–14 days), chemicals are injected with the backwash water to clean the membrane and maintain system performance at low pressure without going offline for a CIP. The application of ultrafiltration (UF) as an alternative to conventional processes for clarification of apple juice was clearly demonstrated (Heatherbell et al., 1977; Short, 1983; Wu et al., 1990). However, the acceptance of UF in the fruit processing industry is not yet complete, because there are problems with the operation and fouling of membranes. During UF two fluid streams are generated: the ultrafiltered solids’ free juice (permeate), and the retentate with variable content of insoluble solids, which, in the case of apple juice, are mainly remains of cellular walls and pectin. Permeate flux (J) results from the difference between a convective flux from the bulk of the juice to the membrane and a counterdiffusive flux or outflow by which the solute is transferred back into the bulk of the fluid. The value of J is strongly dependent on hydrodynamical conditions, membrane properties, and the operating parameters. The main driving force of UF is the transmembrane pressure (DPTM), which in the case of hollow fiber ultrafiltration systems (HFUF) can be defined as: DPTM ¼

(Pi þ Po ) Pext 2

(2:1)

Table 2.7. Advantages and disadvantages of the different UF configurations. UF MEMBRANE CONFIGURATION

3

Pack density (m2 =m ) Fouling resistance Cleaning facility Relative cost

Flat/press

Spiral

Tubular

Hollow fibers

300–500 Good Good High

200–800 Moderate Fair Low

30–200 Very good Very good High

500–9000 Poor Poor Low

Source: Cheryan, 1986; Zeman and Zydney, 1996.

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where Pi is the pressure at the inlet of the fiber, Po the outlet pressure, and Pext the pressure on the permeate side. In practice, the J values obtained with apple juice are much less than those obtained with water only. This phenomenon is attributable to various causes, including resistance of gel layer, concentration in polarization boundary layer (defined as a localized increase in concentration of rejected solutes at the membrane surface due to convective transport of solutes (Porter, 1972)), and plugging of pores due to fouling, where some of these phenomena are reversible and disappear after cleaning of the UF membranes while others are definitively irreversible. 2.2.6.1. Stationary Permeate Flux It is well known (Iritani et al., 1991) that the transmembrane pressure-permeate flux characteristic for ultrafiltration shows a linear dependence of J with DPTM at lower values of pressure (1st region), while the permeate flux approaches a limiting value (Jlim ) independent of further increase in pressure at higher pressures (2nd region). The last situation was assumed to be controlled by mass transfer. Figures 2.22 and 2.23 show the variation of J with DPTM as a function of VCR or volume concentration ratio (defined as the initial volume divided by retentate volume at any time), which is a measurement of the retentate concentration, and recirculating flow rate, Qr, respectively (Constenla and Lozano, 1996). Pressure independence (2nd region) was observed to occur at a higher pressure at higher Qr. The point at which the pressure independence is evident is called optimum transmembrane pressure (DPTMo). 80.00

T = 50ⴗC, PC = 50,000 ds, VCR = 1

J,L/hm2

60.00

40.00

Qr = 10 L/min Qr = 12.5 L/min 20.00

Qr = 15 L/min

0.00 0.00

0.40

0.80

∆PTM,

1.20

1.60

Kg/cm2

Figure 2.22. Effect of DPTM and Qr on J at 508C. (Constenla and Lozano, 1996) with permission.

50

Fruit Manufacturing 80.00 T = 50ⴗC, PC = 50,000 ds, Qr = 10 L/min

J,L/hm2

60.00

40.00

VCR = 1 20.00

VCR = 2 VCR = 5

0.00

0.00

0.40

0.80

1.20

1.60

DPTM, Kg/cm2 Figure 2.23. Effect of DPTM and VCR on J at 508C (Constenla and Lozano, 1996). Reprinted from Lebensm. Wiss. u. Technol. 27: 7–14, Constenla, D.T. Lozano, J.E., Predicting stationary permeate flux in the ultrafilteration of apple juice. (copyright) 1996, with permission from Elsevier.

The reduction of Jlim with Qr may be associated with a reduction in the boundary layer due to an increase in the turbulence. On the other hand, the optimal DPTM values were practically independent of VCR at Qr > 10 L/min. A hysteresis effect in the permeate flux, attributable to the consolidation of the gel layer (Omosaiye et al., 1978), has been observed. The area enclosed by the hysteresis loop was greater at lower Qr and VCR values. Traditionally, correlations of J with DPTM and VCR were determined by parameter fitting of the experimental data. Since the polynomial functions have no physical basis, a large number of experimental data are needed for determination of J. Therefore other theoretical and semiempirical approaches should be considered (Constenla and Lozano, 1986). 2.2.6.2. Permeate Flux as a Function of Time Several models are proposed in the literature for representing J, most of them being semiempirical and practical equations (Table 2.8). Membrane fouling mechanisms may be studied through the classical laws of filtration under constant pressure (Table 2.9). During UF process (Iritani et al., 1991) J behaves as in cake filtration only at the very beginning, attributable to the formation of the gel layer with minor counterdiffusion flux.

2

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51 Table 2.8. Some equations representing permeate flux as a function of time.

No.

Permeate flux equations

(i) (ii) (iii) (iv) (v)

J ¼ J0 exp ( Bt) J ¼ JF þ (J0 JF ) exp ( At) J ¼ (J02 þ 2K t)1=2 J ¼ J0 B ln(VCR) J JF ¼ (J0 JF )(1 exp ( ( t=B) )

Subindex: 0, 1, F are zero, initial, and final time, respectively; Vp ¼ permeate volume; A, B, and K are constants. Sources: Heatherbell et al. (1977), Probstein et al. (1979), Mietton-Peuchot et al. (1984), Koltuniewicz (1992), Constenla and Lozano (1996).

As previously indicated, pectin and other large solutes like starch, normally found when unripen apples are processed, tend to form a fairly viscous and gelatinous-type layer on the ‘‘skin’’ of the asymmetric fiber. Flux decline, due to this phenomenon, can be reduced by increasing flow velocity on the membrane Traditionally correlations of J with DPTM and VCR have been determined by parameter fitting of the experimental data. It was found that the following exponential equation, proposed in the SRT model (Constenla and Lozano, 1996), fitted appropriately: J ¼ JF þ (JO JF ) exp ( At)

(2:2)

Jo , JF , and A values can be obtained at different Qr and constant values of VCR and DPTM. An increase in Qr significantly increases the permeate flux. This behavior was reflected as an extensive increase in the parameter A. 2.2.6.3. Influence of VCR on the Permeate Flux Constenla and Lozano (1996) found that in the case of pseudoplastic fluids, as fruit juice retentates, different operative conditions restrain the VCR up to a maximum of 14. The permeate flux becomes independent of the solute rejection, characteristic of the hollow fibers Table 2.9. Classic filtration models (pseudoplastic fluids). Mechanism

Scheme

Representative equations

(1) Total pore blocking

J0 J ¼ K1 Vp ln (J=J0 ) ¼ K1

(2) Partial pore blocking

ln (J=J0 ) ¼ K2 Vp1 =J 1=J0 ¼ K2 t

. (3) Blocking Progressive pore

J ¼ J0 (1 K3Vp =2)(3n þ 1)=2nJ ¼ J0 (1 þ ((n þ 1)=n)J0 K3 t) (3n þ 1)=(n þ 1)

(4) Cake filtration

(1=J)n ¼ (1=J0 )n þ K4 Vp : (1=J)nþ1 ¼ (1=J0 )nþ1 þ ((n þ 1)=n)K4 t

K 1 , K 2 , K 3 , K 4 : experimental constants; n: flow behavior index. Source: Lozano et al. (2000). (with permission)

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after a few minutes of operation. This effect is commonly attributed to the build up of the concentration polarization/gel layer. During the UF of apple juice in the mass transfer region, a 60% increase in DPTM was reflected only as a 5% increase in J. Acceleration of the fruit juice retentate near the membrane surface removes the accumulated macromolecules, thereby reducing the effect of concentration polarization gel layer. Due to the low diameter of the hollow fibers, high-tangential velocities can be obtained at laminar rates. Equation iv in Table 2.8, fit reasonably well for these types of membranes: J ¼ Jo B In (VCR)

(2:3)

where JO is the initial permeate flux, and B is a constant, which depends on the system, operating conditions, and juice properties. Decrease of flux with concentration is nonlinear, and changes in the rate of permeation were better followed when plotted against ln VCR (Fig. 2.24). The rate of flux decrease J could be divided into three periods. The first period, characterized by a rapid decrease in J, occurred in a few minutes. During the second period (up to VCR ¼ 3 approximately) the variation of J is unstable, depending on fiber cut-off. Then J approached a ‘‘linear’’ steady logarithmic decrease with VCR. This behavior could be explained by considering the resistance to flux as two separate 0 additive resistances in series: (i) the membrane resistance (Rm ); and (ii) the concentration polarization/gel layer resistance (Rp ). During the first period Rp increases very fast reaching a 0 0 value equivalent to that of Rm . In the second region, the Rm value is still an important 100.00

PC = 30,000 ds PC = 50,000 ds PC = 10,000 ds

J, L/hm2 80.00

I 60.00

II 40.00

III

20.00

0.00 1

2

3

45

6

7

8

9 10

VCR Figure 2.24. Decrease of permeate flux with ln VCR for hollow fibers with different MWCO. Full line represents Eq. (2.4). Reprinted from Lebensm. Wiss. u. Technol. 27: 7–14, Constenla, D.T. Lozano, J.E., Predicting stationary permeate flux in the ultrafilteration of apple juice. (copyright) 1996, with permission from Elsevier.

2

.

Processing of Fruits

53

component of the total resistance and J is not completely independent of the properties of the fiber. Finally, during the last period Rp is dominant and the cut-off of the hollow fiber becomes irrelevant.

REFERENCES Alkorta, I., Garbisu, C., Llama, M.J. and Serra, J.L. (1996). Immobilization of pectin lyase from Penicillium Italicum by covalent binding to Nylon. Enzyme Microb. Technol. 18: 141–146. Ates, S. and Pekyardimci, S. (1995). Properties of immobilized Pectinesterase on Nylon. Macromol. Rep. A32: 337–345. Ben-Shalom, N., Levi, A. and Pinto, R. (1986). Pectolytic enzyme studies for peeling of grapefruit segment membrane. J. Food Sci. 51: 421–423. Bump, V.L. (1989). Apple pressing and juice extraction. In Processed Apple Products, Downing, D.L. (ed.). AVI Publishing Company, Van Nostrand Reinhold, New York, pp. 53–82. Ceci, L. and Lozano, J.E. (1998). Determination of enzymatic activities of commercial pectinases. Food Chem. 31(1/2): 237–241. Cheryan, M. (1986). Ultrafiltration Handbook. Technomic Publishing Company, Lancaster. Constenla, D.T. and Lozano, J.E. (1996). Predicting stationary permeate flux in the ultrafiltration of apple juice. Lebensm. Wiss. Technol. 27: 7–14. Dietrich, H., Patz, C., Scho¨pplain, F. and Will, F. (1991). Problems in evaluation and standardization of enzyme preparations. Fruit Process. 1: 131–134. FAOSTAT Data (2005). FAO Statistical Databases. www.fas.usda.gov/htp/Presentations/2005. Felloes, P. (1988). Food Processing Technology: Principles and Practice. Ellis Horwood International Publishers, Chichester, England, pp. 300–310. Grampp, E.A. (1976). New process for hot clarification of apple juice for apple juice concentrate. Fluss. Obst. 43: 382–388. Heatherbell, D.A., Short, J.L. and Stauebi, P. (1977). Apple juice clarification by ultrafiltration. Confructa 22: 157–169. Hui, Y.H. (1991). Data sourcebook for Food Scientists and Technologists. VCH Publishers, New York. Iritani, E., Hayashi, T., and Murase, T. (1991). Analysis of filtration mechanism of crossflow upward and downward ultrafiltration. J. Chem. Eng. Jpn 1: 39–44. Koltuniewicz, A. (1992). Predicting permeate flux in ultrafiltration on the basis of surface renewal concept. J. Membr. Sci. 68: 107–118. Lineback, D.R. and Wongsrikasen, E. (1980). Gelatinization of starch in baked products. J. Food Sci. 45: 71–74. Liu, Y.K. and Luh, B.S. (1978). Purification and characterization of endo-polygalacturonase from Rhizopus arrhizus. J. Food Sci. 43: 721–726. Lozano, J.E. (2003). Separation and clarification. In Encyclopedia of Food Science and Nutrition, Caballero, B., Trugo, L. and Finglas, P. (eds.). AP Editorial, Elsevier, London, UK, pp. 5187–5196. ISBN: 0-12-227055-X. Lozano, J.E., Constenla, D.T. and Carrı´n, M.E. (2000). Ultrafiltration of apple juice. In Trends in Food Engineering, Lozano, J.E., An˜o´n, C., Parada-Arias, E. and Barbosa-Ca´novas, G. (eds.). Food Preservation Technol. Series. Technomics Publishing Company, Inc., Lancaster, Basel, pp. 117–134. McLellan, M.R. (1996). Juice processing, Chapter III. In Processing Fruits: Science and Technology. Biology, Principles and Applications, Vol. 1, Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomic Publishing Company, Inc., Lancaster, Basel. Mietton-Peuchot, M., Milisic, V. and Ben Aim, R. (1984). Microfiltration tangentielle des boissons. Le Lait. 64, 121–128. Nagy, S.; Chen C.S. and Shaw, P.E. (eds.) (1993). Fruit Juice Processing Technology. Agscience, Inc. Auburndale, FL. Omosaiye, O., Cheryan, M. and Mathews, M. (1978). Removal of oligosacharides from soybean water extracts by ultrafiltration. J. Food Sci. 51: 354–358. Porter, M. (1972). Concentration polarization with membrane ultrafiltration. Ind. Eng. Chem.—Prod. Res. Develop. 11(3): 234–248. Probstein, R., Leung, W. and Alliance, Y. (1979). Determination of diffusivity and gel concentration in macromolecular solutions by ultrafiltration. J. Phys. Chem. 83(9): 1228–1236.

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Ramaswamy, H.S. and Abbatemarco, C. (1996). Thermal processing of fruits. In Processing Fruits: Science and Technology, Vol. I, pp. 25–65. Reed, G. (1975). Enzyme in food processing, 2nd ed. Academic Press, New York. Rombouts, F.M. and Pilnik, W. (1978). Enzymes in fruit and vegetable juice technology. Process Biochem. 13: 9–13. Sakai, T., Sakamoto, T., Hallaert, J. and Vandamme, E.J. (1993). Pectin, pectinase, and protopectinase: production, properties, and applications. Adv. Appl. Microbiol. 39: 213–294. Short, J.L. (1983). Juice clarification by ultrafiltration. Process Biochem. 18(5): VI. Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.) (1996). In Processing Fruits: Science and Technology. V.2 Major Processed Products, Technomic Publishing Company, Inc., Lancaster, PA. Spagna, G., Pifferi, P.G. and Martino, A. (1993). Pectinlyase immobilization on epoxy supports for application in the food processing industry. J. Chem. Tech. Biotechnol. 57: 379–385. Toribio, J.L. and Lozano, J.E. (1984). Non enzymatic browning in apple juice concentrate during storage. J. Food Sci. 49: 889–893. Woodroof, J.G. and Luh., B.S. (1986). Commercial fruit processing, 2nd ed. AVI Publishing Company, Westport, CT. Wu, M.L., Zall, R.R. and Tzeng, W.C. (1990). Microfiltration and ultrafiltration comparison for apple juice clarification. J. Food Sci. 55(4): 1162–1163. Zeman, L.J. and Zydney, A.L. (1996). Microfiltration and Ultrafiltration: Principles and Applications. Marcel Dekker, Inc., New York, NY. Zobel, H.F. (1984). Starch gelatinization and mechanical properties. In Starch: Chemistry and Technology, 2nd ed., Whstler, R.L., BeMiller, J.N. and Paschall, E.F. (eds.), Academic Press, Orlando, FL, pp. 300–302.

CHAPTER 3

PROCESSING OF FRUITS: ELEVATED TEMPERATURE, NONTHERMAL AND MISCELLANEOUS PROCESSING 3.1. PASTEURIZATION The process of ‘‘pasteurization’’ pioneered by Louis Pasteur was aimed at the destruction of bacteria, molds, spores, etc. by exposing them to a certain minimum temperature for a certain minimum time; the higher the temperature, the shorter the time required. The term ‘‘pasteurized’’ can be used to refer to products with reduced bacteria. Products with no bacteria are referred to as ‘‘sterile’’ or ‘‘ultrapasteurized.’’ Some products are ‘‘sterilized’’ before they are sold to the public. Most of the fruit juice sold on store shelves is produced this way. These products have relatively unlimited shelf life even without refrigeration. However, the time/temperature combination required to kill 100% of bacteria also destroys some of the flavor components in the juice. There is some dispute over how much flavor degradation actually occurs, and since this is a subjective opinion on the part of the consumer, no definitive data are available. The following methods are commonly accepted for pasteurization. 3.1.1. Batch Pasteurization This is a typical pasteurization process, by heating the product in a batch pan to about 638C for relatively long periods (Table 3.1). This method destroys very common pathogenic bacteria. However, as production demands grow, simply adding more number of pans is usually not feasible. 3.1.2. HTST (Short Time) Pasteurization High-Temperature, Short-Time pasteurization is typically conducted at 728C for 15 s. A hold time of 15 s can easily be achieved in a continuous process by installing a holding tube. The product is then cooled for storage. This method provides the convenience of continuous processing, at a temperature low enough to prevent taste and aroma deterioration. 3.1.3. UHT Pasteurization In UHT pasteurization, the product is brought to over the boiling point (under pressure) for only a fraction of a second. This results in a sterile product that does not require refrigeration 55

56

Fruit Manufacturing Table 3.1. Typical pasteurization methods and conditions. Method Batch HTST (High temperature/short time) UHT (Ultrahigh temperature)

Temperature (8C)

Hold time

63 72 >121

30 min 15 s 0.1 s

Source: Nickerson and Sinskey, 1972; US Department of Health and Human Services, 2004.

later. However, after being brought to this temperature, a slight ‘‘cooked’’ taste is sometimes said to be detectable. Most apple juice producers are relatively familiar with both batch and UHT pasteurization. While smaller producers use batch method, UHT systems are commonly employed by large processors. 3.1.4. Nonthermal Pasteurization Much has been written about ‘‘new’’ pasteurization and sterilization technologies such as irradiation, microwave sterilization, and high-pressure processes that have long been available, but, for various reasons, scarcely applied in food processing. Meanwhile, new or improved thermal and nonthermal technologies have emerged that are available now for pasteurizing, sterilizing, or otherwise reducing microbiological contamination of foods. Moreover, some new nonthermal pulsed technologies have been cleared by the FDA for antimicrobial applications (Bolando-Rodriguez et al., 2000). Traditional thermal sources, such as steam, are being engineered into new and improved processes to destroy pathogens with minimal heat, as for example the continuous steamfusion cookers. This technique involves a variable-speed agitator, which rotates on an axis parallel to product flow through a vertical tube. A row of steam injectors along each side of the tube provides rapid heating, while turbulence created by the agitator fuses steam to evenly heat the product as it flows through the tube. The product goes up to cooking temperature in just a few seconds, and steam is combined with the product to avoid overcooking. Cooker surfaces are kept at practically the same temperature as the product. Pressurized juice should be preserved under chilled conditions to retain its fresh flavor and taste. Low temperature also helps to reduce the development of precipitates, since lowtemperature storage keeps pectin esterase activity low; thus, pectin esterase cannot participate in the formation of a precipitate. New nonthermal technologies with potential for processing juices to retain flavor and extend shelf life include ultrahigh pressure (UHP), pulsed electric fields (PEF), ultraviolet (UV) light, electric pulse, and carbon dioxide (CO2 ) (Ohlsson and Bengtsson, 2002). Over the past several years, intense R&D efforts have aimed at validating and commercializing these technologies. The low-temperature storage is important to other nonthermally treated fruit products. Pulsed electric-fields’ method is a cold-pasteurization process for antimicrobial treatment of liquids and pumpable foods. It is based on the application of short-duration, high-intensity electric-field pulses to kill both spoilage and pathogenic organisms without affecting product taste or color. The use of high-intensity pulsed light to control microorganisms on food surfaces applies nonionizing, high-intensity flashes of broad-spectrum light to reduce microbial populations

3

.

Processing of Fruits

57

on foods and packaging materials. Each flash is approximately 20,000 times the intensity of sunlight, with wavelengths ranging from ultraviolet to near infrared (Barbosa-Canovas et al., 1997). Radio frequency (RF) energy has been investigated as a nonthermal alternative to thermal pasteurization (Geveke et al., 2002). Electric-field strengths of 14–30 kV/cm generated with RF power supply systems at frequencies in the range of 20 kHz–27 MHz were applied to suspensions of Saccharomyces cerevisiae in water over a temperature range of 28–558C. The population of S. cerevisiae was reduced by >5 log following 30 exposures to a 100-kHz, 25-kV/cm field at 288C.

3.2. STERILIZATION OF FOOD BY HIGH PRESSURE The basis for applying high pressure to food is to compress the water surrounding the food. A decrease in volume of water with increasing pressure is very minimal compared to gases. The volume decreased for water is approximately 4% at 100 MPa, 7% at 200 MPa, and 11.5% at 400 MPa at 228C. Above 1,000 MPa and at room temperature, however, water changes to a solid (type VI ice), whose compressibility is very small. Usually irreversible effects on biological materials are observed at pressure >100 MPa. Therefore, pressure of 100 –1,000 MPa could be useful in food treatment. For reversible effects, pressure up to 200 MPa may be used. Microbial death at higher pressure is considered to be due to changes in permeability of cell membranes (Farr, 1990). Bacteria, yeasts, and molds in foods, such as meat, fish, and agricultural products, are sterilized by high-pressure treatment at 400–600 MPa. The pressurization of mandarin or orange juices at 300–400 MPa for 10 min is enough to sterilize vegetative microorganic cells, although spores of Bacillus sp. are not killed. This retains good taste and flavor of the juice, and allows to store it at room temperature for 5 months. When pressure was applied at 458C, the results were considerably better than that at the room temperature. The major use of the high-pressure sterilization is in partially prepared foods or ovenready foods. Pressure treatment preserves flavor, taste, and natural nutrients, but bacterial spores are not killed. Hence, these foods require chilled transportation. 3.2.1. High-Pressure Equipment and the System Test equipment for foods have been developed by several equipment industries and are available on the market. A typical equipment has 500-ml capacity, is made of stainless steel, and works at a maximum pressure of 700 MPa. It takes only 90 s to attain the maximum pressure. Temperature of the inside water, used as the pressure-transducing medium, is regulated by an electric heater outside the pressure vessel. Thus, the hydrostatic pressure is directly applied to foods placed in the pressure vessel at high speed under regulated temperature without any harmful contaminants. Several food companies and government institutions in Japan have been equipped with high-pressure test machines in recent years and are performing research and development of new food products based on the high-pressure processing (Barbosa-Canovas et al., 2004) Industrial equipment for high-pressure processing of foods are operational in several food industries: a batchwise system of 10–50 l capacity and a semicontinuous system of 1– 4 ton/h treatment. The former is used for the processing and sterilization of packed foods and the latter for the treatment of liquid foods. These machines are as small as an

58

Fruit Manufacturing

industrial machine, but a pressure vessel of 50 l is similar to a heating vessel of 200 l in capacity. The cycle time for operating the pressure machine is short, generally being 15 min for food sterilization or food processing, while a large pan takes about 1 h for heating and cooling in conventional processing (Cheftel, 1995; Cole, 1997). An industrial system for the high-pressure processing of foods is similar to the conventional heat processing: Raw materials are pretreated, filled in plastic bags, sealed in vacuum, and pressurized. Final products are obtained after drying the bags. Liquid food may be placed directly in the pressure vessel in a semicontinuous way. It was also indicated that the use of pressure processing may save energy and improve sanitary conditions in the use of hightemperature processing. Selection of packaging materials is important for high-pressure food processing. While metal and glass are not suitable for high-temperature processing, plastic films are generally acceptable. Ochiai and Nakagawa (1991) pointed the importance of head space in plastic cups and suggested the use of plastic as a package material because of its heat sealability, hygiene, and safety. Packaging materials, which prevent oxygen permeability and light exposure, should be developed especially for retaining fresh color and flavor of foods. In brief, high pressure similar to high temperature is useful for the purpose of cooking, processing, sterilizing, and preserving food. The advantage of high pressure lies in the fact that it avoids destruction of the covalent bonds and retains natural flavor, taste, color, and nutrients. Thus, high-pressure technology is of great importance to the fruit industry.

3.3. CONCENTRATION BY EVAPORATION Evaporation refers to the process of heating the liquid to boiling point to remove water as vapor. Because fruit products, in particular fruit juices, are heat sensitive, heat damage can be minimized by evaporation under vacuum to reduce the boiling point (Heldman and Singh, 1981). The basic components of this process consist of: (i) heat exchanger, (ii) vacuum system, (iii) vapor separator, and (iv) condenser. The heat exchanger transfers heat from the heating medium, usually low-pressure steam, to the product via indirect contact surfaces. The vacuum system reduces the product temperature. The vapor separator removes juice from the vapors, driving juice back to the heat exchanger and vapors out to the condenser, which condenses the vapors from inside the heat exchanger and may act as the vacuum source. The driving force for heat transfer is the difference in temperature between steam and juice. The steam is produced in large boilers, generally tube and chest heat exchangers. The steam temperature is a function of the steam pressure. Water boils at 1008C at 1 atm., but at other pressures the boiling point changes. At boiling point, the steam condenses in the coils and gives out latent heat. If the steam temperature is too high, burn-on/fouling increases, so there are limits to how high steam temperatures can go. The juice is also at its boiling point, with an increase of solids’ concentration. The most important types of single effect evaporators are described by Minton (1986) and Perry and Chilton (1973). 3.3.1. Batch Pan It consists of spherical-shaped, steam-jacketed vessels (Fig. 3.1). The heat transfer per unit volume is small, requiring long residence times. The heating is due to natural convection. Heat transfer characteristics are poor.

3

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Processing of Fruits

59 Vapor

Steam

Condensate

Concentrate

Figure 3.1. Batch pan or calandria.

3.3.2. Rising Film Evaporator This type of evaporator consists of a heat exchanger isolated from the vapor separator (Fig. 3.2). The heat exchanger consists of 10 –15 m long tubes in a tube chest, which is heated with steam. The liquid rises by percolation from the vapors formed near the bottom of the heating tubes. The thin liquid film moves upward rapidly. The product may be recycled if necessary to arrive at the desired final concentration. Vaccum Feed

Steam Feed

Concentrate Condensate

Figure 3.2. Rising film evaporator (single stage).

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Fruit Manufacturing

3.3.3. Falling Film Evaporator The falling film evaporators are the most widely used type of evaporators in the food industry. It has similar components to the rising-film type except that the thin liquid film moves downward under gravity in the tubes (Fig. 3.3). Specially designed nozzles or spray distributors at the feed inlet permit it to handle products that are more viscous. The residence time is 20–30 s as opposed to 3– 4 min in the rising film type. The vapor separator is at the bottom, which decreases the product holdup during shutdown. The tubes are 8–12 m long and 30–50 mm in diameter. 3.3.4. Scraped-Surface Evaporator Scraped-surface evaporators are designed for the evaporation of highly viscous and sticky products, which cannot be otherwise evaporated. This type of evaporator has been specially designed to provide a high degree of agitation as well as scraping the walls of the evaporator to prevent deposition and subsequent charring of the product. Scraped-surface heat evaporators consist of a cylinder that has an inner tube (heat transfer surface area) and an outer tube. Between the two tubes, there is annular space, where the heating or cooling media flows countercurrent to the product. Inside the inner tube, a bladed shaft rotates and removes the product from the heat transfer wall areas (Fig. 3.4). Scraped-surface heaters improve cooking by allowing better heat transfer to the batch and preventing burn-on. Typical scraped film evaporator application includes processing of fruit pure´es, mashes, pulps, concentrates, and pastes.

Feed

Steam

Vacuum

Condensate

Concentrate Figure 3.3. Falling film evaporator (single stage).

3

Processing of Fruits

.

Steam

61

Rotor

Fruit product

Figure 3.4. Cross section of a scraped-surface evaporator.

3.3.5. Multiple Effect Evaporator Two or more evaporator units can be run in sequence to produce a multiple effect evaporator. Each effect would consist of a heat transfer surface, a vapor separator, as well as a vacuum source and a condenser. The vapors from the preceding effect are used as the heat source in the following effect. There are two advantages to multiple effect evaporators: . .

Economy: They evaporate more water per kilogram steam by reusing vapors as heat sources in subsequent effects Improvement in heat transfer: This is due to the viscous effects of the products as they become more concentrated.

Each effect operates at a lower pressure and temperature than the effect preceding it so as to maintain a temperature difference and continue the evaporation procedure. The vapors are removed from the preceding effect at the boiling temperature of the product, so that no temperature difference would exist if the vacuum were not increased. The operating costs of evaporation are relative to the number of effects and the temperature at which they operate. As evaporation is a very energy-consuming process, the availability and the relative cost of energy determine the design of the evaporation plant. Normally an evaporation plant is designed to use energy as efficiently as possible by using more than one effect. Therefore the following technical solutions are used in order to keep the temperature of the steam high enough to run the process: . . .

Thermal vapor recompression (TVR), Mechanical vapor recompression (MVR), or Combination of both.

3.3.5.1. Thermocompression (TC) This includes the use of a steam-jet booster to recompress part of the exit vapors from the first effect. Through recompression, the pressure and the temperature of the vapors are increased. As the vapors exit from the first effect, they are mixed with very high-pressure steam. The steam entering the first effect is at a slightly less pressure than the supply steam. There are

62

Fruit Manufacturing

usually more vapors from the first effect than can be used by the second effect; usually only the first effect is coupled with multiple-effect evaporators. 3.3.5.2. Mechanical Vapor Recompression (MVR) Whereas only part of the vapor is recompressed using TC, the entire vapor is recompressed in an MVR evaporator. Vapor is mechanically compressed by radial compressors or by simple electrical fans. There are several variations: in single effect, all the vapors are recompressed, therefore no condensing water is needed; in multiple effect, MVR is possible on first effect, followed by two or more traditional effects, or recompress vapors from all effects.

3.4. DEHYDRATION Dehydration refers to the nearly complete removal of water from foods to a level of less than 5%. Dehydrated foods are protected from spoilage by lowering the water activity: aw ¼ pv =po

(3:1)

where pv is the vapor pressure of water in the product; and po the vapor pressure of saturated water at the same temperature. In dry fruits at aw ¼ 0:85 or above, some other form of preservation such as SO2 or potassium sorbate may be used. Some definitions are of interest. . .

.

Dried: Refers to all products with reduced moisture content regardless of the method. Evaporated: Refers to use of sun and forced air driers to evaporate moisture to a fairly stable product. Sun drying in general will not reduce moisture below 15%. Many evaporated fruits will have up to 25% water level. These products have short storage life even when a high level of preservative is added. Evaporated fruits need refrigeration. Dehydrated: Refers to fruits whose moisture has been reduced to 1–5% under carefully controlled conditions. Dehydrated fruits have more than 2 years’ storage life, particularly if stored in modified atmosphere or low-temperature conditions.

Drying rate of fruits depends on particle size and mode of heat transfer. Sliced and diced products dry faster. None of the mechanisms involved in drying of fruits are as simple as they might seem, as foods do not usually deal with a single uniform phase. Two important aspects of mass transfer in dehydration are: (a) movement of water to the surface of material being dried, and (b) removal of water from the surface through the (probably) thin immobile boundary layer. Considering some possible modes of heat transfer that can be applied in fruit drying we find: Phases involved in fruit drying

Examples

Gas–solid Gas–liquid Liquid–gas Liquid–solid Solid–gas Solid–liquid Solid–solid

Conventional dehydration of fruits Concentration of syrup in cascade-type drier Internally during moist fruits’ drying Frying Internally during moist fruits’ drying Drum drying Drum drying of fruit pure´e

3

.

Processing of Fruits

63

Drying Rate

Constant rate period

Falling rate period

Xe

TIME, t Xcr Figure 3.5. Characteristic drying rate curve.

Drying curve relates to the amount of water removal with time (Fig. 3.5) and can be divided into different regions (Crapiste and Rotstein, 1997): (1) An initial period in which evaporation occurs on the surface and temperature wet bulb value. This is the constant-rate period. However, due to water conditions on product surface and shrinkage during drying a pseudoconstant-rate period may be observed. (2) At a point usually called the critical moisture content (Xcr ) the falling rate period starts. When surface moisture is lost, the rate falls as water must diffuse from inside to surface before evaporation can take place. Dominant factor is availability of water at evaporation surface. The process of dehydration slows greatly at this period, until moisture content asymptotically reaches Xe , the equilibrium value at the relative humidity and temperature of the air. Drying rate is affected by drier loading on tray or belt driers. This can be overcome by fluidizing fruit particles. During the initial constant-rate stage hotter is better until falling rate phase is reached. At Xcr , temperature must be reduced to avoid deteriorative reactions. Table 3.2 lists different dehydration systems’ characteristics. Other driers used in the fruit industry are bin driers, simple devices consisting of bins with perforated bottoms, and fluidized bed driers, used for drying food powders. In fluidized bed driers, air is blown up through a wire mesh belt on porous plate that supports and conveys the product. A slight vibration motion is imparted to the food particles. Fluidization occurs when the air velocity is increased to the point where it just exceeds the velocity of free fall of the particles. The fluidization provides intimate contact of each particle with the air. With products that are particularly difficult to fluidize, a vibrating motion of the drier itself is used to aid fluidization—this is called vibrofluidizer. The fluidized solid particles then behave in a manner analogous to a liquid, i.e., they can be conveyed. Air velocities will vary with particle size and density but are in the range of 0.3–0.75 m/s. They can be used not only for drying but also for cooling. If the velocity is too high, the particles will be carried away in the gas stream, therefore, gravitational forces need to be only slightly exceeded. Minimum air velocity to fluidize 10-mm pulp is about 115 m/min. It can be used to dry uniform-sized products.

64

Fruit Manufacturing Table 3.2. Description and schematic diagram of different driers.

Type of drier

Description

Drier sketch

Sun or solar

Simplest systems, consisting of trays laid flat on ground or supported slightly above it. Used for drying apricots, raisins, etc. Stacked tray system for very low humidity areas improves dehydration due to natural air movement

Wind

Cabinet drier

It is a typical batch operation in which air is heated and forced to circulate between trays or through the product by using perforated trays. These driers can process from pulps to solid pieces of fruits

Hot air

Tunnel drier

Tunnel driers have been the most widely used form of fruit dehydrators. They are set up with parallel flow air during the constant-rate period, arranged in such a way that incoming fruit encounters the hottest, driest air, then counterflows so that the outgoing product encounters the driest air. Air velocities of 200 – 400 m/min are commonly used. Initial temperatures of 1008C may be used. Final temperatures are about 708C or less, depending on fruit products

Continuous belt or conveyor driers

Product moving through these driers is exposed to the same successive sets of drying conditions. A single continuous belt or series of small belts may be used. Conveyor may consist of mesh belts or perforated metal plates. Temperature is reduced in the direction of the outlet of the system

Belt driers

Highly efficient device units, occupying a relatively small plant area per ton of product. In these driers high-velocity air (not enough to fluidize the product) passes through the belt. Adequate for dehydro freezing lines. Drying fruits to low moisture is usually attained by a complementary system

Exhaust air

Hot air

Trucks’ direction Hot air

Feeder

Air temperature decrease downstream Drum driers

Originally used for dry milk. Can be operated at atmospheric pressure or under vacuum. In the fruit industry it is used for making apple sauce flakes. Almost any pure´e can be dried if the fiber content is adequate. Control factors include sheet thickness, temperature, drum speed, and air flow over drums

Source: Karel et al., 1975; Crapiste and Rotstein, 1997.

Feeder

3

Processing of Fruits

.

65

Microwave drying (Van Arsdel et al., 1973), osmotic dehydration (Shi and Fito, 1993), explosion puffing (Saca and Lozano, 1992), and freeze drying (Mujumdar and Menon, 1995) have been also applied for fruit dehydration. 3.4.1. Spray Drying Spray driers are one of the most widely used types of air convection drier. Spray drying involves transforming a pumpable food, i.e., juices, low-viscosity pastes, and pure´es into a dry-powdered or particle form. This is achieved by atomizing the fluid into a drying chamber, where the liquid droplets are passed through a hot-air stream (Heldman and Singh, 1981; Green and Maloney, 1999). The objective is to produce a spray of high surface-to-mass ratio droplets and then to uniformly and quickly evaporate the water. Evaporation keeps product temperature to a minimum, so little high-temperature deterioration occurs. In its simplest form, spray drying consists of four separate process stages: . . . .

Atomization of the liquid food feed, Spray-air contact, Drying, Separation of the dried food product from the drying air.

Atomization is generally accomplished by: (i) a single-fluid (or pressure) nozzle, (ii) a two-fluid nozzle, or (iii) a rotary atomizer, also known as a spinning disk or a wheel. The single-fluid nozzle allows more versatility in terms of positioning with the spray chamber, so the spray angle and spray direction can be varied. A typical drying-chamber design used for fruit pulps and juices is the cylindrical flatbottomed drier. A pneumatic powder discharger removes the product, while an air broom cools chamber walls to prevent sticking. This design allows easier access for cleaning. Drying occurs in two phases, and air-temperature control is vital to their control. The first phase is a constant-rate step, in which the moisture rapidly evaporates from the surface, and capillary action takes out the moisture from within the particle. Then, during the ‘‘falling-rate’’ period, diffusion of water to the surface controls the drying rate. As moisture content drops, diffusion rate also decreases. Removing moisture to required values in a single-stage drier is responsible for most of the residence time in the drier. As a rule, the residence time of the air and the particle in a single-stage cocurrent drier is about the same. Since the moisture level is still decreasing toward the end of the process, the outlet temperature must be high enough to continue the drying process. This can be avoided by adding a fluid bed after the drier. The final stage of spray drying is the removing of the dried product from the air. Depending on drier design, the dried product can be separated at the base (as in a flatbottomed drier). While the heavier product is removed by gravity, the smallest particles are pulled together in some type of collection equipment. Otherwise, the entire product and air can be moved to equipment designed to separate particles from air. Fine particles are removed with cyclones, bag filters, electrostatic precipitators, or scrubbers. Fines are bagged or returned to an agglomeration process; air is returned to the system. Drying takes place within a matter of seconds at temperatures approximately 2008C. Although evaporative cooling maintains low product temperatures, rapid removal of the product is still necessary.

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Nozzle

Air outlet

Dry product Figure 3.6. Typical spray drier configuration.

The liquid food is generally preconcentrated by evaporation. The concentrate is then introduced as a fine spray into a tower or chamber with heated air. As the small droplets are put in contact with the heated air, they flash off their moisture and drop to the bottom of the tower and are removed. Principal spray components are: (1) a high-pressure pump, for introducing liquid into the tower, (2) a nozzle for atomizing the feed stream, (3) a blower with a source of hot air, and (4) a system for removing the dried food (Fig. 3.6). Several billion particles per liter ensure a large surface area for exposure to drying forces. Particle size must be reduced in three ways: (1) using a smaller orifice, (2) increasing atomization pressure, or (3) reducing product viscosity, by increasing feed temperature or redilution. The exit air temperature is an important control parameter, which can be used to adjust feed flow rate and inlet temperature. 3.4.2. Powder Recovery Three systems are available (Walas, 1976; Green and Maloney, 1999) for powder recovery from the air stream: (1) Bag filters: Although very efficient they are not very popular due to labor costs and sanitation problems. They are not recommended for hygroscopic particles. (2) Cyclone collector: In this type of powder collector, air enters at tangent at high velocity into a cylinder or cone, which has a much larger cross section. Air velocity is decreased in the cone, permitting settling of solids by gravity. Several cyclones can be placed in series. High air velocity is needed to separate small diameter and light materials, while centrifugal force is important in removing particles from the air stream. To increase centrifugal force cyclone diameter may be reduced. A rotary airlock is used to remove powder from the cyclone. (3) Wet scrubber: Wet scrubbers are the most economical outlet air cleaner. The principle of a wet scrubber is to dissolve any dust powder left in the airstream into either water or the feed stream by spraying the wash stream through the air. Wet scrubbers also recover approximately 90% of the potential drying energy normally lost in exit air. Cyclone separators are hygienic and easy to operate. However, high losses may occur. Either feed stream or water can be used as scrubbing liquor.

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Two- and Three-Stage Drying Processes In single-stage spray drying, the rate of evaporation is particularly high in the first part of the process, and it gradually decreases because of the falling water content of the particle surfaces. Therefore a relatively high outlet temperature is required during the final drying phase. The two-stage drying process was introduced to reduce temperatures and cost of production, and increase product quality. The two-stage drier consists of a spray drier with an external vibrating fluid bed placed below the drying chamber. The product can be removed from the drying chamber with a higher moisture content, and the final drying takes place in the external fluid bed where the residence time of the product is longer and the temperature of the drying air is lower than in the spray drier. This principle forms the basis of the development of the three-stage drier. The second stage is a fluid bed built into the cone of the spray drying chamber. This fluid bed is called the integrated fluid bed. The inlet air temperature can be raised, resulting in improved efficiency in the drying process. The exhaust heat from the chamber is used to preheat the feed stream. The third stage is again the external fluid bed, for final drying and/or cooling the powder. As a result, a higher quality powder with much better rehydrating properties is obtained. Moreover, lower energy consumption and smaller space requirements are obtained.

3.5. MISCELLANEOUS PROCESSING 3.5.1. Size Enlargement Size enlargement operations are used in the food processing industry for improving handling and flowability, producing structural useful forms, enhancing appearance, etc. Food size enlargement operations are known as compaction, granulation, tabletting (palletizing), encapsulation, sintering, and agglomeration. The main objective of agglomeration is to control porosity and density of materials in order to manipulate properties like dispersibility and solubility, known as instantizing, because rehydration is an important functional property in food processes. If size enlargement is used with the objective of obtaining definite shapes, extrusion is the selected process to shape and cook at the same time. 3.5.1.1. Instantizing Powdered fruit juice with particle size less than about 10 mm is considered. Powdered fruit juice is the product obtained from fruit juice of one or more kinds by the physical removal of virtually all the water content. The resulting product will be in the powder form and will require the addition of water before use. This product tends to form lumps when dissolved in water and require strong mechanical stirring for obtaining a homogeneous dispersion. It was proposed that under those conditions water penetrates into the narrow spaces between the particles by capillarity, and the powder starts to dissolve, forming a thick, gel-like mass, which resists further penetration of water. Therefore, fruit particle agglomeration, with a dried core is formed. Moreover, if enough air is locked into these lumps, they will float on the water surface, resisting further dispersion. To avoid this problem, agglomerated powder needs an open structure, allowing water to penetrate before a tightly packed gel layer is formed. To do this the specific surface of the powder has to be reduced and the liquid needs to penetrate more evenly around the particles. In this way the powder can disperse into the bulk of the liquid, and the following steps represent a complete dissolution (Ortega-Rivas, 2005):

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Fruit Manufacturing (1) Granular juice particles are wetted, and water penetrates into the pores of the granule structure.

(2) The wetted particles sink into the water and granules disintegrate into their original smallest particles.

(3) The small, dispersed particles dissolve in the water. The total time required for all these steps should be the criteria used to evaluate a product’s instant properties. 3.5.1.2. Agglomeration Agglomeration can be defined as the process of size enlargement by which particles are joined or bind each other in a random way, finishing with an aggregate of porous structure much larger in size than the original material. Agglomeration is used in food processes mainly to improve properties related to handling and reconstitution, that is to produce a more uniform particle size, increase or decrease particle bulk density, improve solubility and dispersibility, and reduce caking. Dispersibility is defined as the time to dissolve a given weight of material in water. The term agglomeration includes varied unit operations and processing techniques aimed at agglomerating particles (Green and Maloney, 1999). The main attractive forces involved in the agglomeration of food particles are studied by Rumpf (1962) and Rumpf and Schubert (1978): (1) Solid bridges, liquid bridges, and capillary forces. The force exerted by a liquid bridge at rest depends on both the surface tension of the interface and the capillary effects due to the curvature of the bridge. Solvents’ evaporation and treatment of food particles at elevated temperatures form solid bridges, by formation of salt bridges, and partial sintering or melting at the intra-agglomerate contact points. Alternatively chemical bonding may occur with the use of organic binders, like hydrocolloid for hydrophilic materials. If the material has some hydrophobicity, the binder may have to have a wetting agent, e.g., lecithin. Figure 3.7 shows a simplified agglomeration by liquid bridges’ development. After drying soluble solids crystallize out of the liquid bridge, forming solid bridges. (2) Van der Waals’ Forces. These are commonly known as dispersion forces, and are quantum mechanical in origin. These forces of attraction exist between molecules of any kind and constitute a general property of matter. Such attractions will cause particles to stick to each other when they come within a few nm of each other. While

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Wetting

69

Collision

Liquid bridge formation

Figure 3.7. Agglomeration by liquid bridge formation.

van der Waals’ attractions can be strong at short distances (<10 nm), the attraction becomes negligible for particles that are far apart. (3) Electrostatic forces. These tend to be the weakest force between particles. Particles in close proximity will tend to be held together by the difference in electrostatic potential, U. To enable calculation of the strength of the agglomerate and the manner of the bonding, the number of bonds and the mechanics of the breakup need to be considered (Rumpf, 1962). Figure 3.8 summarizes the mechanisms usually involved during food particles’ agglomeration, like partial melting, liquid bridges, molecular, interlocking bonds, and electrostatic and capillary forces (Pietsch, 1991). The agglomeration process causes an increase in the amount of air incorporated between powder particles. More incorporated air is replaced with more water when the powder is reconstituted, which immediately wets the powder particles. Figure 3.8 represents in fact a three-dimensional structure containing a large number of particles. Particles in agglomerates could be quite numerous. The points of interaction can be characterized by contact, or by a distance small enough for the development of binder bridges. Alternatively, sufficiently high attraction forces can be caused by one of the shortrange force fields. The number of all interaction sites of one particle within the agglomerate

Partial melting

Capillary force

+ + +

Interlocking bonds

− − −

Van der Waal and electrostatic forces

Liquid bridge

Figure 3.8. Mechanisms usually involved during food particles’ agglomeration.

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structure is called the coordination number (Green and Maloney, 1999; Pietsch, 1983; OrtegaRivas, 2005). Indirect measurement of the coordination number can be made as a function of other properties of the agglomerate, namely porosity. 3.5.1.3. Agglomeration Process and Equipment Two particles can be made to agglomerate if they are brought into contact. Food particles are then brought into a sticky state, by wetting with the application of a finely dispersed liquid or steam, by heating (thermoplastic materials), or by the addition of binder media (a food adhesive). Use of binder agents is considered a nonagglomeration method. The steam condensation method usually cannot provide enough wetting without adversely heating the material and is used less frequently on newer systems. The food particles’ surface must be uniformly wetted and held wet over a selected period of time to give moisture stability to the clusters formed. The clusters are dried to the desired moisture content and then cooled. Dried clusters are screened and sized to reduce excessively large particles and remove excessively small ones. Subsequently, the particles are placed under such conditions where they can form structures. Successful formation of stable agglomerate structures depends on product solubility and surface tension, as well as on the conditions that can be generated in the process equipment. For most products, combinations of moisture and temperature can be established. Generally agglomeration temperature decreases when particle moisture increases. 3.5.1.4. Agglomeration Equipment Agglomeration is a complicated process. However, it is possible to agglomerate powder foods by means of comparatively simple equipment (Pietsch, 1991), which involves the use of a fluidized bed for rewetting and particle contact phase, followed by a belt or a fluid bed for moisture removal (Fig. 3.9). The process must be strictly controlled to avoid deposit formation in the chamber by overwetting, and weak agglomerates due to insufficient liquid or vapor rate.

Water/vapor Air outlet Powder in

Hot air

Agglomerated product

Figure 3.9. Typical agglomeration system.

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Figure 3.10. Gear pelletizer.

Deposit formation will always be a concern in agglomeration equipment. Proper implementation of a fluid bed agglomeration system requires detailed knowledge of the fluidization technology, including fluidization velocities, bed heights, air-flow patterns, and residence time distribution. These processes have allowed the manufacturing of powders with better reconstitution properties, such as fruit juice powder. During agglomeration, the powder is wetted, uniformly but not excessively, with water or steam. Other agglomeration methods are compaction, extrusion, melt forming, mixing, tumbling, and sintering. During pressure agglomeration particles with only slight amounts of moisture are formed into tablets, and briquettes into stamp presses, tablet presses, and roller presses. The principal binding force is van der Waals’ attraction. Figure 3.10 shows a typical gear pelletizer. 3.5.1.5. Selective Agglomeration (Spherical Agglomeration) In the latest agglomeration process, a second immiscible phase is added to the suspension. This wets the solid phase and binds the particles together by means of capillary forces. As a result, rounded flocks or agglomerates form with diameters up to 5 mm. Selective agglomeration can be achieved for mixtures of solids.

REFERENCES Barbosa-Canovas, G.V., Palou, E., Pothakamury, U.R. and Swanson, B.G. (1997). Application of light pulses in the sterilization of foods and packaging materials. Nonthermal Preservation of Foods. Marcel Dekker, New York, Chapter 6, pp. 139–161. Barbosa-Ca´novas, G.V., Maria Tapia and Pilar Cano, M. (eds.) (2004). Novel Food Processing Technologies. CRC Press, Boca Raton, FL.

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Bolando-Rodrı´guez, S., Go´ngora-Nieto, M.M., Pothakamury, G.V., Barbosa-Ca´novas, G. and Swanson, B.G. (2000). A review of nonthermal technology. In Trends in Food Engineering. Aspen Publishers, Inc., Maryland, EEUU, pp. 117–134. Cheftel, J.C. (1995). High-pressure, microbial inactivation and food preservation. Food Sci. Technol. Int. 1: 75–90. Cole, R. (1997). High pressure processing: a technology of the future. Food Manuf. 72: 21–26. Crapiste, G.H. and Rotstein, E. (1997). Design and performance evaluation of driers. In Handbook of Food Engineering Practice, Valentas, K.J., Rotstein, E. and Singh, R.P. (eds.). CRC Press, Boca Raton, Chapter 4, pp. 125–165. Farr, D. (1990). High pressure technology in the food industry. Trends Food Sci. Technol. 1: 14–16. Geveke, D., Kozempel, M., Scullen, O.J. and Brunkhorst, C. (2002). Radio frequency energy effects on microorganisms in foods. Innov. Food Sci. Emerg. Technol. 3: 133–138. Green, D.W. and Maloney, J.O. (1999). Perry’s Chemical Engineers’ Handbook. McGraw-Hill, New York. Heldman, D.R. and Singh, R.P. (1981). Food Processing Engineering, 2nd ed. AVI Publishing Company, Inc., Westport, USA. Karel, M., Fennema, O.R. and Lund, D.B. (1975). Principles of Food Science. Part. 2. Physical Principles of Food Dehydration. O.R. Fennema Ed. M. Dekker Inc. NY. Minton, P.E. (1986). Evaporator types and applications. In Handbook of Evaporation Technology. William Andrew Publishing/Noyes. Mujumdar, A.S. and Menon, A.S. (1995). Drying of solids: principles, classification and selection of driers. In Handbook of Industrial Drying, Mujumdar, A.S. (ed.). Marcel Dekker, Inc., New York, Chapter 1, pp. 1–40. Nickerson and Sinskey (1972). Microbiology of Foods and Food Processing. American Elsevier Publishing Company, NY. Ochiai S, Nakagawa, Y. (1991). High Pressure Science for Food. Hayashi Edition, Kyoto, Japan. Ohlsson, T. and Bengtsson, N. (2002). Minimal processing of foods with non-thermal methods. In Minimal Processing Technologies in the Food Industry, Thomas Ohlsson and Nils Bengtsson (eds.). CHIPS, Texas, USA. Ortega-Rivas, E. (2005). Handling and processing of food powders and particulates. In Encapsulated and Powdered Foods, Onwulata, C.I. and Konstance, R.P. (eds.). Marcel Dekker, New York, in press. Perry, R.H. and Chilton, C.H. (1973). Chemical Engineers’ Handbook, 5th ed. McGraw-Hill Book Company, New York, pp. 11–27. Pietsch, W. (1983). Low-energy production of granular NPK fertilizers by compaction-granulation. Proceedings of Fertilizer’83. British Sulphur Corp., London, UK, pp. 467–479. Pietsch, W. (1991). Size enlargement by agglomeration. John Wiley & Sons Ltd., Chichester, England. Rumpf, H. (1962).The strength of granules and agglomerates. In Agglomeration, Knepper, W.A. (ed.). Interscience, New York, pp. 379–418. Rumpf, H., Schubert, H. (1978). Adhesion forces in agglomeration processes. In Onada & Hench: Ceramic processing before firing. J. Wiley and Sons, Inc., London. Saca, A. and Lozano, J.E. (1992). Explosion puffing of bananas. Int. J. Food Sci. Technol. 27: 419–423. Shi, X.Q. and Fito, P. (1993). Vacuum osmotic dehydration of fruits. Drying Technol. 11: 1429–1442. US Department of Health and Human Services (2004). Juice HACCP Hazards and Controls Guidance. Guidance for Industry, 1st ed. Food and Drug Administration, Center for Food Safety and Applied Nutrition (CFSAN). Van Arsdel, W.B., Copley, M.J. and Morgan Jr., A.I. (1973). Food Dehydration, Vols. 1 and 2. AVI Publishing Company, Inc., Westport, CN. Walas, S.M. (1976) Spray driers. In Encyclopedia of Chemical Processing and Design. Vol. 53. J.J. McKetta Ed. Marcel Dekker Inc. NY. pp. 22–44.

