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Drying and Storage of Cereal Grains
Drying and Storage of Cereal Grains
Second Edition
B. K. Bala
Department of Agro Product Processing Technology
Jessore University of Science and Technology
Jessore, Bangladesh
This edition first published 2017
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Contents
Foreword to the Second Edition xi
Foreword to the First Edition xiii
Preface xv
1 Principles of Drying 1
1.1 Introduction 1
1.2 Losses of Crops 2
1.3 Importance of Drying 2
1.4 Principles of Drying 2
Reference 3
Further Reading 3
2 Moisture Contents and Equilibrium Moisture Content Models 5
2.1 Introduction 5
2.2 Moisture Content Representation 5
2.3 Determination of Moisture Content 7
2.3.1 Direct Methods 8
2.3.2 Indirect Methods 10
2.4 Grain Sampling 12
2.5 Equilibrium Moisture Content 12
2.6 Determination of Static Equilibrium Moisture Content 17
2.7 Static Equilibrium Moisture Content Models 20
2.8 Net Isosteric Heat of Sorption 22
Exercises 28
References 28
3 Psychrometry 31
3.1 Introduction 31
3.2 Psychrometric Terms 31
3.2.1 Humidity Ratio 32
3.2.2 Relative Humidity 32
3.2.3 Specific Volume 33
3.2.4 Vapour Pressure 33
3.2.5 Dry Bulb Temperature 33
v
3.2.6 Dew Point Temperature 33
3.2.7 Wet Bulb Temperature 34
3.2.8 Enthalpy 34
3.2.9 Adiabatic Wet Bulb Temperature 35
3.2.10 Psychrometric Wet Bulb Temperature 36
3.3 Construction of Psychrometric Chart 38
3.4 Use of Psychrometric Chart 39
3.4.1 Sensible Heating and Cooling 39
3.4.2 Heating with Humidification 40
3.4.3 Cooling with Humidification 41
3.4.4 Cooling with Dehumidification 41
3.4.5 Drying 42
3.4.6 Mixing of Air Streams 43
3.4.7 Heat Addition with Air Mixing 45
3.4.8 Drying with Recirculation 46
Exercises 52
References 54
Further Reading 54
4 Physical and Thermal Properties of Cereal Grains 55
4.1 Introduction 55
4.2 Structure of Cereal Grains 55
4.3 Physical Dimensions 55
4.4 1000 Grain Weight 56
4.5 Bulk Density 57
4.6 Shrinkage 57
4.7 Friction 58
4.7.1 Angle of Internal Friction and Angle of Repose 59
4.7.2 Coefficient of Friction 59
4.8 Specific Heat 61
4.9 Thermal Conductivity 63
4.9.1 Theory 63
4.9.2 Apparatus and Measurement 65
4.10 Latent Heat of Vaporization of Grain Moisture 66
4.10.1 Determination of Latent Heat of Vaporization of a Grain 67
4.11 Heat Transfer Coefficient of Grain Bed 69
4.11.1 Dimensional Analysis 70
4.11.2 Comparison of Theory and Experiment 70
4.11.3 Determination of Volumetric Heat Transfer Coefficient 72
Exercises 76
References 78
Further Reading 80
5 Airflow Resistance and Fans 81
5.1 Airflow Resistance 81
5.1.1 Non-linear Airflow Analysis 83
5.2 Fans 91
Contentsvi
5.2.1 Fan Performance 92
5.2.2 Centrifugal Fan Laws 95
5.2.3 Fan Selection 97
5.2.4 Effect of Change in Fan Speed 98
5.2.5 Effect of Change in Speed and System Resistance 99
5.2.6 Fans in Series and Parallel 99
5.3 Duct Design for On-Floor Drying and Storage System 102
Exercises 103
References 105
6 Thin Layer Drying of Cereal Grains 107
6.1 Theory 107
6.2 Thin Layer Drying Equations 109
6.2.1 Empirical Drying Equations 109
6.2.2 Theoretical Drying Equations 110
6.2.3 Semi-Theoretical Drying Equations 113
6.2.4 Comparison of Thin Layer Drying Equations 114
6.3 Development of Thin Layer Drying Equations 116
6.3.1 Drying Rate 119
6.4 Drying Parameters 119
6.4.1 Drying Rate Constant and Diffusion Coefficient 120
6.4.2 Dynamic Equilibrium Moisture Content 127
6.5 Finite Element Modelling of Single Kernel 133
6.5.1 Finite Element Model Formulation 133
6.5.2 Finite Difference Solution in Time 138
6.5.3 Discretization of the Domain 138
Exercises 140
References 142
Further Reading 145
7 Deep-Bed and Continuous Flow Drying 147
7.1 Introduction 147
7.2 Deep-Bed Drying Models 147
7.2.1 Logarithmic Models 148
7.2.2 Partial Differential Equation Models 148
7.2.3 Comparison of Deep-Bed Drying Models 149
7.3 Development of Models for Deep-Bed Drying 149
7.3.1 Logarithmic Model 150
7.3.2 Partial Differential Equation Model 156
7.3.3 Method of Solution 160
7.3.4 Condensation Procedure 161
7.3.5 Sensitivity Analysis 169
7.3.6 Comparison of Simulated Drying with Experimental Results 169
7.3.7 Comparison of Direct, Indirect and Recirculating Direct Fired Drying 170
7.4 Development of Models for Continuous Flow Drying 171
7.4.1 Crossflow Model 173
7.4.2 Fluidized Bed Drying Model 180
Contents vii
7.5 CFD Modelling of Fluidized Bed Drying 185
7.5.1 Continuity Equation 185
7.5.2 Momentum Conservation Equations 186
7.5.3 Energy Conservation Equation 186
7.5.4 User-Defined Scheme (UDS) 187
7.5.5 CFD Analysis 187
Exercises 190
References 193
Further Reading 194
8 Grain Drying Systems 195
8.1 Introduction 195
8.2 Solar Drying Systems 195
8.3 Batch Drying Systems 199
8.4 Continuous-Flow Drying Systems 200
8.4.1 Crossflow Dryer 200
8.4.2 Concurrent Flow Dryer 200
8.4.3 Counterflow Dryer 202
8.5 Safe Temperature for Drying Grain 202
8.6 Hydrothermal Stresses during Drying 203
8.7 Energy and Exergy Analysis of Drying Process 204
8.7.1 Drying Efficiency 205
8.7.2 Exergy Analysis through the Analysis of Second Law of Thermodynamics 205
8.8 Neural Network Modelling 206
8.8.1 Structure of ANN Model 207
8.8.2 Training of ANN Model 208
8.9 Selection of Dryers 209
Exercises 211
References 212
Further Reading 213
9 Principles of Storage 215
9.1 Introduction 215
9.2 Principles of Storage 215
9.3 Interrelations of Physical, Chemical and Biological Variables in the
Deterioration of Stored Grains 218
9.4 Computer Simulation Modelling for Stored Grain Pest Management 219
References 220
Further Reading 221
10 Temperature and Moisture Changes During Storage 223
10.1 Introduction 223
10.2 Qualitative Analysis of Moisture Changes of Stored Grains in
Cylindrical Bins 223
10.3 Temperature Changes in Stored Grains 225
10.4 Temperature Prediction 225
Contentsviii
10.4.1 The Differential Equation of Heat Conduction
in Cylindrical
Coordinate System 226
10.4.2 Numerical Method 227
10.5 Numerical Solution of One-Dimensional Heat Flow 227
10.6 Numerical Solution of Two-Dimensional Heat and Moisture Flow 232
10.6.1 Heat Transfer Equation 233
10.6.2 Mass Transfer Equation 234
10.7 Simultaneous Momentum, Heat and Mass Transfer during Storage 249
10.7.1 The Energy Balance Equation 250
10.7.2 The Mass Balance Equation 251
10.7.3 The Momentum Balance Equation 251
10.7.4 Finite Difference Formulation 252
10.8 CFD Modelling of Grain Storage Systems 258
10.8.1 Continuity Equation 258
10.8.2 Momentum Conservation Equations 258
10.8.3 Energy Conservation Equation 258
10.8.4 User-Defined Function 258
Exercises 260
References 262
Further Reading 262
11 Fungi, Insects and Other Organisms Associated with Stored Grain 263
11.1 Introduction 263
11.2 Fungi 263
11.2.1 Field Fungi 265
11.2.2 Intermediate Fungi 265
11.2.3 Storage Fungi 265
11.3 Insects 267
11.3.1 Insect Species 268
11.3.2 Grain Temperature and Moisture Content 269
11.4 Mites 270
11.5 Rodents 270
11.6 Respiration and Heating 270
11.7 Control Methods 271
References 272
Further Reading 272
12 Design of Grain Storages 273
12.1 Introduction 273
12.2 Structural Requirements 273
12.2.1 Janssen Equation 273
12.2.2 Rankine Equation 277
12.2.3 Airy Equation 278
12.3 Construction Materials 280
Exercises 288
References 288
Contents ix
13 Grain Storage Systems 289
13.1 Introduction 289
13.2 Traditional Storage Systems 290
13.3 Modern Storage Systems 290
13.3.1 Bagged Storage System 290
13.3.2 Silo Storage System 291
13.3.3 Airtight Grain Storage 292
13.3.4 Aerated Storage System 297
13.3.5 Low-Temperature Storage System (Grain Chilling by Refrigeration) 301
13.3.6 Controlled Atmosphere Storage Systems 304
13.3.7 Damp Grain Storage System with Chemicals 306
References 310
Further Reading 313
Appendix A: Finite Difference Approximation 315
Appendix B: Gaussian Elimination Method 317
Appendix C: Finite Element Method 321
Appendix D: Computational Fluid Dynamics 325
Index 333
Contentsx
Foreword to the Second Edition
Drying of cereal grains is an important preservation method prior to storage of the grain.
