Organisation: International Rice Research Institute, Philippines (IRRI)
1.2 World Trade
1.3 Primary product
1.4 Secondary and derived product
1.5 Requirements for export and quality assurance
1.6 Consumer preference
Rice (Oryza sativa L.) is a staple food of over half the world's people and is grown on approximately 146 million hectares, more than 10 percent of total available land. Total world production is about 535 million tons of unmilled or rough rice (paddy). Ninety seven percent (97 percent) of the world's rice is grown by less developed countries, mostly in Asia. China and India produce about 55 percent of the total crop (IRRI, 1997).
Asian farmers plant 89 percent of the world's harvested rice accounting for 91 percent of global rice production. In Bangladesh, Cambodia, Indonesia, Lao PDR, Myanmar, Thailand and Vietnam, rice provides 56 to 80 percent of the total calories consumed. In the tropics, rice is the primary source of human nutrition (See Table 1.0.1, Annex 1.0). With the exception of the highest income countries, per capita rice consumption has remained stable in Asia over the past 30 years. In most African and Latin American countries, rice is less important than other crops (Henry and Kettlewell, 1996). On average, rice contributes 10 percent or less of the total calorie intake, although in Guinea, Guyana, Surinam, Liberia, Madagascar, and Sierra Leone, 31 to 45 percent of the calories eaten come from rice.
Rice production and consumption are often associated with low incomes and poverty. Most of the major rice-producing countries are developing countries categorised by the World Bank as "low income economies". For these countries, rice is not only their staple food, but also a major economic activity and source of employment and income for the rural population.
By 2025, more than 5 billion of the world's anticipated 10 billion people will depend on rice as their principal food. Recent projections indicate that the world will need about 880 million tons of rice in 2025 - 92 percent more rice than was consumed in 1992. In South Asia where poverty is extensive, the need for rice is expected to double over the next 40 years. Production requirements will be even higher, to provide stocks, seed and for non-food uses.
Rice is one of the cheapest sources of food energy and protein. Most rice is consumed as white polished grain, despite the valuable food content of brown rice. These nutrients are lost when bran is removed in milling. Brown rice, once the form of rice eaten before the advent modern rice mills, has lost its appeal due to consumer preference changes for colour, nutty taste and other traits. Health-conscious people in European countries where rice is not a staple prefer brown rice. Drawbacks of brown rice are it requires more fuel for cooking than white rice, it may cause digestive disturbances and the oil in the bran tends to turn rancid and reduce storage life (Henry and Kettlewell, 1996).
Annex 1.0 includes tables on population demographics and agricultural production in 41 important rice-producing countries plus the current, forecast and historical production and consumption of rice.
1.2 World Trade
In most of Asia, rice is grown on small, one to three-hectare farms. Farms can be less than one hectare in more densely populated countries. A typical Asian farmer plants rice primarily to meet the family's basic needs. In Brazil, 70 percent of the rice cultivated is on commercial farms of more than 50 hectares (IRRI, 1997).
Less than 5 percent of the worlds rice production is traded internationally. For example, Basmati, the high-quality rice produced in Pakistan and Northwest India, commands an international market price four times higher than the domestic price of the coarse, local rice which the low-income people eat. In 1993, the major rice exporters were Thailand, (31 percent of the world market), the United States (16 percent), Vietnam (11 percent), China (9 percent), Pakistan (6 percent), and India (5 percent). Myanmar is an emerging exporter of rice.
With this narrow and volatile global market for rice, most countries cannot depend on imports to meet the food needs of their people. Self-sufficiency in rice production in order to maintain price levels, is an important political objective in most rice-dependent countries. For example, if China wanted to buy 10 percent of its domestic consumption, the demand for rice in the world market would increase by more than 88 percent and that would dramatically affect international prices. (Henry and Kettlewell, 1996). Few developing countries have adequate foreign exchange for major international purchases.
There is variation in the price of rice brought by farmers to market effected by annual climate changes. This situation makes domestic prices highly unstable. Price controls through maintenance of large stocks can benefit urban consumers, but often keep farm prices below a profitable level. The world market is small and few reserves are held.
World trade is projected to increase at around 2.5 percent per annum to admit 17 million tons by the year 2000. This exceeds the 2 percent annual growth during 1978-1988. Some countries in the Far East, particularly those that have recently achieved self-sufficiency, are expected to emerge as exporters in the next decade. In most other regions, demand for rice would generally continue to exceed domestic production, stimulating a global rice import demand.