CHAPTER 4

THERMODYNAMICAL, THERMOPHYSICAL, AND RHEOLOGICAL PROPERTIES OF FRUITS AND FRUIT PRODUCTS 4.1. INTRODUCTION Most processed and many freshly consumed fruits receive some type of heating or cooling during handling or manufacturing. Design and operation of processes involving heat transfer needs special attention due to heat sensitivity of fruits. Both theoretical and empirical relationships used when designing, or operating, heat processes need knowledge of the thermal properties of the foods under consideration. Food thermal properties can be defined as those properties controlling the transfer of heat in a specified food. These properties are usually classified (Perry and Green, 1973) into thermodynamical properties, viz, specific volume, specific heat, and enthalpy; and heat transport properties, namely, thermal conductivity and thermal diffusivity. When considering the heating or cooling of foods, some other physical properties must be considered because of their intrinsic relationship with the ‘‘pure’’ thermal properties mentioned, such as density and viscosity. Therefore, a group of thermal and related properties, known as thermophysical properties, provide a powerful tool for design and prediction of heat transfer operation during handling, processing, canning, and distribution of foods (Fig. 4.1). Abundant information on thermophysical properties of food (Polley et al., 1980; Wallapapan et al., 1983; Choi and Okos, 1986; Rahman, 1995) is available to the design engineer. However, finding relevant data is usually the controlling step in the design of a given food operation, and the best solution may be the experimental determination. This chapter provides data and information for thermal process calculation for fruits and fruit products, including a brief description of more commonly used methods for measurement and determination of thermophysical properties.

4.2. THERMOPHYSICAL PROPERTIES’ IDENTIFICATION Thermophysical properties include different types of parameters associated to the heat transfer operations present during fruit processing. It is well known that heat can be transferred by three ways: radiation, conduction, and convection. Radiation is the transfer of heat by electromagnetic waves. The range of wavelength 0.8–400 mm is known as thermal radiation, since this infrared radiation is most readily 73

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Thermodynamical properties “Pure” Thermophysical properties

Specific volume

n (m3/kg)

Specific heat

cp (kJ/kg−1/ ⬚ C−1)

Enthalpy

∆H (kJ/kg−1)

Thermal conductivity

k (W/m−1/K−1)

Thermal diffusivity

a (m2/ s−1)

Density

r (kg/m−3)

Porosity

e

Viscosity

m (Pa s)

Heat transport properties

Physical properties

Figure 4.1. Thermophysical properties associated to fruit processing.

absorbed and converted to heat energy. A body emitting or absorbing the maximum possible amount of radiant energy is known as a ‘‘black body.’’ Energy emitted by a black body is given by the Stefan–Boltzmann law: Q ¼ sAT 4

(4:1)

where s is the Stefan–Boltzmann constant; A the area of transfer, and T the absolute temperature. For no ‘‘perfect’’ black bodies, as real bodies are, Eq. (4.1) is corrected by as factor known a emissivity («): Q ¼ s«AT 4

(4:2)

Emissivity values of foods are in the range 0.5–0.97 (Karel et al., 1975). Conduction is the movement of heat by direct transfer of molecular energy within solids (for example, heating of a fruit pulp by direct fire through metal containers). Convection is the transfer of heat by groups of molecules that move as a result of a gradient of density or agitation (for example, the stirring of tomato pure´e). Heat transfer may take place: (i) in steady-state way by keeping constant the temperature difference between two materials or (ii) under unsteady-state way when the temperature is constantly changing. Calculation of heat transfer under these conditions is extremely complicated but is simplified by making a number of assumptions or giving approximate solutions from prepared graphic or tabulated information. Table 4.1 shows some common simplified equations used for the calculation of heat transfer. Most of the thermophysical properties are required to solve the heat transfer equations by conduction and convection. During processing, the temperature within a fruit, changes continuously depending on the temperature of the heating medium, and two properties of the fruit: the thermal conductivity (k) and the specific heat Cp . On the other hand thermal diffusivity is related to k and Cp : a ¼ k=rcp

(4:3)

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Table 4.1. Simplified equations used for heat conduction and convection calculations (Lozano 2005, with permission). 8 < Steady state: Conduction (unidirectional )

: Unsteady state: 8 Natural > > < > > :

Convection

Forced

Q ¼ kA(u1 u2 )=x 2 2 8 du=dt ¼ ad u=dx < Q ¼ hs A(DT) Gr ¼ r2 gbl 3 DT=m2 : 8 Nu ¼ K(GrPr) < Nu ¼ f (Re, Pr) ¼ hl=k Pr ¼ cm=k : Re ¼ lnr=m

Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’ Edited by Gustaro V. Barbosa-Ca´novas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd. Gr, Nu, Pr, and Re are the Grashoff, Nusselt, Prand, and Reynolds’ numbers, respectively.

When a solid piece of food is heated or cooled by a fluid, the resistance to heat transfer, which are the surface heat transfer coefficient and k may be related as follows: Bi ¼ hl=k 2

(4:4)

1

where h (W=m =K ) is the heat transfer coefficient, f the characteristic half-dimension, and k (W=m1 =K1 ) the thermal conductivity. At small Bi (<0:2) the surface film is the predominant resistance, while for Bi > 0:2 it is the thermal conductivity, which limits the rate of heat transfer (Urbicain and Lozano, 1997).

4.3. FRUITS AND FRUIT PRODUCTS’ PROPERTIES 4.3.1. Fruit and Fruit Products’ Properties During Freezing It must be considered that thermophysical properties of foods change dramatically during the freezing process. One of the characteristics of food freezing is that temperature changes gradually with the phase change, which implies that the fraction of water frozen always changes continuously with temperature below the freezing point. Depression of the initial freezing temperature can be predicted from (Heldman and Singh, 1981): l 1 1 (4:5) ¼ ln XA R TAo TA where l is the molal latent heat of fusion, R the universal gas constant, TAo the freezing temperature of water, TA the freezing temperature of food product, and XA the mole fraction of water. In most cases Eq. (4.5) is used to predict the unfrozen water fraction with the initial freezing temperature, through the determination of apparent molecular weights for the mass fraction of total solids in the product. 4.3.2. Water Content Though water content (Xw ) is not a thermophysical property, it significantly influences all thermophysical properties (Lozano 2005). As food is a living commodity, its water content changes with maturity, cultivar, growth, and harvest and storage conditions. Values of most of the thermophysical properties can be calculated directly from the water content. Xw is usually expressed as water mass fraction [kg water per kg food; wet basis].

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4.4. EXPERIMENTAL DATA AND PREDICTION MODELS Fruits and fruit products show extended variability in composition and structure, which must be kept in consideration when modeling their thermal properties. Fruits are generally nonhomogeneous, varying in composition and structure not only between products but also within a single product. Some thermophysical properties for fruits were modeled only as a function of the water content (Alvarado, 1991; Gupta, 1990). However, the presence of proteins, fats, and carbohydrates, as major components besides water, differs from one fruit to another. These compounds have variable effects on the properties of the complex fruit structure (Sweat, 1995). As a result some proposed thermophysical properties models applied to fruit and fruit products include a combination of the properties of water, fats, proteins, carbohydrates, and/ or ash (Oguntunde and Akintoye, 1991; Rahman, 1995). Literature provides a large volume of experimental thermophysical food properties data (Dickerson, 1968; Mohsenin, 1980; Jowitt et al., 1983; Rahman, 1995; Urbicain and Lozano, 1997). However, as the amount of thermal properties data required for describing any foodstuff under the varied handling, processing, and storage condition is practically infinite, modeling and prediction of such properties is a must. Thermophysical properties of any material control the thermal energy transport and/or storage within it, as well as the transformations undergone by the material under the action of heat. Fruits are no exception; they are dependent on the temperature and the material’s chemical composition, and physical structure. Since fruits are complex materials, relevant information on their properties is given as average or effective value. For this reason the generation of predictive models requires a physical representation of the material under study. Fruits show three different levels of complexity. First, microscopically, fruits may look as a continuous and homogeneous single phase. However, they are composed of different chemical compounds including proteins, carbohydrates, fats, fiber, water, and other minor components. For this reason models proposed for the prediction of a given property must consider the individual contribution of such compounds. It may be done by ‘‘weighing factors’’ accounting for the proportion in which they are present. In the second place, some fruits or fruit products can be considered as a solid matrix of the continuous described above and a disperse phase of air or water, respectively. This description corresponds typically to porous fruits and fruit powders. In this case both the volumetric fraction and the spatial distribution of each phase are to be considered, which is done by means of distribution factors adequately described. Finally, a third level of complexity is achieved when different food materials, including fruits, are processed together to give composite food. This group includes all kinds of canned and packed foods, pastries, confectioneries, and a wide variety of prepared foods. Once more, modeling requires the information of the mean or effective values of the components together with the representation of the physical structure. As previously mentioned, the value of the thermophysical property will be a function of the temperature, through the dependence of the components, and porosity or water content, for porous or composite foodstuffs. Since water can be either liquid or solid, particular attention is paid to frozen fruit products. Available information may be contradictory, due to the different conditions at which thermophysical properties were gathered, as well as to the differences among fruits of different origin, composition, and structure.

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4.4.1. Density* Density (r) is the unit mass per unit volume. SI unit for density is [kg=m3 ]. In particular, when the fruit or fruit product is a porous solid, density plays an important role in heat transfers intrinsically or through the definition of porosity. A few definitions are necessary (Lozano, 2005): . .

.

Substance density: rs (or true density), is the density measured when the substance has been broken, milled, or mashed to ensure that no pores remain. Particle density: rp , is the density of a sample that has not been structurally modified. In the case of pores not externally connected to the surrounding atmosphere, particle density will include these close pores. Bulk density: rb (or apparent density), is the density measured so as to include the volume of the solid and liquid materials, and all pores, closed or open to the surrounding atmosphere.

As other authors (Maroulis and Saravacos, 1990; Farkas and Singh, 1991) have used different terms for the same condition, it is recommended to verify the definition of density before using density data. 4.4.1.1. Porosity* Porosity indicates the volume fraction of air (or void space). On the basis of the given densities, the following definitions of porosity have been proposed (Lozano et al., 1980; Lozano, 2005): .

.

Total porosity («t ) is the ratio of air space volume to total volume: «t ¼ (rs rb )=rs

(4:6)

Open pore porosity («a ) is the ratio of the volume of pores connected to the outside to the total volume: «a ¼ (rp rb )=rp (4:7)

As in the case of density, it is recommended to verify definitions before using porosity data. 4.4.1.2. Density Measurement Methods Techniques developed for density measurement are basically methods for the measurement of volume, weight being easily measured with different types of precision balances. The principal measurement techniques applied for volume (and density) determination in fruit and fruit products are: .

Hydrometric method: The bulk density is calculated from the apparent weight of the sample and the buoyant force E. (4:8) rb ¼ rliq (Wair =E)

Fruit tissue must be coated to avoid mass loss by dilution or pore inundation (Lozano et al., 1980). * Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods’’. Edited by Gustaro V. Barbosa-Ca´novas. Paris UNESCO Publishing. ISBN 92-3-1039997pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.

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Geometric method: The volume is calculated from the dimensions (L) of the food sample. rb ¼ L3 =W

(4:9)

It is not suitable for soft and irregular solid foods. .

Pycnometry: Liquid pycnometry. The pycnometer is a calibrated flask that allows the weighing of an exactly known volume of liquid, which in turn gives the density. The weight is determined by difference with the empty flask. To set the experimental temperature, the pycnometer is immersed in a constant temperature bath and filled with the sample.

Gas pycnometry. Using perfect gas law it is possible to determine the volume of open pores (Vpore ) in food by determining the gas volume in a chamber (Vch ) with and without the sample: Vpore

Pi Vi ¼ mRT ¼ Vch (P1 P2 )=P2

(4:10)

P1 and P2 are pressure in empty space and in sample chamber,respectively (Mohsenin, 1980; Lozano et al., 1980). Table 4.2 lists bulk density values of two fruits (apple and pear) and fruit tissue components. These values cover a relatively wide range of density, illustrating the influence of water and air (porosity) in this property. 4.4.1.3. Empirical Equations and Theoretical Density Models Some empirical equations for the calculation of bulk density of selected fruits, in terms of temperature and water content, are listed in Table 4.3. Constenla et al. (1989) proposed a theoretical approach by considering the thermodynamic expression for the specific volume of a multicomponent solution (reciprocal of density) in terms of partial specific volumes: X V ¼ 1=r ¼ w i vi (4:11) where wi and vi ; represent the mass fraction and the partial specific volume of the i-component in solution, respectively. Sugars, organic acids, and different macromolecules interact with a substantial number of water molecules, resulting in a nonideal solution behavior. Therefore, specific volume is not necessarily equal to the specific volume of the pure component. Table 4.2. Bulk density of selected fruits and fruit products or components. Fruit/component Apple (GS; Xw ¼ 0:86) Pear Cellulose Fat Glucose (solid) Protein Water

Temperature/range (8C)

Bulk density (kg=m3 )

25 25 — — — — 4

837 990 1,550 900/950 1,560 1,400 1,000

Reference Lozano et al. (1980) Lozano et al. (1983) Kirk–Othmer (1964) Lewis (1987) Kirk–Othmer (1964) Lewis (1987) Perry and Green (1973)

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Table 4.3. Empirical equations to calculate bulk density of selected fruit products or fruit components. Application range Equation

Xw (kg=kg) rb ¼ a þ T þ cT 2 rb ¼ a þ bXw þ þcXw2

rb rb rb rb

Parameter

Product

¼ a þ b ln Xw ¼ a bxr þ c 106 e(1:33xX =Xo ) ¼ a þ þbe(0:01 BrixþcT) ¼ 0:852 0:462e0:66X( Brix)

n. a. Pistachio Coconut (shredded) Orange juice Apple (G. Smith) Pear Apple juice Apple

T (8C)

0.05/0.40 —

— —

0.09/0.65 0.8/6.6 0.15/7.0 0.25/1.0

21 Ambient Ambient 10/90 Ambient

a

b

439 236

5.003 440

0.994 0.636 1.251 0.828 0.852

0.307 0.102 0.153 0.3471 0.462

c

— — 0.282 — 0.107 5:4794

From Lozano et al. (1979, 1983, 2002, 2005), Constenla et al. (1989), Moresi and Spinosi (1980), Jindal and Murakami (1984), Hsu et al. (1991).

In the dilute limit, vw (water) has contributions mainly from structured free-solvent regions, while vs (solute) is affected by hydration and water–solute interactions. In the concentrated limit, vw is defined by water–solute aggregations, i.e., hydrogen bonded to hydroxyl groups. For these reasons, in sugar solutions both vw and vs are functions of concentration and temperature (Taylor and Rowlinson, 1955; Maxwell et al., 1984). Constenla et al. (1989) also suggested that the thermal effect on density could be significantly reduced by referring the specific volume to that of pure water vwo , so Eq. (4.11) can be written as: V =vwo ¼ Vw =IVwo þ ws (vs vw )=vwo

(4:12)

Although according to the above discussion the partial specific volumes depend on concentration, from a practical point of view a linear relationship as suggested in Eq. (4.11) can be used to correlate density data, as Constenla et al. have found for apple juice: r ¼ rw =(0:992417 3:7391 103 X )

(4:13)

r2 ¼ 0:9989

Predictions of this equation were also extrapolated to temperatures in the range 10–908C. Perez and Calvelo (1984) proposed the following semiempirical equation for the bulk density calculation of beef muscle, during cooking: rb ¼

h

(1 Xw ) 1

rbo (1 Xwo ) rbo rw

v wo ) þ rrbo (1X (1Xw ) þ u(Xwo Xw )

i

(4:14)

b

where u and v are empirical parameters. Changes in food density by freezing were predicted by Hsieh et al. (1977) as follows: 1 1 1 1 ¼ Mu þ Ms þ MI (4:15) r ru rs rI where M is the mass fraction of unfrozen water (u ), ice (I ) and solids (s ). The most significant change in density occurs immediately below the initial freezing temperature.

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4.4.2. Specific Heat Specific heat is the amount of heat required to increase the temperature of unit mass by unit degree at a given temperature. SI unit for Cp is [kJ/kg/K]. Specific heat of solids and liquids depends upon temperature but is not sensitive to pressure, as it is incompressible to practical purposes. It is common to use the constant pressure specific heat, Cp , which thermodynamically represents the change in enthalpy for a given change in temperature when it occurs at constant pressure: Cp ¼ (dH=dP)p

(4:16)

where H is enthalpy. 4.4.2.1. Measurement Methods Several methods have been used for the specific heat measurement (Rahman, 1995). Both differential scanning calorimetry (DSC) and the method of mixtures are commonly used techniques (Table 4.4). The advantages of DSC are that measurement is rapid, and a very small sample can yield accurate results for homogeneous products (Wang and Kolbe, 1991). Specific heat of selected fruits is listed in Table 4.5. 4.4.2.2. Prediction Models and Empirical Equations Mohsenin (1980) proposed an equation valid for the calculation of Cp of meats, fruits, vegetables, and other foods, which equals the sum of the specific heat of water (Cpw ) and solid matter (Cpsm ): Cp ¼ Cpsm þ (Cpw Cpsm )xW

(4:17)

Table 4.4. Methods for specific heat measurement (Lozano 2005, with permission). Method

Principle of operation

Mixture

A sample of known mass (Ws ) and temperature (Ts ) is dropped into a calorimeter of known specific heat, containing a liquid (usually water) of known mass (Wref ) and temperature(Tref ). Temperature of the mixture is recorded until equilibrium (Teq ) This method is based on the determination of the amount of heat required to raise the temperature of sample of known mass (Ws ), at a given rate within a given interval. The measurement requires an external standard of known mass (Wref ) and specific heat (Cpref )

DSC

Governing equation

Additional comments

Cp ¼

Cpref Wref (Tref Teq ) Ws (Teq TS )

Numerous calorimeters were developed to reduce heat loss, heat generation by mixture and mixing problems

Cp ¼

Cpref Wref d Ws dref

Reduced sample size and escape of water vapor during heating are important limitations of this technique

Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’ Edited by Gustaro V. Barbosa-Ca´novas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.

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Table 4.5. Specific heat of selected fruits and fruit products and components. Product Apple juice

Apple, RD whole Apple sauce Banana Orange juice Sugar

1

Xw

DT (8C)

Cp (kJ=kg1 =K )

758Brix 758Brix 108Brix 0.75/0.85 — 74.8 0.105 0.133

30 90 30 — — — 20/40 59.1

2.805 2.973 3.894 3.95 3.73 3.35 1.85 1.256

From Polley et al. (1980), Constenla et al. (1989), Gupta (1990), and Rahman (1995).

A theoretical approach can also be used to predict the specific heat of a solution in terms of partial specific heats of individual components as follows: X Cp ¼ Cpi wi (4:18) A linear relationship of Cp with concentration as suggested by Eq. (4.18) is the basis for most of the existing correlations to evaluate specific heat of liquid foods (Choi and Okos, 1983). However, due to water–solute interactions, Cpi differs from the specific heat of pure component and usually changes with the concentration of soluble solids. Actually, the resulting values of Cps for sugar solutions are significantly higher than those corresponding to crystalline sugar at the same temperature (Taylor and Rowlinson, 1955; Pancoast and Junk, 1980), while at high water contents Cpw approximates to the heat capacity of pure water Cpwo . In addition, while Cpwo remains almost constant with temperature, the specific heat of the solution increases with this variable following the same pattern as that of crystalline sugar. This behavior was also observed in apple juice (Constenla et al., 1989), so no improvement in the correlation ability of Eq. (4.18) may be obtained by using the ratio Cp =Cpwo . Heldman (1975) proposed an expression for heat-capacity calculation of foods, based on the composition: Cp ¼ 4:180 (0:34Xca þ 0:37Xp þ 0:4XFA þ 0:2Xas þ 1:0Xw

(4:19)

Although widely used, Eq. (4.19) shows deviation when compared with experimental values, due to, among other conditions, the variation of Cp between bound and free water, and the excess specific heat due to the interaction of the component phases. Rahman (1993) proposed corrections reducing some of the limitations of Eq. (4.19). Below freezing point the calculation is more difficult. Rahman (1995) cited Van Beek equation, which is considered to be applicable to all foods below the freezing temperature: Cp ¼ Cpso (1 Xwo ) þ Cpw Xwo (Tfreezing =T) þ Cpice Xwo (1 Tfreezing =T) LXwo (Tfreezing =T 2 )

(4:20)

Schwartzberg (1976), Chang and Tao (1981), and Mannapperuma and Singh (1989), among others, presented more complete equations for the prediction of heat capacity of foods below freezing point. Some other models applicable for different foods and conditions are listed in Table 4.6.

82

Fruit Manufacturing Table 4.6. Models for predicting heat capacity of fruit products and components. Application range

Equation

Parameter

Fruit/component Xw (Kg=kg)

Cp ¼ a þ bT þ cT 2

Cp ¼ a þ bXW Cp ¼ a þ bXw þ T Cp ¼ 1:56e0:945X w

Sugars Ash Fiber Fats Ice Proteins Water Fruits and vegetables Food in general Fruit pulps

T (8C)

a

b

c 106

40/150

1.548 1.093 1.846 1.984 2.063 2.008 4.176 1.670

1:962:103 1:890:103 1:831:103 1:473:103 6:077:103 1:209:103 9:086:105 2.500

5.934 3.682 4.651 4.801 — 1.313 5.473 —

2.477 —

2.356 —

40/150 >0.5 0.001/0.80 0.012/0.945

30/60 20/40

3.791 —

* Adapted from Dickerson (1969), Mohsenin (1980), Gupta (1990), Alvarado (1991), Choi and Okos (1986), Lozano (2005).

4.4.3. Thermal Conductivity Thermal conductivity (k) is an intrinsic property of material and represents the quantity of heat that flows in unit time through a plate of unit thickness and unit area having unit temperature difference between faces. SI unit for k is [W/m/K]. Figure 4.2 shows thermal conductivity values measured in selected fruits at ambient temperature, while Fig. 4.3 shows the influence of temperature on k. 4.4.3.1. Measurement Methods Techniques for measurement of thermal conductivity can be classified as: (i) steady state, (ii) quasisteady state, and (iii) transient. 0.7 0.6 78%

87% 88%

85%

89%

0.5

k (W/m K)

100%

86% 0.4 0.3

30%

0.2 0.1

Water

Strawberry

Orange juice

Pear

Apple

Pineapple

Applesauce

Plum, dried

0

Figure 4.2. Measured k of selected fruits and fruit products at ambient temperature.

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Thermodynamical, Thermophysical, and Rheological Properties

83

2

Sucrose (Xw=75) Apple (GS)

k, W/m K

1.5

1

0.5

0 60

40

20

0

20

40

60

80

Temperature, ⴗC Figure 4.3. Influence of temperature in the thermal conductivity of selected foods (Keppler and Boose, 1970; Kent et al., 1984; Constenla et al., 1989).

These techniques were extensively reviewed and compared by Rahman (1995). Table 4.7 describes different methods for k determination. Steady-state techniques (SST) are wellestablished methods based on the determination of constant heat flux under constant temperature gradient and the solution of the unidirectional steady-state heat conduction equation (Fourier’s equation) for k calculation. Although SST are mathematically simple, highly experiment controlled, and precise, they are practically not applicable in foods due to long equilibrium time (biological active samples, e.g., fruits, deteriorate), the need of geometrically defined samples (most foods are amorphous and/or soft), hence convection must be avoided (this excludes liquids or high-moisture foods). 4.4.3.2. Prediction Models and Empirical Equations Several empirical, semiempirical, and theoretical models and equations have been developed to predict thermal conductivity of composed material in general, and foodstuffs, in particular. Tables 4.8 and 4.9 list the most commonly used predictive models and empirical equations, respectively, valid for fruits and fruit products. 4.4.4. Thermal Diffusivity Thermal diffusivity (a) is a combination of three thermophysical properties that result from the derivation of the Laplace equation of heat conduction (Fourier equation in three dimensions): a ¼ k=rcp

(4:21)

Physically it represents the change in temperature produced in a unit volume of unit surface and unit thickness, containing r[kg] of matter, by heat flowing in the unit time through the unit face under unit temperature difference between opposite faces. Figure 4.4 shows thermal diffusivity values measured in selected fruits at ambient temperature.

> > > > > > > > > :

Thermal comparator

8 > Line source > > > > > > > > <

8 > > > : modifications

Method 8 > Guarded hot > > > > plate > > > > > > > > > < Concentric > > cylinders > > > > > > > > > > > > : Heat flux

Zuritz et al. (1987) and Rahman (1991) modified Fitch’s method for small individual food particles and frozen foods Thermal conductivity probe is a test body made basically of a line source, providing a constant amount of heat and a temperature-measuring device. Alternative designs of the instrument have been discussed by Sweat (1974) and Hayashi et al. (1974) A probe is equilibrated to a higher temperature than the sample. Then the probe is placed in contact with food and it changes temperature at a different rate increasing emf, which is related by calibration to k

T and T1 , and t and t1 , are temperatures and times corresponding to final and initial time, Q is heat produced per unit length of probe

Calibration is required with a number of materials of known thermal conductivity

emf ﬃ k1=2

T i , Ts , mco , and cpco are initial temperature, source temperature, copper mass, and copper Cp , respectively.

T T1 ¼ (Q=4kK) ln (t=t1 ) To do this, time and temperature are correlated by the model equation: k ¼ Q=4ks

ln [(T i Ts )=(T Ts )] ¼ kAt=lmco cpco k is calculated from the slope of the plot of ln [(T i Ts )=(T Ts )]vs:t

k ¼ Wl=(Ti To )

W is the heat flux (W=m2 ).

To calculate k, inner and outer temperatures (Ti ,To ), sample thickness (l) and heat quantity (Q) must be measured Same as previous

Q ¼ kA(Ti To )=l

Heat source surrounded by sample, and in turn surrounded by a heat sink. Insulation is located at ends to avoid heat loss and ensure unidirectional heat conduction Usually heat source is the outer cylinder and heat sink the inner cylinder. Heat absorbed by coolant is the same as the heat conducted through sample. It is based on temperature-gradient determination across sample. k is evaluated at (Ti To )=2 Sample is ‘‘sandwiched’’ in between a constant temperature heat source and a copper plug as heat sinks insulated on all faces but one Q ¼ kAo [(To Ti )= ro ln (ri =ro )]

Additional comments

Governing equation

Principle of operation

Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’ Edited by Gustaro V. Barbosa-Ca´novas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.

Transient

Quasisteady state

Steady state

Technique

Table 4.7. Description of different methods for k determination (Lozano 2005, with permission).

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85

Table 4.8. Predictive models for thermal conductivity estimation applicable to fruit products (Lozano 2005, with permission). Model

Description

Equation

Series

In series distribution, layers of components are thermally in series with respect of heat flow. It is applied in food gels. Layers are considered as thermally in parallel with respect to the direction of heat flow. It is Proposed for liquid and powder foods Based on random distribution of noninteractive spherical particles in a continuous medium The simplest is based on the weighed geometric mean of components, using the volume fraction as the weighing factor A model of statistical nature that considers a heterogeneous medium as represented by a virtual homogeneous one with the same properties. kp is the thermal conductivity of pores

1=ke ¼

Parallel

Maxwell Random

Effective medium theory

ke ¼

P

P

(fi =ki )

(fi ki )

km ¼ kc

kd þ 2kc 2fa (kc ka ) kd þ 2kc þ 2fd (kc kd )

k ¼ Pfi i P

fi [(k ki )=ki 2k)] ¼ 0 For porousp materials: ﬃﬃ k ¼ kp [b (b2 þ 2m)] b ¼ 3e 1 þ [3(1 e) 1]

Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’ Edited by Gustaro V. Barbosa-Ca´novas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.

4.4.4.1. Measurement Techniques The estimation of thermal diffusivity of foods can be done by: (i) direct measurement or (ii) indirect calculation using Eq. (4.21). Several direct methods for a determination were proposed (Rahman, 1995) based on the solution of one-dimensional unsteady state heat transport equation with the appropriate boundary conditions for different geometries. Fourier equation has been solved for numerous conditions, and graphical solutions are also available (Bird et al., 2002). Fourier equation is limited to temperatures above freezing and restricted to homogeneous, isotropic substances. However, analytical and numerical solutions of the one-dimensional heat conduction equation have been used to determine the thermal behavior of foods. These techniques are similar to those used for k determinations, in particular the thermal probe. Table 4.10 lists some of the proposed direct techniques for the determination of a. Indirect method for determination of r, k, and Cp values needs more time and instrumentation. It was indicated that indirect determination yielded statistically more accurate values of a (Drouzas et al., 1991). Andrieu et al. (1989) found roughly 3% difference when determining thermal diffusivity in potato using both pulse and hot wire (probe) methods. 4.4.4.2. Empirical Equations Several empirical equations have been developed to predict thermal conductivity of fruits, and fruit products and components. Table 4.11 lists some empirical equations. 4.4.5. Viscosity Fluid and semisolid foods exhibit a variety of rheological behaviors ranging from Newtonian to time dependent and viscoelastic. Whereas an ideal elastic solid produces an elastic

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Fruit Manufacturing

Table 4.9: Empirical equations for calculating thermal conductivity of fruits and fruit components. Application range Equation* k ¼ a þ bT þcT 2

Fruits or fruit component

Xw (kg=kg)

40/150

Carbohydrates Ash Fiber Fats Ice Proteins Water Apple (GD)

k ¼ a þ bXw þcXw2 k ¼ a þ bXw þcT

k ¼ kwo r=rw [a bXw ( Brix)]

¼ 1:821:66 [esp(0:85) XXwow ] keff ¼ 0:58Xw þ0:155Xpr þ0:25Xca þ þ0:135Xas þ0:16XFA kef ko

T (8C)

40/150 25/0 0/25 30/70

Starch (gelatinized) Starch Pears

Ambient

Tomato paste Apple juice Apple

0.54/0.71 0/758Brix Fresh

Apple juice

0/758Brix

30 10/80 T > Tfreezing T Tfreezing T < Tfreezing 10/80

Sucrose Glucose Fructose Fruits and meats

0/75 0/75 0/75 0.05/0.88

10/80 10/80 10/80 20/25

Liquid and solid foods (more than 400 data)

—

Parameters a 0.2014 0.3296 0.1833 0.1807 2.2196 0.1788 0.5712 1.290 0.394 0.210 0.478 0.4875 0.029 0.2793 0.994 10.3 0.378 2.5289

b 103 1.387 1.401 1.250 0.276 6.249 1.196 1.763 9.50 2.120 0.410 6.90 0.057 Xw 2 0.793 3.572 (8Brix) 15.9 0.133 1.376 1.0052

c 106 4.331 2.907 3.168 0.177 101.5 2.718 1.015 — — — — 0.0227 ( ln Xw )Xw2 1.135 (K) 2.500 10.300 0.930 T 2 —

2.6122 2.5617 2.4153 —

1.0381 0.9725 0.9807 —

— — — —

—

—

—

*Equations must be checked for application ranges and influence of porosity. Adapted from Ramaswamy and Tung (1981, 1984), Drouzas and Saravacos (1985), Mattea et al. (1986), Sweat (1986), Constenla et al. (1989), Marouilis et al. (1991), Rahman (1991), Renaud et al. (1992), Choi and Okos (1996).

displacement when shear stress is applied, a fluid produces viscous flow. If a liquid is held between two parallel infinite plates and the top plate moves at a velocity U (length/time) relative to the bottom plate, the force required to maintain this motion will produce a viscous flow and a velocity gradient, which is equivalent to the shear rate (g ¼ dU=dx) developed. Under this condition viscosity can be defined as: g ¼ t=ma

(4:22)

where ma is called the apparent viscosity and is constant only for this one value of g. If ma is a constant at different values, then: t¼m where m is the Newtonian viscosity of the fluid.

(4:23)

4

Thermodynamical, Thermophysical, and Rheological Properties

.

87

Thermal diffusivity (107 m2/s)

1.7 1.6

Water

1.5 Sucrose

1.4

Strawberry

1.3

Apple

Peach

1.2 Banana

1.1 0.7

0.75

0.8

0.85

0.9

0.95

1

Xw Figure 4.4. Thermal diffusivity of selected fruits and fruit components.

Newtonian behaviors indicate that the viscosity of the food is shear-independent. Other than water, Newtonian flow is exhibited by sugar solutions and vegetable oils also. Viscosity of Newtonian foods has the unit Pas in the International System. Table 4.12 lists definitions of interest in the rheological study of foodstuffs. Most of the foods show more complicated relationships between shear rate and shear stress. It is no longer feasible to talk in terms of viscosity, since m varies with the rate of shear. Table 4.13 shows types of non-Newtonian fluids: (1) Those whose properties are independent of time of duration of shear, (2) Those whose properties are dependent on time of shear, and (3) Those exhibiting characteristics of a solid. 4.4.5.1. Measurement Techniques Methods of viscometry (measurement of apparent viscosity) are described in Table 4.14. Viscometers are based on the measurement of either the resistance to flow in a capillary tube, Table 4.10. Principle of operation and governing equation for the two most commonly used methods for the determination of thermal diffusivity in fruit products. Method

Principle of operation

Analytical solution of Eq. (4.9)

Sample is located in a cylinder (L=D > 4) immersed in a water bath at constant temperature. Thermocouples located at the center of the sample (axis) and surface of cylinder measure are T vs. t. After transition, both temperature gradients are time independent Similar to thermal conductivity probe, with additional thermocouple placed at a known distance in the sample

Probe method

Governing equation a ¼ VR2 =[4(Ts Tc )]

T ¼ (l=2pk)[ 0:58=2 ln g þ g 2 pﬃﬃﬃ =2:1 g 4 =4:2! þ . . . ]g ¼ r=(2 at)

Dickerson (1965), Hayakawa (1973), Uno and Hayakawa (1980), Singh (1982), Gordon, Lozano (2002) and Thorne (1990); Nix et al. (1967).

88

Fruit Manufacturing Table 4.11. Empirical equations for the calculation of the thermal diffusivity of selected fruits and fruit products. Application range

Equation

Product/component

a ¼ a þ T þ cT 2

Ash Carbohydrates Fats Fiber Ice Proteins Water Apple

a ¼ a þ bXw þ cXw2 a ¼ a þ bXw þ cT a ¼ 0:88:107 þ aw Xw 0:88:107 :Xw

Corn Multiple regression Multiple regression

Parameter

T ( C)

a 10

b 104

c 106

40/160 40/160 40/160 40/160 40/160 40/160 40/160 25=10 10=Tfreezing T > Tfreezing 20/90 8/23 — 0/80

12.46 8.08 9.88 7.39 1.17 6.87 13.17 1:22:105 4:37:105 1:39:105 1:51:105 9:56:105 0:786:105 —

3.73 5.30 1.26 5.19 6.08 4.76 6.25 0:187:103 0:437:103 0:278:105 1:34:106 1:83:102 0:574:103 —

1.22 2.32 0.039 2.22 95.0 1.46 2.40 — — — — 0.44 0:29:103 —

2

Adapted from Lozano (2002), Riedel (1980), Ramaswamy and Tung (1981), Choi and Okos (1986), and Rahman (1995).

or the torque produced by the movement of an element through the fluid. There are three main categories of commercially available viscometers applicable to foodstuffs: Capillary, falling-ball, and rotational viscometers. Of late, food scientists and technologists use rheometers available at a relatively low cost, which can measure over wide ranges of shear behavior and perform complete rheograms, including thixotropic recovery, stress relaxation, or oscillatory experiment at programmed temperature sweep. For Newtonian liquid foods it is sufficient to measure m as the ratio t=g. Besides the ratio of shear stress and rate of shear, the properties required to describe a non-Newtonian material can be measured by: (a) (b) (c) (d)

compression (force–deformation relationship), creep test (stress versus strain as a function of time), stress relaxation (stress required to maintain a constant strain), and dynamic test (deformation by a time variable stress, generally oscillatory stress).

Table 4.12. Definition of different types of viscosities. Name

Equation

Comments

Kinematic viscosity Relative vis The reference Dickerson (1969) is not given in the reference list. Please check.cosity Specific viscosity Reduced viscosity Intrinsic viscosity

y ¼ m=r mr ¼ m=mo

In (cm2 =s stoke); where r is density It is the ratio of solute to solvent viscosity at equal temperatures

msp ¼ mr 1 mred ¼ msp =c [m] ¼ ( ln mr =c)[c!o]

Where c is the concentration of solute Also called limiting viscosity number, which is usually correlated with molecular weight

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89

Table 4.13. Types and examples of non-Newtonian foods (Lozano 2005, with permission). Rheological classification

Description

Descriptive diagram

Non-Newtonian time-independent foods Bingham plastic foods (ideal plastic material)

Linear relationship between g and t does not go through the origin. The t value at g ¼ 0 is the yield value or yield stress (ty )

Shear-thinning behavior (pseudoplastic foods)

When g increases with t. When a food is pseudoplastic above yield stress is also known as mixed-type plastic

Shear-thickening behavior (dilatant foods)

Their rheological behavior is opposite to the pseudoplastic in which g decreases as t increases

Examples Most foods

t

Cocoa butter

ty .

γ

t

.

γ

t .

γ

Most of non-Newtonian foods (fruit pure´es, condensed milk, ketchup, etc.)

Starch suspensions and some chocolate syrups exhibit dilatant flow

Time-dependent foods Thixotropic foods

Rheopectic materials

Semisolid foods

Rate the shear stress value decrease with time at constant shear, while the structure collapses

Include those few materials that are able to build up (or set up) while submitted to a shear stress at constant. These foods show both solid (elasticity) and fluid (viscosity) behavior when they are subjected to a sudden, instantaneous, constant shear stress; sufficient time is allowed for the test; and the stress is large enough to prevent the food showing pure elasticity. During flow, normal stresses s are built up

ma

Some fruit pulps

t ma t

The viscoelastic behavior of foodstuffs is commonly explained by two basic tests: stress relaxation and creep (increase of strain with time)

Whey protein polymers have strong rheopectic properties

Cheeses

Reproduced with permission from Thermal Properties of Foods. In ‘‘FOOD ENGINEERING 1. Engineering Properties of Foods.’’ Edited by Gustaro V. Barbosa-Ca´novas. Paris UNESCO Publishing. ISBN 92-3-103999-7, pp.45-64. All of which is part of Encyclopedia of Life Support Systems (EOLSS). http://www.eols.net (copyright) EOLLS Publishers Co.Ltd.

4.4.5.2. Newtonian Fruit Products Liquid foods, such as clarified fruit juice, exhibit Newtonian behavior. As an early approximation viscosity of Newtonian foods can be estimated as the viscosity of water (mw ) and that of the prevalent soluble solids. Different empirical equations relating liquid food viscosity with both soluble solids and temperature were published (Rao, 1977). The viscosity of water,

90

Fruit Manufacturing Table 4.14. Methods for determination of viscosity.

Method

Description

Governing equation*

Capillary tube viscometers

The time for a standard volume of fluid to pass through a length of capillary tube is measured. The driving force is gravity, gas pressure, a descending piston, or partial vacuum at the exit. The force used to create flow is usually gravity as seen in Ostwald and Cannon–Fenske viscometers

m ¼ pDPr4 t=8Vl

Falling-ball viscometers

Rotational viscometers

A standardized tube is filled with the product, and the time under the influence of gravity for a ball to pass between two specified points is measured. The falling ball reaches a limiting velocity when the acceleration is exactly compensated by the friction of the fluid on the ball. The rolling-ball instrument uses a tube inclined at a 10-degree angle, which allows the ball to remain in contact with the inner tube surface. The drawn-ball type uses a ball mechanically pulled upward through the tube. These viscometers accurately measure low- to medium-viscosity Newtonian fluids. The drawn ball may allow the measurement of opaque fluids These instruments can determine the viscosity of Newtonian and non-Newtonian fluids contained between two coaxial cylinders (bob and cup), or different geometries as the cone and plate geometry, by measuring the drag of the fluid on a mobile member (cylinder or cone) while the other member (cylinder or plate) remains stationary. They produce precise measurements of absolute viscosity for a wide range of viscosities. Because the shear rate can be varied, it is possible to plot the flow curves of non-Newtonian fluids. Time effects can be studied either manually or automatically with computerized controls

where DP is the driving force (pressure), r the capillary radius, V the volume, and l the capillary length. For Newtonian liquid foods n ¼ Kt where K is a constant m ¼ K(rball rs )=n where K ¼ (0:374gD(D þ d) sin u, and d is the diameter of the ball, D the diameter of the tube, g the acceleration of gravity, and u the angle of tilting of the tube

Coaxial cylindersþ : m ¼ GT (1=R21 1=R22 )=4plv Cone plate: m ¼ 3GT u=(2pR3 v) where GT is the torque of the bob, v the angular velocity, R1 the radius of the bob, Ro the radius of the cup, and R the radius of the plate u the cone angle, v the angular velocity, l the cylinder length.

*Adapted from Slattery (1961), Van Waser et al. (1963), Johnson et al. (1975), and Bourne (1982).

and salt and sucrose solutions, major solutes in foodstuffs, can be calculated by Eqs. (4.24), (4.25), and (4.26), respectively (Kubota et al., 1980): ln mw ¼ 0:266 2:02 102 T þ 4:4 105 T 2

(4:24)

where T is the temperature in 8C. m ¼ a exp (b=T n ) (283:2K < T < 323:2K)

(0 < X < 40 Brix) where log a ¼ 0:00458X 1:15 3:05 and b ¼ 9:90 104 X 1:51 þ 6:1 107

(4:25)

4

.

Thermodynamical, Thermophysical, and Rheological Properties m ¼ aeb=T

91

n

(283:2K < T < 323:2K) (0 wt% < X < 24 wt%)

(4:26)

where log a ¼ 3:59103 X 1:33 2:0 and b ¼ 3:09105 X 1:59 þ 6:1107 In another approach Rao et al. (1984) reported that the effect of concentration on viscosity of fruit juices at constant temperature may be represented by an exponential-type relationship. Constenla et al. (1989) modified the Mooney (1951) equation in order to express the concentration on a weight basis (X), as 8Brix, and to take into account the effect of temperature: ln

m A(T)X ¼ mw 100 B(T)X

(4:27)

where coefficients A and B are temperature dependent (Table 4.15). As Fig. 4.5 shows, viscosity of apple juice expressed as reduced viscosity (m=msucrose ), lies between the corresponding curves for sucrose and reducing sugars. The same figure also includes the predictions of Eq. (4.27), for a model system made with sugars in the same proportions present in apple juice. Although Eq. (4.27) gives a reasonable estimate of the behavior of apple juice, differences are not insignificant and were associated with the presence of nonsugar organic components, which usually tend to increase the viscosity. Some discrepancies were attributable to the effect of malic acid on the refractometric measurement of soluble solids (Millies and Burkin, 1984). It was observed that viscosity data of clarified pear juice (Ibarz et al., 1987) also lie between those of sucrose and reducing sugars. On the other hand, grape and orange juices containing some suspended colloids, mainly pectin, tartrates, and citrates, were more viscous than sucrose solutions; and cloudy apple, grape, and orange juices, which contained a significant amount of pulp and suspended particles, were pseudoplastic (Saravacos, 1970; Moresi and Spinosi, 1980, 1984; Rao et al., 1984). Thus, it appears that Eq. (4.27) and sugar solutions’ data can be applied to estimate viscosity only in the case of clarified fruit juices. The presence of suspended material not only increased the viscosity but also changed the rheological behavior of the product, so a different approach must be used in that case. 4.4.5.3. Non-Newtonian Fruit Products Viscolastic and semisolid foods have been extensively studied during the last few decades. Rheological characterizations of non-Newtonian foods have been in the form of t versus g curves, dynamic characteristic, time effect on h at constant, g, etc. Values for these parameters were compiled by different authors (Rao, 1977; Kokini, 1992). The following creep

Table 4.15. Parameters of Eq. (4.27) valid for the determination of viscosity at 208C. Coefficient A B

Glucose

Fructose

Sucrose

2.562 0.972

2.415 0.981

2.612 1.038

Adapted from Constenla et al., 1989.

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Fruit Manufacturing 1.1

Reduced viscosity (m/m sucrose)

1 0.9 0.8 0.7 Apple juice

0.6

Fructose Eq.(4.27)

0.5

Glucose

0.4 0.3 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Concentration (w/w) Figure 4.5. Effect of concentration on the reduced viscosity of apple juice at 208C (Constenla et al., 1989) with permission.

compliance vs. time equation (Sherman and Sherman, 1966), proposed for the description of the rheological behavior of ice cream, is a representative example of a model for description of the viscoelastic behavior of semisolid foods: J(t) ¼ J0 þ J1 (1 ett ) þ J2 (1 ett2 ) þ t=h

(4:28)

where J ¼ g=t is the creep compliance, J0 is the instantaneous elastic compliance, J1 and J2 are the compliances associated with retarded elastic behavior, t 1 and t 2 are retardation times, associated with retarded elasticity, and h is the viscosity associated with Newtonian flow. In Table 4.16 selected experimental data of viscosity and values of power law, and other rheometric parameters for fruit and tomato products are listed. 4.4.5.4. Effect of Temperature and Pressure on the Viscosity of Foodstuffs Viscosity–temperature dependence is frequently represented by the Arrhenius–type equation: ln m ¼ k0 þ DEa =RT

(4:29)

where k0 is a pre-exponential factor, DEa is the activation energy of flow, and R is the gas constant. Ultracki (1974) presented the following empirical equation: ln m ¼ k0 þ A=(T T0 )

(4:30)

where k, A, and T0 are constants. During extrusion and other food processing operations relatively high pressures are applied. In such a case, the calculation of the viscosity at pressures different from those published may be necessary: m ¼ m0 eaP

(4:31)

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Thermodynamical, Thermophysical, and Rheological Properties

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Table 4.16. Viscosity, power-law (n and K) parameters, and yield stress (t y ) of selected foods at ambient temperature. Fruit product

m or ma , mPa/s

Apple sauce Pear juice (708Brix) Pectin, 0.5 (wt%) Tomato concentrate (30 wt%) Tomato paste (308Brix)

n

K Pa=sn

tyPa

0.15–0.24

40.6–76.9

18.4–50.7

0.40 0.28

187 139–252

78–212

1.15 4.5 (500 s1 ) Serum:4.3–140

Adapted from Qiu and Rao (1888), Da Silva et al. (1992), and Harper and El Sahrigi (1965).

where m0 is the viscosity at reference pressure and a is a parameter for the sample food. For non-Newtonian foods, either t or g must be specified as a parameter. The composition of apple juices from different sources has been reported by Mattick and Moyer (1983). Fructose, glucose, and sucrose are the most important constituents of clarified apple juice, accounting for more than 90% of soluble solids, so thermophysical properties should be largely determined by the type and concentration of sugars. The resulting coefficients of the above equations for different sugars at 20–258C, as they compare with those obtained for apple juice, are presented in Table 4.17. Thermophysical properties’ data for sugar solutions were obtained from Riedel (1949), Honig (1953), Taylor and Rowlinson (1955), Pancoast and Junk (1980), and Weast (1985). The properties of fructose, glucose, and sucrose solutions appear to be very similar. Thermophysical properties of clarified apple juice A difference at the 1% level between density data of apple juice and sugar solutions was observed. At the same concentration expressed as mass fraction, the juice has a density a little higher than the sugar solutions, which means an average specific volume of solids smaller than those corresponding to sugars. Thus, it seems that Eq. (4.11) slightly underpredicts the density of apple juice. If the effect of the minor components is taken into account, a decrease in density should be expected since a typical value of vs for proteins, the most important minor component in apple juice, is about 0:73 cm3 =g (Kunz and Kauzmann, 1974). The observed behavior was explained by considering the influence of organic acids in the refractometric reading of soluble solids. Millies and Burkin (1984) reported that a reduction up to 3.5% in the refractometric value was found in concentrated apple juice with a malic acid content similar to that of the juice samples used in this work. Therefore, the reported value of Table 4.17. Values of coefficients for evaluating thermophysical properties with some proposed models at 208C (Constenla et al., 1989) with permission. Equation number 4.11 4.27 Parallel model 4.18

Coefficient

Sucrose

Glucose

Fructose

Apple juice

ns nw A B ks kw Cps Cpw

0.6261 0.9956 2.6122 1.0381 0.2090 0.9790 0.4400 0.9971

0.6323 0.9962 2.5617 0.9725 0.2142 0.9810 0.4425 0.9989

0.6277 0.9950 2:4 53 0.9807 – – – –

0.6145 0.9921 2.5289 1.0052 0.2070 0.9789 0.5242 0.9773

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density for a 68.58Brix could be comparable to a sugar solution of 70.8% soluble solids. No significant differences between densities of grape juice, orange juice, and sucrose solution at 208C were found by Moresi and Spinosi (1980, 1984), the8Brix reading being corrected by acidity. Thus, it appears that density of fruit juices can be predicted by Eq. (4.11) using their main sugar composition if the correction by acidity in the refractometric reading is taken into account. While Constenla et al. (1989) found the specific heat of apple juice decreased less rapidly with soluble solids than those of pure sugar solutions, Moresi and Spinosi (1980, 1984) reported the opposite behavior for orange and grape juices at 208C and concluded that the minor components were responsible for the reduction in Cp values. However, using suggested values for Cpi (Kunz and Kauzmann, 1974; Choi and Okos, 1983) in Eq. (4.18), it can be estimated that the influence of these components on Cp of fruit juices is practically negligible. Obviously more work is needed in this area to explain the above discrepancies. 4.4.6. Boiling Point Rise Concentration of fruit products by evaporation is conducted by boiling off the water, which occurs at the boiling point of the solution. It is well known that the presence of solute results in depression of partial pressure of the solvent below its vapor pressure. Depression of vapor pressure and freezing point, and elevation of boiling point and osmotic pressure belong to the group of colligative properties, depending on the number on molecules in solutions and not on the concentration of these species by weight (Fig. 4.6). 8 700 mbar 7

473 mbar 311 mbar

6

Boiling Point Rise, ⴗC

199 mbar 123 mbar

5

73 mbar 4

Sacrose, 700 mbar Reducing sugar, 700 mbar

3

2

1

0 0

20

40

60

80

Concentration, ⴗBrix Figure 4.6. Effect of concentration on the rise of boiling point of clarified apple juice at different pressures (Crapiste and Lozano, 1988 with permission).