Basic knowledge about fundamentals in drying technology and advanced details about
physical and thermal processes during drying and storage of grain is very important
not only for students, scientists and engineers in post-harvest technology but also for
drying and storage facility managers.
Cereals are globally one of the most important arable crops produced directly for
human nutrition. A significantly increasing amount of cereal production worldwide is
designated for feeding animals in livestock breeding to produce meat, eggs, milk and
so on, finally as an additional nutrition for humans. Higher production of cereals is
expected in the future to meet the demand for a sufficient nourishment of humans espe-
cially in developing countries. FAO estimates that in the next few decades the production
of cereal grains will need to be augmented by one billion tonnes per year to meet the
demand for sufficient nutrition of population increasing in the future.
Additionally to these purposes, grain has increasingly grown in the recent decades
as a source for bioenergy facilities to produce biofuel and biogas to more and more
partly replace mineral oil. For any of these purposes, high quality of traded pure grain
and cereal products is requested from manufacturers, wholesalers, retailers and end
consumers. Spoiled grain with fungi or pest infection stored in silos or heaps desig-
nated even for combustion or biogas production may cause risk of environmental
impact or pollution.
High quality of produce is mandatory for assuring nutrition value. It is important to
avoid development of microorganisms and fungi and contamination with toxins from
it during storage and transport which can cause health risks and may get in conflict with
national or international quality standard regulations. Overdried or overheated grain like
wheat or barley will result in loss of germination capacity or backing capability.
It is still a severe problem that inappropriate post-harvest conditions during cleaning,
sorting, drying, storage, transport, packing and marketing often cause high amount of
losses. Deficient or unavailable drying and storing facilities are a problem especially in
subtropic or tropic areas. Since the capabilities to increase the productivity of cereals
are limited, it is compulsory to reduce their post-harvest losses. Therefore, it is crucial
to know well the influences on drying and storing procedures to make the best decisions
for installing appropriate equipment and to set the correct parameters for optimal drying
and storage of cereals.
Besides post-harvest losses, incorrect operation of drying may cause waste and
ineffective consumption of energy which results directly in monetary loss.
xi
Only a profound knowledge about the physical, thermal, (bio)chemical and aeration
processes will give the designing engineer and the operator the ability to prevent pro-
blems which are associated with drying and storing of grain and to guarantee high quality
of the produce and to reduce loss.
The theory in this textbook is comprehensibly outlined beginning with principles
about drying and physical and thermal basics related to drying and ventilation, continu-
ing with principles of storage and finally with explanations about proper design of drying
and storing facilities.
In some chapters the understanding of the theory is supported by modelling examples
which are coded in the simple but comprehensive programming language BASIC. Enthu-
siasts in programming can easily find software engineering development packages and
tools for programming in BASIC. A good number of software packages are available
in the Internet for free download. Anyway, the program listings in the chapters of this
book can easily be reformatted in other more ‘modern’ programming languages like
C, Pascal, Java and Python, which are now becoming more popular among students
programming games, but are also well suited for programming numerical methods.
A number of well-known and frequently used software packages of so-called computer
algebra systems (CAS) with the advantage of built-in graphic capabilities can also be
used here excellently for modelling. A number of widespread commercial systems like
MATLAB,Maple,Mathcad,Mathematica and so on, are frequently used at universities
and in industry. But others are open-source packages like Octave, Scilab, Maxima and
more. To the student or young scientist, a good number of great possibilities to study and
experiment the theory with computer simulation methods using a CAS accessible for
free from the Internet are therefore given. With the programming examples in this book,
the reader can easily try for themselves, the many variants of different operating condi-
tions for drying and ventilation of grain and then consequently profit from an ease of
understanding of the drying processes of cereals.
It should be emphasized here that a profound knowledge of drying and storage pro-
cesses of grain is important not only for engineers, manufacturers and operators to be
able to design and operate properly drying and storage facilities but also for purchasing
agents of equipment to be able to make correct and reliable decisions when investing in
drying and storage technology.
This book of Professor B. K. Bala about cereal grain drying and storage will excellently
give the ability to students, researchers and operators to study the needed fundamental
background and detailed theory and practices about appropriate cereal grain drying and
storage. The book will also greatly encourage the student to experiment and study the
subjects with computer simulation methods.
Prof. Dr.-Ing. Klaus Gottschalk
Leibniz-Institut für Agrartechnik
Potsdam-Bornim e.V.
Foreword to the Second Editionxii
Foreword to the First Edition
One of the great miracles of nature, the carbon–oxygen
cycle, poses two great challenges
to mankind:
• How to maximize the amount of solar energy fixed in the photosynthesis phase ofthe cycle
• How to control the general well-being, the processes of ‘breakdown’, which is nature’sway of recycling the products of photosynthesis
The first challenge is being met by a large corps of husbandry specialists who have
selected the plants and proposed methods of husbandry which exploit fully the local
environmental potential towards maximizing the yield at harvest. However, even in
the most favourable environment we can only expect three harvests in a year. The other
challenge is how to reconcile the supply of a perishable commodity which is produced
only once, twice or, at most, three times a year with a demand which is relatively rising
from 1 day to the next. This is the task of post-harvest technology: how to conserve eco-
nomically, in nutritious and palatable form, the hard-won fruits from one harvest until
the next. Unfortunately, the level of avoidable post-harvest losses is still unacceptably
high. This is partly due to the fact that losses accumulate along a long chain of distribu-
tion between the producer and consumer and partly because losses due to fungal and
mite attacks in stores cause less public outrage than if a crop was left to rot in the field.
There is really only one method of conserving cereal grains between harvest and uti-
lization and that is drying, which is a complicated process. Although drying of cereals has
been practised since prehistoric times, it is still not completely understood. It involves
both energy and mass transfer in complex biological material, which can be easily
damaged. Energy is needed to change water from the liquid phase to the vapour phase.
Mass transfer is involved in the migration of water from within the grain to its surface
and in the removal of the vapour from the surface of each grain by a stream of air which
transports the moisture into the atmosphere. An effective drying operation is one which
gets the balance between the two processes of heat and mass transfer just right. In grain
drying, the needs to preserve germination, taste and baking quality and to prevent the
cracking of the kernels are all constraints on the drying process.
Professor B. K. Bala has written what deserves to be the standard textbook on drying
and storage of cereal grains. The approach is comprehensive but accessible. There are
good general descriptions of the problems to be tackled, which leads the reader towards
an understanding of the broad physical principles that are involved. These principles are
then translated into quantitative terms, on which a systematic approach to design is
xiii
formulated. There are plenty of worked examples, which help the reader to understand
how the principles are applied to a wide range of practical problems. The survey of the
background literature is masterly in that it gives recognition to the research workers, who
have contributed to the science of drying and storage, while, at the same time, blending
the various contributions into a coherent whole.
This book is a major contribution in an important but under-resourced area of
post-harvest technology. It unravels the complicated links in the long chain of events
between harvest and utilization. It focuses attention on the critical processes at every
stage and presents, in an accessible way, design procedures, based on sound scientific
principles.
It is a matter of great pride to my colleagues and me, who have worked with Professor
Bala in the Newcastle Drying Group, that we have stimulated him to writing such a
useful book.
J. R. O’Callaghan
University of Newcastle upon Tyne
Foreword to the First Editionxiv
Preface
This book has been written primarily for undergraduate and graduate students in agri-
cultural engineering and food engineering. It is the outcome of several years of teaching
and research work carried out by the author.
The book covers a very wide spectrum of drying and storage studies which is probably
not available in a single book. Chapters 1–8 deal with air and grain moisture equilibria,
psychrometry, physical and thermal properties of cereal grains, principles of airflow and
detailed analyses of grain drying, and Chapters 9–13 deal with temperature andmoisture
in grain storages, fungi and insects associated with stored grain, design of grain storages
and a comprehensive treatment of modern grain storage systems. Chapters 7 and 10 have
been primarily devoted to the application of simulation techniques using digital compu-
ters. New sections on net heat of sorption in Chapter 2, finite element modelling of single
kernel in Chapter 6, CFD modelling of fluidized bed drying in Chapter 7, exergy analysis
and neural network modelling in Chapter 8 and numerical solution of two-dimensional
temperature and moisture changes in stored grain have been included in this second
edition of the book. A good number of problems have been solved to help understand
the relevant theory. At the end of each chapter, unsolved problems have been provided
for further practice. The References and Further Reading will help the reader to find
detailed information on various topics of his interest.
