Drying and Storage of Cereal Grains E-Book

B. K. Bala

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 470

Veröffentlichungsjahr: 2016

Beschreibung

Finite Element Analysis and Computational Fluid Dynamics have been introduced in modelling and simulation of drying and storage systems, these techniques are expected to dominate the future research and development of drying and storages, and should reduce losses and improve the quality of agricultural products, enhancing food security globally.

Drying and Storage of Cereal Grains, Second Edition, covers the wide spectrum of drying and storage methods applied to economically important cereal produce, providingnumerical examples for better understanding the complexity in drying and storage systems through modelling and simulation, aiding design and management of drying and storage systems. Chapters 1 to 8 look at air and grain moisture equilibria, psychrometry, physical and thermal properties of cereal grains, principles of air flow, and provide detailed analyses of grain drying.Chapters 9 to 13 focus on temperature and moisture in grain storages, and provide comprehensive treatment of modern grain storage systems. The book also includes a number of unsolved problems at the end of each chapter for further practice.

This revised second edition includes new sections on -

• heat of sorption
• finite element modeling of single kernel
• CFD modeling  of fluidized bed drying
• exergy analysis and neural network modeling
• numerical solution of two dimensional temperature and moisture  changes in stored grain

This book will provide students in agricultural engineering and food engineering with a wide spectrum of drying and storage studies previously unavailable in a single monograph. It will also serve as an excellent reference for practicing agricultural engineers, food engineers and food technologists.