4

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Thermodynamical, Thermophysical, and Rheological Properties

95

Two different methods can be used to describe the boiling point elevation of sugar solutions, applicable to fruit juices as a function of pressure (or boiling point of water) and concentration of soluble solids. In the first approach, empirical correlations are presented to fit the experimental data on vapor–liquid equilibrium of solutions. The use of expressions suitable for describing the vapor pressure of pure water can be extended to aqueous solutions. A correlation derived from the Clausius–Clapeyron equation can be expressed in the form ln P ¼ A(W ) B(W )=T

(4:32)

or the Antoine equation, which can be written as: ln P ¼ A(W ) B(W )=(T þ C(W ))

(4:33)

where P is the pressure (mbar), T is the boiling temperature (K), and W is the mass concentration of soluble solids (% by weight or8Brix). More complex and accurate correlations have been proposed. However, coefficients like A, B, and C in Eqs. (4.31) and (4.32) result in complex functions of concentration, and only in particular cases can the boiling point be explicitly obtained from those expressions (Moresi and Spinosi, 1984). In addition, best representation can be obtained if the boiling point rises instead the temperature of ebullition is used in fitting the data. For the above reasons an empirical equation of the form (Crapiste and Lozano, 1998): DTr ¼ aW b exp (gW )Pd

(4:34)

was proposed, where DTr is the boiling point rise (8C) and the parameters a, b, g, and d are evaluated from experimental information. Parameters of Eq. (4.34) are listed in Table 4.18. Since the sugars are the most important component of fruit juices DTr was determined largely by the type and concentration of sugars. Thermophysical properties of foods are well documented, and the measurement of most of them is a matter of routine. However, despite the fairly large amount of data collected on some particular foods, they are sometime contradictory due to the different conditions at which they are gathered, as well as to the differences among the same foods of different origin, composition, and structure. Considerable progress is being made toward explaining the influence of individual components on effective properties. Moreover, as the amount of thermal property data required to describe any foodstuff under the varied handling, processing, and storage conditions is practically infinite, modeling and prediction of such properties is a must.

Table 4.18. Value of parameters for evaluating rise of boiling point of apple juice and related sugar solutions with Eq. (4.34) (Crapiste and Lozano, 1988 with permission)

Sucrose Reducing sugars Apple juice

a 102

b

g 102

d

r2

s

3.0612 2.2271 1.3602

0.0942 0.5878 0.7489

5.329 3.593 3.390

0.1356 0.1186 0.1054

0.999 0.997 0.998

0.083 0.078 0.062

r2 is the multiple correlation coefficient (squared); s is the standard error.

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Karel, M., Fennema, O.R. and Lund, D.B. (1975). Heat transfer in foods. In Principles of Food Science. Part II: Physical Principles of Food Preservation, Fennema, O. (ed.). Marcel Dekker, Inc., NY, pp. 11–30. Kent, M., Christiansen, K., van Haneghem, I.A., Holtz, E., Morley, M.J., Nesvadba, P. and Poulsen, K.P. (1984). Cost 90 collaborative measurements of thermal properties of foods. J. Food Eng. 3(2): 117–150. Keppler, R.A. and Boose, J.R. (1970). Thermal properties of frozen sucrose solutions. Trans. ASAE 13(3): 335–339. Kirk-Othmer Encyclopedia of Chemical Technology (1964). 2nd. ed. John Willey and Sons, Inc., NY, London, Sydney. Kokini, J.L. (1992). Rheological properties of foods. In Handbook of Food Engineering, Heldman, D.R. and Lund, D.B. (eds.). Marcel Dekker, Inc., New York, pp. 1–39. Kubota, K., Matsumoto, T., Kurisu, S., Suzuki, K. and Hosaka, H. (1980). The equation regarding temperature and concentration of the density and viscosity of sugar, salt and skim milk solutions. J. Fac. Appl. Biol. Sci. 19: 133–145. Kunz, I.D. and Kauzmann, W. (1974). Hydration of proteins and polypeptides. Adv. Protein Chem. 28: 239–242. Lewis, M.J. (1987). Physical Properties of Foods and Food Processing Systems. Ellis Horwood Ltd/ VCH Verlagsgessellschaft, GmbH, England/FRG. Lozano J.E. (2005). Thermal properties of Foods. In Food Engineering ed. by Gustavo V. Barbosa-Ca´novas, and Pablo Juliano, in Encyclopedia of Life Support Systems (EOLSS). Developed under the Auspices of the UNESCO, EOLLS Publishers, Oxford, UK. Lozano, J.E., Urbicain, M.J. and Rotstein, E. (1979). Thermal conductivity of apples as a function of moisture content. J. Food Sci. 14(1): 198–199. Lozano, J.E., Urbicain, M.J. and Rotstein, E. (1980). Total porosity and open pore porosity in the drying of fruits. J. Food Sci. 45: 1403–1407. Lozano, J.E., Urbicain, M.J. and Rotstein, E. (1983). Shrinkage, porosity and bulk density of foodstuffs at changing moisture content. J. Food Sci. 48: 1497–1502, 1553. Mannapperuma, J.D. and Singh, R.P. (1989). A computer aided method for the prediction of properties and freezing/ thawing times of foods. J. Food Eng. 9(4): 275. Maroulis, S.N. and Saravacos, G.D. (1990). Density and porosity in drying starch materials. J. Food Sci. 55(5): 1367–1375. Maroulis, Z.B., Shah, K., Saravacos, G.D. (1991). Thermal conductivity of gelatinized starches. J. Food Sci. 56(3). 773–776. Mattea, M., Urbicain, M.J. and Rotstein, E.R. (1986). Prediction of thermal conductivity of vegetable foods by the effective medium theory. J. Food. Sci. 51(1): 113–115, 134. Mattea, M., Urbicain, M.J. and Rotstein, E.R. (1989). Effective thermal conductivity of cellular tissues during drying: prediction by a computer assisted model. J. Food Sci. 54(1): 194–197, 204. Mattick, L.R. and Moyer, J.C. 1983. Composition of apple juice. J. Assoc. 0:T Anal. Chem. 66: 1251. Maxwell, J.C. (1904). A Treatise on Electricity and Magnetism, Vol. 1, 3rd ed. The Clarendon Press, Oxford, p. 440. Maxwell, J.L., Kurt, F.A., and Strelka, B.J. (1984). Specific Volume (density) of saccharine solutions (corn syrups and blends) and partial specific volumes of saccharide water mixtures. J. Agric. Food Chem. 32: 974–982. Millies, K. and Burkin, M. (1984). Amending der refractometric our produckcontrolle in FuchtsaftbetriebenMeBertkorrecturen. Flussigest Obst. 12: 629–636. Mohsenin, N. (1980) Thermal Properties of Food and Agricultural Materials. Gordon and Breach Science Publishers, NY. Mooney, M. (1951). The viscosity of concentrated suspensions of spherical particles. J. Colloid Sci. 6: 162–167. Moresi, M. and Spinosi M. (1980). Engineering Factors in the Production of Concentrated Fruit Juices. l) Fluid Physical Properties of Orange Juices. J. Fd. Technol., 15: 265–276. Moresi, M. and Spinosi, M. (1984). Engineering factors in the production of concentrated fruit juices. 1. Fluid physical properties of grape juices. J. Food Technol. 19: 519–527. Nix, G.H., Lowery, G.W., Vachon, R.I., Tanger, G.E. (1967). Direct determination of thermal diffusivity and conductivity with a refined line-source technique. Prog. Aeronaut. Astronaut: Thermophys. Spacecraft Planet. bodies 20: 865–878,New York, Academic Press. Oguntunde, A.O. and Akintoye, O.A. (1991). Measurement and comparison of density, specific heat and viscosity of cow’s milk and soymilk. J. Food Eng. 13(3): 221–227. Pancoast, H.M. and Junk, W.R. (1980). Handbook of Sugars, 2nd ed. AVI Publishing Company, Wesport, CT, USA. Perez, M.G. and Calvelo, A. (1984). Modeling the thermal conductivity of cooked meat. J. Food Sci. 49: 152–158. Perry, R.H. and Green, C.H. (1973). Chemical Engineers’ Handbook, 5th ed. McGraw-Hill Book Company, New York.

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Polley, S.L., Snyder, O.P. and Kotnour, P. (1980). A compilation of thermal properties of foods. Food Technol. 34(11): 76–94. Qiu, C.G. and Rao, M.A. (1888). Role of pulp content and particle size in yield stress of apple sauce. J. Food Sci. 53: 1165–1169. Rahman, M.S. (1991). Evaluation of the precision of the modified Fitch method for thermal conductivity measurements of foods. J. Food Eng. 14: 71. Rahman, M.S. (1992). Thermal conductivity of four food materials as a single function of porosity and water content. J. Food Eng. 25: 261–266. Rahman, M.S. (1993). Specific heat of selected fresh seafoods. J. Food Sci. 58(3): 522–524, 566. Rahman, M.S. (1995). Food Properties Handbook. CRC Press, Inc., Florida, USA. Rahman, M.S. and Driscoll, R.H. (1991). Thermal conductivity of seafoods: calamari, octopus and king prawn. Food Aust. 43(8): 356–359. Ramaswamy, H.S. and Tung, M.A. (1981). Thermophysical properties of apples in relation to freezing. J. Food Sci. 46: 724–728. Rao, M.A. (1977). Rheology of liquid foods. A review. J. Textural Stud. 8: 135–168. Rao, M.A., Cooley, H.J. and Vitali, A.A. (1984). Flow properties of concentrated juices at low temperatures. Food Technol. 38: 113–118. Renaud, T, Briery, P., Andrieu, J. and Laurent, M. (1992). Thermal properties of model foods in the frozen state. J. Food Eng. 15(2): 83–89. Riedel, L. 1949. Warmeleitfahigkeitsmessungen an Zuckerlosungen Fruchtsaften, und Milch. Chem. Ing. Tech. 21: 340. Saravacos, G.D. 1970. Effect of temperature on viscosity of fruit juices and puree. J. Food Sci. 35: 122. Schwartzberg, H. (1976). Effective heat capacities for the freezing and thawing of food. J. Food Sci. 41(1): 152–156. Sherman, F. and Sherman, P. (1966). The texture of ice-cream. 2. Rheological properties of frozen ice cream. J. Food Sci. 31: 699–706. Singh, P. (1982). Thermal diffusivity in food processing. Food Technol. 2: 36–87. Slattery, J.C. (1961). Analysis of the cone-plate viscometer. J. Colloid Sci. 16: 431– 437. Sweat, V.E. (1974). Experimental values of thermal conductivity of selected fruits and vegetables, J. Food Sci. 39: 1080–1083. Sweat, V.E. (1986). Thermal properties of foods. In: Engineering Properties of Foods (Rao MA; Rizvi SSH, (Eds), p 49. Marcel Dekker, New York. Sweat, V.E. (1995). Thermal properties of foods. In Engineering Properties of Foods, 2nd ed. Rao, R.A. and Rizvi, S.S. (eds.). Marcel Dekker, Inc., NY, pp. 99–157. Tang, J., Sojhansanj, S., Yannacopoulus, S. and Kasap, S.O. (1991). Specific heat capacity of lentil seeds by differential scanning calorimetry. Trans. ASAE 34(2): 517. Taylor, J.B. and Rowlinson, J.S. (1955). The thermodynamic properties of aqueous solutions of glucose. Trans. Faraday Soc. 51: 1186–1190. Uno, J. and Hayakawa, K. (1980). A method for estimating thermal diffusivity of heat conduction food in cylindrical can. J. Food Sci. 45: 692–697. Urbicain, M.J. and Lozano, J.E. (1997). Definition, measurement and prediction of thermophysical and rheological properties. In The CRC Handbook of Food Engineering Practice. CRC Press, Inc., USA. ISBN:0–8493–8694–2/. Van Waser, J.R., Lyons, J.W., Kim, K.Y. and Colwell, R.E. (1963). Viscosity and Flow Measurement. Interscience, NY. Wallapapan, K., Sweat, V.E., Diehl, K.C. and Engler, C.R. (1983). Thermal properties of porous foods. ASAE Paper No. 83–6515. Wang D.Q., Kolbe E. (1991). "Thermal Properties of Surimi Analyzed Using DSC", J. Food Sci, 56(2): 302–308. Weast, R.C. 1985. "Handbook of Chemistry and Physics. "66th ed. CRC Press Inc., Boca Raton, FL. Woodside, W. and Messmer, J.H. (1961). Thermal conductivity of porous media. I. Unconsolidated sands. J. Appl. Phys. 32(9): 1688–1699. Zuritz, C.A., Sastry, S.K., McCoy, S., Konanayakan, M. and Crawford, J. (1987). A revised theory for improvement of the Fitch method of thermal conductivity measurement. ASAE Paper 97–6540.

CHAPTER 5

COLOR, TURBIDITY, AND OTHER SENSORIAL AND STRUCTURAL PROPERTIES OF FRUITS AND FRUIT PRODUCTS 5.1. INTRODUCTION Measurements of color and turbidity are analytical problems confronting the food technologists working with fruit and fruit products. The appearance of fruit products plays an important role in determining whether or not a consumer will purchase them (Francis and Clydesdale, 1975). In the case of fruit juices, opacity, color, and homogeneity contribute to the overall appearance. Moreover, the kinetics of deterioration can be followed through color measurement, which is a simple and effective way for studying the phenomenon. The complexity of the reactions and the various compounds involved (Hodge, 1953; Spark, 1969) make it difficult to study the reactions by means of a simple analytical chemical method. Two major approaches have been used to evaluate color changes in fruits and fruit products (Francis and Clydesdale, 1975; Pomeranz and Meloan, 1994): .

.

Measurements based on absorbance spectrophotometry: The absorption of light depends on the type and concentration of the chromophores present. A variety of different types of spectrophotometers have been developed to measure transmission of light from liquids as a function of wavelength both in the UV and visible regions. Measurements based on tristimulus colorimetry: The basis of these methods is that colors can be simulated by combining red (R), green (G), and blue (B), in the appropriate ratios and intensities.

Other methods used the absorbency measurement of soluble pigments by spectrophotometry at 360 –500 nm, often near 400 nm. The susceptibility of apples to browning was adequately determined by simultaneous measurement of soluble (Abs400 ) and insoluble (L ) brown pigments (Amiot et al., 1992). It was indicated that for some berry products color deterioration cannot be characterized by changes in total anthocyanin (TA) alone. Most of the anthocyanin was polymerized, rather than lost during storage (Ochoa et al., 1999). Percentage of polymeric color (%PC) is a measure of the pigment resistance to bleaching, and indicates, in some degree, the anthocyanin polymerization. Increases in %PC values followed a near-zero reaction kinetics throughout the storage period. As pointed out by Labuza and Riboh (1982) most of the quality-related reaction rates are either 0 or firstorder reactions, and statistical difference between both types may be small. 99

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5.2. MEASUREMENT OF COLOR Color and opacity are the result of interaction between light and food. Incident light may be transmitted, reflected, or scattered before being detected by the human eye. The interaction of light with matter is fundamental for qualitative and quantitative analyses of fruit juices in particular. Color can be quantified in several ways. Measurement of the color of foods can be by visual systems (by comparison with colored references), spectrophotometry, tristimulus colorimetry, or specialized instrumentation for particular foodstuffs. (Hutchings, 1994; McClements, 1999) As Fig. 5.1 shows, incident electromagnetic waves may be partly reflected and partly transmitted (or refracted). Refractive index, angle of incidence, and surface topography determine the relative importance of these phenomena. In Fig. 5.1, Io is the radiant power arriving at the cuvette, I is the radiant power leaving the cuvette, and b is the path length. Reflection of light may be specular (angle of incidence, fin , is equal to angle of reflection, fref ), or diffuse (light reflected over many different angles). While the former is the predominant form of reflection of smooth surfaces, the latter is most important in the case of rough surfaces. Particles in suspension may be responsible for light scattering, depending on the particle size and the wavelength of the incident radiation. 5.2.1. Absorbance Spectrophotometry Spectrophotometry is based on the fact that substances of interest selectively absorb or emit electromagnetic energy at different wavelengths, in the range of the ultraviolet (200–400 nm), the visible (400–700 nm), or the near infrared (700–800 nm). The basic principle of spectrophotometry is that the energy-absorption properties of a substance can be used to measure the concentration of the substance. In most cases, the sample is the product of reactions between the original substance and reagents, and absorbs light selectively according to Beer’s law, which states that equal thickness of an absorbing material will absorb a constant fraction of the energy incident upon it. Similarly, Lambert’s law stated that light absorbance is proportional to path length. These relationships are described in Table 5.1. A cuvette is designed to keep b as nearly constant as possible. Therefore changes in I should reflect changes in the concentration (C ) of the absorbing substance in the sample. Since I and b are kept constant, the absorbance varies with C. The concentration of an b fin

Scattered

I

Io Transmitted

Incident fref Reflected (specular)

Figure 5.1. Simplified scheme for light reflection and transmission through a cuvette filled with a turbid liquid.

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Table 5.1. Light absorbance relationships. Law name

Formula

Nomenclature

Beer

A/C

C is the concentration of the absorbing substance, mole/L or g/L Io is the radiant power arriving at the cuvette, I the radiant power leaving the cuvette A is absorbance

%T ¼ I=Io 100

Lambert Beer–Lambert

A ¼ log (Io =I) ¼ log (100=%T) ¼ 2 log (%T) A/b A ¼ «cb or A ¼ ab

b is path length, cm « is molar extinction coefficient ¼ a molecular mass a ¼ absorptivity

unknown can be determined by determining the absorbance (Abs) of a standard with known concentration (Cs ) of the substance of interest. The concentration of the unknown substance (Cu ) can be calculated from the following relationship: Cu ¼ Cs (Absu =Abss )

(5:1)

If the relationship holds over the possible range of concentrations of the substance of interest, then the determination is said to obey Beer’s law. If the relationship does not hold due to absorption by the solvent or reflections of the cuvette, then a relatively large number of standards with known concentration values must be used to compute a calibration curve of concentration versus absorbance. The absorbance, also called extinction or optical density, is linearly correlated to concentration. The law is valid only monochromatic light and for diluted solution. 5.2.1.1. Spectrophotometer Components Different types of spectrophotometers have been developed to measure the transmission and reflection of light from objects as a function of wavelength (Clydesdale, 1969; Francis and Clydesdale, 1975; Hutchings, 1994). A typical visible light spectrophotometer is shown in Fig. 5.2. A good light source should emit a continuous, featureless spectrum, with no exaggerated change of intensity as the wavelength is varied.

Reference photo tube

Light source Collimator Lens Lens Filter

Sample

Diffraction grid

Measurement photo tube Figure 5.2. Schematic representation of a visible light spectrophotometer.

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For ultraviolet (UV) spectrophotometry deuterium lamp is used. In this lamp electrical discharge causes D2 gas to emit continuous spectrum in the UV region (200 –350 nm). For visible light spectrophotometry tungsten filament lamp (350 –800 nm range) or tungsten– halogen lamp (tungsten filament embedded in a quartz–iodide matrix) is used. Two classes of devices, filters (glass and interference) and monochromators, are used to select those portions of the power spectrum produced by the power source that are to be used to analyze the sample. Some instruments related to spectrophotometer use filters to select wavelength. Devices that use filters as their wavelength selectors are called colorimeters or photometers. Colorimeter uses colored glass filters. It is low cost and has a wide band of wavelengths (often >70 –100 nm deviation from Beer’s law). Glass filters function by absorbing certain wavelengths (e.g., red region) and transmitting others (e.g., blue-green region, the customary bandwidth of glass filters, is 50 nm). Peak transmittance decreases as bandwidth decreases. At a bandwidth of 30 nm, the peak transmittance is about 10%, which is too low for most applications. Some photometric instruments use interference filters. Interference filters used selectively spaced reflecting surfaces to reinforce the wavelength of interest and cancel others. Harmonic frequencies can be eliminated by glass-cutoff filters. Glass filters are used in applications in which only modest accuracy is required, while interference filters are used in many spectrophotometers. Monochromators are tunable wavelength selectors. Monochromators use prisms and diffraction gratings, which disperse incident light into its component spectrum to provide very narrow bandwidths by dispersing the input beam spatially as a function of wavelength. A mechanical device then allows wavelengths in the band of interest to pass through a slit. In diffraction grating wavelength is selected by constructive interference (reinforcement) at chosen wavelength, while the other wavelengths are cancelled by destructive interference. Sample cells: Rectangular cuvettes of 10 mm path length are most common (1– 40 mm sizes are available). Cylindrical tubes (cf. test tubes) are common for some instruments. However, positioning a cylindrical tube in light beam is very critical. Table 5.2 lists materials and principal characteristics of cuvettes. While plastic and glass cuvettes are used only in the visible range (> 340 nm), silica cuvettes have an extended wavelength range (200 –1000 nm). Detectors: Spectrophotometers use photoelectric devices that convert radiant energy into an electrical signal. These detectors use: (a) silicon photodiode (solid-state detector for general purpose photometer), (b) vacuum phototube, or (c) photomultiplier tube—this is a very

Table 5.2. Different spectrophotometer cuvettes. Plastic

Glass

Quartz (silica)

Cheap (disposable), not breakable, easily scratched, aqueous, any pH

Medium price

Expensive

Breakable on impact More resistant to scratching Aqueous or organic solvents (concentrated alkalis must be avoided)

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sensitive detector and is used if the photometric instrument needs to measure very low-light intensities. Finally a Readout Device (usually a digital voltmeter or analog-to-digital data acquisition system) is used to quantify amplified signal from detector. Modern instruments usually incorporate time averaging or damping to counteract instrument noise. 5.2.1.2. Improved Spectrophotometers Most spectrophotometers use two lamps: a deuterium lamp with high UV output, and a tungsten lamp for high visible output (double-beam spectrophotometer). Photodiode array technology positions multiple detectors side by side on a silicon crystal, with a capacitor to convert light to electric discharge. Polychromic light from the grating can now be detected in the same time it takes to measure a single wavelength with a conventional spectrophotometer. 5.2.1.3. Turbidity and Scattering An object that allows all the light to pass through it, is referred to as being transparent, whereas an object that scatters or absorbs all the light is referred to as being opaque (Clydesdale, 1975). Many dilute suspensions fall somewhere between these two extremes and are therefore referred to as being translucent. The opacity of most food suspensions is determined mainly by the scattering of light from the particles: the greater the scattering, the greater the opacity (Hernandez and Baker, 1991; Dickinson, 1994). When a light wave impinges on a food suspension, like a cloudy fruit juice, all the different wavelengths are scattered by the particles, and so the light cannot penetrate very far into the juice. As a consequence, the juice appears to be optically opaque (Farinato and Rowell, 1983). The extent of light scattering by a suspension is determined mainly by the relationship between the droplet size and wavelength. Scattering due to turbidity in a cuvette gives apparent absorbency, which is proportional to the density of suspended particles. For fruit juices spectrophotometry samples must be centrifuged or filtered to remove suspended particles. Light-scattering techniques may be used to determine the size distribution of the particles in a food suspension. Knowledge of the particle size distribution enables one to predict the influence of particles on light scattering and therefore on the turbidity of a suspension. Two analytical methods must be differentiated: (i) turbidimetry, for the measuring of apparent absorbency and (ii) nephelometry, for the measuring of scattered light. A nephelometer measures the intensity of light that is scattered at an angle of 908 to the incident beam. The intensity of light scattered by a sample is compared with that scattered by a standard material of known scattering characteristics (Hernandez et al., 1991). Because small particles scatter light more strongly at wide angles than large particles, the nephelometer is more sensitive to the presence of small particles than turbidity measurements. 5.2.1.4. Reflection Spectrophotometer The reflectance (R) of a material was defined as the ratio of the intensity of the light reflected from the sample (Rs ) to the intensity of the light reflected from a reference material of known reflectance (RR ): R ¼ RS =RR (Francis and Clydesdale, 1975). For specular reflection, the

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intensity of the reflected light is usually measured at an angle of 908 to the incident wave; whereas for diffuse reflection, the sum of the intensity of the reflected light over all angles is measured using an integrating sphere. A reflectance spectrum is obtained by carrying out this procedure across the whole range of wavelengths in the visible region. The transmittance and reflectance spectra obtained from a sample can be used to calculate the relative magnitudes of the absorption and scattering of light by an emulsion as a function of wavelength. Alternatively, the color of a product can be specified in terms of trichromatic coordinates by analyzing the spectra using appropriate mathematical techniques (McClements et al., 1998). The details of these techniques have been described elsewhere and are beyond the scope of this book (Francis and Clydesdale, 1975).

5.2.1.5. Tristimulus and Special Colorimeters Since visual color judgments can be affected by several factors (lighting conditions, angle of observation, individual differences in perception) instruments for color measurement provide a subjective alternative. Due to the difficulty in objectively describing the colors of materials using everyday language, a number of standardized methods have been developed to measure and specify color in a consistent way (Francis and Clydesdale, 1975; Hutchings, 1994). These methods are based on the principle that all colors can be simulated by combining three selected colored lights (red, green, and blue) in appropriate ratios and intensities. This trichromatic principle means that it is (almost) possible to describe any color in terms of just three mathematical variables (i.e., hue, value, and chroma) (Francis and Clydesdale, 1975). The color of a food suspension is determined by the absorption and scattering of light waves from both the particles and continuous phase (Dickinson, 1994). The absorption of light depends on the type and concentration of chromophores present, while the scattering of light depends on the size, concentration, and relative refractive index of any particulate matter. Whether a suspension appears ‘‘red,’’ ‘‘orange,’’ ‘‘yellow,’’ ‘‘blue,’’ etc. depends principally on its absorption spectra. Under normal viewing conditions, a suspension is exposed to white light from all directions. When this light is reflected, transmitted, or scattered by the suspension, some of the wavelengths are absorbed by the chromophores present. The color of the light that reaches the eye, is a result of the nonabsorbed wavelengths (e.g., a juice appears red if it absorbs all the other colors) (Francis and Clydesdale, 1975). The color of a suspension is modified by the presence of the particles or any other particulate matter. As the concentration or scattering cross-section of the particles increases, a suspension becomes lighter in appearance because the scattered light does not travel very far through the emulsion and is therefore absorbed less by the chromophores. It is therefore possible to modify the color of a suspension by altering the characteristics of the emulsion particles or other particulate matter. Tristimulus colorimeters employ three glass filters (red, green, and blue) corresponding to the response of the cones in the human eye, a light source, and a detector system. Recombination of these three primary colors (RGB) can match almost any unknown color (Fig. 5.3). As not all colors can be obtained by the addition of three primaries, the problem was overcome by adding one of the primaries to the unknown color and matching the combined color by the addition of the other two primaries. In this way an imaginary mathematically negative color was generated and RGB source was changed to XYZ coordinates. Moreover, a trained panel can match the spectrum color using the RGB sources, and

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B G R

i Figure 5.3. Color match with addition of primary lights, red (R), green (G), and blue (B).

data can be recalculated in terms of XYZ and a curve of the standard observer can be obtained (Fig. 5.4). The response of the human eye was standardized, giving origin to the CIE system. The Commission Internationale de L’Eclerage (CIE) also specifies the following for color measurement (King, 1980): (i) The use of standard light sources, (ii) the conditions for the observation and measurement of color, and (iii) the use of ‘‘standard observer’’ curves. The amounts of the three theoretical primaries or tristimulus values required to match a given color can then be determined. The most common approach to determining the tristimulus values of a material is the weighted ordinate method (Nassau, 1996). CIE XYZ coordinates are given by the following equations: 750 ð

X¼

RE x dl

(5:2)

380

Relative response

z

y

x

x

400

550 Wavelength (nm)

Figure 5.4. Standard observer curves.

700

106

Fruit Manufacturing 750 ð

Y¼

REy dl

(5:3)

REz dl

(5:4)

380 750 ð

Z¼ 380

where R is the reflectance (or transmittance) of the sample, E is the energy distribution of the standard light source, and l is the wavelength. Tristimulus colorimeters replace the integration with the mentioned filters. Spectrophotometers introduced microelectronics to perform the tristimulus values’ calculation also. It is recommended that reflecting materials be illuminated at an angle of 458 and viewed at an angle of 908. When defining colors there are a number of attributes that need to be accounted for: (1) Hue, which is the color of the material black; white and grays are colors devoid of hue. (2) Lightness or luminosity is simply the brightness of a color, the paler colors have greater lightness than the dark colors. (3) Saturation is used to indicate the strength of the chromatic response: pale or pastel colors have low saturation, while deep and vivid colors have high saturation. These three independent variables allow colors to be arranged logically in a threedimensional space. Although the X, Y, Z tristimulus values can be used to define a color, new parameters are required because interpreting the appearance of that color from the former is very difficult. The parameters usually chosen are the chromaticity coordinates: (Nassau, 1996): x¼

X Y Z ; y¼ ; z¼ X þY þZ X þY þX X þY þX

(5:5)

As x þ y þ z ¼ 1 only two of the coordinates need to be specified, generally x and y. Thus the three parameters required to define a color are x, y and Y; x, and y to define the hue and saturation and Y the brightness. If the x, y chromaticity are plotted, the CIE horseshoeshaped spectrum locus is obtained (Fig. 5.5), including all real colors. The lightness of the color will be represented by an axis perpendicular to the x, y plane. Colors can be located in a three-dimensional space (color solid). 5.2.1.6. CIELAB Method In 1976 a useful method for quantifying the appearance of a surface color was introduced. Three new parameters L , a , and b were defined as: L ¼ 116(Y =Yn )1=3 when (Y=Yn > 0:008856) L ¼ 903:3(Y =Yn )when (Y=Yn < 0:008856) a ¼ 500[ f (X =Xn ) f (Y =Yn )] b ¼ 200[ f (Y =Yn ) f (Z=Zn )] where f(Y =Xn ), f(Y =Yn ) and f(Z=Zn ) are defined elsewhere (King, 1980).

(5:6)

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0.9 515 520

0.8

530

505 0.7

545

500

555

0.6 495

565

0.5

575

Y

C Illuminant 490

590

0.4

605

485

0.3

780

480

0.2

470

0.1

380

0 0

0.2

0.4

0.6

0.8

X Figure 5.5. CIE chromaticity diagram.

X, Y, and Z are the tristimulus values of the material, while Xn , Yn , and Zn are the tristimulus values of a white object, and these correspond to the normalized tristimulus values of the illuminant. L , a , and b are the axes of a three-dimensional color space. It has the advantage of having the same configuration as those derived logically by arranging colors visually. Hunter parameters (Fig. 5.6) are based in similar consideration (Hunter, 1975). The system measures the degree of lightness (L), the degree of redness or greenness (a), and the degree of yellowness or blueness ( b). Additionally Psychometric chroma (C ) and hue (H) were defined as:

L = 100

White +b Yellow

–a

Red +a

Green Blue −b

L=0

Black

Figure 5.6. Hunter color space.

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Fruit Manufacturing C ¼ [(a )2 þ (b )2 ]1=2

(5:7)

H ¼ tan1 (t =a )

(5:8)

The total difference AE between two colors each given in terms of L , a , and b can be calculated from: AE [(D L )2 þ (Da )2 þ (Db )2 ]1=2

(5:9)

Other possible values are lightness difference DL , chroma difference DC , and hue difference DH : DL ¼ L sample L standard

(5:10)

DC ¼ C sample C standard

(5:11)

DH ¼ [(DE)2 (DL )2 (DC )2 ]1=2

(5:12)

5.2.1.8. Measurement of Tristimulus Values Reflectance and transmittance measurements over the visible spectrum can be readily made using a spectrophotometer. The surface of the sample will not be subjected to elevated temperatures, which could cause a change in coloration. When the sample is fluorescent, as most spectrophotometers irradiate the sample with monochromatic radiation, the higher energy-excitation wavelengths will not be incident on the sample when wavelengths in the emission region are being measured. The problem will not occur in instruments that irradiate the sample with polychromatic light. Commercial systems for tristimulus measurements were compared by Francis and Clydesdale (1975) and Little (1976) among others. In all cases, tristimulus values, Hunter values, etc. have to be computed from reflectance or transmittance data. A number of tristimulus colorimeters are available; these are instruments that, after irradiation of the sample with polychromatic light, examine the reflected light with three or four filters and a photocell. Visual colorimeters are also used, e.g., the Lovibond Tintometer, in which red, yellow, and blue filters are used to obtain a visual match with the sample. The color is then defined in Lovibond units of red, yellow, and blue; conversion of these units into X, Y, and Z values is possible by graphical methods. 5.2.1.9. Application of Colorimetry Colorimetry provides an objective method for specifying the color of food. Most fruits have been subjected to an investigation of their color properties. Francis and Clydesdale (1975) have reviewed the application of colorimetry to most foods, including tomatoes and tomato products, green vegetables, citrus products, potato products, cereal products, meat color, sugars, beer and wine, and tea and coffee. Kramer (1976) has discussed the application of colorimetry to quality control. Many studies relating pigment concentration to color have been made. The color of a material is dependent on a number of factors, e.g., the pigment concentration, the nature of the surface and particle size, and as a result of these factors light scattering may also be significant (McClements et al., 1998). Scattering may alter the observed color of the food considerably.

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5.3. FOOD DISPERSIONS 5.3.1. Definitions The classical definition of the colloidal state is in terms of size alone; the lower limit is ˚ , and the upper size limit, as 1---5 mm. generally taken to be in the neighborhood of 10–50 A (Fig. 5.7) Table 5.3 shows how any two of three states of matter (solid, liquid, and gas) can be mixed to form a colloid. Substances in the same state (except gases) can also be mixed to form colloids. Particles in a colloid may adhere together and form aggregates of increasing size ( flocculation), which may settle down due to gravity. If flocks change to a much denser form it is said to undergo coagulation, which is an irreversible process. A typical example of food dispersion is a cloudy or opalescent fruit juice. A cloudy juice is a colloidal system where the dispersing medium is water, and the dispersed (colloidal) matter is formed by the rest of cellular tissue released after fruit processing (milling and pressing). During cloudy juice processing a small percentage of insoluble particles is retained in suspension, giving it a light opaque color. Nagel (1992) described cloudy apple juice as a light, whitish-yellow juice, clearly showing cloudiness, which presents no sedimentation, is full bodied and juicy, but has no astringent or bitter taste. Figure 5.8 compares cloudy and clarified apple juice. Typical cloud fruit particles are also included in Fig. 5.8. Particularly, cloudy fruit juices have solids of various dimensions distributed in a serum (mainly sugars and organic acids) or clarified juice (Moyls, 1966; Beveridge and Tait, 1993). One of the main problems with cloudy juice production is the assurance of cloud stability (Chobot and Horulaba, 1983; Beveridge and Harrison, 1986; Gierschner, and Baumann, 1988; Genovese et al., 1997). Even after prolonged storage none or only a very small part of the cloud particles should precipitate. Colloidal stability. Derjaguin, Verway, Landau, and Overbeek (McClements, 1999) developed a theory, which explains the colloidal stability as the result of the sum of the electrical double layer repulsive and van der Waals’ attractive forces that the particles experience as they approach one another (Fig. 5.9).

µ Å

0.0001

0.001

0.01

0.1

1

1

10

100

1000

10,000

Molecular Dispersion

10

100

1000

Suspensions

Colloidal Dispersion

Bacteria Yeast

Turbidity Figure 5.7. Particle classification by size.

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Fruit Manufacturing Table 5.3. Different types of colloidal systems. Dispersed phase

Dispersing phase

Type of system

Familiar examples

Gas Gas Liquid Liquid Liquid Solid Solid Solid

Aerosol Solid aerosol Foam Emulsion Sol, Suspension Solid foam Gel Solid sol

Fog Smoke Whipped cream Mayonnaise, milk Paint, ink Cloudy juice Foam rubber, Marshmallow Gelatin, cheese Colored gemstones, Some alloys

Liquid Solid Gas Liquid Solid Gas Liquid Solid

b

Apple

Juice

r x 0.5 µm

a

Clarified

Cloudy

Figure 5.8. (a) Visual comparison between cloudy and clarified apple juice, and (b) cloudy apple juice particles. r is the particle radius, and x the particle separation. (Genovese and Lozano, 2005)

- - + -+ -

- + -+ Hydrophobic and + Hydration interactions --+ + -- + + --- Steric - Repulsion London-van der Waals - +- - + --- - +--- ++ --+ - - ++ --- + +- - - Electrostatic forces --

Figure 5.9. Possible forces acting on particles in colloidal dispersion.

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The DVLO theory proposes that an energy barrier resulting from the repulsive force prevents two particles approaching one another and adhering together. However, if two particles collide with sufficient energy to overcome that barrier, the attractive force will pull them into contact and they will adhere strongly and irreversibly together. For colloidal stability, the repulsive forces must be dominant. There are different mechanisms that affect dispersion stability: Steric repulsion: This involves macromolecules absorbed onto the particle surface and causing repulsion. Electrostatic (charge) repulsion: This is the effect on particle interaction due to the distribution of charged species in the system. Figure 5.10 shows a submicron positive fruit particle completely surrounded by negatively charged pectin (based on Yamasaki et al., 1964, proposal). Development of a net charge at the particle surface affects the distribution of ions in the surrounding interfacial region, resulting in an increased concentration of counterions (ions of opposite charge to that of the particle) close to the surface. Thus an electrical double layer exists around each particle. The liquid layer surrounding the particle exists as two parts: an inner region (Stern layer), where the ions are strongly bound; and an outer (diffuse layer) region, where they are less firmly associated. Within this diffuse layer is a hypothetical boundary within which the particle acts as a single entity. The potential at this boundary is the Z-potential (z). In Fig. 5.10 k1 is the Debye length, a measure of the ‘‘thickness’’ of the electrical double layer. Non-DLVO interaction forces have been found in aqueous suspensions of very hydrophobic or very hydrophilic particles (Molina-Bolı´var and Ortega-Vinuesa, 1999). Recently, Genovese and Lozano (2005) have claimed that repulsive hydration forces might play a significant role in the stability of CAJ. 5.3.2. Food Dispersion Characterization The proper characterization of food suspensions depends on the purposes for which the information is sought, because the total description is an enormous task (Trottier, 1997).

Diffuse layer + + Pectin + + + κ

−1

+

+ +

+

+

+

+

+

+

+

+

+ +

+ +

+ + + + +

+

+

+

+ + +

+ +

+

+

+

+

+

+ + + + +

Nerst layer

+

+ +

+ +

Shear plane Figure 5.10. Charges at a particle surface.

Particle

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Anyway, one or more of the following properties should be considered: size and size distribution, shape, number and size distribution of pores, morphology of the primary particles, surface area, state of agglomeration, and or chemical and phase composition. 5.3.3. Particle Size, Shape, and Size Distribution Particle size and particle size distribution (PSD) are fundamental properties, which have a strong influence on many other properties and can be used to predict them. While the size of spheres or cubes can be completely specified by a unique dimension (diameter or length), food engineers will rarely be so fortunate as to be dealing with regularly shaped particles. Although an irregularly shaped particle does not possess a unique linear dimension, its size is usually expressed as the diameter equivalent sphere. At least three possibilities for spheres that are equivalent to a given particle may be considered: A sphere: (1) (2) (3) (4)

With With With With

the same volume (dv ), the same surface area (ds ), the same projected area as that of the particle (da ), and the sieve diameter (dA ).

The sieve diameter, which is the width of the smallest aperture through which a particle can pass, has little application in food suspensions at the colloidal level. This diameters and a number of additional equivalent diameter are listed in Table 5.4. PSD is usually inferred, via Stokes’ law, from the sedimentation time of dispersed particles. The most direct methods for determining the particle size, shape, and distribution of food dispersions are scanning and transmission electron microscopy. Indirect methods for determining size and particle size distribution include sedimentation and centrifugation, conductimetric techniques, light scattering, X-ray diffraction, gas and solute adsorption, ultrafiltration, and diffusiometric methods (McClements, 1999). The analytical choice depends on the physical properties of the food dispersion. Modal, median, and mean diameters should also be calculated. Modal diameter is the diameter that occurs most frequently in the sample, median diameter is the diameter such that 50% of the sample diameters are smaller than it, and mean diameter is obviously the sum of all the sample values divided by the size of the sample. 5.3.3.1. Electron Microscopy Microscopy is the only method in which particles are observed and classified individually. The lower limit of particle size that can be resolved by the microscope using visible light is about 0:2 mm. Colloids and many suspensions require transmission or scanning electron microscopy, which would permit the measurement of smaller particles. Electron microscopy is applied when particle identification and shape, as in the case of most food particles, are important in addition to size. Sample preparation is in general more complex, which involves drying and gold covering in a metal evaporator. Recent developement of enviromental scanning electron microscopes (ESEM) has simplified sample preparation. The increasing power of data processing software, coupled with the falling cost of TV cameras and scanners, had led to a generalization of image processing systems. Three basic types of diameters are used in particle diameter analysis, Martin, Feret, and equivalent surface area, and are described in Table 5.4. (Trottier, 1997)

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Table 5.4. Diameters frequently used in particle size analysis. Diameter

Name

Definition

dv

Equivalent volume

Diameter of a sphere having the same volume as the particle

ds

Equivalent surface

da

Projected area

Diameter of a sphere having the same surface as the particle Diameter of a circle having the same surface as the particle

dst

Stokes

Diameter of a sphere with the same Stokes’ free falling velocity, as the particle in the laminar regime

dF

Feret

Distance between two parallel tangents on opposite sides of particle profile

dM

Martin or horizontal

Distance between opposite sides of the projected particle

dc

Perimeter

Diameter of a circle with the same perimeter as the projected particle

Representation

Source: Trottier, 1997; Genovese and Lozano, 2000.

The processing of hundreds, even thousands, of particles may be required to establish statistical significance. Digital images of food particles can be statistically analyzed with powerful software, such as the AnalySIS 2.1 (Soft Imaging Software GmbH) version. 5.3.3.2. Sedimentation Measurement of the settling rate for particles under gravitational or centrifugal acceleration in a quiescent liquid is the basis of techniques for determining particle size and size distribution. The particle size determination by sedimentation is based on an equivalent spherical

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diameter. The upper size limit for sedimentation methods is established by the value of the particle Reynolds’ number: Re ¼ dvr=m

(5:13)

where d is the particle diameter, r is the particle density, m is the viscosity, and v the terminal velocity of the particle, which can be determined from Stokes’ law, given by equation: 1=2 18h h (5:14) d¼ (r rl )g t where rl is the dispersant density. Instruments of different configurations are used to determine particle diameter (McClements, 1999). Centrifugal particle size analyzers utilize the sedimentation method and detects particle concentration photometrically (Fig. 5.11). A builtin microcomputer converts the absorbance changes into particle size distribution, based on Stokes’ law. It must be noted that variation in distance from the center of rotation and the location of particle may not be correctly introduced by the manufacturers of the analyzer, and deviations up to 12.5% can be expected. When using the same cuvette and sample volume the mentioned error is systematic and constant, and it can be easily overcome by calibrating the equipment with standards of known particle size. Centrifugal sedimentation under constant speed rotation is described by the following equation: ! rp rl du 18:m ¼ u (5:15) :R:v2 dt rp :d 2 rp where v is the angular velocity, d the particle diameter, R the distance from sedimentation uppermost surface to detection beam, rp the particle density, rl the dispersant density, m the dispersant viscosity, t the time, and u the sedimentation velocity of particles. As Eq (1) shows, particle density must be known. Genovese et al. (1997) used starch granules of known size and density for calibration of the method.

Detector

Sample

Light source Figure 5.11. Scheme of a simplified photometric/sedimentation method for particle size determination.

5

Color, Turbidity, and Other Sensorial and Structural Properties

.

115

5.3.3.3. Photon Correlation Technique Photon correlation is a technique for (among other things) measuring the size of colloidal particles, from a few nanometers to approximately 1 mm. It should not be confused with laser diffraction, which measures from 1 mm upward to several hundred micrometers. Photon correlation operates by measuring the temporal fluctuations in the light scattered by the particles, while diffraction measures the angular distribution of scattered light. A basic PCS setup is shown in Fig. 5.12. A laser illuminates the sample, which is a dilute suspension of the particles to be measured. The scattered light is viewed by a photomultiplier, usually at a 908 angle. The light intensity is not constant, but varies randomly, as the particles diffuse around in the beam, and the wavefronts of light scattered from them overlap and interfere. The photomultiplier sees a time-varying signal, not a constant one. If the particles are small, they move around by diffusion and the scattered light shows rapid fluctuations. Contrarily large particles diffuse slowly, and light scattered from such particles varies on a slower time scale. Variations in the light level give information about the particle size. Problems arise when the particles are all not of the same size, in other words there is a particle size distribution. PCS is less reliable in suspensions with broad size distribution, because of difficulties associated with interpreting the more complex autocorrelation decay curves (Horne, 1995). PCS is capable of measuring the size of extremely small particles, up to individual large macromolecules. In brief: . . .

Photon correlation analysis is fast and reliable. Range: 0:01---5:00 mm. Electron microscopy needs more sample treatment. New software and computational peripherics made the particle analysis easier. Range: 0:002---15 mm. Sedimentation methods need a standard and knowledge of the density of the particle. Range: 0:02---500 mm.

5.3.4. Cloudy Fruit Juice Viscosity It is useful to study the flow behavior of cloudy juices under well-defined conditions and to link the rheological behavior with the microstructure. Particle size, shape, and

Laser

Sample

Correlator

Photomultiplier Figure 5.12. Simplified scheme of the photon correlation technique.

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volume fraction, as well as electroviscous effects, modify juice viscosity, compromising the colloidal system stability. Rao (1987) reviewed the flow properties of plant food suspensions. While the rheology of fruit pulps and cloudy citrus juices has received continuous attention (Vitali and Rao, 1984; Nogueira et al., 1985; Rao et al., 1985; Ibarz and Lozano, 1992), there are also some published works on the viscoelastic properties of cloudy juices (Saravacos, 1970; Ibarz and Graell, 1986; Genovese et al., 1997; Genovese and Lozano, 2000). Characterizing the cloudy juice microstructure is difficult, but most of the variables involved are reflected in one parameter: the volume fraction of particles (f). Moreover, diluted and moderately concentrated regions can be modeled in terms of the intrinsic viscosity [m] (Sherman, 1970), which depends on particle shape and size distribution, and also on the applied shear stress (t). Other factors influencing the viscosity of cloudy juices are serum viscosity (mo ), pH, electrolyte concentration, and electroviscous effects. Because of the absence of distortion of the cloudy particles as a result of strong electrostatic forces, two electroviscous effects (Krieger, 1972) may be present: (a) When diluted dispersions are sheared, the electrical double layer (shear layer) is distorted. This distortion leads to an increased viscosity, or first electroviscous effect. (b) In more concentrated dispersions, viscosity increases due to particle repulsion effect, which was claimed to be proportional to f2 and inversely proportional to pH (Sherman, 1970). This effect is known as second electroviscous effect. The liquid microstructure under given conditions of stress, particle concentration, shape, size distribution, and interparticle affinity will be the primary determinant of the rheology. The viscosity of a diluted dispersion (m) containing spherical nondeformable particles is given by the well-known Einstein relationship (Metzner, 1985): mr ¼ (1 þ af)

(5:16)

where mr is the relative viscosity, f is the particle volume fraction, and a is a constant. Provided that f is low enough to prevent particle interaction, the distance between particles is much greater than their diameter, there is no slippage at the particle–fluid interface, and m arises only from viscous drag, then a ¼ 2:5, independent of particle size. Sherman (1970) found that for f$0:05 then a ¼ [m], where [m] is the intrinsic viscosity: hr 1 [m] ¼ Lim (5:17) f!0 f On the other hand, the first electroviscous effect may significantly increase the a value in the case of water dispersion of charged particles. It was then proposed (Russel, 1980) to modify Eq. (5.3) as follows: mr ¼ 1 þ 2:5bo f

(5:18)

where bo is a coefficient that takes into account the mentioned electroviscous effect. However, in the case under study bo will also be a function of the sphericity, size, and distribution of particles in the juice. Genovese and Lozano (2000) found that cloudy apple juice particles could be considered ellipsoidal rather than spherical. In this case, the axial ratio of the particle (pa ) should be considered: pa ¼ La =Ba

(5:19)

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Color, Turbidity, and Other Sensorial and Structural Properties

117

where La and Ba are the major and minor axis, respectively. Mooney (1951) worked on a different extension of Eq. (5.16): hr ¼ 1 þ af þ 0:4075(pa 1)1:508 f (5:20) when 1 < pa < 15; and: p2a 1 1 þ hr ¼ 1 þ 1:6f þ f 5 3( ln 2pa 1:5) ln 2pa 0:5

(5:21)

when pa > 15. Depending on the axial ratio of particles Eq. (5.20) or (5.21) gives information about the influence of shape (sphericity) on the viscosity of cloudy juice. Figure. 5.13 shows a typical frequency histogram obtained through the statistical analysis of apple juice cloud particles (Genovese et al., 1997). Cloudy apple juice resulted in a suspension of irregular shape particles ranging from 0.25 to 5 mm in size, with a mean diameter of f ¼ 0:84 mm. Calculated maximum and minimum mean diameters resulted in La ¼ 1:01 m and Ba ¼ 0:74 mm, respectively It was found that relative cloud material in apple juice was < 0.5% (Ruck and Kitson, 1965; Sta¨hle-Hamatschek, 1989). A 108Brix juice had a particle volume fraction of fo ¼ 3:93 103 (Genovese and Lozano, 2000). 50 %N

40

30

20

10

D, µm 0 0

1

2

3

4

5

Figure 5.13. Particle size distribution histogram: particle relative number (%N) versus particle diameter (D) (Genovese et al., 1997 with permission).

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Fruit Manufacturing

Figure 5.14 shows a typical log shear stress (t) versus log shear rate (g_ ) curve for different apple juice soluble solids’ concentration. It can be observed that t increased linearly with g_ and all curves go to the origin. Cloudy apple juice was shown to be Newtonian up to 508 Brix. Genovese and Lozano (2000) also compared viscosity with soluble solids for both clarified and cloudy apple juice (Fig. 5.15). Fitting relative viscosity values versus f data to the modified Einstein’s equation (5.18) resulted in an equation with slope a ¼ 96:23. A particle suspension with f < 0:05 (cloudy juice) conformed with the Sherman (1970) assumption [m] ¼ a ¼ 96:23. Calculated bo resulted in bo ¼ hr =2:5 ¼ 37:43. This elevated value indicates that the first electroviscous effect cannot be neglected in this type of products. Finally, with the estimated La and Ba particle axial values, pa parameter resulted equal to 1.365 and Eq. (3.20) should be considered. It can be easily calculated that the term accompanying a coefficient in Eq. (5.20) is practically irrelevant and the effect produced in the viscosity due to nonsphericity of particles can be neglected. When the size of particles is considered, below about 0:5 mm diameter, a higher relative viscosity is always to be expected.

5.4. FRUIT AROMA 5.4.1. Activity Coefficients of Fruit Juice Aroma It is well known that citrus fruits contain peel oil, the essence from which oil is obtained during the juice processing. This oil is rich in terpene hydrocarbons (limonene), which 100.00

t, Pa 25ⴗC

10.00

ⴗBrix 10 20 30 40

1.00

50

0.10

g, s−1 0.01

10

100

1000

Figure 5.14. Log–log plot of shear rate (g_ ) and shear stress (t) at 258C for cloudy apple juice at various soluble solids (Genovese and Lozano, 2000 with permission).