Imports in Africa are forecast to rise by a slightly faster rate than the previous decade to 4 million tons more than the present. Although output of paddy is projected to increase at 4 percent per annum, demand for rice would be marginally larger. As a result, Africa is likely to continue to rely on imports to fulfil over 80 percent of its total demand.
Latin America will most likely remain a large net importer of rice, and many countries in North America and Europe will also show significant increase in imports (ASEAN, 1992).
Annex 2.0 shows tables regarding the world trade in rice.
1.3 Primary product
Parboiling is the hydrothermal treatment of paddy before milling. The three important steps are:
(a) Soaking (sometimes called steeping) paddy in water to increase its moisture content to about 30 percent.
(b) Heat treating wet paddy, usually by steam to complete the physical-chemical change;
(c) Drying paddy to a safe moisture level for milling.
Parboiling of paddy is an age-old process in parts of Asia. Africa, and to a limited extent today, in some European countries and America. The advantages of parboiling are improved recovery percentage, salvaged poor quality or spoiled paddy, and met demand or preferences by certain consumers. The process causes certain changes in the milled rice; physical-chemical and aesthetic. The changes include the following:
(a) Change in taste and texture of the rice, preferred by some consumers and disliked by others;
(b) Gelatinization of starch, making the grain translucent, hard and resistant to breakage during milling. Thus, milling recovery rates for head rice and total rice yields are improved;
(c) Inactivating of all enzymes; all biological process and fungus growth
(d) Easier removal of the hull during milling but more difficult bran removal;
(e) More rice swelling during cooking and less starch in the cooking water.
All the above changes affect the results obtained during milling, storage and cooking and ultimately affect consumer preferences. (Wimberly, 1983).
Parboiling paddy has advantages over ordinary drying without parboiling and also some disadvantages as follows:
Advantages of parboiling
1. Milling or dehusking is easier; costs less.
Bran removal is more difficult and costs more.
2. Milled rice has fewer brokens; is nutritious.
Cannot be used in starch making or brewing.
3. Increased head and total rice out-turn.
Doubles the total processing cost.
4. Rice more resistant to storage insect pests.
Rice easily becomes rancid.
5. Bran contains more oil.
Requires large capital investment.
6. Cooking losses less starch and keeps longer.
Takes longer to cook and uses more fuel.
Some research studies report that parboiled rice retains more protein, vitamins and minerals and thus is more nutritious than a raw milled rice. (Wimberly, 1983). However, other studies show no significant nutritional differences between the two.
The process involved in soaking, steaming, and re-drying is expensive. In many cases, however, poor quality paddy (improperly cleaned, dried, handled, and stored) can be improved by parboiling. Properly parboiled rice receives a premium price, again offsetting the added cost of parboiling.
Paddy or the rice grain consists basically of the hull or husk (l8 - 28 percent) and the caryopsis or the brown rice (72 - 83 percent). The brown rice consists of a brownish outer layer (pericarp, tegmen and aleurone layers) called the bran (5 - 8 percent), the germ or embryo (2 - 3 percent) connected on the ventral side of the grain, and the edible portion (endosperm, 89 - 94 percent).
Rice milling is the removal or separation of the husk (dehusking) and the bran (polishing) to produce the edible portion (endosperm) for consumption. This process has to be accomplished with care to prevent excessive breakage of the kernel and improve the recovery of the paddy. Actual milling process, however, removes also the germ and a portion of the endosperm as broken or powdery materials reducing the quantity of grains recovered in the process. The extent of losses on the edible portion of the grain during milling depends on so many factors as variety of paddy, condition of paddy during milling, degree of milling required, the kind of rice mill used, the operators, insect infestation and others. What comes out during the milling operation are the husk or hull, milled rice or the edible portion, germ, bran and the brokens. Depending on the rice mill used, the by-products come out from the mill as mixed or separated. Milling is usually done when paddy is dry (about l4 percent moisture content). Wet soft grain will be powdered. Very dry brittle grain will break and produce brokens and powdery materials during the milling operations.
Losses in milling could be qualitative and quantitative in nature. Quantitative or physical losses are manifested by low milling recovery while quality losses are manifested by low head rice recovery or high percentage of broken grains in the milled product.