I have great pleasure in acknowledging what I owe to many persons in writing this
book. I am deeply indebted to my teacher Professor J. R. O’Callaghan of the University
of Newcastle upon Tyne, United Kingdom, for writing the foreword of the first edition of
this book. Also I sincerely express my acknowledgements to Professor, Dr.-Ing. Klaus
Gottschalk, Leibniz Institute for Agricultural Engineering, Potsdam, Germany, for
writing the foreword of the second edition of this book. At the Jessore University of
Science and Technology, I acknowledge the encouragement and assistance received
from Professor M. A. Satter, Vice Chancellor, Jessore University of Science and
Technology, Jessore, Bangladesh.
B. K. Bala
xv
1
Principles of Drying
1.1 Introduction
Drying is a common activity which has its origin at the dawn of the civilization. It is
interesting to note that the knowledge of how to dry and store crops developed enough
before how to cultivate crops was discovered. But scientific studies on crop production
started before such studies on drying and storage. However, considerable research
has been done on drying but surprisingly limited research work on storage has been
carried out.
Annual loss of grain from harvesting to consumption is estimated to be 10–25%. The
magnitude of these losses varies from country to country. These losses are significantly
high in the developing countries because of favourable climates which cause deteriora-
tion of stored grains and also because of lack of knowledge and proper facilities for drying
and storage. Great efforts are beingmade to increase crop production, but until now little
or no effort is being made to improve drying and storage facilities, especially in develop-
ing countries. Most developing countries are facing acute shortage of food and they need
food, not production statistics. The post-harvest loss is proportional to production and
increases with increased production. A programme to reduce drying and storage loss
could probably result in 10–20% increase in the food available in some of the developing
countries, and the increased food supply could be used for the nourishment of hungry
people in the developing countries.
Drying and storage are a part of food production system consisting of two subsystems –
crop production and post-harvest operation. Efficiency of the system can only be
increased by a coordinated effort of a multidisciplinary team consisting of agriculturists,
agricultural engineers, economists and social scientists for increased crop production
and reduction of post-harvest losses. The reduction in post-harvest losses depends on
the proper threshing, cleaning, drying and storage of the crops. A reduction in crop loss
at one stage may have a far-reaching effect on the overall
reduction of the loss. For exam-
ple, overdrying of paddy will increase the storage life but it will also increase the breakage
percentage of the rice during milling. This suggests that a systems approach is essential
for increasing the efficiency of food production system. Food security can be increased
through increasing production and reducing post-harvest losses of the crops (Majumder
et al., 2016). This implies that considerable emphasis should be given not only on crop
production but also on drying and storage process.
1
Drying and Storage of Cereal Grains, Second Edition. B. K. Bala.
© 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd.
1.2 Losses of Crops
As mentioned earlier proper harvesting, drying and storage are essential to reduce losses
of farm crops. Loss of harvested crops may be quantitative or qualitative and may occur
separately or together. One of the basic problems in loss estimates is the definition of the
term ‘loss’. The following brief descriptions are intended to demonstrate the different
types of loss.
Weight Loss: Weight or quantity loss is the loss of weight over a period under inves-
tigation. There are two types of weight loss – apparent weight loss and real weight loss.
Apparent weight loss is the loss of weight during any post-harvest operation under study.
This loss does not consider the effect of the moisture content or the contamination with
insects, fungi and foreign materials. The real weight loss is the apparent weight loss with
the correction for any change in moisture content, plus dust, frass, insects and so on.
Nutritional Loss: Any loss in weight of the edible matter involves a loss of nutrients.
Thus, weight loss can be used to estimate nutritional loss.
Quality Loss: Damaged grains and contaminants, such as insect fragments, rodent
hairs and pesticide residues, within the grain cause the loss of quality, resulting in mon-
etary loss. Similarly, changes in the biochemical composition, such as increase in free
fatty acid content, may also rank as losses in quality.
Loss of Viability: Loss in viability of seed is one of the losses easiest to estimate and is
apparent through reduced germination, abnormal growth of rootlets and shoots and
reduced vigour of the plant.
Indirect Loss: Indirect losses involve commercial relationship which may not be quan-
tified easily. This includes goodwill loss and social loss.
The crop losses discussed in the preceding text are mainly quantitative and qualitative
losses. The major factors in quality loss appear to be from insect damage, damage by
fungi, broken grain, dust and other foreign materials.
1.3 Importance of Drying
Drying has the following important advantages:
1) Drying permits the long-time storage of grains without deterioration of quality.
2) Drying permits farmers to have better-quality product for their consumption
and sale.
3) Drying permits the continuous supply of the product throughout the year and takes
advantage of higher price after harvesting season.
4) Drying permits the maintenance of viability and enables the farmers to use and sell
better-quality seeds.
5) Drying permits early harvest which reduces field damage and shatter loss.
6) Drying permits to make better use of land and labour by proper planning.
1.4 Principles of Drying
Drying is the removal of moisture to safe moisture content and dehydration refers to the
removal of moisture until it is nearly bone dry. Generally, drying is defined as the removal
of moisture by the application of heat, and it is practised to maintain the quality of grains
Drying and Storage of Cereal Grains2
during storage to prevent the growth of bacteria and fungi and the development of
insects and mites. The safe moisture content for cereal grain is usually 12–14% moisture
on a wet basis.
Heat is normally supplied to the grains by heated air naturally or artificially, and the
vapour pressure or concentration gradient thus created causes the movement of mois-
ture from inside of the kernel to the surface. Themoisture is evaporated and carried away
by the air.
Drying capacity of the air depends on air temperature, moisture content of the grain,
the relationship between the moisture content of the grain and the relative humidity of
the drying air and grain type and maturity. The temperature of the drying air must be
kept below some recommended values depending on the intended use of the grain. Safe
maximum temperature of drying seed grains and paddy grains is 43 C, and for milling
wheat the maximum recommended temperature is 60 C. Excessive high-temperature
drying causes both physical and chemical changes and, especially in the case of rice,
increases the percentage of breakage of whole rice and reduces the quantity and quality
of rice. However, in cases of malt and tea, high-temperature drying is essential for desired
physical and chemical changes for their ultimate use as drinks.
Reference
Majumder, S., Bala, B.K., Fatimah, M.A., Hauque, M.A. and Hossain, M.A. 2016. Food
security through increasing technical efficiency and reducing post harvest losses of rice
production systems in Bangladesh. Food Security, 8(2): 361–374.
Further Reading
Adams, J.M. 1977. A review of the literature concerning losses in stored cereals and pulses
published since 1964. Tropical Science, 19(1): 1–28.
Bala, B.K. 1997. Drying and storage of cereal grains. Oxford & IBH Publishing Co, New Delhi.
Hall, C.W. 1980. Drying and storage of agricultural crops. AVI Publishing Company Inc,
Westport, CT.
Principles of Drying 3
2
Moisture Contents and Equilibrium Moisture Content Models
2.1 Introduction
Moisture contained in a grain is an indicator of its quality and a key to safe storage and
can be of two types: ‘water of composition’, called absorbed water, which is contained
within the plant cells of which the grain kernel is composed of and adsorbed water which
is present on the surface but not within the cells. The moisture content of the grains may
be determined on farms, in stores and under laboratory conditions. These necessitate
some standards for representation of moisture content and methods of its measurement.
2.2 Moisture Content Representation
Moisture content is usually expressed in per cent of moisture present in the grain, and
there are two methods for expressing these percentages: (i) wet basis and (ii) dry basis.
Moisture content of a grain on a wet basis is expressed as the ratio of the weight of
water present to the total weight of the grain. It is normally expressed in per cent. Mois-
ture content on a wet basis is used for commercial designation and also universally by
farmers, agriculturalists and merchants. This method of expression tends to give incor-
rect impression when applied to drying since both moisture content and the basis on
which it is computed change as drying proceeds. For this reason moisture content on
a dry basis is used in many engineering calculations and mainly used by researchers.
Moisture content on a wet basis is given by
Mw =
Ww
Ww +Wd
2 1
Alternatively, moisture content on a dry basis compares the weight of the moisture
present with the weight of dry matter in the grain. This can be expressed as
Md =
Ww
Wd
2 2
It may be necessary to convert moisture content from wet basis to dry basis, and vice
versa. To convert moisture content from a wet basis to a dry basis, subtract each side of
Equation 2.1 from 1.
Drying and Storage of Cereal Grains, Second Edition. B. K. Bala.
© 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd.
5
1−Mw = 1−
Ww
Ww +Wd
2 3
This equation on simplification gives
Md =
Mw
1−Mw
2 4
To convert moisture content from a dry basis to a wet basis, Equation 2.2 may be
rewritten as
1 +Md = 1 +
Ww
Wd
2 5
This equation on simplification yields
Mw =
Md
1 +Md
2 6
Example 2.1
2000 kg of freshly harvested paddy with a moisture content of 25% (d.b.) is dried to a
moisture content of 14% (d.b.). Determine the final weight of the grain after drying.
Solution
From Equation 2.5, we can write
Ww +Wd
Wd
= 1 +Md
Here W25 +Wd = 2000 kg and Md = 0.25
Hence Wd =
2000
1 + 0 25
= 1600kg
Again for 12% m.c.,
Wd +W12 = 1600 × 1+ 0 12 = 1792 kg
Hence the final weight of the dried grain is 1792 kg.
Example 2.2
8000 kg of paddy with a moisture content of 0.12 (d.b.) is required for a research project on
grain storage. It was decided that the available freshly harvested paddy with a moisture
content of 0.20 (w.b.) should be procured, and then it will be dried to a moisture content
of 12% on a dry basis. Howmany kilograms of freshly harvested paddy are to be procured?