5

.

Color, Turbidity, and Other Sensorial and Structural Properties

119

40

h, cp

25ⴗC

30

Cloudy juice

20

Clarified juice

10

X, ⴗBrix

0 10

20

30

40

50

Figure 5.15. Viscosity of cloudy and clarified apple juice as a function of soluble solids at 258C (Genovese and Lozano, 2000 with permission).

contribute little to the aroma and are susceptible to oxidation. However, many aldehydes, esters, and alcohols contribute to the aroma of citrus oils. Other fruits, like apples and pears, contain much less volatile compounds and cannot form essential oils in distillates but essence can be used as flavorants after separation and rectification. Processing of fruit juices often involves aroma recovery, an operation by which volatile aromatic compounds contained in natural juice are stripped, together with a certain amount of water vapor, by thermal evaporation. Fruit aroma are very dilute solutions of esters, aldehydes, and alcohols, never exceeding levels of few parts per million in the juice (Carelli and Lozano, 1989). This water solution of aromatics, which can be considered at infinite dilution from a practical standpoint, is further rectified in a packed column up to a concentration of 150 –200 fold, condensed and cooled to avoid evaporation of more volatile compounds. Design and optimization of aroma recovery operations requires the knowledge of thermodynamic properties. To design or optimize the performance of evaporation units and rectification columns in aroma concentration information on relative volatilities of the aroma compounds is needed. The rate of the partial pressure of a given volatile in the vapor phase to its mole fraction in the liquid phases is called volatility of the volatile. The ratio of the volatilities of two compounds (a and b) is called the relative volatility: 0

aa=b 0

P g ¼ b0 b Pa g a

(5:22)

where Pi are the vapor pressure of pure compounds and gi are the activity coefficients.

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Fruit Manufacturing

Activity coefficients were introduced to extend Raults’ law application to real solutions. Relative volatility may be calculated if the activity coefficients of the substances are known. Volatile aroma compounds are present in fruit products at very low concentrations and solute–solute interactions may be neglected. Therefore, at infinite dilution the activity coefficient is a constant value g1 . From reliable values of these infinite dilution coefficients it is possible to predict the vapor–liquid equilibrium over the entire range of compositions (Loncin and Merson, 1979). While data on g1 of many compounds in organic solvents are easily available (Tiegs et al., 1987), information on fruit aroma compounds’ in aqueous solution is scarce. Aroma compounds values of g 1 may be obtained experimentally or estimated through thermodynamic models. 5.4.2. Experimental Method Carelli et al. (1991) measured the infinite dilution coefficients (g1 ) of aroma compounds in model solutions simulating apple juice, by following the dilution exponential method (Leroi et al., 1977). This method, also called the dynamic method, is based on the stripping of a solute from a solvent by a constant flow of inert gas. The variation of solute concentration in the carrier gas is then measured by gas–liquid chromatography. Figure 5.16 shows a sketch of a typical equilibrium cell used in this method. A working equation for g1 calculation valid for volatile solvent can be derived from the equilibrium conditions and mass balance in the cell (Duhem and Vidal, 1978; Carelli et al., 1991): g1 ¼ [(X (t)=Y (t)) þ 1] (Pav =Ps )

(5:23)

where: X (t) ¼ ln (Si =Sit¼0 ) Y (t) ¼ ln [1 (Pt DPav t=( pt Pav )NRT)]

Carrier gas outlet

Septum

Aroma solution Carrier gas inlet

Fritted disk

Water bath

Figure 5.16. Sketch of a typical equilibrium cell used for aroma stripping. Temperature and pressure must be carefully controlled.

5

.

Color, Turbidity, and Other Sensorial and Structural Properties

121

and N is the total moles of solvent in the dilution cell, D is the flow of carrier gas through the cell (cm3 =min), R is the gas constant, T is the cell temperature (K), Si is the area of solute peak, Pt is the total pressure in the cell (kPa), Pav and Ps are the vapor pressure of solvent and solute at T (kPa), respectively, and t is time (min). Vapor pressure of fruit aroma compounds may be calculated with the Antoine equation (Reid et al., 1977) or calculated from nonlinear regression of Ps versus T data. Table 5.5 lists Antoine constants of some volatile compounds, valid at the usual processing temperatures. Vapor pressure of solvent may be estimated from nonlinear regression of Psv versus T data of glucose solutions (Taylor and Rowlinson, 1955), a reasonable assumption taking into account the concentration and composition of fruit juices (Crapiste and Lozano, 1988). The g 1 values were obtained from linear regression of X(t) versus Y(t) data. A more simplified expression of Eq. (5.23) has been previously used for g1 calculation of volatile in food model systems (Lebert and Richon, 1984; Sorrentino et al., 1986), but it failed to represent solvent volatility. 5.4.3. Thermodynamic Models Several models have been proposed for estimating activity coefficients. Carelli et al. (1991) compared experimental g 1 values of apple aroma with those predicted from one-parameter Wilson, NRTL, and UNIQUAC equations. In food engineering practice, UNIFAC has achieved general acceptance (Saravacos et al., 1990; Sancho et al., 1997). 5.4.3.1. Wilson Equation The Wilson expression for a binary mixture with the solute at infinite dilution is (Kruming et al., 1980) ln g 1 ¼ ln (1 A12 ) þ A21

(5:24)

where Aji ¼ 1 (Vj =Vi ) exp [ (gji gii )=RT]

(5:25)

and Table 5.5. Antoine constants of aroma compounds. Compound Benzaldehyde Butanol Butyl acetate Butyl isobutyrate Ethanol Ethyl acetate Ethyl butyrate Ethyl valerie Hexanal Hexanol 2-Methyl- 1- butanol Pentyl acetate Propanol Trans-2-hexenal . .

A

B

C

14.3351 15.2010 14.1686 14.8788 16.8969 14.1366 13.9837 15.3818 15.4971 16.0848 14.2558 15.3830 15.5285 15.3857

3748.62 3137.02 3151.09 3515.11 3803.98 2790.50 3127.60 4074.13 3952.08 4055.45 2752.19 4103.45 3166.38 4007.22

66:12 94:43 69:15 40:76 41:68 57:15 60:15 40:59 38:12 76:49 116:30 40:94 80:15 47:56

Adapted from Carelli et al. (1991). Antoine equation: In P ¼ A B=(C þ T); P is vapor pressure in kPa; T is temperature in K.

122

Fruit Manufacturing g21 ¼ g12 ¼ (g11 g22 )1=2 (1 c12 )

(5:26)

In these equations Vij is the liquid molar volume of components i and j (m3 =mol), gii is a constant proportional to the energy of interaction between molecules of species, and C12 is a parameter to be determined experimentally. Hiranuma and Honma (1975) proposed that for systems where the infinite dilution activity coefficient is of the order of 10 or greater values gii ¼ DUi=3

(5:27)

Vj =Vi ¼ 1

(5:28)

and

can be used, where AU is the energy of vaporization of the ith component (J/mol), and can be calculated from: DUi ¼ [RT 2 d( ln Poi )=dT] RT where

Poi

(5:29)

is the vapor pressure of the ith component.

5.4.3.2. NRTL Equation The NRTL equation is a one-parameter expression valid at infinite dilution that can be expressed, after the nonrandomness parameter a12 is set equal to 0.4 and on the basis of V2 > V1 , as ln g 1 2 ¼ (V2 =V1 0:4RT)(g12 g22 ) þ exp [ (g21 g11 )=RT](g12 g11 )

(5:30)

where the parameter gij was assumed to be given by Eq. (5.26) and the parameter gii by the expression: gii ¼ 0:08 DUi =qij

(5:31)

q12 ¼ 1; q21 ¼ (V2 =V1 )1=2

(5:32)

where

5.4.3.3. UNIQUAC Model The UNIQUAC model was derived from statistical mechanical arguments (Abrams and Prausnitz, 1975). It was expressed as the combination of a combinatorial term, which takes into account liquid-phase nonidealities due to differences in molecular size and shape; and a residual term, which takes into account nonidealities due to intermolecular interactions. When activity coefficients are calculated at infinite dilutions, UNIQUAC equations can be simplified considerably to give the expressions: ln g1 ¼ ln (r2 =r1 ) þ 5q2 ln (q2 r1 =q1 r2 ) þ 5(r2 q2 ) (r2 1) r2 =r1 [5(r1 q1 r1 1)]

(5:33)

ln g1 ¼ q2 (1 þ g12 =RT g22 =RT) exp [ (g12 g11 )=RT ]

(5:34)

ln g 1 2

¼

ln g 1 c2

þ

ln g1 r2

Where gii ¼ 0:5 DUi =qi

(5:35)

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Color, Turbidity, and Other Sensorial and Structural Properties

123

On the other hand, gij is given by Eq. (5.26); r and q are pure components structural parameters, obtained from Gmehling et al. (1982). 5.4.3.4. UNIFAC Model This model calculates the activity coefficients as the sum of two contributions: molecular size and molecular interactions. A general equation for predicting infinite dilution coefficients based in the UNIFAC model was used by Reid et al. (1987): ln g1 1 ¼ A1,2 þ B2 n1 þ C 1 =n1 þ F 2 =n2

(5:36)

where n1 and n2 are the total number of carbon atoms in molecules 1 and 2. The other constants are temperature dependent and specific for each binary system. Values of g 1 of selected aroma compounds obtained experimentally and predicted with the UNIFAC model are presented in Fig. 5.17 (Sancho et al., 1997). 5.4.4. Fruit Aroma Properties Carelli et al. (1991) found that the activity coefficients at infinite dilution for alcohols, esters, and aldehydes increase with the length of the carbon chain, particularly in the case of esters and alcohols. The behavior is in agreement with the trends obtained in previous studies (Lebert and Richon, 1984; Sorrentino et al., 1986). Activity coefficients of lower alcohols and esters also increased with temperature. Contrarily, heavier aromas reduced their values when the temperature was increased. On the other hand, g 1 values for ethyl butyrate, butyl acetate, ethyl isobutyrate, and butanol remained practically constant with temperature in the range of practical interest.

6000 Experiemental

5000

UNIFAC 4000 3000 2000 1000

Hexenal

Trans- 2hexenal

Pentyl acetate

Butyl acetate

Ethyl butyrate

Ethyl acetate

0

Figure 5.17. g1 values of some aroma compounds predicted with the UNIFAC model (adapted from Sancho et al., 1997).

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Fruit Manufacturing

The observed behavior for alcohols is in agreement with g 1 in pure water calculated from Pierotti et al. (1959), which predicted increasing values of g1 with temperature for ethanol, propanol, and butanol and a decrease in g 1 for hexanol. Kieckbusch and Judson King (1979) studied the effect of temperature on partition coefficients of n-acetates in water and polysaccharide solutions. The authors reported that partition coefficients for the series methyl acetate–pentyl acetate increase with temperature in the range 25–408C. According to this, it appears that a change of behavior could be expected at higher temperatures. Since the slope of ln g1 ’versus 1/T is associated with the excess molar enthalpy, this would imply a change from exothermic to endothermic mixtures. Experimental values of activity coefficients at 20–258C of some aroma compounds diluted in pure water and sugars-organic acid solution simulating apple juice are presented in Figs. 5.18 and 5.19. It can be seen that g1 values in model solutions of most of the volatiles are higher than those reported in pure water. It appears that the presence of apple juice solutes leads to an increase in activity coefficients of aroma components. EXAMPLE Aroma stripping by flash condensation Due to the heat sensitivity of fruit juices, multiple-effect evaporators with aroma recovery are commonly used (see Chapter 2). Single-strength fruit juices are evaporated, and volatile is captured by flash condensation. This process, based more on industrial practice than theory, is schematized in Fig. 5.20 (Carelli et al., 1996), and is presented as a computer program for the simulation of fruit aroma recovery by flash evaporation. The PC program can estimate the volatile composition of the flash outlet streams for different juice composition and flash temperatures. The program is valid for adiabatic (using a preheater) and isothermal flash processes. Figure 5.21 shows a printed sheet of results for the flash recovery of a specified fruit juice. 9000 8000

Water

7000

Model solution

ga

6000 5000 4000 3000 2000 1000 Hexanal

Trans- 2-hexenal

Pentyl acetate

Ethyl valerate

Butyl acetate

Ethyl butyrate

Ethyl acetate

0

Figure 5.18. Activity coefficients at infinite dilution of different fruit volatile in water and solution simulating fruit juice. Adapted from Pierotti et al. (1959) and Carelli et al. (1991).

5

.

Color, Turbidity, and Other Sensorial and Structural Properties

125

1200

Water Model solution

1000

800

g a 600

400

200

0 Ethanol

Propanol

Butanol

Hexanol

Figure 5.19. g 1 values of some alcohols present in fruit juice aroma in pure water and model solution. Adapted from Sorrentino et al. (1986), Chandrasekaran and Judson King (1972), Lebert and Richon (1984), and Carelli et al. (1991).

Clarified fruit juice

Evaporator

Flash separator

Vapor and volatiles

Stripped fruit juice

Rectification column Concentrated essence Figure 5.20. Volatile captured by flash condensation of a single-strength fruit juice.

5.4.5. Fruit Shrinkage During Dehydration Shrinkage of fruits during dehydration is an observable physical phenomenon that occurs simultaneously with moisture diffusion, and has a significant effect not only in drying process but also in product quality. Shrinkage directly determines the structural properties of the product as well as its rehydration characteristics, while it indirectly influences flavor and taste

126

Fruit Manufacturing AROMA RECOVERY BY FLASH *************************** FLASH STREAMS **************** FEED LIQUID Q(kg/h)

VAPOR

10000.00

9000.00

Q(kmol/h)

502.83

447.37

1000.00 55.47

P(mm. Hg)

4578.44

758.54

758.54

T(K)

433.10

373.80

373.80

BRIX

11.00

12.2

----

HEAT REQUIREMENT ISOTHERMAL FLASH : 29921.67 KCAL/H AROMA COMPOSITION (ppm) ************************************ VOLATILE

FEED

LIQUID

VAPOR

RECOVERED (%)

ETHANOL

64.93

4.64

407.45

66.01

PROPANOL

2.00

0.56

14.23

74.8

BUTANOL

6.00

1.15

47.2

82.78

2METHYL-BUTANOL

1.86

0.24

15.60

88.24

HEXANOL

2.03

0.21

17.51

90.75

ETHYL ACETATE

1.39

0.05

12.81

96.90

ETHYL ISOBUTYRATE

12.69

0.14

119.43

99.00

ETHYL BUTYRATE

8.38

0.1

78.69

98.77

BUTYL ACETATE

4.58

0.08

42.86

98.44

PENTYL ACETATE

5.28

0.08

49.49

98.59

ETHYL VALERATE

10.26

0.13

96.40

98.83

HEXANAL

5.69

0.1

53.09

98.14

TRANS-2-HEXENAL

5.55

0.35

49.80

94.38

BENZALDEHYDE

0.47

0.06

3.9

87.83

AROMA CONCENTRATION: 7.69 FOLD Figure 5.21. Volatile recovery by flash condensation, estimated with Carelli et al. (1996) computer program. (This software is downloadable free of charge at the site http://www.upv.es/dtalim/)

too. The details of fruit structure at a cellular level determine the pathway of water occurring in fruit processing. In drying fruits the development of the physical structure is characterized by indices such as bulk, particle density, porosity, and shrinkage. Shrinkage affects the product quality in terms of loss of rehydration capacity and decrease in rehydration rate (Mavroudis et al., 1998). Some generalized correlations that predict bulk shrinkage coefficient by taking into account only the initial moisture content of the food were proposed (Lozano et al., 1983; Ratti, 1994; Mavroudis et al., 1998). Drying is a key food preservation process of many food products. During drying, stresses are formed due to nonuniform shrinkage resulting from nonuniform moisture and/or temperature distributions. This may lead to stress crack formation, when stresses exceed a critical level, resulting in products of undesired quality.

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5.4.5.1. Shrinkage coefficient, sb The shrinkage coefficient may be defined as the ratio between the bulk volume of the sample after processing and that of the fresh sample: sb ¼

Vb r ¼ bo (Xs þ Xw Xwo ) Vbo rb

(5:37)

where Vb , rb , and X are bulk volume, bulk density, and mass fraction, respectively. Subscripts o, s, and w are for initial (fresh product), solids, and water, respectively. Experimental data for shrinkage of fruits during processing were previously reported (Lozano et al., 1980, 1983). They showed shrinkage is dependent on processing conditions. A few models for shrinkage were also published (Suzuki et al., 1976; Lozano et al., 1980, 1983; Ratti, 1991). Different equations for calculating shrinkage during dehydration of selected fruits are listed in Table 5.6. Fictitious length model z (Roman et al., 1982) transforms every change of real length Dx into change of fictitious length. The above model requires data on porosity and bulk density as a function of moisture content. Kilpatrick et al. (1975) studied volume shrinkage of potatoes and other vegetables as drying proceeds. Charm (1978) reported on volumetric contraction of meat and potatoes. Suzuki et al. (1976) developed three equations that are applicable to three different drying models: uniform drying, core drying, and semicore drying. The first model results in two alternate equations: one needs data for equilibrium moisture contents and bulk density, while the other requires the initial moisture content and bulk density of the material. The second and third models need the initial and equilibrium values for moisture and bulk density. Shrinkage of fruits at different moisture contents were reported by Lozano et al. (1980, 1983), who also provided equations to predict Sb in the entire range 0 < Xw < Xwo for a variety of foods, requiring only knowledge of the fresh food moisture content. The linear relation between Sb and water content is well established for a wide variety of fruits in air drying. Figure 5.22 shows the change in bulk shrinkage coefficients of pear Table 5.6. Literature equations for the calculation of shrinkage during fruit dehydration. Model

Basic equation

Description

Reference

Fictitious length

Dz ¼ rb Dz=[r(1 þ x)]

X ¼ real length

Roma´n et al. (1982)

Early stage of drying

Sb ¼ (Xx þ 0:8)=(Xwo þ 0:8)

r ¼ rb (1 «X¼0 ) X ¼ water mass fraction

Uniform drying

Sb ¼ (Xx þ a)=(Xwo þ a)

Core drying

Sb ¼ kXw =Xwo þ 1

Semicore drying Volume shrinkage modeling

Sb ¼ rXw =Xwo þ n0 Sb ¼ 0:161 þ 0:816=Xw =Xwo ) þ0:022 e0:018=(Xwþ0:025) þf (1 (Xw =Xwo )

Kilpatric et al. (1955)

Xo ¼ Initial water mass fraction a ¼ Xe (1=rb,e 1) þ 1=rb,e * Subscript ‘‘e’’ is for equilibrium condition. (Xw,e þ 1)rb,o Xo k¼1 (Xo þ 1)re Xo Xo,e

Suzuki et al. (1976)

R and n0 are complex functions of (Xw , Xwo , Xe , rb,o rb,e ) f ¼ 0:209 Sb,f Sb,f ¼ 0:966=(Xo þ 0:796)

Lozano et al. (1983)

Suzuki et al. (1976)

128

Fruit Manufacturing 0.9

Sb 0.8

Pear 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.2

0.4

0.6

0.8

1

X/X o Figure 5.22. Bulk shrinkage coefficient of pear as a function of moisture content (adapted from Lozano et al., 1983).

(Lozano et al., 1983), as a result of drying. For some products the slope of Sb becomes noticeably less steep for X =X0 < 0:15. This is important because it indicates that all linear predictive equations for Sb will fail to cover the entire range 0 < X =X0 < 1. No less important is the fact that the range 0 < X =X0 < 0:15 is very significant in modeling drying operations. In other words, this range of X at which there is a change of slope in Sb is the one where most of the modeling and drying simulation is done. While Suzuki’s and Kilpatrick’s equations fit the Sb data reasonably well at high X =X0 values, they fit the data less well than Lozano et al.’s (1983) model when X =X0 < 0:15. Moreover, they fail to indicate the curvature in Sb versus X =X0 , which is encountered when X =X0 < 0:15. A valid question is how sensitive the data are to different drying conditions and sample shape. Data by Kilpatrick et al. (1975), Suzuki et al. (1976), and Mazza and Lemaguer (1980) are quite close to those reported here. All authors used conventional air drying. Kilpatrick et al. (1975) did not report sample shape or drying conditions, although they referred to tunnel drying. Suzuki et al. (1976) used 408C dry bulb temperature, 30% relative humidity and air at 0.6 – 0.7 m/sec. Mazza and Lemaguer (1980) used 40.5 – 608C, an unreported relative humidity (36%) and air at 0.30 – 0.55 m/s. Thus, as long as it is conventional air drying and changes in drying conditions are not too drastic, the results are valid. As far as the authors know, there are no similar data available for other drying procedures. As to the influence of sample shape, this study reports data corresponding to cylinders of 1 cm in diameter, 4 cm in length and, in the case of garlic, there are additional data corresponding to slicing the original cylinder. Suzuki et al. (1976) used 1 in. cubes, while Mazza and Lemaguer (1980) dried onion slices. The implication is that the correlations suggested are not sensitive to shape.

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5.4.6. Structural Damage During Freezing Freezing is not a common operation in the fruit and fruit juice industry. During freezing the ice crystals grow to a size that depends on the rate of heat removal. When heat is rapidly removed, ice crystals tend to be small. On the contrary, during slow cooling the ice crystals grows slowly outside the cell. Under such conditions cell shrinks, a phenomenon associated to the osmotic transfer of water from inside the cell to the forming ice. In addition to this shrinkage, there are other mechanisms of freezing damage (Reid, 1996). (1) Cells may be destroyed during freezing due to the increasing concentration of the unfrozen matrix, especially at high salt concentration. (2) During fast freezing, the ice crystal formation within the cell may destroy the membrane structure and organelles of the cell. This may result in the liberation of enzymes responsible for undesirable reactions. (3) While nondesirable enzymatic reactions may be controlled by blanching, this heat process cannot prevent loss of cell turgor, associated to changes in the semipermeable properties of the cell membrane. Loss of turgor due to freezing is more evident in fruits that are eaten raw.

REFERENCES Abrams, D.S., Prausnits, J.M. (1975). Statistical thermodynamics of liquid mixtures: a new expression for the excess Gibbs energy of partly or completely miscible systems. AIChEJ. 21: 116–128. Amiot, M.J., Tacchini, M., Aubert, S. and Nicolas, J. (1992). Phenolic composition and browning susceptibility of various apple cultivars and maturity. J. Food Sci. 57: 958–962. Beveridge, T. and Harrison, J.E. (1986). Clarified natural apple juice: production and storage stability of juice and concentrate. J. Food Sci. 51: 411– 414. Beveridge, T and Tait, V. (1993). Structure and composition of apple juice haze. Food Structure 12: 195–198. Carelli, A., Lozano, J.E. (1989). Apple aroma from Argentina: quality evaluation by capillary gas chromatography. HRC CC 12: 490–493. Carelli, A., Crapiste, G.H. and Lozano., J.E. (1991). Activity coefficients of aroma compounds in model solutions simulating apple juice. J. Agric. Food Chem. 39: 1636 –1640. Carelli, A., Crapiste, G.H. and Lozano, J.E. (1996). Simulacio´n de la recuperacio´n de aromas de fruta por evaporacio´n flash. In Herramientas de Ca´lculo en Ingenierı´a de Alimentos, Vol. 3, pp. 66–78 (SPUPV-96.3032). Chandrasekaran, S.K., Judson King, C. (1972). Multicomponent diffusion and vapor-liquid equilibria of dilute organic components in aqueous sugar solutions. AIChEJ. 18: 513–526. Charm, E. (1978). The Fundamentals of Food Engineering, 3rd ed. AVI Publishing Company, Inc., Westport, CT. Chobot, R. and Horulaba, A. (1983). Stabilization of naturally cloudy apple juices by mechanical and heat treatment of must. Przemysl Spozywczy 37: 409– 411. Clydesdale, F.M. (1969). The measurement of color. Food Technol. 23: 16 –22. Clydesdale, F.M. (1975). Methods and measurements of food color. In Theory, Determination and Control of Physical Properties of Food Materials, Rha C. (ed.). D. Reidel Publishing Company, Dordrecht, Holland/Boston, USA, Chapter 14, pp. 274–289. Crapiste, C.H., Lozano, J.E. (1988). Effect of concentration and pressure on the boiling point rise of apple juice and related sugar solutions. J. Food Sci. 53: 865–895. Dickinson, E. (1994). Colloidal aspect of food beverages. Food Chem. 51: 343–348. Duhem, P., Vidal, J. (1978). Extension of the dilutor method to measurement of high activity coefficients at infinite dilution. Fluid Phase Equilib. 2: 231–235. Farinato, R.S. and Rowell, R.L. (1983). Optical properties of emulsions. In Encyclopedia of Emulsion Technology, Vol. 1. Basic Theory, Becher, P. (ed.). Marcel Dekker, New York, NY, pp. 439– 478. Francis, F.J. and Clydesdale, F.M. (1975). Food Colorimetry: Theory and Applications. AVI Publishing Company, Inc., Westport, CT, USA.

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Genovese, D.B., Elustondo, M.P. and Lozano, J.E. (1997). Color and cloud stabilization in cloudy apple juice by steam heating during crushing. J. Food Sci. 62: 1171–1175. Genovese, D.B. and Lozano, J.E. (2000). Effect of cloud particle characteristics on the viscosity of cloudy apple juice. J. Food Sci. 65(4): 641– 645. Genovese, D.B. and Lozano, J.E. (2005). Stability of cloudy apple juice colloidal particles modeled with the extended DLVO theory. In Water Properties of Food, Pharmaceutical, and Biological Materials, Bruera, P., WeltiChanes, J., Lillford, P. and Corti, H. (eds.). CRC Press, Boca Raton, FL (Cat. #2993), in press. ISBN:0849329930. Genovese, D.B. and Lozano, J.E. (2006). Contribution of colloidal forces to the viscosity and stability of cloudy apple juice. Food Hydrocolloids (In Press - On line Sept. 2005). Genovese, D.B., Elustondo, M.P. and Lozano, J.E. (1997). Clor and cloud stabilization in cloudy apple juice by steam heating during crushing. J. Food Sci. 62: 1171–1175. Gierschner, K. and Baumann, G. (1988). New method of producing stable cloudy fruit juices by the action of pectolytic enzymes. Ind. Obst-Gemuesev. 54: 217–218. Gmehling, J., Rasmussen, P., Fredenslund, A. (1982). Vapor–liquid equilibria by UNIFAC Group contribution, revision and extension 2. Ind. Eng. Chem. Process Des. Dev. 21: 118–127. Hernandez, E. and Baker, R.A. (1991). Turbidity of beverages with citrus oil clouding agents. J. Food Sci. 56: 1024–1031. Hernandez, E., Baker, R.A. and Crandall, P.G. (1991). Model for evaluating the turbidity in cloudy beverages. J. Food Sci. 56: 747–753. Hiranuma, M. and Honma, K. (1975). Estimation of unlike-pair potential parameter in single parameter Wilson equation. Ind. Eng. Chem. Process Des. Dev. 14: 221–226. Hodge, J.E. (1953). Dehydrated foods. Chemistry of browning reactions in model systems. J. Agr. Food Chem. 1(15): 928–936. Horne, D.S. (1995). Light scattering studies of colloid stability and gelation. In New Physicochemical Techniques for the Characterization of Complex Food systems, Dickinson, E. (ed.). Blakie Academic and Professional, London, Chapter 11. Hunter, R.S. (1975). Scales for measurement color differences. In Measurement of appearance. J. Wiley Ed., Interscience, NY. Hutchings, J.B. (1994). Food Colour Appearance. Blakie Academic and Professional, London. Ibarz, A. and Graell, J. (1986). Evolucio´n del comportamiento reolo´gico del zumo de manzana. Alimentaria 4: 89–92. Ibarz, A. and Lozano, J.E. (1992). Rheology of concentrated peach and plum pulps. Rev. Espan˜ola Cienc. Tecnol. Aliment. 32(1): 85–94. Kramer, A. (1994). Use of color measurements in quality control of food. Food Technol., 48(10): 63–71. Kieckbusch, T.G. and King, C.J. (1979). Partition coefficients for acetates in food systems. J. Agric. Food Chem., 27: 504–507. Kilpatrick, P.W., Lowe, E. and Van Arsdel, W.B. (1975). Tunnel dehydration for fruits and vegetables. Adv. Food Res. 50: 385. King, R.D. (1980). The determination of food colours. In Development of food analysis techniques—2. Applied Science Publishers, London, pp. 79–106. Krieger, I.M. (1972). Rheology of monodisperse lattices. Adv. Colloid Interf. Sci. 3: 111–136. Kruming, A.E., Rastogi, A.K., Rusak, M.E., Tassios, D. (1980). Prediction of binary vapor–liquid equilibrium from one parameter equations. Can. Chem. Eng. 58: 663– 669. Labuza, T.P. and Riboh, D. (1982). Theory and application of Arrhenius kinetics to the prediction of nutrient losses in foods. Food Technol. 36(10): 66 –72. Lebert, A., Richon, D. (1984). Infinite dilution activity coefficients of n-alcohols as a function of dextrin-concentration in water dextrin systems. S. Agric. Food Chem. 32: 1156 –1161. Leroi, J.C., Manon, J.C., Renon, H., Sannier, H. (1977). Accurate measurement of activity coefficients at infinite dilution by inert gas stripping and gas chromatography. Ind. Eng. Chem. Process Des. Dev. 16: 139–144. Little, C. (1976). Physical measurements as predictors of visual appearance. Food Technol. 10: 74–82. Loncin, M. and Merson, R.L. (1979). Equilibrium between phases. In Food Engineering Principles and Selected applications. Academic Press, Inc., London, pp. 175–202. Lozano, J.E., Rotstein, E. and Urbicain, M.J. (1980). Total porosity and open pore porosity in the drying of fruits. J. Food Sci. 45: 1403–1407. Lozano, J.E., Rotstein, E. and Urbicain, M.J. (1983). Shrinkage, porosity and bulk density of foodstuffs at changing moisture contents. J. Food Sci. 48: 1497–1553.

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Mavroudis, N.E., Gekas, V. and Sjo¨jolm, I. (1998). Osmotic dehydration of apples. Shrinkage phenomena and the significance3 of initial structure on mass transfer rate. J. Food Eng. 38: 101–123. Mazza, G. and Lemaguer, M. (1980). Dehydration of onion: some theoretical and practical considerations. J. Food Technol. 15: 181–187. McClements, D.J. (1999). Characterization of emulsion properties. In Food Emulsions. Principles, Practice and Techniques. CRC Press, Boca Raton, FL, USA, pp. 295–339. McClements, D.J., Chantrapornchai, W. and Clydesdale, F. (1998). Prediction of food emulsion color using light scattering theory. J. Food Sci. 63(6): 935–939. Metzner, A.B. (1985). Rheology of suspensions in polymeric liquids. J. Rheol. 29: 739–747. Molina-Bolı´var, J.A. and Ortega-Vinuesa, J.L. (1999). How proteins stabilize colloidal particles by means of hydration forces. Langmuir 15: 2644 –2653. Mooney, M. (1951). The viscosity of concentrated solutions of spherical particles. J. Colloid Sci. 6: 162–167. Moyls, A.W. (1966). Opalescent apple juice concentrate. Food Technol. 20(5): 121–123. Nagel, B. (1992). Continuous production of high quality cloudy apple juices. Fruit Process. 1: 6 –8. Nassau, K. (1996) Color. In Encyclopedia of Chemical Technology. Vol. 6, 4th edition. Kirk-Othmer (eds.). John Wiley & Sons. pp. 841–876. Nogueira, J.N, McLellan, M.R. and Anantheswaran, R.C. (1985). Effect of fruit firmness and processing parameters on the particle size distribution in applesauce of two cultivars. J. Food Sci. 50: 744 –749. Ochoa, M.R., Kesseler, A.G., Vullioud, M.B. and Lozano, J.E. (1999). Physical and chemical characteristics of raspberry pulp: storage effect on composition and color. Lebensm. Wiss. Technol. 32(3): 149–153. Pierotti, O.J., Deal, C.H., Den, E.L. (1959). Activity coefficients and molecular structure. Ind. Eng. Chem. 51: 95–102. Pomeranz, U.E. and Meloan, C.E. (1994). Food Analysis, 3rd ed. Chapman and Hall, NY. Rao, M.A. (1987). Predicting the flow properties of food suspensions of plant origin. Food Technol. 41(3): 85–88. Rao, M.A, Cooley, H.J., Nogueira, J.N. and McLelland, M.R. (1985). Rheology of apple sauce: effect of apple cultivar, firmness and processing parameters. J. Food Sci. 51(1), 176–179. Ratti, C. (1994). Shrinkage during drying of foodstuffs. J. Food Eng. 23: 91–105. Reid, D.S. (1996). Fruit freezing. In Processing Fruits: Science and Technology, Vol. 1, Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomics Publishing Company, Lancaster, PA, USA. Reid, R., Sherwood, T.and Prausnitz, J. (1977). The Properties of Gases and Liquids. McGraw-Hill, New York. Reid, R.C., Prausnitz, J.M. and Poling, B.E. (1987). The Properties of Gases of Liquids, 4th ed., McGraw-Hill, New York. Roma´n, G., Urbicain, M.J. and Rotstein, E. (1982). Kinetics of the approach to sorptional equilibrium by a foodstuff. AICHE J. Ruck, J.A. and Kitson, J. (1965). Seasonal variation in the soluble solids and total acid content of opalescent apple juice. Wissen. Techn. Kommission Intarn. Fruchtsaft-Union 433–438. Russel, W.B. (1980). Review of the role of colloidal forces in the rheology of suspensions. J. Rheol. 24(3): 287–317. Sancho, M.F., Rao, M.A. and Downing, D.L. (1997). Infinite dilution activity coefficients of apple juice aroma compounds. J. Food Eng. 34: 145–158. Saravacos, G.D. (1970). Effect of temperature on viscosity of fruit juice and purees. J. Food Sci. 35: 122–127. Saravacos, G.D., Karathanos, V., and Marino-Kouris, D. (1990). Volatility of fruit aromacompounds in sugar solution, in C. Charalambous (Ed.), Proceedings of the Sixth International Favour Conference (p. 729). Elsevier: Rethymnon, Crete, Amsterdam. Sherman, P. (1970). Rheology of dispersed systems. In Industrial Rheology. Academic Press, Inc., London, pp. 97–183. Sorrentino, F., Voilley. A., Richon, D. (1986). Activity coefficients of aroma compounds in model food systems. AIChE J. 32: 1988 –1993. Spark, A.A. (1969). Role of amino acids in nonenzymatic browning. J. Sci. Food. Agric. 20(5): 308–313. Sta¨hle-Hamatschek, S. (1989). Cloud composition and its influence on cloud stability in naturally cloudy apple juice. Fluss. Obst. 56: 543 –544. Suzuki, K., Kubota, K., Hasegawa, T. and Hosaka, H. (1976). Shrinkage in dehydration of root vegetables. J. Food Sci. 41: 1189. Taylor, J.B., Rowlinson, J.S. (1955). The thermodynamic properties of aqueous solutions of glucose. S. Trans. Faraday Soc. 51: 1183–1192. Tiegs, D., Gmehling, J. Rasmussen, P. and Fredeeslund, A. (1987). Vapor–liquid equilibria by UNIFAC group contribution. 4. Revision and extension. Ind. Eng. Chem. Res. 26: 159–170. Trottier, R. (1997). Size measurement of particles. In Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 22, 4th ed. John Wiley & Sons, Inc., NY, USA, pp. 258–277.

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Vitali, A.A. and Rao, M.A. (1984). Flow properties of low-pulp concentrated orange juice: serum viscosity and effect of pulp content. J. Food Sci. 49: 876 –881. Yamasaki, M., Yasui, T. and Arima, K. (1964). Pectic enzymes in the clarification of apple juice. Part I. Study on the clarification reaction in a simplified model. Agr. Biol. Chem. 28(11): 779–787.

CHAPTER 6

CHEMICAL COMPOSITION OF FRUITS AND ITS TECHNOLOGICAL IMPORTANCE In spite of their popularity, fruits are relatively unimportant as major nutritional items. Fruits are selected largely for their agreeable taste. Most fruits are juicy, with high water and sugar content, and they become important mainly for the vitamins, minerals, and fibers they contain. Fruits add variety and flavor to the diet. Whole fruit may be fresh, frozen, canned, dried, made into preserves or a variety of desserts. Concentrated fruit flavors are also used in food and drinks. Figure 6.1 shows a simplified scheme of apple components. Fruits are living complex systems, and it is obvious that after the liberation of these chemical reactive components during size reduction, mashing, trimming, and any other destructive process, different deteriorative reactions will take place. Therefore, studies on the composition and changes occurring during processing and storage might be equally helpful to the nutritionist and the processor, the latter to optimize the processing parameters to avoid browning and other undesirable reactions affecting organoleptic properties.

6.1. PROXIMATE COMPOSITION OF FRUIT AND FRUIT PRODUCTS In the early 1990s the long-standing, traditional basic four food groups, consisting of meat, dairy products, grains, and fruits and vegetables, were reworked into a balanced and healthy food guide pyramid. This pyramid has as its base the grain group; on the second level are the fruit and vegetable groups; on the third level are the meat and dairy groups; and at the top is the fats, oils, and sweets’ group (Anonymous, 1992). The sources of most vitamins and minerals belong to fruits and vegetables. This pyramid suggests three to five servings (One serving ¼ half cup) of vegetables and two to four servings of fruit should be eaten every day. They also provide fiber, which contains no nutrients but aids in moving food through the digestive system (Fig. 6.2). More recently, a new version of food pyramid, was published by the USDA (2005). This new pyramid (Fig. 6.3) symbolizes a personalized approach to healthy eating and physical activity, and was developed to ensure consumers make healthy food choices and active every day. The widths of food group bands suggest how much food should be chosen from each group. This food pyramid recommends two cups of fruits every day. Personalized information on the amounts of food to eat each day may be accessed at the website, mypyramid.gov. A vast amount of data have been accumulated on the compositional characteristic in different fruits (Nagy et al., 1990, 1992; Somogy et al., 1996). Sugar and organic acids are the 133

Water (84%)

Solids (soluble and insoluble : 16%)

Aldehydes Ethers Volatiles Esters

Malic Alcohols Organic acids

Citric

Waxes and essential oils

Ethylene

Vitamins

Proteases Enzymes

Nitrogen

Catalases Oxidases

Amino

Tannins

Amino acids

Diastases Pectinases

Basic

Amides

Lysine, arginine, hystidine, aspartic acid, etc.

Asparagine

Pigments Minerals Anthocyanins Ca, K, Na, Mn, Mg, S, P, etc.

Chlorophylls Flavonoids Carbohydrates

Dextrins

Pectin

Sugars

Starch

Cellulose

Figure 6.1. Simplified schematic representation of the most remarkable components of a fruit.

Fat, oil sweets

Milk & cheese group

Meat, eggs,

Fruits Vegetables

Bread, cereals, pasta

Figure 6.2. Food guide pyramid (USDA,1992).

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Figure 6.3. USDA new food pyramid (USDA, 2005).

major constituents of soluble substances. The nutrients known to be essential for humans are proteins, carbohydrates, fats and oils, minerals, vitamins, and water. Compositions of fruit not only vary according to botanical variety, cultivation practices, and weather, but also change with the degree of maturity prior to harvest, the condition of ripeness, and storage conditions. Most fresh fruits are high in water content, and low in protein and fat. In these cases water contents will be greater than 70% and frequently greater than 85%. Fruits are also important sources of both digestible and indigestible carbohydrates. The digestible carbohydrates are present largely in the form of sugars and starches, while indigestible cellulose provides fibers that are important to normal digestion (Table 6.1). Fruits are also important sources of minerals and certain vitamins, especially vitamins A and C. It is well known that citrus fruits are excellent sources of vitamin C. Beta-carotene and certain carotenoids, and vitamin A precursors are present in the yellow-orange fruits. Table 6.1. Typical percentage composition of edible portion selected fruits. Fruit Bananas Oranges Apples Strawberries

Carbohydrate

Protein

Fat

Ash

Water

24.0 11.3 15.0 8.3

1.3 0.9 0.3 0.8

0.4 0.2 0.4 0.5

0.8 0.5 0.3 0.5

73.5 87.1 84.0 89.9

Source: Chen (1992) and Konja and Lovric (1993).

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6.1.1. Proteins and Amino acids Nitrogen-containing substances are found in fruits in different combinations: proteins, amino acids, amides, amines, nitrates, etc. In fruits, nitrogen-containing substances are less than 1% in most cases. Among nitrogen-containing substances proteins are most important (Dauthy, 1995). Proteins are colloidal in structure, and heating makes them insoluble above approximately 508C. This behavior needs to be considered in heat processing of fruits. Proteins are source of amino acids, necessary for growth and tissue repair. However, fruit proteins are less valuable than animal proteins due to the lack of essential amino acids. Good plant sources of proteins are beans, peas, nuts, bread, and cereals. Amino acids are defined as any group of organic molecules that consist of a basic amino group (NH2 ), an acidic carboxyl group (COOH), and a specific organic side chain that is unique to each amino acid. Arginine, glycine, cystine, histidine, and tryptophan are a few examples of amino acids. The human body is unable to synthesize the so-called nine essential amino acids. In the case of fruits they provide less than 3 g/100 g of proteins (Fig. 6.4). 6.1.2. Organic Acids Fruit contains organic acids, such as citric acid in oranges and lemons, malic acid in apples, and tartaric acid in grapes (Dauthy, 1995). These acids give fruits, tartness and slow down bacterial spoilage. Acidity and sugars are the main elements determining the taste of fruits, and sugar/acid ratio is very often used in order to give technological characterization of fruit products.

Protein (g) Apple Tomato 3.5 Strawberry 3 Quince Pumpkin 2.5 Pomegranate Plum Pineapple Persimmon

Apricot Avocado Banana Carambola Cherry

2

Fig

1.5 1

Grape

0.5

Grapefruit 0

Pear

Guava

Peach

Jackfruit

Passion fruit Orange Olive green Nectarine Mulberry

Kiwifruit Lemon Lime Lychee Mandarin Mango Melon honeydew

Figure 6.4. Protein content of fresh fruits (g/100g) (Wills, 1987; Nagy, 1990; Somogyi et al., 1996).

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Malic acid is found in juices and fruits, such as apples, gooseberries, rhubarbs, and grapes. Tartaric acid is a widely distributed plant acid with many food and industrial uses, and is obtained from by-products of wine fermentation. Its forms include several salts (Fig. 6.5), cream of tartar (potassium hydrogen tartrate), and Rochelle salt (potassium sodium tartrate). It is used in effervescent tablets, gelatin desserts, fruit jellies, and as an acidifying agent in carbonated drinks; and belongs to dicarboxylic group. Carambola fruit is rich in oxalic acid (Swi-Bea Wu et al., 1992). Among other organic acids present in minor amounts lactic, succinic, pyruvic, glyceric, shikimic, maleic, and isocitric acids must be included (Fig. 6.6). One of the consequences of the organic acid content in fruits is the relatively wide range of pH encountered in fruit products (Table 6.2). 6.1.3. Carbohydrates Carbohydrates are the main component of fruits, representing more than 90% of their dry matter. They are produced by the process of photosynthesis and function as structural

Figure 6.5. SEM micrograph of grape juice tartrates (Buglione, 2005).

800 700 600 500 400 mg/100 g 300 200 100 0 Citric rry

e

ap

ac h

a Pe r

Ki

Tartaric

ple

Ap

Gr Pe

Malic

e Ch

Quim

wi ry

ge

er

wb

it

fru

ra St

an

Or

Figure 6.6. Organic acid content of selected fruits (Wills, 1987; Nagy, 1990; Somogyi et al., 1996).

138

Fruit Manufacturing Table 6.2. pH values of selected fruit products. Food product

pH

Apple butter Apple sauce Apples Apricots Cherries Cucumbers Grapefruits Lemons Olives Oranges

3.1–3.5 3.6 –3. 9 3.0 –3.3 3.7–3.8 3.4 –4.0 3.0 –3.5 3.2–3.5 2.3 –2.6 2.9–3.2 3.2–38

Food product

pH

Orange juice Peaches Peas Pineapple juice Plums, currants Prune juice Pumpkins Raisins Strawberries Tomato juice

3.7– 4.1 3.4 –3.6 6.1–6.4 3.3 –3.6 2.9 –3.2 3.7– 4.1 4.1– 4.4 3.6– 4.2 3.3 –3.4 4.0 – 4.5

Source: Dennis, 1983; Friend, 1982; Goodenough and Atkin, 1981; Jackson and Shinn, 1979; Salunkhe, 1991; Wills et al., 1989; Hui, 1991.

components as in the case of cellulose. On the other hand, as starch, carbohydrates account for the energy reserves; they function as essential components of nucleic acids as in the case of ribose, and also as components of vitamins such as ribose and riboflavin (Dauthy, 1995). Carbohydrates account for more than half of the calories’ intake for most people, daily adult intake should contain about 500 g carbohydrates. Starches and sugars are the main source of the body’s energy. However, sugars are not essential foods; they provide energy but not nutrients. Figure 6.7 shows that fruit sugars mainly consist of glucose, fructose, and sucrose. Maltose and minor

12

10

8

% 6 4

Apple Cherry Grape

2

Peach Pear 0

Kiwifruit Fructose Glucose

Strawberry Sucrose

Sorbitol

Figure 6.7. Sugar content of selected fruits.

6

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Chemical Composition of Fruits and its Technical Importance

139

Figure 6.8. Scanning electron photomicrograph of an isolated apple starch granule (5 kV 4,400).

percentage of other mono- and oligosaccharides are also present in fruits (McLellan and Acree, 1992). In mango pure´e heptulose and xylose have been detected. Carambola juice is rich in arabinose (Swi-Bea Wu et al., 1992). 6.1.3.1. Starch Starches provide a reserve energy source in plants and supply energy in nutrition; they are found in seeds and tubers as characteristic starch granules. Apple juice is one of the juices that can contain considerable amounts of starch, particularly at the beginning of the season. Unripe apples contain as much as 15% starch (Reed, 1975). Apple starch granules could be considered practically spherical (Fig. 6.8). In this case, major (La ¼ 9:21mm) and minor axes (Ba ¼ 7:86mm) are very similar (Carrı´n et al., 2004). The starch content varies from fruit to fruit (Table 6.3), variety to variety, and season to season within a given fruit. As the fruit matures on the tree, starch hydrolyzes into sugars. Table 6.3. Starch content of selected fruits. Fruit Guava Apple Jackfruit Carambola Mango Pumpkin Banana

Starch (g/kg) 1 2 4 5 5 17 31

Source: Somogyi et al., 1996; Sanchez-Castillo et al., 2000; Carrin et al., 2004.

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Fruit Manufacturing

Decrease in starch usually begins a few weeks before harvest. Apple starch resulted in particles of regular shape with a mean diameter D ¼ 9:21mm (s ¼ 2:74) (Carrin et al., 2004) 6.1.3.2. Pectin Pectin is a ‘‘gum’’ found naturally in fruits that causes jelly to change to gel. Tart apples, crab apples, sour plums, Concord grapes, quinces, gooseberries, red currants, and cranberries are especially high in pectin. Apricots, blueberries, cherries, peaches, pineapples, rhubarbs, and strawberries are low in pectin. Underripe fruit has more pectin than fully ripe fruit. Pectin consists of a backbone, in which ‘‘smooth’’ a-d-(1–4)-galacturonan regions are interrupted by ramified rhamnogalacturonan regions, highly substituted by neutral sugar side chains (Oakenfull, 1991) (Fig. 6.9). An important feature of galacturonans is the esterification of the galacturonic acid residues with methanol. The degree of methoxylation (DM) is defined as the number of moles of methanol per 100 moles of galacturonic acid. 6.1.4. Lipids Fats and oils are a concentrated source of energy. Fats make certain vitamins available for use in the body, they cushion vital organs, help to maintain body temperature, and make up part of all body cells. Most fruits have fat content <0.5 g/100 g edible portion (Watt and Merrill, 1963). However, significant quantities are found in nuts (55%), apricot kernel (40%), grape seeds (16%), apple seeds (20%), and tomato seeds (18%). Table 6.4 lists fruits with relatively high fat content. 6.1.5. Minerals Most of foods contribute to a varied intake of essential minerals. Calcium builds bones and teeth, and it is necessary for blood clotting. The best sources are milk and hard cheese. Others are leafy greens, nuts, and small fishes (sardines) with bones that can be eaten. Calcium content in fruits generally rarely exceeds 40 mg/100 g edible portion (Fig. 6.10). (Dauthy, 1995) 6

4

COOCH3 O OH

OH 1

OH

α

OH O

O COOH

O

COOCH3 O OH

OH O

OH

O

OH O COOCH3

Figure 6.9. Polygalacturonic acid molecule.

Table 6.4. High fat content fruits. Fruit Avocado Lychee Olive green Coconut Source: Watt and Merrill, 1963.

Fat (g/100g) 16 1 13 35

6

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Chemical Composition of Fruits and its Technical Importance

141

Calcium Apple Tangerine Sapote Sapodilla Raspberry

40

Apricot Avocado Banana Blackberry

30

Quince Plum

Breadfruit Carissa

20

Pineapple

Cherimoya

10

Pear

Cherry,Sour Red

0

Peach

Cherry, Sweet

Passion -Fruit

Crabapple

Papaya

Fig

Orange

Gooseberry

Mulberry

Grapefruit

Meloncantaloupe Mango Loquat

Guava Jackfruit Lime Lemon

Jujub Kiwifruit

Figure 6.10. Calcium content in 100 g edible portion of fruits (Watt and Merrill, 1963; Wills, 1987).

Mineral substances are present as salts of organic or inorganic acids, or as complex organic combinations (chlorophyll, lecithin, etc.); they are in many cases dissolved in cellular juice. Mineral-rich fruit includes strawberries, cherries, peaches, and raspberries. Important quantities of potassium (K) and absence of sodium chloride (NaCl) give a high dietectic value to fruits and to their processed products. Phosphorus is mainly supplied by vegetables. Potassium is a major mineral present in fruits and ranges from 30 (cherimoya) to 600 mg/ 100 g (avocado) edible portion (Fig. 6.11). Phosphorus works with calcium to make strong bones and teeth. A diet that has enough protein and calcium also provides enough phosphorus (Fig. 6.12). Fruits with relatively high levels of sodium and magnesium are shown in Figs. 6.13 and 6.14, respectively. Copper is of particular interest since it is a cofactor for PPO and can also serve as a catalyst for numerous oxidative reactions. Iron helps to build red blood cells and aids the blood in carrying oxygen to the cells. Iron content of selected fruits is given in Fig. 6.15.

6.1.6. Vitamins Many reactions in the body require several vitamins, and the lack or excess of any one can interfere with the function of another. As the body cannot manufacture all vitamins, it must absorb them from food. Vitamins are also added to fruit products mainly for nutritional purposes.

142

Fruit Manufacturing Potassium LONGANS Watermelon Tangerine

400

Loquat Lychee Mango

300

Strawberry Soursop

Melon honeydew

200

Mulberry

Sapote

100

Sapodilla

Nectarin

0 Orange

Rose apple

Papaya

Rhubarb

Passion fruit Raspberry Peach

Quince

Pear Persimmon

Pomegranate Plum

Pineapple Figure 6.11. Potassium content of selected fruits (100g edible portion) (Watt and Merrill, 1963; Wills, 1987).

High P fruits (mg/kg) Apricot Strawberry Soursop

700 600

Avocado Banana

500 Sapote

Blackberry

400 300 200

Passion fruit

Breadfruit

100 0

Mulberry

Cherimoya

Lychee

Cherry, sweet

Loquat

Gooseberry Guava

Longans Kiwifruit

Jackfruit Jujub

Figure 6.12. Selected fruits with high phosphorous content (Watt and Merrill, 1963; Wills, 1987).