Leseprobe

Table of Contents

Cover

Title Page

Foreword to the Second Edition

Foreword to the First Edition

Preface

1 Principles of Drying

1.1 Introduction

1.2 Losses of Crops

1.3 Importance of Drying

1.4 Principles of Drying

Reference

Further Reading

2 Moisture Contents and Equilibrium Moisture Content Models

2.1 Introduction

2.2 Moisture Content Representation

2.3 Determination of Moisture Content

2.4 Grain Sampling

2.5 Equilibrium Moisture Content

2.6 Determination of Static Equilibrium Moisture Content

2.7 Static Equilibrium Moisture Content Models

2.8 Net Isosteric Heat of Sorption

Exercises

References

3 Psychrometry

3.1 Introduction

3.2 Psychrometric Terms

3.3 Construction of Psychrometric Chart

3.4 Use of Psychrometric Chart

Exercises

References

Further Reading

4 Physical and Thermal Properties of Cereal Grains

4.1 Introduction

4.2 Structure of Cereal Grains

4.3 Physical Dimensions

4.4 1000 Grain Weight

4.5 Bulk Density

4.6 Shrinkage

4.7 Friction

4.8 Specific Heat

4.9 Thermal Conductivity

4.10 Latent Heat of Vaporization of Grain Moisture

4.11 Heat Transfer Coefficient of Grain Bed

Exercises

References

Further Reading

5 Airflow Resistance and Fans

5.1 Airflow Resistance

5.2 Fans

5.3 Duct Design for On-Floor Drying and Storage System

Exercises

References

6 Thin Layer Drying of Cereal Grains

6.1 Theory

6.2 Thin Layer Drying Equations

6.3 Development of Thin Layer Drying Equations

6.4 Drying Parameters

6.5 Finite Element Modelling of Single Kernel

Exercises

References

Further Reading

7 Deep-Bed and Continuous Flow Drying

7.1 Introduction

7.2 Deep-Bed Drying Models

7.3 Development of Models for Deep-Bed Drying

7.4 Development of Models for Continuous Flow Drying

7.5 CFD Modelling of Fluidized Bed Drying

Exercises

References

Further Reading

8 Grain Drying Systems

8.1 Introduction

8.2 Solar Drying Systems

8.3 Batch Drying Systems

8.4 Continuous-Flow Drying Systems

8.5 Safe Temperature for Drying Grain

8.6 Hydrothermal Stresses during Drying

8.7 Energy and Exergy Analysis of Drying Process

8.8 Neural Network Modelling

8.9 Selection of Dryers

Exercises

References

Further Reading

9 Principles of Storage

9.1 Introduction

9.2 Principles of Storage

9.3 Interrelations of Physical, Chemical and Biological Variables in the Deterioration of Stored Grains

9.4 Computer Simulation Modelling for Stored Grain Pest Management

References

Further Reading

10 Temperature and Moisture Changes During Storage

10.1 Introduction

10.2 Qualitative Analysis of Moisture Changes of Stored Grains in Cylindrical Bins

10.3 Temperature Changes in Stored Grains

10.4 Temperature Prediction

10.5 Numerical Solution of One-Dimensional Heat Flow

10.6 Numerical Solution of Two-Dimensional Heat and Moisture Flow

10.7 Simultaneous Momentum, Heat and Mass Transfer during Storage

10.8 CFD Modelling of Grain Storage Systems

Exercises

References

Further Reading

11 Fungi, Insects and Other Organisms Associated with Stored Grain

11.1 Introduction

11.2 Fungi

11.3 Insects

11.4 Mites

11.5 Rodents

11.6 Respiration and Heating

11.7 Control Methods

References

Further Reading

12 Design of Grain Storages

12.1 Introduction

12.2 Structural Requirements

12.3 Construction Materials

Exercises

References

13 Grain Storage Systems

13.1 Introduction

13.2 Traditional Storage Systems

13.3 Modern Storage Systems

References

Further Reading

Appendix A: Finite Difference Approximation

Appendix B: Gaussian Elimination Method

Appendix C: Finite Element Method

References

Appendix D: Computational Fluid Dynamics

Further Reading

Index

End User License Agreement

List of Tables

Chapter 02

Table 2.1 Grain equilibrium moisture content, % (wb).

Table 2.2 Relative humidity of saturated salt solutions at different temperatures.

Table 2.3 Relative humidity of different concentrations of aqueous acid solution at various temperatures, %.

Chapter 03

Table 3.1 Composition of dry air.

Chapter 04

Table 4.1 Specific heat of some common agricultural crops.

Table 4.2 Thermal conductivity of wheat and rough rice.

Table 4.3 Latent heat of vaporization of some agricultural crops.

Table 4.4 Heat transfer coefficients of grain bed of some agricultural crops.

Chapter 06

Table 6.1 Selected thin layer drying models.

Chapter 07

Table 7.1 The effect of operating conditions on air heating energy use, drying time and equilibrium moisture content.

Chapter 08

Table 8.1 Maximum safe drying temperatures.

Chapter 12

Table 12.1 Properties of crop seeds related to bin load.

Table 12.2 Grain to bin wall friction coefficients.

Chapter 13

Table 13.1 Maximum temperature for chilled storage of grain.

Table 13.2 Results of two grain refrigeration trials with wheat in an insulated 1700 tonne steel silo, 14.5 m diameter and 13.1 m wall height at Lah, Victoria.

Table 13.3 Exposure periods proposed for modified atmosphere disinfestation of grains (<12% moisture content).

Table 13.4 Propionic acid treatment of grain.

List of Illustrations

Chapter 02

Figure 2.1 Sorption isotherm, desorption isotherm and hysteresis of moisture sorption of wheat at 35°C.

Figure 2.2 Adsorption and desorption isotherms of hybrid rice at 30°C, 40°C and 50°C.

Figure 2.3 Desorption and adsorption isotherms of rough rice at 30°C.

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.

Figure 2.5 Comparison of net isosteric heat of sorption of hybrid rice with those of other varieties and crops. des – desorption, ads – adsorption.

Chapter 03

Figure 3.1 Adiabatic saturation chamber.

Figure 3.2 Adiabatic saturation. Chart has obliquely inclined scale.

Figure 3.3 Humidity–temperature chart illustrating the adiabatic saturation process.

Figure 3.4 Use of a psychrometric chart.

Figure 3.5 Sensible heating and cooling on a skeleton psychrometric chart.

Figure 3.6 Heating with humidification on a skeleton psychrometric chart.

Figure 3.7 Cooling with humidification.

Figure 3.8 Cooling with dehumidification on a skeleton psychrometric chart.

Figure 3.9 Sensible heating of air and adiabatic drying.

Figure 3.10 Development of drying zone and drying front. (a) Deep bed driver. (b) Drying front.