Solution
From Equation 2.5, we can write
Ww +Wd
Wd
= 1 +Md
Here Wd +W12 = 8000 kg and Md = 0.12
Hence Wd =
8000
1 + 0 12
= 7142 86 kg
Drying and Storage of Cereal Grains6
Again from Equation 2.4,
Md =
Mw
1−Mw
For Mw = 0.20,
Md =
0 20
1−0 20
= 0 25
Again for 25% m.c. (d.b.),
Wd + W25 = Wd × 1 + M25 = 7142 86 × 1 25 = 8928 58 kg
Hence 8928.58 kg of freshly harvested paddy is to be procured.
Example 2.3
Ten tonnes of rice is dried from an initial moisture content of 22.0 to 12% (w.b.) in a
batch dryer using diesel fuel. Calculate (i) how much diesel is needed and (ii) cost of
drying per kg. Use latent heat of vaporization of moisture = 10MJ/kg, heating value of
diesel = 42.7 MJ/l and price of diesel = Tk. 55.0/l.
Solution
The initial moisture content on a dry basis is
Md =
Mw
1−Mw
=
0 22
1−0 22
= 0 2821
and the final moisture content on a dry basis is
Md =
Mw
1−Mw
=
0 12
1−0 12
= 0 1364
Moisture in the grain is
Moisture present = 10,000 × 0 22 = 2,200 kg
and the dry matter is
Wd = 10,000−2,200 = 7,800 kg
The moisture removal is given by
Moisture removal = dry matter× Mo−Mf = 7800× 0 2821−0 1364 = 1136 46 kg
Diesel needed is given by
Diesel needed =
1136 46 × 10
42 7
= 266 15l
Cost of drying per kg is given by
Cost perkg =
266 15 × 55
10,000
= 1 46Tk kg
2.3 Determination of Moisture Content
Determination of moisture content of a grain is essential to know its keeping quality. It is
also important to know the moisture content during drying and storage. Price of grains
depends on moisture content. Again, if the farmers sell overdried grains, they sell the dry
Moisture Contents and Equilibrium Moisture Content Models 7
matter of grains for the price of water. For underdried grains the farmers are offered
lower prices. Also the quality of the grains will deteriorate soon during storage. These
also emphasize further the need to determine the moisture content of cereal grains.
The methods of determining moisture content can be classified as (i) direct method
and (ii) indirect method. Direct method involves the actual removal of moisture and
its measurement. The moisture contents are expressed either on a wet basis or on a
dry basis. The following are the common methods for direct determination of moisture
content:
1) Oven method
2) Infrared lamp method
3) Brown–Duvel method
Indirect method involves the measurement of some properties related to the moisture
in grains. Themoisture content is expressed on a wet basis. This method is much quicker
but less accurate. The instrument has to be calibrated against a primary method. The
following are the commonmethods for determiningmoisture content through themeas-
urement of some parameters of moisture contained in the grains:
1) Resistance method
2) Capacitance method
3) Chemical method
4) Relative humidity method
2.3.1 Direct Methods
2.3.1.1 Oven Methods
Operating conditions and procedures are different for various materials. Air oven
method or water ovenmethodmay be used for direct determination of moisture content,
but the air oven method is commonly used for grains.
Air Oven Method, 130 ± 1 C
a) One-stage method (for grains under 13% moisture content)
a) Grind duplicate samples of 2–3 g each and weigh accurately.
b) Heat for 1 h at 130 C.
c) Remove from oven and place in a desiccator. Then reweigh. Samples should be
within 0.1% moisture content of each other.
Moisture content w b =
initial weight of sample− final weight of sample
initial weight of sample
×100
b) Two-stage method (for grains over 13% moisture content)
a) Weigh accurately a 25–30 g sample of whole grain.
b) Place in the oven for 14–16 h.
c) Remove from the oven and place in a desiccator. Then reweigh.
d) Grind a sample of the partially dried grain and proceed for the one-stage
method.
Drying and Storage of Cereal Grains8
Water Oven or Air Oven method, 100 C
a) Weigh two 25–30 g samples accurately and place them in the oven.
b) Heat for 72–96 h at 90–100 C.
c) Remove from the oven and place in the desiccator. Then reweigh. Sample should be
within 0.1% moisture content of each other.
An alternative approach is to use a vacuum oven. Grain is ground and placed in the
oven at 100 C and 25mm pressure for approximately 5 h.
The grain sample should be in the oven until weight loss stops. It is practically impos-
sible to remove all the moisture from grains without their deterioration. If the grain sam-
ples are kept too long in the oven, organic materials of the samples will be lost, and these
will appear as moisture loss and give inaccurate value. So moisture content should be
determined according to the standards set by the professional organization and/or gov-
ernment regulations.
2.3.1.2 Infrared Lamp Method
Moisture meter employing infrared lamp is available commercially. Moisture content is
measured directly by the evaporation of water from the grain sample by heating with an
infrared lamp. Milling of grain sample is not essential, but this will reduce the time
required for the evaporation of water from the grain sample.
This meter consists of a balance and an infrared lamp. The pan of the balance is coun-
terbalanced by a fixed and an adjustable weight along a lever. There is also a scale cali-
brated in moisture content. The infrared lamp is mounted on a swivelling arm above
the pan.
The procedures for measuring moisture content are as follows:
1) Set the balance at a zero position by placing the moisture content indicator at zero
position.
2) Weigh a fixed amount of the sample accurately.
3) Place the sample on the pan of the balance at zero position and place equal weight on
the counterbalance such that the balance indicates zero position and the moisture
content indicator is at 0% moisture level.
4) Heat the sample by infrared lamp until weight loss stops.
5) Set the balance again at the zero position by shifting the moisture content indicator.
6) Read the moisture content in per cent from the calibrated scale indicated by the
pointer.
The range of moisture content that can be read is from 0 to 100%. If the sample to be
tested is so wet that it cannot bemilled, it should be pre-dried to reduce the time required
for moisture content determination. It is interesting to note that this meter does enable
the determination of high moisture contents.
2.3.1.3 Brown–Duvel Method
In this method a sample of grains is placed in an oil bath and heated to a temperature
above that of boiling water but below the distillation temperature of the oil. The weight of
water vapour driven off is determined by collecting the condensed vapour in a measuring
cylinder or by measuring the weight of the sample. A grain sample of 100 g is heated in a
flask containing 150 ml of oil for about 1 h.
Moisture Contents and Equilibrium Moisture Content Models 9
In modified Brown–Duvel method the water driven off from the sample being tested
by heating in vegetable oil is measured, and the moisture content is determined.
The apparatus required are:
1) A balance capable of weighing 1.5 kg accurately to at least 1 g
2) One 2 l saucepan
3) One thermometer graduated at 0–220 C in 1 C intervals
4) One stirring rod
5) Supply of vegetable oil such as domestic cooking oil
6) Source of heat such as electric hot plate or camping stove
The test procedures are as follows:
1) Place the saucepan containing the stirring rod on the balance and add oil
until it is
approximately half full. Additional quantity of oil should be added until the balance
is ‘tared’.
2) Add exactly 100 g of the grain sample in the oil, when the total weight of the saucepan,
oil and sample is 900–1500 g.
3) Heat the saucepan and stir the sample regularly until the temperature reaches 190 C.
This should take 10–15 min.
4) Reweigh the saucepan.
With 100 g sample the moisture content is equal to the loss in weight. When the sam-
ple is not exactly 100 g, the moisture content should be calculated. The principal advan-
tage of this method is that the moisture contents beyond the range of indirect electric
moisture meters can be measured.
2.3.2 Indirect Methods
2.3.2.1 Resistance Methods
The electrical resistance or conductivity of a material varies with its moisture content.
The electrical resistance-type moisture meters measure the electrical resistance of grain
as the criterion of grain moisture content, and these meters are calibrated against a
standard method. For wheat it has been found that the logarithm of electrical resistance
is linearly related to moisture content over the range 11–16%. The electrical resistance of
grain varies with temperature, and the reading of the meter should be corrected for tem-
perature when the temperature of the operating condition differs from the calibrated
conditions. Most of the models of this type of meters incorporate a compression cell
which may be an integral part of the meter or remote from it. The electrical resistance
of grains decreases when pressure is increased. The cell contains two electrodes between
which the electrical resistance of the sample is measured. Such cells incorporate a device
which ensures that the sample is consistently compressed to a predetermined extent.
Above 17% ofmoisture content, the relationship between themoisture content and the
logarithm of electrical resistance is parabolic. Most meters do not give reading below 7%
because there is little change in electrical resistance with moisture content.
The accuracy of the resistance-type meters is dependent on the uniform distribution of
the moisture throughout the grain. Recently dried grains tend to give low readings if the
surface of the grain is disproportionately dry. Conversely, freshly wet grains may give
high readings. Some models of this type of meter can be used with either milled or
Drying and Storage of Cereal Grains10
unmilled grain, the former being more recommended for greater accuracy. This type of
meter is most accurate at the moisture content which is required for prolonged storage
of grains in bulk.