6

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Chemical Composition of Fruits and its Technical Importance

143

Sodium Avocado

30

Strawberry

Mammy apple

20

Soursop

Melon cantaloupe

10 Sapote

Melon honeydew

0

Sapodilla

Mulberry Passion fruit

Rhubarb Quince

Persimmon

Figure 6.13. Selected fruits with high sodium content (100g edible portion) (Watt and Merrill, 1963; Wills, 1987).

Magnesium Longans Watermelon Tangerine Strawberry

30

Loquat Lychee

25

Mango

20 Soursop Sapote

Melon honeydew

15

Mulberry

10 5

Rose apple

Nectarine

0 Rhubarb

Orange

Raspberry

Papaya Passion fr, purple

Quince Pomegranate

Peach Plum Pineapple

Pear Persimmon japan

Figure 6.14. Selected fruits with high magnesium content (100g edible portion) (Watt and Merrill, 1963; Wills, 1987).

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Fruit Manufacturing 2

Metal concentration (mg/kg)

1.8

Copper

1.6

Zinc

1.4

Iron

1.2 1 0.8 0.6 0.4 0.2

Mulberry

Avocado

Sapodilla

Lemon

Soursop

Breadfruit

Raspberry

Kiwifruit

Cherimoya

Fig

Stberry

Gooseberry

Cherry, Sour

Pomegranate

Rhubarb

Carambola

Cranberry

Watermelon

Mango

Persimmo

Grapefruit

Papaya

Tangerine

Rose apple

0

Figure 6.15. Metal composition of selected fruits (Wills, 1987; Nagy, 1990; Somogyi et al., 1994).

Vitamin B group and vitamin C (or ascorbic acid) are water-soluble vitamins that are not stored in the body for long, hence should be consumed every day. Table 6.5 lists major vegetable source of vitamins, and Fig. 6.16 shows a selection of fruits with relatively high ascorbic acid content. Some vitamins also serve multiple functional purposes: vitamins C and E act as antioxidants, prevent undesirable color changes, and retard the development of rancidity. Provitamin A (or b-carotene) and riboflavine (vitamin B2 ) are used as natural colorants. Four vitamins (A, D, E, and K) are known as the fat-soluble vitamins. They are digested and absorbed with the help of fats that are in the diet. 6.1.7. Water Water plays an active part in many chemical reactions and is needed to carry other nutrients, regulate body temperature, and help eliminate wastes (Dauthy, 1995). Water makes up about 60% of an adult’s body weight. Requirements for water are met in many ways. Most fruits are more than 80% water (Fig. 6.17). 6.1.8. Aroma Aroma components exist in very small quantities in fruits and are composed of various chemical species: alcohols, aldehydes, esters, terpenes, etc. Ripe fruits, especially bananas, Table 6.5. Major vegetable sources of vitamins. Vitamin

Major vegetable sources

Vitamin B1 (thiamine), mg Vitamin B6 , mg Folic acid, mg Vitamin C, mg Vitamin E, IU Vitamin K

Cereal grains, nuts Cereal grains Green leafy vegetables, wheat bran and germ Citrus fruits, green peppers, broccoli, cantaloupe Vegetable oils, margarine, cereal grains Green leafy vegetables, vegetable oils

IU is international unit, mg stands for milligram, indicates no data available. Source for RDA data: US Department of Health, Education, and Welfare.

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Chemical Composition of Fruits and its Technical Importance

145

300 250 200 150 100

Tomato

Passion Fruit

Cherry

Pineapple

Mango

Carambola

Grapefruit

Quince

Lime

Lemon

Lychee

Orange

Kiwifruit

Guava

0

Mandarin

50

Strawberry

Ascorbic acid (g/kg)

6

Figure 6.16. Ascorbic acid content of selected fruits (Watt and Merrill, 1963; Wills, 1987).

Water Apple Tomato Strawberry Quince Pumpkin Pomegranate Plum

95 90

Apricot Avocado Banana

85

Carambola

80

Cherry

75

Fig

70 Pineapple

Grape

65

Persimmon

Grapefruit

60

Pear

Guava

Peach

Jackfruit Kiwifruit

Passion Fruit

Lemon

Orange

Lime

Olive, green Nectarine Melon honeydew

Lychee Mango

Mandarin

Figure 6.17. Water content of fruits (100g edible portion) (Wills, 1987; Nagy, 1990; Somogyi et al., 1994).

146

Fruit Manufacturing Table 6.6. Esters usually found in fruits. Name

Formula

Identified in

Butyl-acetate Octyl-acetate Ethyl-butyrate

CH3 COOC4 H9 CH3 COOC8 H17 CH3 COOC2 H5

Bananas Oranges Pineapples

oranges, and pineapples, owe their odors to the presence of esters. Some common esters formed with acetic acid (CH3 COOH) are found in Table 6.6. Fruit processing techniques must be designed and operated to reduce loss and modification of aroma components. The volatile components of fruit aroma are usually recovered by removing them by a partial evaporation of the fresh juice, prior to the clarification and/or concentration operations. These dilute aqueous aromas are usually concentrated by distillation and then returned to the juice. The process of aroma recovery and concentration can be optimally designed and efficiently operated if the composition of the aroma is known. Figure 6.18 compares the aroma quality of volatile extracts obtained from whole and peeled apples, based only on the most desirable compounds (Carelli and Lozano, 1989). Results indicated that, in the case of the GS aroma, only those volatiles with relatively high retention times increased their relative composition when the whole fruit was processed. On the contrary, ‘‘whole RD apple’’ aroma was rich in those desirable compounds with low retention times. However, organoleptic assessment indicated ‘‘whole apple’’ aroma to be more ‘‘fruity’’ and ‘‘characteristic’’ than that of peeled apples independent of the apple variety. Figure 6.19 compares the volatiles present in the aroma extracted from a commercial apple essence. 120

1/2 h

1h

Recovery (%)

100

80 60 40

20

Benzanal

Hexanol

Ethyl acetate

2-Methyl-i-butanol

Butanol

Hexanol

Ethanol

Ethylbutyrate

0

Figure 6.18. Contribution of peel to aroma: comparison between desirable volatiles of ‘‘whole apple’’ and ‘‘peeled apple’’ aroma. Expressed as relative area (A%) of total desirable (Adapted from Carelli and Lozano, 1989.) A very significant increase in ethanol content was also determined and attributed to extensive fermentation during the grinding and pressing operations.

6

Chemical Composition of Fruits and its Technical Importance

.

147

1000

Volatile (ppm)

100

10

1 0.1

0.01

Ethanol

Butanol

Ethyl acetate

Propanol+ethly butyrate

Hexanal

Ethyl valerate +pentyl acetate

2-Methyl-1-butanol

Butyl acetate

Hexanol

Trans-2-hexenal

Ethylisobutyrate

Trans-2-hexnol

Hexyl acetate

Benzanal

Acetophenone

4-Methoxyallyl-benzene

0.001

Figure 6.19. Volatile composition of apple aroma (Source: Carelli and Lozano, 1989).

The values indicated that substantial losses of very valuable components (e.g., ethylisobutyrate, pentyl acetate, and trans-2-hexenal) occurred during the industrial process. 6.1.9. Color compounds Different pigments complete the proximate composition of fruits. Color is an important aspect of both natural and processed fruits. Natural colorants are in general unstable, and color of fruits and fruit products may change during processing and storage. Natural pigments may be defined as the pigments occurring in unprocessed fruits, as well as those formed upon processing and storage. Major fruits and fruit products pigments can be grouped into chlorophylls, carotenoids, flavonoids (anthocyanins and anthoanthins), Melanoidins, and caramels (Dauthy, 1995). .

.

Chlorophyll. This is the most abundant of all these pigments. In living plant tissues, chlorophyll is present in chloroplast cells as colloidal suspension and is associated to carbohydrates and protein. There are two types of chlorophyll: (a) Blue-green. Its chemical formula is C55 H72 O5 N4 Mg. (b) Yellow-green. Found in most green tissues; its formula is C55 H70 O6 N4 Mg. In many fruits chlorophyll is present in the unripe state and gradually disappears during ripening. Chlorophyll is water insoluble. Sodium copper chlorophyllin salt, obtained after the hydrolysis of chlorophyll with sodium hydroxide and the replacement of magnesium with copper, is a heat-stable coloring food. Carotenoids. Pigments belonging to this group are fat soluble and range in color from yellow through orange to red. Important fruit carotenoids include the orange carotenes

148

Fruit Manufacturing

.

of apricot, peach, and citrus fruits; the red lycopene of watermelon and apricot; and the yellow-orange xanthophyll of peach. These and other carotenoids seldom occur singly within plant cells. The content of these pigments rarely exceeds 0.1%. In fruits, b-carotene is an indication of provitamin A content. The carotenoids g, b-carotene, and phytofluene were reported as the three main carotenoids in passion fruit (purple) (Chan, 1994). Flavonoids. Flavonoids are polyphenolic compounds possessing 15 carbon atoms; two benzene rings joined by a linear three-carbon chain. This class of pigments are water soluble and commonly present in the juices of fruits. Flavonoids include the purple, blue, and red anthocyanins of grapes, berries, plump, eggplants, and cherries; the yellow anthoxanthins of apples, and the colorless catechins and leucoanthocyanins, which are tannins present in apples and grapes. These colorless tannin compounds are easily converted to brown pigments upon reaction with metal ions. Anthocyanin pigments (red and purple) occur in the sap of cells. Anthocyanins give the familiar color to fruits such as red apples, blueberries, cherries, cranberries, strawberries, and plums. Anthocyanins are responsible for color in most berries. Anthocyanin concentration is usually expressed as cyanidin-3-glucoside/100 g pulp sample. The color of concord grapes is due to anthocyanin pigments, the major contributor being delphidin monoglucoside (McLelland and Acree, 1992).

Phenolic acids are not practically found in free forms in plants because the carboxyl groups are very active and easily transform into esters or amides when combined with aliphatic alcohols and phenols or amino compounds. Phenolic acids are divided into two subgroups, which are the derivatives of hydroxybenzoic acid and hydroxycinnamic acid. The most important derivatives of hydroxybenzoic acid are ferulic acid, caffeic acid, and coumaric acid, which occur in trace amounts naturally in plants. The caffeic acid ester of d-quinic acid is one of the most important polyphenols naturally occurring in apples. Flavonoids are divided into five subgroups according to their chemical structure (Fig. 6.20). All flavonoids are derived from flavan (2-phenol-benzo-dihydropyran). The general structure (C6 C3 C6 ) is given in Fig. 6.21. Anthocyanidins Anthocyanidins are found in nature as glycosides and are called anthocyanins. Anthocyanins are natural color pigments, which have a color range varying from rich red to blue, the characteristic color of most fruits. Flavones and Flavonols Flavones and flavonols, which have slight yellow color, are practically found in all plants. Flavones differ from flavonols in the absence of OH-group at C3 -atom of the center ring. Flavonones Flavonones do not have a double bond at the center ring. Their glycosides are mainly found in citrus fruits, like naringin (Fig. 6.22).

6

.

Chemical Composition of Fruits and its Technical Importance

Anthocyanidins

Flavones and flavanols Flavanones Flavonoids

Found in nature as glycosides called anthocyanins

Familiar Color of red apples, blueberries, cherries, cranberries, strawberries, and plums

Flavones and flavonols (slight yellow in color— practically found in any plants)

Flavones differ from flavonols in the absence of OH-group at C3-atom of the center ring

Flavonones do not have a double bond at the center ring

Their glycosides are mainly found in citrus fruits

149

The most frequent catechins are diastereoisomer pair (+)– catechin and (–)–epicatechin, as well as (+)-gallocatechin and (–)–pigallocatechin

Catechins and leucoanthocyanidins

Catechins are very reactive if exposed to atmospheric oxygen

Proanthocyanidins

They can condense chemically and enzymatically into oligomers and polymers, forming proanthocyanidins

Figure 6.20. General classification of flavonoids (Source: Herrmann, 1976; Cook and Samman, 1996).

8

2

1 o 2 1

7 6

4 6

3 5

3

5

4 o

Figure 6.21. General structural formula of flavonoids.

O

Aa-L/ Rh(1-6)-b-Gl-O

OH

OH

O

Figure 6.22. Chemical structure of naringine.

Catechins and Leucoanthocyanidins Catechins are flavan-3-ol monomers comprising one OH-group at C3 -atom. The leucoanthocyanidins flavan-3,4-diols, have one OH-group at C3 - and C4 -atoms. The most frequent catechins are diastereoisomer pair (þ)-catechin and ()-epicatechin, as well as (þ)gallocatechin and ()-epigallocatechin. Catechins are very reactive if exposed to atmospheric

150

Fruit Manufacturing

oxygen. They can condense chemically and enzymatically into oligomers and polymers forming proanthocyanidins. Caramel. It is an amorphous dark brown-coloring material formed by heating saccharides, alone or in the presence of amino products or selected accelerators. Maillard-type reactions are involved and the products are extremely complex in composition. Detailed information on nonenzymatic browning is given in Chapter 7. Finally, proanthocyanidins are colorless if their chain is short. Yellowish to brown color is formed with increasing polymerization. When they are heated in acidic media, they transform into corresponding anthocyanidins, getting the typical reddish-violet color.

6.2. INFLUENCE OF PROCESSING AND STORAGE ON THE COMPOSITION OF FRUITS As soon as fruits are harvested, deterioration of quality attributes or nutrients begins and increases with time. Nutrients are lost by: (a) (b) (c) (d)

Food processing operations Sensitivity of nutrients to pH, oxygen, light, and heat Enzyme action Measures to control enzyme activity, e.g., blanching.

6.2.1. Vitamin Destruction During Processing and Storage Vitamin losses may occur in canned fruits when stored at high temperatures (>378C). Vitamin C (ascorbic acid) is probably the most unstable vitamin, and it is readily oxidized by many nonenzymatic processes. Although frozen storage temperatures between 18 and 288C result in satisfactory vitamin C retention levels in fruits, at temperatures above 108C, it is easily oxidized and will be drastically reduced in a short period of time. The use of package materials impermeable to oxygen and light is recommended. The enzyme ascorbate oxidase, which is not present to any great extent in vitamin C sources, causes oxidation. It was also found that the mixing of orange juice with mashed bananas naturally containing phenolase, act similar to ascorbate oxidase. All vitamins are subject to enzyme hydrolysis and the above illustrates the point. Particular food combinations can lead to nutrient loss. Benterud (1977) studied thermal stability of vitamins in a hot melt of carbohydrates free of oxygen. The author found some vitamins were extremely heat resistant (e.g., vitamin E, riboflavine), whereas thiamine was the most labile of the vitamins. Some vitamins (A, D, B12 , and C) show a gradual degradation as temperature is raised from 100 to 1308C. However, in fruit processing vitamin stability condition is more complicated because pH, oxygen, ions, reducing agents, etc. influence the rate of decomposition. Figure 6.23 shows the loss in canned fruit products. Canned or bottled fruit juice stored at ambient temperature for extended period is likely to lose all its vitamin C content (Cameron, 1978). The principal causes for vitamin C destruction in bottled juice are oxidation by residual air in the head space, anaerobic decomposition, and the effect of light. It was observed that after some fruit processing operation significant loss of ascorbic acid occurs (Nagy, 1980). Up to 47% loss in vitamin C occurred in canned fruits after two years’ storage at 278C. Adisa (1986) studied the influence of storage and molds on the ascorbic acid content of orange and pineapple fruits. He found that about 40% loss of ascorbic acid was recorded in both fruits stored at 308C for 8 weeks. The rate of loss of vitamin C was observed to be faster in fruits infected with mold than in healthy fruits.

6

.

Chemical Composition of Fruits and its Technical Importance

151

120

Vitamin,%retained

Vit.C 100

B6 Panth.acid

80 60 40 20 0

Furit juices

Grapefurit juice

Orange juice

Peaches

Apricots

Figure 6.23. Loss of vitamins C and B6 , and panthotenic acid in canned fruit and fruit juices (adapted from Benterud, 1977).

Maeda and Mussa (1986) indicated that ascorbic acid depletion with time in bottled and canned orange juice was almost linear. The ascorbic acid levels in canned orange juice stored for 8 weeks at room temperature were significantly higher than in bottled (glass) juice. Ascorbic acid is also present in relatively high concentration in raspberry. Ochoa et al. (1999) measured the percentage of residual ascorbic acid during long-term storage of raspberry pulp at 4, 20, and 378C (Fig. 6.24). These values are in general agreement with those

Ascorbic acid (%)

100

10

37ⴗC 20 ⴗ C 4ⴗC

1

56

52

48

44

40

36

32

28

24

20

16

12

8

4

0

0.1 Storage time(days) Figure 6.24. Semilogarithmic plot of the ascorbic acid reduction during storage of raspberry pulp at 48, 208 and 378C. Reprinted from Lebensm. Wilss. u Technol. 32(3): 149–153. Ochoa, M.R. Kesseler, A.G., Vullioud, M.B. and Lozano, J.E. Physical and chemical characteristics of raspberry pulp: storage effect on composition and color. (copyright) 1999, with permission from Elsevier.

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reported for other fruits and vegetables in the same range of temperatures (Villota and Hawkes, 1992). Ochoa et al. (1999) found that ascorbic acid decrease in raspberry pulp obeys the following linear regression equation: AA(%) ¼ 100ekt

(6:1)

where AA is the ascorbic acid content (%), k is the fitted reaction rate constant, and t is the storage time (days). Parameters and correlation coefficients of Eq. (6.1) are k37 C ¼ 0:0980 (r2 ¼ 0:978), k20 C ¼ 0:0424 (r2 ¼ 0:979), and k4 C ¼ 0:0275 (r2 ¼ 0:995). Calculated values of the rate constant, k, were temperature dependent and the effect of this variable was calculated according to the Arrhenius model. The calculated activation energy for ascorbic acid degradation in raspberry pulp was Ea ¼ 6:45 kcal=mol (r2 ¼ 0:970). 6.2.2. Effect of Storage on Metal Content Acid liquid foods, as many fruit juices are, interact with the components of the container. In the case of canned juices, corrosion of tinplate increases the heavy metal content, especially tin, lead, and iron. Table 6.7 lists the increase in heavy metal content in canned orange juice. Packing of fruit juice in tin cans causes a higher contamination with heavy metals than that in paperboard boxes or laminated pouches. 6.2.2.1. Influence of Storage on Fruit Juice Aroma The effect of storage condition on fruit juice aroma has been extensively studied. Velez et al. (1993) studied changes of orange juice aroma due to storage time and temperature, by gas chromatographic analysis of nearly 40 volatile constituents. The authors found that at 08C changes in orange volatile were very slow and difficult to detect. Contrarily, at 358C the quality of the orange aroma deteriorates very fast. Figure 6.25 shows orange aroma deterioration of the principal volatile, at 208C after 3 months’ storage. During aroma stripping of the single-strength juice when the amount of evaporated water is in the 10 –12% range, up to 90% of the aroma compound evaporates. Moreover, only a slightly greater evaporation of volatile may be achieved by doubling the amount of water evaporated. Ve´lez et al. (1993) also found that only 7 out of 38 volatile were significantly affected by storage time and temperature.

Table 6.7. Heavy metal content of orange juice when affected by type containers. Metal Tin Lead Iron Zinc Copper

Fresh fruit

Tin can

Other containers

– – 0.1 0.07 0.045

55 0.16 2.75 1.05 0.09

0.4 0.1 0.16 0.12 0.03

Adapted from Mesallam, 1987.

6

.

Chemical Composition of Fruits and its Technical Importance

153

Acetoin Acetoin Terpinolene

Volatile (mg/L)

Linalool Alfa-pinene Ethyl butyrate Octanal Terpinene-4-ol

Initial 1 month

Dodecanal

2 months

Citronellon 3 months 0

1

2

3

Figure 6.25. Changes in the main orange volatile after 3 months’ storage at 208C (adapted from Kirchner and Miller, 1957; Petersen et al., 1998).

6.2.3. Fruit Juice Change in Amino Acid Content During Storage During storage fruit juices are exposed to temperatures that have an adverse influence on quality. In these fruit juices the major constituents believed to be involved in browning are the reducing sugars, amino acids, polyphenols, and organic acids (Joslyn, 1956; Cornwell and Wrolstad, 1981). Wolfrom, Kashimura, and Norton (1974) studied browning mixtures constituted by different amino acids and glucose in 1:1 molar ratio simulating orange juice stored at 658C and reported that g-aminobutiric acid and l-arginine were the main contributors to browning. del Castillo et al. (1998) found a loss of 65.1% of original amino acids’ content, when storing dehydrated orange juice (aw ¼ 0:44) at 508C for 14 days. About 78% of the loss was attributed to the amino acids in major proportions, namely proline, arginine, asparagine, and g-aminobutiric acid. Babsky et al. (1986) evaluated changes in the composition of clarified apple juice concentrate during prolonged storage at 378C. Results showed that storage caused an 87% loss in the total free amino acids, which was mostly due to decreases in glutamic acid, asparagine, and aspartic acid (Fig. 6.26). The major constituents were asparagine (Asn), aspartic acid (Asp), and glutamic acid (Glu). The other individual amino acids amounted to less than 10%. These values are similar to those found by Burroughs (1957) and Czapski (1975). Bielig and Hofsommer (1982), working with about 90 samples of apples, apple juices, and concentrates, found that every apple juice has a characteristic amino acid spectrum and no mean value can be specified. The concentration changes of total amino acids during storage of apple juice were very large (Fig. 6.27). Asp and Glu decreased more markedly. Several studies (Warmbier et al.,

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Total % Retention

400

Amino acids (mg/L)

350 300 250 200 150 100 50 0 0

50

100

150

Time (days) Figure 6.26. Free amino acid composition of apple juice concentrate (758Brix). Variation during storage at 378C (adapted from Babsky et al., 1986).

350 Asn 300

Glu

AA (mg/L)

250

Asp

200 150 100 50 0 0

20

40

60 Time (days)

80

100

120

Figure 6.27. Decreases in glutamic acid, asparagine, and aspartic acid (adapted from Babsky et al., 1986).

1976; Spark, 1969; Eichner and Karel, 1972; Reyes et al., 1982) reported Maillard browning of reducing sugars with only one or two amino acids other than Asp, Asn, and Glu. Wolfrom et al. (1974) and Ashoor and Zent (1984) studied the influence of different amino acids in model systems. None of these studies have shown Glu and Asn to be very high browning producing compounds.

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Buedo et al. (2001) studied the change of free amino acid (AA) composition of peach juice concentrate (PJC), as a result of the Maillard reactions. The authors observed that total AA content decreased 8, 35, and 60%, after 112 days of storage at 15, 30, and 378C, respectively (Fig. 6.28). The main constituent, asparagine, contributed 71% of the total loss while aspartic acid increased its concentration, probably as a result of the asparagine degradation. Buedo et al. (2001) also found that during storage of peach juice at 378C glutamine concentration drops 60 fold, while alanine reduces only to half at the same time and conditions. However, it must be taken into account that the contribution of each AA to the juice browning is not directly related to the consumption rate, as each AA can produce different chromophores, having different light absorbance characteristics (Labuza and Baisier, 1992). 6.2.4. Effect of Storage on Fruit Sugars Sucrose in an acid media, as many fruit products are, can hydrolyze under a rate corresponding to a first-order process (Babsky et al., 1986). The reducing sugars increased at a rate determined by the inversion of sucrose. It is well known that the rate of hydrolysis is a function of the concentration of reactants, temperature, and acid–catalyst concentration (Glasstone, 1946). If excess water is present the rate of disappearance of sucrose can be represented by a pseudo first-order reaction rate equation: S ¼ S0 exp (Kt)

(6:2)

where S0 is the initial sucrose concentration, moles/100 g concentrate, S is the sucrose concentration at time t; K is the rate constant (0:00822 day1 under studied conditions), t is the time, min. Hydrolysis, also called inversion because it is accompanied by an inversion of the angle of polarization, yields two simple sugars, d-glucose and d-fructose. The rate of appearance representing total reducing sugars is described by Eq. (5.3): R ¼ 2So (1 eKt ) þ Ro

(6:3)

where: R ¼ reducing sugars (glucose þ fructose) concentration at time t moles/100 g concentrate, Ro is the reducing sugar concentration at t ¼ t0 ; and t is time, mm.

AA (mg/L) after 11 days, storage

6000 Peach juice: 12ⴗBrix

5000 4000 3000 2000 1000 0 Initial

37ⴗC

30ⴗC

15ⴗC

Figure 6.28. Decrease in PJC’s total AA content during storage at 37, 30, and 158C (adapted from Buedo et al., 2001).

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Schoebel et al. (1969) obtained experimental data on the dependence of the first-order reaction rate on pH. Figure 6.29 shows the development of sucrose and total reducing sugars during storage of concentrated apple juice, which increased in concentration in accordance with the predicted kinetics (Eq. 6.3). Hence, hydrolysis appeared to be the major cause of sucrose reduction (and reducing sugars increase) in apple juice at a rate determined by pH and temperature. Akhavan and Wrolstad (1980) verified that slight losses (6%) in total sugars occur after 112 days of storage at 378C of pear concentrate. Stadtman (1948) considered the possibility that relatively small chemical changes are required to produce brown pigment of intense color. If this is the case, the changes in reducing sugars necessary to produce large changes in color might be hard to be detectable. Beveridge and Harrison (1984) detected no loss of reducing sugar after heating 72.50 Brix-pear juice at temperatures up to 808C for 2 h. Reyes et al. (1982) found that glucose undergoes more browning than fructose with glycine at 608C and pH 3.5. Any detectable variation in the fructose/glucose ratio may indicate unbalanced consumption of these reducing sugars due to nonenzymatic browning reaction.

0,4

0,35

0,3

Sugar (mol/100 g)

0,25

Reducing sugars

0,2

Sucrose

0,15

0,1

0,05

0 0

20

40

60 Time (days)

80

100

120

Figure 6.29. Sucrose hydrolysis and increase in reducing sugars in apple juice as a function of time of storage, at 378C (from Babsky et al., 1986 with permission).

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6.2.5. Effect of Processing and Storage on Fruit Pigments Changes in fruits and fruit products color are extensively considered in Chapter 7. However, some aspects on fruit pigment degradation are revised here. Pigments may oxidize resulting in color fading of highly colored canned fruits. As carotenoids are highly sensitive to oxygen and light, particularly in the presence of metals such as iron, copper, and manganese, processing and storage can produce carotenoid degradation. Anthocyanins show low stability in products manufactured from fruits. Moreover, the stability of these natural pigments is poor in dehydrated fruits, unless packaged in inert gas. Temperature, light, and initial composition of fruits are considered as responsible factors for the instability of fruit anthocyanins. For many apple varieties, red skin color is important for marketability. Apple harvest is largely based on the amount of red color consistent with the natural tendency of the variety. Unfortunately, this may not be the best time to harvest for optimum quality after storage. Whereas color development of fruit maturing on the tree generally increases with time, the fruit also begins to ripen and will not store as well. Different apple varieties showed different storage temperature optima for red color development. Precooling the tissue for 48 h at 28C (to simulate cold nights) increased the amount of red pigment that accumulated. 6.2.6. Changes in Organic Acid Content The role of organic acids appears to be essentially catalytic (Reynolds, 1965). Reduction of organic acids in apple juice was only 9% (Babsky et al., 1986). The slight decrease in acidity might be partly due to copolymerization of organic acids with products of the browning reactions. Lewis et al. (1949) also suggested that organic acids can react with reducing sugars to produce brown pigments. Sample pH did not change during storage, keeping its initial value of 3.72 + 0.02 almost constant. Urbicain et al. observed that the titratable acidity in peach juice rose with time and temperature. Major organic acids present in stone-free peaches are malic, citric, and quinic (Wang et al., 1993). The basic amino groups disappear during Maillard reaction, hence pH lowers as the reaction proceeds in systems with no buffers (Spark, 1969). In the case of peach juice, the organic acids present in major proportion act as a strong buffer, hence no variations of pH may be expected. Moreover, the consumption of AA would increase titrable acidity. Spark (1969) has reported that buffers increase browning rate, which boosts the color damage in concentrated peach juice. 6.2.7. Changes in Phenolic Compounds Phenolic compounds present in fruit products may react to form brown polymeric compounds (Abers and Wrolstad, 1979). If this reaction plays any role in the color development of apple juice, total phenolic content will not increase during storage as Babsky et al. (1986) found, using the Folin–Ciocalteau reagent (Singleton and Rossi, 1965). Cornwell and Wrolstad (1981) proposed that reductone compounds present in the juices interfere with the Folin–Ciocalteau reagent increasing the apparent phenolic contents. Market demands for ‘‘natural’’ juices and pulps devoid of food additives have prompted food scientists to study the quality deterioration of fruits during processing and storage. The raspberry (Rubus ideaus) is a bush fruit of the rosaceous family, whose economic importance is increasing because of the use of raspberry products in the food industry.

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Major problems confronted in the production of raspberry pulp include operations that may affect their properties. The color of raspberry fruits is not significantly affected by freezing and cold storage. However, when crushed, most fruit berries yield a highly pectinous pulp, releasing little free run juice with poor color stability on storage. Anthocyanins, which are responsible for color in most berries, easily degrade following various reaction mechanisms affected by oxygen, ascorbic acid, pH, and temperature among other variables (Abers and Wrolstad, 1979; Skrede, 1985). Ochoa et al. (1999) found that total anthocyanin (TA) pigment in raspberries decreased significantly through storage, at a rate strongly dependent on temperature. After 40 days, pulp stored at 378C had lost the majority of the anthocyanins. Semilogarithmic plots of percentage of residual anthocyanin during long-term storage of raspberry pulps were linear (Fig. 6.30), showing that decrease followed first-order reaction kinetics, in accordance with previous findings in other berries (Skrede, 1985). It was suggested that anthocyanins may be destroyed either through direct oxidation by quinones formed from catechin by PPO action or through copolymerization of anthocyanins into brown pigments (tannins) formed from catechin–quinone polymerization (Jackman et al., 1987). Pigment instability is an undesirable consequence of processing of canned syrup strawberries and products that contain them (Garcı´a-Viguera et al., 1999). Processing was found to cause a 50% decrease in the flavanol concentration and the formation of a polar compound. The conversion of leucoanthocyanidin to anthocyanin when heated at acidic condition (Lee and Wicker, 1991) was responsible for the pink discoloration in canned fruits like lychee. A number of researchers have shown that the rate of ascorbic acid oxidation influences total anthocyanin loss in strawberry products. Loss of natural color was reported to be affected by AA content by Skalsky and Sistrunk (1973). However, studies on the effect of ascorbic acid on the destruction of anthocyanin pigment were in general carried out at elevated temperatures and under relatively low storage temperatures. No significant correlations were found between ascorbic acid content and any of the other quality factors.

% Antocyanin

100

10 37 ⬚C 20⬚C 4⬚C

1 0

10

20

30 40 Storage time (days)

50

60

Figure 6.30. Percentage of retention of anthocyanin in heritage raspberry pulp at 4, 20, and 378C during storage (from Ochoa et al., 1999). Reprinted from Lebensm. Wilss. u Technol. 32(3): 149–153. Ochoa, M.R. Kesseler, A.G., Vullioud, M.B. and Lozano, J.E. Physical and chemical characteristics of raspberry pulp: storage effect on composition and color. (copyright) 1999, with permission from Elsevier.

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REFERENCES Abers, J.E. and Wrolstad, R.E. (1979). Causative factors of color deterioration in strawberry preserves during processing and storage. J. Food Sci. 44: 75–78. Adisa, V.A. (1986). The influence of molds and some storage factors on the ascorbic acid content of orange and pineapple fruits. Food Chem. 22: 139–146. Akhavan, I. and Wrolstad, R.E. (1980). Variation of sugars and acids during ripening of pears and in the production and storage of pear concentrate. J. Food Sci. 45: 499–506. Anonymous. (1992). Hot topic: food guide pyramid replaces the basic 4 circle. Food Technology, 46(7): 64 –67. Ashoor, S.H. and Zent, J.B. (1984). Maillard browning of common amino acids and sugars. J. Food Sci. 49: 1206– 1211. Babsky, N., Toribio, J.L. and Lozano, J.E. (1986). Influence of storage on the composition of clarified apple juice concentrate. J. Food Sci. 51: 564 –567. Benterud, A. (1977). Vitamin losses during thermal processing. In Physical, Chemical and Biological Changes in Foods Caused by Thermal Processing, Hoyem, T. and Kvale, O. (eds.). Applied Science Publishers Ltd, Essex, UK, pp. 185–201. Beveridge, T. and Harrison, J.E. (1984). Nonenzymatic browning in pear juice concentrate at elevated temperatures. J. Food Sci. 49: 1335–1339. Bielig, H.J. and Hofsommer, H.J. (1982). On the importance of the amino acid spectra in apple juices. Flussiges Obst. 2: 50–56. Buedo, A.P., Elustondo, M.P. and Urbicain, M.J. (2001). Amino acid loss in peach juice concentrate during storage. Innov. Food Sci. Emerg. Technol. 1: 281–288. Buglione, M.B. (2005). Chemical Changes During Grape Juice Processing and Storage. Doctoral Thesis. Universidad Nacional del Sur, Bahı´a Blanca, Argentina. Burroughs, L.F. (1957). The amino-acids of apple juices and ciders. J. Sci. Food Agric. 3: 122. Cameron, D.J. (1978). Variation on storage of ascorbic acid levels in prepared infant feeds. Food Chem. 3(2): 103–110. Carelli, A.A. and Lozano, J.L. (1989). Apple aroma from Argentina: quality evaluation by capillary gas chromatography. HRC CC 12: 488– 490. Carrı´n, M.E. Ceci, L. and Lozano, J.E. (2004). Characterization of starch in apples and its degradation with amylases. Food Chem. 62: 215–223. Chan, H.T. (1994). Passion fruit, papaya and guava juices. In Fruit Juice Processing Technology, Nagy, S., Chen, C.S. and Shaw, P.E. (eds.). Agscience, Inc., Auburndale, FL, USA, pp. 378– 435. Chen, S.C. (1992). Physicochemical principles for the concentration and freezing of fruit juices. In Fruit Processing Technology, Nagy, S., Chen, C.S., and Shaw, P.E., Editors. Agscience, Inc., Auburndale, Florida. 23–25. Cook, N.C., and Samman, S. (1996). Flavonoids – chemistry, metabolism, cardioprotective effects, and dietary services. J. Nutr. Biochem. 7: 66–76. Cornwell, C.J. and Wrolstad, R.E. (1981). Causes of browning in pear juice concentrate during storage. J. Food Sci. 46: 515–519. Czapski, J. (1975). Wplw wolnych aminokwasdw na zmiany jakdsci zageszcconych sokdw joblkowych podczas prcechownwania. Prezem. Ferm. Ilolny 9: 19–26. Dauthy, M.E. (1995). Fruit and vegetable processing. Fao Agricultural Services Bulletin. 119 Food and Agriculture Organization of the United Nations, Rome. In: http://www.fao.org/documents del Castillo, M.D., Corzo, N., Polo, M.C., Pueyo, E. and Olano, A. (1998). Changes in amino acid composition of dehydrated orange juice during accelerated nonenzymic browning. J. Agric. Food Chem. 46: 277–280. Dennis, C. (ed.) (1983). Post-harvest Pathology of Fruit and Vegetables. Academic Press, San Diego, CA. Eichner, K. and Karel, M. (1972). The influence of water content on the amino browning reaction in model systems under various conditions. J Agr. Food Chem. 20: 218–223. Friend, J. (ed.) (1982). Recent advances in Biochemistry of Fruits and Vegetables. Academic Press, San Diego. Garcı´a-Viguera, C., Zafrilla, P., Arte´s, F., Romero, F., Abella´n, P., Toma´s-Barbera´n, F.A. (1999). Colour and anthocyanin stability of red raspberry jam. J. Sci. Food. Agric. 78(4): 565–573. Glasstone, S. (1946). Textbook of Physical Chemistry. D. Van Nostrand, Princeton, NJ. Goodenough, P.W. and Atkin, R.K. (eds.) (1981). Quality in Stored and Processed Vegetables and Fruit. Academic Press, New York, NY, 398 pp. Herrmann, K. (1976). Flavonols and flavones in food plants: a review. J Food Technol. 11: 433–448. Hui, Y.H. (1991). Data Sourcebook for Food Scientists and Technologists. VCH Publisher, Inc., New York, pp. 331–410. Jackman, R.L. Yada, R.Y., Tung, M. and Speers, R.A. (1987). Anthocyanins as food colorants: a review. J. Food Biochem. 11: 201–247.

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Jackson, J.M. and Shinn, B.M. (1979). Fundamentals of Food Canning Technology. AVI Publishing Company, Westport, CT. Joslyn, M.A. (1956). Role of amino acids in the browning of orange juice. Adv. Food Res. 22: 1–9. Kirchner, J.G., Miller, J.M. (1957). Canning and storage effects, volatile water-soluble and oil constituents of valencia orange juice. J.Agric. Food Chem. 5: 283–288. Konja, G., and Lovric, T. (1993). Berry Fruit Juices. In Fruit Juice Processing Technology, Ed. By Seven Nagy, Chin Shu Chen and Philip E. Shaw, Agscience, Inc. Auburndale, Florida. Labuza, T.P. and Baisier, W.M. (1992). The kinetics of nonenzymatic browning. In Physical chemistry of foods, Schwartzberg, H.G. and Hartel, R.W. (eds.). Marcel Dekker, New York, pp. 595–647. Lee, H. and Wicker, L. (1991). Anthocyanin pigments in the skin of lychee fruit. J. Food. Sci. 56: 466–468. Lewis, V.M., Esselen, W.B. and Fellers, C.R. (1949). Nonenzymatic browning of foodstuffs. Nitrogen free carboxylic acids in the browning reaction. Ind. Eng. Chem. 41: 2591–2599. Maeda, E. and Mussa, D. (1986). The stability of vitamin C (l-ascorbic acid) in bottled and canned orange juice. Food Chem. 22: 51–58. Mesallam, A.S. (1987). Heavy metal content of canned orange juice as determined by direct current plasma atomic emission spectrophotometry (DCPAES). Food Chem. 26(1): 47–58. Mc Lellan, R. and Acree, T.E. (1992). Grape juice. In Fruit Juice Processing Technology, Nagy, S., Chen, C.S. and Shaw, P.E. (eds.). Agscience, Inc., Auburndale, FL, USA, pp. 318–333. Nagy, S. (1980). Vitamin C contents of citrus fruit and their products: a review. J. Agric. Food Cherm. 28(1): 8–18. Nagy, S., Shaw, P.E. and Wardowski, W.F. (1990). Fruits of Tropical and Subtropical Origin. Composition, Properties and Uses. FSS, Florida Science source, Inc., Lake Alfred, FL, USA. Nagy, S., Chen, C.S. and Shaw, P.E. (1992). Fruit Processing Technology. Nagy, S., Chen, C.S. and Shaw, P.E., (eds.) Agscience, Inc., Auburndale, Florida. Oakenfull, D.G. (1991). The chemistry of high-methoxyl pectins. In The Chemistry and Technology of Pectin. R.H. Walter Ed. Academic Press Inc., San Diego, CA. 87–108. Ochoa, M.R. Kesseler, A.G., Vullioud, M.B. and Lozano, J.E. (1999). Physical and chemical characteristics of raspberry pulp: storage effect on composition and color. Lebensm. Wiss. u Technol. 32(3): 149–153. Petersen, M.A. Tønder, D. and Poll, L. (1998). Comparison of normal and accelerated storage of commercial orange juice. Changes in flavour and content of volatile compounds. Food Quality Pref. 9(1–2): 43–51. Reed, G. (1975). Enzymes in Food Processing, 2nd ed. Academic Press, London. Reyes, F.G.R., Poocharoen, B. and Wrolstad, R.E. (1982). Maillard browning reaction of sugar-glycine model systems: changes in sugar concentration, color and appearance. J. Food Sci. 47: 1376–1380. Reynolds, T.H. (1965). Chemistry of nonenzymatic browning II. Adv. Food Res. 14: 167–171. Salunkhe, D.K., Bolin, H.R. and Reddy, N.R. (1991). Storage, Processing, and Nutritional Quality of Fruits and Vegetables, 2nd ed., Vol. 1: Fresh Fruits and Vegetables (323 p.) and Vol. 2: Processed Fruits and Vegetables (195 pp.). CRC Press, Boca Raton, FL. Sanchez-Castillo, C.P., Dewey, P.J.S., Lara, J.J., Henderson, D.L., de Lourdes Solano and W. James, W.P. (2000). The Starch and sugar content of some mexican cereals, cereal products, pulses, snack foods, fruits and vegetables. J. Food Comp. Analysis, 13: 157–170. Schoebel, T., Tannenbaum, S.R. and Labuza, T.P. (1969). Reaction at limited water concentration. 1. Sucrose hydrolysis. J. Food Sci. 34: 324 –329. Singleton, V.I. and Rossi, J.A. (1965). Colorimetry of total phenolics with phosphomolybdic–phosphotungstic acid reagents. Am. I. Enol. Vitcul. 16: 144 –151. Skalsky, C. and Sistrunk, W.A. (1973). Factors influencing color degradation in concord grape juice. J. Food Sci. 38: 1060–1066. Skrede, G. (1985). Color quality of blackcurrant syrups during storage evaluated by Hunter L0 , a0 , b0 values. J. Food Sci. 50: 514–525. Somogy, L.P., Ramaswamy, H.S. and Hui, Y.H. (1996). Processing Fruits: Science and Technology, Vol. 2, Major Processed Products. Technomics Publishing Company, Inc., Lancaster, PA, USA. Spark, A.A. (1969). Role of amino acids in nonenzymic browning. J. Sci. Food Agric. 20: 308–312. Stadtman, E.R. (1948). Nonenzymatic browning in fruit products. Adv. Food Res. 1: 325–331. Swi-Bea Wu, J., Ming-jen Sheu and Tzuu-tar Fang (1992). Oriental fruit juices: carambola, Japanese apricot (Mei), lychee. In Fruit Juice Processing Technology, Nagy, S., Chen, C. S. and Shaw, P.E. (eds.). Agscience, Inc., Auburndale, FL, USA. USDA (1992). U. S. Development of Agriculture, Human Nutrition Information Service. The Food Guide Pyramid. Home and Garden Bulletin No. 252, Washington, D.C.: Government Printing Office, August.

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USDA (2005). My pyramid. U.S. Department of Agriculture (USDA) and U.S. Department of Health and Human Services (HHS), January. http://www.nal.usda.gov/fnic/Fpyr/pyramid.html Velez, C., Costell, E., Orlando, L., Nadal, M.I., Sendra, J.M. and Izquierdo, L. (1993). Multidimensional scaling as method to correlate sensory and instrumental data of orange juices aromas. J. Sci. Food Agric. 61: 41– 46. Villota, R. and Hawkes, J.G. (1992). Reaction kinetics in food systems. In Handbook of Food Engineering, Heldman, D.R. and Lund, D.B. (eds.). Marcel Dekker, Inc., New York, pp. 65 –72. Wang, T., Gonzalez, A.R., Gbur, E.E. and Aselage, J.M. (1993). Organic acid changes during ripening of processing peaches. J. Food Sci. 58, 631–632. Warmbier, H.C., Schnickels, R.A. and Labuza, T.P. (1976. Nonenzymatic browning kinetics in an intermediate moisture model system. Effect of glucose to lysine ratio. J. Food Sci. 41: 981–985. Watt, B.K. and Merrill, A.L. (1963). Composition of Foods: Raw; Processed; Prepared. Agriculture Handbook No. 8. Consumer and Food Economics Research Division, Agricultural Research Service, USDA, Washington, DC. Wills, R.B.H. (1987). Composition of Australian fresh fruits and vegetables. Food Technol. Australia 39(11): 523–530. Wills, R.B.H., McGlasson, W.B., Graham, D., Lee, T.H. and Hall, E.G. (1989). Post harvest-An Introduction to the Physiology and Handling of Fruits and Vegetables. AVI Book, Van Nostrand Reinhold, New York. Wolfrom, M.L., Kashimura, N. and Horton, D. (1974). Factors affecting the Maillard browning reaction between sugars and amino acids. Studies on the nonenzymatic browning of dehydrated orange juice. J. Agr. Food Chem. 22: 796 –800.

CHAPTER 7

FRUIT PRODUCTS, DETERIORATION BY BROWNING 7.1. INTRODUCTION Food processing, defined by the predictability of the product–process interactions, is turning into a science. Processing and storage of fruit products affect composition in many ways. How to develop the best process knowledge of the reaction and a description of the engineering processes involved, the latter already studied in previous chapters, is required. Knowledge of deterioration factors, including the rates of deterioration, means that it is possible to find ways of lowering or stopping those deteriorative actions, thereby gaining fruit preservation. In order to maintain their nutritional value and organoleptic properties and because of technical–economical considerations, not all the identified methods against deterioration actually have practical applications. 7.1.1. Different Mechanisms of Deterioration Appearance, which is significantly impacted by color, is one of the first attributes used by consumers in evaluating food quality. Color may be influenced by naturally occurring pigments such as chlorophylls, carotenoids, and anthocyanins in fruits; or by pigments resulting from browning reactions. Browning of fruits and fruit products is one of the major problems in the fruit industry and is believed to be probably the first cause of quality loss during postharvest handling, processing, and storage. Browning can also adversely affect flavor and nutritional value. Extraction of fruit juices, or elaborating fruit pulps and pure´es, requires the grinding of fruit, which results in the rupturing of the fruit cells and the mixing of the fruit components with the atmospheric oxygen. The same is valid for the cutting of fruits before dehydration. This incorporation of oxygen into the fruit pulp, or on the surface of the cut fruit, causes the oxidation of the phenolic compounds to quinones, which are naturally present in the fruit tissue. The endogenous enzyme polyphenoloxidase (PPO) catalyzes this oxidation, known as enzymatic browning (EB). EB is one of the most devastating reactions for many exotic fruits, in particular tropical and subtropical varieties. It is estimated that over 50% loss in fruits occurs as a result of EB (Whitaker and Lee, 1995). Projected increases in fruit markets will not occur if EB is not understood and controlled. On the other hand, browning reactions of nitrogen compounds, mainly free amino acids and proteins, with carbohydrates cause deterioration by color and off-flavor development during processing and storage of fruits. These deteriorative reactions are generally known as nonenzymatic browning (NEB) reactions. However, browning reactions in fruits are more 163

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complex than suggested by the simple classification as enzymatic or nonenzymatic, because of the large number of secondary reactions that may occur. This is reflected in the range of color produced even in the same product (e.g., raspberries may develop red or brown discolorations; Ochoa et al., 1999). Browning may occur in some fruits in which endogenous ascorbic acid (AA) is oxidized to dehydroascorbic acid (DHAA), which then reacts with free amino acids to yield deep brown colors by the Maillard reaction (Kacem et al., 1987). These deteriorative reactions need to be described in some more detail. Much research has been concentrated in recent years to find effective and economical ways to prevent browning in various fruit products. Concerted efforts have been made to understand the basic biochemistry involved in enzymatic browning reactions and to find practical techniques to prevent the browning reactions in fresh and processed products. On the other hand, the complexity of EB and NEB reactions, and the various compounds involved (Hodge, 1953; Spark, 1969) makes it difficult to study the reactions by means of a simple analytical chemical method. However, kinetic approach can be used in solving the problem.

7.2. ENZYMATIC BROWNING EB occurs in fruits after bruising, cutting, or during storage, and its control during the processing of fruits is of great importance to fruit manufacturing. EB is a significant problem in apples, pears, bananas, peaches, and grapes, particularly. Acceptability of browning also depends on the product: . .

In clarified fruit juice, like apple juice, a little browning is accepted and the typical amber-like hue is commercially desirable. However, both apple pure´e and cloudy juice must retain the yellowish or greenish color, which characterizes the fresh product.

It must be remembered that enzymatic problem is not always a problem to be avoided: the color of products such as raisins and prunes is obtained thanks to a controlled PPO reaction (Va´mos-Vigya´zo´, 1981). 7.2.1. Phenolic Compounds and Oxidases POP is an example of an enzyme that can lower the quality of a food product by catalyzing the oxidation of phenolic compounds. The susceptibility to browning may depend on PPO activity and/or phenolic content (Coseteng and Lee, 1987). Polyphenol oxidase catalyzes the initial step in the polymerization of phenolics to produce quinones, which undergo further polymerization to insoluble dark brown polymers known as melanins. These melanins form barriers and have antimicrobial properties, which prevent the spread of infection or bruising in plant tissues. The formation of yellow and brown pigments in fruit products during EB reactions is controlled by the levels of phenols, the amount of PPO activity, and the presence of oxygen (Spanos and Wrolstad, 1992). The phenolic composition of apple, pear, and white grape juices was reviewed by Spanos and Wrolstad (1992) (see also Chapter 6). These authors classified the phenolic constituents of importance in fruit juices into two groups: (a) phenolic acids and related compounds, and (b) flavonoids.

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During the browning of fruit tissue, the enzyme PPO, also called orthodiphenol oxidase or catecholase, catalyzes the oxidation of phenolic compounds related to catechol and containing two o-dihydroxy groups to the corresponding o-quinone (Joslyn and Ponting, 1951; Vamos-Vigyazo, 1981). Relatively few of the phenolic compounds in fruits serve as substrates for polyphenol oxidase (Table 7.1). Compounds with minor differences may or may not be substrates for polyphenol oxidase. For example, Shannon and Pratt (1967) found that when comparing quercetin and dihydroquercetin, differing only in the bonding between carbons at the 2 and 3 positions, only the latter was a substrate for apple polyphenol oxidase. It was assumed that quercetin is more stable than dihydroquercetin, due to the presence of a double bond conjugated to an aromatic ring, affecting compound solubility. Most raw fruits contain polyphenols and PPOs, located in different compartments in the cell structure. When through damaging or processing (e.g., milling) enzyme, substrates and oxygen come into contact with each other, and a lot of reactions start that finally lead to the formation of insoluble brown pigments (melanins). The EB of fruit and vegetables is always considered as a quality loss of both fresh and processed food products. Simple representation of EB reactions is given in Fig. 7.1. 7.2.2. Kinetics of Enzymatic Browning PPO activity, as for most of enzymes, may be minimized by reducing agents, heat inactivation, lowering the pH of the fruit product, and the presence of enzyme inhibitors, among other techniques, which are reviewed in Chapter 8. To effectively inhibit or control the EB in fruit products, an accurate determination of the kinetics of these catalyzed-oxidative reactions is required. The kinetics of deterioration can be followed through color measurements, which is a simple and effective way for studying the phenomenon. The substrate specificity of polyphenol oxidase varies in accordance with the source of the enzyme. Phenolic compounds and polyphenol oxidase are in general directly responsible for EB reactions in damaged fruits, during postharvest handling and processing. The relationship of

Table 7.1. Phenolic substrates of PPO in fruits. Fruit

Phenolic substrates

Apple

Chlorogenic acid (flesh), catechol, catechin (peel), caffeic acid, 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxy benzoic acid, p-cresol, 4-methyl catechol, leucocyanidin, p-coumaric acid, flavonol glycosides Isochlorogenic acid, caffeic acid, 4-methyl catechol, chlorogenic acid, catechin, epicatechin, pyrogallol, catechol, flavonols, p-coumaric acid derivatives 4-methyl catechol, dopamine, pyrogallol, catechol, chlorogenic acid, caffeic acid, DOPA 3,4-dihydroxyphenylethylamine (dopamine), leucodelphinidin, leucocyanidin Chlorogenic acid, caffeic acid, coumaric acid, cinnamic acid derivatives Catechin, chlorogenic acid, catechol, caffeic acid, DOPA, tannins, flavonols, protocatechuic acid, resorcinol, hydroquinone, phenol Dopamine-HCl, 4-methyl catechol, caffeic acid, catechol, catechin, chlorogenic acid, tyrosine, DOPA, p-cresol Chlorogenic acid, pyrogallol, 4-methyl catechol, catechol, caffeic acid, gallic acid, catechin, dopamine Chlorogenic acid, catechol, catechin, caffeic acid, DOPA, 3,4-dihydroxy benzoic acid, p-cresol Chlorogenic acid, catechin, caffeic acid, catechol, DOPA

Apricot Avocado Banana Eggplant Grape Mango Peach Pear Plum

Adapted from Marshall, Kim and Wei, 2000.

