Rice Postproduction Technology in the Tropics 9780824886059

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Esmay Rice Paperback Edition September 1979 3170 $7.00

THE EAST-WEST CENTER-officially known as the Center for Cultural and Technical Interchange Between East and West—is a national educational institution established in Hawaii by the U.S. Congress in 1960 to promote better relations and understanding between the United States and the nations of Asia and the Pacific through cooperative study, training, and research. The Center is administered by a public, nonprofit corporation whose international Board of Governors consists of distinguished scholars, business leaders, and public servants. Each year more than 1,500 men and women from many nations and cultures participate in Center programs that seek cooperative solutions to problems of mutual consequence to East and West. Working with the Center's multidisciplinary and multicultural staff, participants include visiting scholars and researchers; leaders and professionals from the academic, government, and business communities; and graduate degree students, most of whom are enrolled at the University of Hawaii. For each Center partici-

pant from the United States, two participants are sought from the Asian and Pacific area. Center programs are conducted by institutes addressing problems of communication, culture learning, environment and policy, population, and resource systems. A limited number of "open" grants are available to degree scholars and research fellows whose academic interests are not encompassed by institute programs. The U.S. Congress provides basic funding for Center programs and a variety of awards to participants. Because of the cooperative nature of Center programs, financial support and cost-sharing are also provided by Asian and Pacific governments, regional agencies, private enterprise and foundations. The Center is on land adjacent to and provided by the University of Hawaii. East-West Center Books are published by The University Press of Hawaii to further the Center's aims and programs.

RICE POSTPRODUCTION TECHNOLOGY IN THE TROPICS

RICE POSTPRODUCTION TECHNOLOGY IN THE TROPICS

M E R L E ESMAY, SOEMANGAT, ERIYATNO, AND A L L A N PHILLIPS

X An East- West Center Book from the East-West Food Institute Published for the East-West Center by The University Press of Hawaii Honolulu Copyright © 1979 by the East-West Center All rights reserved. No part of this work may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage or retrieval system, without permission in writing from the publisher. Manufactured in the United States of America

Library of Congress Cataloging in Publication Data Main entry under title: Rice postproduction technology in the tropics. Bibliography: p. Includes index. 1. Rice processing—Tropics. 2. R i c e Tropics. I. Esmay, Merle L., 1920TS2159.R5R52 633'.18'60913 79-15428 ISBN 0-8248-0638-7

Contents

Preface Drying Systems

A Note on Terminology

7

1 Introduction 2 Physicochemical Properties of Rice 17

77

8 Storage Systems

88

Milling

39

Economic Considerations References

Drying Principles

105

10

4

Psychrometrics

Storage Principles

9

3

Harvesting and Threshing

57

46

Index

121

133 139

Preface

The inspiration for this text originated in 1973 when Dr. Esmay collaborated with Professor Soemangat in presenting a three-month course at the University of Gadjah Mada for agricultural engineering instructors from outlying Indonesian universities. The teaching notes on rice postproduction technology prepared for that short course became the first draft of a manuscript for this book. In late 1975, the Midwest University Consortium for International Activities (MUCIA) financed a program under which Professor Soemangat came to Michigan State University and worked for five months on the preparation of the manuscript. On Professor Soemangat's way to the United States, Dr. Esmay met him in Hawaii where they both participated in a seminar on postharvest crop protection sponsored by the Food Institute of the East-West Center. While at the seminar an arrangement for collaboration with Dr. Allan Phillips of the East-West Food Institute was established for final preparation of the manuscript and its publication as an East-West Center book by The University Press of Hawaii. This book discusses all of the critical postproduction operations for rice. The principles and concepts of harvesting, threshing, handling, drying, storing,

and milling systems, along with their application, are presented in a semitechnical manner. The book, therefore, is intended for undergraduate agricultural engineering students, although much of the material will be understood by nonengineers. The book should therefore fulfill a need as a reference for many people involved in rice postproduction operations. The subject matter of this book is applicable to tropical conditions, which present unique problems of handling, storing, and processing as compared to temperate climatic areas. Instructors should supplement the text material whenever possible with discussion of conditions unique to their specific local area. The book includes some sample problems. Instructors are encouraged to supplement these examples with local or regional problems designed to provide the students with analytical experiences in agricultural engineering. This book has been prepared using the metric system of measurement, but it still may be in the wrong language for use in many countries. Translation and publication of this text into the appropriate language for your use is encouraged.

