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Food processing handbook


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Food processing handbook

  1. 1. Food Processing Handbook Edited by James G. BrennanFood Processing Handbook. Edited by James G. BrennanCopyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 3-527-30719-2
  2. 2. Further of InterestW. Pietsch G.-W. OetjenAgglomeration in Industry Freeze-DryingOccurrence and Applications Second, Completely Revised Edition2004 2004ISBN 3-527-30582-3 ISBN 3-527-30620-XK. J. Heller (Ed.) O.-G. Piringer, A. L. Baner (Eds.)Genetically Engineered Food Plastic Packaging MaterialsMethods and Detection for Food and Pharmaceuticals2003 2007ISBN 3-527-30309-X ISBN 3-527-31455-5E. Ziegler, H. Ziegler (Eds.) K. Bauer, D. Garbe, H. SurburgHandbook of Flavourings Common FragranceProduction, Composition, Applications,Regulations and Flavor Materials Preparation, Properties and UsesSecond, Completely Revised Edition Fourth, Completely Revised Edition2006ISBN 3-527-31406-7 2001 ISBN 3-527-30364-2J. N. Wintgens (Ed.) F. Müller (Ed.)Coffee: Growing, Processing, AgrochemicalsSustainable Production Composition, Production, Toxicology,A Guidebook for Growers, Processors, ApplicationsTraders and Researchers 20002005 ISBN 3-527-29852-5ISBN 3-527-30731-1
  3. 3. Food Processing HandbookEdited byJames G. Brennan
  4. 4. Editor n All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, andJames G. Brennan publisher do not warrant the information contained16 Benning Way in these books, including this book, to be free ofWokingham errors. Readers are advised to keep in mind thatBerks statements, data, illustrations, procedural details orRG40 1 XX other items may inadvertently be inaccurate.UK Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at <> © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Typesetting K+V Fotosatz GmbH, Beerfelden Printing Strauss GmbH, Mörlenbach Binding Litges & Dopf Buchbinderei GmbH, Heppenheim Printed in the Federal Republic of Germany Printed on acid-free paper ISBN-13: 978-3-527-30719-7 ISBN-10: 3-527-30719-2
  5. 5. VContents Preface XXI List of Contributors XXIII1 Postharvest Handling and Preparation of Foods for Processing 1 Alistair S. Grandison1.1 Introduction 11.2 Properties of Raw Food Materials and Their Susceptibility to Deterioration and Damage 21.2.1 Raw Material Properties Geometric Properties Colour Texture Flavour Functional Properties 51.2.2 Raw Material Specifications 61.2.3 Deterioration of Raw Materials 71.2.4 Damage to Raw Materials 71.2.5 Improving Processing Characteristics Through Selective Breeding and Genetic Engineering 81.3 Storage and Transportation of Raw Materials 91.3.1 Storage Temperature Humidity Composition of Atmosphere Other Considerations 131.3.2 Transportation 131.4 Raw Material Cleaning 141.4.1 Dry Cleaning Methods 141.4.2 Wet Cleaning Methods 181.4.3 Peeling 201.5 Sorting and Grading 211.5.1 Criteria and Methods of Sorting 21Food Processing Handbook. Edited by James G. BrennanCopyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 3-527-30719-2
  6. 6. VI Contents 1.5.2 Grading 24 1.6 Blanching 26 1.6.1 Mechanisms and Purposes of Blanching 26 1.6.2 Processing Conditions 27 1.6.3 Blanching Equipment 28 1.7 Sulphiting of Fruits and Vegetables 29 References 30 2 Thermal Processing 33 Michael J. Lewis 2.1 Introduction 33 2.1.1 Reasons for Heating Foods 33 2.1.2 Safety and Quality Issues 34 2.1.3 Product Range 35 2.2 Reaction Kinetics 36 2.2.1 Microbial Inactivation 36 2.2.2 Heat Resistance at Constant Temperature 36 2.3 Temperature Dependence 39 2.3.1 Batch and Continuous Processing 41 2.3.2 Continuous Heat Exchangers 43 2.4 Heat Processing Methods 48 2.4.1 Thermisation 48 2.4.2 Pasteurisation 48 HTST Pasteurisation 49 Tunnel (Spray) Pasteurisers 53 2.4.3 Sterilisation 53 In-Container Processing 53 UHT Processing 61 Special Problems with Viscous and Particulate Products 67 2.5 Filling Procedures 68 2.6 Storage 68 References 69 3 Evaporation and Dehydration 71 James G. Brennan 3.1 Evaporation (Concentration, Condensing) 71 3.1.1 General Principles 71 3.1.2 Equipment Used in Vacuum Evaporation 73 Vacuum Pans 73 Short Tube Vacuum Evaporators 74 Long Tube Evaporators 75 Plate Evaporators 76 Agitated Thin Film Evaporators 77 Centrifugal Evaporators 77 Ancillary Equipment 78
  7. 7. Contents VII3.1.3 Multiple-Effect Evaporation (MEE) 783.1.4 Vapour Recompression 793.1.5 Applications for Evaporation 803.1.5.1 Concentrated Liquid Products 803.1.5.2 Evaporation as a Preparatory Step to Further Processing 823.1.5.3 The Use of Evaporation to Reduce Transport, Storage and Packaging Costs 833.2 Dehydration (Drying) 853.2.1 General Principles 853.2.2 Drying Solid Foods in Heated Air 863.2.3 Equipment Used in Hot Air Drying of Solid Food Pieces 883.2.3.1 Cabinet (Tray) Drier 883.2.3.2 Tunnel Drier 893.2.3.3 Conveyor (Belt) Drier 893.2.3.4 Bin Drier 903.2.3.5 Fluidised Bed Drier 903.2.3.6 Pneumatic (Flash) Drier 933.2.3.7 Rotary Drier 933.2.4 Drying of Solid Foods by Direct Contact With a Heated Surface 943.2.5 Equipment Used in Drying Solid Foods by Contact With a Heated Surface 953.2.5.1 Vacuum Cabinet (Tray or Shelf) Drier 953.2.5.2 Double Cone Vacuum Drier 953.2.6 Freeze Drying (Sublimation Drying, Lyophilisation) of Solid Foods 963.2.7 Equipment Used in Freeze Drying Solid Foods 973.2.7.1 Cabinet (Batch) Freeze Drier 973.2.7.2 Tunnel (SemiContinuous) Freeze Drier 983.2.7.3 Continuous Freeze Driers 993.2.7.4 Vacuum Spray Freeze Drier 993.2.8 Drying by the Application of Radiant (Infrared) Heat 1003.2.9 Drying by the Application of Dielectric Energy 1003.2.10 Osmotic Dehydration 1023.2.11 Sun and Solar Drying 1043.2.12 Drying Food Liquids and Slurries in Heated Air 1053.2.12.1 Spray Drying 1053.2.13 Drying Liquids and Slurries by Direct Contact With a Heated Surface 1103.2.13.1 Drum (Roller, Film) Drier 1103.2.13.2 Vacuum Band (Belt) Drier 1123.2.14 Other Methods Used for Drying Liquids and Slurries 1133.2.15 Applications of Dehydration 1143.2.15.1 Dehydrated Vegetable Products 1143.2.15.2 Dehydrated Fruit Products 1163.2.15.3 Dehydrated Dairy Products 117
  8. 8. VIII Contents Instant Coffee and Tea 118 Dehydrated Meat Products 118 Dehydrated Fish Products 119 3.2.16 Stability of Dehydrated Foods 119 References 121 4 Freezing 125 Jose Mauricio Pardo and Keshavan Niranjan 4.1 Introduction 125 4.2 Refrigeration Methods and Equipment 125 4.2.1 Plate Contact Systems 126 4.2.3 Immersion and Liquid Contact Refrigeration 127 4.2.4 Cryogenic freezing 127 4.3 Low Temperature Production 127 4.3.1 Mechanical Refrigeration Cycle 129 4.3.1 2 The Real Refrigeration Cycle (Standard Vapour Compression Cycle) 131 4.3.2 Equipment for a Mechanical Refrigeration System 132 Evaporators 132 Condensers 133 Compressors 135 Expansion Valves 135 Refrigerants 136 4.3.3 Common Terms Used in Refrigeration System Design 137 Cooling Load 137 Coefficient of Performance (COP) 137 Refrigerant Flow Rate 138 Work Done by the Compressor 138 Heat Exchanged in the Condenser and Evaporator 138 4.4 Freezing Kinetics 138 4.4.1 Formation of the Microstructure During Solidification 140 4.4.2 Mathematical Models for Freezing Kinetics 141 Neumann’s Model 141 Plank’s Model 142 Cleland’s Model 142 4.5 Effects of Refrigeration on Food Quality 143 References 144 5 Irradiation 147 Alistair S. Grandison 5.1 Introduction 147 5.2 Principles of Irradiation 147 5.2.1 Physical Effects 148 5.2.2 Chemical Effects 152 5.2.3 Biological Effects 153
  9. 9. Contents IX5.3 Equipment 1545.3.1 Isotope Sources 1545.3.2 Machine Sources 1575.3.3 Control and Dosimetry 1595.4 Safety Aspects 1605.5 Effects on the Properties of Food 1605.6 Detection Methods for Irradiated Foods 1625.7 Applications and Potential Applications 1635.7.1 General Effects and Mechanisms of Irradiation 1645.7.1.1 Inactivation of Microorganisms 1645.7.1.2 Inhibition of Sprouting 1665.7.1.3 Delay of Ripening and Senescence 1665.7.1.4 Insect Disinfestation 1665.7.1.5 Elimination of Parasites 1675.7.1.6 Miscellaneous Effects on Food Properties and Processing 1675.7.1.7 Combination Treatments 1675.7.2 Applications to Particular Food Classes 1675.7.2.1 Meat and Meat Products 1675.7.2.2 Fish and Shellfish 1695.7.2.3 Fruits and Vegetables 1695.7.2.4 Bulbs and Tubers 1705.7.2.5 Spices and Herbs 1705.7.2.6 Cereals and Cereal Products 1705.7.2.7 Other Miscellaneous Foods 170 References 1716 High Pressure Processing 173 Margaret F. Patterson, Dave A. Ledward and Nigel Rogers6.1 Introduction 1736.2 Effect of High Pressure on Microorganisms 1766.2.1 Bacterial Spores 1766.2.2 Vegetative Bacteria 1776.2.3 Yeasts and Moulds 1776.2.4 Viruses 1786.2.5 Strain Variation Within a Species 1786.2.6 Stage of Growth of Microorganisms 1786.2.7 Magnitude and Duration of the Pressure Treatment 1796.2.8 Effect of Temperature on Pressure Resistance 1796.2.9 Substrate 1796.2.10 Combination Treatments Involving Pressure 1806.2.11 Effect of High Pressure on the Microbiological Quality of Foods 1806.3 Ingredient Functionality 1816.4 Enzyme Activity 1836.5 Foaming and Emulsification 185