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Fruit Manufacturing OH O

HO

OH

PPO

HO

OH OH

OH

O O

PPO. O2

O

O

HO

OH OH

OH OH

OH

OH O

HO

OH OH

o-dihydroxyphenol

OH

o-quinone

Melanins

Figure 7.1. Simplified mechanism for the transformation of a diphenol to dark colored melanins by PPO.

the rate of browning to phenolic content and polyphenol oxidase activity, has been reported for various fruits such as apples (Coseteng and Lee, 1987), grapes (Lee and Jaworski, 1988), and peaches (Lee et al., 1990). In addition to serving as polyphenol oxidase substrates, phenolic compounds act as inhibitors of polyphenol oxidases (Walker, 1995). Their inhibitory action decreased in the following order: cinnamic acid > p-coumaric acid > ferulic acid > benzoic acid. Although relatively few of the phenolic compounds in fruits serve as substrates for polyphenol oxidase, as catechins, cinnamic acid esters, 3,4-dihydroxy phenylalanine (DOPA), and tyrosine (Table 7.1), the stecheometry of complex reactions like EB in fruits as substrate is practically unknown. Therefore, instead of determination of consumption of reactives (phenols), or formation of products (melanins), the kinetics of color development is commonly used for studying the browning reactions. As found by Sapers and Douglas (1987), tristimulus reflectance values were strongly nonlinear, and changes in the rate of browning may be better understood when plotting colorimetric parameters against log time. EB in Golden Delicious apple juice was monitored by measuring CIE L value (Lozano et al., 1994). The authors observed a significant influence of degree of ripeness on the rate of EB. Pulp made with unripe (GA) apples browned at a faster rate. This behavior was attributable to differences in AA content and PPO activity in young fruits. Figure 7.2

70

T = 5˚C OA MA GA

65

CIE L*

60 55 50 45 40 35 30 1

10 Time, min

100

Figure 7.2. Relationship of CIE ¼ l value in Golden Delicious apple pulp to time and degree of ripeness at 58C. GA stands for green apple, MA for mature apple, and OV for overmature apple (Lozano et al., 1994 with permission).

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shows the variation of CIE L parameter as a function of log time with degree of ripeness of processed apple as a parameter, at constant temperature. The rate of luminosity decrease can be divided into three periods: (i) the first period, characterized as an induction or flat period, attributable to the inhibition action of the naturally occurring ascorbic acid (Ponting and Joslyn, 1948), (ii) the second period, which looks linear when represented in this semilog plot, attributed to the consumption of the enzymes’ substrates (Sapers and Douglas, 1987), and (iii) the third period that approaches a plateau at a time depending on the degree of ripeness of apples. It was also observed (Fig. 7.2) that the lower induction time corresponded to unripe fruits. Koch and Bretthauer (1956) also found considerable seasonal variations in the amount of AA and DHAA in apples. Another relevant information given by Koch and Bretthauer (1956) is that PPO activity is greatest in young fruits than in fully ripe apples. Lozano et al. (1994) found that a reduction in b values and change in sign (from to þ) in a parameter clearly indicated that browning development occurred in apple pulp. Negative a values were given by the green pigmentation of apples and it was pronounced in green samples. 7.2.2.1. Effect of the Temperature in the Color Change Color development during pulping of fruits certainly includes Michaelis–Menten type reaction kinetics followed by several reactions, both reversible and irreversible, up to the formation of dark brown pigments. The combined effect of these browning reactions may result in a nonlinear behavior strongly dependent on temperature. Experimental data obtained with apples on the dependence of the rate of CIELAB L with temperature were fitted by Lozano et al. (1994) to the equation: L ¼ a k log t

ð7:1Þ

where a and k are fitting parameters and t is the time in min. The selected range is in agreement with the second linear period in the L versus log t plot. However, it must be noted that browning mechanism can be strongly nonlinear and simplified kinetics equations may not be applicable. Kinetic measurements on complex systems, such as fruit pulp, usually give reaction constant values, which may or may not be the dissociation constant, but it is frequently the combination of the rate constants for several steps. In general, for production of light-colored apple pulp, the time between milling of the fruit and the heat treatment must be as short as possible.

7.3. NONENZYMATIC BROWNING NEB via Maillard-type reactions is the most important route of color deterioration in fruit juices. The reaction is followed by undesirable color, odor, and flavor changes (Pribella and Betusova, 1978; Toribio and Lozano, 1984; Cornwell and Wrolstad, 1981). Three basic NEB reactions have been identified (Fig. 7.3): . .

Pyrolysis: which results in a burnt and inedible flavor; Caramelization: when the simpler sugars lose water molecules from their structure, through a 1:2 and 2:3-enolization. This process is affected by pH. Through many

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Fruit Manufacturing Pyrolysis Involves the total loss of water from the sugar molecule and the breaking of carbon–carbon linkages

NEB Caramelization Heat-induced transformation of reducing sugars alone in a concentrated solution

Maillard Browning reactions involve simple sugars, and amino acids and simple peptides

Figure 7.3. Basic NEB reactions.

.

intermediates, and in the pH 2–7 range, d-fructose for example can give rise to furans, isomaltol, and maltol, well-known bread crust flavor/aromas; Maillard-type reaction: of amino acids and proteins with carbohydrates, which is discussed extensively in the following section.

7.3.1. Maillard Reactions The reaction begin to occur at lower temperatures and at higher dilutions than caramelization, as in clarified fruit juices (Toribio and Lozano, 1984). The rate can increase by 2–3 times for each 108C rise in temperature. Maillard reactions have three basic phases (Fennema, 1986): .

.

.

.

The initial reaction is the condensation of an amino acid with a simple sugar, which loses a molecule of water to form N-substituted aldosylamine. This is unstable and undergoes the Amadori rearrangement to form 1-amino- 1-deoxy-2-ketoses (or ketosamines), which can cause complex subsequent dehydration, fission, and polymerization reactions. One of the Maillard paths is a simple caramel reaction, catalyzed by amino acids. The ketosamine products of the Amadori rearrangement can then react three ways in the second phase. One is simply further dehydration into reductones and dehydroreductones, which are essentially caramel products. Second is the production of shortchain hydrolytic fission products such as diacetyl, acetol, pyruvaldehyde, etc. These then undergo Strecker degradation with amino acids to aldehydes and by condensation to aldols. Negative aromas like 2 and 3-methyl-butanal are also formed. Third path is the Schiff’s base/furfural path. This involves the loss of 3 water molecules, then a reaction with amino acids and water. These also undergo aldol condensation and polymerize further into true melanoidins. These products react further with amino acids in the third phase to form the brown pigments and flavor active compounds collectively called melanoidins. These can be off-flavors. The outcome will depend not only on which amino acids and sugars are available, but also on pH, temperature, and concentration.

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In general, high levels of amino acids favor both caramel and Maillard reactions, but dilution eliminates caramel reactions. At temperatures >1008C pyrazines are produced. High levels of polyphenols favor Strecker degradation. Table 7.2 lists principal reactions and characteristics of identified NEB reactions. Fruit juice concentrates containing more than 65% total solids are normally stable from the standpoint of fermentation at any temperature, but when stored at relatively high temperatures, NEB reactions occur. NEB via Maillard-type reactions is the most important route of color deterioration in apple juice (Czapski, 1975; Toribio and Lozano, 1984). The reaction takes place between amino acids and reducing sugars present in the juice, decreasing the alpha-amino nitrogen content followed by undesirable color, odor, and flavor changes (Pribella and Betusova, 1978; Toribio and Lozano, 1984). The same behavior was found in pear juice concentrate (Cornwell and Wrolstad, 1981), citrus juices (Kanner et al., 1982; Cornwell and Wrolstad, 1981), and intermediate moisture foods (Resnik and Chirife, 1979; Waletzko and Labuza, 1976). Color deterioration was reported for many fruit products, such as citrus juices (Reynolds, 1965; Kanner et al., 1982; Cornwell and Wrolstad, 1981), intermediate moisture foods (Waletzko and Labuza, 1976; Johnson et al., 1969; Czapski, 1975), and apple juice (Toribio and Lozano, 1984, 1986). Babsky et al. (1986) studied the effect of storage on the composition of clarified apple juice concentrate and concluded that natural juices were very complex mixtures in which the role of the many components in browning reactions was difficult to elucidate. Kinetics of NEB in fruits and fruit products can be simplified as seen in the following scheme (Fig. 7.4).

Table 7.2. Basic Maillard-type reactions (adapted from Fennema, 1985). Stage

Principal reactions

Characteristics

Initial (Colorless)

Condensation, enolization, Amadori rearrangement. With proteins, glucose and free amino groups combine in 1:1 ratio

Intermediate (Strong absorption in near-ultraviolet range)

Sugar dehydration to 3-deoxyglucosone and its -3, 4-ene, HMF, and 2-(hydroxyacetyl)furan; sugar fragmentation; formation of alpha-dicarbonyl compounds, reductones, and pigments Aldol condensations, polymerization, Strecker degradation of alpha amino acids to aldehydes and N-heterocyclics at elevated temperatures. Carbon dioxide evolves

Reducing power in alkaline solution increases. Storage of colorless 1:1 glucose–protein product produces browning and insolubility Addition of sulfite decolorizes, reducing power in acidic solution develops, pH decreases, sugars disappear faster than amino acids. Positive test for amino sugars (Amadori compounds) Acidity, caramel-like and roasted aromas develop, colloidal and insoluble melanoidins form, fluorescence, reductone reducing power in acid solution, addition of sulfite does not decolorize

Final (Red-brown and dark-brown color)

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Fruit Manufacturing Reactives Intermediates Hexoses Amino-acids Organic acids Glucids Vitamins Anthocyanins, etc.

Standard methods of determination are available

Final products CO2 Amadori compounds Aldehydes

Most of them very reactives and unstable (Exceptions: CO2 and 5-HMF)

Melanoidines

Large Absorbance on visible range

Figure 7.4. Simplified scheme for NEB reactions in most fruit juices.

7.3.1.1. Tristimulus Parameters and Absorbance as a Measurement of Browning in Fruit Juices The absorption at one fixed wavelength, although reliable for kinetics studies, is not adequate for comparing the visual color changes in both browned apple juice and model systems. For this reason, the Hunter a, b, L color parameters and C.I.E. x, y, z parameters are also measured (see also Chapter 5). In general during NEB of fruit juices, except for the differences given by the initial color, the tristimulus values are grouped within a narrow band drawn from the standard light source ‘‘C’’ so as to approach asymptotically the spectrum red locus (Fig. 7.5). This behavior could denote that the same NEB reactions occurred in juices and model systems, resulting in polymers (melanoidines) with the same color attributes. 7.3.1.2. Kinetics of Nonenzymatic Browning (NEB) The kinetics of NEB is generally dependent upon product characteristics and storage conditions, including: . . . .

Influence of temperature, soluble solids’ concentration, pH, acidity, and water activity. Amino acids and reducing sugars’ content. 5-HMF formation during NEB Effect of polyphenols, galacturonic acid, and other minor compounds.

As pointed out by Labuza and Riboh (1982) most of the quality-related reaction rates are either zero- or first-order reactions, and the statistical difference between both types may be small. Besides real fruit products, the technique of using simplified model food systems to simulate the effect of storage and processing on quality has been widely used. 7.3.1.3. Effect of Soluble Solids It is well known that by increasing the concentration of food solids (or reducing water content) browning reactions are significantly enhanced (Eichner and Karel, 1972). The

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0.9 515 520

0.8 505 0.7

530 Green 545

500

555

0.6 495

565

0.5 Y

Yellow

575

490 0.4

590 Daylight

485

0.3

605

Pink Red

780

480

0.2

Blue 470

0.1

380

0 0

0.2

0.4

0.6

0.8

X Figure 7.5. Effect of browning during storage of apple juice (adapted from Toribio and Lozano, 1987).

occurrence of a maximum reaction rate at a certain water activity was also described (Labuza et al., 1970). A further increase in solids’ content resulted in rate decrease. It was suggested that at these high concentrations, the rate of reaction was controlled by the mobility of the reactants. Toribio et al. (1994) found that clarified apple juice has a slower nonenzymatic browning reaction rate (NEBr) at low water activities increasing up to the maximum point between aw 0:53---0:55 (about 828Brix) as shown in Fig. 7.6. A further increase in aw significantly reduces the color formation. It is assumed, in this case, that the increase in aw tends to dilute the concentration of reactants, decreasing chemical reaction rate. This NEBr maximum is typical in nonenzymatic Maillard-type reactions. The aw values at maximum NEBr for model mixtures was found to be aw ¼ 0:87 (Labuza et al., 1970). 7.3.1.4. Effect of Reducing to Total Sugars’ Ratio (R/T) The rate of sucrose hydrolysis is a function of the concentration of reactants, temperature, and acid–catalyst concentration. However, if excess water is present as in fruit juices, the rate of disappearance of sucrose follows a pseudo-first-order reaction rate equation. As expected, an increase in the reducing sugars (fructose þ glucose) content, when keeping the total sugars’ content constant, resulted in a faster browning of model mixtures (Lozano, 1991). 7.3.1.5. Effect of the Fructose to Glucose Ratio (F/G) Lozano (1991) found that glucose was more reactive than fructose, at least during the zeroorder reaction rate period in model solutions simulating apple juice. Wolfrom et al. (1974)

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Fruit Manufacturing 0.025 37ⴗC

NEBr (Abs 420nm/day)

0.02

0.015

0.01 Storage time 0 days

0.005

40 days 80 days 0 0

0.1

0.2

0.3

0.4 aw

0.5

0.6

0.7

0.8

Figure 7.6. NEBr as a function of water activity (aw ) and time of storage (adapted from Toribio et al., 1984).

working with a simulated orange juice, found that fructose had higher initial rate of browning than glucose during the initial stage of reaction but was dependent on the kind of amino acid. 7.3.1.6. Effect of Amino Acids (AA) Lozano (1991) also found that an increase of total amino acids’ content (AA) from 4.34 to 6.51 g/L, resulted in a noticeable increase in the browning rate. It can be calculated that for 1.5 times greater AA content, there was an approximately 1.5 increase of the reaction constant K. Similar results were also found during storage (Babsky et al., 1986) and processing (Toribio and Lozano, 1987) of clarified apple juice. Asparagine was found to represent nearly 70% of the total amino compounds in clarified apple juice, and 90% of it disappeared after 100 days of storage at 378C (Babsky et al., 1986). Calculated increase in reaction constant K was 5% when the Asn content was increased from 60 to 70%. Del Castillo et al., (1998) found a loss of 65.1% of original AA content, when storing dehydrated orange juice (aw ¼ 0:44) at 508C for 14 days. About 78% of that loss was attributed to the AA in major proportions, namely proline, arginine, asparagine, and g-aminobutiric acid. Buedo et al. (2001) studied the change of free amino acids’ (AA) composition of peach juice concentrate (PJC), as a result of the Maillard reactions, in particular the effect of the storage at 15, 30, and 378C for 6 weeks, since those are conditions likely to be found in commercial practice. A decrease in total AA content was observed to be 8, 35, and 60%, after 112 days of storage at those temperatures, respectively. The main constituent, asparagine, contributed to 71% of the total loss, while aspartic acid increased its concentration, probably as a result of the asparagine degradation. Decrease in total AA content was exponential, pH remained constant during the storage, while titratable acidity increased with both time and temperature, assumed by disappearance of amino groups.

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7.3.1.7. Effect of the Content of Organic Acids The malic acid participation in the NEB, via Maillard reaction, was judged to be essentially catalytic (Reynolds, 1965). However, Lozano (1991) found that: (i) an increase in malic acid actually accelerated the rate of browning during storage, and (ii) the pH was reduced only by 0.1 unit when malic acid content was increased from 2 to 6 g/L. It must be noted that NEB reaction is not very sensitive to low pH changes in the pH ¼ 2– 4 range (O’Beirne, 1986). Titratable acidity in peach juice was observed to rise with time and temperature. Also, at 30 and 378C the increase rate is maximum on the initial storage days. Major organic acids present in stone-free peaches are malic, citric, and quinic (Wang et al., 1993). 7.3.1.8. Effect of Other Minor Components NEBr increased with ascorbic acid, because of the participation of vitamin C in the Maillard reactions. During the enzymatic clarification process of fruit juices, the natural pectic substances (mainly polymers of the galacturonic acid) are broken by specific enzymes (pectinases), which are able to hydrolyze pectin to their basic units. This treatment is also applied during the pressing stage, to improve the juice extraction, and depending on fruit variety and maturity, considerable amount of free galacturonic acid could be present in clarified fruit juices after enzyme treatments. Lozano (1991) found that the adding of 60 mg/L of galacturonic acid to a model sugar– malic acid–amino acid solution accelerated the color formation. It can be concluded that galacturonic acid, produced during the enzymatic treatment of pulps and juices, may accelerate browning, thus reducing the storage capacity of apple juice concentrate. 7.3.1.9. Effect of Temperature Figure 7.7 shows the change in absorbance at 420 nm for different apple juice concentrates and a model solution over 120 days, at 378C. The rate of NEB of this model solution can be divided into the following two stages: Red Del. Granny Smith Model system

Absorbance (420 nm)

2

1.5

1

0.5 37ⴗC 0 0

25

50 75 100 Storage time (days)

125

Figure 7.7. Color development as a function of time of apple juice (708Brix) and a model solution (fructose/glucose: 3.13, reducing/total sugars: 0.90, total amino acids: 3.5 g/L, malic acid: 6.4 g/L at 128Brix) (adapted from Toribio and Lozano, 1984; Lozano, 1991).

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Fruit Manufacturing (1) An induction period, already observed in other model food systems (Warmbier et al., 1976), is attributed to the formation of colorless intermediates. This period was exponential rather than linear and the color development could be expressed as a typical first-order reaction equation. (2) A linear period of reaction where the color formation follows a zero-order kinetics. It was assumed that the behavior with temperature would follow the same trend at any given time when the storage temperature was reached.

Moreover, when concentrate was stored for 30 days at 378C and the next 30 days at 208C, color formation was 40% higher than that during the inverse condition (first 30 days at 208C and the next 30 days at 378C). Clearly this shows how important it is to cool the product as soon as it is produced. The information is relevant because a sizeable amount of concentrate is made during the summer season and the product is stored in the open air. When the color formation during storage of model solutions at any temperature is compared with the color development of a natural apple juice, some differences are readily observed (see Fig. 7.7): (1) Color increase was very much higher in AJC than in model solutions, (2) Induction time was not detected during the AJC storage, (3) Obtained maximum NEBr had different values. The first two differences could be attributable both to the influence of minor components like galacturonic acid and to the heating during processing (clarification, aroma recovery, and concentration stages), which could reduce or eliminate the induction time. Eichner (1975) and Eichner and Karel (1972) found in very viscous food systems that the viscosity remarkably affected the NEBr. As the viscosity of both the model systems and apple juice had practically the same values at the same soluble solids, it is difficult to exclusively attribute the limitation of NEB reactions and the occurrence of a maximum to the reactant mobility. It must be noticed that the use of liquid model systems for kinetic studies, besides the previously considered limitations, resulted in a very good tool to individually quantify the participation of juice components in deteriorative reactions. To put it succinctly, browning rates in fruit juice are mostly dependent on reducing sugars and amino acids’ content. However, independent of juice composition, the lower the storage temperature, the less is the darkening. Thus, from a practical standpoint, thermal history is crucial in obtaining lighter color products.

7.3.2. 5-HMF Formation During Storage and Processing of Fruit Products The Maillard reaction among hexoses and amino components leads to the formation of 5-hydroxymethylfurfural (5-HMF) (Shallenberger and Mattick, 1983) as intermediate. The HMF increase during processing and storage of fruit products was positively identified and quantified (Resnik and Chirife, 1987; Toribio and Lozano, 1979). Figure 7.8 shows the HMF increase of apple and grapefruit juice during prolonged storage. As Fig. 7.8 shows, the rate of accumulation can be divided into three periods. The first period is characterized as an induction time of approximately 2 weeks. During the second

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175

Kcal/mol.

HMF (mg/100 g)

50 40 30 20 Apple, 37ⴗC

10

Grapefruit, 40ⴗC

0 0

25

50 75 100 Time (days)

125

Figure 7.8. Rate of accumulation of 5-HMF with time of storage in apple and grape juices (Saguy et al., 1978; Babsky et al., 1986).

period the rate showed a rapid increase of HMF with a maximum at about 7–8 weeks. After that maximum the rate of formation diminished rapidly, and the HMF production approached a plateau. A similar behavior attributable to a second-order autocatalytic reaction (Frost and Pearson, 1961), was recognized by Schallenberger and Mattick (1983) during the acidic degradation of hexoses. It would appear that after 50 days of storage under the present conditions, HMF started to form brown pigments (melanoidins) in apple juice at such a rate that after some period the consumption equaled the formation via Amadori rearrangement of hexose degradation. Calculated activation energy/valid for the HMF formation in the range of temperatures considered, resulted Ea ¼ 35 Kcal=mol. Similar results were obtained by O’Beirne (1986) for apple juice concentrate. Petriella et al. (1985) found that the NEBr in different food systems reduced with decreasing pH. However, the range of work was pH ¼ 5 –7 and the authors showed an apparent change in the browning mechanism at pH ¼ 5 and lower. Although Wolfrom et al. (1974) also found that the browning rate decreased with pH, they worked with apple juice at pH ¼ 6 –7. It was speculated that an increase in the malic acid content was more effective in accelerating NEB than the consequent pH reduction. Concentration by evaporation ideally reduces costs and increases shelf life by removing water without changing the solid composition. However, in practice, clarified fruit juices are susceptible to color and flavor changes during evaporation. In the previous section we considered the reaction of the hexoses and amino components present in apple juice, leading to the formation of 5-hydroxymethylfurfural. HMF can also be produced by acid-catalyzed splitting of sugars (Shallenberger and Mattick, 1983). Extrapolation of the straight-line portion to the time axis, gives a value for the induction period. A similar induction period in the formation of HMF was observed by Shallenberger and Mattick (1983) and was attributed to some autocatalytic mechanism. Only an initial flat period and rapidly increasing rates at the outset of reaction were observed in apple juice (Toribio and Lozano, 1987). The formation of 5-HMF was also proposed to be used to complement color data in estimating the severity of heating during processing and storage of fruit juices (Askar,

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EXAMPLE 7.1 NEB estimation during storage Urbicain and Lozano devised a nomogram for calculating the relative color increase from the initial one, with concentration (in Brix), time (in days), and temperature (in8C) given. This is shown in Fig. 7.9. In this example, the increase in color after 60 days’ of storage of a 708Brix apple juice concentrate at 308C needs to be estimated. A line is drawn passing through 708Brix and 60 days’ points, and extrapolated to the reference line. A second line is plotted joining the focus point (Fo) with the intersection between the first plotted line and the reference line. Finally, a line normal to temperature line, beginning at the intersection between (Fo-Ref) line and 308C line, indicates DC value at the corresponding scale. The result indicates an absorbance increase DC ¼ 0:68 can be estimated.

1984; Toribio and Lozano, 1987). The formation of HMF depends on the duration and temperature of processing and storage. In fresh, untreated juices the HMF content is practically 0 (Babsky et al., 1986; Askar, 1984). The HMF level is important because it indicates the severity of heating that has been applied during processing, as reported for milk (Burton, 1984), honey (Jeuring and Kuppers, 1980), dehydrated apples (Resnik and Chirife, 1979), and tomato paste (Allen et al., 1980). Both HMF and furfural (2-furaldehyde) are useful as indicators of temperature abuse in orange juice (Meydav and Berk, 1978).

8Brix t days 65

DC Ref

T 8C

Fo 5 10 20 25

Result: DC = 067

100 30 1 37

200

15

Figure 7.9. Nomogram to estimate the relative color increase (DC) in Abs. 420 nm of concentrate as a function of time of storage or transportation, concentration (8Brix), and temperature.

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EXAMPLE 7.2 NEB during refrigeration of a juice barrel 50-gallon plastic barrels, although practically displaced by high-volume containers, are still used in small processing plants. The problem is to estimate the necessary time to reduce the center temperature to values low enough to retard the NEB. Urbicain and Lozano (1992) found that by maintaining clarified concentrated apple juice (CAJ) for 60 min at 508C (outlet evaporator temperature), 10% increase in color and 5-HMF formation occurs. Barrels are filled at 508C to facilitate pumping and improve sanitary conditions. A typical barrel is 1 m high and has 0.55 m diameter. As viscosity increases during cooling from 0.06 Pas to 2.0 Pas, convective movement is rapidly restricted, and barrels are normally piled in threes. The well-known solutions for conductive cooling of an infinite cylinder can be applied (Carslaw and Jaeger, 1965; Welty et al., 1976). Assuming the following average properties for CAJ: r ¼ 1,360kg=m3 k ¼ 0:86 kcal=sm C cp ¼ 0:64 kcal=kg C And considering a convective coefficient Hc as (Charm, 1963): Hc ¼ 0:095(Ti Ta)r1 (BTU=h ft F) where r is the barrel radius. By solving analytically or graphically, temperature reduction at different barrel diameters was calculated and plotted in Fig. 7.10. Under normal storage condition, a CAJ barrel may need up to 2 days to reduce its temperature to sufficiently low NEB rate. It must be indicated that any efforts made to optimize the process will become useless if packaging and refrigeration is not properly done.

The presence of excessive amounts of HMF is considered evidence of overheating. Askar (1984) indicated that HMF is also responsible for the cooked taste of apple juice. Multipleeffect evaporators were designed to concentrate apple juice at reduced temperatures, but in practice temperatures become very high in the initial effects (Lozano et al., 1984). Toribio and Lozano (1987) heated apple juice in a set of thin, rectangular cells, made of stainless steel, designed to have a relatively high sample capacity and a short come-up time (less than 40 s under the more adverse conditions) to evaluate the buildup of HMF in apple juices at the soluble solid concentrations and temperatures that usually prevail during concentration by evaporation. The various apple juices were heated in the range 100 –1088C for selected times (from 4 to 80 min). HMF, which has been shown to be essentially absent in fresh manufactured singlestrength apple juice (Askar, 1984), increased to significant amounts during high-temperature heating. The relationship between buildup of HMF and time and temperature, and soluble solids is shown in Fig. 7.11. These results indicate that HMF increases linearly with time after an initial induction period, which depends on soluble solids and temperature. More research is needed to establish relationships between material properties and rates of oxidation, NEB, and enzymatic changes.

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Fruit Manufacturing 55 CAJ Barrel distance from center

Temperature (ⴗC)

45

r = 0.01 m r = 0.11 cm r = 0.25 cm

35

25

15

5

−5 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Time (h)

Figure 7.10. Cooling rate of a 708Brix CAJ in barrel, stored in a –58C cooling chamber.

The induction periods are comparable with the residence times in individual evaporator effects. Kurudis and Mauch (21) reported 32.5 min as the mean residence time in an industrial sugar evaporator similar to those commonly found in the fruit juice industry. 160 140

70ⴗBrix 108ⴗC 70ⴗBrix 100ⴗC 30ⴗBrix 108ⴗC 30ⴗBrix 100ⴗC

HMF (mg/L)

120 100 80 60 40 20 0 0

10

20

30

40 50 Time (min)

60

70

80

Figure 7.11. HMF formation as a function of time and temperature, and soluble solids, for clarified apple juice (adapted from Toribio and Lozano, 1987).

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The effect of variety on HMF formation was noticeable. The results show that the rate of buildup of HMF was especially dependent on juice composition. It was three to four times more rapid in Granny Smith than in Red Delicious apple juice. As noted previously, HMF can be formed either by heating of reducing sugars in acid solution, or by the reaction between hexoses and amino acids. Both pH and total amino acid content favor the formation of HMF in fruit juice. Calculated activation energies for HMF formation in apple juice and dehydrated apples ranged from 33.8 to 46.8 kcal/mol. (Resnik and Chirife, 1979; Toribio and Lozano, 1987). Toribio and Lozano (1987) considered 30 mg/l of HMF as a reasonable limit after heat treatment of apple juice. Time of heating required at various temperatures to attain this level is plotted in Fig. 7.12. The data in Fig. 7.12 indicate that variety has a more drastic effect on HMF buildup than on the development of NEB. Therefore, the measurement of HMF in fruit juice may provide a useful complement to color data. Depending on the composition of the juice, the HMF level can reach very high values. As HMF is an important intermediate in NEB via Maillard (Babsky et al., 1986), its production during processing may accelerate browning during storage. HMF content can then be used to complement color data in estimating the severity of heating during processing and the storage capacity of fruit juice concentrates. Nonenzymatic browning kinetics as affected by glass transition Although glass transition may control physical changes in foods, its effect on reaction kinetics is not well established. Significant issues are whether reactions become diffusion controlled and importance of glass transition on reaction rates. Lievonen et al. (1998) studied the effects of physical state, water plasticization, and glass transition on kinetics of NEB in a water solution, and concentrated polyvinylpyrrolidone (PVP) and maltodextrin (MD) model systems with 0.23, 0.33, and 0.44 aw at 248C with the same concentration of reactants, xylose, and lysine 1:1, (10%, w/w) in the water phase. Water contents of the MD and PVP systems increased from 6.3 to 9.7 and from 8.2 to 17.3 g H2 O/ 100

Time to attain 30 mg/L HMF (min)

Granny Smith Red Delicious

10 98

102 106 Temperature (ⴗC)

110

Figure 7.12. Effect of temperature on rate of accumulation of 5-HMF in apple juice for Granny Smith (698Brix) and Red Delicious (70.68Brix) varieties (adapted from Toribio and Lozano, 1987).

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100 g dry matter, respectively, as aw increased from 0.23 to 0.44. The rate of NEB was the highest at all temperatures (10 –1008C) in water solution. The rate in PVP systems (Tg ranging from 30 to 608C) was higher than in MD systems (Tg ranging from 30 to 808C) both as a function of temperature at constant water content and as a function of water content at a constant temperature. Above Tg , reaction rates increased more rapidly than below Tg . These results may be useful in controlling NEB in processing and storage of concentrated food materials.

REFERENCES Allen, B.H. and Chin, H.B. (1980). Rapid HPLC determination of hydroxymethylfurfural in tomato paste. J.Assoc. Off. Anal. Chem. 63: 1974 –1976. Askar, A. (1984). Flavor alterations during production and storage of fruit juices. Flussiges Obst. 11: 564 –569. Babsky, N., Toribio, J.L. and Lozano, J.E. (1986). Influence of storage on the composition of clarified apple juice concentrate. J.Food Sci. 51: 564 –567. Buedo, A. Elustondo, M.P. and Urbicain, M.J. (2001). Non enzymatic browning of peach juice concentrate. Innov. Food Sci. Emerg. Technol. 1: 255–260. Burton, H. (1984). Reviews of the progress of dairy science: the bacteriological, chemical, biochemical and physical changes that occur in milk at temperatures of 100 –1508C. Dairy Res. 51: 341–363. Carslaw, H.S. and Jaeger, J.C. (1969). Conduction of Heat in Solids, 2nd Edition. London: Oxford University Press, Inc. Charm, S. (1963). A Method For Experimentally Evaluating Heat-Transfer Coefficients In Freezers And Thermal Conductivity Of Frozen Foods. Food Technology 17: 1305. Cornwell, C.J. and Wrolstad, R.E. (1981). Causes of browning in pear juice concentrate during storage. J.Food Sci. 46: 515–518. Coseteng, M.Y. and Lee, C.Y. (1987). Changes in apple polyphenoloxidase and polyphenol concentrations in relation to degree of browning. J.Food Sci. 52: 985. Czapski. J. (1975). Wpliw wolnych aminokwasow na zmiany jal czageszczonych sokow iablkowYch podczas przechowy. Prze Mysi, Ferment I Rolny 8 –9: 19–23. del Castillo, M.D., Corzo, N, Polo, M.C., Pueyo, E. and Olano, A. (1998). Changes in amino acid composition of dehydrated orange juice during accelerated nonenzymic browning. J.Agric. Food Chem. 46: 277–280. Eichner, K. (1975). The influence of water content on non-enzymatic browning reactions in dehydrated foods and model systems. In Water Relations in Foods, Duckworth, R. (ed.). Academic Press, NY. Eichner, K. and Karel, M. (1972). The influence of water content on the amino browning reaction in model systems under various conditions. J.Agron. Food Chem. 20: 218–223. Fennema, O.R. (1985). Food Chemistry. New York, Mercel Decker, 991p. Frost, A.A. and Pearson, R.G. (1961). Kinetics and Mechanism, 2nd. ed. John Wiley & Sons, New York, NY. Hodge, J.E. (1953). Dehydrated foods. Chemistry of browning reactions in model systems. J.Agr. Food Chem. 1(15): 928–935. Jeuring, H.J. and Kuppers, F.J.E.M. (1980). High performance liquid chromatography of furfural and hydroxymethylfurfural in spirits and honey. J.Assoc. Off. Anal. Chem. 63: 1215–1218. Johnson, G., Donnelle Y.J. and Johnson, D.K. (1969). Proantho-cyanidins as related to apple juice processing and storage. J.Food Sci. 33: 254 –257. Joslyn, M.A. and Ponting, J.D. (1951). Enzyme-catalyzed oxidative browning of fruit products. Adv. Food Res. 3: 1–7. Kacem, B., Cornell, J.A., Marshall, M.R., Shiremen, R.B. and Matthews, R.F. (1987). Nonenzymatic browning in aseptically packed orange drinks: effect of ascorbic acid, amino acids and oxygen. J.Food Sci. 52(6): 1668–1672. Kanner, J., Fishbein, J., Shalom, P., Harel, S. and Ben-Gera, I. (1982). Storage stability of orange juice concentrate packaged aseptic. J.Food Sci. 47: 429– 433. Koch, J. and Bretthauer, G. (1956). The vitamin C content of ripening fruits. Landwirstsch. Forsch. 9: 51–63. Labuza, T.P. and Riboh, D. (1982). Theory and application of Arrhenius kinetics to the prediction of nutrient losses in foods. Food Technol. 36(10): 66 –74. Labuza, T.P., Tannenbaum, S.R. and Karel, M. (1970). Water content and stability of low moisture and intermediate moisture of foods. Food Technol. 24: 543–550.

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Lee, C.Y. and Jaworski, A. (1988). Phenolics and browning potential of white grapes grown in New York. Am. J. Enol. Vitic. 39: 337–340. Lee, C.Y., Kagan, V. Jaworski, A.W. and Brown, S.K. (1990). Enzymatic browning in relation to phenolic compounds and polyphenoloxidase activity among various peach cultivars. J.Agric. Food Chem. 38: 99–191. Lievonen, S.M., Laaksonen, T.J. and Roos, Y.H. (1998). Glass Transition and Reaction Rates: Nonenzymatic Browning in Glassy and Liquid Systems. J. Agric. Food Chem. 46(7): 2778–2784. Lozano, J.E. (1991). Kinetics of non enzymatic browning in model systems simulating clarified apple juice. Lebensm. Wiss. Technol. 24: 355–360. Lozano, J.E., Elustondo, M.P. and Romagnoli, J.A. (1984). Control studies in an industrial apple juice evaporator. J.Food Sci. 49: 1422–1427. Lozano, J.E., Biscarri R.D. and Ibarz, A. (1994). Enzymatic browning in apple pulps. J. Food Sci. 59: 1– 4. Marshall, M.R., Kim, J. and Cheng-I, W. (2000). Enzymatic Browning in Fruits, Vegetables and Seafoods. FAO. In http://www.fao.org/ag/ags /agsi/ENZYMEFINAL/. Meydav, S. and Berk Z. (1978). Colorimetric determination of browning precursors in orange juice products. J.Agric. Food Chem. 26: 282–285. O’Beirne, D. (1986). Effects of pH on non-enzymatic browning during storage in apple juice concentrate prepared from Bradley’s Seedling Apples. J.Food Sci. 51: 1073–1076. Ochoa, M.R. Kesseler, A.G., Vullioud, M.B. and Lozano J.E. (1999). Physical and chemical characteristics of raspberry pulp: storage effect on composition and color. Lebensm. Wiss. Technol. 32(3): 149–153. Petriella, C., Resnik, S.L., Lozano, R.D. and Chirife, J. (1985). Kinetics of deteriorative reactions in model food systems of high water activity: color changes due to non-enzymatic browning. J.Food Sci. 50: 625–630. Ponting, J.D and Joslyn, M.A. (1948). Ascorbic acid oxidation and browning in apple tissue extracts. Arch. Biochem. 19: 47–51. Pribella, A. and Betusowa, M. (1978). Veranderungen in Geha Sticktoffhaltingen Soffen bei der Lagerung von Obstsaft-ko traten. Fruchtsaft-lndustrie 9(1): 15–19. Resnik, S. and Chirife, J. (1979). Effect of moisture content and temperature on some aspects of non-enzymatic browning in dehydrated apple. J.Food Sci. 44: 601–606. Reynolds, T.H. (1965). Chemistry of non-enzymatic browning II. Adv. Food Res. 14: 167–210. Saguy, L., Kopelman, I.J. and Mizrahi, S. (1978). Extent of nonenzymatic browning in grapefruit juice during thermal and concentration processes: Kinetics and prediction. J. Food Proc. Pres 175–184. Sapers, G.M. and Douglas Jr., F.W. (1987). Measurement of enzymatic browning at cut surfaces and in juice of raw apple and pear fruits. J. Food Sci. 52: 1258–1263. Shallenberger, R.S. and Mattick, L.R. (1983). Relative stability of glucose and fructose at different acid pH. Food Chem. 12: 159–166. Shannon, C.T. and Pratt, D.E. (1967). Apple polyphenol oxidase activity in relation to various phenolic compounds. J. Food Sci. 32: 479–483. Spanos, G.A. and Wrolstad, R.E. (1992). Phenolics of apple, pear, and white grape juice and their changes with processing and storage: a review. J.Agric. Food Chem. 40: 1478–1487. Spark, A.A. (1969). Role of amino acids in nonenzymatic browning. J.Sci. Food. Agric. 20(5): 308–314. Toribio, J.L. and Lozano, J.E. (1984). Nonenzymatic browning in apple juice concentrate during storage. J.Food Sci. 49: 889–892. Toribio, J.L. and Lozano, J.E. (1986). Heat induced browning of clarified apple juice at high temperatures. J.Food Sci. 51: 172. Toribio, J.L. and Lozano, J.E. (1987). Formation of 5-hydroxymethylfurfural in clarified apple juice during heating at elevated temperatures. Lebensm. Wiss. Technol. 20: 59–63. Toribio, J.L., Nunes, R.V. and Lozano, J.E. (1984). Influence of water activity on the nonenzymatic browning of apple juice concentrate during storage. J.Food Sci. 49: 1630 –1632. Urbicain M.J., and Lozano, J.E. (1992). Damage of concentrated apple juice during processing and storage. Lebensm. Technol. (25), 194–204. Va´mos-Vigya´zo´, L. (1981). Polyphenol oxidase in fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 15: 49–127. Waletzko, P. and Labuza, T.P. (1976). Accelerated shelf-life testing an intermediate moisture food in air and in an oxygen free-free sphere. J. Food Sci. 41: 1338. Walker, J.R.L. (1995). Enzymatic browning in fruits: Its biochemistry and control. In Enzymatic Browning and Its Prevention, Lee, C.Y. and Whitaker, J.R. (eds.). ASC Symposium Series 600, American Chemical Society, Washington, DC, pp. 8–22. Wang, T., Gonzalez, A.R., Gbur, E.E. and Aselage, J.M. (1993). Organic acid changes during ripening of processing peaches. J. Food Sci. 58: 631–632.

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Warmbier, H.C., Schnickels, R.A. and Labuza, T.P. (1976). Non-enzymatic browning kinetics in an intermediate moisture model system. Effect of glucose to lysine ratio. J.Food Sci. 41: 981–983. Welty, J.R., Wicks, C.E. and Wilson, R.E. (1976). Fundamentals of Momentum, Heat and Mass Transfer, 2nd ed., Wiley. Whitaker, J.R. and Lee, C.Y. (1995). Recent advances in enzymatic browning. In Enzymatic Browning and Its Prevention, Lee, C.Y. and Whitaker, J.R. (eds.). American Chemical Society Symposium Series 600, 2–7. American Chemical Society, Washington, DC. Wolfrom, M.L., Schuertz, R.D. and Cavalieri, L.F. (1974). Factors affecting the Maillard browning reaction between sugars and aminoacids. Studies on the non-enzymatic browning of dehydrated orange juice. J.Agr. Food Chem. 22: 796 –801.

CHAPTER 8

INHIBITION AND CONTROL OF BROWNING 8.1. INTRODUCTION As previously discussed in this book (Chapter 6) fruits are complex systems, which, after size reduction (pulping, milling, or cutting) are transformed into a mixture of chemical and biochemical active components reacting in aqueous media. Moreover, process conditions cover a wide range of temperatures (Fig. 8.1), which make the modeling and prediction of deteriorative reactions even more difficult. Voluminous literature on the interplay of these parameters during processing of foodstuffs is available. Browning of fruits is a major problem in the fruit industry and is believed to be one of the main causes of quality loss during postharvest handling and processing. The mechanism of browning in fruits and fruit products is well characterized and can be enzymatic or nonenzymatic in origin (Chapter 7).

8.2. INHIBITION AND CONTROL OF ENZYMATIC BROWNING Enzymatic browning (EB) is the result of fast reactions. Even an optimized processing technology cannot completely avoid the EB during pulping and pressing of fruit juice, unless special care is taken to avoid oxygen. Enzymes and reactions responsible of discoloration in fruits are described in Chapter 7. In brief, during EB reactions, polyphenol oxidase catalyzes the oxidation of phenols to o-quinones, which are highly reactive compounds. o-Quinones thus formed undergo spontaneous polymerization to produce high molecular weight compounds or brown pigments (melanins). These melanins may in turn react with amino acids and proteins leading to enhancement of the brown color produced. Many studies have focused on either inhibiting or preventing polyphenol oxidase activity in foods. Va´mos-Vigya´zo´ (1995) classified the principles of EB prevention into: . .

Inhibition or inactivation of the enzyme, and Elimination or transformation of the substrate.

The author indicated that it is not easy to classify an inhibitor as belonging only to one of these categories. Moreover, many inhibitors act on both enzyme and substrate. It must be emphasized that EB in fruits and fruit products can be controlled or reduced also by: . . .

Selecting cultivates of slight browning tendency. Improving the agricultural techniques. Identifying PPO activity, phenolic composition, and browning kinetics. 183

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Removal of heat

Ambient temperature

Application of heat

Freeze drying & freeze Concentration Freezing Chilling Storage

−358C

Raw material preparation Size reduction Mechanical separation Enzyme treatment Use of membranes Dehydration Blanching Pasteurization Evaporation Ultra high temperature Processes

150 8C

Figure 8.1. Range of temperatures during typical fruit processing and storage.

Various techniques and mechanisms have been developed for controlling EB. These techniques attempt to eliminate one or more of the essential components (oxygen, enzyme, copper, or substrate) from the reaction. As EB is an oxidative reaction, it can be retarded by the elimination of oxygen from the cut surface of the fruit. However, browning restarts rapidly when oxygen is reintroduced. Oxygen exclusion is possible by immersion in syrup, deoxygenated water, or the coating of fruit with films not permeable to that gas (McEvily et al., 1992). As copper prosthetic group of polyphenol oxidases must be present for the EB reaction to occur, these chelating agents capable of removing Cu may be effective to control ED deterioration. Inactivation of the polyphenol oxidase by heat treatments, such as steam blanching, is effectively applied for the control of browning in fruits that are to be canned or frozen. Chemical modification of phenolic substrates, such as chlorogenic acid, caffeic acid, and tyrosine, can however prevent oxidation. Certain chemical compounds react with the products of polyphenol oxidase activity and inhibit the formation of the colored compounds produced in advanced, nonenzymatic reaction steps, which finally lead to the formation of brown compounds. Other techniques, such as the use of naturally occurring enzyme inhibitors and ionizing radiation, have been used as alternatives to heat treatment and the health risks associated with certain chemical treatments. It must be realized that inactivation of enzymes responsible for browning in fruits can be irreversible (e.g., heat treatment) or reversible (e.g., use of ascorbic acid). A general classification of the methods used to inhibit EB is sketched in Fig. 8.2. 8.2.1. Thermal Treatments 8.2.1.1. Elevated Temperatures Although steam blanching is one of the most effective methods for controlling EB in canned or frozen fruits (Va´mos-Vigya´zo´, 1995), it is not a practical alternative for treatment of fresh

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185

Thermal inactivation

EB Inhibition and control Chemical treatment

Nontraditonal methods

Figure 8.2. General classification of methods for inhibition of enzymatic browning.

foods. In such cases, the exclusion of oxygen and/or the application of inhibitors should be considered. Moreover, blanching should not be used as it affects the texture and flavor of fruit products. Adams (1991) reviewed enzyme inactivation during heat processing of foodstuffs. He concluded that enzymes have complex covalent and noncovalent structures, which are susceptible to heat-induced chemical degradation and disruption. In general enzyme inactivation as a function of temperature can be described by the Arrhenius or activated-complex model. It is also known that refrigeration (0– 48C) retards browning; however during fruit juice processing the cellular tissue is practically destroyed, and low temperatures are not enough to control oxidation. It is also true that about 10 s at 908C inactivates PPO (Dimick et al., 1951), which are conditions easily provided during heating of pulps. However, in practice a long delay occurs between crushing (or pulping) and thermal processing. Yemeniciogı¨lu et al. (1997) studied the heat-inactivation kinetics of crude polyphenol oxidase (PPO) from six apple cultivars (Golden Delicious, Starking Delicious, Granny Smith, Gloster, Starckinson, and Amasya) at three temperatures (688, 738, and 788C). PPO activity initially increased and then decreased with heat, following a first-order kinetic model (Fig. 8.3). The authors attributed the increase in activity to the presence of latent PPO. Calculation of activation energies (54.7–77.2 kcal/mol) indicated that PPO in apples was generally more heat stable than PPO in other fruits, like banana (Galeazzi et al., 1981), grape (Lee et al., 1983), and pear (Halim and Montgomery, 1978). Thermal enzymatic inactivation is described in accordance with kinetic parameters such as decimal reduction times (D), inactivation rate constant (k), z values (z), and activation energies (Ea ). The D value, or decimal reduction value, is defined as the time required to inactivate 90% of the original enzyme activity at a given temperature. An inactivation reaction, which follows first-order kinetics, has a D value equivalent to 2.303/k. Temperature dependence of the D value is given by the z value, which represents the temperature increase required in order to obtain a 10-fold (1-log cycle) decrease in D value. For a first-order decay process, the D value is equivalent to ln (10) k. Similar to the z value is the activation energy (Ea ), which expresses the temperature dependence of the k value as indicated in the Arrhenius relationship: ln k ¼ Ea =RT þ ln A k ¼ A(eEa =RT )

186

Fruit Manufacturing 1000

Activity, as % of original activity

68ⴗC

100 73ⴗC

78ⴗC

10 0

10

20 30 Time (min)

40

50

Figure 8.3. Effect of heating time and temperature on PPO in apple (Gloster cultivar) (Yemeniciogı¨lu et al., 1997) with permission.

The Q10 value is the change in the rate of a reaction that occurs with a 108C change in temperature and can be related to the Arrhenius equation as, Q10 ¼ e10Ea =RT (Tþ10) (Labuza and Riboh, 1982). While chemical reactions of single polyphenols have been described step by step, in complex food systems only secondary effects, such as color development, can be recorded. Color parameter was useful for studying the kinetics of EB reactions. Sapers and Douglas (1987) reported that decreases in the CIE L value correlated well with increases in fruit browning. Labuza et al. (1990) proposed the normalized DL=L0 (%) values as a measure of browning, when initial Hunter L0 values varied slightly between samples. Genovese et al. (1997) used deviation from initial Hunter parameters to study the EB in cloudy apple juice. These authors prepared different types of samples: natural juice (without steam treatment during crushing), not centrifuged (NJnC); natural juice, centrifuged (NJnC); cloudy juice (steam treated), not centrifuged (CJnC); and cloudy juice, centrifuged (CJnC). Figure 8.4 shows the variation of DL ¼ L L0 with time, for the different cloudy apple juice assayed. Luminosity decreases monotonically in the case of centrifuged natural juice (NJC) and remains practically constant for cloudy juices, either centrifuged (CJC) or not (CJnC). The small increase in DL for CJnC samples was attributed to partial precipitation of insoluble particles. Not centrifuged natural juices (NJnC) showed a completely different behavior and the rate of luminosity variation could be divided into two periods. The first period was characterized as a rapid increase in DL, attributed to the fast precipitation of unstable particles. During the second period, after a maximum at 10 min the luminosity decreased exponentially due to the oxidative darkening. The combined effect of particle precipitation and EB resulted in a strong nonlinear behavior. Hunter hue angle and saturation index (Chapter 5) was practically constant in all cases other than natural juices without treatments, in this case it is attributable to particle

8

.

Inhibition and Control of Browning

187

8

NJnC NJnC

NJC CJnC

6 Precipitation

CJC

4 Enzymatic browning DL

2

CJnC 0 CJC

NJC 2 0

10

20

30

40

50

Time (min) Figure 8.4. Variation of DL with time, for the different samples assayed (Genovese et al., 1997) with permission.

precipitation. While hue angle decreases, the saturation index increases with time for NJnC samples. The color difference (DE) development in apple juice samples is shown in Fig. 8.5. Analysis of the data concerning the color deterioration of apple juice suggests that cloud characteristics and EB effect on hue should not be independently considered. Moreover, steam treatment of juice was very effective not only in inactivating oxidative enzymes, but also in stabilizing cloudiness. Similar results were obtained by McKenzie and Beveridge (1988), during the blanching of Spartan apple juice. The authors attributed apple particulate stabilization to the formation of a protective colloid that prevented aggregation. It was also observed that centrifugation (4,200g per 5 min) had a positive effect in controlling, or at least in retarding, color changes, when applied to natural juices without heat treatment (NJC). Table 8.1 lists some thermal fruit treatments for the inhibition of EB.

60

188

Fruit Manufacturing 12 NJnC NJC CJnC

10

CJC

8 DE 6

4

2

0 0

10

20

30 Time (min)

40

50

60

Figure 8.5. Total color difference (DE) as a function of time, for natural and cloudy apple juices, at 208C (Genovese et al., 1997) with permission.