A Note on Terminology

Within the English language there is a wide variation in terms used to refer to rice in its different stages of production and processing. Moreover, additional foreign words are readily adopted and widely used. Persons working with rice production systems will soon become familiar with the terminology, but for the purpose of clarity in reading this book, the following comments on terminology are offered. Paddy can refer to the entire growing plant or to the harvested grains which have not been milled. The word is sometimes spelled padi. It is synonymous with the word palay, which is of standard use in the English spoken in the Philippines. The term paddy is often used with the same meaning as rough rice, which refers to the unmilled rice. After the outer

husk is removed, the grain is called brown rice. After the bran layer is removed by polishing or whitening, the grain is called milled rice or white rice. The processes described in subsequent chapters of this book should give the reader a good understanding of the precise meanings of these terms. When dealing with quantities of materials, such as yield data, process capacities, or market volumes, it is important to differentiate properly between rough rice, milled rice, and other terms. The term stalk paddy refers to the rice plant which has been cut but not threshed. Sometimes the term paddy variety is used with the same meaning as rice variety. In several Asian countries, government agencies carry the title of "Paddy Marketing Board."

1

Introduction

The urgent need to increase food production to meet the requirements of a rapidly growing world population is widely recognized. Rice is one of the most important food crops in the world, and the major item in the diet of half the world's population. The availability of an adequate supply of rice and other food commodities means more than simply providing for people's nutritional needs; rice has economic importance in local and international trade with significant political and social implications, and for many people rice production is an integral part of their culture. The production of rice is a predominant part of the agriculture of most Asian countries, and in many cases national policy is directed toward self-sufficiency in rice production. In 1975, world production of paddy rice was 342,592,000 metric tons, with Asia producing 311,625,000 metric tons or 90 percent of the world total (FAO 1976a). The percentage of rice production involved in international trade is relatively small. Total imports to Asia in 1974 were 1,316,200 metric tons of milled rice equivalent, corresponding to about one half of one percent (0.5 percent) of the production in Asian countries. The United States is the leading exporter of rice (1,725,600 metric tons exported in 1974). Among Asian countries, Thailand, Pakistan, and Burma export large quantities while most other Asian countries are net importers of rice. Italy, Egypt, and Australia are also major exporters (FAO 1976a). In the developing countries, progress toward selfsufficiency in food production is rapidly offset by population growth. Approximately half of the world's 4 billion human inhabitants eat rice as their staple food. With the population increasing at an annual rate of 2 percent, the total number of rice eaters increases by 40 million each year. By the time the earth completes one revolution about its axis, there

will be 100,000 more people eating rice. These sobering statistics give a sense of urgency to our efforts to increase the available supply of food. There are two general ways in which the supply of rice can be increased. One way is to increase production; the other is to reduce postproduction losses. Research and development efforts aimed at increasing rice production have been carried out vigorously during recent years with a notable degree of success. Breeding programs have created high-yielding varieties (HYV) which provide a high yield potential when provided with suitable production inputs of fertilizer and water. These high-yielding varieties also incorporate other desirable characteristics, such as short stems to resist lodging, disease resistance, and tolerance to unfavorable soil conditions. These modern varieties, when used in conjunction with the recommended cultural practices and provided with adequate fertilizers, water management practices, and pest control, have produced remarkable results. The term Green Revolution has been used to describe the impact of the high-yielding varieties on the world's agriculture. The solution to the problem of low yields can lead to other difficulties in rice production: the "secondgeneration" problems. For example, the development of large-scale irrigation projects in combination with the adoption of high-yielding varieties leads to double cropping of rice in many regions. As a result, large acreages of crops are harvested during the wet season, thus placing a heavy demand on drying and storage facilities. Another of the second-generation problems concerns labor availability. The combination of more intensive cropping schedules, shorter growing seasons, and more intensive production inputs increases the demand for labor at critical times, and the rice production system must be carefully

