FOOD PROCESS ENGINEERING AND TECHNOLOGY
Food Science and Technology
International Series
Series Editor
Steve L. Taylor
University of Nebraska – Lincoln, USA
Advisory Board
Ken Buckle
The University of New South Wales, Australia
Mary Ellen Camire
University of Maine, USA
Roger Clemens
University of Southern California, USA
Hildegarde Heymann
University of California – Davis, USA
Robert Hutkins
University of Nebraska – Lincoln, USA
Ron S. Jackson
Quebec, Canada
Huub Lelieveld
Bilthoven, The Netherlands
Daryl B. Lund
University of Wisconsin, USA
Connie Weaver
Purdue University, USA
Ron Wrolstad
Oregon State University, USA
A complete list of books in this series appears at the end of this volume
Food Process
Engineering and

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ISBN: 978-0-12-373660-4
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09 10 11 12 13 10 9 8 7 6 5 4 3 2 1
To my students
Contents
Introduction – Food is Life 1
1 Physical properties of food materials 7
1.1 Introduction 7
1.2 Mechanical properties 8
1 . 2 . 1 D e fi nitions 8
1.2.2 Rheological models 9
1.3 Thermal properties 10
1.4 Electrical properties 11
1.5 Structure 11
1.6 Water activity 13
1.6.1 The importance of water in foods 13
1.6.2 Water activity, defi nition and determination 14
1.6.3 Water activity: prediction 14
1.6.4 Water vapor sorption isotherms 16
1.6.5 Water activity: effect on food quality and stability 19
1.7 Phase transition phenomena in foods 19
1.7.1 The glassy state in foods 19
1.7.2 Glass transition temperature 20

3.2 Basic relations in transport phenomena 69
3.2.1 Basic laws of transport 69
3.2.2 Mechanisms of heat and mass transfer 70
3.3 Conductive heat and mass transfer 70
3.3.1 The Fourier and Fick laws 70
3.3.2 Integration of Fourier’s and Fick’s laws for
steady-state conductive transport 71
3.3.3 Thermal conductivity, thermal diffusivity
and molecular diffusivity 73
3.3.4 Examples of steady-state conductive heat and
mass transfer processes 76
3.4 Convective heat and mass transfer 81
3.4.1 Film (or surface) heat and mass transfer coeffi cients 81
3.4.2 Empirical correlations for convection heat and mass
transfer 84
3.4.3 Steady-state interphase mass transfer 87
3.5 Unsteady state heat and mass transfer 89
3.5.1 The 2 nd Fourier and Fick laws 89
3.5.2 Solution of Fourier’s second law equation for an
infi nite slab 90
3.5.3 Transient conduction transfer in fi nite solids 92
3.5.4 Transient convective transfer in a semi-infi nite body 94
3.5.5 Unsteady state convective transfer 95
3.6 Heat transfer by radiation 96
3.6.1 Interaction between matter and thermal radiation 96
3.6.2 Radiation heat exchange between surfaces 97
3.6.3 Radiation combined with convection 100
3.7 Heat exchangers 100
3.7.1 Overall coeffi cient of heat transfer 100
3.7.2 Heat exchange between fl owing fl uids 102

5.5.1 First order response 133
5.5.2 Second order systems 135
5.6 Control modes (control algorithms) 136
5.6.1 On-off (binary) control 136
5.6.2 Proportional (P) control 138
5.6.3 Integral (I) control 139
5.6.4 Proportional-integral (PI) control 140
5.6.5 Proportional-integral-differential (PID) control 140
5.6.6 Optimization of control 141
5.7 The physical elements of the control system 142
5.7.1 The sensors (measuring elements) 142
5.7.2 The controllers 149
5.7.3 The actuators 149
6 Size reduction 153
6.1 Introduction 153
6.2 Particle size and particle size distribution 154
6.2.1 Defi ning the size of a single particle 154
6.2.2 Particle size distribution in a population of particles;
defi ning a ‘ mean particle size ’ 155
6.2.3 Mathematical models of PSD 158
6.2.4 A note on particle shape 160
Contents vii
6.3 Size reduction of solids, basic principles 163
6.3.1 Mechanism of size reduction in solids 163
6.3.2 Particle size distribution after size reduction 163
6.3.3 Energy consumption 163
6.4 Size reduction of solids, equipment and methods 165
6.4.1 Impact mills 166
6.4.2 Pressure mills 167
6.4.3 Attrition mills 168

