DEHYDRATION

OF FOODS

DEHYDRATION OF FOODS

SERIES EDITOR

GUSTAVO V. BARBOSA-CÂNOVAS, WASHINGTON STATE UNIVERSITY

EDITORIAL BOARD

JOSE M. AGUILERA UNIVERSIDAD CATOLICA

RICHARD W. HAR TEL UNIVERSITY OF WISCONSIN

JOZEF L. KOKINI RUTGERS UNIVERSITY

MICHAEL J. McCARTHY UNIVERSITY OF CALIFORNIA

MARTIN R. OKOS PURDUE UNIVERSITY

MICHA PELEG UNIVERSITY OF MASSACHUSETTS

M. ANANDHA RAO CORNELL UNIVERSITY

JORGE WELTI-CHANES UNIVERSIDAD DE LAS AMERICAS

DEHYDRATION

OF FOODS

GUST A VO V. BARBOSA-CĂNOV AS WASHINGTON STATE UNIVERSITY

HUMBERTO VEGA-MERCADO MERCK SHARP & DOHME OUIMICA DE PUERTO RICO

SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

I(!)F International Thomson Publishing

Cover design: Trudi Gershenov

Copyright © 1996 by Springer Science+Business Media Oordrecht Originally published by Chapman & Hali in 1996

AII righls reserved. No part of this book covered by the copyright hereon may be reproduced or used in any fann or by any means--·graphic, electronic, Of mechanical, including photocopying, recording, taping, Of information storage and retrieval systems-without the written permission of the publisher.

23456789 10 XXX 01 00999897

Library of Congress Catalogil1g-in-Publication Data

Barbosa-Cănovas, Gustavo V. Dehydration of foods / Guslavo V. Barbosa-Canovas and H umberto

Vega-Mercado. p. cm.

Includes bibl:ographical references and index.

L Food··Drymg. 1. Vega·Mercado, Humberio. TI. Title. TP371.5.B365 1996 668' .0284--dc20

95-44230 CIP

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ISBN 978-1-4419-4723-9 ISBN 978-1-4757-2456-1 (eBook) DOI 10.1007/978-1-4757-2456-1

Ta aur families

CONTENTS

CHAPTER 1. INTRODUCTION TO DEHYDRATION OF FOOD 1

1.0 Introduction 1 1.1 Theoretical Aspects 3 1.2 Dryers 4 1.3 Final Remarks 7 1.4 References 7

CHAPTER 2. FUNDAMENTALS OF AIR-WATER MIXTURES AND IDEAL DRYERS 9

2.0 Introduction 9 2.1 Fundamentals of Air-Water Mixtures 9

2.1.1 Ideal Gas Relationships 10 2.1.2 Moisture Content of Air 12 2.1.3 Psychrometric Chart 13

2.1.3.1 Dry Bulb Temperature 13 2.1.3.2 Relative Saturation or Relative Humidity 15 2.1.3.3 Percentage Saturation ar Percentage

Absolute Humidity 15 2.1.3.4 Humid Volume ar Specific Volume 15 2.1.3.5 Humid Heat 16 2.1.3.6 Enthalpy of a Vapor-Gas Mixture 16

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viii Con ten ts

2.1.3.7 Dew Point 16 2.1.4 The Saturated Condition 17 2.1.5 Adiabatic Saturation Temperature 18 2.1.6 Wet Bulb Temperature 18

2.2 Mixing Two Streams of Air 22 2.3 Reat and Mass Balances in Ideal Dryers 24

2.3.1 Continuous Dryer Without Recirculation 24 2.3.2 Continuous Dryer With Recirculation 26

2.4 Concluding Remarks 27 2.5 References 27

CHAPTER 3. PHYSICAL, CHEMICAL, AND MICROBIOLOGICAL CHARACTERISTICS OF DEHYDRATED FOODS 29

3.0 Introduction 29 3.1 Water Content of Foods 30 3.2 Determination of Water Content 31 3.3 Water Activity 32

3.3.1 Thermodynamic's of Water Activity 33 3.3.1.1 Raoult's Law 35

3.3.2 Theoretical and Empirical Models to Predict aw and Applications 36

3.3.2.1 Nonelectrolytic Solutions 37 3.3.2.2 Electrolytic Solutions 40 3.3.2.3 MulticOIuponent Mixtures 44

3.3.3 Measurement ofWater Activity 53 3.3.3.1 Vapor Pressure 54 3.3.3.2 Freezing Point Depression

and Boiling Point Elevation 54 3.3.3.3 Osmotic Pressure 55 3.3.3.4 Dew Point Rygrometer 56 3.3.3.5 Thermocouple Psychrometer 56 3.3.3.6 Isopiestic Methods and