INTRODUCTION

2

designed so as to achieve appropriate levels of productive capacity during critical operations. These considerations are especially important in relation to harvesting, drying, and transportation, which are the operations required to get the crop product into a suitable condition for storage. The problem of reducing postproduction losses has until recently been given little attention. Fortunately, interest in research, development, and education activities related to postproduction crop protection is growing rapidly. The fact that this book has been written and is now being read is evidence of that growing interest. For each postproduction operation there is a possibility of some loss either in quantity or in quality of crop product. Losses of quantity occur through spillage or consumption by insects, birds, and rodents. Losses of quality occur when the product's nutritive

value or economic value is diminished due to the effects of insects, birds, rodents, bacteria, mold, physiological changes, or contaminants. For cereal grains, the overall postharvest losses are usually estimated to be in the range of 10 to 20 percent. Occasionally much higher losses occur. One of the difficulties in reducing losses is gaining an accurate determination of the magnitude of loss. Most often, accurate measurements of losses are very difficult to obtain. During harvesting, for example, grains of paddy rice are lost when they "shatter" and fall to the ground. If these fallen grains were counted over a measured surface area of the field, accurate determinations of grain losses could be made. This is a very tedious procedure and is therefore seldom done. Instead most determinations of loss are based upon estimates rather than actual measurements whereby grains are either counted or weighed. Never-

Drying of stalk paddy under the sun in Indonesia. (Photo courtesy of Dante de Padua)

3

INTRODUCTION

theless, there have been some painstaking studies in which the actual grains of paddy rice on the plant stalk were carefully counted before each harvesting operation (Pelayo 1966; Ruiz and Castro 1965). The measured losses were surprisingly high, especially for varieties which are susceptible to shattering, and the losses increase rapidly when the time of harvesting is delayed (Figure 1.1). These studies show that the losses during harvest are substantial. They also indi701

60

50

g, Cfl W C/3 W O -I

40

-7

0

7

14

21

D A Y S FROM MATURITY FIGURE 1.1 Grain losses of three rice varieties harvested at different stages of maturity. (Data for the IR-8 variety are from Cristal 1967; data for peta, from Pelayo 1966; and for Raminad Str. 3 from Ringor 1966.)

cate that an earlier harvest will help to reduce quantitative loss. From a physical point of view, the engineering principles to reduce postproduction losses are quite simple. The product must be kept in an environment which has the right combination of temperature and humidity, and it must be isolated from damaging pests and pathogens. However, these relatively simple engineering principles often must be applied under conditions where the physical resources are extremely limited. Electrical energy, fuel, and construction materials are often in short supply or not available in many of the rice-producing regions. In the face of limited physical resources, the expeditious use of intellectual resources becomes even more important. To elaborate further on the points made in the preceding paragraph, it is important that the student who uses this textbook gain a thorough understanding of the individual principles of physics, chemistry, biology, economics, and engineering upon which postproduction technologies are based. It is also important to understand the interactions among these principles when applying them to design and operations. Equipped with this understanding, the engineer can then make use of information provided by experience, observations, handbooks, manufacturer's specifications of product performance, and other sources. Because of the extremes in the conditions under which rice is grown, processed, and consumed, it is very difficult to write simple instructions for rice equipment design and operations. In some cases the same hardware is used with different operating practices according to local conditions. The design of equipment depends on its operation, and good twoway communication between manufacturers and users is necessary. In addition to the physical question " W h a t happens to the crop?", there is the question " W h o does what to the crop?" This question relates to the institutional aspects of distribution—that is, the market channels that handle the crop and how much of the crop they handle. If we were to trace the channels