2.3.2.2 Capacitance Methods
The dielectric properties of products depend on the moisture content. Hence, the capac-
itance of an electrical condenser varies with the moisture content, with the product
placed between its plates. Wet materials have a high dielectric constant, whereas dry
materials have a low dielectric constant. Water has a dielectric constant of 80 at
20 C, and most grains have a value less than 5. The measurement of capacitance is
an indirect measurement of the moisture content.
This type ofmeters often incorporates a chamber wherein thematerials to be tested are
placed. Two sides of the chamber form the plates of a condenser between which a high-
frequency current is passed to measure the capacitance of the sample. Capacitance
meters are generally capable of determining a wider range of moisture content than a
resistance type, that is, for wheat 8–40%. These meters are also less susceptible than
resistance meters to errors arising from uneven moisture distribution within the sample
but tend to be more difficult to keep in adjustment.
2.3.2.3 Chemical Methods
Water from a grain sample can be removed by adding a chemical which decomposes
and combines with water. When calcium carbide is mixed with the grain sample,
the moisture in the grain reacts with the chemical, resulting in the production of
acetylene gas.
CaC2 + 2H2O Ca OH 2 +C2H2
The volume of gas produced is proportional to the moisture in the grain. Moisture
meters are available which operate on this principle.
Approximately 30 g of the grain sample is mixed in a sealed vessel with an excess quan-
tity of calcium carbide. After 15–20 min the calcium carbide reacts with the moisture
present to liberate acetylene. The pressure generated by the gas enables a direct reading
of moisture content on the pressure gauge mounted on the base of the meter.
2.3.2.4 Relative Humidity Methods
The air in the inter-granular space in a grain mass reaches a state of equilibrium with the
moisture content of the grain. Hence, when stable conditions are established, it is feasible
to measure the relative humidity and express the result as a moisture content.
A simple device of this type consists of a probe which encapsulates sensitive hair ele-
ments. Ventilation is provided through the holes of the probe, which aids the circulation
of air around the sensing elements. The sensing elements expand and contract in
response to the changes in the relative humidity in the air. The elements are connected
to a needle on a scale and calibrated in both relative humidity and moisture content.
This type of meter takes up to 30min to respond to the relative humidity of the air in a
grain mass, and the readings should not be taken until the pointer is stable. Exact accu-
racy cannot be anticipated in this type of meter. When moisture content exceeds 22%,
inaccuracy can be anticipated, because the relative humidity becomes almost 100%
Moisture Contents and Equilibrium Moisture Content Models 11
regardless of grain moisture content. If an inexpensive direct reading instrument is
required, this type should be considered.
2.4 Grain Sampling
The accuracy of determination of grain moisture content depends not only on the
accuracy of the moisture meters but also on the method of sampling, size and number
of samples. It is important that the sample be a representative of the product.
When the moisture content of a batch of grains is assessed, representative samples of
that batch should be ensured. The extent to which the samples are representative of the
whole batch depends on the variability of that batch and the thoroughness of sampling.
Various organizations recommend precise methods of sampling, such as British Stan-
dards Institute.
A primary sample is defined as a small quantity of grain taken from a single position in
the lot, and all primary samples should be of similar size and weight. Sampling spears are
available and standard samplers will normally penetrate a depth of approximately 3 m.
The selection of correct locations is also of significance. The sampling position should be
selected impartially so that the top, bottom and sides of the grain mass are equally repre-
sented. Special care should be taken to ensure that a disproportionate quantity of grain
from its exposed surfaces is not included in the samples. It is also important that mois-
ture content of the grains be maintained from the time of sampling until its determina-
tion. Standard metal containers and film bags prescribed by the government or
professional organizations should be used for holding the samples.
2.5 Equilibrium Moisture Content
The equilibriummoisture content (EMC) of a cereal grain is defined as themoisture con-
tent of the material after it has been exposed to a particular environment for an infinitely
long period of time. EMC is dependent upon the relative humidity and temperature con-
ditions of the environment and upon the species, variety and maturity of the grain. EMC
may be classified as (i) static EMC and (ii) dynamic EMC. The concept of dynamic EMC
was introduced by McEwen et al. (1954). The dynamic EMC is obtained best by fitting
the thin layer drying equation to experimental data, whereas the static EMC is obtained
after a prolonged exposure of the product to a constant atmosphere. McEwen, Sim-
monds and Ward further suggested that dynamic and static EMCs should be used for
drying
and storage design, respectively.
The relationship between the moisture content of any material and its equilibrium rel-
ative humidity at a constant temperature can be represented by a curve (Figure 2.1) called
an isotherm. At normal atmospheric pressure, the values of moisture content which are
reached are mainly dependent on relative humidity and to a much lesser extent on tem-
perature. Often it is seen in practice that temperature is the more important factor. This
is because of the relative humidity of the moist air which is temperature dependent.
However, there is generally a reduction in moisture content for a fixed relative humidity
as the temperature is increased. Figure 2.2 shows EMC changes with relative humidity of
Drying and Storage of Cereal Grains12
hybrid rice at 30, 40 and 50 C and shows that EMC decreases with the increase in tem-
perature. EMC of major cereal grains is shown in Table 2.1.
A sorption isotherm is a plot of EMC versus relative humidity at a given temperature
for a material which has been subjected to a wetting environment. A desorption isotherm
is a similar plot for a material which has been subjected to a drying environment. The
difference between the desorption and adsorption isotherms at a given temperature
for a material is termed as the hysteresis effect (Figure 2.1), and this suggests that the
EMC of a given material is not only a function of its immediate environment but also
affected by the previous moisture condition. Several theories and hypotheses have been
advanced to explain the hysteresis and subsequent disappearance of hysteresis loop ini-
tially exhibited in successive adsorption and desorption cycles. Chung and Pfost (1967c)
explained these phenomena of hysteresis effect and disappearance of hysteresis loop by
the concepts of molecular shrinkage in adsorption caused by activation and crack
30
25
20
15
10
M
oi
st
ur
e 
co
nt
en
t, 
%
5
0
0 20 40 60 80 100
Relative humidity, %
Adsorption
Desorption
Figure 2.1 Sorption isotherm, desorption
isotherm and hysteresis of moisture sorption
of wheat at 35 C.
Desorption
0
5
10
15
20
25
30
30 40 50 60 70 80 90 100
M
oi
st
ur
e 
co
nt
en
t, 
%
 
0
5
10
15
20
25
30
M
oi
st
ur
e 
co
nt
en
t, 
%
 
Relative humidity, % 
30 40 50 60 70 80 90 100
Relative humidity, % 
Adsorption
30°C
40°C
50°C
30°C
40°C
50°C
Figure 2.2 Adsorption and desorption isotherms of hybrid rice at 30 C, 40 C and 50 C.
Moisture Contents and Equilibrium Moisture Content Models 13
Ta
b
le
2.
1
G
ra
in
eq
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lib
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m
m
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)
Ta
b
le
2.
1
(c
on
tin
ue
d
)
M
at
er
ia
l
Te
m
p
er
at
ur
e,
C
Re
la
ti
ve
hu
m
id
it
y,
%
10
20
30
40
50
60
70
80
90
10
0
W
he
at
So
ft
-r
ed
w
in
te
r
−
7
11
.3
12
.8
14
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15
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17
.0
0
11
.0
12
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13
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14
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10
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21
9.
7
11
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12
.4
14
.0
25
4.
3
7.
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8.
8
9.
7
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11
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13
.6
15
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25
.6
H
ar
d-
re
d
w
in
te
r
25
4.
4
7.
2
8.
5
9.
7
10
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12
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19
.7
25
.0
H
ar
d-
re
d
sp
ri
ng
25
4.
4
7.
2
8.
5
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8
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13
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.7
25
.0
W
hi
te
25
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7.
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8.
6
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.0
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.3
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ur
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25
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7.
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8.
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8
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7
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So
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ce
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al
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(1
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7)
.
formation caused by wetting during the successive cycles of the sorption process, respec-
tively. Figure 2.3 shows desorption and adsorption isotherms of hybrid rough rice at
30 C, and desorption isotherm is at higher moisture content than adsorption isotherm.
2.6 Determination of Static Equilibrium Moisture Content
There are two general methods for determining the static EMC: (i) static method and
(ii) dynamic method. In the static method the grain is allowed to come to equilibrium
in still and moist air. Saturated salt or acid solution is normally used to maintain the rel-
ative humidity in this method. The dynamic method is quicker but complicated in design
and instrumentation. In dynamic method the air is mechanically moved around the sam-
ple in a closed chamber containing a dew point apparatus or an either salt or acid solu-
tion. It is also important to measure moisture content as accurately as determining and
maintaining relative humidity in the closed chamber. The dew point method is exten-
sively used in the United Kingdom, whereas the chemical method of known relative
humidity maintained by a saturated salt or acid solution is extensively used in the United
States (static method).
The dynamic method requires a couple of days or less, but the static method may take
several weeks. At high relative humidities mold may grow in the grains and give appar-
ently a highmoisture content. Themoisture content obtained above 80%
relative humid-
ity is not a true moisture content. The dynamic method is faster and is extensively used.
A saturated salt or acid solution may be used to maintain the relative humidity in the
closed chamber. A salt solution is more stable, less corrosive and often less expensive.
A saturated salt solution can be maintained easily and remains saturated even after
the evaporation of water but precipitates some of the salt. To maintain relative humidity
between 0 and 100%, a large number of salts are required, but a saturated salt solution
can easily be prepared by dissolving all the salts at a temperature above the desired tem-
perature. Table 2.2 lists the relative humidity values at different temperatures for a large
number of salts (Hall, 1980).
