Fruit Manufacturing

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 min
1 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

10

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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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11

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,

14

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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15

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 cm
2 s
1 cmHg
1 ] 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

22

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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Processing of Fruits

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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41

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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Fruit Manufacturing

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.

2

Processing of Fruits

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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.

2

Processing of Fruits

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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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Fruit Manufacturing

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.

2

Processing of Fruits

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47

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.

2

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49

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 (J0
2 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

.

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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Fruit Manufacturing Air blower Feed

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=m
1 =K
1 ) 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=m
3 )

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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79

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  10
6 e(1:33xX =Xo )  ¼ a þ þbe(0:01 BrixþcT)  ¼ 0:852
0:462e
0:66X( 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
5:479
4

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  10
3 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 (1
X (1
Xw ) 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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Fruit Manufacturing

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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81

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=kg
1 =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

1:962:10
3 1:890:10
3 1:831:10
3 1:473:10
3 6:077:10
3 1:209:10
3 9:086:10
5 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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Thermodynamical, Thermophysical, and Rheological Properties

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

86

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 1:82
1:66 [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 )Xw
2 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 0:88:10
7 þ aw Xw
0:88:10
7 :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
1:22:10
5
4:37:10
5
1:39:10
5 1:51:10
5 9:56:10
5 0:786:10
5 —

3.73 5.30 1.26 5.19 6.08 4.76 6.25
0:187:10
3
0:437:10
3
0:278:10
5
1:34:10
6
1:83:10
2 0:574:10
3 —

1.22 2.32 0.039 2.22 95.0 1.46 2.40 — — — — 0.44 0:29:10
3 —

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

4

.

Thermodynamical, Thermophysical, and Rheological Properties

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  10
2 T þ 4:4  10
5 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 3:5910
3 X 1:33
2:0 and b ¼ 3:0910
5 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
e
tt ) þ J2 (1
e
tt2 ) 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

93

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 s
1 ) 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

RE
y dl

(5:3)

RE
z 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 tan
1 (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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111

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 k
1 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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Fruit Manufacturing

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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Fruit Manufacturing

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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Fruit Manufacturing

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  10
3 (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)

5

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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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Chemical Composition of Fruits and its Technical Importance

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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