Rice mills being used in rice-producing countries vary from the manually operated hammer beam pounder or mortar and pestle to the very sophisticated rice mill used in big commercial or government installations. In remote areas where power is not available the beam hammer pounder or the mortar and pestle are used by farmers usually operated by the female members of the family. When an engine powered single -pass rice mill is brought to the community by enterprising individuals, the manually operated mills disappear. Women bring their paddy for milling to reduce their workload and have time to socialise with their neighbours in the rice mills.
As the volume of grain being milled increase and people become knowledgeable and concerned with the milled rice recovered from paddy, rice millers upgrade their machines and prospective entrepreneurs acquire a bigger and efficient machine to satisfy the demand of the customers.
Table 3.8.1, Annex 3.8 includes the machines that are involved in the actual removal or separation of the husk and bran during the milling process. The extent of rice mill sophistication to improve quantity and quality of grain processed depends on the number of these motorised equipment included in the table used in series and parallel installations together with ancillary components. The ancillary equipment not included in the table are paddy cleaner, husk and bran aspirator, destoner, paddy separator, automatic weighing device, brown rice thickness grader, automatic weighing and bagging, grain elevators and conveyors.
The milling of rice involves at least two basic operations. i.e., removing the outer covering called the husk, or hull and removing the seed coat called the bran. The former called dehusking or dehulling while the latter, is polishing or whitening process. Different methods of accomplishing these two operations range from the traditional hand pounding using pestle and mortar to high capacity sophisticated milling systems.
Pestle and mortar. This process is a manual form of milling, which is found in isolated and remote areas in most of the Third World countries. In this process, milling is accomplished by the impact and friction acting between the paddy kernels. The grain is dehulled and whitened every time it is pounded in the mortar. However, excessive impact and pressure result in high breakage of the milled rice. This method has been losing popularity since a wide range of size of milling systems had been introduced at the grass-roots level. small and large capacity processing systems. Small- and large- capacity processing systems such as steel hullers, rubber roll type mills and other systems consisting combinations of efficient milling equipment, have become available.
Steel huller. The steel huller, sometimes referred to as Engleberg steel huller rice mill, is more efficient than the pestle and mortar. The impact force in the steel huller is absent. A rotating steel roller inside a screen cylinder provides pressure and friction among the grains and effect simultaneous dehulling and whitening or polishing of the kernels. Tests have shown that it yields 3 - 5 percent more total rice with 15 to 25 percent fewer brokens (Wimberly, 1983). Studies in the Philippines indicated that the milling recovery of steel hullers vary from 60 to 63 percent depending upon the variety and condition of paddy (PRRPO). Among the4 commercial rice mills surveyed steel hullers exhibited the lowest milling and head rice yield, averaging 66.23 percent and 41.70 percent, respectively.
Under-run disk shellers. The under-run disk sheller, often referred to as a disc sheller, consists of two horizontal iron discs partly coated with an abrasive layer. Paddy is fed into the centre of the machine and moves outwards by centrifugal force. It is evenly distributed over the surface of the rotating disc. Under the centrifugal pressure and friction of the disc, most of the grains are dehusked.
The main advantages of the disc sheller are its operational simplicity and its lower running cost since the abrasive coating can be remade at the site with inexpensive materials. The main disadvantages are grain breakage and the abrasions caused to the outer bran layers.
Rubber roll paddy huskers. The rubber roll paddy husker, referred to also as huller or sheller, consists of two rubber rolls rotating in opposite directions at different speeds. One roll moves about 25 percent faster than the other. The difference in peripheral speeds subjects the paddy grains falling between the rolls to a shearing action that strips off the husk.
Compared with the disc sheller, the rubber roll husker has the advantage of reducing grain breakage, loss of small brokens, and risk of damage to the grain and machine by unskilled operators. It does not remove the germ and therefore sieving the resulting brown rice is unnecessary. Its hulling efficiency is high and it does not require a beard-cutting machine. The main disadvantage is the cost of replacing the rubber rolls as they wear. That is offset, however, by the reduction of breakage and increased total rice overturn.
Multiple machine mills. The large capacity multiple machine rice mill uses a different machine for each processing step: cleaning, dehusking, separating, bran removal and grading. These processes are integrated into one system by bucket elevators linking machine to machine to accomplish each stage of processing to the end of the output polished rice.