4 through which the rice crop moves after harvesting, we may find many people involved in the various marketing and distribution processes (Getubig 1975). These channels include farmers, country buyers, millers, processors, wholesalers, and retailers. It is therefore useful to approach the evaluation of crop losses, and the design and application of processing operations, within a framework that systematically identifies the important processes and market channels involved in moving the crop from farmer to consumer. The magnitude of the postproduction management problems in the developing countries can be illustrated by an example. Sri Lanka, a small country with a population of 13 million, is compelled to import rice and wheat to meet food requirements. In 1974, Sri Lanka imported 308,000 tons of rice at $115

INTRODUCTION

million. At the same time, approximately 25 percent of the paddy produced is lost in postproduction management. This loss is equivalent to 250,000 tons of imported rice valued at $75 million (Wimberly 1975). These losses and consequent excessive expenditures can be greatly reduced with the wise use of existing technology. The following chapters of this book give a presentation of rice postproduction technology relevant to conditions in the tropical regions of the world. The various steps from harvest to milling are described, and the engineering principles and data which are basic to the design and analysis of operations and systems are presented in a concise and comprehensive form.

2

Physicochemical Properties of Rice

Rice plants possess certain specific physical and chemical properties that are relevant to optimizing the harvesting and processing operations. These properties vary between varieties and within varieties and, moreover, are affected by the environmental conditions during the growing period. New varieties with improved grain quality and quantity have been developed in recent years. Other properties have also been changed, however, and a large portion of the increased yields have been lost after production because of a lack of understanding of these basic physical and chemical properties. Time of harvest is critical as there is an optimum period for recovery of a maximum yield and top quality. Field shattering and grain cracking increase as harvest is delayed beyond this optimum period. The preharvest and postharvest grain checks and cracks end up as broken kernels after milling. Rice kernel deterioration often begins while the crop ripens in the field. Bacteria, fungi, birds, insects, and rodents may attack various parts of the plant before harvest. Postharvest losses in storage consist of dry matter losses (such as chemical changes in protein, carbohydrate, and oil) and contamination by various chemical toxins, insect fragments, and rodent urine and feces. Environmental conditions of temperature and humidity along with grain moisture content govern the biochemical changes as well as microorganism and insect losses during storage. A thorough knowledge of the physical and chemical properties of paddy rice grain is then necessary in order to minimize losses and maintain the top quality of the rice product until consumption. The introduction of mechanical harvesting and processing equipment has further increased the need for a basic knowledge of the physical properties of rice. Much of

the present postproduction technology has been developed by the industrial nations for temperate climates. Inasmuch as tropical conditions are greatly different, as well as the rice varieties with their corresponding chemical and physical properties, the direct transfer of technology has only limited application. Most of the postproduction problems must be studied and solved within the tropical countries. Paddy Morphology The morphology of each rice variety, as to the time periods for all stages of growth, must be understood and considered with reference to harvest. These periods of growth, even within varieties, may however vary, depending on season, temperature, rainfall, available sunlight, and cultural practices. Paddy rice varieties in the tropics have growing periods from 100 to 210 days, with the majority falling between 110 and 150 days. The growth cycle of the paddy plant may be logically divided into three main phases, as shown in Figure 2.1. The approximate duration of each growth phase is: 1. Vegetative phase: 25 to 65 days (depending upon the variety) 2. Reproductive phase: about 35 days (regardless of variety) 3. Ripening phase: 25 to 35 days (regardless of variety) The vegetative phase of the paddy plant varies most between varieties. It consists of the following periods: (1) nursery period, during which the seedling roots and the first five leaves develop; (2) transplanting period, which extends from the time the seedling is uprooted from the nursery until transplanted and fully recovered (seed sown directly in the field does not suffer this transplanting period); (3) tillering pe-