  10. 10. X Contents 6.6 Gelation 187 6.7 Organoleptic Considerations 189 6.8 Equipment for HPP 190 6.8.1 ‘Continuous’ System 190 6.8.2 ‘Batch’ System 191 6.9 Pressure Vessel Considerations 193 6.9.1 HP Pumps 194 6.9.2 Control Systems 195 6.10 Current and Potential Applications of HPP for Foods 195 References 197 7 Pulsed Electric Field Processing, Power Ultrasound and Other Emerging Technologies 201 Craig E. Leadley and Alan Williams 7.1 Introduction 201 7.2 Pulsed Electric Field Processing 203 7.2.1 Definition of Pulsed Electric Fields 203 7.2.2 Pulsed Electric Field Processing – A Brief History 203 7.2.3 Effects of PEF on Microorganisms 204 Electrical Breakdown 204 Electroporation 205 7.2.4 Critical Factors in the Inactivation of Microorganisms Using PEF 205 Process Factors 205 Product Factors 206 Microbial Factors 206 7.2.5 Effects of PEF on Food Enzymes 206 7.2.6 Basic Engineering Aspects of PEF 208 Pulse Shapes 208 Chamber Designs 210 7.2.7 Potential Applications for PEF 211 Preservation Applications 211 Nonpreservation Applications 212 7.2.8 The Future for PEF 213 7.3 Power Ultrasound 214 7.3.1 Definition of Power Ultrasound 214 7.3.2 Generation of Power Ultrasound 215 7.3.3 System Types 216 Ultrasonic Baths 216 Ultrasonic Probes 216 Parallel Vibrating Plates 217 Radial Vibrating Systems 217 Airborne Power Ultrasound Technology 217 7.3.4 Applications for Power Ultrasound in the Food Industry 218 Ultrasonically Enhanced Oxidation 218
  11. 11. Contents XI7.3.4.2 Ultrasonic Stimulation of Living Cells 2187.3.4.3 Ultrasonic Emulsification 2207.3.4.4 Ultrasonic Extraction 2207.3.4.5 Ultrasound and Meat Processing 2207.3.4.6 Crystallisation 2207.3.4.7 Degassing 2217.3.4.8 Filtration 2217.3.4.9 Drying 2227.3.4.10 Effect of Ultrasound on Heat Transfer 2227.3.5 Inactivation of Microorganisms Using Power Ultrasound 2227.3.5.1 Mechanism of Ultrasound Action 2227.3.5.2 Factors Affecting Cavitation 2237.3.5.3 Factors Affecting Microbiological Sensitivity to Ultrasound 2247.3.5.4 Effect of Treatment Medium 2247.3.5.5 Combination Treatments 2257.3.6 Effect of Power Ultrasound on Enzymes 2277.3.7 Effects of Ultrasound on Food Quality 2277.3.8 The Future for Power Ultrasound 2287.4 Other Technologies with Potential 2297.4.1 Pulsed Light 2297.4.2 High Voltage Arc Discharge 2307.4.3 Oscillating Magnetic Fields 2307.4.4 Plasma Processing 2307.4.5 Pasteurisation Using Carbon Dioxide 2317.5 Conclusions 231 References 2328 Baking, Extrusion and Frying 237 Bogdan J. Dobraszczyk, Paul Ainsworth, Senol Ibanoglu and Pedro Bouchon8.1 Baking Bread 2378.1.1 General Principles 2378.1.2 Methods of Bread Production 2388.1.2.1 Bulk Fermentation 2398.1.2.2 Chorleywood Bread Process 2398.1.3 The Baking Process 2428.1.3.1 Mixing 2428.1.3.2 Fermentation (Proof) 2428.1.3.3 Baking 2438.1.4 Gluten Polymer Structure, Rheology and Baking 2448.1.5 Baking Quality and Rheology 2498.2 Extrusion 2518.2.1 General Principles 2518.2.1.1 The Extrusion Process 2528.2.1.2 Advantages of the Extrusion Process 253
  12. 12. XII Contents 8.2.2 Extrusion Equipment 254 Single-Screw Extruders 255 Twin-Screw Extruders 256 Comparison of Single- and Twin-Screw Extruders 258 8.2.3 Effects of Extrusion on the Properties of Foods 259 Extrusion of Starch-Based Products 259 Nutritional Changes 264 Flavour Formation and Retention During Extrusion 267 8.3 Frying 269 8.3.1 General Principles 269 The Frying Process 270 Fried Products 270 8.3.2 Frying Equipment 272 Batch Frying Equipment 272 Continuous Frying Equipment 272 Oil-Reducing System 273 8.3.3 Frying Oils 274 8.3.4 Potato Chip and Potato Crisp Production 275 Potato Chip Production 276 Potato Crisp Production 277 8.3.5 Heat and Mass Transfer During Deep-Fat Frying 278 8.3.6 Modelling Deep-Fat Frying 279 8.3.7 Kinetics of Oil Uptake 280 8.3.8 Factors Affecting Oil Absorption 280 8.3.9 Microstructural Changes During Deep-Fat Frying 281 References 283 9 Packaging 291 James G. Brennan and Brian P. F. Day 9.1 Introduction 291 9.2 Factors Affecting the Choice of a Packaging Material and/or Container for a Particular Duty 292 9.2.1 Mechanical Damage 292 9.2.2 Permeability Characteristics 292 9.2.3 Greaseproofness 294 9.2.4 Temperature 294 9.2.5 Light 295 9.2.6 Chemical Compatibility of the Packaging Material and the Contents of the Package 295 9.2.7 Protection Against Microbial Contamination 297 9.2.8 In-Package Microflora 297 9.2.9 Protection Against Insect and Rodent Infestation 297 9.2.10 Taint 298 9.2.11 Tamper-Evident/Resistant Packages 299 9.2.12 Other Factors 299
  13. 13. Contents XIII9.3 Materials and Containers Used for Packaging Foods 3009.3.1 Papers, Paperboards and Fibreboards 3009.3.1.1 Papers 3009.3.1.2 Paperboards 3019.3.1.3 Moulded Pulp 3029.3.1.4 Fibreboards 3029.3.1.5 Composite Containers 3039.3.2 Wooden Containers 3039.3.3 Textiles 3039.3.4 Flexible Films 3049.3.4.1 Regenerated Cellulose 3059.3.4.2 Cellulose Acetate 3069.3.4.3 Polyethylene 3069.3.4.4 Polyvinyl Chloride 3069.3.4.5 Polyvinylidene Chloride 3079.3.4.6 Polypropylene 3079.3.4.7 Polyester 3089.3.4.8 Polystyrene 3089.3.4.9 Polyamides 3089.3.4.10 Polycarbonate 3099.3.4.11 Polytetrafluoroethylene 3099.3.4.12 Ionomers 3099.3.4.13 Ethylene-vinyl Acetate Copolymers 3099.3.5 Metallised Films 3109.3.6 Flexible Laminates 3109.3.7 Heat-Sealing Equipment 3119.3.8 Packaging in Flexible Films and Laminates 3129.3.9 Rigid and Semirigid Plastic Containers 3149.3.9.1 Thermoforming 3149.3.9.2 Blow Moulding 3159.3.9.3 Injection Moulding 3159.3.9.4 Compression Moulding 3159.3.10 Metal Materials and Containers 3159.3.10.1 Aluminium Foil 3169.3.10.2 Tinplate 3169.3.10.3 Electrolytic Chromium-Coated Steel 3199.3.10.4 Aluminium Alloy 3199.3.10.5 Metal Containers 3209.3.11 Glass and Glass Containers 3229.4 Modified Atmosphere Packaging 3259.5 Aseptic Packaging 3299.6 Active Packaging 3319.6.1 Background Information 3319.6.2 Oxygen Scavengers 3349.6.3 Carbon Dioxide Scavengers/Emitters 337
  14. 14. XIV Contents 9.6.4 Ethylene Scavengers 337 9.6.5 Ethanol Emitters 339 9.6.6 Preservative Releasers 340 9.6.7 Moisture Absorbers 341 9.6.8 Flavour/Odour Adsorbers 342 9.6.9 Temperature Control Packaging 343 9.6.10 Food Safety, Consumer Acceptability and Regulatory Issues 344 9.6.11 Conclusions 345 References 346 10 Safety in Food Processing 351 Carol A. Wallace 10.1 Introduction 351 10.2 Safe Design 351 10.2.1 Food Safety Hazards 352 10.2.2 Intrinsic Factors 354 10.2.3 Food Processing Technologies 355 10.2.4 Food Packaging Issues 355 10.3 Prerequisite Good Manufacturing Practice Programmes 355 10.3.1 Prerequisite Programmes – The Essentials 357 10.3.2 Validation and Verification of Prerequisite Programmes 361 10.4 HACCP, the Hazard Analysis and Critical Control Point System 362 10.4.1 Developing a HACCP System 362 10.4.2 Implementing and Maintaining a HACCP System 370 10.4.3 Ongoing Control of Food Safety in Processing 370 References 371 11 Process Control In Food Processing 373 Keshavan Niranjan, Araya Ahromrit and Ahok S. Khare 11.1 Introduction 373 11.2 Measurement of Process Parameters 373 11.3 Control Systems 374 11.3.1 Manual Control 374 11.3.2 Automatic Control 376 On/Off (Two Position) Controller 376 Proportional Controller 377 Proportional Integral Controller 378 Proportional Integral Derivative Controller 379 11.4 Process Control in Modern Food Processing 380 11.4.1 Programmable Logic Controller 381 11.4.2 Supervisory Control and Data Acquisition 381 11.4.3 Manufacturing Execution Systems 382 11.5 Concluding Remarks 384 References 384
  15. 15. Contents XV12 Environmental Aspects of Food Processing 385 Niharika Mishra, Ali Abd El-Aal Bakr and Keshavan Niranjan12.1 Introduction 38512.2 Waste Characteristics 38612.2.1 Solid Wastes 38712.2.2 Liquid Wastes 38712.2.3 Gaseous Wastes 38712.3 Wastewater Processing Technology 38712.4 Resource Recovery From Food Processing Wastes 38812.5 Environmental Impact of Packaging Wastes 38912.5.1 Packaging Minimisation 38912.5.2 Packaging Materials Recycling 39012.6 Refrigerents 39212.7 Energy Issues Related to Environment 39412.8 Life Cycle Assessment 396 References 39713 Water and Waste Treatment 399 R. Andrew Wilbey13.1 Introduction 39913.2 Fresh Water 39913.2.1 Primary Treatment 40013.2.2 Aeration 40113.2.3 Coagulation, Flocculation and Clarification 40113.2.4 Filtration 40313.2.5 Disinfection 40613.2.5.1 Chlorination 40613.2.5.2 Ozone 40813.2.6 Boiler Waters 40913.2.7 Refrigerant Waters 41013.3 Waste Water 41013.3.1 Types of Waste from Food Processing Operations 41113.3.2 Physical Treatment 41213.3.3 Chemical Treatment 41313.3.4 Biological Treatments 41313.3.4.1 Aerobic Treatment – Attached Films 41413.3.4.2 Aerobic Treatment – Suspended Biomass 41713.3.4.3 Aerobic Treatment – Low Technology 41913.3.4.4 Anaerobic Treatments 41913.3.4.5 Biogas Utilisation 42413.4 Sludge Disposal 42513.5 Final Disposal of Waste Water 425 References 426