A number of manufacturers have introduced small capacity (500 to 1000 kg/h) rice mills. They fill the gap between the small capacity single machine and the large capacity multiple machine mills.
The modern multiple machine rice mill is more efficient than the traditional steel huller and consumes about one-half to two-thirds the power of the steel huller operating at the same capacity. The rice recovery rate is considerably higher in terms of total rice and head yields.
Recovering the maximum amount of endosperm or the edible portion of paddy grain with none or minimum brokens is the main objective of rice milling. The removal or separation of the husk and bran to get the edible portion of the grain is done in many ways. In remote areas where no power is available some farmers just remove the husk and cook while others use the mortar and pestle or the beam hammer pounder to remove the husk and bran. This process is very laborious; recovery of milled rice is low; and presence of broken grains is high. Farmers do not mind the losses probably because of low volume processed and there is no other better alternative to mill paddy in remote areas. The steel huller or the Engleberg rice mill is the most common rice mill in the rural areas where power can be made available. The process of husk and bran removal by this machine is through intense pressure and friction in a single pass over a very short period of time. The result is milled rice with high percentage of brokens and low grain recovered.
In the combined rubber roll husker and friction polisher, husk is aspirated after the grain pass through the rubber roll husker and the brown rice-paddy mixture is fed directly to the friction polisher (one pass) to produce the milled rice. This machine performs better than the steel huller mill because of separate husking and polishing, lower quantity of abrasive husk in the polisher reducing friction and pressure in the process.
In the under-runner disc husker and the pearling cone polisher, the pearling cone can not separate husk. A paddy separator is added to separate brown rice before feeding to the pearling cone. Separated paddy is returned to the disc husker. This machine combination performs still better than the steel huller mill because of the separate husk and bran removal. To further improve the milling performance, a series of up to three pearling cones is installed to have a gradual removal of the bran layer. Although milling efficiency is improved, the proper setting and adjustment of the disc huller and the pearling cone by the operator is the critical factor in the milling operation.
Modern commercial rice mills employ more sophisticated equipment to clean, sort, weigh, separate paddy from brown rice, separate brokens and others. Several rubber roll huskers in parallel to increase capacity, abrasive whiteners and friction polisher in series are employed to subject the grain to less pressure and friction in removing the bran. This controlled milling operation results in more grain recovered, more whole grains and less brokens. The design and operation of big commercial rice milling complexes produce quality milled rice with minimum loss. The solvent extractive rice milling developed in Houston, Texas (USA) is another step taken by the industry to improve and refine the milling operations in order to improve the process and increase further the milling efficiency and reduce losses.
A large portion of the paddy harvested in the Asian region is retained and milled in the farm. The steel huller (kiskisan or Engleberg) is the most common mill found in the rural areas. This mill was reported to produce milled rice lower by 4.1 percent and 6.6 percent compared with the disc sheller-pearling cone and rubber roller-abrasive and friction whiteners, respectively. Farmers do not mind the milling loss in the steel huller milling because of small quantities milled at a time and the utilisation of the husk-bran mixture for animal feed.
Studies have been conducted in improving the design of the steel huller but results were not promising because of process limitation. That is, husk and bran are removed in a single process over a very short period. Excessive pressure, friction and rubbing of the grain results in sudden increase in grain temperature, increase brokens, powdering of the endosperm and ultimately reduce grain recovery. Attempts have been made to separate husking and whitening in regular milling using the steel huller mill. Two steel hullers in series, the first as husker and the second as polisher, rubber roller husker-steel huller, emery stone disc-steel huller, centrifugal husker-steel huller are some of the tests conducted. Other combinations of rubber roller and disc husker combined with pearling cone whitener and friction polisher.
Improvements in the milling performance were observed when husking and whitening or bran removal was made as two separate operations. The best performance was observed when rubber roll was used as the husker. The hard and rough surface of the disc huller scratches and breaks some grain during the husking process affecting the quantity of milled rice afterwards.
It is recommended that the removal of husk and bran in a single operation as the steel huller should be avoided. The minimum milling operation should be husking and polishing or whitening as separate processes which could be assembled in a single frame. Machine with separate husking and polishing in a single frame is already commercially available. A steel huller can be used as husker and as polisher but a rubber roll husker is preferred for husking. Rubber roll is presently readily available.