6

PHYSICOCHEMICAL PROPERTIES OF RICE

Plant Height Dying and Nonproductive Panicle N u m b e r

Tiller

Panicle Length D a y s f r o m Seeding

Preemergence

FIGURE 2.1

G r o w t h stages a n d d e v e l o p m e n t o f 1 2 0 - d a y rice variety.

riod, which begins with the appearance of the first tiller1 from the auxiliary bud in one of the lowermost nodes and extends until the total number of tillers for that plant have been formed (see Figure 2.2). The reproductive phase begins with the initiation of the first panicle, which may in some cases commence before the maximum number of tillers have formed. Panicle2 growth begins 70 to 75 days prior to the expected date of maturity of any variety and is mainly dependent on day length and the environment (Figure 2.3). The internode elongation differs among varieties. The heading period begins with the emergence of a panicle tip out of the flag leaf sheath. The flowering stage occurs about 25 days after panicle initiation regardless of variety. Flowering continues 'The tiller is the vegetative branch of the rice plant, typically including roots, culm, and leaves, but which may or may not develop a productive panicle. : The panicle is a group of spikelets borne on the uppermost node of the culm.

until all spikelets5 in the panicles have bloomed and is followed by pollination and fertilization (Figure 2.4). The ripening phase is the most important with reference to the harvesting operation. Ripening occurs, regardless of variety, over a period of 25 to 35 days in the tropics, while in cooler climates it may take up to 60 days. The ripening grain undergoes the following progressive stages: (1) the milk stage, in which the starchy portion of the kernels changes from a watery fluid to a milky consistency; (2) the dough stage, in which the milky caryopsis (the starchy portion of the grain) turns to a soft dough; (3) the maturation stage, in which the caryopsis of each individual kernel becomes fully developed—hard, clear, and free of any greenish tint. The grain is mature by definition when more than 90 percent of the kernels in all panicles are fully developed. In the tropics, the grain kernel 'The spikelet consists of two very small "outer glumes" (sterile lemmas) with all other floral parts lying between or above them. It is borne on the pedicel which connects the panicle branch.

P H Y S I C O C H E M I C A L P R O P E R T I E S OF R I C E

7

Leaf Sheath

Prophyllum

Iniernode Leaf Sheath h Pulvinus

Spikelet Secondary Branch

Nodal Septum

Internode

Internode Tiller

Panicle Axis Primary Branch Panicle Base Rag Leaf

Adventitious Roots

Uppermost Internode

FIGURE 2.2 Parts of a primary tiller and its secondary tiller. (From Rice Production Manual 1970)

FIGURE 2.3 Component parts of a panicle (partly shown). (From Rice Production Manual 1970)

weight reaches an optimum level 28 days after flowering. All three growth phases affect grain yield, as yield is a function of the number of panicles per plant, the number of filled spikelets per panicle, and the weight of all grain kernels.

cent, and the endosperm 65-67 percent. The outermost kernel tissue, commonly known as the husk or hull, is formed from two specialized plant leaves. The husk consists mostly of cellulosic and fibrous tissue covered with very hard glasslike spines which a f f o r d some protection for the enclosed kernel. The husk is structurally separate f r o m the rest of the seed and thus can be mechanically removed by the milling process. If all conditions have been favorable, this process leaves the body of the seed intact. The husk is not impervious to moisture; thus moisture passes through it during the drying or wetting processes.