Figure 3.11 Mixing of two air streams.

Figure 3.12 The psychrometric process of mixing.

Figure 3.13 Supersaturated air mixture.

Figure 3.14 Non-adiabatic mixing of moist air streams. (a) Heat gain. (b) Heat loss.

Figure 3.15 State paths for air when drying with recirculation.

Figure E3.2 Illustration of Example 3.2 on a skeleton psychrometric chart.

Figure E3.3 Illustration of Example 3.3 on a skeleton psychrometric chart.

Figure E3.4 Illustration of Example 3.4 on a skeleton psychrometric chart.

Figure E3.5 Illustration of Example 3.5 on a skeleton psychrometric chart.

Chapter 04

Figure 4.1 Anatomical structure of (a) wheat, (b) rice and (c) corn.

Figure 4.2 Angle of repose of grains.

Figure 4.3 Forces and angle of friction.

Figure 4.4 Device for measuring the angle of static friction.

Figure 4.5 Thermal conductivity measurement apparatus.

Figure E4.5 Othmer plots for equilibrium moisture content data of malt.

Figure 4.6 Theoretical non-dimensional air temperatures for different values of

Z

and for values of

Y

ranging from 2 to 16.

Figure E4.7 Computed and observed temperature history of air.

Figure P4.2 Temperature of the calorimeter against time.

Chapter 05

Figure 5.1 Resistance of grains and seeds to airflow.

Figure 5.2 Elemental area.

Figure 5.3 (a) Division of a region into grid and (b) isopressure lines and direction of airflow.

Figure 5.4 Velocity and pressure gradient vectors.

Figure 5.5 (a) Grid point notation, (b) impervious boundary condition and (c) impervious boundary condition at a corner.

Figure E5.1 Pressure pattern for a duct covered with wheat.

Figure 5.6 (a) Propeller fan, (b) axial flow fan and (c) centrifugal fan.

Figure 5.7 (a) Forward curved blade fan, (b) radial blade fan and (c) backward curved blade fan.

Figure 5.8 Axial flow fan characteristics.

Figure 5.9 Forward curved centrifugal fan characteristics.

Figure 5.10 Backward curved centrifugal fan characteristics.

Figure 5.11 Fan and system.

Figure 5.12 Effect of speed change.

Figure 5.13 Effect of change of speed and system resistance.

Figure 5.14 Fans in series.

Figure 5.15 Fans in parallel.

Figure E5.5 (a) Fan arrangement, (b) equivalent fan arrangement and (c) equivalent fan characteristics.

Chapter 06

Figure 6.1 Drying rate curve.

Figure 6.2 Drying rate versus moisture content.

Figure 6.3 A typical thin layer drying apparatus.

Figure 6.4 Temperature effect of drying performance of rice, wheat and hybrid rice with time.

Figure 6.5 Variation of germination percentage with drying temperature.

Figure 6.6 Predicted and observed drying rates.

Figure 6.7 Determination of

k

from semi-logarithmic plot.

Figure 6.8 Determination of

k

and

u

from log–log plot.

Figure E6.4 Moisture content against time.

Figure E6.5 Predicted and observed moisture content: • experimental and - predicted.

Figure 6.9 Finite element grid for one quarter of a single kernel.

Chapter 07

Figure 7.1 Moisture ratio versus time variable.

Figure 7.2 Temperature ratio versus time variable.

Figure E7.1 Moisture content variation with time at the middle position.

Figure E7.2 Temperature variation with time at the middle position.

Figure 7.3 Element of bed.

Figure 7.4 Finite difference grid for the deep-bed drying equation.

Figure 7.5 The principle of condensation procedure on a skeleton psychrometric chart.

Figure E7.3 (a) Moisture content changes with time. (b) Air temperature changes with time.

Figure 7.6 Predicted and observed mean moisture contents.

Figure 7.7 Predicted and observed moisture content distributions.

Figure 7.8 Predicted and observed temperature variations with time at a number of bed depths.

Figure 7.9 Temperature variations with time as predicted by model: b, bottom; m, middle; t, top. (Top) gas-fired. (Bottom) recirculated gas-fired.