  16. 16. XVI Contents 14 Separations in Food Processing 429 James G. Brennan, Alistair S. Grandison and Michael J. Lewis 14.1 Introduction 429 14.1.1 Separations from Solids 430 Solid-Solid Separations 430 Separation From a Solid Matrix 430 14.1.2 Separations From Liquids 430 Liquid-Solid Separations 431 Immiscible Liquids 431 General Liquid Separations 431 14.1.3 Separations From Gases and Vapours 432 14.2 Solid-Liquid Filtration 432 14.2.1 General Principles 432 14.2.2 Filter Media 434 14.2.3 Filter Aids 434 14.2.4 Filtration Equipment 435 Pressure Filters 435 Vacuum Filters 439 Centrifugal Filters (Filtering Centrifugals, Basket Centrifuges) 440 14.2.5 Applications of Filtration in Food Processing 442 Edible Oil Refining 442 Sugar Refining 442 Beer Production 443 Wine Making 443 14.3 Centrifugation 444 14.3.1 General Principles 444 Separation of Immiscible Liquids 444 Separation of Insoluble Solids from Liquids 446 14.3.2 Centrifugal Equipment 447 Liquid-Liquid Centrifugal Separators 447 Solid-Liquid Centrifugal Separators 448 14.3.3 Applications for Centrifugation in Food Processing 450 Milk Products 450 Edible Oil Refining 451 Beer Production 451 Wine Making 451 Fruit Juice Processing 451 14.4 Solid-Liquid Extraction (Leaching) 452 14.4.1 General Principles 452 14.4.2 Extraction Equipment 455 Single-Stage Extractors 455 Multistage Static Bed Extractors 456 Multistage Moving Bed Extractors 457 14.4.3 Applications for Solid-Liquid Extraction in Food Processing 459 Edible Oil Extraction 459
  17. 17. Contents XVII14.4.3.2 Extraction of Sugar from Sugar Beet 45914.4.3.3 Manufacture of Instant Coffee 45914.4.3.4 Manufacture of Instant Tea 46014.4.3.5 Fruit and Vegetable Juice Extraction 46014.4.4 The Use of Supercritical Carbon Dioxide as a Solvent 46014.5 Distillation 46214.5.1 General Principles 46214.5.2 Distillation Equipment 46614.5.2.1 Pot Stills 46614.5.2.2 Continuous Distillation (Fractionating) Columns 46614.5.3 Applications of Distillation in Food Processing 46714.5.3.1 Manufacture of Whisky 46714.5.3.2 Manufacture of Neutral Spirits 46914.6 Crystallisation 47114.6.1 General Principles 47114.6.1.1 Crystal Structure 47114.6.1.2 The Crystallisation Process 47114.6.2 Equipment Used in Crystallisation Operations 47514.6.3 Food Industry Applications 47614.6.3.1 Production of Sugar 47614.6.3.2 Production of Salt 47714.6.3.3 Salad Dressings and Mayonnaise 47714.6.3.4 Margarine and Pastry Fats 47714.6.3.5 Freeze Concentration 47714.7 Membrane Processes 47814.7.1 Introduction 47814.7.2 Terminology 47914.7.3 Membrane Characteristics 48014.7.4 Flux Rate 48114.7.5 Transport Phenomena and Concentration Polarisation 48114.7.6 Membrane Equipment 48314.7.7 Membrane Configuration 48314.7.8 Safety and Hygiene Considerations 48614.7.9 Applications for Reverse Osmosis 48814.7.9.1 Milk Processing 48814.7.9.2 Other Foods 48914.7.10 Applications for Nanofiltration 48914.7.11 Applications for Ultrafiltration 49014.7.11.1 Milk Products 49014.7.11.2 Oilseed and Vegetable Proteins 49214.7.11.3 Animal Products 49214.7.12 Applications for Microfiltration 49314.8 Ion Exchange 49514.8.1 General Principles 49514.8.2 Ion Exchange Equipment 497
  18. 18. XVIII Contents 14.8.3 Applications of Ion Exchange in the Food Industry 500 Softening and Demineralisation 500 Decolourisation 502 Protein Purification 502 Other Separations 503 14.8.4 Conclusion 504 14.9 Electrodialysis 504 14.9.1 General Principles and Equipment 504 14.9.2 Applications for Electrodialysis 506 References 507 15 Mixing, Emulsification and Size Reduction 513 James G. Brennan 15.1 Mixing (Agitation, Blending) 513 15.1.1 Introduction 513 15.1.2 Mixing of Low and Moderate Viscosity Liquids 513 Paddle Mixer 515 Turbine Mixer 515 Propeller Mixer 516 15.1.3 Mixing of High Viscosity Liquids, Pastes and Plastic Solids 517 Paddle Mixers 519 Pan (Bowl, Can) Mixers 519 Kneaders (Dispersers, Masticators) 519 Continuous Mixers for Pastelike Materials 519 Static Inline Mixers 520 15.1.4 Mixing Dry, Particulate Solids 520 Horizontal Screw and Ribbon Mixers 521 Vertical Screw Mixers 522 Tumbling Mixers 522 Fluidised Bed Mixers 523 15.1.5 Mixing of Gases and Liquids 523 15.1.6 Applications for Mixing in Food Processing 524 Low Viscosity Liquids 524 Viscous Materials 524 Particulate Solids 524 Gases into Liquids 524 15.2 Emulsification 524 15.2.1 Introduction 524 15.2.2 Emulsifying Agents 526 15.2.3 Emulsifying Equipment 527 Mixers 527 Pressure Homogenisers 528 Hydroshear Homogenisers 530 Microfluidisers 530 Membrane Homogenisers 530
  19. 19. Contents XIX15.2.3.6 Ultrasonic Homogenisers 53015.2.3.7 Colloid Mills 53115.2.4 Examples of Emulsification in Food Processing 53215.2.4.1 Milk 53215.2.4.2 Ice Cream Mix 53315.2.4.3 Cream Liqueurs 53315.2.4.4 Coffee/Tea Whiteners 53315.2.4.5 Salad Dressings 53415.2.4.6 Meat Products 53415.2.4.7 Cake Products 53515.2.4.8 Butter 53515.2.4.9 Margarine and Spreads 53615.3 Size Reduction (Crushing, Comminution, Grinding, Milling) of Solids 53715.3.1 Introduction 53715.3.2 Size Reduction Equipment 54015.3.2.1 Some Factors to Consider When Selecting Size Reduction Equipment 54015.3.2.2 Roller Mills (Crushing Rolls) 54115.3.2.3 Impact (Percussion) Mills 54415.3.2.4 Attrition Mills 54615.3.2.5 Tumbling Mills 54815.3.3 Examples of Size Reduction of Solids in Food Processing 55015.3.3.1 Cereals 55015.3.3.2 Chocolate 55215.3.3.3 Coffee Beans 55415.3.3.4 Oil Seeds and Nuts 55415.3.3.5 Sugar Cane 555 References 556 Subject Index 559
  20. 20. XXIPrefaceThere are many excellent texts available which cover the fundamentals of foodengineering, equipment design, modelling of food processing operations etc.There are also several very good works in food science and technology dealingwith the chemical composition, physical properties, nutritional and microbiolog-ical status of fresh and processed foods. This work is an attempt to cover themiddle ground between these two extremes. The objective is to discuss the tech-nology behind the main methods of food preservation used in today’s food in-dustry in terms of the principles involved, the equipment used and the changesin physical, chemical, microbiological and organoleptic properties that occurduring processing. In addition to the conventional preservation techniques, newand emerging technologies, such as high pressure processing and the use ofpulsed electric field and power ultrasound are discussed. The materials andmethods used in the packaging of food, including the relatively new field of ac-tive packaging, are covered. Concerns about the safety of processed foods andthe impact of processing on the environment are addressed. Process controlmethods employed in food processing are outlined. Treatments applied to waterto be used in food processing and the disposal of wastes from processing opera-tions are described. Chapter 1 covers the postharvest handling and transport of fresh foods andpreparatory operations, such as cleaning, sorting, grading and blanching, ap-plied prior to processing. Chapters 2, 3 and 4 contain up-to-date accounts ofheat processing, evaporation, dehydration and freezing techniques used for foodpreservation. In Chapter 5, the potentially useful, but so far little used processof irradiation is discussed. The relatively new technology of high pressure pro-cessing is covered in Chapter 6, while Chapter 7 explains the current status ofpulsed electric field, power ultrasound, and other new technologies. Recent de-velopments in baking, extrusion cooking and frying are outlined in Chapter 8.Chapter 9 deals with the materials and methods used for food packaging andactive packaging technology, including the use of oxygen, carbon dioxide andethylene scavengers, preservative releasers and moisture absorbers. In Chapter10, safety in food processing is discussed and the development, implementationand maintenance of HACCP systems outlined. Chapter 11 covers the varioustypes of control systems applied in food processing. Chapter 12 deals with envi-Food Processing Handbook. Edited by James G. BrennanCopyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 3-527-30719-2
  21. 21. XXII Preface ronmental issues including the impact of packaging wastes and the disposal of refrigerants. In Chapter 13, the various treatments applied to water to be used in food processing are described and the physical, chemical and biological treat- ments applied to food processing wastes are outlined. To complete the picture, the various separation techniques used in food processing are discussed in Chapter 14 and Chapter 15 covers the conversion operations of mixing, emulsif- ication and size reduction of solids. The editor wishes to acknowledge the considerable advice and help he re- ceived from former colleagues in the School of Food Biosciences, The Univer- sity of Reading, when working on this project. He also wishes to thank his wife, Anne, for her support and patience. Reading, August 2005 James G. Brennan
  22. 22. XXIIIList of ContributorsDr. Araya Ahromrit Mr. James G. Brennan (Editor)Assistant Professor 16 Benning WayDepartment of Food Technology WokinghamKhon Kaen University Berkshire, RG40 1XXKhon Kaen 40002 UKThailand Dr. Brian P. F. DayProfessor Paul Ainsworth Program Leader –Department of Food and Consumer Minimal Processing & PackagingTechnology Food Science AustraliaManchester Metropolitan University 671 Sneydes Road (Private Bag 16)Old Hall Lane WerribeeManchester, M14 6HR Victoria 3030UK AustraliaProfessor Dr. Ing. Ali Abd El-Aal Bakr Dr. Bogdan J. DobraszczykFood Science and Technology School of Food BiosciencesDepartment The University of ReadingFaculty of Agriculture P.O. Box 226Minufiya University WhiteknightsShibin El-Kom Reading, RG6 6APA. R. Egypt UKDr. Pedro Bouchon Dr. Alistair S. GrandisonDepartamento de Ingeniera Quimica School of Food Biosciencesy Bioprocesos The University of ReadingPontificia Universidad Católica P.O. Box 226de Chile WhiteknightsVicuña Mackenna 4860 Reading, RG6 6APMacul UKSantiagoChileFood Processing Handbook. Edited by James G. BrennanCopyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 3-527-30719-2