Adsorption
Desorption
0
5
10
15
20
25
30
30 40 50 60 70 80 90 100
M
oi
st
ur
e 
co
nt
en
t, 
%
 
Relative humidity, % 
Figure 2.3 Desorption and adsorption isotherms of rough rice at 30 C.
Moisture Contents and Equilibrium Moisture Content Models 17
Table 2.2 Relative humidity of saturated salt solutions at different temperatures.
Salt
Temperature
( C)
RH
%
BaCl2 • 2H2O 29.4 88.0
(Barium chloride)
(Washburn 1927)
CaCl2
(Calcium chloride) −6.7 44.0
(Thompson & Shedd 1954) 0 41.0
10 40.0
21 35.0
CaCl2 • 6H2O 5 39.8
(Calcium chloride) 20 32.3
(Washburn 1927) 24.4 31.0
CaSO4• 5H2O 20 98.0
(Calcium sulphate)
(Washburn 1927)
Ca(NO3)3 −6.7 64.0
(Calcium nitrate) 0 64.0
(Thompson & Shedd 1954) 10 59.0
21 55.0
Ca(NO3)2 • 4H2O 20 53.6
(Calcium nitrate) 25 50.4
(Wink & Sears 1950) 30 46.6
35 42.0
37.8 38.9
KBr 20 84.0
(Potassium bromide) 100 69.2
(Washburn 1927)
KC2 H3O2 20 23.2
(Potassium acetate) 25 22.7
(Wink & Sears 1950) 30 22.0
37.8 20.4
K2SO4 0 99.1
(Potassium sulphate) 10 97.9
(Wexler & Hasegawa 1954) 20 97.2
30 96.6
40 96.2
50 95.8
Salt
Temperature
( C)
RH
%
KNO2
(Potassium nitrite) 20 49.0
(Wink & Sears 1950) 25 48.2
30 47.2
37.8 45.9
KNO3 0 97.6
(Potassium nitrate) 10 95.5
(Wexler & Hasegawa 20 93.2
1954) 30 90.7
40 87.9
50 85.0
K2CO3 20 43.9
(Potassium carbonate) 25 43.8
(Wink & Sears 1950) 30 43.6
37.8 43.4
K2CrO4 20 86.6
(Potassium chromate) 25 86.5
(Wink & Sears 1950) 30 86.3
37.8 85.6
K2CO3 • 2H2O 18.9 44.0
(Potassium carbonate) 24.4 43.0
(Washburn 1927)
KCNS 20 47.6
(Potassium thiocyanate) 25 45.7
(Wink & Sears 1950) 30 43.8
37.8 41.1
LiCl • H2O 0 14.7
(Lithium chloride) 20 12.4
(Wexler & Hasegawa 1954) 30 11.8
40 11.6
50 11.4
LiCl 20 11.2
(Lithium chloride) 25 11.2
(Wink & Sears 1950) 30 11.2
37.8 11.2
Drying and Storage of Cereal Grains18
Acid solution of various concentrations can be used to obtain desired relative humidity
in a closed chamber. Sulfuric acid is usually used, but will corrode metal easily, and there
is a danger in handling it. Table 2.3 gives the relative humidity above some acid solutions
at various strengths (Hall, 1980).
Salt
Temperature
( C)
RH
%
MgCl2 22.8 32.9
(Magnesium chloride) 30 32.4
(Wink 1946) 37.8 31.9
MgCl2 • 6H2O 0 35.0
(Magnesium chloride) 20 33.6
(Wexler & Hasegawa 30 32.8
1954) 40 32.1
50 31.4
Mg(NO3)2 22.8 53.5
(Magnesium nitrate) 30 51.4
(Wink 1946) 37.8 49.0
Mg(NO3)2 • 6H2O 0 60.9
(Magnesium nitrate) 20 54.9
(Wexler & Hasegawa 30 52.0
1954) 40 49.2
50 46.3
Mg(NO3)2 22.8 53.5
(Magnesium nitrate) 30 51.4
(Wink 1946) 37.8 49.0
Mg(NO3)2 • 6H20 0 60.9
(Magnesium nitrate) 20 54.9
(Wexler & Hasegawa 1954) 30 52.0
40 49.2
50 46.3
NaC2H3O2 22.8 74.8
(Sodium acetate) 30 71.4
(Wink 1946) 37.8 67.7
(continued overleaf )
Salt
Temperature
( C)
RH
%
NaC2H3O2.3H2O 20 76.0
(Sodium acetate) 25 73.7
(We 1950) 30 71.3
37.8 67.6
NaCr2O2 • 2H2O 0 60.6
(Sodium dicromate) 20 55.2
(Wexler & Hasegawa 1954) 30 52.5
40 49.8
50 46.3
Na2Cr2O7 22.7 54.1
(Sodium dichromate) 30 52.0
(Wink 1946) 37.8 50.0
NaBr 20 59.2
(Sodium bromide) 25 57.8
(Wink & Sears 1950) 30 56.3
37.8 53.7
NH4H2PO4 20 93.2
(Ammonium
monophosphate)
25 92.6
30 92.0
37.8 91.1
(NH4)2 SO4 0 83.7
(Ammonium sulphate) 20 80.6
(Wexler & Hasegawa 1954; 30 80.0
Wink & Sears 1950) 40 79.6
50 79.1
NH4C1 −6.7 82.0
(Ammonium chloride) 0 83.0
(Thompson & Shead 1954) 10 81.0
21.1 75.0
Source: Bala (1997).
Table 2.2 (continued)
Moisture Contents and Equilibrium Moisture Content Models 19
2.7 Static Equilibrium Moisture Content Models
The relationship between the EMC and the relative humidity is usually represented by a
sigmoid-shape curve which is not easy to draw or manipulate. Several theoretical, semi-
theoretical and empirical models have been proposed to describe the isotherm curves.
Some important isotherm models are discussed in the succeeding text:
Brunauer et al. (1938) derived the Brunauer–Emmett–Teller (BET) equation using
kinetic approach, which is a multilayer homogeneous equation. This equation is popular
due to its thermodynamic base and assumes homogeneous sorption, whereas the water
sorption in food materials is heterogeneous.
PV
V PVS−PV
=
1
Vmc
+
c−1 PV
Vmc PVS
2 7
where c is the product constant related to heat of adsorption of the water vapour and PVS
is saturated vapour pressure.
This equation gives best agreement in the range of 10–50% relative humidity (Coulson
et al., 1971; Iglesias and Chirife, 1978; Labuza, 1968; Okos et al., 1992).
Table 2.3 Relative humidity of different concentrations of aqueous acid solution at various
temperatures, %.
Acid Temperature, C
Acid by weight, %
10 20 30 40 50 60 70 80
H2SO4
(sulfuric)
−17.8 87.3 55.7 15.0 3.14
10 87.4 56.6 15.8 3.88
20 87.7 56.7 16.3 4.76
30 87.5 56.6 17.0 5.75
40 87.6 57.5 17.8 6.88
44 88.8 58.2 18.8 8.20
HNO3 (nitric) −17.8 89.2 78.4 65.3 45.7
10 86.7 77.0 63.0 45.6
20 86.6 75.2 61.5
30 86.6 74.9 61.3
40 85.9 74.1 60.5
44 86.5 74.6
60 86.9 75.6
HCl
(hydrochloric)
−17.7 83.5 56.0 27.4 8.9
10 83.5
20 83.2
30 84.24
Source: Bala (1997).
Drying and Storage of Cereal Grains20
Smith (1947) had shown that the final portion of the water sorption isotherm of bio-
logical materials such as starch and cellulose is described by the following equation:
Msw =Mb−b ln 1−rh 2 8
It has been shown that this equation fails to take into account the progressive
enlargement of the effective sorbing surface of the gel which accompanies swelling when
moisture content is expressed on a dry basis. It was also demonstrated that the plot
of moisture content (w.b.) versus –ln(1 − rh) is linear between relative humidities of
0.5–0.95 for cellulose. Becker and Sallans (1956) have shown that the Smith equation
is applicable for desorption isotherms of wheat for the relative humidity range of
0.5–0.95 when moisture content is expressed on a weight basis.
Henderson (1952) proposed the following semi-empirical model for EMC of cereal
grains:
1−rh= exp −cTabMsd
n 2 9
Henderson’s equation in the form described in the preceding text has been found
inadequate for cereal grains (Brooker et al., 1974; Pichler, 1957). Day and Nelson
(1965) modified Henderson’s equation to describe wheat:
1−rh= exp −aMsd
b 2 10
Where a and b are functions of temperature. Zuritz and Singh (1985) recommended
the following equation for calculating EMC values of rough rice:
ln erh = −c0T
c1
ab exp c2T
c3
abMe 2 11
where
c0 = 3 88368E9, c1 = −3 52486
c2 = −1 1205E−2, c3 = 1 30047
Strohman and Yoerger (1967) proposed the following equation to represent the equi-
librium relative humidity of corn at various moisture contents:
rh = exp exp bMsd lnPvs + cexp dMsd 2 12
This equation is valid over the whole range of moisture content, relative humidity and
temperature.