Figure 7.10 Moisture content variation with time as predicted by model: b; bottom; m, middle; t, top. (Top) gas-fired. (bottom) recirculated gas-fired.

Figure 7.11 Moisture content variation with time as predicted by model ———: -, direct gas; · - · - · - · - · - · -, indirect; - - - - - - - - recirculated direct gas; b, bottom; m, middle; t, top.

Figure 7.12 Elemental volume of the bed of the drier.

Figure E7.4 Moisture content variations along the length of the bed 150 min after the start of drying.

Figure 7.13 Conditions of air and the product around a single-stage fluidized bed drier.

Figure E7.5 (a) Moisture content variation with time. (b) Temperature variation with time.

Chapter 08

Figure 8.1 A typical solar grain drying system.

Figure 8.2 Solar tunnel dryer.

Figure 8.3 Greenhouse solar dryer.

Figure 8.4 Batch drying and storage system. (a) Dry when the bin is full and store in the same bin. (b) Dry in layers and store in the same bin.

Figure 8.5 Batch drying system.

Figure 8.6 Moisture content and temperature changes in crossflow dryers.

Figure 8.7 Moisture content and temperature changes in concurrent flow dryers.

Figure 8.8 Moisture content and temperature changes in counterflow dryers.

Figure 8.9 A hypothetical stress distribution of residual desorption within a rice kernel.

Figure 8.10 The structure of a typical ANN model.

Figure 8.11 Variation of predicted moisture content and observed moisture content of jackfruit leather with drying time for the test data.

Chapter 09

Figure 9.1 Damp grain heating.

Figure 9.2 Loss of germination and baking quality.

Figure 9.3 Damage by insects.

Figure 9.4 Damage by mites.

Figure 9.5 Safe storage conditions.

Figure 9.6 A diagrammatic representation of interrelations among the grain bulk, organisms and their abiotic environment in the spoilage of stored grain (from Sinha and Muir, 1973).

Chapter 10

Figure 10.1 Convection air currents with warm grain in the bin with cooler surrounding air.

Figure 10.2 Convection air currents with cold grain in the bin with warmer surrounding air.

Figure 10.3 Heat conduction in an element of grain in cylindrical coordinate.

Figure 10.4 Finite difference grid.

Figure 10.5 Energy balance at the convection boundary.

Figure 10.6 Energy balance at the convection boundary with a fictitious temperature outside the boundary.

Figure 10.7 Predicted and measured temperature of stored grain in cylindrical bins.

Figure E10.10 Simulated temperature changes with time at a radius of 0.30 m.

Figure 10.8 A cylindrical bin showing two-dimensional flow directions.

Figure 10.9 Schematic diagram of the storage system (a) and grid structure of the finite difference formulation of the grain storage system model (b).

Figure 10.10 The experimental and simulated moisture content (% w.b.) distribution of rough rice during storage in a traditional cylindrical bin – experimental and - - - - - simulated.

Chapter 11

Figure 11.1 A schematic diagram of the ecosystem.

Figure 11.2 A schematic diagram of the grain ecosystem in the storage bin.

Figure 11.3 The environment of decomposers and consumers in a bin of stored grain.

Figure 11.4 (a) Stored product insect species. (b) Stored product insect and mite species.

Figure 11.5 Spoilage of grain due to temperature gradients, moisture content and localized development of fungi and insects.

Chapter 12

Figure 12.1 Relations of plane of rapture to bin height. (a) Shallow bins. (b) Deep bins.

Figure 12.2 Bin, wall and loading of grain.

Figure 12.3 Lateral pressure chart for rice and wheat for variable height and diameter.

Figure 12.4 A shallow bin containing grain and the failure plane.

Figure 12.5 A shallow bin with a wedge shaped mass of grain.

Figure 12.6 Lateral pressure chart for rice and wheat for variable height and diameter.

Figure E12.2 (a) Depth versus lateral pressure. (b) Depth versus hoop tension.

Figure E12.4

y

/

Y

against PJ.

Figure E12.5

y

/

Y

versus lateral pressure PL.

Chapter 13

Figure 13.1 A typical grain storage facility (high-throughput, short-term storage).