  23. 23. XXIV List of Contributors Dr. Senol Ibanoglu Professor Keshavan Niranjan Department of Food Engineering School of Food Biosciences Gaziantep University The University of Reading Kilis Road P.O. Box 226 27310 Gaziantep Whiteknights Turkey Reading, RG6 6AP UK Dr. Ashok Khare School of Food Biosciences Dr. Jose Mauricio Pardo The University of Reading Director P.O. Box 226 Ingenieria de Produccion Whiteknights Agroindustrial Reading, RG6 6AP Universidad de la Sabana UK A. A. 140013 Chia Mr. Craig E. Leadley Columbia Campden & Chorleywood Food Research Association Dr. Margaret F. Patterson Food Manufacturing Technologies Queen’s University, Belfast Chipping Campden Department of Agriculture and Rural Gloucestershire, GL55 6LD Development UK Agriculture and Food Science Center Newforge Lane Professor Dave A. Ledward Belfast, BT9 5PX School of Food Biosciences Northern Ireland The University of Reading UK Whiteknights Reading, RG6 6AP Mr. Nigel Rogers UK Avure Technologies AB Quintusvägen 2 Dr. Michael J. Lewis Vasteras, SE 72166 School of Food Biosciences Sweden The University of Reading P.O. Box 226 Mrs. Carol Anne Wallace Whiteknights Principal Lecturer Reading, RG6 6AP Food Safety Management UK Lancashire School of Health & Postgraduate Medicine Mrs. Niharika Mishra University of Central Lancashire School of Food Biosciences Preston, PR1 2HE The University of Reading UK P.O. Box 226 Whiteknights Reading, RG6 6AP UK
  24. 24. List of Contributors XXVMr. R. Andrew Wilbey Dr. Alan WilliamsSchool of Food Biosciences Senior Technologist & HACCPThe University of Reading SpecialistP.O. Box 226 Department of Food ManufacturingWhiteknights TechnologiesReading, RG6 6AP Campden & Chorleywood FoodUK Research Association Group Chipping Campden Gloucestershire, GL55 6LD UK
  25. 25. 11Postharvest Handling and Preparation of Foods for ProcessingAlistair S. Grandison1.1IntroductionFood processing is seasonal in nature, both in terms of demand for productsand availability of raw materials. Most crops have well established harvest times– for example the sugar beet season lasts for only a few months of the year inthe UK, so beet sugar production is confined to the autumn and winter, yet de-mand for sugar is continuous throughout the year. Even in the case of raw ma-terials which are available throughout the year, such as milk, there are estab-lished peaks and troughs in volume of production, as well as variation in chem-ical composition. Availability may also be determined by less predictable factors,such as weather conditions, which may affect yields, or limit harvesting. Inother cases demand is seasonal, for example ice cream or salads are in greaterdemand in the summer, whereas other foods are traditionally eaten in the win-ter months, or even at more specific times, such as Christmas or Easter. In an ideal world, food processors would like a continuous supply of raw ma-terials, whose composition and quality are constant, and whose prices are pre-dictable. Of course this is usually impossible to achieve. In practice, processorscontract ahead with growers to synchronise their needs with raw material pro-duction. The aim of this chapter is to consider the properties of raw materialsin relation to food processing, and to summarise important aspects of handling,transport, storage and preparation of raw materials prior to the range of proces-sing operations described in the remainder of this book. The bulk of the chapterwill deal with solid agricultural products including fruits, vegetables, cerealsand legumes; although many considerations can also be applied to animal-basedmaterials such as meat, eggs and milk.Food Processing Handbook. Edited by James G. BrennanCopyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 3-527-30719-2
  26. 26. 2 1 Postharvest Handling and Preparation of Foods for Processing 1.2 Properties of Raw Food Materials and Their Susceptibility to Deterioration and Damage The selection of raw materials is a vital consideration to the quality of processed products. The quality of raw materials can rarely be improved during processing and, while sorting and grading operations can aid by removing oversize, under- size or poor quality units, it is vital to procure materials whose properties most closely match the requirements of the process. Quality is a wide-ranging con- cept and is determined by many factors. It is a composite of those physical and chemical properties of the material which govern its acceptability to the ‘user’. The latter may be the final consumer, or more likely in this case, the food pro- cessor. Geometric properties, colour, flavour, texture, nutritive value and free- dom from defects are the major properties likely to determine quality. An initial consideration is selection of the most suitable cultivars in the case of plant foods (or breeds in the case of animal products). Other preharvest fac- tors (such as soil conditions, climate and agricultural practices), harvesting methods and postharvest conditions, maturity, storage and postharvest handling also determine quality. These considerations, including seed supply and many aspects of crop production, are frequently controlled by the processor or even the retailer. The timing and method of harvesting are determinants of product quality. Manual labour is expensive, therefore mechanised harvesting is introduced where possible. Cultivars most suitable for mechanised harvesting should ma- ture evenly producing units of nearly equal size that are resistant to mechanical damage. In some instances, the growth habits of plants, e.g. pea vines, fruit trees, have been developed to meet the needs of mechanical harvesting equip- ment. Uniform maturity is desirable as the presence of over-mature units is as- sociated with high waste, product damage, and high microbial loads, while un- der-maturity is associated with poor yield, hard texture and a lack of flavour and colour. For economic reasons, harvesting is almost always a ‘once over’ exercise, hence it is important that all units reach maturity at the same time. The predic- tion of maturity is necessary to coordinate harvesting with processors’ needs as well as to extend the harvest season. It can be achieved primarily from knowl- edge of the growth properties of the crop combined with records and experience of local climatic conditions. The ‘heat unit system’, first described by Seaton [1] for peas and beans, can be applied to give a more accurate estimate of harvest date from sowing date in any year. This system is based on the premise that growth temperature is the overriding determinant of crop growth. A base tem- perature, below which no growth occurs, is assumed and the mean temperature of each day through the growing period is recorded. By summing the daily mean temperatures minus base temperatures on days where mean temperature exceeds base temperature, the number of ‘accumulated heat units’ can be calcu- lated. By comparing this with the known growth data for the particular cultivar, an accurate prediction of harvest date can be computed. In addition, by allowing
  27. 27. 1.2 Properties of Raw Food Materials and Their Susceptibility to Deterioration and Damage 3fixed numbers of accumulated heat units between sowings, the harvest seasoncan be spread, so that individual fields may be harvested at peak maturity. Sow-ing plans and harvest date are determined by negotiation between the growersand the processors; and the latter may even provide the equipment and labourfor harvesting and transport to the factory. An important consideration for processed foods is that it is the quality of theprocessed product, rather than the raw material, that is important. For mini-mally processed foods, such as those subjected to modified atmospheres, low-dose irradiation, mild heat treatment or some chemical preservatives, the char-acteristics of the raw material are a good guide to the quality of the product.For more severe processing, including heat preservation, drying or freezing, thequality characteristics may change markedly during processing. Hence, thoseraw materials which are preferred for fresh consumption may not be mostappropriate for processing. For example, succulent peaches with delicate flavourmay be less suitable for canning than harder, less flavoursome cultivars, whichcan withstand rigorous processing conditions. Similarly, ripe, healthy, well col-oured fruit may be perfect for fresh sale, but may not be suitable for freezingdue to excessive drip loss while thawing. For example, Maestrelli [2] reportedthat different strawberry cultivars with similar excellent characteristics for freshconsumption exhibited a wide range of drip loss (between 8% and 38%), andhence would be of widely different value for the frozen food industry.1.2.1Raw Material PropertiesThe main raw material properties of importance to the processor are geometry,colour, texture, functional properties and flavour. Geometric PropertiesFood units of regular geometry are much easier to handle and are better suitedto high speed mechanised operations. In addition, the more uniform the geom-etry of raw materials, the less rejection and waste will be produced during prep-aration operations such as peeling, trimming and slicing. For example, potatoesof smooth shape with few and shallow eyes are much easier to peel and washmechanically than irregular units. Smooth-skinned fruits and vegetables aremuch easier to clean and are less likely to harbour insects or fungi than ribbedor irregular units. Agricultural products do not come in regular shapes and exact sizes. Size andshape are inseparable, but are very difficult to define mathematically in solidfood materials. Geometry is, however, vital to packaging and controlling fill-inweights. It may, for example, be important to determine how much mass orhow many units may be filled into a square box or cylindrical can. This wouldrequire a vast number of measurements to perform exactly and thus approxima-tions must be made. Size and shape are also important to heat processing and