Chung and Pfost (1967a, b) developed the following equation based on potential theory
and a simplified equation of state:
ln rh = exp
−A
R0Tab
exp −BMsd 2 13
This equation assumes that the free energy function or useful work decreases exponen-
tially the increasing thickness of adsorbed layer and the adsorbed is directly related to
moisture content. Chung–Pfost equation fits grain EMC data
over the 20–90% relative
humidity range. Gustafson and Hall (1974) had shown that the constants A and B are
Moisture Contents and Equilibrium Moisture Content Models 21
temperature dependent for shelled corn. Chung and Pfost (1967a, b) equation was also
modified in the form of
aw = exp
−b0
T + b1
exp −b2Me 2 14
Chung and Pfost (1967a, b) equation is recommended for cereal grains (Pixton and
Howe, 1983).
Although many studies have been reported to suggest numerous isotherm models
for food materials (Kaymak-Ertekin and Gedik, 2004; Lahsasni et al., 2004; Lomauro
et al., 1985; Mir and Nath, 1995; Sun and Woods, 1994; Van den Berg, 1984), the
Guggenheim–Anderson–de Boer (GAB) model has been proposed by food engineers
as the universal model to fit the sorption data for all foods and is the most widely used
and versatile model. This equation has the form of
Me =
b0b1b2aw
1−b2aw 1−b2aw + b1b2aw
2 15
Actually the GAB equation is a modification of the BET equation with one additional
energy constant by which the GAB equation gains its greater versatility. Lomauro et al.
(1985) reported that moisture sorption of foods can be described by more than one
sorption model, and the GAB gives the best fit for more than 50% of the fruits, meats
and vegetables analyzed (Lomauro et al., 1985). Reddy and Chakraverty (2004) reported
that the GAB model fitted the best to the sorption isotherms of rough rice, parboiled
rough rice, brown rice and rice bran. This equation is recommended for calculating
EMC values of food materials.
2.8 Net Isosteric Heat of Sorption
The net isosteric heat of sorption is also an important information for drying. It can be
used to determine the energy requirements and provide information on the state of water
within the dried product. The moisture content level of a product at which the net isos-
teric heat of sorption reaches the value of latent heat of sorption is often considered as
the indication of the amount of bound water existing in the product (Wang and
Brennan, 1991).
The net isosteric heat of sorption phenomenon can be explained by the Clausius–
Clapeyron equation (Hossain et al., 2001; Mohamed et al., 2005; Phomkong et al., 2006):
∂ln RH
∂Tab
=
Qst
R0 T2ab
2 16
Integrating Equation 2.16 and assuming that the isosteric heat of sorption (Qst) is inde-
pendent of temperature gives Equation 2.17:
ln RH = −
Qst
R0
1
Tab
+K 2 17
where K is a constant. The value of Qst is calculated from the slope of Equation 2.17.
Drying and Storage of Cereal Grains22
The net isosteric heats of adsorption and desorption of hybrid rice kernels (i) rough,
(ii) brown and (iii) milled rice for different moisture contents are presented in Figure 2.4.
The net isosteric heat of hybrid rice seed decreases with increase in moisture content.
The heat of sorption is higher at lower moisture contents than at higher moisture con-
tents. The net isosteric heats of desorption are higher than those of adsorption within the
moisture content range of 20–12% (d.b.) for all types of kernels. This indicates
the requirement of higher energy in the desorption process. The difference between
the heats of adsorption and desorption converges as moisture content increases. These
changes are probably due to changes in molecular structures during sorption which
affect the degree of activation of sorption sites. Hysteresis effect on sorption isotherms
also might have influenced these differences. Rough rice had the highest net isosteric
heat during sorption followed by brown rice and milled rice. The comparisons of net
isosteric heats of sorption are shown in Figure 2.5. The variation in net isosteric heat
of sorption might he attributed to the physicochemical properties of the different culti-
vars which are responsible for variations in isotherms.
Several researchers reported the isosteric heat of sorption as an empirical function of
moisture content (Hossain et al., 2001; Öztekin and Soysal, 2000; Wang and Brennan,
1991). The net isosteric heat of sorption of hybrid rice kernels as a function of EMC
in the following forms has been reported:
Qst = a∗exp Me 2 18
and
Qst = −aM
3
e + bM
2
e −cMe + d 2 19
whereQst is the net isosteric heat of sorption, kJ/mol;Me is the EMC, % (d.b.); and a, b, c,
and d are the equation parameters.
The intercept K of Equation 2.17 can be expressed as a function of EMC in the form
suggested by Sutherland et al. (1971) as follows:
K = mMne 2 20
where Me is the EMC, % (d.b.) and m and n are the two parameters.
0
2
4
6
8
10
10 12 14 16 18 20 22 24
Moisture content, % (d.b.)
H
ea
t o
f s
or
pt
io
n,
 k
J/
m
ol
r-des
r-ads
b-des
b-ads
m-des
m-ads
Figure 2.4 Comparison of adsorption and desorption net isosteric heat values for hybrid rice kernels.
m – milled rice, b – brown rice, r – rough rice, ads – adsorption, des – desorption.
Moisture Contents and Equilibrium Moisture Content Models 23
Example 2.4
A model for EMC in the form (1 − rh) = exp (−c Tab Me
n) is to be used. It is known that
one type of grain has an EMC of 30% (d.b.) at 90% rh and 20% (d.b.) at 70% rh. If the
temperature is assumed constant (26.66 C), estimate the values of c and n for the model
in the preceding text.
Solution
Taking the logarithm of both sides of Equation 2.9,
ln 1−rh = −cTabM
n
e
For Me = 0 30, rh = 0 9, Tab = 273 + 26 66
ln 0 1 = −c× 299 6 × 30 n 2 21
For Me = 0 20, rh = 0 7, Tab = 273 + 26 66
ln 0 3 = −c× 299 6 × 20 n 2 22
Dividing Equation 2.22 by Equation 2.21
ln 0 3
ln 0 1
=
20
30
n
0
2
4
6
8
10
12
14
8 10 12 14 16 18 20 22 24 26
Moisture content, % (d.b.)
N
et
 Q
st
, k
J/
m
ol
Rough rice (des), this study
Rough rice (ads), this study
Rice total (des), Oztekin and Soysal (2000)
Rice total (ads), Oztekin and Soysal (2000)
Rough rice (des), Hunter (1987)
Rough rice (ads), Hunter (1987)
Shelled corn (des), Oztekin and Soysal (2000)
Shelled corn (ads), Oztekin and Soysal (2000)
Malt, Bala (1997)
Wheat (des), Oztekin and Soysal (2000)
Wheat (ads), Oztekin and Soysal (2000)
Figure 2.5 Comparison of net isosteric heat of sorption of hybrid rice with those of other varieties and
crops. des – desorption, ads – adsorption.
Drying and Storage of Cereal Grains24
n=
ln 0 5228
ln 2 3
= 1 5995
For n= 1 5995, we have from Equation 2.21,
c= −
ln 0 1
299 66 × 30 n
= 3 3337 × 10−5
Example 2.5
Paddy grain is stored in bulk in a bin with the top side open. The temperature and relative
humidity of outside environment are 25 C and 70%, respectively. What should be the
moisture content of the grain so that there would be no mass transfer from the top
surface of the grain in the bin? Use Zuritz–Singh equation.
Solution
Here, given Tab = 273 + 25 = 298 K, rh = 0.70
Rearranging and taking the logarithm of both sides of Zuritz–Singh equation, we have
Me =
− ln erh c0 T
c1
ab
c2 T
c3
ab
We have
− ln erh = − ln 0 7 = 0 3566
c0T
c1
ab = 3 8836 × 10
9 298 −3 52486 = 7 3785
c2T
c3
ab = −1 1205 × 10
−2 298 1 30047 = −18 4950
Hence
Me =
ln 0 3566 7 3785
−18 4950
= 0 16 d b
Example 2.6
If the Chung–Pfost EMC model is used instead of Henderson’s model in Example 2.4,
determine the parameters of the Chung–Pfost equation, R0 = 8.315 kJ/kg mol K.
Solution
ln erh =
−A
R0Tab
exp −BMe
For the same temperature and for two sets of erh and Me, we have
ln erh1 =
−A
R0Tab
exp −BMe1 2 23
ln erh2 =
−A
R0Tab
exp −BMe2 2 24
Moisture Contents and Equilibrium Moisture Content Models 25
Dividing Equation 2.23 by Equation 2.24
ln erh1
ln erh2
=
exp −BMe1
exp −BMe2
Or exp −B Me1−Me2 =A
ln erh1
ln erh2
Substituting erh1 = 0 90, Me1 = 30 and erh2 = 0 70, Me2 = 20
exp −B 30−20 =
ln 0 90
ln 0 30
exp −10B = 0 0875
Hence B= ln 0 0875 10 = 0 2436
From Equation 2.23 we have
A=
ln erh1 ×R0Tab
exp −BMe
=
ln 0 90 × 8 315 × 299 66
exp −0 2436 × 30
= 391,735 56
The required parameters are A = 391735.56 and B = 0.2436.
Example 2.7
Two tonnes of rice at a moisture content of 20.0% (d.b.) is dried in a batch dryer using
diesel fuel, and the drying air
temperature and relative humidity are 38 C and 70%,
respectively. Calculate (i) how much diesel is needed and (ii) cost of drying per kg.