Rice Grain Anatomy The rice kernel is composed mainly of the hull, pericarp, endosperm, and germ or embryo (Figure 2.5). By portion of the total kernel weight, the hull comprises 15-30 percent, the pericarp coats 4-5 percent, aleurone layer 12-14 percent, the embryo 2-3 per-

The pericarp consists of three layers of cells

Awn

Anther-

Stamen

Filament

Apiculi

Palea

1 emma Nerves

Stigma

Ovary Sterile

Rachiila

Pedicel

Parts of a spikelet. (From Rice Production Manual 1970) FIGURE 2 . 4

Removing rice seedlings f r o m the seedbed in preparation for transplanting in Taiwan. (Photo by Allan Phillips)

9

P H Y S I C O C H E M I C A L P R O P E R T I E S OF R I C E

TABLE 2 . 1

Composition of a Typical Rice Kernel ("la by weight) Type R o u g h rice B r o w n rice M i l l e d rice

Pericarp

Aleurone Layer Endosperm

Testa

Embryo

FIGURE 2.5

Structure of the rice kernel.

around the main body of the seed which form a protective covering. The pericarp is further classified into an outer pericarp, the hypoderm or mesocarp, and the cross layer. The thin outer pericarp layer consists of a very hard tissue which is quite impermeable to the movement of oxygen, carbon dioxide, and water vapor. The pericarp, therefore, provides some protection from oxidative and enzymatic deterioration. Beneath the pericarp is the tegmen or interseed coat, which consists of a layer several cells in thickness. These cell layers, although a part of the seed coat, are less fibrous than the outer pericarp layer. The tegmen contains a fatty material which is high in oil and protein and low in starch.

Moisture

Protein

Fat

Carbohydrates

Fiber

Ash

12.55 11.68 12.90

6.35 7.71 6.47

2.14 1.19 0.46

65.19 77.79 79.43

7.84 0.70 0.25

5.93 0.93 0.49

Both the endosperm and the embryo are enclosed by the aleurone layer, which lies beneath the tegmen. The aleurone layer of cells is very rich in protein, oil, and vitamins and low in starch. The environment and climatic temperatures during the ripening period affect the number of these aleurone layers. The endosperm consists of starch granula that are low in protein. The starch cells are somewhat hexagonal in shape, but from the center there are elongated walls radiating outward. These radial walls of the starch cells form thousands of potential cleavage planes which may enhance checks or cracks in the grain kernels resulting from mechanical, thermal, or moisture stresses. The significant characteristic of the chemical composition of rice is its lower protein content and higher starch content as compared with other cereal crops. The proportion of the various chemical constituents depends on the rice variety, climatic environment, and agronomic conditions of growing and maturing. Table 2.1 presents the composition of rice in the various processing stages. As the layers are successively removed, the proportions of protein, fats, and vitamins in the remaining kernel decrease while the proportion of carbohydrate increases. The cooking behavior of rice is mostly a function of the characteristics of the starch, especially as related to amylose content, gelatinization temperature, and pasting response. Long-grain varieties usually have 23 to 25 percent amylose content, whereas medium-grain varieties have 15 to 16 percent. The Husk The husks make up about one-fifth of the total grain paddy weight. Husk weight may vary appreciably with the variety, cultural practices, geographic loca-

IO

P H Y S I C O C H E M I C A L P R O P E R T I E S OF R I C E

tion, season, and temperature. Juliano and coworkers (1964) through milling tests on 55 rice varieties in five Asian countries found that the husk weight varied from 16 to 26 percent of the total rough rice weight. Since husks make up about 20 percent of grain paddy, considerable effort has been made to find uses for them. Research on the composition, properties, and possible use of husks started almost a century ago (Houston 1972). Husks have a woody abrasive nature, low nutritional value, resistance to weathering, great bulk density, and high ash content, none of which seems to add much to its economic utilization. Most husks remain as unused waste disposed of by burning. Rice husks contain 15 to 18 percent silica. This silica is heavily concentrated on the inner and outermost surfaces as a cellulose-silica membrane. As a consequence of this high silica content, rice husks are very abrasive in character and auger-type conveying equipment for handling rough rice wears rapidly. Rice husks are composed of very small fibers about 0.50 mm in length. Unground rice husks have a bulk density of approximately 100 to 150 kg/m J . They can readily be compressed to a density of about 400 kg/ m 3 . The angle of repose of the unground husks is about 35° compared with 38° for bran. The fuel value of husks is approximately 3300 to 3600 kcal/ kg. The composition of rice husks as reported by Houston (1972) is given in Table 2.2. TABLE 2.2