1.4 Secondary and derived product
Straw, husk, bran and brokens are the by-products resulting from harvesting of paddy in the field and in its processing into milled rice the final consumable staple food, are. Rice and its by-products can either be used directly or further processed for other uses.
The whole grains can be transformed into flakes or popped rice. Brown rice has become a speciality rice, which is attractively packaged and marketed as health food in developed countries. Special upland coloured rice varieties are preferred by farmers in Lao PDR that practice slash and burn cultivation in the mountainous areas. These are fermented and made into rice toddy (lao hai) and liquor (lao khao). In Japan, rice wine or sake is a part of meals and social gatherings. Rice is used as an ingredient for brewing beer in some parts of the rice-producing states of USA.
Applications of rice by-products include:
Rice straw: Animal feed, sometimes treated with urea to improve its digestibility; Thatch, fuel strips with cowdung as binder (Bangladesh); mushroom bed; mulch in horticulture; processed into paper or compost.
Husk: Fuel used directly or in briquettes or cakes with dung binder in cooking stoves and furnaces for dryers, brick kilns, steam boilers, as gasified fuel for engines and burners, as insulating boards, packing material in transporting eggs and other delicate products, bedding materials in the livestock and poultry industry, ash cement or component for making light weight cement blocks for partition walls, in tile industry, oil absorbent, washing powder, and mulch.
Bran: Commonly used as animal feed in most developing countries, either directly or mixed with other ingredients as done by commercial feed producers. Rice bran has a high percentage of oil, which can be extracted by solvents. Edible bran oil is used extensively in Japan and India. Bran is also used in food processing industry for making biscuits and speciality cookies. Spoiled bran can be used in compost and used as organic fertiliser. De-oiled bran can be safely used as animal feed. China, Indonesia, Malaysia, Sri Lanka, Thailand and Vietnam produce and export edible bran oil. India also produces edible bran oil and plans to increase output over the next few years.
Rice brokens: Used in breakfast cereals, baby foods and for making several food items with rice flour base, such as noodles, rice cakes and rice delicacies. Rice brokens are an export item from large commercial rice mills in Thailand.
Annex 2.1 gives information on the production, utilisation of by-products of rice post-harvest processing in various countries. It also gives information on the Asian trade of edible bran oil. Other internationally traded by-products are hand-made paper products using rice straw as one of the raw materials.
1.5 Requirements for export and quality assurance
Major problems in quality arise from lack of incentives to farmers. Especially noteworthy is the corresponding price for value added in drying paddy. Manifestations of poor quality are yellow rice, brokens, contaminants, ageing, storage changes, variety mixing and mislabelling, lack of screening methods to differentiate among rice with similar starch properties and among special rice for rice food products. Parboiled rice is susceptible to high aflatoxin level from fungal growth.
International standards for export of rice are particular about the permissible dirt, moisture, pesticide residue and pest situation in the traded rice. Table 2.2.1 and Table 2.2.2, Annex 2.2 give the quality standards for international trade in rice.
Some developing countries with chronic rice shortages specify lower than international standards in terms of brokens but at least at par with the national standards to obtain cheaper price than the national counterpart rice.
The importer of high quality rice may specify one variety. However, Thailand, a major exporter of rice faces the problem of mixed varieties because paddy is collected from different areas and different farmers. Thailand was faced with complaints from discriminating Japanese consumers that Thai rice exported to Japan did not meet the taste, texture and sanitation standards in Japan (The Bangkok Post, 1994). It was reported that some complaints were politically motivated because trade liberalisation would be inimical to the interests of Japanese farmers who received as much as 10 times the international market price for rice.
1.6 Consumer preference
Quality of milled rice means different things to different consumers in various Asian countries. One of the distinguishing factors influencing consumer preferences is the type of amylose content of rice. Table 2.2.3, Annex 2.2 shows the distribution of such preferences in various Asian countries.
Quality has also different meanings to different people involved in the processing and trading industry, to producers and to consumers. The following are broad meanings of quality:
(a) Producer: Quality grain is of good variety, filled, well-ripened, winnowed and cleaned, commands high farm gate price and in demand by traders, millers and consumers;
(a) Trader: Quality means dry, insect-free, undamaged grain, which will store well;
(b) Miller: Quality means grain batch is of pure or homogenous variety and yields a high percentage of finished products or has high milling recovery:
(c) Consumer: Quality of milled rice means that it has good appearance (polish or whiteness, wholeness, uniformity, purity and attractive packaging for more sophisticated consumers) and the preferred texture (see Table 2.2.3, Annex 2.2), flavour, and cooking properties (also high nutritional value for health-conscious consumers).