  28. 28. 4 1 Postharvest Handling and Preparation of Foods for Processing freezing, as they will determine the rate and extent of heat transfer within food units. Mohsenin [3] describes numerous approaches by which the size and shape of irregular food units may be defined. These include the development of statistical techniques based on a limited number of measurements and more subjective approaches involving visual comparison of units to charted standards. Uniformity of size and shape is also important to most operations and pro- cesses. Process control to give accurately and uniformly treated products is al- ways simpler with more uniform materials. For example, it is essential that wheat kernel size is uniform for flour milling. Specific surface (area/mass) may be an important expression of geometry, especially when considering surface phenomena such as the economics of fruit peeling, or surface processes such as smoking and brining. The presence of geometric defects, such as projections and depressions, com- plicate any attempt to quantify the geometry of raw materials, as well as pre- senting processors with cleaning and handling problems and yield loss. Selec- tion of cultivars with the minimum defect level is advisable. There are two approaches to securing the optimum geometric characteristics: firstly the selection of appropriate varieties, and secondly sorting and grading operations. Colour Colour and colour uniformity are vital components of visual quality of fresh foods and play a major role in consumer choice. However, it may be less impor- tant in raw materials for processing. For low temperature processes such as chilling, freezing or freeze-drying, the colour changes little during processing, and thus the colour of the raw material is a good guide to suitability for proces- sing. For more severe processing, the colour may change markedly during the process. Green vegetables, such as peas, spinach or green beans, on heating change colour from bright green to a dull olive green. This is due to the conver- sion of chlorophyll to pheophytin. It is possible to protect against this by addi- tion of sodium bicarbonate to the cooking water, which raises the pH. However, this may cause softening of texture and the use of added colourants may be a more practical solution. Some fruits may lose their colour during canning, while pears develop a pink tinge. Potatoes are subject to browning during heat processing due to the Maillard reaction. Therefore, different varieties are more suitable for fried products where browning is desirable, than canned products in which browning would be a major problem. Again there are two approaches: i.e. procuring raw materials of the appropri- ate variety and stage of maturity, and sorting by colour to remove unwanted units.
  29. 29. 1.2 Properties of Raw Food Materials and Their Susceptibility to Deterioration and Damage TextureThe texture of raw materials is frequently changed during processing. Texturalchanges are caused by a wide variety of effects, including water loss, protein de-naturation which may result in loss of water-holding capacity or coagulation,hydrolysis and solubilisation of proteins. In plant tissues, cell disruption leadsto loss of turgor pressure and softening of the tissue, while gelatinisation ofstarch, hydrolysis of pectin and solubilisation of hemicelluloses also cause soft-ening of the tissues. The raw material must be robust enough to withstand the mechanical stres-ses during preparation, for example abrasion during cleaning of fruit and vege-tables. Peas and beans must be able to withstand mechanical podding. Raw ma-terials must be chosen so that the texture of the processed product is correct,such as canned fruits and vegetables in which raw materials must be able towithstand heat processing without being too hard or coarse for consumption. Texture is dependent on the variety as well as the maturity of the raw materialand may be assessed by sensory panels or commercial instruments. One widelyrecognised instrument is the tenderometer used to assess the firmness of peas.The crop would be tested daily and harvested at the optimum tenderometerreading. In common with other raw materials, peas at different maturities canbe used for different purposes, so that peas for freezing would be harvested at alower tenderometer reading than peas for canning. FlavourFlavour is a rather subjective property which is difficult to quantify. Again, fla-vours are altered during processing and, following severe processing, the mainflavours may be derived from additives. Hence, the lack of strong flavours maybe the most important requirement. In fact, raw material flavour is often not amajor determinant as long as the material imparts only those flavours whichare characteristic of the food. Other properties may predominate. Flavour is nor-mally assessed by human tasters, although sometimes flavour can be linked tosome analytical test, such as sugar/acid levels in fruits. Functional PropertiesThe functionality of a raw material is the combination of properties which deter-mine product quality and process effectiveness. These properties differ greatlyfor different raw materials and processes, and may be measured by chemicalanalysis or process testing. For example, a number of possible parameters may be monitored in wheat.Wheat for different purposes may be selected according to protein content.Hard wheat with 11.5–14.0% protein is desirable for white bread and somewholewheat breads require even higher protein levels, 14–16% [4]. In contrast,soft or weak flours with lower protein contents are suited to chemically leavenedproducts with a lighter or more tender structure. Hence protein levels of 8–11%
  30. 30. 6 1 Postharvest Handling and Preparation of Foods for Processing are adequate for biscuits, cakes, pastry, noodles and similar products. Varieties of wheat for processing are selected on this basis; and measurement of protein content would be a good guide to process suitability. Furthermore, physical test- ing of dough using a variety of rheological testing instruments may be useful in predicting the breadmaking performance of individual batches of wheat flours [5]. A further test is the Hagberg Falling Number which measures the amount of a-amylase in flour or wheat [6]. This enzyme assists in the break- down of starch to sugars and high levels give rise to a weak bread structure. Hence, the test is a key indicator of wheat baking quality and is routinely used for bread wheat; and it often determines the price paid to the farmer. Similar considerations apply to other raw materials. Chemical analysis of fat and protein in milk may be carried out to determine its suitability for manufac- turing cheese, yoghurt or cream. 1.2.2 Raw Material Specifications In practice, processors define their requirements in terms of raw material speci- fications for any process on arrival at the factory gate. Acceptance of, or price paid for the raw material depends on the results of specific tests. Milk deliveries would be routinely tested for hygienic quality, somatic cells, antibiotic residues, extraneous water, as well as possibly fat and protein content. A random core sample is taken from all sugar beet deliveries and payment is dependent on the sugar content. For fruits, vegetables and cereals, processors may issue specifica- tions and tolerances to cover the size of units, the presence of extraneous vege- table matter, foreign bodies, levels of specific defects, e.g. surface blemishes, in- sect damage etc., as well as specific functional tests. Guidelines for sampling and testing many raw materials for processing in the UK are available from the Campden and Chorleywood Food Research Association ( Increasingly, food processors and retailers may impose demands on raw material production which go beyond the properties described above. These may include ‘environmentally friendly’ crop management schemes in which only specified fertilisers and insecticides are permitted, or humanitarian con- cerns, especially for food produced in Third World countries. Similarly animal welfare issues may be specified in the production of meat or eggs. Another im- portant issue is the growth of demand for organic foods in the UK and Western Europe, which obviously introduces further demands on production methods, but are beyond the scope of this chapter.