8.2.1.2. Refrigeration Temperatures In general, for every 108C reduction in temperature a similar decrease in the rate of enzymecatalyzed reactions occurs, which is referred to as the temperature coefficient (Q10 ). This effect was attributable to a decrease in both mobility and ‘‘effective collisions’’ necessary for the formation of enzyme–substrate complexes and their products. Freezing temperatures of 188C or below are often used for the long-term preservation of food. Some fruits (berries) may be precooled or stored at chilling temperatures. However, others like bananas, mangoes, and avocados are susceptible to chill injury and should

Table 8.1. Inhibition of EB by thermal treatment. Fruit/product

Inhibition method

Reference

Apricot substrate is catechin and chlorogenic acid Plum juice Cloudy apple juice

808C at 10 min

Dijkstra and Walker (1991)

658C at 20 min Steam heating of the mash in the range 65 –70C for 15–20 s

Siddiq et al. (1994) Genovese et al. (1997)

8

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Inhibition and Control of Browning

189

therefore not be stored below their respective critical temperatures (Fennema, 1975). Cold preservation and storage during distribution and retailing are necessary for the prevention of browning in fruit, since refrigerated temperatures are effective in lowering polyphenol oxidase activity. 8.2.2. Chemical Inhibition Chemical antibrowning agents have been commonly used to prevent browning of fruits and fruit products. Antibrowning agents are compounds that either act primarily on the enzyme or react with the substrates and/or products of enzymatic catalysis in a manner that inhibits colored product formation. The enzyme PPO can be inhibited by acids, halides, phenolic acids, chelating agents, sulfites, reducing agents such as ascorbic acid, quinone couplers such as cysteine, and some other substrate-binding compounds. Figure 8.6 shows the evolution of the chemical methods for the inhibition of EB applied to fruit products. The most widespread methodology used in the fruit industry for control of EB is the addition of sulfiting agents. The major effect of sulfites on EB is described in Fig. 8.7. As a reducing agent, sulfites reduce the o-quinone produced by PPO catalysis to the less reactive diphenol, preventing the development of later condensation of complex brown melanins. Inactivation of PPO by application of sulfur dioxide (SO2 ) has been successful in preventing EB, but its use was restricted by regulations. Sulfites have been linked to allergic reactions, the Food and Drug Administration (FDA) prohibited the use of sulfite preservatives in fresh vegetables and fruits (Langdon, 1987). The effect of reducing agents is temporary because these compounds oxidize irreversibly by reaction with pigments, enzymes, and metals. Their role is based in their ability to reduce o-quinones (Fig. 8.7). Sulfydryl compounds transform o-quinones in stable, colorless products.

More traditional

More innovative Cyclodextrin

EDTA Organic acids

Sulfiting Cysteine Glutathione Ascorbic acid and analogs

Chitosan Chelating agents

Reducing agents

Chemical inhibition of EB

Aromatic enzyme inhibitor

Proteolitic enzymes Acidulant Citric acid Phosphoric acid

Peptides

Hexylresorcinol Aliphatic alcohols Anions

Ficin Papain Bromelain

Carbohydrate derivatives

Honey

Figure 8.6. Description of chemical methods for the inhibition of EB.

190

Fruit Manufacturing Reducing agent

OH PPO + O2 R

R

Monophenol

OH PPO + O2

O

OH

O

R

Melanins

o-Quinone

Diphenol

Figure 8.7. Effect of reducing agents on the first stages of EB.

Alternative inhibitors of PPO were investigated extensively (Shannon and Pratt, 1967; Park and Luh, 1985; Sapers and Ziolkowski, 1987; Oszmianski and Lee, 1990; Siddiq et al., 1994; Lozano-de-Gonzalez et al., 1993). Tables 8.2–8.4 list some alternatives to sulfite antibrowning agents, according to their primary mode of action (McEvily et al., 1992): reducing agents, chelating agents, enzyme inhibitors, complexing agents, and miscellaneous methods. Different EB inhibitors were assayed in raw apple juice and on cut surfaces of apple plugs, using quantitative measurements of color changes to evaluate treatment effectiveness during storage by Sapers et al. (1989). While ascorbic acid-6-fatty acid esters showed

Table 8.2. Nonsulfite antibrowning agents applied to fruits and fruit products. Reducing agents. Name

Mechanism of inhibition

Comments

Reference

Ascorbic acid

Free radical scavenger. Reduces o-quinone to diphenols.

Effect on PPO activity is controversial. It is easily decomposed to form dehydroascorbic acid. Insufficient penetration into the cellular matrix of fruits

Gola-Goldhirsh et al. (1984)

Erythorbic acid

Idem

Ascorbyl phosphate esters (APE) and ascorbyl fatty acid esters (AFAE)

Releases ascorbic acid when hydrolyzed by acid phosphatases

Sulfydryl compounds

These agents react with o-quinones to produce stable, colorless products

Erythorbic and ascorbic acid application depends on the fruit. One compound cannot be substituted for the other without prior evaluation In APE inhibition power depends on the acidity of the fruit and the activity of endogenous acid phosphatase. AFAE needs emulsifying agents, which have detrimental effect on antibrowning ability This category is reduced to sulfur-containing amino acids (e.g., cysteine and methionine). High concentrations affect taste of treated fruit products

Janovitz-Klapp et al. (1990) Borestein (1965)

Seib and Liao (1987)

Sapers et al. (1989) Sapers et al. (1991) Pierpoint (1966)

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Inhibition and Control of Browning

191

Table 8.3. Nonsulfite antibrowning agents applied to fruits and fruit products: Chelating agents and enzyme inhibitors. Name

Mechanism of inhibition

Comments

Reference

Ethylenediamine tetraacetic acid

Chelating agents bind to the active site of PPO, or reduce Cu availability for the enzyme Idem

EDTA or its sodium salt is used in the food industry as a metal chelating agent

McEvily et al. (1992)

Acidic polyphosphate mixture has been evaluated as EB inhibitor in combination with ascorbic acid 4-hexylresorcinol inhibits browning, is water soluble, stable, and nontoxic (GRASS)

Ashoor and Zent (1984)

Phosphate-based agents

Substituted resorcinols

Frankos et al. (1991)

Table 8.4. Nonsulfite antibrowning agents applied to fruits and fruit products. Miscellaneous agents. Name

Mechanism of inhibition

Comments

Reference

Enzyme treatments

o-Methyl transferase converts PPO substrates to ferulic acid (inhibitor of PPO) Sodium, calcium, and zinc chloride are pH-dependent inhibitors of PPO, explained by the interaction between charges of halides and active site of PPO

Method too expensive

Finkle and Nelson (1963)

The order of decreasing inhibition power of halides is F > Cl > Br > I

Martinez et al. (1986)

Anions

Janovitz-Klapp et al. (1990)

antibrowning activity in juice, ascorbic acid-2-phosphate (AAP) and -triphosphate was effective for cut fruit surfaces. Combinations of ascorbic acid (AA) with an acidic polyphosphate were highly effective with both juice and cut surfaces. Cinnamate and benzoate inhibited browning in juice, but induced browning when applied to cut surfaces. On the contrary, combinations of betacyclodextrin with AA were effective in juice, but not on cut surfaces. Sapers (1991) infiltrated ascorbic acid-2-phosphate (AAP) and ascorbic acid into apple tissue to control browning. AAP hydrolysis by endogenous acid phosphatase (APase) yielded AA, which became oxidized to dehydroascorbic acid. APase activity varied greatly with commodity, method of sample preparation, and sample pH. Variation in the ability of AAP to inhibit browning in different products could be explained by these factors. Montgomery (1983) treated pear juice concentrates with 0.2 mM cysteine and observed color changes of concentrates during storage for 6 months at different temperatures. Initial browning of concentrates was eliminated by cysteine treatment of pear juice. Cysteine appeared to retard Maillard reaction in pear juice concentrates and no deleterious changes in flavor intensity were noted. During milling and finishing operations EB is difficult to control even with high levels of SO2 or vitamin C because of the incorporation of air. Regarding opalescent and cloudy juices,

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several production procedures effective for reducing EB reactions were published (Chobot and Horulaba, 1983), including the use of ascorbic acid and nitrogen (Fukutani et al., 1986), blanching of the pulp (McKenzie and Beveridge, 1988), and controlled pectolytic enzyme treatment (Gierschner and Baumann, 1988). Montgomery and Petropakis (1980) found that the amount of ascorbic acid required to prevent EB in pear juice is dependent on the length of time between milling and heating. 8.2.3. Effect of the Ascorbic Acid (AA) Content in Color Change As previously described ascorbic acid (AA) does not inhibit polyphenol oxidase directly but acts as a reducing compound and reduces the orthoquinones to dehydroxyphenols. This action will continue as long as the concentration of ascorbic acid is sufficient to maintain a low concentration of quinones. As the concentration of AA is decreased, the quinone concentration increases and causes the formation of the brown pigments. Sapers and Douglas (1987) studied the effectiveness of ascorbic acid (AA) in cut surfaces and apple and pear juices, finding that 40 ppm AA inhibited approximately 60% in raw Granny Smith juice, but only ﬃ20% in Red Delicious juice, after 90 min at 208C. Sapers and Douglas (1987) also evaluated the effectiveness of sodium bisulfite (NaHSO3 ) and ascorbic acid (AA) in cut surfaces and apple and pear juices. The authors found that EB in apple juice was completely inhibited by the addition of 10 ppm SO2 . The effectiveness of ascorbic acid (AA) and erythorbic acid (EA) in inhibiting EB at cut surfaces of apple and in raw apple juice was determined by tristimulus colorimetry by Sapers and Ziolkowski (1987). Lozano et al. (1995) studied the color changes of apple pulp treated with the various AA concentrations at 188C (Fig. 8.8). They found three linear regions in the Hunter L versus log t plot, and a very well-defined breaking point was observed at the point where the AA loses its inhibitory properties after which browning proceeds at the usual rate. Similar behavior was

70 65

AA ppm

60

100 ppm 300 ppm 600 ppm 900 ppm 1200 ppm

CIE L*

55 50 45 40 35 30 1

10

100 Time (min)

1000

Figure 8.8. Influence of ascorbic acid on enzymatic browning in apple pulp (Lozano et al., 1994) with permission.

8

.

Inhibition and Control of Browning 200

(MA)

175

18ⴗC Breaking point t at L*max

150 Time (min)

193

125 100 75 50 25 0 0

2

4

6 8 10 AA levels (ppm)

12

14

Figure 8.9. Time to reach L ¼ 55 and breaking point (end of browning inhibition) for MA sample as related to AA concentration, at 188C (Lozano et al., 1994) with permission.

reported by Matsui et al. (1957), studying the oxidative darkening during the processing of ‘‘natural apple juices.’’ Experimental breaking points as a function of AA levels are plotted in Fig. 8.9. The data in Fig. 8.9 indicate that the addition of AA at levels greater than 600 ppm results in a linear increase of the browning breaking point. Visual observation of apple pulp samples treated with AA by trained judges indicated that for L values greater than 55 the browning can be considered unacceptable. Time required at 188C to attain this level at various AA amounts is also plotted in Fig. 8.9. These values indicate that overtreatment with AA (>600 ppm) will not proportionally increase the time required to reach a maximum level of browning for commercial acceptability. To make some contribution to processing of cloudy apple juice, experiments regarding prevention of EB by adding ascorbic acid were also carried out. When retention of apple pulp in maceration tanks is required, as in the case of cloudy juice production, the amount of ascorbic acid necessary to inhibit EB must be estimated in accordance with the maceration time and temperature. 8.2.4. Nonconventional Chemical Inhibition of EB Table 8.5 lists a selection of chemical methods commonly used to inhibit EB. However, many other nonconventional methods have been developed simultaneously. Vacuum and pressure infiltration were investigated (Sapers et al., 1989) as a means of applying ascorbate- or erythorbate-based EB inhibitors to apple cut surfaces. Apple plugs infiltrated at 34 kPa pressure showed more uniform uptake of treatment solution and less extensive water logging than plugs vacuum infiltrated at 169–980 mbar. Delicious and Winesap plugs and dice gained 3–7 days of storage life at 48C when treated by pressure infiltration, compared to dipping. However, infiltrated dice required dewatering by centrifugation or partial dehydration to prevent water logging. Red Delicious and Winesap plugs, dipped for 90 s in 0.8–1.6% solutions of AA or EA, showed longer lags before the onset of browning with the former compound. AA and EA were similar in the effectiveness in apple juice. Because the relative effectiveness of AA and EA depends on the system in which they are compared, the

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Table 8.5. Classical chemical methods used for the inhibition of EB in selected fruits and fruit products. Fruit/product

Inhibition method

Apple (GD) Apple (McItosh) Apple (GS)

N-acetyl-l-cysteine (25 mM). Reduced glutathione (50 mM) Chitosan Ascorbyl-6-fatty acid esters

Apple

Ascorbic acid derivatives

Apple juice and dices Apple pulp

0.5% amylose sulfate, 0.5% xylen sulfate Ascorbic acid

Apple slices

Ascorbic acid and 4-hexylresorcinol

Avocado Banana Model apple juice solution Carambola slices

High pressure 800 MPa; 258C l-cysteine Citric acid þ ascorbic acid

Pear juice (D’Anjou)

l-cysteine Ascorbic acid (AA)

Plum juice

0.5 mM cysteine, 1.0 Na metabisulfite

Comments

Treatment with 200 ppm chitosan Use of 1.14 mM inhibited EB for 6 h (equivalent to 0.02% ascorbic acid) 0.250% erythorbic acid The inhibitory effect could be enhanced by 0.5% citric acid 100 –1000 ppm, depending on the required breaking point Mixture of 0.01% 4-HR and 0.5% AA reduces EB as 0.1% SO2 5 mM gives 100% inhibition Found oxidation depends on total phenols Treating slices with 1.0 or 2.5% citric acid 1 0.25% ascorbic acid (in water) prior to packaging was very effective in limiting browning 10 mM inhibits 100% PPO, 1 mM AA inhibits 25% PPO, 1 mM l-cysteine inhibits 57% PPO Substrate is phenolics and chlorogenic acids

Reference Molnar-Perl and Friedman (1990) Sapers (1991) Sapers et al. (1989)

Sapers and Ziolkowski (1987) Va´mos-Vigya´zo´ (1995) Lozano et al. (1994) Luo and Barbosa-Canovas (1996) Weemaes (1998) Kahn (1985) Goupy et al. (1995) Weller et al. (1997)

Siddiq et al. (1994)

authors indicated they should not be used interchangeably as sulfite alternatives without experimental verification of equivalence. ¨ zoglu and Bayindirli (2002), using the response surface methodology, found that the O ascorbic acid, l-cysteine, and cinnamic acid combination provided better results as EB inhibitor than as individual compounds. The authors found 0.49 mM AA, 0.42 mM l-cysteine, and 0.05 mM cinnamic acid in cloudy apple juice inhibited browning for 2 h at 258C. 8.2.4.1. Honey The use of honey as a natural browning inhibitor was demonstrated in apple slices, grape juice, and model systems (Oszmianski and Lee, 1990). The browning of apple slices was inhibited to a greater extent by using 10% honey, than by a sucrose solution containing an equivalent sugar concentration. Analysis of honey revealed that a small peptide is responsible for the inhibition of polyphenol oxidase. The efficacy of honey in inhibiting polyphenol oxidase activity varied in accordance with the variety of honey (Chen et al., 1998).

8

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Inhibition and Control of Browning

195

8.2.4.2. Aromatic Carboxylic Acids Cinnamic acid and its analogs, p-coumaric, ferulic, and sinapic acids, were found to be potent inhibitors of apple polyphenol oxidases (Pifferi et al., 1974; Walker and Wilson, 1975). Cinnamic acid at levels of 0.01% was observed to be effective in providing long-term inhibition of polyphenol oxidase in apple juice (Walker, 1976). 8.2.4.3. Proteases Some plant proteases like ficin, papain, and bromelain are sulfydryl enzymes (Labuza et al., 1992; Taoukis et al., 1990), which are very effective as browning inhibitors. Pineapple juice was found to be effective in inhibiting browning in apple rings (Lozano-de-Gonzalez et al., 1993). Bromelain, organic acids, sulfydryl compounds, and certain metallic constituents of pineapple juice are believed to be responsible for this inhibitory effect. Polyphenol oxidase activity in plum juice was significantly reduced when the juice was treated in a column containing immobilized proteases (Arnold et al., 1992). 8.2.5. Miscellaneous Methods The market for lightly processed apples has increased rapidly (Schlimme, 1995). The development of retail and institutional precut apple products has been limited by browning, which can be controlled or minimized by using modified atmosphere packaging (MAP) with selected chemical treatments (El-Shimi, 1993). Lakakul et al. (1999) studied the use of plastic films to control moisture loss and respiration rate of cut apples. Raghavan et al. (1996) reported that damage to apple tissue texture could be reduced by calcium treatment and proper storage temperature. Figure 8.10 shows different treatments that have been experimented during the last few years to inhibit EB in fruit products. Although some of these methods have been impractical or expensive up to now, they are sufficiently innovative and safe for use in foods.

Ultrafiltration and nanofiltration

Sonication irradiation

Supercritical CO2 Miscellaneous and non conventional EB treatments

High pressure treatment

Blanching during size reduction

Figure 8.10. Miscellaneous and nonconventional methods for the treatment of enzymatic browning.

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Zemel et al. (1990) showed that PPO activity could be irreversibly inhibited by temporarily lowering the pH to 2.0 with HCl. However, the pH must to be adjusted to its initial value by the addition of NaOH solution. This treatment inhibited EB and stabilized the apple juice, but was unfavorable as it affected the flavor. Tronc et al. (1997) used electrodialysis (ED) to prevent EB in cloudy apple juice, without employing additives, by temporarily acidifying the juice and then readjusting its pH to the initial value. ED is a membrane technique that results in the separation of ions (Lopez-Leiva, 1988). This method has been used to regenerate mineral acids and bases (Mani, l991). The application of ED with bipolar membranes proved to be a convincing approach for achieving changes in pH of cloudy apple juice. The authors indicated that in this way, the pH could be varied by the gradual incorporation of protons and hydroxyl ions derived from the dissociation of water, without affecting juice flavor by salt formation. Sapers et al. (1989) have shown that b-cyclodextrins inhibit browning of raw apple juice, and also found that ultrafiltration (UF) reduces EB in fruit products (Goodwin and Morris, 1991). Recent studies have shown that the simultaneous application of heat and ultrasonic waves had a synergistic effect on PPO inactivation. The process called mano-thermosonication needs special equipment and can be used with foods that are damaged by drastic heat treatment (Chen et al., 1992a,b). Supercritical carbon dioxide (SC-CO2 ) treatment was tried for inactivation of PPO in vegetables (Chen et al., 1992). In fruits SC-CO2 was used for the inactivation of pectinesterase (Arreola et al., 1991). With the exception of UF the other nonconventional methods involve heat treatment. 8.2.5.1. Irradiation Food irradiation is increasingly recognized as a method for reducing postharvest food losses and ensuring hygiene. Food irradiation is effective in securing the long-term preservation of foods through the inactivation of microorganisms. Fruits can be preserved by irradiation, thereby delaying their maturation or sprouting. Browning reduction in tropical fruits by gamma irradiation was reviewed by Thomas (1984, 1986). Browning was minimized by controlling the dosage level of the applied radiation. However, ionizing radiation at doses exceeding 1 kGy can introduce various types of physiological disorders in food products. Free radicals produced during the treatment of food with ionizing radiation, are capable of reacting with various food constituents and inducing undesirable side effects, such as tissue darkening, lipid oxidation, and decreased vitamin content. Nonenzymatic browning (NEB) reactions of free amino acids and proteins with reducing sugars, such as glucose, may be responsible for this discoloration. The sensitivity of enzymes to ionizing radiation is defined as the dose required to inactivate 63% of the original activity of the enzyme, D37 . Table 8.6 lists D37 values of some enzymes in model systems measured in the dry state (Roozen and Pilnik, 1971). Combined treatments using both irradiation and heat or other methods have demonstrated a synergistic effect. 8.2.5.2. Ultrafiltration (UF) UF has shown to be effective in stabilizing the color of fruit juices (Flores et al., 1988; Sims et al., 1989). Moreover, Goodwin and Morris (1991) studied UF as an alternative to sulfiting for the control of EB. UF is believed to remove polyphenol oxidase, but not lower molecular

8

.

Inhibition and Control of Browning

197

Table 8.6. Ionizing radiation doses to inactivate 63% (D37 ) of some selected enzymes. Enzyme Alkaline phosphatase Pectin esterase Peroxidase

D37 (kGy) 40 –50 60 30 –70

weight polyphenols or Maillard-reaction precursors, which could undergo NEB during storage. Galeazzi et al. (1981) found that banana PPO fractions had molecular weights >30 kDa, within the range of molecular weight cut-offs for UF membranes. 8.2.5.3. High-pressure treatments High-pressure treatments reduce microbial counts and enzyme activity, and affect product functionality (Farr, 1990; Hoover et al., 1989; Cheftel, 1991). This provides a good basis for development of new processes for food preservation or product modifications (Mertens and Knorr, 1992). The first commercial products made, using high-pressure treatments, have been almost exclusively plants or product-containing plants (Knorr, 1995). Effects of high-pressure treatments on enzymes may be related to reversible or irreversible changes in protein structure (Cheftel, 1992). However, loss of catalytic activity can differ depending on type of enzyme, the nature of substrates, and the length and temperature of processing (Cheftel, 1992; Kunugi, 1992). Ogawa et al. (1990) reported the effect of high-pressure treatments on pectinesterase and peroxidase activity in model systems from mandarin juice. Cano et al. (1997) determined the effects of high-pressure treatments up to 400 MPa combined with mild heat treatments up to 6078C on different enzymes, including polyphenol oxidase (PPO) in strawberry pure´e and orange juice. Pressurization/depressurization treatments caused a significant loss of strawberry PPO (60%) up to 250 MPa. Although neither enzyme, including PPO, was completely inactivated after pressurization from 100 to 400 MPa, no recovery of enzyme activity was observed during storage. Degree of inactivation varied depending on the type of fruit and vegetable products studied (Knorr, 1995), and strong enzyme activation could be observed in cell-free extracts (Anese et al., 1995). Weemaes et al. (1998) found PPO from apples, avocados, grapes, pears, and plums was rather pressure stable. While inactivation of PPO from apple became noticeable at 600 MPa (258C), for pear PPO, pressures as high as 900 MPa were required (Fig. 8.11). Simultaneous application of mild heat increased the PPO inactivation rate constant. Table 8.7 lists some high-pressure treatments of fruit, which were positive for PPO inactivation. The inactivation of a pure enzyme by pressure is dependent on the immersion medium, the pH, as well as the temperature and duration of the treatment. Moreover, food constituents may show protective effects on enzymes during high-pressure treatment (Fig. 8.12). Mushroom polyphenol oxidase shows very high-pressure stability, although it is a thermosensitive enzyme that is readily inactivated by temperatures exceeding 508C (Weemaes et al., 1997).

8.3. INHIBITION AND CONTROL OF NONENZYMATIC BROWNING (NEB) The extent of NEB on fruit products depends on product composition, water activity, storage time, and temperature, as previously discussed (Chapter 7). NEB in fruit products may be

198

Fruit Manufacturing 0 −0.1

25ⴗC

Log relative activity

−0.2 −0.3 −0.4 −0.5 −0.6 −0.7 Apple pear

−0.8 −0.9 −1 0

25

50

75

100 125 Time. min.

150

175

200

225

Figure 8.11. Inactivation of PPO from apple and pear, at 800 and 900 MPa, respectively (adapted from Weemaes et al., 1998).

inhibited or reduced by refrigeration, control of water activity in dehydrated fruits (Labuza and Saltmarch, 1981), reduction of amino nitrogen in juices (Prı´ncipe and Lozano, 1991), and use of different chemical inhibitors. Two basic treatments have been used for the control or reduction of nonenzymatic reactions (Fig. 8.13): . .

Preventive methods, which are those that limit the advance of NEB reactions; while Restorative methods, which basically let the NEB to develop, reducing later the product of the deteriorative reactions.

8.3.1. Preventive Methods 8.3.1.1. Temperature Control It is well known (Reynolds, 1965) that NEB reaction is retarded by reducing the temperature. Figure 8.14 shows a t–T plot of the several conditions apple juice undergoes from milling to distribution. In the particular case of processing apples to obtain juice concentrate, it may be expected that color, taste, and flavor would be undesirably modified and the questions are: Table 8.7. Effective high-pressure treatments for fruit PPO inactivation. Fruit

Treatment

Reference

Apple (pH 4.5) Avocado Pear in slices White grapes

>500 MPa/258C/1 min 800 MPa/258C 900 MPa/258C (slight inactivation) 700 MPa/258C

Anese et al. (1995) Weemaes et al. (1998) Weemaes et al. (1998) Weemaes et al. (1998)

8

Inhibition and Control of Browning

.

199

1 600 MPa

0.9 0.8

Activity (relative)

0.7 0.6

700 MPa

0.5 0.4 0.3

800 MPa pH=7

0.2

45ⴗC

0.1

900 MPa

0 0

5

10

15 20 Time (min)

25

30

35

Figure 8.12. Effect of high hydrostatic pressure on relative activity of polyphenol oxidase at pH ¼ 7 (458C) (adapted from Seyderhelm et al., 1996).

. .

How much, in any suitable unit, is the damage introduced by a particular operation? Moreover, can it be quantitatively related to the magnitude of treatment, that magnitude being also measured in any conventional units, such as temperature, time, concentration, and the like?

In the case of juices, more precisely apple juice concentrate, out of the those sensorial properties mentioned above, color is the one that can be submitted to a more objective measurement by well-known techniques. Flavor and taste, though perhaps more relevant from the hedonic point of view, are more subjective in nature.

Temperature control Process optimization Preventive

Ion exchange treatment Use of chemical inhibitors

Neb treatment Synthetic adsorbers Restorative

Nanofiltration Activated charcoal

Figure 8.13. Basic nonenzymatic browning treatments.

200

Fruit Manufacturing 10000000

Transportation

1 month

Time (s)

1000000

1 day 30%

100000 10000

10% Storage

1h

1000 100

Clarification Evaporation

10 1 0

1 min

20

40

60

80

100

120

Temperature (ⴗC) Figure 8.14. Time–temperature plot during the processing of clarified apple juice concentrate (broken lines indicate percentage of color increase).

Hence it is common to consider the browning expressed as the absorbance or optical density at a given light wavelength, namely 420 nm, when measured on the pure or singlestrength juice. Less common, but still well known, is the qualification of the color through their three-tristimulus parameters (e.g., Hunter L, a, and b). 8.3.1.2. Process Optimization Concentration by evaporation is a very common practice in the fruit juice and pulp industry. Multiple-effect evaporators used in the fruit juice processing plants were designed to eliminate water under vacuum at relatively low temperatures. However, it is not unusual in practice to find very high temperatures in the first stage of processing. This can lead to changes both in color and flavor of the juice, mainly due to NEB. Process control has been increasingly adopted in the food industry during the last 30 years both to improve quality and reduce energy costs (Frost, 1977; Lozano et al., 1984). In order to properly adopt control strategies it is necessary to obtain either the empirical or the simulated dynamic model of the process by itself, without considering any control loop. Tonelli et al. (1990) presented a versatile computer package useful for the simulation of the open-loop dynamic response of a triple-effect evaporator for the concentration of fruit juice. In addition to fluid dynamics and thermal considerations some attention should be paid to the potential damage, which could be induced during concentration. However, selection of the pair values t–T to perform a given process is not a trivial task, but a trade-off between opposite considerations. For instance, low temperatures are more expensive because of the demand for a larger area, but high temperatures introduce the risk of scaling. Morgan (1967) showed that process side heat transfer coefficient dropped 10 times after 1 h operation in a tomato paste evaporator as a consequence of fouling. It is also known that heat transfer coefficient increases with the temperature difference between the heating medium and the solution, which makes advisable to work at high vacuums, low temperatures. But that means high vapor volumes, and larger piping and related pieces of equipment to maintain pressure drop within practical limits, which in turn means larger investment capital. If, alternatively, higher-pressure steam is used, the process is more expensive.

8

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Inhibition and Control of Browning

201

The formation of 5-hydroxy-methyl-2-furfuraldehyde (5-HMF), an intermediate of browning reactions in apple juice, has been directly related to the severity of heating in fruits and honey (Chapter 7). Toribio and Lozano (1986) found that this reaction follows a zero-order kinetics, after an induction period where no buildup of 5-HMF is detected. Babsky et al. (1986) also found that accumulation of 5-HMF achieves maximum accumulation after a long period of storage. These results may indicate that rate of formation of 5-HMF is similar that of a second-order autocatalytic reaction. However, no further advances were made to develop a complete and realistic mechanism of reaction based on the theory available. From an operational point of view and damage introduced by the thermal treatment, it is apparent that time is more critical than temperature: residence times of several minutes are common in evaporators for which relatively small changes in temperature may produce a dramatic increase in HMF concentration. Alternatively, if time can be kept within small values, the same temperature step does not provoke a significant HMF growth (Toribio and Lozano, 1986). This confirms the accepted practice of moving to lower time–higher temperature combination whenever foods are to be thermally processed. It is experimentally known that actual residence times are several times longer than those calculated on the assumption of piston flow. For instance, in recirculating equipment, Moore and Pinkel (1968) showed that the actual holding time for 97% replacement of the liquid volume is 3.6 times greater than the average calculated as plug flow. There are, in addition, dead times in the distributors, pipes, pumps, and vapor separators, in which the juice is at the same temperature, which should be taken into account, as the damage is also in progress. However, this analysis is restricted to the liquid transit along the tubes since those times cannot be accurately estimated, and can be assumed to be the same in both configurations for comparison purposes. EXAMPLE 8.1 Application to multiple-effect evaporator design: evaporator selection Selection of a given type of evaporator is a task governed by many considerations. When fruit juice damage is considered, the falling film evaporator is an attractive alternative, since it imposes only short residence times to the solution, allowing for relatively low temperatures. Tonelli et al. (1990) simulated an actual 3-effect falling film unit, by means of a program specially formulated for that purpose, which provides mass flow rates and concentrations, at the entrance and discharge of each effect, as well as temperatures live steam consumption, and mass and enthalpy balances, for both backward and forward feed. It takes into account the rise in boiling point of solutions, and allows for feed preheating and vapor thermal recompression. The computer package simulates a tripleeffect horizontal flash concentrator (Fig. 8.15), with a capability to concentrate about 7,600 kg/h of a 16.38Brix clarified apple juice (Table 8.8). A more complete description of the simulation model and the industrial unit was given previously (Tonelli et al., 1990). Once flow rates and temperatures were known, physical properties and Reynolds numbers in the tubes were easily computed. Film widths were calculated by means of the equation presented by Sideman (1981), at entrance, discharge, and arithmetic average. Holdup was calculated as a single value for average conditions. Finally, velocities and residence times were calculated for average conditions. All data are presented in Table 8.9.

202

Fruit Manufacturing Tap water

Vapors

Steam

Barometric foot

1st effect Preheating 2nd effect

Concentrate Feed

3rd effect

Figure 8.15. Sketch of a triple-effect flash concentrator.

By applying the correlation for damage as a function of time, temperature, and concentration to each effect for both backward and forward flow arrangements, the values of HMF formation shown in Table 8.8 are obtained. It is apparent that forwarded flow is much less harmful, the concentration being one order of magnitude lower than that produced in the countercurrent or backward arrangement. This is proof of the generalized practice in favor of the parallel flow for heat-sensitive materials. In both cases the first effect becomes the most damaging one, while in the countercurrent one, the reduction in viscosity of the more concentrated product due to the higher temperature, is not enough as to reduce the residence time significantly, so the juice is submitted to the most unfavorable combination of the relevant variables. It is seen that while in the forward flow the first effect provokes 78% of the damage, in the backward one it is 99.3%, the absolute figure being ten times greater. Regarding the aroma-stripping operation the calculation on the commercial unit installed with the 3-effect evaporator did not indicate HMF formation for the prevailing conditions. Lozano et al. (1995) studied the open-loop dynamic response of an apple juice evaporator, based on the kinetics of 5-HMF formation. Results indicated that 5-HMF content of concentrated juice is strongly dependent on the temperature at the 1st effect. Level of 5-HMF was below 30 mg/L, a proposed reasonable limit (Toribio and Lozano, 1987) for clarified apple juice. Table 8.9 also lists the estimated increase in 5-HMF as affected by a variation in the temperature at the first effect of the simulated evaporator and with the set of industrial

Table 8.8. Experimental industrial operating conditions. Feed flow rate (kg/h) Feed concentration (8Brix) Feed temperature (8C) Steam pressure (kPa) Thirst effect pressure (kPa)

7650.0 16.3 45.0 182.0 10.0

8

.

Inhibition and Control of Browning

203

Table 8.9. Formation of 5-HMF as affected by temperature at the first effect. Temperature,8C 1st effect

Soluble solids,8C 3rd effect

5-HMF, mg/L

60 60 60 70 70 71 75 75 75

5.02 7.80 12.63 6.6 10.8 16.8 7.4 12.5 18.6

100.1 104.4 109.1 100.1 104.4 109.2 100.0 105.1 109.1

operation conditions given in Table 8.8. The authors concluded that the knowledge of the dynamic response of heat-exchange equipment, like multiple-effect evaporators, together with the appropriate kinetics equations of deteriorative reactions is important to estimate and reduce the heat damage. This reduction can be achieved by the implementation of an appropriate control configuration. 8.3.1.3. Ion Exchange Treatment Ion exchange resins have been used in the industry for discoloration of syrups (Harris, 1986), the hydrolysis of lactose (Guerin and Lancrenon, 1982), and the anthocyanin recovery from fruit bagasses (Chiriboga and Francis, 1973) among other applications. Ion exchange treatments of liquid foods are legally permitted in several countries (Rankine, 1986; Johnson and Chandler, 1986). Ion exchange resins, as well as different types of adsorbers, have also been used in fruit juices (Withy et al., 1978) to elucidate the role of amino acids and polyphenols in the formation of brown color polymers (Cornwell and Wrolstad, 1981) and for deacidification and debittering (Johnson and Chandler, 1986). Prı´ncipe and Lozano (1990) studied the effect of such treatments on the quality of the juices, as well as the operative applications of these processes. The exchange process is largely confined to a narrow region in the resin bed, which, within a short time after the liquid to be treated is flowing, moves down the bed at a constant rate and leaves the column at a point called the break through point. At that point the absorbed compound suddenly increases its concentration in the effluent, resulting in a typical S-curve. In order to calculate bed capacities the following equation can be used: ð Ve Ct ¼ (X X0 )dV=Vra (8:1) 0

where Ct is the total capacity of the column, Vra the average volume occupied by resin in the bed, X and X0 are the concentrations of compound to be absorbed in effluent and influent, respectively, V the volume eluted at any time, and Ve the volume at which exhaustion of column results. If integration takes place up to the breakthrough point (Vt ) only, the resulting column capacity is called effective capacity. Figure 8.16 shows the reduction in the total amino acids (AA) when apple juice (158 Brix; pH ¼ 3:8) is passed through a cation exchange column (DOWEX 50 8), as a function of the volume of effluent collected per gram of dry resin (Prı´ncipe and Lozano, 1990).

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Fruit Manufacturing

Amino acids, mg/100ml

1.6

0.9

3rd 2nd

0.2

1st

Vt

50

100

150

Effluent, ml/g Figure 8.16. Effluent amino compound concentration versus effluent volume per gram of cation resin, with the number of column regenerations as a parameter. Vt is the volume at which break through point occurred (Reprinted from Lebens. Wiss. Technol., 24, Prı´ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).

In general, successive regenerations did not reduce the column capacity and the resin may be used many times. Progress of the ion exchange may be monitored by pH readings in the outlet juice variable (Fig. 8.17). In order to recover the original juice pH, the cationexchanged juice may be passed through an anion-exchange resin. The anion-exchange column also reduced the color of the juice. PH

4.0

3.0

Anion exchange 2.0 Cation exchange

20

40 60 Effluent, ml/g

80

100

Figure 8.17. pH of column effluent as a function of effluent volume per gram of resin (Reprinted from Lebens. Wiss. Technol., 24, Prı´ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).

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8.3.2. Restorative Methods During fruit juice discoloration, chromophoric components are eliminated, without modifying if possible, the other components of the product. While in the case of apple juice polyphenols are to be removed, red coloration in orange juice is caused by anthocyanins. The adsorption capacity of certain substances eliminating coloring matter by adsorption is exploited for discoloration. Properties such as grain size, surface area, and porosity define adsorbers’ capacity. This could be attributed to polyphenol adsorption. Adsorption forces are ruled by weak Van der Waals’ forces, which are temperature dependent. Adsorption of color is usually performed by adding activated carbon (AC) as a slurry to the juice, since this gave better dispersion than the addition of dry carbon. Table 8.10 lists the average characteristics of a typical AC used for apple juice discoloration (Prı´ncipe and Lozano, 1990). The ACs listed in Table 8.10 had practically the same adsorption capacity and kinetics, which was in accordance with the similarity in their characteristics. Absorption of solutes from a dilute solution, as in the case of brown compounds in apple juice, can be described by an empirical isotherm similar to that attributed to Freundlich: Y ¼ m Xn

(8:2)

where Y ¼ C=C0 , color at the equilibrium/initial color; X is the units of color adsorbed (L juice/g AC), and m and n are constants experimentally determined. Discoloration is basically a batch operation where the amount of insoluble adsorbent (activated carbon) is very small with respect to the amount of product treated and the highly colored compounds removed are much more strongly adsorbed than the other juice constituents (sugars, acids, etc.). A solute, or color, balance is: A=J ¼ (Y0 Yf )=(Xf X0 )

(8:3)

where A is the mass of activated carbon, J the volume of juice to be treated; X0 and Xf are the initial and final color adsorbed/mass of carbon, and Y0 and Yf are the initial and final color of the treated juice. Since the AC used ordinarily is fresh (X0 ¼ 0), substitution of (8.3) in (8.2) gives: A=J ¼ (Y0 Yf )=(Yllm)l=n

(8:4)

This equation permits the calculation of the carbon to juice ratio, for a given change in the juice color from Y0 to Yf . Table 8.10. Properties of activated carbons. Appearance

Black powder

pH Activity (blue methylene test) Water content (%) Particle size (%): mesh 200 mesh 325 Density (apparent, kg/l) Ash (%) Sulfates (%) Iron (ppm)

4.5–5.5 20(minimum) 10 –15 5 10 0.34 – 0.38 7 (maximum) 0.6 (maximum) 100

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Example 8.2 Application of activated charcoal for apple juice discoloration Consider the need to reduce by 78% the color of a juice with activated charcoal. Adsorption of color by AC is fast and strongly dependent on the amount of AC used. On the other hand, the influence of temperature between acceptable working values (40 –808C) was practically negligible. The application of the Freundlich-type equation (8.2) to the equilibrium data resulted in: Y (C=C0 ) ¼ 44:38X 4:42 (C=gr:C)

(8:5)

This equation was plotted in Fig. 8.18, which represents a typical equilibrium curve for a single-stage discoloration process. Figure 8.18 also shows the operating line between the initial relative color (Y0 ¼ 1) and the coordinates of point (Xl , Yl ). If sufficient contact time is allowed and equilibrium is reached, the operating line intersects the Freundlich isotherm at Xl . The operating line in the example was a slope A=J ¼ 2:75, which directly determined the necessary amount of AC. 8.3.2.1. Effect of Storage Figure 8.19 shows the effect of prolonged storage on clarified apple juice color, after deamino acid treatment. Sample ‘‘a’’ browned at approximately the same rate as the control juice, while ‘‘b’’ showed a reduced rate of browning. This behavior can be explained by considering that free amino acids remained in juice. Therefore, the heated juice had practically the same amino acid content as the fresh juice, which was demonstrated to directly enhance the rate of NEB (Babsky et al., 1986). 1.0 n = 4.52

Y , C/Co

.8

.6

.4

Operating line. slope = A/J

.2

(X1 , Y1)

.3

.1 X , color / g c.a

Figure 8.18. Nonenzymatic browning of apple juice concentrate as a function of time of storage, at 378C. Control: untreated juice. Sample ‘‘a’’: Fresh processed juice discolored with AC. Sample ‘‘b’’: Long-term storage, highly colored, AJC rediluted and treated with AC (Reprinted from Lebens. Wiss. Technol., 24, Prı´ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).

8

.

Inhibition and Control of Browning Red Del.: 75 Brix 37⬚C

1.8 Absorbance 420 Nm

207

Sample “a”

1.4

1.0 Sample “b”

o.6

o.2 0

40

120

80

Storage time, days Figure 8.19. Freundlich equilibrium curve for a single-stage discoloration of apple juice (Reprinted from Lebens. Wiss. Technol., 24, Prı´ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).

On the other hand, in sample ‘‘b’’ most of the amino compounds, already reacted to form melanoidins, were adsorbed by the ACs, and NEB reactions scarcely developed. Figure 8.20 shows a typical browning curve during storage of apple juice with different levels of total amino acid content.

RED DELICIOUS 37⬚C

1.8

Control Abs., 420 nm

1.4

1.0

Sample “a”

0.6 Sample “b” 0.2 0

20

40

60

80

Time of storage, days Figure 8.20. Color of ion exchange-treated AC as a function of storage time, at 378C. Sample ‘‘a’’: Initial amino acid content, 57 mg/L. Sample ‘‘b’’: Initial amino acid control, 38 mg/l (Reprinted from Lebens. Wiss. Technol., 24, Prı´ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).

208

Fruit Manufacturing Table 8.11. Effect of ion-exchange treatment on apple juice.

Total amino acids (meq/l) Acidity (g/l) Malic acid, L (g/l) Total phenols (ppm) Calcium (mg/100 g) Sodium (mg/100 g) Iron (ppm) Soluble solids (8Brix) Initial color (Abs4z()

Before

After

Values determined in natural apple juice in natural

7.0 3.7 4.1 39.3 30.1 23.s 12.4 15.4 0.337

5.8 4.5 4.0 15.4 0.7 0.1 1.2 15.3 0.124

3–30 2.4–7.6 — < 300 >3 — < 18 — < 0:5

*Babsky et al. (1986); Prı´ncipe and Lozano (1991).

Partially treated juice suffered some compositional changes, with the major components retaining acceptable values, but significant reductions in concentrations of calcium and iron were observed. Results of the experimental treatment of apple juice with ion-exchange resins are listed in Table 8.11. Calcium, amino acids, and other nutrients have been shown to affect the growth of microorganisms. Treated juices became much less susceptible to microbiological spoilage. On the other hand, Fe levels greater than 8 ppm are practically unacceptable in clarified apple juice. Adsorption, or ion exchange treatment, can modify the color attributes of the apple juice. Treated juices were more purple than fresh juice, and displayed a more natural chromaticity after several days of storage (Fig. 8.21). Prı´ncipe and Lozano (1990) concluded that AC treatment should only be used to reduce the color of fruit juice concentrates subjected to prolonged and/or high temperature conditions of storage. Carbon adsorption of fresh juices will not reduce the rate of NEB.

Yellow Db

Orange

20

78 days

10 15 days

Green –5

0

0

5

10

15

Da red

7 days

–10 Purple

–20 blue Figure 8.21. Hunter Da and Db parameters with time of storage at 378C. (&) Untreated juice, (O) AC-treated juice, (x) ion-exchange treated juice (Reprinted from Lebens. Wiss. Technol., 24, Prı´ncipe, L. and Lozano, J.E., Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange, pp. 34–38 (copyright) 1991, with permission from Elsevier).

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The process of discoloration with AC included tedious steps like clarification with bentonite and gelatin, filtration with a filterpress or vacuum filter, and reconcentration to original soluble solids’ content. On the other hand, fresh clarified fruit juice can be treated with cation þ anion exchange resins in order to reduce the amino compound content to levels low enough to satisfactorily reduce the rate of Maillard-type browning. Additionally, ion exchange treatment can also adjust the pH values and reduce the amount of micronutrients, which could make the juice more stable from a microbiological standpoint. AC and resins are readily available, and resins can be reactivated economically and simply, and used over and over again without significant working capacity reduction. 8.3.2.2. Use of PVPP (Polyvinyl Polypyrrolidone) The synthetic polymer polyvinyl polypyrrolidone (PVPP), which has been used in the beverage industry since several years, is an absorbent, which shows selective affinity to polyphenols and tannins (Binnig, 1992). PVPP is used not only for discoloration of fruit juices, but also to prevent haze formation after processing. The regenerability of PVPP is the advantage of this product compared with the AC treatment. The application of PVPP can be realized either by batch process or with continuous dosage using a precoat filter for the elimination. For a significant discoloration as much as 3 g/L must be added (Hoffsommer and Cook, 1991). Regeneration of PVPP is done by alkaline solution followed by an acid neutralization. Polyvinyl polypyrrolidone (PVPP) shows a high selectivity for adsorption of polyphenols and has been established as a final stabilization treatment after UF (Gu¨nther and Stocke´, 1995). 8.3.3. Miscellaneous Methods for Inhibition and Control of Nonenzymatic Browning 8.3.3.1. Color Reduction by Combined Methods Various pre- and post-treatments are available to avoid post-turbidity and discoloration of fruit juices. Stabilization of beverages by gelatin, bentonite, and silica gel is a widespread conventional treatment. Pretreatment techniques, including hyperoxidation of raw juice with PPO prior to UF, have been used as an alternative (Giovanelli and Ravasini, 1993; Maier et al., 1994). The use of combined adsorbent resins for clear apple juice stabilization has gained increasing importance as a final treatment after clarification (Schobinger et al., 1995; Weinand, 1995). However, such treatments imply an additional cost in existing juice processing lines. Polyphenols that are responsible for haze formation and browning during storage of clear apple juice and concentrate, could be selectively removed by an UF process using membranes of polyethersulfone and polyvinylpyrrolidone (Go¨kmen et al., 1998). The authors compared their results with those from commercial UF membranes made out of regenerated cellulose acetate. The effects of laccase treatment on removal of polyphenols and color in apple juice were also investigated. Custom membranes were effective in reducing the amount of polyphenols. A remarkable desired color removal of apple juice could also be achieved using these membranes. Resulting products were stable in color and had clarity at 508C for up to 6 weeks. Laccase treatment increased the percentage removal of polyphenols from apple juices. However, laccasetreated samples were more susceptible to coloration and haze formation during storage. Kacem et al. (1987) studied the NEB during the storage of single-strength orange juice and synthetic orange drinks under aerobic and anaerobic conditions. The effect of free amino acids on browning was linear, with concentration being more pronounced in the presence of high

210

Fruit Manufacturing 0.7

0.6

4.2 mg/100mL 38 mg/100mL

0.5 71.8 mg/100mL 0.4

0.3

0.2

0.1

0 0

4

8

12

16

20

Figure 8.22. Effect of ascorbic acid concentration on browning of orange drinks with 0.66% amino acids. Solid line indicates juice stored in retort pouch, while dashed line represents juice stored in polyethylene pouch (Kacem et al., 1987 with permission).

levels of ascorbic acid. Ascorbic acid was found to be the most reactive constituent of orange juice (Fig. 8.22). Packaging in polyethylene pouch greatly accelerates loss of ascorbic acid. 8.3.3.2. Use of Chemical Inhibitors Bolin and Steele (1987) investigated the effect of various treatments on NEB of dried apples, and determined that cysteine incorporation did not reduce browning during storage. The same was valid for manganese and tin addition. Many nonsulfite compounds have been shown to exhibit NEB protection in a variety of foods. Trehalose has been found to retard reaction between dry proteins and reducing sugars (Loomis et al., 1979). Bolin et al. (1976) used packaging with nitrogen headspace, to reduce the darkening rate in sulfured dried peaches. The authors attributed only 20% of the NEB to Maillard-type reactions. Tamaoka et al. (1991) studied the effect of high pressure (up to 500 MPa at 508C) on Maillard reaction between amino compounds with carbonyl compounds. Results indicate that the high pressure may suppress the browning process.

8.4. CONCLUSIONS The heating of the fruit mash or juice immediately after crushing the fruit appears to be the most effective way to control EB in many fruit products. The addition of sulfur dioxide, ascorbic acid, or cysteine has been used to retard browning during the heating period. The effect of a definite amount of AA in apple fruit pulp showed a very well-defined breaking point after which browning proceeds at the usual rate. Nontraditional methods, like UF of liquid fruit products, use of supercritical carbon dioxide, or sonication, in combination with heat treatments have been used by researchers since the last decade.

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Although foreign additives are considered ever more undesirable in foods in general (for health reasons), the inhibition of chemicals will play primary role in the prevention of EB, at least in the very near future. The search for effective safe inhibitors of the NEB is stimulated by the need to: (1) prevent undesirable Maillard reactions, and (2) find alternatives to the use of sulfites. Organoleptic considerations are the major barrier to the use of some Maillard inhibitors (O’Brien and Labuza, 1994). Cysteine appears to be a good alternative to the use of sulfite in foods at present.

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Labuza, T.P. and Riboh, D. (1982). Theory and Application of Arrhenius kinetics to deterioration of foods. Food Technol. 36: 66–74. Labuza, T.P., Lillemo, J.H. and Taukis, P.S. (1990). Inhibition of polyphenol oxidase by proteolytic enzymes. Fruit Process. 2: 9–13. Lakakul, R., Beaudry, R.M. and Hernandez, R.J. (1999). Modeling respiration of apple slices in modified-atmosphere packages. J. Food Sci. 64: 105–110. Langdon, T.T. (1987). Prevention of browning in fresh prepared potatoes without the use of sulfiting agents. Food Technol. 41(5): 64 –67. Lee, C.Y., Smith, N.L. and Pennesi, A.P. (1983). Polyphenol oxidase from DeChaunac grapes. J. Sci. Food Agric. 34: 987–991. Loomis, S.H., O’Dell, S.J. and Crowe, J.H. (1979). Anhydrobiosis in nematodes: inhibition of the browning reaction of reducing sugars with dry protein. J. Exp. Zool. 208: 355–360. Lopez-Leiva, M. (1988). The use of electrodialysis in food processing. Part 1: Some theoretical concepts. Lebens. Wiss. Technol. 21: 119–125. Lozano, J.E., Elustondo, M.P. and Romagnoli, J.A. (1984). Control studies in an industrial apple juice evaporator. J. Food Sci. 49: 1422–1427. Lozano, J.E., Biscarri, R.D. and Ibarz, A. (1994). Enzymatic browning in apple pulps. J. Food Sci. 59: 1– 4. Lozano, J.E., Porras, J.A., Errazu, A. and Tonelli, S. (1995). Prediction of 5-HMF formation in an industrial apple juice evaporator. J. Food Sci. 60: 1292–1294. Lozano-de-Gonzalez, P.G, Barrett, D.M., Wrolstad, R.E., Durst, R.W. (1993). Enzymatic browning inhibited in fresh and dried apple rings by pineapple juice. J. Food Sci. 58(2): 399– 404. Luo, Y. and Barbosa-Ca´novas, G.V. (1996). Preservation of apple slices using ascorbic acid and 4-hexylresorcinol. Food Sci. Technol. Int. 2: 315. Maier, G., Frei, M., Wucherpfenning, K., Dietrich, H. and Ritter, G. (1994). Innovative processes for production of ultrafiltrated apple juices and concentrates. Fruit Process. 5: 134 –136. Mani, K.N. (1991). Electrodialysis water splitting technology. J. Membrane Sci. 58: 117–138. Martı´nez, J.H., Solano, F., Penadiel, R., Galindo, J.D., Iborra, J.L. and Lozano, J.A. (1986). Comparative study of tyrosinase from different sources: relationship between halide inhibition and the enzyme active site. Comp. Biochem. Physiol. 83B: 633. Matsui, S., Ito, S. and Murata, N. (1957). Studies on apple juice processing. 1: Prevention of oxidative darkening in apple juice during processing. Tokai Kinki Nosikenkyu Hokoku 4: 1–18. McEvily, A.J., Iyengar, R. and Otwell, W.S. (1992). Inhibition of enzymatic browning in foods and beverages. Crit. Rev. Food Sci. Nutr. 32: 253–273. McKenzie, D.L. and Beveridge, T. (1988). The effect of storage, processing and enzyme treatment on the microstructure of cloudy Spartan apple juice particulate. Food Microstruct. 7: 195–203. Mertens, B. and Knorr, D. (1992). Development of nonthermal processes for food preservation. Food Technol. 46(5): 124 –133. Molnar-Perl and Friedman, M. (1990). Inhibition of browning by sulfur amino acids, apples and potatoes. J Agric. Food Chem. 38(8): 1652–1656. Montgomery, M.W. (1983). Cysteine as an inhibitor of browning in pear juice concentrate. J. Food Sci. 48: 951–952. Montgomery, M.W. and Petropakis, H.J. (1980). Inactivation of Bartlett pear polyphenol oxidase with heat in the presence of ascorbic acid. J. Food Sci. 45: 1090 –1091. Moore, J.K. and Pinkel, E.B. (1968). When to use single pass evaporators. Chem. Eng. Prog. 64(7): 39–44. Morgan, A.I. (1967). Evaporation concepts and evaporators design. Food Technol. 21: 1353–1359. O’Brien, J.M., Labuza, T.P. (1994). Symposium provides new insights into nonenzymatic browning reactions. Food Technol. 48(7): 56 –58. Ogawa, H., Fukuhisa, K., Kubo, Y. and Fukumoto, H. (1990). Pressure inactivation of yeast, molds and pectinesterase in Satsuma mandarin juice, effect of juice concentration, pH and organic acids and comparison with heat sanitation. Agric. Biol. Chem. 54(5): 1219–1225. Oszmianski, J. and Lee, C.Y. (1990). Inhibition of polyphenol oxidase activity and browning by honey. J. Agric. Food Chem. 38: 1892–1898. ¨ zoglu, H. and Bayindirli, A. (2002), Inhibition of enzymic browning in cloudy apple juice with selected antibrownO ing agents. Food Control 13(4 –5): 213–221. Park, E.Y. and Luh, B.S. (1985). Polyphenoloxidase of kiwifruit. J. Food Sci. 50: 678–683. Pierpoint, W.S. (1966). The enzymatic oxidation of chlorogenic acid and some reactions of the quinone produced. Biochem. J. 98: 567.