Use latent heat of vaporization of moisture = 10MJ/kg, heating value of diesel = 42.7
MJ/l and price of diesel = Tk. 55.0/l.
Solution
Initial moisture content on a wet basis is
Me w b =
0 20
1 + 0 20
= 0 1667
and hence the moisture in 2 tonnes of rice is
Moistureinrice = 2000 × 0 1667 = 333 4 kg
Dry matter = 2000−333 4 = 1666 6 kg
Here Tab = 38 + 273 = 311 K and rh = 0.70
Rearranging and taking the logarithm of both sides of Zuritz–Singh equation, we have
Me =
ln − ln erh c0 T
c1
ab
c2 T
c3
ab
Drying and Storage of Cereal Grains26
We have
− ln erh = − ln 0 7 = 0 3566
c0T
c1
ab = 3 8836 × 10
9 311 −3 52486 = 6 3475
c2T
c3
ab = −1 1205 × 10
9−2 311 1 30047 = 19 5511
Me =
ln 0 3566 6 3475
−19 5511
= 0 1473
The moisture to be removed is
Moisture removed = dry matter × M0−Me
= 1666 6 × 0 20−0 1473
= 87 83 kg
Diesel needed is given by
Diesel needed =
87 83 × 10
42 7
= 20 57l
Cost of drying per kg is given by
Cost perkg =
20 57 × 55
2000
= 0 56Tk kg
Key to Symbols
a, b, c, d, n constants
b0, b1, b2, b3 constants
c0, c1, c2, c3 constants
A, B, K constants
Aw water activity, decimal
erh equilibrium relative humidity, decimal
Mb bound moisture content (w.b.), ratio or %
Md moisture content (d.b.), ratio
Me equilibrium moisture content, ratio or %
Msd static equilibrium moisture content (d.b.), ratio or %
Msw static equilibrium moisture content (w.b.), ratio or %
Mw moisture content (w.b.), ratio
Pv vapour pressure, Pa
Pvs saturated vapour pressure, Pa
Qst isoteric heat of sorption, kJ/mol
rh relative humidity, decimal
R0 universal gas constant, Nm/kgmolK
RH relative humidity, %
Ta temperature, C
Tabl temperature, K
V volume of moisture, m3
Moisture Contents and Equilibrium Moisture Content Models 27
Vm volume of moisture in mololayer, m
3
Wd weight of bone dry material, kg
Ww weight of moisture, kg
Exercises
2.1 Parboiled paddy at a moisture content of 45% (w.b.) is dried on a paved yard using
solar energy to a moisture content of 12% (w.b.). Determine the amount of mois-
ture to be removed and the amount of energy used to evaporate this amount of
moisture from grain.
2.2 100 kg of wheat at a moisture content of 14% (d.b.) was mixed with 2000 kg of
wheat at a moisture content of 14% (w.b.). Then it was again dried to a moisture
content of 12% (d.b.). What will be the final weight of the grain?
2.3 8000 kg of paddy was stored in a godown at 12% moisture content on a wet basis.
About 0.2% of the dry matter was lost during storage, and the moisture content of
the top one-third of the grain rises to 14% (w.b.) during storage. Determine the
weight of the grain left.
2.4 Determine the equilibrium moisture content at RH = 65% and Ta = 35 C using
Henderson’s equation (c = 5.59 × 10−7 and n = 3.03).
2.5 Assume that Chung–Pfost equation fits the following data: (i) RH = 20%, Me =
6.5%, Ta = 25 C and (ii) RH = 70%, Me = 12%, Tu = 25 C. Determine the values
of parameters A and B.
References
Bala, B.K. 1997. Drying and storage of cereal grains. Oxford & IBH Publishing Co, New Delhi.
Becker, H.A. and Sallans, H.R. 1956. A study of a desorption isotherms of wheat at 25 C and
50 C. Cereal Chemistry, 33(2): 79–90.
Brooker, D.B., Bakker-Arkema, F.W. and Hall, C.W. 1974. Drying cereal grains. AVI
Publishing Company, Inc, Westport, CT.
Brunauer, S., Emmett, P.H. and Teller, E. 1938. Adsorption in multimolecular layers. Journal
of American Chemical Society, 60: 309–319.
Chung, D.S. and Pfost, H.B. 1967a. Adsorption and desorption of water by cereal
grains and their products. Part I: Heat and free energy changes of adsorption and
desorption. Transactions of the American Society of Agricultural Engineers, 10(4):
549–551 and 555.
Chung, D.S. and Pfost, H.B. 1967b. Adsorption and desorption of water vapour by cereal
grains and their products. Part II: Development of general isotherm equation. Transactions
of the American Society of Agricultural Engineers, 10(4): 552–555.
Drying and Storage of Cereal Grains28
Chung, D.S. and Pfost, H.B. 1967c. Adsorption and desorption of water vapour by cereal
grains and their products. Part III: A hypothesis for explaining the hysteresis effect.
Transactions of the American Society of Agricultural Engineers, 10(4): 556–557.
Coulson, J.M., Richardson, J.F. and Peacock, D.G. 1971. Chemical Engineering, Vol. 3.
Pergamon Press, Oxford/New York.
Day, D.L. and Nelson, G.L. 1965. Desorption isotherms of wheat. Transactions of the
American Society of Agricultural Engineers, 8(2): 293–297.
Gustafson, R.J. and Hall, G.E. 1974. Equilibrium moisture content of shelled corn from 50 to
155 F. Transactions of the American Society of Agricultural Engineers, 17: 120–124.
Hall, C.W. (1980). Drying and Storage of AgricMural Crops. AVI Publishing Company, Inc,
Westport, CT.
Henderson, S.M. 1952. A basic concept of equilibrium moisture content. Agricultural
Engineering, 33: 29–32.
Hossain, M.D., Bala, B.K., Hossain, M.A. and Mondol, M.R.A. 2001. Sorption isotherms and
heat of sorption pineapple. Journal of Food Engineering, 48(2): 103–107.
Hunter, A.J. 1987. An isostere equation for some common seeds. Journal of Agricultural
Engineering Research 37: 95–105.
Iglesias, H.A. and Chirife, J. 1978. Isosteric heats of water vapour on dehydrated foods.
Lebensmittel-Wissenschaft und-Technologie, 9: 116–122.
Kaymak-Ertekin, F. and Gedik, A. 2004. Sorption isotherms and isosteric heat of sorption for
grapes, apricots, apples and potatoes. Lebensmittel-Wissenschaft und-Technologie, 37:
429–438.
Labuza, T.P. 1968. Sorption phenomena in foods. Journal of Food Technology, 22: 263–272.
Lahsasni, N., Kouhila, M. andMahrouz, M. 2004. Adsorption–desorption isotherms and heat
of sorption of prickly pear fruit (Opuntia ficus-indica). Energy Conservation and
Management, 45: 249–261.
Lomauro, C.J., Bakshi, A.S. and Labuza, T.P. 1985. Evaluation of food moisture sorption
isotherm equations. Part 1. Fruit, vegetable andmeat products. Lebensmittel-Wissenschaft
und-Technologie, 18: 111–117.
McEwen, E., Simmonds, W.H.C. and Ward, G.T. 1954. The drying of wheat grain. Part III:
Interpretation in terms of biological structure. Transactions of the Institution of the
Chemical Engineers, 32: 115–120.
Mir, M.A. and Nath, N. 1995. Sorption isotherms of fortified mango bars. Journal of Food
Engineering, 25: 141–150.
Mohamed, L.A., Kouhila, M., Lahsasni, S., Jamali, A., Idlimam, A., Rhazi, M., Aghfir, M. and
Mahrouz, M. 2005. Equilibrium moisture content and heat of sorption of Gelidium
sessquipedale. Journal of Stored Products Research, 41(2):199–209.
Okos, M.R., Narsimhan, B., Singh, R.P.and Weimauer, A.C. 1992. Food dehydration. In:
Handbook of Food Engineering. (Eds.) D.R. Heldman and D.B. Lund. Marcel Dekker, Inc.,
New York, pp. 437–562.
Oztekin, S. and Soysal, Y.A. 2000. Comparison of adsorption and desorption isosteric heats of
some grains. Agricultural Engineering International, II: 1–17.
Pichler, H.J. 1957. Sorption isotherms of grain and rape seeds (translation). Journal of
Agricultural Engineering Research, 2: 159–165.
Pixton, S.W. and Howe, R.W. 1983. The suitability of various linear transformations to
represent the sigmoid relationship of humidity and moisture content. Journal of Stored
Products Research, 19(1): 1–18.
Moisture Contents and Equilibrium Moisture Content Models 29
Phomkong, W., Srzednicki, G. and Discroll, R.H. 2006. Desorption isotherms of stone fruit.
Drying Technology, 24(2): 201–210.
Reddy, B.S. and Chakraverty, A. 2004. Equilibrium moisture characteristics of raw and
parboiled paddy, brown rice, and bran. Drying Technology, 22(4): 837–851
Smith, S.E., 1947. Sorption of water vapour by high polymers. Journal of American Chemical
Society, 69: 646–651.
Strohman, R.D. and Yoerger, R.R. 1967. A new equilibrium moisture content equation.
Transactions of the American Society of Agricultural Engineers, 10(5): 675–667.