8.5.2 Mechanisms 211
8.5.3 Applications and equipment 213
9 Centrifugation 217
9.1 Introduction 217
9.2 Basic principles 218
9.2.1 The continuous settling tank 218
9.2.2 From the settling tank to the tubular centrifuge 220
9.2.3 The baffl ed settling tank and the disc-bowl centrifuge 223
9.2.4 Liquid–liquid separation 224
viii Contents
9.3 Centrifuges 226
9.3.1 Tubular centrifuges 227
9.3.2 Disc-bowl centrifuges 228
9.3.3 Decanter centrifuges 230
9.3.4 Basket centrifuges 230
9.4 Cyclones 231
10 Membrane processes 233
10.1 Introduction 233
10.2 Tangential fi ltration 234
10.3 Mass transfer through MF and UF membranes 235
10.3.1 Solvent transport 235
10.3.2 Solute transport; sieving coeffi cient and rejection 237
10.3.3 Concentration polarization and gel polarization 238
10.4 Mass transfer in reverse osmosis 241
10.4.1 Basic concepts 241
10.4.2 Solvent transport in reverse osmosis 242
10.5 Membrane systems 245
10.5.1 Membrane materials 245
10.5.2 Membrane confi gurations 247
10.6 Membrane processes in the food industry 249

12.5.3 Application: Water softening using ion exchange 292
12.5.4 Application: Reduction of acidity in fruit juices 293
13 Distillation 295
13.1 Introduction 295
13.2 Vapor–liquid equilibrium (VLE) 295
13.3 Continuous fl ash distillation 298
13.4 Batch (differential) distillation 301
13.5 Fractional distillation 304
13.5.1 Basic concepts 304
13.5.2 Analysis and design of the column 305
13.5.3 Effect of the refl ux ratio 310
13.5.4 Tray confi guration 310
13.5.5 Column confi guration 311
13.5.6 Heating with live steam 311
13.5.7 Energy consider ations 312
13.6 Steam distillation 313
13.7 Distillation of wines and spirits 314
14 Crystallization and dissolution 317
14.1 Introduction 317
14.2 Crystallization kinetics 318
14.2.1 Nucleation 318
14.2.2 Crystal growth 320
14.3 Crystallization in the food industry 323
14.3.1 Equipment 323
14.3.2 Processes 325
14.4 Dissolution 328
14.4.1 Introduction 328
14.4.2 Mechanism and kinetics 328
15 Extrusion 333
15.1 Introduction 333

enzymes 356
17.2.1 The concept of decimal reduction time 356
17.2.2 Effect of the temperature on the rate of thermal
destruction/inactivation 358
17.3 Lethality of thermal processes 360
17.4 Optimization of thermal processes with respect to quality 363
17.5 Heat transfer considerations in thermal processing 364
17.5.1 In-package thermal processing 364
17.5.2 In-fl ow thermal processing 369
18 Thermal processes, methods and equipment 375
18.1 Introduction 375
18.2 Thermal processing in hermetically closed containers 375
18.2.1 Filling into the cans 376
18.2.2 Expelling air from the head-space 378
18.2.3 Sealing 379
18.2.4 Heat processing 380
18.3 Thermal processing in bulk, before packaging 386
18.3.1 Bulk heating – hot fi lling – sealing – cooling in container 386
18.3.2 Bulk heating – holding – bulk cooling – cold fi lling – sealing. 386
18.3.3 Aseptic processing 388
19 Refrigeration, chilling and freezing 391
19.1 Introduction 391
19.2 Effect of temperature on food spoilage 392
19.2.1 Temperature and chemical activity 392
19.2.2 Effect of low temperature on enzymatic spoilage 395
19.2.3 Effect of low temperature on microorganisms 396
19.2.4 Effect of low temperature on biologically active
(respiring) tissue 398
19.2.5 The effect of low temperature on physical properties 399
19.3 Freezing 400

21.6 Evaporators in the food industry 448
21.6.1 Open pan batch evaporator 448
21.6.2 Vacuum pan evaporator 449
21.6.3 Evaporators with tubular heat exchangers 449
21.6.4 Evaporators with external tubular heat exchangers 451
21.6.5 Boiling fi lm evaporators 451
21.7 Effect of evaporation on food quality 454
21.7.1 Thermal effects 454
21.7.2 Loss of volatile fl avor components 457
22 Dehydration 459
22.1 Introduction 459
22.2 Thermodynamics of moist air (psychrometry) 461
22.2.1 Basic principles 461
22.2.2 Humidity 461
22.2.3 Saturation, relative humidity (RH) 462
22.2.4 Adiabatic saturation, wet-bulb temperature 462
22.2.5 Dew point 463
22.3 Convective drying (air drying) 464
22.3.1 The drying curve 464
22.3.2 The constant rate phase 467
22.3.3 The falling rate phase 470
22.3.4 Calculation of drying time 472
22.3.5 Effect of external conditions on the drying rate 475
xii Contents
22.3.6 Relationship between fi lm coeffi cients in convective drying 476
22.3.7 Effect of radiation heating 477
22.3.8 Characteristic drying curves 477
22.4 Drying under varying external conditions 478
22.4.1 Batch drying on trays 478
22.4.2 Through-fl ow batch drying in a fi xed bed 480