The condition of the grain at harvest which is a result of influencing factors such as climate, soil, production management, as well as harvest operations and post-harvest techniques, can only be sustained at best and may no longer be improved by processing. However, processing could increase the value of the raw product. On the contrary, improper and incomplete post-harvest operations could cause a deterioration of quality, which could have been potentially excellent.
Grain deterioration may be measured in terms of losses in quantity and quality of the final milled rice product. For example, the nutritional value of rice is reduced when it has turned yellowish caused by stackburning. Table 2.2.4, Annex 2.2 shows that protein of yellow rice has a lower lysine content than that of sound grain. Rat experiments showed that the net protein utilisation and protein quality were also lower in yellow rice than in white rice (Eggum et al. 1984).
Table 2.2.5, Annex 2.2 shows the effects of the environment, processing, and variety on grain quality at different steps in the post-harvest system (Juliano, 1996).
Some of the factors contributing to deterioration are impurity, too high or too low moisture content, immature and unfilled grain, cracked kernel, chalky grain, and red rice and other impurities. The following describes how each major factor affects the grain quality:
(a) Moisture content of grain. Too high rather than too low (overdried) moisture content is the common problem encountered among the traded paddy because it is more expensive to overdry the paddy except when the method used is sundrying. High moisture content results in the rapid deterioration of the paddy because the grains continue to respire and heat builds up giving favourable conditions for mould to grow, fermentation to set and micro-organisms to multiply. Insects and mites will be most active when the equilibrium relative humidity inside the grain mass gets to about 60-80 percent aggravated also by the biological activities. The result is yellowed and damaged grains.
(b) Temperature. High grain temperature has damaging effects on the grain because of the increased respiration and reproductive activities of insects. Most insects infesting paddy complete their life cycles at temperatures of 15-45oC while moulds and bacteria have a wide range of temperature (0-60oC) for their activities.
(c) Insects and micro-organisms. Insects cause damage to the paddy by eating the food matter, causing reduced weight and volume of the grain bulk, as well as indentation and deformation of the kernels, which reduce the milling recovery. They also leave black marks on the kernels and increase the temperature in the bulk grain. They also contaminate it with their wastes and dead bodies. At high grain moisture content (25-30 percent) and high relative humidity (70-75 percent) the activity of fungi and bacteria also increase and cause further damage by discolouring of the grain, giving off or bad odour, causing off-flavour and producing mycotoxins.
(d) Impurities. Matter other than grain such as stones, dirt, sand, plastics, glass and metal bits as well as organic materials such as chaff, straw, empty grains, red kernels and seeds of weeds and other crops, animal and insect parts and even human hair constitute the common impurities in the grain bulk. The inorganic materials damage the mill and the organic ones rot rapidly, cause uneven distribution of moisture content and induce the growth of micro-organisms.
(e) Immature grains. While not exactly impurities, immature grains do lower grain quality by causing uneven distribution of moisture and themselves the favoured food of insects, hence causing a chain of actions leading to increased infestation and quality deterioration.
(f) Thermal and mechanical stresses. The rapid rate of moisture removal induces stresses in the grain because of the differential expansion and contraction of the inner and outer layers of the grain. Fissuring occurs and eventually during milling, the grain breaks along the fissure lines. Accidental or unavoidable re-wetting by rain of dried grain as during retrieval from the sundrying floor, also cause stresses and eventually fractured kernels. A worse situation occurs when the grain is chalky or already damaged by insects and water as well as by mechanical handling and processing such as in threshing or sundrying.
(g) Mixed varieties. Harvests from different fields get mixed in the rice mill compound for various reasons. Milling of different sized grains result in poor quality rice because no one adjustment of the mill may satisfy the requirements of the non-uniform grain sizes.
Table 2.2.6, Annex 2.2 shows the different quality indicators during each post-harvest operation from harvesting to marketing.