Graphic Interpolation 57 3.3.3.7 Electric and Rair Rygrometers 58 3.3.3.8 Water Potential 59

3.4 Sorption Phenomena and Sorption Isotherms 59 3.4.1 Rysteresis Phenomenon 60

Con ten ts ix

3.4.2 Temperature Effect on Sorption 62 3.4.3 Brunauer-Emmett-Teller (BET) Equation 63 3.4.4 Heat of Sorption and Free Energy Change 65 3.4.5 Empirical and Theoretical

Models of Sorption Isotherms 66 3.4.5.1 Henderson Model 66 3.4.5.2 Iglesias-Chirife Model 67 3.4.5.3 Guggenheim-Anderson-de Boer

(GAB) Model 67 3.4.5.4 Smith Model 68

3.4.6 Sorption Isotherm and Water Activity Standards 68

3.5 Food Stability 70 3.5.1 Microbial Spoilage 70 3.5.2 Enzymatic Reactions 72 3.5.3 Nonenzymatic Reactions 75 3.5.4 Lipid Oxidation 78 3.5.5 Physical and Structural Phenomena 78

3.5.5.1 Glass Trasition Temperature 80 3.5.5.2 Techniques to Study Food Structure 89

3.5.6 Destruction of Nutrients, Aroma, and Flavor 90 3.6 Concluding Remarks 94 3.7 References 95

CHAPTER 4. DEHYDRATION MECHANISMS 101

4.0 Introduction 101 4.1 The Drying Process 103

4.1.1 Constant Rate Period 107 4.1.2 Falling Rate Period 111

4.1.2.1 Diffusion Theory 111 4.1.2.2 Capillary Theory 118 4.1.2.3 Evaporation-Condensation Theory 118 4.1.2.4 The Luikov Theory 120 4.1.2.5 The Philip and De Vries Theory 124 4.1.2.6 The Berger and Pei Theory 127 4.1.2.7 The Whitaker Theory 130 4.1.2.8 Strongin-Borde Model 141 4.1.2.9 Regular Regime Theory 143

x Contents

4.2 The Drying Process and Water Activity 148 4.3 Concluding Remarks 150 4.4 Nomenclature 151 4.5 References 153

CHAPTER 5. CABINET AND BED DRYERS 157

5.0 lntroduction 157 5.1 Fundamentals 158

5.1.1 Components of a Dryer 159 5.2 Mass and Heat Balances 160

5.2.1 Batch Dryers 160 5.2.2 Through Circulation Batch Dryer 163 5.2.3 Continuous Dryers 168

5.3 Description of Dryers 170 5.3.1 Batch Dryers 170

5.3.1.1 Kiln Dryer 170 5.3.1.2 Cabinet or Tray Dryer 171 5.3.1.3 Rotary Dryer 172

5.3.2 Continuous Dryers 173 5.3.2.1 Tunnel Dryers 175 5.3.2.2 Belt ar Conveyar Dryers 176

5.3.3 Steam Dryers 181 5.3.4 Heat Pumps 182

5.4 Concluding Remarks 183 5.5 Nomenclature 183 5.6 References 184

CHAPTER 6. SPRAY DRYING 185

6.0 lntroduction 185 6.1 Fundamentals 187

6.1.1 Pressure Nozzles 187 6.1.2 Rotary Atomizers 191 6.1.3 Pneumatic Atomizers-Two Fluids 195

6.2 lnteraction Between Droplets and Drying Air 197 6.3 Heat and Mass Balances 199 6.4 Drying of Droplets 200

6.4.1 Pure Liquid Droplets 201

6.4.2 Droplets with Dissolved Solids 6.4.3 Droplets with Insoluble Solids

6.5 Microstructure of Spray Dried Product 6.6 Reconstitution of Spray Dried Products 6.7 Foam Spray Drying 6.8 Applications in the Food Industry

6.8.1 Milk Products 6.8.2 Instant Coffee 6.8.3 Instant Tea 6.8.4 Dry Eggs 6.8.5 Spray Drying of Enzymes 6.8.6 Microorganisms and Yeasts 6.8.7 Spray Drying of Whey Proteins