  31. 31. 1.2 Properties of Raw Food Materials and Their Susceptibility to Deterioration and Damage 71.2.3Deterioration of Raw MaterialsAll raw materials deteriorate following harvest, by some of the following mecha-nisms:– Endogenous enzymes: e.g. post-harvest senescence and spoilage of fruit and vegetables occurs through a number of enzymic mechanisms, including oxi- dation of phenolic substances in plant tissues by phenolase (leading to brown- ing), sugar-starch conversion by amylases, postharvest demethylation of pectic substances in fruits and vegetables leading to softening tissues during ripen- ing and firming of plant tissues during processing.– Chemical changes: deterioration in sensory quality by lipid oxidation, non- enzymic browning, breakdown of pigments such as chlorophyll, anthocya- nins, carotenoids.– Nutritional changes: especially ascorbic acid breakdown.– Physical changes: dehydration, moisture absorption.– Biological changes: germination of seeds, sprouting.– Microbiological contamination: both the organisms themselves and toxic prod- ucts lead to deterioration of quality, as well as posing safety problems.1.2.4Damage to Raw MaterialsDamage may occur at any point from growing through to the final point of sale.It may arise through external or internal forces. External forces result in mechanical injury to fruits and vegetables, cerealgrains, eggs and even bones in poultry. They occur due to severe handling as aresult of careless manipulation, poor equipment design, incorrect containerisa-tion and unsuitable mechanical handling equipment. The damage typically re-sults from impact and abrasion between food units, or between food units andmachinery surfaces and projections, excessive vibration or pressure from overly-ing material. Increased mechanisation in food handling must be carefully de-signed to minimise this. Internal forces arise from physical changes, such as variation in temperatureand moisture content, and may result in skin cracks in fruits and vegetables, orstress cracks in cereals. Either form of damage leaves the material open to further biological or chem-ical damage, including enzymic browning of bruised tissue, or infestation ofpunctured surfaces by moulds and rots.
  32. 32. 8 1 Postharvest Handling and Preparation of Foods for Processing 1.2.5 Improving Processing Characteristics Through Selective Breeding and Genetic Engineering Selective breeding for yield and quality has been carried out for centuries in both plant and animal products. Until the 20th century, improvements were made on the basis of selecting the most desirable looking individuals, while in- creasingly systematic techniques have been developed more recently, based on a greater understanding of genetics. The targets have been to increase yield as well as aiding factors of crop or animal husbandry such as resistance to pests and diseases, suitability for harvesting, or development of climate-tolerant vari- eties (e.g. cold-tolerant maize, or drought-resistant plants) [7]. Raw material quality, especially in relation to processing, has become increasingly important. There are many examples of successful improvements in processing quality of raw materials through selective plant breeding, including: – improved oil percentage and fatty acid composition in oilseed rape; – improved milling and malting quality of cereals; – high sugar content and juice quality in sugar beets; – development of specific varieties of potatoes for the processing industry, based on levels of enzymes and sugars, producing appropriate flavour, texture and colour in products, or storage characteristics; – brussels sprouts which can be successfully frozen. Similarly traditional breeding methods have been used to improve yields of animal products such as milk and eggs, as well as improving quality, e.g. fat/lean content of meat. Again the quality of raw materials in relation to processing may be im- proved by selective breeding. This is particularly applicable to milk, where breed- ing programmes have been used at different times to maximise butterfat and pro- tein content, and would thus be related to the yield and quality of fat- or protein- based dairy products. Furthermore, particular protein genetic variants in milk have been shown to be linked with processing characteristics, such as curd strength during manufacture of cheese [8]. Hence, selective breeding could be used to tailor milk supplies to the manufacture of specific dairy products. Traditional breeding programmes will undoubtedly continue to produce im- provements in raw materials for processing, but the potential is limited by the gene pool available to any species. Genetic engineering extends this potential by allowing the introduction of foreign genes into an organism, with huge potential benefits. Again many of the developments have been aimed at agricultural improvements, such as increased yield, or introducing herbicide, pest or drought resistance, but there is enormous potential in genetically engineered raw materials for processing [9]. The following are some examples which have been demonstrated: – tomatoes which do not produce pectinase and hence remain firm while col- our and flavour develop, producing improved soup, paste or ketchup; – potatoes with higher starch content, which take up less oil and require less energy during frying;
  33. 33. 1.3 Storage and Transportation of Raw Materials 9– canola (rape seed) oil tailored to contain: (a) high levels of lauric acid to im- prove emulsification properties for use in confectionery, coatings or low fat dairy products, (b) high levels of stearate as an alternative to hydrogenation in manufacture of margarine, (c) high levels of polyunsaturated fatty acids for health benefits;– wheat with increased levels of high molecular weight glutenins for improved breadmaking performance;– fruits and vegetables containing peptide sweeteners such as thaumatin or monellin;– ‘naturally decaffeinated’ coffee.There is, however, considerable opposition to the development of geneticallymodified foods in the UK and elsewhere, due to fears of human health risksand ecological damage, discussion of which is beyond the scope of this book. Ittherefore remains to be seen if, and to what extent, genetically modified rawmaterials will be used in food processing.1.3Storage and Transportation of Raw Materials1.3.1StorageStorage of food is necessary at all points of the food chain from raw materials,through manufacture, distribution, retailers and final purchasers. Today’s con-sumers expect a much greater variety of products, including nonlocal materials,to be available throughout the year. Effective transportation and storage systemsfor raw materials are essential to meet this need. Storage of materials whose supply or demand fluctuate in a predictable man-ner, especially seasonal produce, is necessary to increase availability. It is essen-tial that processors maintain stocks of raw materials, therefore storage is neces-sary to buffer demand. However, storage of raw materials is expensive for tworeasons: firstly, stored goods have been paid for and may therefore tie up quan-tities of company money and, secondly, warehousing and storage space areexpensive. All raw materials deteriorate during storage. The quantities of rawmaterials held in store and the times of storage vary widely for different cases,depending on the above considerations. The ‘just in time’ approaches used inother industries are less common in food processing. The primary objective is to maintain the best possible quality during storage,and hence avoid spoilage during the storage period. Spoilage arises throughthree mechanisms:– living organisms such as vermin, insects, fungi and bacteria: these may feed on the food and contaminate it;
  34. 34. 10 1 Postharvest Handling and Preparation of Foods for Processing – biochemical activity within the food leading to quality reduction, such as: res- piration in fruits and vegetables, staling of baked products, enzymic browning reactions, rancidity development in fatty food; – physical processes, including damage due to pressure or poor handling, phys- ical changes such as dehydration or crystallisation. The main factors which govern the quality of stored foods are temperature, moisture/humidity and atmospheric composition. Different raw materials pro- vide very different challenges. Fruits and vegetables remain as living tissues until they are processed and the main aim is to reduce respiration rate without tissue damage. Storage times vary widely between types. Young tissues such as shoots, green peas and imma- ture fruits have high respiration rates and shorter storage periods, while mature fruits and roots and storage organs such as bulbs and tubers, e.g. onions, pota- toes, sugar beets, respire much more slowly and hence have longer storage peri- ods. Some examples of conditions and storage periods of fruits and vegetables are given in Table 1.1. Many fruits (including bananas, apples, tomatoes and mangoes) display a sharp increase in respiration rate during ripening, just be- fore the point of optimum ripening, known as the ‘climacteric’. The onset of the climacteric is associated with the production of high levels of ethylene, which is believed to stimulate the ripening process. Climacteric fruit can be har- vested unripe and ripened artificially at a later time. It is vital to maintain care- ful temperature control during storage or the fruit will rapidly over-ripen. Non- climacteric fruits, e.g. citrus fruit, pineapples, strawberries, and vegetables do not display this behaviour and generally do not ripen after harvest. Quality is therefore optimal at harvest, and the task is to preserve quality during storage. With meat storage the overriding problem is growth of spoilage bacteria, while avoiding oxidative rancidity. Cereals must be dried before storage to avoid Table 1.1 Storage periods of some fruits and vegetables under typical storage conditions (data from [25]). Commodity Temperature (8C) Humidity (%) Storage period Garlic 0 70 6–8 months Mushrooms 0 90–95 5–7 days Green bananas 13–15 85–90 10–30 days Immature potatoes 4–5 90–95 3–8 weeks Mature potatoes 4–5 90–95 4–9 months Onions –1 to 0 70–80 6–8 months Oranges 2–7 90 1–4 months Mangoes 5.5–14 90 2–7 weeks Apples –1 to 4 90–95 1–8 months French beans 7– 8 95–100 1–2 week
  35. 35. 1.3 Storage and Transportation of Raw Materials 11germination and mould growth and subsequently must be stored under condi-tions which prevent infestation with rodents, birds, insects or moulds. Hence, very different storage conditions may be employed for different rawmaterials. The main methods employed in raw material storage are the controlof temperature, humidity and composition of atmosphere. TemperatureThe rate of biochemical reactions is related to temperature, such that lower stor-age temperatures lead to slower degradation of foods by biochemical spoilage,as well as reduced growth of bacteria and fungi. There may also be limited bac-teriocidal effects at very low temperatures. Typical Q10 values for spoilage reac-tions are approximately 2, implying that spoilage rates would double for each10 8C rise, or conversely that shelflife would double for each 10 8C reduction.This is an oversimplification, as Q10 may change with temperature. Most insectactivity is inhibited below 4 8C, although insects and their eggs can survive longexposure to these temperatures. In fact, grain and flour mites can remain activeand even breed at 0 8C. The use of refrigerated storage is limited by the sensitivity of materials to lowtemperatures. The freezing point is a limiting factor for many raw materials, asthe tissues will become disrupted on thawing. Other foods may be subject toproblems at temperatures above freezing. Fruits and vegetables may displayphysiological problems that limit their storage temperatures, probably as a re-sult of metabolic imbalance leading to a build up of undesirable chemical spe-cies in the tissues. Some types of apples are subject to internal browning below3 8C, while bananas become brown when stored below 13 8C and many othertropical fruits display chill sensitivity. Less obvious biochemical problems mayoccur even where no visible damage occurs. For example, storage temperatureaffects the starch/sugar balance in potatoes: in particular below 10 8C a buildup of sugar occurs, which is most undesirable for fried products. Examples ofstorage periods and conditions are given in Table 1.1, illustrating the wideranges seen with different fruits and vegetables. It should be noted that pre-dicted storage lives can be confounded if the produce is physically damaged, orby the presence of pathogens. Temperature of storage is also limited by cost. Refrigerated storage is expen-sive, especially in hot countries. In practice, a balance must be struck incorpor-ating cost, shelflife and risk of cold injury. Slower growing produce such asonions, garlic and potatoes can be successfully stored at ambient temperatureand ventilated conditions in temperate climates. It is desirable to monitor temperature throughout raw material storage anddistribution. Precooling to remove the ‘field heat’ is an effective strategy to reduce the peri-od of high initial respiration rate in rapidly respiring produce prior to transpor-tation and storage. For example, peas for freezing are harvested in the cool earlymorning and rushed to cold storage rooms within 2–3 h. Other produce, such