23.3 Heat and mass transfer in freeze drying 512
23.4 Freeze drying, in practice 518
23.4.1 Freezing 518
23.4.2 Drying conditions 518
23.4.3 Freeze drying, commercial facilities 518
23.4.4 Freeze dryers 519
23.5 Freeze concentration 520
23.5.1 Basic principles 520
23.5.2 The process of freeze concentration 521
24 Frying, baking, roasting 525
24.1 Introduction 525
Contents xiii
24.2 Frying 525
24.2.1 Types of frying 525
24.2.2 Heat and mass transfer in frying 526
24.2.3 Systems and operation 527
24.2.4 Health aspects of fried foods 528
24.3 Baking and roasting 528
25 Ionizing irradiation and other non-thermal preservation processes 533
25.1 Preservation by ionizing radiations 533
25.1.1 Introduction 533
25.1.2 Ionizing radiations 533
25.1.3 Radiation sources 534
25.1.4 Interaction with matter 535
25.1.5 Radiation dose 537
25.1.6 Chemical and biological effects of ionizing irradiation 538
25.1.7 Industrial applications 540
25.2 High hydrostatic pressure preservation 541
25.3 Pulsed electric fi elds (PEF) 542
25.4 Pulsed intense light 542

xiv Contents
Appendix 575
Table A.1 Common conversion factors 576
Table A.2 Typical composition of selected foods 577
Table A.3 Viscosity and density of gases and liquids 578
Table A.4 Thermal properties of materials 578
Table A.5 Emissivity of surfaces 579
Table A.6 US standard sieves 579
Table A.7 Properties of saturated steam – temperature table 580
Table A.8 Properties of saturated steam – pressure table 581
Table A.9 Properties of superheated steam 581
Table A.10 Vapor pressure of liquid water and ice below 0°C 582
Table A.11 Freezing point of ideal aqueous solutions 583
Table A.12 Vapor–liquid equilibrium data for ethanol–water
mixtures at 1 atm 583
Table A.13 Boiling point of sucrose solutions at 1 atm 584
Table A.14 Electrical conductivity of some materials 584
Table A.15 Thermodynamic properties of saturated R-134a 584
Table A.16 Thermodynamic properties of superheated R-134a 585
Table A.17 Properties of air at atmospheric pressure 586
Figure A.1 Friction factors for fl ow in pipes 587
Figure A.2 Psychrometric chart 587
Figure A.3 Mixing power function, turbine impellers 588
Figure A.4 Mixing power function, propeller impellers 588
Figure A.5 Unsteady state heat transfer in a slab 589
Figure A.6 Unsteady state heat transfer in an infi nite cylinder 589
Figure A.7 Unsteady state heat transfer in a sphere 590
Figure A.8 Unsteady state mass transfer, average concentration 590
Figure A.9 Error function 591
Index 593

Food Process Engineering and Technology Copyright © 2009, Elsevier Inc.
ISBN: 978-0-12-373660-4 All rights reserved
Table I.1 Unit operations of the food processing industry by principal groups
Group Unit operation Examples of application
Cleaning Washing
Peeling
Removal of foreign bodies
Cleaning in place (CIP)
Fruits, vegetables
Fruits, vegetables
Grains
All food plants
Physical separation Filtration
Screening
Sorting
Membrane separation
Centrifugation
Pressing, expression
Sugar refi ning
Grains
Coffee beans
Ultrafi ltration of whey
Separation of milk
Oilseeds, fruits
Molecular (diffusion based)
separation
Adsorption
Distillation
Extraction
Bleaching of edible oils

cooking, volume and mass reduction,
improving the fl avor etc.)
Thermal processing
(blanching, pasteurization,
sterilization)
Pasteurized milk
Canned vegetables
Chilling Fresh meat, fi sh
Freezing Frozen dinners
Ice cream
Frozen vegetables
Concentration Tomato paste
Citrus juice concentrate
Sugar
Addition of solutes Salting of fi sh
Jams, preserves
Chemical preservation Pickles
Salted fi sh
Smoked fi sh
Dehydration Dried fruit
Dehydrated vegetables
Milk powder
Instant coffee
Mashed potato fl akes
Freeze drying Instant coffee
Packaging Filling
Sealing
Wrapping
Bottled beverages
Canned foods