Figure 13.2 Mortality of adult and immature stages of granary weevils (

Sitophilus granarius

). (a) Oxygen concentration (%). (b) Carbon dioxide concentration (%).

Figure 13.3 Depletion of oxygen and production of carbon dioxide in a sealed container of wheat infested with adult granary weevils.

Figure 13.4 Concentration of oxygen with a slight leak (0.5% entry of oxygen/24 h) with different populations of granary weevils.

Figure 13.5 Concentration of oxygen and carbon dioxide with a slight leak filled with wheat infested with adults of granary weevils.

Figure 13.6 Production of carbon dioxide in completely sealed container of wheat at the moisture contents shown.

Figure 13.7 Depletion of oxygen in the same container as in Figure 13.6.

Figure 13.8 Simplified psychrometric chart illustrating grain cooling with evaporation of small quantities of water from the grain.

Figure 13.9 Simplified psychrometric chart illustrating grain cooling with very small quantities of water absorbed by the grain.

Figure 13.10 Vertical duct used for aeration.

Figure 13.11 Horizontal ducts for aeration.

Figure 13.12 Layout of ducting in 1700 tonne capacity insulated silo for grain refrigeration trials at Lah, Victoria (1) Refrigeration unit, (2) atmospheric air intake, (3) return air intake and (4) perforated air distribution duct.

Figure 13.13 Grain temperature contours at central measuring section of 15,000 tonne capacity insulated horizontal storage at Gravesend, NSW, showing effect of increased airflow on cooling rate at the centre of bulk.

Figure 13.14 Selecting a method for applying CO

2

.

Figure 13.15 Reduction coefficient of the number of bacteria present on maize grains of moisture contents 21% w.b. (curve 1) and 35% w.b. (curve 2) as a function of the propionic acid concentration used.

Figure 13.16 Storage diagram. Propionic acid concentration to be applied as a function of moisture content in order to ensure the preservation of maize for a given period (Fink, 1970). Abscissa: duration of storage in months. Ordinate: propionic acid level as a percentage of the mass of maize.

Figure 13.17 Growth of moulds in batches of moist maize grains (moisture content 35%) as a function of propionic acid concentration, during open atmosphere storage at 13°C. 1, control; curves 2–5 relate to propionic acid concentrations of 0.2, 0.5, 0.75 and 1%, respectively. Abscissa: duration of storage in days. Ordinate: number of moulds propagules per gramme.

Appendix A

Figure A.1 Illustration of central difference, forward difference and backward difference formulae.

Appendix C

Figure C.1 Finite element grid for one quarter of a single fruit of mango.

Appendix D

Figure D.1 Mass flows in and out of the fluid element.

Figure D.2 Stress components in the

x

-direction.

Figure D.3 Components of the heat flux vector.

Guide

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Drying and Storage of Cereal Grains

Second Edition

B. K. Bala

Department of Agro Product Processing TechnologyJessore University of Science and TechnologyJessore, Bangladesh

This edition first published 2017© 2017 John Wiley & Sons, Ltd

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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 especially 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 designated 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.

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 problems 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, continuing 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. Enthusiasts 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 conditions 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 processes 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 AgrartechnikPotsdam-Bornim e.V.

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 of the cycle

How to control the general well-being, the processes of ‘breakdown’, which is nature’s way 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 economically, 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 distribution 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 utilization 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 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

Preface

This book has been written primarily for undergraduate and graduate students in agricultural 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 and moisture 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 computers. 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

1Principles 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 deterioration of stored grains and also because of lack of knowledge and proper facilities for drying and storage. Great efforts are being made to increase crop production, but until now little or no effort is being made to improve drying and storage facilities, especially in developing 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 example, 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.

1.2 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 investigation. 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 monetary 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 quantified 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:

Drying permits the long-time storage of grains without deterioration of quality.

Drying permits farmers to have better-quality product for their consumption and sale.

Drying permits the continuous supply of the product throughout the year and takes advantage of higher price after harvesting season.

Drying permits the maintenance of viability and enables the farmers to use and sell better-quality seeds.

Drying permits early harvest which reduces field damage and shatter loss.

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 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 moisture from inside of the kernel to the surface. The moisture 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.