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Pifferi, P.G., Baldassari, L. and Cultrera, R. (1974). Inhibition by carboxylic acids of an o-diphenol oxidase from Prunus avium fruit. J. Sci. Food Agric. 25: 263–270. Prı´ncipe, L. and Lozano, J.E. (1991). Reduction and control of non-enzymatic browning in clarified apple juice by absorption and ion-exchange. Lebens. Wiss. Technol. 24: 34 –38. Raghavan, G.S.V., Alvo, P., Garie´pi, Y. and Vigneault, C. (1996). Refrigerated and Controlled Modified Atmosphere Storage. in Processing Fruits: Science and Technology. 1. Biology, Principles, and Applications, Somogyi, L.P., Ramaswamy, H.S. and Hui, Y.H. (eds.). Technomics Publishing Company, Inc., Lancaster, USA, pp. 135 –167. Rankine, B. (1986). Using ion-exchange to alter acidity. Aust. Grapegrower Winemaker 22, 9–10. Reynolds, T.H. (1965). Chemistry of nonenzymatic browning—II. Food Res. 14: 167–175. Roozen, J.P. and Pilnik, W. (1971). On the stability of adsorbed in water deficient systems. V. The effect of storage irradiation by electrons on the stability of alkaline phosphatase, Lebens. Wiss. Technol. 4: 196–200. Sapers, G.M. (1991). Control of enzymatic browning in raw fruit juice by filtration and centrifugation. J. Food. Proc. Preserv. 15: 443–456. Sapers, G.M. and Douglas, F.W. (1987). Uptake and fate of ascorbic acid-2-phosphate in infiltrated fruit and vegetable tissue. Measurement of enzymatic browning at cut surfaces and in juice of raw apple and pear fruits. J. Food Sci. 52: 1258–1262. Sapers, G.M. and Ziolkowski, M.A. (1987). Comparison of erythorbic and ascorbic acid as inhibitors of enzymatic browning in apples. J. Food Sci. 52: 1732–1737. Sapers, G.M., El-Atawy, Y.A., Hicks, K.B. and Garzarella, L. (1989). Effect of emulsifying agents on inhibition of enzymatic browning in apple juice by ascorbyl palmitate, laurate and decanoate. J. Food Sci. 54: 1096. Sapers, G.M., Miller, R.L., Douglas, F.W. and Hicks, K.B. (1991). Uptake and fate of ascorbic acid-2-phosphate in infiltrated fruit and vegetable tissue. J. Food Sci. 56: 419. Schlimme, D.V. (1995). Marketing lightly processed fruits and vegetables. HortScience 30: 15–17. Schobinger, U., Barbic, I., Durr, P. and Waldvogel, R. (1995). Phenolic compounds in apple juice. Positive and negative effects. Fruit Process. 6: 171–174. Seib, P.A. and Liao, M.L. (1987). Ascorbate-2-phosphate esters and method of making the same. US Patent 4,647,672. Seyderhelm, I., Boguslawski, S., Michaels, G. and Knorr, D. (1996). Pressure induced inactivation of selected food enzymes. J. Food Sci. 61: 308–310. Shannon, C.T. and Pratt, D.E. (1967). Apple polyphenol oxidase activity in relation to various phenolic compounds. J. Food Sci. 32: 479– 483. Siddiq, M., Arnold, J.F., Sinha, N.K and Cash, J.N. (1994). Effect of polyphenol oxidase and its inhibitors on anthocyanin changes in plum juice. J. Food Proc. Preserv. 18: 75–84. Sideman, S. (1981). Film evaporation and condensation in desalination. In Heat Exchangers: Thermal-hydraulic fundamentals and design, Kakac et al. (eds.). Hemisphere Publishing Company, p. 357. Sims, C.A., Johnson, R.P. and Bates, R.P. (1989). Quality of a non-sulfited vitis rotundifolia and a Euvitis hybrid white wine produced from ultrafiltered juice. Am. J. Enol. Vitic. 40: 272–276. Tamaoka, M., Itoh, N. and Hayashi, R. (1991). High pressure effect on Maillard reaction. Agric. Biol. Chem. 55: 2071–2074. Taoukis, P.S., Labuza, T.P., Lillemo, J.H. and Lin, S.W. (1990). Inhibition of shrimp melanosis (black spot) by ficin. Lebens. Wiss. Technol. 23: 52–54. Thomas, P. (1984). Radiation preservation of food of plant origin. Part 1. Potatoes and other tuber crops. Crit. Rev. Food Sci. Nutr. 19: 327–379. Thomas, P. (1986). Radiation preservation of food of plant origin. Part 3. Tropical fruits: bananas, mangoes, and papayas. Crit. Rev. Food Sci. Nutr. 23: 147–205. Tonelli, S., Romagnoli, J.A. and Porras, J.A. (1990). Computer package for transit analysis of industrial multipleeffect evaporators. J. Food. Eng. 12: 267–281. Toribio, J.L. and Lozano, J.E. (1986). Heat induced Browning of clarified apple juice at high temperatures. J. Food Sci. 51(1): 172–177. Toribio, J. and Lozano, J. (1987). Formation of 5-HMF in clarified apple juice during heating at elevated temperatures. Lebens. Wiss. Technol. 20: 59–63. Tronc, J.S., Lamarche, F. and Makhlouf, J. (1997). Enzymatic Browning inhibition in cloudy apple juice by electrodialysis. J. Food Sci. 62: 75–78. Va´mos-Vigya´zo´, L. (1995). Prevention of enzymatic browning in fruits and vegetables. In Enzymatic Browning and its Prevention, Lee, C.Y. and Withaker, J.R. (eds.). ACS Symposium Series 600, American Chemical Society, Washington, DC.

8

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Inhibition and Control of Browning

215

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INDEX A Absorbance spectrophotometry, 99–104 Acetic acid, as preservative, 8 Acetoin, 153 N-Acetyl-L-cysteine, as browning inhibitor, 192 Acidity, 157 measurement of, 7 ripening-related decrease in, 21 as taste component, 136 Activated carbon, use in browning control, 203–207 Activity coefficients, of aroma components, 118–125 experimental values, 124, 125 NRTL model, 121, 122 UNIFAC model, 121, 123 UNIQUAC model, 121–123 Wilson equation, 121, 122 Agglomeration, 67–71 selective (spherical), 71 Air, chemical composition of, 12, 13 Aligning, as fruit sorting method, 29 Alkaline phosphatase, radiation-related inactivation of, 195 Aluminum cans, 6 Amino acids, 136. See also names of specific amino acids in browning, 164, 165, 199, 200 effect of ascorbic acid on, 207, 208 in fruit juice, storage-related loss of, 153–155 g-Aminobutyric acid, 153, 170 Amylase, 40 Amyloglucosidase, 35 AnalySIS 2.1, 113 Anions, as browning inhibitors, 187, 189 Anthocyanidins, 149, 150 Anthocyanins, 99, 148, 149, 157 recovery from fruit bagasses, 199 storage-related loss of, 158 Anthoxanthins, 148 Antibrowning agents, 187–193 nonconventional, 187, 191–193 Antoine equation, 95, 121 Appearance, as food quality indicator, 99, 161 Apple butter, pH of, 138 Apple juice amino acid content of, 153 as browning cause, 204

Apple juice (cont.) storage-related loss of, 153, 154 browning in, 162, 204 canned, 24 clarification of, 35–37 clarified boiling point rise in, 94, 95 comparison with cloudy apple juice, 110 concentration of, 198–200 effect of storage on, 204–207 5-hydroxymethylfurfural in, 175, 176 thermophysical properties of, 93–95 cloudy, 109 browning inhibition in, 186, 194 comparison with clarified apple juice, 110 pH of, 194 starch granules in, 40, 41 viscosity of, 116–119 color difference development in, 186 concentration of, 36 density of, 93, 94 enzymatic browning in, 193 enzymatic browning inhibition in, 188–192 enzymatic processing of, 36–38 frozen, 24 nonenzymatic browning in 5-hydroxymethylfurfural in, 173, 175–178 kinetics of, 169–171 Maillard-type reactions in, 167 storage-related, 204–207 nonenzymatic browning inhibition in, 196–198 with activated carbon, 203–205 with ion-exchange resins, 201, 202, 205–207 organic acid content of, 157 pasteurization of, 56 pH of, 35, 40 effect of ion exchange on, 202 reducing sugars in, 156 specific heat of, 81 starch content of, 40, 41 sucrose hydrolysis in, 156 sugar content of, as natural preservative, 36 viscosity of, 91, 92 Apple juice concentrate, nonenzymatic browning in, 171, 172 Apple pulp enzymatic browning inhibition in, 192 217

218 Apple pulp (cont.) enzymatic processing of, 36, 37 light-colored, 165 Apples aroma/aroma components of, 119, 146, 147 browning in, 99 bulk density of, 78 canned, 22 chemical composition of, 134, 135 cooling methods for, 12 dried, 22, 23 enzymatic browning in, 162 inhibition of, 192, 195, 196 luminosity in, 165 measurement of, 164, 165 in unripe fruit, 164, 165 frozen, 22 harvest time for, 157 lightly processed, 193 major commercial applications of, 4 milling processing of, 30, 31 pectin content of, 140 pH of, 138 pigment content of, 134 polyphenol oxidase content of heat-inactivation kinetics of, 183 substrates for, 163 protein content of, 136 red skin color of, 157 scientific name of, 4 specific heat of, 81 starch content of, 35, 139 starch granules, 139, 140 storage life of, 12 storage temperature for, 9, 157 unripe, starch content of, 35 washing of, 27 water content of, 134 world production of, 2, 4 Apple sauce, 22 pH of, 138 specific heat of, 81 viscosity of, 93 Apricot products, enzymatic browning inhibition in, 186 Apricots dried, 22 frozen, 22 major commercial applications of, 4 pectin content of, 140 pH of, 138 polyphenol oxidase phenolic substrates in, 163 protein content of, 136 scientific name of, 4 world production of, 4 Arabanase, 37, 38 Arabans, 40 Arabinose, 139

Index Arginine, 135, 136, 170 L-Arginine, 153 Aroma formation during ripening, 21 properties of, 123 Aroma compounds, 144, 146, 147 activity coefficients of, 118–125 experimental values, 124, 125 NRTL model, 121, 122 UNIFAC model, 121, 123 UNIQUAC model, 121–123 Wilson equation, 121, 122 effect of storage on, 152, 153 volatile, 119–120 infinite dilution coefficients of, 120, 121 relative volativity of, 119 vapor pressure of, 121 Aroma stripping and recovery, 26, 35, 119–125, 146 by flash condensation, 124–126 Arrhenius relationship, 183, 184 Ascorbic acid in browning, 162, 206, 207 Maillard reactions in, 171 as browning inhibitor, 187–192 fruit content of, 144, 145 processing and storage-related destruction of, 150–152 vegetable content of, 144 Ascorbic acid derivatives, as browning inhibitors, 192 Ascorbic acid-6-fatty acid esters, as browning inhibitors, 188, 189 Ascorbic acid-2-phosphate esters, as browning inhibitors, 188, 189 Ascorbic acid-2-triphosphate esters, as browning inhibitors, 188, 189 Ascorbyl fatty acid esters, as browning inhibitors, 188–189 Ascorbyl-6-fatty acid esters, as browning inhibitors, 192 Ascorbyl phosphate esters, as browning inhibitors, 188 Asparagine, 153–155, 170 Aspartic acid, 153, 154 Aspergillus niger, 38 Atomization, in spray drying, 65 Avocados enzymatic browning inhibition in, 192, 195, 196 fat content of, 140 major commercial applications of, 4 polyphenol oxidase phenolic substrates in, 163 protein content of, 136 scientific name of, 4 storage temperature for, 186–176 world production of, 4

B Bag filters, for powder recovery, 65, 66 Bananas browning in, 162

Index Bananas (cont.) chemical composition of, 135 enzymatic browning inhibition in, 192, 195 esters content of, 144, 146 major commercial applications of, 4 polyphenol oxidase content of heat-inactivation kinetics of, 183 phenolic substrates of, 163 protein content of, 136 scientific name of, 4 specific heat of, 81 starch content of, 139 storage temperature for, 186–176 world production of, 2, 4 Basic Four food guide, 6 Beans, protein content of, 136 Beer-Lambert law, 101 Beer’s law, 100–102 Belt conveyors, 29 Bentonite as browning inhibitor, 207 as clarification agent, 35, 36 Benzaldehyde, flash condensation recovery of, 126 Benzoic acid, as browning inhibitor, 164 Berries frozen, 22 refrigeration of, 186 Beta-carotene, 135 Binder media, in agglomeration, 68, 70 Bin food driers, 63 Bins, use in food processing, 27 Birdseye, Charles, 6 Blackberries, 4 Blanching, as enzymatic browning control method, 182, 183 Blueberries cooling methods for, 12 pectin content of, 140 storage life of, 12 Boiling point rise, 94, 95 Boysenberries, 4 Bread, protein content of, 136 Breadfruit, 4 Brix-to-acid ratio, 7 Bromelain, as browning inhibitor, 187, 193 Brown extractors, 30 Browning as deterioration cause, 161–179 enzymatic. See Enzymatic browning inhibition and control of, 181–212 nonenzymatic. See Nonenzymatic browning reducing sugars in, 156 Bruising, in harvested fruits, 7 Butanol, as aroma component, 125 flash condensation recovery of, 126 Butyl acetate, as aroma component, 124 flash condensation recovery of, 126

219 C Caffeic acid, 148 Cage presses, 32, 33 Calcium as apple preservative, 193 fruit content of, 140, 141 Calcium chloride treatment, of harvested fruits, 7, 8 Calorimetry, differential scanning, 80 Candy, 3 Canned fruits, 21, 22 Carambola juice, sugar content of, 139 Carambolas enzymatic browning inhibition in, 192 oxalic acid content of, 137 protein content of, 136 starch content of, 139 Caramel, 150 Caramelization, in nonenzymatic browning, 165–167 Carbohydrates. See also Sugars fruit content of, 134, 135, 137–140 production and function of, 137, 138 Carbon dioxide diffusion through packaging film, 14–17 effect on plant respiration, 13 in fruit storage facilities, 14 in modified atmosphere packaging, 15 supercritical, 194 Carbon dioxide processing, 56 b-Carotene, 148 Carotenoids, 135, 147–148 degradation of, 157 Catechins, 148–150, 163, 164 Catecholase. See Polyphenol oxidase Cellars/caves, as fruit storage areas, 11 Cellulases, 37 Cellulose, 21, 78, 135, 137, 138 Centrifugation as enzymatic browning control method, 185 as extraction process, 34 Cereals, protein content of, 136 Champagne, 3 Chelating agents, as browning inhibitors, 187 Chemical composition, of fruits and fruit products, 133–160 amino acids, 136 carbohydrates, 137–140 effects of processing or storage on, 150–158 organic acids, 136, 137 pectin, 140 proteins, 136 proximate, 133–150 starch, 138–140 Chemical peeling, 29

220 Chemical preservatives, See also names of specific preservatives 7,8 definition of, 7, 8 for semiprocessed fruit products, 17, 18 Chemical treatment, of harvested fruits, 7, 8 Cherries frozen, 22 major commercial applications of, 4 mineral content of, 141 pectin content of, 140 pH of, 138 protein content of, 136 scientific name of, 4 world production of, 4 Chill injury, 186, 187 Chitosan, as browning inhibitor, 187, 192 Chlorophyll, 147 breakdown during ripening, 21 CIELAB method, for surface color quantification, 106–108 Cinnamate, as browning inhibitor, 189, 192 Cinnamic acid, as browning inhibitor, 164, 192 Cinnamic acid esters, as polyphenol oxidase substrates, 163, 164 Citric acid, 21, 136, 157 as browning inhibitor, 192 in peaches, 171 as preservative, 8 Citronellon, 153 Citrus fruit. See also Lemons; Limes; Oranges aroma components of, 118, 119 Citrus juice. See also Lemon juice; Orange juice nonenzymatic browning in, 167 Citrus juice extractors, 30 Clarification processes, 26, 35, 36 centrifugation, 35 concentration, 26, 35 of partial concentrates, 35, 36 temperature for, 40 Clausius-Clapeytron equation, 95 Clostridium botulinum, 13 Coagulation, 109 Coconuts, 4 Cold wall forced-air cooling, 10 Colloidal particles as fruit juice viscosity cause, 115–118 size, shape, and distribution measurement of with electron microscopy, 112, 115 with photon correlation, 115 with sedimentation methods, 113, 114 Colloidal stability, 109, 111 DVLO theory of, 111 Colloidal systems, types of, 110 Colloids, 109 Color, of fruits and fruit products, 161. See also Browning; Pigments

Index Color, of fruits and fruit products. See also Browning; Pigments (cont.) attributes of, 106 effect of processing and storage on, 147 measurement of, 7, 99–108 absorbance spectrophotometry-based, 99–104 tristimulus colorimetry-based, 99–104 visual systems-based, 100 relationship to pigment concentration, 108 Color compounds. See Pigments Colorimeters/colorimetry applications of, 108 definition of, 102 tristimulus, 99, 104–108 CIELAB method, 106–108 Commission Internationale de L’Eclerage (CIE) system, 106–108 Compaction, as size enlargement process, 67 Concentration, of fruit juices, 26, 35–36 through evaporation, 175, 198 effect of boiling point rise on, 94, 95 Concentrators, triple-effect, 198–201 Condensation, formation on stored fruit, 8 Conduction, 74, 75 Laplace equation of, 83 Continuous belt presses, 32, 33 Continuous pressure filters, 42 Convection, 74, 75 Cooling, of harvested fruits, 8–12 methods, 9–12 alternative methods, 11 forced-air cooling, 9, 10, 12 hydrocooling, 9–12 package icing, 9 room cooling, 9, 10, 12 top icing, 9, 11 vacuum cooling, 9 purpose of, 9 Copper, 141, 144, 152, 157 Coring, of harvested fruit, 29 Coumaric acid, 148 p-Coumaric acid, as browning inhibitor, 164, 192 Crab apples, 4 pectin content of, 140 Cranberries, 4 pectin content of, 140 Cranberry sauce, 22 Cream of tartar (potassium hydrogen tartrate), 137 Cucumbers, pH of, 138 Currants, 4 pectin content of, 140 pH of, 138 Cutting, of harvested fruit, 29 b-Cyclodextrans, as browning inhibitors, 194 Cyclodextrin, as browning inhibitor, 187 Cyclone collectors, for powder recovery, 65, 66

Index

221

Cysteine, as browning inhibitor, 187, 189, 192 Cystine, 136

D Deacidification, 199, 200 Debittering, 199, 200 Debye length, 111 Deep freezing, invention of, 6 Dehydrated products, definition of, 62 Dehydration, as fruit processing method, 62–67 critical moisture content (Xcr) in, 63 definition of, 62 driers for, 63–67 belt, 64 bin, 63 cabinet, 64 continuous belt or conveyor, 64 drum, 64 explosion puffing, 65 fluidized bed, 63, 65 microwave, 65 osmotic bed, 65 spray, 62, 65–67 sun or solar, 64 three-stage, 67 tunnel, 64 two-stage, 67 vibrofluidizers, 63 drying curve in, 63 fruit shrinkage during, 125–128 shrinkage coefficient, 127, 128 Dehydroascorbic acid, in browning, 162 Dehydroascorbic acid (DHAA), 165 Density, 73, 74, 77–79 bulk, 77–79 definition of, 77 of fruit juice, 93, 94 measurement of, 77–79 particle, 77 substance, 77 Depectinization, 35, 36, 40 Destarching, of fruit juices, 36 Deterioration, of fruits and fruit products browning-related, 161–179 color measurement of, 99 mechanisms of, 161, 162 postharvest, 6, 7 DHAA (dehydroascorbic acid), 165 Diffusion, as extraction process, 34 Dihydroquercetin, 163 3,4-Dihydroxy phenylalanine, as polyphenol oxidase substrate, 163, 164 Diphenols, transformation to melanins, 164 Dispersibility, 68 Dispersions. See Food dispersions

Dodecanal, 153 Dried fruits, 21, 23 Dried products, definition of, 62 Drying, as preservation method dehydration-related shrinkage during, 125–128 historical development of, 3

E Eggplants, polyphenol oxidase phenolic substrates in, 163 Egyptians, 3 Elderberries, 4 Electrodialysis, as enzymatic browning inhibition method, 194 Electromagnetic waves, 73, 74, 100 Electron microscopy, of colloidal particles, 112, 113, 115 Electrostatic (charge) repulsion, in food dispersions, 111 Electrostatic forces, 69 Encapsulation, as size enlargement process, 67 Endocarp, 1, 2 Enthalpy, 73, 74, 80 Enzymatic browning, 161–165 effect of temperature on, 165 inhibition and control of, 163, 181–195 with chemical treatments, 183, 187–190 with color measurement, 163–165 with miscellaneous methods, 193–195 with nonconventional chemical treatments, 183, 191–193 with thermal treatments, 182–187 kinetics of, 163–165 phenolic compounds and oxidases in, 161–163 susceptibility to, 162 Enzymes. See also specific enzymes apple content of, 134 in fruit and fruit juice processing, 36–41 inactivation of Arrhenius relationship in, 183, 184 with chemical agents, 183, 187–190 for enzymatic browning control, 182–187 with miscellaneous methods, 193–195 with refrigeration, 186, 187 with thermal methods, 182–187 Erythorbic acid, as browning inhibitor, 188, 190–192 Essential amino acids, 136 Essential oils, in apples, 134 Esters, 144, 146 Ethanol, as aroma component, 125 flash condensation recovery of, 126 Ethyl acetate, as aroma component, 124 flash condensation recovery of, 126 Ethyl butyrate, as aroma component, 124, 153 flash condensation recovery of, 126 Ethylene, 13, 21 Ethylenediamine tetraacetic acid (EDTA), as browning inhibitor, 187, 189

222

Index

Ethyl valerate, as aroma component, 124 flash condensation recovery of, 126 Evaporated products, definition of, 62 Evaporation, 58–62 as concentration method, 175, 198 effect of boiling point rise on, 94, 95 condensers for, 58 definition of, 58 heat exchangers for, 58–60 vacuum system for, 58 vapor separator for, 58–60 Evaporators, 58–62 batch pan (calandria), 58, 59 falling film, 60 multiple-effects, 61, 62, 198–200 with aroma recovery, 124–126 mechanical vapor recompression, 61, 62 thermocompression, 61, 62 rising film, 59 scraped-surface, 60, 61 Exocarp, 1, 2 Extraction processes, 30–34 centrifugation, 34 for citrus fruits, 30 diffusion, 34 for pome fruits, 30, 31 pressing, 32, 33

F Fats bulk density of, 78 fruit content of, 140 Feret diameter, of colloidal particles, 112, 113 Fermentation, 3 Ferulic acid, 148 as browning inhibitor, 164, 192 Fiber, dietary, 133 Ficin, as browning inhibitor, 187, 193 Figs, 4 dried, 22 protein content of, 136 Filter presses, 32–33 Filtration methods and filters, 42–53 driving force-type, 42 filter aid and processing, 42, 43 filtrate in, 42 membrane-type, 45–53 hollow fiber membranes, 47 microfiltration membranes (MF), 46, 47 module structures of, 47, 48 nanofiltration membranes (NF), 46 reverse osmosis (RO) membranes, 46 ultrafiltration membranes (UF), 46–53 operating cycle-type, 42 pressure-type, 42–45 candle filters, 44, 45

Filtration methods and filters (cont.) filter press (plate and frame), 43 vacuum filters, 45 vertical and horizontal pressure leaf, 43, 44 Fining, 26, 35, 36 Fining agents, 35 Flavan, 148 Flavones, 148, 149 Flavonoids, 162 Flavonols, 148, 149 Flocculating agents, 35 Flocculation, 109 FMC citrus juice extractors, 30 Folic acid, vegetable content of, 144 Folin-Ciocalteau reagent, 157 Food dispersions, 109–118 characterization of, 111, 112 Debye length in, 111 definitions of, 109–111 electrostatic (charge) repulsion in, 111 particle size, shape, and size distribution in, 112–114 stability of, 109, 111 steric repulsion in, 111 Z-potentials in, 111 Food guides Basic Four, 6 Food Guide Pyramid, 6, 133–135 Forced-air cooling, 9, 10, 12 Formic acid, as preservative, 17, 18 Free radicals, ionizing radiation-related production of, 194 Freeze-drying, invention of, 6 Freeze-thawing peeling, 29 Freezing, of fruits and fruit products of semiprocessed fruit products, 17, 18 structural damage during, 129 thermophysical properties during, 75 Freundlich equilibrium curve, 203, 204 Frozen fruits, 21–23 Fructose, 138 D-Fructose, 155 Fructose/glucose ratio, 156 Fruit products. See also specific fruit products pH of, 138 Fruits. See also specific fruits biology of, 1, 2 classification of, 1, 3 definition of, 1 recommended daily servings of, 133 world production of, 1 Fruit salads, 22 Fruor, 1

G Galacturonic acid in browning, 171, 172

Index

223

Galacturonic acid (cont.) methoxylation of, 140 Gelatin as browning inhibitor, 207 as clarification agent, 36 as fining agent, 35 Genetics, development of, 6 Geometric method, of bulk density measurement, 77, 78 Glass transition, effect on nonenzymatic browning, 177, 178 Glucose, 138 bulk density of, 78 D-Glucose, 21, 155 Glucose oxidase, 38 Glutamic acid, 153, 154 Glutamine, 155 Glutathione, as browning inhibitor, 187, 192 Glyceric acid, 137 Glycine, 136 Gooseberries, 4 pectin content of, 140 Granulation, as size enlargement process, 67 Grapefruit frozen, 22 major commercial applications of, 4 pH of, 138 protein content of, 136 world production of, 4 Grape juice canned, 24 density of, 93, 94 frozen, 24 5-hydroxymethylfurfural content of, 173 tartartrates content of, 137 viscosity of, 91 Grapes enzymatic browning in, 162 inhibition of, 195, 196 major commercial applications of, 4 pectin content of, 140 polyphenol oxidase content of heat-inactivation kinetics of, 183 phenolic substrates for, 163 protein content of, 136 world production of, 2, 4 Grater mills, 31 Greeks, ancient, 3 Grinding mills, 31 Guavas, 4 protein content of, 136 starch content of, 139

H Hammer mills, 31 Harvesting, of fruits, 6–8 Harvest time, 11

Heat capacity, prediction of, 81, 82 Heating, as juice haziness cause, 36 Heat transport, 73, 74 Heat transport properties calculation of, 75 definition of, 73 thermal conductivity, 73 definition of, 82 measurement and prediction of, 82–86 thermal diffusivity, 73–75 definition of, 83 measurement and prediction of, 85, 88 Heavy metals, 152 Heptulose, 139 Hexanal, as aroma component, 124 flash condensation recovery of, 126 Hexanol, as aroma component, 125 flash condensation recovery of, 126 Hexylresorcinol, as browning inhibitor, 187, 192 High-intensity pulsed light, 56, 57 High-pressure methods, for browning inhibition, 195–197 High-pressure sterilization, 56–58 Histidine, 136 Honey as browning inhibitor, 187, 192 as preservative, 3 Horizontal pack presses, 32 Hot gas peeling, 29 Hue, 106, 107 Hue angle, 184, 185 Humidity in fruit storage facilities, 8, 9 during postharvest cooling, 9, 10 Hunter color values, 107, 108 Hydraulic presses, 33, 34 Hydrocooling, 9–12 Hydrometric method, of bulk density measurement, 77, 78 Hydroxybenzoic acid, 148 Hydroxycinnamic acid, 148 5-Hydroxymethylfurfural, 199–201 formation of, 172–178 during nonenzymatic browning, 168, 172–178 during processing, 175–178 during storage, 172–176

I Ion exchange resins, as browning inhibitors, 199–202, 205–207 Iron effect on carotenoid degradation, 157 fruit content of, 141, 144 Irradiation as browning inhibition method, 193–195 as food preservation method, 6, 56 Irrigation, invention of, 3

224

Index

Isinglass, as clarification agent, 36 Isobutyrate, flash condensation recovery of, 126 Isocitric acid, 137

J Jackfruits protein content of, 136 starch content of, 139 Juice, 24. See also Apple juice; Grape juice; Lemon juice; Orange juice; Peach juice; Pineapple juice; Plum juice; Prune juice; Tangerine juice amino acid content of, effect of storage on, 153–155 appearance of, 99 aroma of, effect of storage on, 152, 153 aroma recovery in, 119 canned, 24 categorization of, 21 clarification temperature for, 40 cloudy, 109 processing of, 25, 26 viscosity of, 115–119 enzymatic browning in, 162 effect on luminosity, 184, 185 inhibition of, 184–186 enzymatic hydrolysis of starches in, 40, 41 frozen concentrated, 24 ‘‘natural,’’ 157 nonenzymatic browning inhibition in, 196–208 pasteurization of, 55 powdered, instantizing of, 67, 68 pressurized, 56 processing of, 21–54 centrifugation method, 34 diffusion method, 34 extraction processes, 30–34 final grading, inspection, and sorting, 28, 29 front-end operations, 27–29 nonthermal, 56 pressing method, 32, 33 reception procedures, 27, 28 stages of, 25, 26 semiprocessed, 17 storage-related vitamin loss in, 150 world trade of, 21

K Kayleigh scattering, 103 Kieselsol, 36 Kiwifruits, 5 protein content of, 136 Kumquats, 5

L Laccase, 207 Lactic acid, 137

Lactose, hydrolysis of, 199 Lambert’s law, 100, 101 Laplace equation, of heat conduction, 83 Lead, 152 Lemonade, 3 Lemon juice, canned, 24 Lemons frozen, 22 major commercial applications of, 5 pH of, 138 protein content of, 136 world production of, 5 Leucoanthocyanins, 148–150 Lightness. See Luminosity Limes, 5 protein content of, 136 Limonene, 118, 119 Linalool, 153 Liquefaction, enzymatic, 37 Longans, 5 Loquats, 5 Lovibond Tintometer, 108 Luminosity, 106 effect of enzymatic browning on, 165, 184, 185 Lychees fat content of, 140 major commercial applications of, 5 protein content of, 136 world production of, 5 Lycopene, 147, 148

M Maceration, enzymatic, 37 Magnesium, fruit content of, 141, 143 Magnus-Taylor pressure tester, 27 Maillard reactions, 162 cysteine-related inhibition of, 189 in nonenzymatic browning, 165–172 basic reactions, 167, 168 effect of amino acids on, 168, 170, 172 effect of fructose-to-glucose ratio on, 169, 170 effect of organic acids on, 171 effect of reducing sugars on, 168, 169, 172 effect of soluble solids on, 168, 169 effect of temperature on, 171, 172 kinetics of, 168 Maleic acid, 137 Malic acid, 21, 136, 137, 157 in nonenzymatic browning, 171 in peaches, 171 Maltodextran, 177, 178 Maltose, 138, 139 Manganese, effect on carotenoid degradation, 157 Mangoes major commercial applications of, 5 polyphenol oxidase phenolic substrates in, 163 protein content of, 136

Index

225

Mangoes (cont.) starch content of, 139 storage temperature for, 186–176 world production of, 5 Mango puree´, sugar content, 139 Marmalade, 24 Martin diameter, of colloidal particles, 112, 113 Mature fruit, definition of, 21 Meat Inspection Act, 6 Melanins, 162, 181 Melanoidins, 166, 168, 204 Melons frozen, 23 honeydew, protein content of, 136 major commercial applications of, 5 world production of, 5 Mendel, Gregor, 6 Mesocarp, 1, 2 Metabisulfite, as browning inhibitor, 192 Metals, fruit content of, 141, 144 2-Methyl-butanol, flash condensation recovery of, 126 Methyl paraben, as preservative, 8 Microwave food driers, 65 Microwave technology, 6, 56 Milling and millers, 30, 31 Molds (fungi). See also Yeast in fruit storage rooms, 8 Monochromators, 102 Mulberries, protein content of, 136 Mushrooms, polyphenol oxidase in, 195

N Nanofiltration, 46 Nano-thermosonication, 194 Nectarines protein content of, 136 world production of, 5 Nephelometry, 103 Nitrogen apple content of, 134 as browning inhibitor, 189, 190 in modified atmosphere packaging, 15 Nitrogen compounds, browning reactions of, 161 Nitrogen-containing substances, in fruits, 136 Nonenzymatic browning, 161, 162, 165–178, 194 absorbance in, 168 caramelization in, 165–167 effect of glass transition on, 177, 178 effect of pH on, 171 5-hydroxymethylfurfural in, 168, 172–178 inhibition and control of, 195–208 factors affecting, 195 with ion exchange, 199–202 miscellaneous methods, 207, 208 preventive methods, 196–202 with process optimization, 197–199

Nonenzymatic browning (cont.) restorative methods, 196, 197, 203–207 with temperature control, 196–198 Maillard reactions in, 165–172 basic reactions, 167, 168 effect of amino acids on, 168, 170, 172 effect of fructose-to-glucose ratio on, 169, 170 effect of organic acids on, 171 effect of reducing sugars on, 168, 169, 172 effect of soluble solids on, 168, 169 effect of temperature on, 171, 172 kinetics of, 168 phases in, 166 pyrolysis in, 165, 166 tristimulus parameters for, 168 Nuts fat content of, 140 protein content of, 136

O Octanal, 153 Oils, fruit content of, 140 Olives fat content of, 140 major commercial applications of, 5 pH of, 138 protein content of, 136 world production of, 5 Opacity, 100, 103 Orange juice aroma of, effect of storage on, 152, 153 dehydrated, amino acid content of, 170 density of, 93, 94 5-hydroxymethylfurfural in, 176 nonenzymatic browning in, 207, 208 pH of, 138 specific heat of, 81 viscosity of, 91 Orange marmalade, 24 Oranges chemical composition of, 135 ester content of, 144, 146 major commercial applications of, 5 mandarin, protein content of, 136 navel, 6 pH of, 138 storage-related vitamin loss in, 150, 151 world production of, 2, 5 Organic acids. See also Citric acid; Malic acid; Quinic acid in browning, 157 as browning inhibitors, 187 fruit content of, 133, 135–137 processing and storage-related changes in, 157 Orthodiphenol oxidase. See Polyphenol oxidase

226

Index

Oxygen diffusion through packaging film, 14–17 effect on plant respiration, in fruit storage facilities, 14 in modified atmosphere packaging, 15

P Package icing, 9 Packaging, modified atmosphere (MAP), 13–17 advantages of, 14 browning inhibition inside, 193 disadvantages of, 15 effect on respiration in fruits, 13–16 Packaging materials, for high-pressure food processing, 58 Palletizing (tabletting), as size enlargement process, 67 Panthothenic acid, 151 Papain, as browning inhibitor, 187, 193 Papayas, 5 irradiation of, 6 Particles. See Colloidal particles Passion fruits, 5 protein content of, 136 Pasteur, Louis, 6, 55 Pasteurization, 6, 55–57 batch, 55 high-temperature, short-time (HTST), 55 invention of, 6 nonthermal, 56, 57 of semiprocessed fruit products, 17, 18 UHT (ultra-high temperature), 55–56 Pasteurized products, 55 Peaches browning in, 162 cooling methods for, 12 dried, 23 frozen, 23 major commercial applications of, 5 mineral content of, 141 organic acid content of, 171 pectin content of, 140 pH of, 138 polyphenol oxidase phenolic substrates in, 163 protein content of, 136 storage life of, 12 world production of, 5 Peach juice acidity of, 171 amino acid content of, 155, 157 nonenzymatic browning in, 170, 171 Pear juice enzymatic browning inhibition in, 189, 192 nonenzymatic browning in, 167 reducing sugars in, 156 viscosity of, 91, 93

Pears bulk density of, 78 dried, 23 enzymatic browning in, 162 inhibition in, 195, 196 major commercial applications of, 5 milling processing of, 30, 31 polyphenol oxidase content of heat-inactivation kinetics of, 183 phenolic substrates for, 163 protein content of, 136 world production of, 5 Peas pH of, 138 protein content of, 136 Pectic enzymes. See Pectinases Pectic substances, ripening-related hydrolysis of, 21 Pectin chemical structure of, 140 fruit content of, 140 viscosity of, 93 Pectinases, 36, 37 activity determination of, 38, 39 as browning inhibitors, 189, 190 as clarification agents, 36, 171 Pectinesterase, 36, 38–40 Pectin esterase, 56 radiation-related inactivation of, 195 Pectinlyase, 36, 38, 39 Peeling methods, 29 Peel oil, 118, 119 Penicillum italicum, 39 Penicillum spp., 39 Pentyl acetate, as aroma component, 124 flash condensation recovery of, 126 Pericarp, 1, 2 Permeate flux, in ultrafiltration membrane filtration, 48–53 effect of volume concentration ratio (VCR) on, 50–53 as function of time, 50, 51 stationary, 49, 50 Peroxidase, radiation-related inactivation of, 195 Persimmons, 5 fat content of, 140 pH effect on pectic enzyme activity, 39, 40 for enzymatic browning inhibition, 194 of fruit products, 138 Phenolase, 150 Phenolic acids in enzymatic browning, 162, 163 subgroups of, 148 Phenolic compounds in browning, 157, 164 as browning inhibitors, 164 as polyphenol oxidase inhibitors, 164

Index Phenolic compounds (cont.) as polyphenol oxidase substrates, 163, 164 processing and storage-related changes in, 157, 158 Phosphate-based agents, as browning inhibitors, 189 Photometers, 102 Photon correlation technique, 115 Phytofluene, 148 Pigments, 147–150 apple content of, 134 concentration of, relationship to color, 108 effect of processing and storage on, 157 natural, 147–150 Pineapple juice as browning inhibitor, 193 pH of, 138 Pineapples ester content of, 144, 146 frozen, 23 major commercial applications of, 5 pectin content of, 140 protein content of, 136 storage-related vitamin loss in, 150, 151 world production of, 5 a-Pinene, 153 Pitting, of harvested fruit, 29 Plastic, as high-pressure processing packaging material, 58 Pliny the Elder, 3 Plum juice, enzymatic browning in, 193 inhibition of, 186, 192 Plums enzymatic browning inhibition in, 195 frozen, 23 major commercial applications of, 5 pectin content of, 140 pH of, 138 polyphenol oxidase phenolic substrates in, 163 protein content of, 136 world production of, 5 Polyethersulfone membranes, 207 Polygalacturonase, 36, 38–40 Polymer films, permeability coefficients of, 16, 17 Polyphenol oxidase (PPO) as browning catalyst, 161–163 inhibition of, 181 with chemical antibrowning agents, 187–193 with cold temperatures, 186, 187 with miscellaneous methods, 193–195 with thermal inactivation, 183 phenolic compound-oxidizing activity of, 162 phenolic substrates for, 163, 164 Polyphenols, 37 in juice, ion exchange resin control of, 199, 200 Polyvinyl polypyrrolidone (PVPP), 177, 178, 207 Polyvinyl pyrrolidone membranes, 207 Pomegranates, 5 protein content of, 136

227 Porosity, 77, 78 Postharvest handling, of fruits, 8–12 Potassium, fruit content of, 141, 142 Potassium hydrogen tartrate (cream of tartar), 137 Potassium sorbate, 8 Potassium tartrate (Rochelle salt), 137 PPO. See Polyphenol oxidase Preservation, of fruits and fruit products. See also Dehydration; Freezing; Storage with chemical preservatives, 7, 8 of semiprocessed fruit products, 17, 18 Pressing, as extraction method, 32, 33, 37 Pressure, effect on viscosity, 92, 93 Pressure infiltrates, of antibrowning agents, 191, 192 Pressure sterilization, 56–58 Proanthocyanidins, 149, 150 Processing, of fruits and fruit juices, 21–54 clarification and fining processes, 26, 35, 36 centrifugation, 35 concentration, 26, 35 filtration, 42–53 of partial concentrates, 35, 36 enzymatic, 36–41 extraction processes, 30–34 centrifugation, 34 for citrus fruits, 30 diffusion, 34 for pome fruits, 30, 31 pressing, 32, 33 front-end operations, 27–29 final grading, inspection, and sorting, 28, 29 reception procedures, 27, 28 history of, 2–4 overview of, 1–19 Processing facilities, 24 Process optimization, 197–199 Proline, 153, 170 Propranol, as aroma component, 125 flash condensation recovery of, 126 Proteases, as browning inhibitors, 193 Proteins bulk density of, 78 fruit content of, 135, 136 Proteolytic enzymes, as browning inhibitors, 187 Provitamin A, 148 Prune juice, pH of, 138 Prunes, 5, 22 color of, 162 Pulps, 17 apple enzymatic browning inhibition in, 192 enzymatic processing of, 36, 37 light-colored, 165 raspberry color stability of, 158 effect of processing and storage on, 158 storage-related vitamin C loss in, 151, 152

228

Index

Pulsed electric fields (PEF), 56 Pulsed technologies, 56, 57 Pumpkins pH of, 138 protein content of, 136 starch content of, 139 Puree processing of, 24–26 Puree´ mango, sugar content of, 139 Pure´es-marks, 17 Pure Foods Act, 6 Pycnometry, 78 Pyrolysis, in nonenzymatic browning, 165, 166 Pyruvic acid, 137

Q Q10 (temperature coefficient), 184, 186 Quercetin, 163 Quinces, 5 pectin content of, 140 protein content of, 136 Quinic acid, 157, 171 o-Quinone, 163, 187 Quinones, 158, 162

R Rack and cloth presses, 32 Radiation, 73, 74 Radiofrequency (RF) energy, 57 Raisins, 23 color of, 162 pH of, 138 Raman scattering, 103 Raspberries economic importance of, 157 mineral content of, 141 Raspberry pulp color stability of, 158 effect of processing and storage on, 158 storage-related vitamin C loss in, 151, 152 Rasp mills, 31 Reel washers, 28 Refrigeration, 8-12. See also Freezing as enzymatic browning control method, 186, 187 Refrigerators, invention of, 6 Rehydration, 67 Relative volatility, 119 Resorcinols, as browning inhibitors, 189 Respiration, in harvested fruits, 7, 8 in controlled atmosphere storage (CAS), 12, 13 in modified atmosphere packaging (MAP), 13–16 Retrograding, of starches, 40, 41 Reynolds’ number, 114 Rhubarb, pectin content of, 140 Ripe fruit, pectin content of, 140

Ripening in harvested fruits, 7 process of, 21 Rochelle salt (potassium tartrate), 137 Romans, 3 Room cooling, 9, 10, 12 Rotary drum pressure filters, 42 Rotary presses, 30

S Saccharomyces cerevisae, 57 Sapotes, 5 Saturation, 106 Saturation index, 184, 185 Scalding, 198 Screw presses, 32, 33 Sedimentation methods, for particle size determination, 113, 114 Seeds, of fruits, 1 Semiprocessed fruit products categories of, 17 preservation of, 17, 18 Senescence, in fruits, 21 Serpentine forced-air cooling, 10 Shade, 11 Shikimic acid, 137 Shimadzu Centrifugal Particle Size Analyzer, 114 Shrinkage, in dehydrated fruits, 125–128 shrinkage coefficient, 127, 128 Sieve diameter, 112 Silica gel, as browning inhibitor, 207 Silo systems, 27 Sinapic acid, as browning inhibitor, 192 Sintering, as size enlargement process, 67 Size enlargement processes, 67–72 agglomeration, 67–71 selective (spherical), 71 compaction, 67 encapsulation, 67 granulation, 67 instantizing, 67, 68 sintering, 67 tabletting (palletizing), 67 Sodium, fruit content of, 141, 143 Sodium benzoate, as preservative, 8, 17, 18 Sodium bisulfite, as browning inhibitor, 190 Sodium diacetate, as preservative, 8 Sodium nitrate, as preservative, 8 Sodium propionate, as preservative, 8 Soft drinks invention of, 3 in plastic bottles, 6 Sorbates, as preservatives, 17, 18 Sorbic acid, as preservative, 17, 18 Sorting, of harvested fruit, 28, 29 Sparkolloid, 36 Specific heat, 73, 74, 80, 81

Index Specific heat (cont.) measurement of, 80–82 Specific volume, 73 relationship to density, 79 Spectrophotometers, 101–104 components of, 101–103 ultraviolet (UV), 102 visible light, 101, 102 Spectrophotometry, absorbance, 99–104 Spray drying, 65, 66 powder recovery process in, 65–67 Starches fruit content of, 138–140 fruit juice content of enzymatic hydrolysis of, 40, 41 as haziness cause, 35, 36 retrograding of, 40, 41 ripening-related decrease in, 21 Steam, use in fruit processing, 56 Steam blanching, as enzymatic browning control method, 182, 183 Steam peeling, 29 Stefan-Boltzmann law, 74 Steric repulsion, 111 Sterile products, 55 Sterilization techniques, 56, 57 Stokes’ law, 112–114 Storage, 7 controlled atmosphere (CAS), 12, 13 refrigerated, 8–12, 186, 187 relative humidity during, 8, 9 temperature during, 9 effect on ascorbic acid (vitamin C) content, 150–152 Storage life, postharvest, 6, 7 Strawberries canned, pigment instability in, 158 chemical composition of, 135 mineral content, 141 pectin content, 140 protein content, 136 cooling methods for, 12 major commercial applications of, 5 pH of, 138 storage life of, 12 world production of, 5 Stress crack formation, 126 Succinic acid, 137 Sucrose, 138 hydrolysis of, 155, 156, 169 Sugar/acid ratio, 6, 7, 21, 136 Sugars. See also Fructose; Glucose effect of storage on, 155, 156 as energy source, 138 fruit content of, 133, 135 hydrolysis products of, 155 ripening-related increase in, 21 specific heat of, 81

229 as taste components, 136 Sulfiting agents, as browning inhibitors, 187 Sulfur dioxide as browning inhibitor, 187 as preservative, 8, 17 Sulfydryl compounds, as browning inhibitors, 188, 193 Sumerians, 3 Surfactants, 7, 8 Sweetness, of fruit, 21 Syrups, ion exchange treatment of, 199

T Tabletting (palletizing), as size enlargement process, 67 Tangerine juice, 24 Tangerines, world production of, 5 Tannins, 35, 134, 158 Tartaric acid, 21, 136, 137 Tartness, 21, 136 Taste acidity-based, 136 sugar-based, 136 Temperature for agglomeration, 70 in dehydration processes, 64 effect on enzymatic browning, 165 effect on 5-hydroxymethylfurfural formation, 175, 176 effect on modified atmosphere packaged fruit, 16 effect on viscosity, 92, 93 for fruit juice clarification, 40 in fruit storage facilities, 9 in pasteurization, 55, 56 Temperature coefficient (Q10), 184, 186 Terpinene-4-ol, 153 Terpinolene, 153 Texture, measurement of, 7 Thermal conductivity, 73 definition of, 82 measurement and prediction of, 82–86 Thermal diffusivity, 73–75 definition of, 83 measurement and prediction of, 85, 88 Thermal methods for browning control D value in, 183 elevated temperatures, 182–186 in enzymatic browning, 182–187 Q10 value in, 184 refrigeration-related methods, 186–187 as scalding cause, 198 Thermal radiation, 73, 74 Thermodynamical properties definition of, 73 enthalpy, 73, 80 specific heat, 73, 74, 80, 81 measurement of, 80–82 specific volume, 73 relationship to density, 79

230

Index

Thermophysical properties, 73–98 density, 73, 74, 77–79 enthalpy, 73, 74, 80 experimental data and prediction models for, 76–95 during freezing, 75 identification of, 73–75 specific volume, 73, 74 viscosity, 73, 74 of cloudy fruit juices, 115–118 definition of, 85, 86 effect of temperature and pressure on, 92, 93 measurement of, 87–94 Newtonian, 85–87, 89–91 Tin, 152 Tomato concentrate, viscosity of, 93 Tomatoes, protein content of, 136 Tomato juice, pH of, 138 Tomato paste, viscosity of, 93 Top icing, 9, 11 Total anthocyanin (TA), 99 Trans-2-hexenal, as aroma component, 124 flash condensation recovery of, 126 Translucency, 103 Transparency, 103 Trimming, of harvested fruit, 29 Tristimulus colorimeters/colorimetry, 99, 104–108 CIELAB method, 106–108 Trucks, refrigerated, 12 Tryptophan, 136 Turbidimetry, 103 Turbidity, 103 Tyrosine, as polyphenol oxidase substrate, 163, 164

U Ultrafiltration as enzymatic browning inhibition method, 193–195 polyvinyl polypyrrolidine-based stabilization of, 207 Ultrafiltration membrane filtration, 46–53 permeate flux in, 48–53 as function of time, 50, 51 influence of volume concentration ratio (VCR) on, 50–53 stationary, 49, 50 Ultrapasteurized products, 55 Ultraviolet light processing, 56 Ultraviolet pressure processing, 56 United Nations Food and Agricultural Organization (FAO), 1, 2 Unripe fruit enzymatic browning rate in, 164, 165 pectin content of, 140

V Vacuum cooling, 9 Vacuum infiltrates, of antibrowning agents, 191, 192

Van der Wall’s (dispersion) forces, 68, 69, 203 Vegetables. See also names of specific vegetables fruits classified as, 1 vitamin content of, 144 Viscosity of cloudy fruit juices, 115–118 definition of, 85, 86 effect of temperature and pressure on, 92, 93 measurement of, 87–94 in Newtonian fruit products, 89–91 in non-Newtonian foods, 88, 89 in non-Newtonian fruit products, 91, 92 Newtonian, 85–87, 89–91 Vitamin(s), 141 fruit content of, 135, 144, 145 processing and storage-related destruction of, 150–152 vegetable content of, 144 Vitamin B1, vegetable content of, 144 Vitamin B6 processing and storage-related destruction of, 151 vegetable content of, 144 Vitamin C. See Ascorbic acid Vitamin E, vegetable content of, 144 Vitamin K, vegetable content of, 144 Volatile compounds, of fruit aroma, 146–147 Volatility of the volatile, 119 Volume fraction of particles (Ø), 116

W Washing, of harvested fruit, 27, 28 Water bulk density of, 78 fruit content of, 21, 133–135, 145 Water content, effect on thermophysical properties, 75 Watermelons cooling methods for, 12 storage life of, 12 Web scrubbers, for powder recovery, 65, 66 Well water, 11 Whiskey distilleries, 3

X Xanthophyll, 147–148 Xylose, 139

Y Yeast, removal from fruit juice, 35

Z Zinc fruit content of, 144 orange juice content of, 152 Z-potentials, 111