food processes and includes chapters on the physical properties of foods, momentum
transfer (fl ow), heat and mass transfer, reaction kinetics and elements of process con-
trol. The rest of the book deals with the principal unit operations of food processing.
Batch and Continuous Processes
Processes may be carried-out in batch, continuous or mixed fashion.
In batch processing , a portion of the materials to be processed is separated from
the bulk and treated separately. The conditions such as temperature, pressure, compo-
sition etc. usually vary during the process. The batch process has a defi nite duration
and, after its completion, a new cycle begins, with a new portion of material. The
batch process is usually less capital intensive but may be more costly to operate and
involves costly equipment dead-time for loading and unloading between batches. It is
easier to control and lends itself to intervention during the process. It is particularly
4 Introduction
suitable for small-scale production and to frequent changes in product composition
and process conditions. A typical example of a batch process would be the mixing of
fl our, water, yeast and other ingredients in a bowl mixer to make a bread dough. After
having produced one batch of dough for white bread, the same mixer can be cleaned
and used to make a batch of dark dough.
In continuous processing , the materials pass through the system continuously,
without separation of a part of the material from the bulk. The conditions at a given
point of the system may vary for a while at the beginning of the process, but ide-
ally they remain constant during the best part of the process. In engineering terms, a
continuous process is ideally run at steady state for most of its duration. Continuous
processes are more diffi cult to control, require higher capital investment, but pro-
vide better utilization of production capacity, at lower operational cost. They are
particularly suitable for lines producing large quantities of one type of product for a
relatively long duration. A typical example of a continuous process would be the con-
tinuous pasteurization of milk.
Mixed processes are composed of a sequence of continuous and batch processes.
An example of a mixed process would be the production of strained infant food. In

Refining
Conching
Tempering
Molding
Cocoa
Cocoa mass
Sugar
Cocoa butter
Other ingredients
Chocolate
Figure I.1 Block diagram for the chocolate manufacturing process
3
7
4
85
1
6
2
Figure I.2 Some symbols used in process fl ow diagrams: 1: Reactor; 2: Distillation column; 3: Heat
exchanger; 4: Plate heat exchanger; 5: Filter or membrane; 6: Centrifugal pump; 7: Rotary positive
displacement pump; 8: Centrifuge
Process Flow Diagrams 5
6 Introduction
equipment in space. A simplifi ed pictorial equipment fl ow diagram for the chocolate
production process is shown in Figure I.3 .
The next step of process development is the creation of an engineering fl ow dia-
gram . In addition to the items shown in the equipment fl ow diagram, auxiliary or sec-
ondary equipment items, measurement and control systems, utility lines and piping
details such as traps, valves etc. are included. The engineering fl ow diagram serves as
a starting point for the listing, calculation and selection of all the physical elements of

prediction of the response of foods to processing, distribution and storage con-
ditions. These are sometimes referred to as ‘ engineering properties ’ , although
most physical properties are signifi cant both from the quality and engineering
points of view.
In recent years, the growing interest in the physical properties of foods is con-
spicuously manifested. A number of books and reviews dealing specifi cally with the
subject have been published (e.g. Mohsenin, 1980 ; Peleg and Bagley, 1983 ; Jowitt,
1983 ; Lewis, 1990 ; Rahman, 1995 ; Balint, 2001 ; Scanlon, 2001 ; Sahin and Sumnu,
2006 ; Figura and Teixeira, 2007 ). The number of scientifi c meetings on related
Food Process Engineering and Technology Copyright © 2009, Elsevier Inc.
ISBN: 978-0-12-373660-4 All rights reserved
8 Physical Properties of Food Materials
subjects held every year is considerable. Specifi c courses on the subject are being
included in most food science, engineering and technology curricula.
Some of the ‘ engineering ’ properties will be treated in connection with the unit
operations where such properties are particularly relevant (e.g. viscosity in fl uid fl ow,
particle size in size reduction, thermal properties in heat transfer, diffusivity in mass
transfer etc.). Properties of more general signifi cance and wider application are dis-
cussed in this chapter.
1.2 Mechanical Properties
1.2.1 Defi nitions
By mechanical properties, we mean those properties that determine the behavior of
food materials when subjected to external forces. As such, mechanical properties are
relevant both to processing (e.g. conveying, size reduction) and to consumption (tex-
ture, mouth feel).
The forces acting on the material are usually expressed as stress , i.e. intensity of
the force per unit area (N.m
Ϫ 2
or Pa.). The dimensions and units of stress are like
those of pressure. Very often, but not always, the response of materials to stress is

0
ϭ original length.
● Plastic deformation : deformation does not occur as long as the stress is below
a limit value known as yield stress . Deformation is permanent, i.e. the body
does not return to its original size and shape when the stress is removed.