Composition Data for Rice Husks (%o by weight) Constituent

Range

H;0 Crude protein Crude fat Nitrogen-free extract Crude fiber Ash Pentotane Cellulose Other

2-11 2- 7 0.4- 3 25-39 32-50 13-30 17-22 34-44 21-47

Physical Properties Paddy rice is subjected to many postproduction operations. After being harvested and threshed, the paddy must be cleaned, transported, dried, stored, and milled. These operations subject the grain to various mechanical forces and some heat. Consequently, the chemical and physical properties of the grain may be changed. The heat treatments and mechanical forces must be maintained below a critical level in order to sustain grain quality. The physical properties of the grain must therefore be understood. There are more than 10,000 varieties of rice cultivated around the world. Thus specific identification is difficult. One of the more important and practical ways to identify paddy rice physically is by the kernel dimensions of length, width, and thickness. Other grain properties of air dynamics, bulk density, flow, water sorption, thermal characteristics, and deformation under different mechanical forces are all related to the individual dimensions of the grain kernels. DIMENSIONS

The grading of milled rice is based quite directly on the dimensions of length, width, and thickness of the kernels. Milled rice is broadly classified as a long, medium, or short-grain variety. Several countries have formulated additional grades based on the length/width ratio. Dimensions are fairly easy to measure; thus they are quite well established for many rice varieties. See Table 2.3 for FAO standards. The dimensions and shape of the rice grain are essential for proper design of processing equipment. Kernels can be separated from foreign material by forced air because of density differences or by vibration because of differing friction, size, and density characteristics. Husking machines must be properly designed and adjusted to remove the husks without overstressing the kernels. Milling machines for polishing and whitening function efficiently only if adjusted to the size of the kernels. Uniform size of kernels within a variety is thus very important.

P H Y S I C O C H E M I C A L P R O P E R T I E S OF R I C E

TABLE 2 . 3

Classification of Milled Rice (FA O standard) Size

Extra long Long Medium Short

Length ( m m )

more than 7.0 6.0-7.0 5.5-5.9 less than 5.5

Shape

Length/width ( m m )

Slender Medium Bold (coarse) Round

more than 3.0 2.4-3.0 2.0-2.39 less than 2.0

Modern rice processing equipment has been developed mainly in the industrial countries, and the design is based upon grain size, shape, specific gravity, and surface characteristics. Van Ruiten (1974) pointed out that the Japanese rubber-roll huller was designed for the short-grain rice variety prevalent in Japan. The huller is, however, now being used in the tropics where long and medium-length rice grain varieties are cultivated. The long-grain varieties place a greater contact area on the rubber rolls and, consequently, the wear on the rolls is high and the life considerably shorter. Several methods have been devised to measure the dimensions of rice grain (Brandenburg and Hamond 1966; Wratten et al. 1969). The dimensions of rice grain are illustrated in Figure 2.6. Table 2.4 shows the length, breadth, and thickness at various moisture levels of the IR-8 variety.