  36. 36. 12 1 Postharvest Handling and Preparation of Foods for Processing as leafy vegetables (lettuce, celery, cabbage) or sweetcorn, may be cooled using water sprays or drench streams. Hydrocooling obviously reduces water loss. Humidity If the humidity of the storage environment exceeds the equilibrium relative hu- midity (ERH) of the food, the food will gain moisture during storage, and vice versa. Uptake of water during storage is associated with susceptibility to growth of microorganisms, whilst water loss results in economic loss and more specific problems, such as cracking of seed coats of cereals, or skins of fruits and vege- tables. Ideally, the humidity of the store would equal the ERH of the food so that moisture is neither gained nor lost, but in practice a compromise may be necessary. The water activity (aw) of most fresh foods (e.g. fruit, vegetables, meat, fish, milk) is in the range 0.98–1.00, but they are frequently stored at a lower humidity. Some wilting of fruits or vegetable may be acceptable in prefer- ence to mould growth, while some surface drying of meat is preferable to bacte- rial slime. Packaging may be used to protect against water loss of raw materials during storage and transport, see Chapter 9. Composition of Atmosphere Controlling the atmospheric composition during storage of many raw materials is beneficial. The use of packaging to allow the development or maintenance of particular atmospheric compositions during storage is discussed in greater de- tail in Chapter 9. With some materials, the major aim is to maintain an oxygen-free atmo- sphere to prevent oxidation, e.g. coffee, baked goods, while in other cases ade- quate ventilation may be necessary to prevent anaerobic fermentation leading to off flavours. In living produce, atmosphere control allows the possibility of slowing down metabolic processes, hence retarding respiration, ripening, senescence and the development of disorders. The aim is to introduce N2 and remove O2, allowing a build up of CO2. Controlled atmosphere storage of many commodities is dis- cussed by Thompson [10]. The technique allows year-round distribution of apples and pears, where controlled atmospheres in combination with refrigera- tion can give shelflives up to 10 months, much greater than by chilling alone. The particular atmospheres are cultivar specific, but in the range 1–10% CO2, 2–13% O2 at 3 8C for apples and 0 8C for pears. Controlled atmospheres are also used during storage and transport of chill-sensitive crops, such as for transport of bananas, where an atmosphere of 3% O2 and 5% CO2 is effective in prevent- ing premature ripening and the development of crown rot disease. Ethene (ethylene) removal is also vital during storage of climacteric fruit. With fresh meat, controlling the gaseous environment is useful in combina- tion with chilling. The aim is to maintain the red colour by storage in high O2 concentrations, which shifts the equilibrium in favour of high concentrations of
  37. 37. 1.3 Storage and Transportation of Raw Materials 13the bright red oxymyoglobin pigment. At the same time, high levels of CO2 arerequired to suppress the growth of aerobic bacteria. Other ConsiderationsOdours and taints can cause problems, especially in fatty foods such as meatand dairy products, as well as less obvious commodities such as citrus fruits,which have oil in the skins. Odours and taints may be derived from fuels or ad-hesives and printing materials, as well as other foods, e.g. spiced or smokedproducts. Packaging and other systems during storage and transport must pro-tect against contamination. Light can lead to oxidation of fats in some raw materials, e.g. dairy products.In addition, light gives rise to solanine production and the development ofgreen pigmentation in potatoes. Hence, storage and transport under dark condi-tions is essential.1.3.2TransportationFood transportation is an essential link in the food chain and is discussed in de-tail by Heap [11]. Raw materials, food ingredients, fresh produce and processedproducts are all transported on a local and global level, by land, sea and air. Inthe modern world, where consumers expect year-round supplies and nonlocalproducts, long distance transport of many foods has become commonplace andair transport may be necessary for perishable materials. Transportation of foodis really an extension of storage: a refrigerated lorry is basically a cold store onwheels. However, transport also subjects the material to physical and mechani-cal stresses, and possibly rapid changes in temperature and humidity, which arenot encountered during static storage. It is necessary to consider both the stres-ses imposed during the transport and those encountered during loading andunloading. In many situations, transport is multimodal. Air or sea transportwould commonly involve at least one road trip before and one road trip afterthe main journey. There would also be time spent on the ground at the port orairport where the material could be exposed to wideranging temperatures andhumidities, or bright sunlight, and unscheduled delays are always a possibility.During loading and unloading, the cargo may be broken into smaller unitswhere more rapid heat penetration may occur. The major challenges during transportation are to maintain the quality of thefood during transport, and to apply good logistics – in other words, to move thegoods to the right place at the right time and in good condition.
  38. 38. 14 1 Postharvest Handling and Preparation of Foods for Processing 1.4 Raw Material Cleaning All food raw materials are cleaned before processing. The purpose is obviously to remove contaminants, which range from innocuous to dangerous. It is im- portant to note that removal of contaminants is essential for protection of pro- cess equipment as well as the final consumer. For example, it is essential to remove sand, stones or metallic particles from wheat prior to milling to avoid damaging the machinery. The main contaminants are: – unwanted parts of the plant, such as leaves, twigs, husks; – soil, sand, stones and metallic particles from the growing area; – insects and their eggs; – animal excreta, hairs etc.; – pesticides and fertilisers; – mineral oil; – microorganisms and their toxins. Increased mechanisation in harvesting and subsequent handling has generally led to increased contamination with mineral, plant and animal contaminants, while there has been a general increase in the use of sprays, leading to in- creased chemical contamination. Microorganisms may be introduced preharvest from irrigation water, manure, fertiliser or contamination from feral or domes- tic animals, or postharvest from improperly cleaned equipment, wash waters or cross-contamination from other raw materials. Cleaning is essentially separation in which some difference in physical prop- erties of the contaminants and the food units is exploited. There are a number of cleaning methods available, classified into dry and wet methods, but a combi- nation would usually be employed for any specific material. Selection of the ap- propriate cleaning regime depends on the material being cleaned, the level and type of contamination and the degree of decontamination required. In practice a balance must be struck between cleaning cost and product quality, and an ‘ac- ceptable standard’ should be specified for the particular end use. Avoidance of product damage is an important contributing factor, especially for delicate mate- rials such as soft fruit. 1.4.1 Dry Cleaning Methods The main dry cleaning methods are based on screens, aspiration or magnetic se- parations. Dry methods are generally less expensive than wet methods and the ef- fluent is cheaper to dispose of, but they tend to be less effective in terms of clean- ing efficiency. A major problem is recontamination of the material with dust. Pre- cautions may be necessary to avoid the risk of dust explosions and fires. Screens are essentially size separators based on perforated beds or wire mesh. Larger contaminants are removed from smaller food items: e.g. straw from cere-
  39. 39. 1.4 Raw Material Cleaning 15al grains, or pods and twigs from peas. This is termed ‘scalping’, see Fig. 1.1 a.Alternatively, ‘dedusting’ is the removal of smaller particles, e.g. sand or dust,from larger food units, see Fig. 1.1 b. The main geometries are rotary drums (also known as reels or trommels),and flatbed designs. Some examples are shown in Fig. 1.2.Fig. 1.1 Screening of dry particulate materials: (a) scalping, (b) dedusting.Fig. 1.2 Screen geometries: (a) rotary screen, (b) principle of flatbed screen.
  40. 40. 16 1 Postharvest Handling and Preparation of Foods for Processing Fig. 1.2 (b) Abrasion, either by impact during the operation of the machinery, or aided by abrasive discs or brushes, can improve the efficiency of dry screens. Screening gives incomplete separations and is usually a preliminary cleaning stage. Aspiration exploits the differences in aerodynamic properties of the food and the contaminants. It is widely used in the cleaning of cereals, but is also incor- porated into equipment for cleaning peas and beans. The principle is to feed the raw material into a carefully controlled upward air stream. Denser material will fall, while lighter material will be blown away depending on the terminal
  41. 41. 1.4 Raw Material Cleaning 17velocity. Terminal velocity in this case can be defined as the velocity of upwardair stream in which a particle remains stationary; and this depends on the den-sity and projected area of the particles (as described by Stokes’ equation). Byusing different air velocities, it is possible to separate say wheat from lighterchaff (see Fig. 1.3) or denser small stones. Very accurate separations are pos-sible, but large amounts of energy are required to generate the air streams. Ob-viously the system is limited by the size of raw material units, but is particularlysuitable for cleaning legumes and cereals. Air streams may also be used simply to blow loose contaminants from largeritems such as eggs or fruit. Magnetic cleaning is the removal of ferrous metal using permanent or electro-magnets. Metal particles, derived from the growing field or picked up duringtransport or preliminary operations, constitute a hazard both to the consumerand to processing machinery, for example cereal mills. The geometry of mag-netic cleaning systems can be quite variable: particulate foods may be passedover magnetised drums or magnetised conveyor belts, or powerful magnetsmay be located above conveyors. Electromagnets are easy to clean by turning offthe power. Metal detectors are frequently employed prior to sensitive processingequipment as well as to protect consumers at the end of processing lines. Electrostatic cleaning can be used in a limited number of cases where the sur-face charge on raw materials differs from contaminating particles. The principlecan be used to distinguish grains from other seeds of similar geometry but dif-ferent surface charge; and it has also been described for cleaning tea. The feed Fig. 1.3 Principle of aspiration cleaning.