6.9 Concluding Remarks 6.10 Nomenclature 6.11 References

CHAPTER 7. FREEZE DEHYDRATION

7.0lntroduction 7.1 Fundamentals of Freeze Dehydration

7.1.1 Freezing Step 7.1.2 Drying Steps-Primary and

Secondary Drying 7.2 Combined Beat and Mass Transfer

7.2.1 Steady-State Model 7.2.2 Non-Steady-State Model

7.3 Structural Changes and Volatile Retention During Freeze Drying

7.4 Freeze-Dehydration Related Processes 7.4.1 Prefreezing

7.4.1.1 Direct Contact 7.4.1.2 Indirect Contact

7.4.2 Pre-Concentration 7.4.3 Condensation 7.4.4 Defrosting 7.5 Industrial Freeze Dryers 7.6 Microwave Beating in Freeze Drying 7.7 Atmospheric Freeze Drying

Con ten ts xi

207 208 209 210 212 216 216 218 218 219 221 221 223 224 225 226

229

229 231 233

233 235 235 243

245 247 248 248 249 250 250 252 252 256 260

xii Contents

7.8 Concluding Remarks 262 7.9 References 262

CHAPTER 8. OSMOTIC DEHYDRATION 265

8.0 Introduction 265 8.1 Fundamentals 266 8.2 Heat and Mass Balances 268 8.3 Osmotic Solutions 271 8.4 Hurdle Technology and Osmotic Dehydration 271

8.4.1 pH as a Hurdle 273 8.4.2 Antimicrobials as Hurdles 274 8.4.3 Air Drying, Vacuum Drying and

Freeze Drying as Hurdles 275 8.5 Applications of Osmotic Dehydratio

and Hurdle Technology 276 8.5.1 Fruits 279 8.5.2 Vegetables 280 8.5.3 Fish and Meat Products 281 8.5.4 Intermediate Moisture Foods (IMF) 282

8.6 Rehydration of Osmo-Dried Products 283 8.7 Concluding Remarks 284 8.8 References 284

CHAPTER 9. OTHER METHODS OF DEHYDRATION OF FOODS AND PACKAGING ASPECTS 289

9.0 Introduction 289 9.1 Sun Drying 289

9.1.1 Natural Convection or Direct Type 293 9.1.2 Indirect Natural Convection Dryer 295 9.1.3 Wind-Ventilated ar Farced

Convection Solar Dryer 296 9.2 Vacuum Dryers 297 9.3 Drum Dryers 299 9.4 Microwave Drying 301 9.5 Extrusion Cooking 302

9.5.1 Single-Screw Extruders 303 9.5.2 Twin-Screw Extruders 304

9.6 Fluidized Bed Dryers 9.6.1 Batch Fluidized Bed 9.6.2 Continuous Fluid Bed Dryers

9.7 Pneumatic Dryers 9.8 Packaging of Dehydrated Foods 9.9 Concluding Remarks 9.10 References

APPENDIXES

Appendix 1. Properties of superheated steam Appendix 2. Properties of saturated steam Appendix 3. Physical properties of water

at the saturation pressure Appendix 4. Physical properties of dry air at

atmospheric pressure Appendix 5. Properties of ice

Index

Con ten ts xiii

306 307 308 310 311 318 319

322 323

325

326 326

327

PREFACE

Food dehydration is one of the most relevant and chal­lenging unit operations in food processing, as well as a topic of continuous interest in food research. Information on fundamental aspects and applications of food dehy­dration has been disseminated mainly through research and review articles, edited books, trade magazines reports, and symposia presentations. It is apparent that not many books have been written to cover, in a system­atic manner, basic and applied engineering aspects of this unit operation, as well as key issues dealing with physi­cal and biochemical changes occurring during dehydra­tion. This book was designed and developed as a useful reference for those individuals, either in the food indus­try or in academia, interested in an organized and updated review, from an engineering perspective, of the most important aspects of food dehydration. The book, comprising nine chapters, includes several tables, fig­ures, diagrams, photographs, and extensive literature citation, as well as numeric al examples, to facilitate the understanding of each module. A significant effort was

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xvi Preface

ma de to identify, from a broad spectrum of references, those areas in food dehydration that are of current inter­est as well as those essential to a better understanding of the process in general. Some of the fundamental aspects covered in the book are: air-water mixtures; theory, pre­diction, and measurement of water activity in foods; glass transition as applied to food dehydration; the most accepted theories in food dehydration; and ideal dryers with and without recirculation. The most popular tech­niques in food dehydration, such as spray drying, freeze dehydration, tray drying, cabinet drying, and osmotic dehydration are extensively covered. Other approaches, although concisely described, offer the opportunity to appreciate the many options available to effectively dehy­drate food products. The last chapter includes an exten­sive section dealing with the proper selection of packag­ing materials for dehydrated foods.

We sincerely hope this book will be a valuable addi­tion to the food literature and will promote additional interest in food dehydration research, development, and implementation.

Gustavo V. Barbosa-Canovas and Humberto Vega-Mercado