A knowledge and understanding of the factors that induce grain deterioration is essential in achieving the quality of the processed grain at each stage of post-harvest operations. The quality of output at each grain-processing episode determines the quality of output of the next process. The chain of events starts from pre-harvest operations or production of the paddy itself. The complication is that unless a particular rice milling industry firm is integrated with farm production and processing under one management, total quality assurance of the grain is difficult to achieve. Table 2.2.7, Annex 2.2 summarises the methods of preventing grain deterioration and maintaining rice quality.
The rice milling industry has little control if any of the variety supplied by farmers and traders. Moreover, the traders obtain their paddy from different farmers who are likely unaware or could not care less of the importance of purity of the variety they are planting because they are not given incentives by traders or millers for such specification. Red rice or other admixture variety in the field could be rouged or pulled out since the rice plants are likely different in growth characteristics than the particular variety chosen for planting. Thorough weed control is important in preventing contamination of the grain with weed seeds and plant parts.
Chalky, immature and unfilled grains could be avoided by timely harvesting which means also timely planting with respect to climatic conditions, insect infestation and synchrony with the planting by other farmers. A guide for harvesting date is the information on the maturity duration. The exact date however, could vary because of climate and soil conditions. Normally, the grain may be harvested if the hulled test grains from the upper portion of the sample panicle are clear and firm and most of the grains at the base of the panicle are in hard dough stage. The decision to harvest or postpone it is influenced by the risk involved due to weather conditions at the intended time of harvest, the availability of labour or hired threshing services and severity of rodent infestation.
Field drying of the harvested paddy, if it is not a shattering variety, should be practised moderately during the dry season only. If hand-harvested by sickle the grip size bundles are better laid out separated rather than stacked to achieve greater aeration rather than stacked. Stacking of moist paddy will cause heating up of the paddy, increasing the activity of micro-organisms and initiate a major deterioration in quality. A safe way is to thresh the paddy immediately after harvesting.
Threshing by foot is the mildest method and will not cause any mechanical damage to the grain. The risk of grain breakage increases depending upon the method used in threshing. Beating the panicle by a stick, impacting it against a wood block or slatted platform; trampling upon it by buffaloes or cows, rubbing it against a wire-looped drum in a pedal-operated or sheared and beaten between a peg-tooth or raspbar drum and concave in a mechanical-powered thresher incur different degrees of mechanical damage to the grain. The damaged grain is the staging point of attack by insects and micro-organisms and so the better it is if mechanical damage could be minimised or avoided through improved machines and handling. The axial-flow threshing principle, in which the panicle is stripped and the grain is mildly impacted, has been used in IRRI-designed small and large rice threshers since the 1970s.
Immediate drying of the grain after harvest is imperative to avoid its deterioration. The use of mechanical dryers at the small farm level has not caught on because of its high initial cost, uneconomical operation and seasonal utilisation. Sundrying of paddy is still the most popular method even among medium-scale rice mills in developing countries because of the free heat energy although handling costs are high. However, it is unreliable and the drying rate is not as controllable as mechanical dryers.
In the absence of heated-air or mechanical dryer during cloudy or rainy weather after harvest, the paddy should at least be aerated by thinly spreading it on a floor or piling it in small heaps and frequently turning it over. The idea is to increase the surface area of the grain bulk to the air, drain any surface water and prevent the build-up of heat within the grain bulk. Farmers during such emergency situations have resorted to blowing with an electric fan over the surface of the heap or laid wet paddy. Spreading or heaping the wet paddy on a mat of fine net laid over a raised slatted platform would further increase the aeration surface and drip off any free water. This technique will save the crop until sunshine weather comes.
Two-stage drying consisting of flash or high-temperature short-exposure or fast drying to 18 percent during the first stage and low-temperature and slow drying or sundrying to 14 percent during the second stage is another technique to save a large volume of wet grain. Paddy at 18 percent moisture content can be stored for two weeks. However, re-wetting of the grain should be avoided to prevent cracking or fissuring which will have telling effects in milling.
Sanitation, prevention of entry of insect, rodent and bird pests, and prevention of contamination of the grains during storage of the grain should be practised meticulously. Storage in a dry and cool place with proper aeration to bring temperature to about 17oC effectively minimises insect infestation.
Prolonged storage under ordinary conditions even without the presence of insects and micro-organisms will cause grain deterioration in terms of colour, texture, odour, flavour, and nutritive value because of uncontrolled moisture and temperature. The re-entry of moisture in milled rice should be avoided.