  42. 42. 18 1 Postharvest Handling and Preparation of Foods for Processing Fig. 1.4 Principle of electrostatic cleaning. is conveyed on a charged belt and charged particles are attracted to an oppo- sitely charged electrode (see Fig. 1.4) according to their surface charge. 1.4.2 Wet Cleaning Methods Wet methods are necessary if large quantities of soil are to be removed; and they are essential if detergents are used. However, they are expensive, as large quantities of high purity water are required and the same quantity of dirty efflu- ent is produced. Treatment and reuse of water can reduce costs. Employing the countercurrent principle can reduce water requirements and effluent volumes if accurately controlled. Sanitising chemicals such as chlorine, citric acid and ozone are commonly used in wash waters, especially in association with peeling and size reduction, where reducing enzymic browning may also be an aim [12]. Levels of 100–200 mg l–1 chlorine or citric acid may be used, although their ef- fectiveness for decontamination has been questioned and they are not permitted in some countries. Soaking is a preliminary stage in cleaning heavily contaminated materials, such as root crops, permitting softening of the soil and partial removal of stones and other contaminants. Metallic or concrete tanks or drums are em- ployed; and these may be fitted with devices for agitating the water, including stirrers, paddles or mechanisms for rotating the entire drum. For delicate pro- duce such as strawberries or asparagus, or products which trap dirt internally, e.g. celery, sparging air through the system may be helpful. The use of warm water or including detergents improves cleaning efficiency, especially where mineral oil is a possible contaminant, but adds to the expense and may damage the texture.
  43. 43. 1.4 Raw Material Cleaning 19 Spray washing is very widely used for many types of food raw material. Efficiencydepends on the volume and temperature of the water and time of exposure. As ageneral rule, small volumes of high pressure water give the most efficient dirt re-moval, but this is limited by product damage, especially to more delicate produce.With larger food pieces, it may be necessary to rotate the unit so that the wholesurface is presented to the spray (see Fig. 1.5 a). The two most common designsare drum washers and belt washers (see Figs. 1.5 a, b). Abrasion may contributeto the cleaning effect, but again must be limited in delicate units. Other designsinclude flexible rubber discs which gently brush the surface clean. Flotation washing employs buoyancy differences between food units and con-taminants. For instance sound fruit generally floats, while contaminating soil,stones or rotten fruits sink in water. Hence fluming fruit in water over a seriesof weirs gives very effective cleaning of fruit, peas and beans (see Fig. 1.6). A dis-advantage is high water use, thus recirculation of water should be incorporated. Froth flotation is carried out to separate peas from contaminating weed seedsand exploits surfactant effects. The peas are dipped in an oil/detergent emul-sion and air is blown through the bed. This forms a foam which washes awaythe contaminating material and the cleaned peas can be spray washed. Following wet cleaning, it is necessary to remove the washing water. Centrifu-gation is very effective, but may lead to tissue damage, hence dewateringscreens or reels are more common.Fig. 1.5 Water spray cleaning: (a) spray belt washer, (b) drum washer.
  44. 44. 20 1 Postharvest Handling and Preparation of Foods for Processing Fig. 1.6 Principle of flotation washing. Prestorage hot water dipping has been used as an alternative to chemical treatments for preserving the quality of horticultural products. One recent devel- opment is the simultaneous cleaning and disinfection of fresh produce by a short hot water rinse and brushing (HWRB) treatment [13]. This involves plac- ing the crops on rotating brushes and rinsing with hot water for 10–30 s. The effect is through a combination of direct cleaning action plus the lethal action of heat on surface pathogens. Fungicides may also be added to the hot water. 1.4.3 Peeling Peeling of fruits and vegetables is frequently carried out in association with cleaning. Mechanical peeling methods require loosening of the skin using one of the following principles, depending on the structure of the food and the level of peeling required [14]: – Steam is particularly suited to root crops. The units are exposed to high pres- sure steam for a fixed time and then the pressure is released causing steam to form under the surface of the skin, hence loosening it such that it can be removed with a water spray. – Lye (1–2% alkali) solution can be used to soften the skin which can again be removed by water sprays. There is, however, a danger of damage to the prod- uct.
  45. 45. 1.5 Sorting and Grading 21– Brine solutions can give a peeling effect but are probably less effective than the above methods.– Abrasion peeling employs carborundum rollers or rotating the product in a carborundum-lined bowl, followed by washing away the loosened skin. It is effective but here is a danger of high product loss by this method.– Mechanical knives are suitable for peeling citrus fruits.– Flame peeling is useful for onions, in which the outer layers are burnt off and charred skin is removed by high pressure hot water.1.5Sorting and GradingSorting and grading are terms which are frequently used interchangeably in thefood processing industry, but strictly speaking they are distinct operations. Sort-ing is a separation based on a single measurable property of raw material units,while grading is “the assessment of the overall quality of a food using a numberof attributes” [14]. Grading of fresh produce may also be defined as ‘sorting ac-cording to quality’, as sorting usually upgrades the product. Virtually all food products undergo some kind of sorting operation. There area number of benefits, including the need for sorted units in weight-fillingoperations and the aesthetic and marketing advantages in providing units ofuniform size or colour. In addition, it is much easier to control processes suchas sterilisation, dehydration or freezing in sorted food units; and they are alsobetter suited to mechanised operations such as size reduction, pitting or peel-ing.1.5.1Criteria and Methods of SortingSorting is carried out on the basis of individual physical properties. Details ofprinciples and equipment are given by Saravacos and Kostaropoulos [15], Bren-nan et al. [16] and Peleg [17]. No sorting system is absolutely precise and a bal-ance is often struck between precision and flow rate. Weight is usually the most precise method of sorting, as it is not dependenton the geometry of the products. Eggs, fruit or vegetables may be separated intoweight categories using spring-loaded, strain gauge or electronic weighing de-vices incorporated into conveying systems. Using a series of tipping or com-pressed air blowing mechanisms set to trigger at progressively lesser weights,the heavier items are removed first, followed by the next weight category and soon. These systems are computer controlled and can additionally provide data onquantities and size distributions from different growers. An alternative systemis to use the ‘catapult’ principle where units are thrown into different collectingchutes, depending on their weight, by spring-loaded catapult arms. A disadvan-tage of weight sorting is the relatively long time required per unit; and other
  46. 46. 22 1 Postharvest Handling and Preparation of Foods for Processing methods are more appropriate with smaller items such as legumes or cereals, or if faster throughput is required. Size sorting is less precise than weight sorting, but is considerably cheaper. As discussed in Section 1.2, the size and shape of food units are difficult to de- fine precisely. Size categories could involve a number of physical parameters, in- cluding diameter, length or projected area. Diameter of spheroidal units such as tomatoes or citrus fruits is conventionally considered to be orthogonal to the fruit stem, while length is coaxial. Therefore rotating the units on a conveyor can make size sorting more precise. Sorting into size categories requires some sort of screen, many designs of which are discussed in detail by Slade [18], Brennan et al. [16] and Fellows [14]. The main categories of screens are fixed aperture and variable aperture designs. Flatbed and rotary screens are the main geometries of the fixed bed screen and a number of screens may be used in se- ries or in parallel to sort units into several size categories simultaneously. The problem with fixed screens is usually contacting the feed material with the screen, which may become blocked or overloaded. Fixed screens are often used with smaller particulate foods such as nuts or peas. Variable aperture screens have either a continuous diverging or stepwise diverging apertures. These are much more gentle and are commonly used with larger, more delicate items such as fruit. The principles of some sorting screens are illustrated in Fig. 1.7. Shape sorting is useful in cases where the food units are contaminated with particles of similar size and weight. This is particularly applicable to grain which may contain other seeds. The principle is that discs or cylinders with accurately shaped indentations will pick up seeds of the correct shape when rotated through the stock, while other shapes will remain in the feed (see Fig. 1.8). Density can be a marker of suitability for certain processes. The density of peas correlates well with tenderness and sweetness, while the solids content of potatoes, which determines suitability for manufacture of crisps and dried prod- ucts, relates to density. Sorting on the basis of density can be achieved using flo- tation in brine at different concentrations. Photometric properties may be used as a basis for sorting. In practice this usually means colour. Colour is often a measure of maturity, presence of defects or the degree of processing. Manual colour sorting is carried out widely on con- veyor belts or sorting tables, but is expensive. The process can be automated using highly accurate photocells which compare reflectance of food units to pre- set standards and can eject defective or wrongly coloured, e.g. blackened, units, usually by a blast of compressed air. This system is used for small particulate foods such as navy beans or maize kernels for canning, or nuts, rice and small fruit (see Fig. 1.9). Extremely high throughputs have been reported, e.g. 16 t h–1 [14]. By using more than one photocell positioned at different angles, blemishes on large units such as potatoes can be detected. Colour sorting can also be used to separate materials which are to be pro- cessed separately, such as red and green tomatoes. It is feasible to use transmit- tance as a basis for sorting although, as most foods are completely opaque, very
  47. 47. 1.5 Sorting and Grading 23Fig. 1.7 Some geometries of size sorting equipment:(a) concentric drum screen, (b) roller size sorter,(c) belt and roller sorter.