W I T H HUSKS

A

\

= = = =

Length Breadth Thickness L e n g t h o f Beard

L |l>

»

ty B

B

WITHOUT FIGURE 2 . 6

HUSKS

Dimensions o f rice kernels with husks and

after milling. TABLE 2 . 4

HYGROSCOPIC PROPERTIES

T h e sorption phenomena relate to the hygroscopic characteristics of grain and include adsorption, desorption, and chemisorption (Trisvyatskii 1969). A b sorption is defined as water held loosely by capillary force, while adsorption is water held more firmly by the forces of the polar or valency type. Desorption denotes the reverse process characterized by the release of water in the f o r m of vapor. Chemisorption occurs with the fumigation process which places

L B T C

Dimensions of IR-8 Rice Grain at Various Moisture Contents Moisture Content ( % wet basis)

Length

Breadth

Thickness

(mm)

(mm)

(mm)

12.1 14.9 17.2 19.4 23.4 25.7

8.52 8.50 8.59 8.71 8.90 9.04

2.92 2.97 2.94 3.05 3.13 3.18

2.10 2.11 2.13 2.18 2.26 2.35

12

P H Y S I C O C H E M I C A L P R O P E R T I E S OF RICE

gases in contact with the grain. A residue remains attached to the grain kernel surface, part of which can be removed by aeration after fumigation. Some of the gas residue enters the grain kernel and dissolves in the tissue where it may react chemically by a process called chemisorption. The water or moisture content of grain is a matter of importance in determining grain maturity and it also serves as a grain quality factor. Usually the moisture content of grain is expressed as a percentage of moisture based on total weight of grain (wet basis). Thus: fVn W = moisture content, % (wet basis) = weight of water = total weight of wet grain M = 100

whereM Ww W

But in many engineering calculations, moisture content is expressed as a percentage of moisture based on dry matter (dry basis):

where Md W Wd M

= = = =

W M MD = W D moisture content, % (dry basis) weight of wet grain dry weight of grain moisture content, °?o (wet basis)

(2.1)

Wet-basis moisture content is used consistently throughout this text except where noted otherwise. The water sorption properties of rice grain are critical to the drying and conditioning processes as well as for storage. Desorption and adsorption of rice grain begin in the field while the seeds are still attached to the standing plant. The moisture content of the rice grain gradually decreases normally as the plant reaches maturity. The moisture content of the rice stem decreases somewhat more slowly than the moisture content of the grain and remains at a higher level even though the plant is physiologically mature. After the grain moisture content reduces to 30 percent, the moisture level begins to cycle downward during the daylight drying hours and upward during the rewetting night hours.

The drying phenomenon begins even though the water transport process in the plant may still be somewhat active. The moisture transport system cannot eventually keep up with the evaporation demand of the surrounding environmental air; thus daytime moisture losses increase as the plant matures. Nighttime climatic cooling of the air and the consequent increase in relative humidity, however, often exceed the equivalent moisture content of the grain and water is readsorbed. During the night, moreover, the water transport system of the plant, prior to complete maturity, will make up partially for the water depletion of the seeds during the day. The general trend of the cycling moisture content is to lower the level of moisture in the seeds. Water transport within the plant becomes extremely low during the 25 to 35 days after heading. The grain moisture content is then in the range of 20 to 30 percent. From that stage on, the grain as a hygroscopic material fluctuates diurnally with the surrounding air conditions in a continuous attempt to reach equilibrium. The effect of the atmospheric air on the moisture level of preharvested rice grain was observed by Curfs (1974). Figure 2.7 shows measured moisture content levels of rice grain at three different times: 8:00, 11:30, and 14:30. During the harvest period in the tropics, the air temperature may vary 5°C and the relative humidity may range from 65 to 95 percent within 24 hours. Figure 2.7 indicates that the variation in grain moisture content may vary by 5 to 10 percentage points each day. The variation is greatest when harvest is delayed beyond maturity. This excessive cycling of grain moisture content causes many checks and cracks to develop in the seeds. The adsorption or rewetting process that takes place at night is particularly damaging in developing stresses within the seeds that result in checks and cracks. The decrease of grain moisture content is followed by a decrease of moisture content in the rice plant stem, which becomes brittle and less stable. This is a critical period as broken and lodged rice plants result from wind and rain. A delay of harvesting paddy rice beyond the time of maturity will expose the crop to the following types of losses:

PHYSICOCHEMICAL P R O P E R T I E S OF RICE

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