The farming management and field operations as well as the post-field operations determine the quantity and the quality of milled rice, the final product from agricultural production. The influence on such output of the total production and processing system starts from the decision made on the agronomic parameters such as what variety of rice to plant. It goes through the series of decisions and actions made on cultural parameters such as crop establishment and care, harvesting, drying and milling.
Decisions related to production of rice are important in attaining the quality desired for the processed grain. For example, the choice of a variety to be planted determines the stand (erect or lodging), stature (tall or short), maturity (early to late), grain to straw ratio (high to low), shattering characteristics (easy to difficult), husk tightness (loose to tight), amylose content (low to high), grains size (short to long), grain length to width ratio (small to large), panicle stature at maturity (erect to drooping), and other characteristics pertinent to that variety. These characteristics in turn become factors influencing the ease, efficiency, grain loss magnitude, and choice of harvesting and threshing technology. They also affect the rate and quality of the drying process and the quality of dehusked rice (brown rice) and eventually the total recovery and quality of milled rice. The differences in varieties planted in a locality also affect the final product, milled rice, as the high-value rice market usually prefers a pure and single variety. In terms of the desirable bio-diversity for sustainable agriculture however, planting of different varieties in a locality, not necessarily in the same field, is a food security strategy of the government. A high degree of management is required to monitor the plantings of farmers and to make sure that the varieties do not get mixed. In some practices, the high quality and medium quality varieties are deliberately mixed to produce a blend of aroma, flavour, consistency, other characteristics desired or preferred by the consumer for bulk rice but which could not be attained in a single variety.
The degree, extent and efficiency of material, technology and management inputs as well as the timeliness of activities with respect to weather, pest and disease incidence, the growth stage of the crop and all the critical stages of field production and post-harvest processing of the rice have a bearing on the yield and quality of paddy and eventually the amount, quality and cost of milled rice and the by-products. The degree of weed, pest and disease infestations, the type and management of their control, particularly the chemical aspects, the timeliness and techniques used, the harvesting and the handling of paddy even before the processing is done can have telling effects on the consumable product and on the individuals, including women, involved in the operations.
Proper timing is important in harvesting the crop as losses could be incurred if rice is harvested too soon or too late. Immature grains due to too early harvest result in high percentage of brokens and low milling recovery. Delayed harvesting exposes the crop to insect, rodent and bird pests, in addition to increased risks of lodging and grain shattering. The ideal is to be within the window of optimum harvest period.
Table 3.1.1, Annex 3.1 shows the losses incurred when rice is harvested one week early and up to four weeks delayed based on the maturity date of the crop. On this basis, the recommended harvesting time is one week before the maturity date of the particular crop variety. Table 3.1.2, Annex 3.1 confirms the values for two varieties studied in the Philippines.
Table 3.13, Annex 3.1 shows that the least grain losses before and during reaping are incurred when harvesting is done 5 days before the maturity date for the variety. However, while there is less grain shatter, the yield is reduced because of immature kernels. Cutting length of the straw should be as close to the ground as possible and not to be less than half the length of the stalk to reduce grain losses due to unharvested grain (Calpatura, 1978).
More information on losses incurred in harvesting and related activities as affected by the harvesting system used, timing of harvest and variety of the rice crop is given in Table 3.1.4, Table 3.1.5 and Table 3.1.6, Annex 3.1.
The indicators of optimum harvest of the grain are as follows:
(a) The variety has reached the particular date of maturity or number of days after heading, 28 to 34 days, as shown in Figure 3.1.1. The period of flowering and maturity dates of IRRI varieties are shown in Table 3.1.5, Annex 3.1. The field should be drained about 7 to 10 days before this maturity date;
(b) Eighty percent (80 percent) of the grains or the upper portion of the panicle has changed from green to straw colour;
(c) At least 20 percent of the grains at the base is already in hard dough stage;
(d) Grain moisture content ranges from 21 percent to 24 percent, as shown in Figure 3.1.2; and
(e) The hand-dehulled grain, as indicated by daily tests near the projected harvest date, is clear and hard.
Figure 3.1.1, Annex 3.1F shows the optimum time of harvesting rice in terms of grain yield and head rice, germination, moisture content of the grain, and green kernel. Figure 3.1.2, Annex 3.1F shows the optimum time of harvest in terms of moisture content.
Harvesting generally refers to all operations carried out in the field which include cutting the rice stalk or reaping the panicles, either laying out the paddy-on-stalk or stacking it to dry, and bundling for transport. Harvesting and its related handling operations and processes should be understood to prevent considerable amount of post-production losses. There is a positive relationship between the method of handling and the degree of loss as shown by various studies. Too much paddy handling create problems both in quality and quantity. (NAPHIRE, 1997).
Harvesting and its related handling operations are significant points in the post production sequence because grain losses can be incurred. Each additional handling step produces a loss of 1 to 2 percent, for highly shattering varieties (Samson and Duff, 1973). The sequence of manual harvesting, field drying, bundling and stacking in traditional systems can incur losses of from 2 percent to 7 percent (Toquero and Duff, 1974). In-field transport which includes bundling of the cut stalks and done by using manually or animal-pulled sleds can incur losses ranging from 0.11 to 0.35 percent. Field stacking of the harvested stalks incur losses ranging from 0.11 to 0.76 percent. The longer the stack is left in the field, particularly where the grain moisture content is high, the greater is the degree of loss. Stackburning or heating up of the harvested crop stack causes yellowing of the milled rice due to attack of micro-organisms and fermentation. Table 3.2.1, Annex 3.2 shows the grain losses resulting from the traditional method of harvesting for IR-24 variety. Related loss values are given in Annex 4.0.
2.2.1 Harvesting methods
Several methods of harvesting have evolved during the progress of rice production. The most common among the developing countries are still the traditional manual methods.
The traditional methods of harvesting rice are the following:
(a) Panicle reaping. This is accomplished by using a hand-held cutting tool or knife (called yatab in the Philippines and ani-ani in Indonesia and kae in Thailand). The blade quarter-circle blade fixed cross-wise on a wooden, grip-sized handle is passed between the index and the middle fingers which grab the panicle stems and execute the cutting action by pressing the panicle stems against the blade. The method is still used in areas where the traditional varieties are grown which are resistant to shattering, an important feature when handling and transporting the bundles of panicles from the field to the house.
The labour required for panicle reaping (240 labour hours/hectare), done mostly by women and big children, is at least four times that of hand sickle harvesting. Panicle reaping has evolved as a social custom, which provides income generation for the landless rural folks. It is advantageous over the stalk cutting by sickle when fields are flooded or terraced, as in the hilly areas that are inaccessible by wheeled vehicle. The carrying capacity of transport labour is more than that when the straw is cut long by sickle.
(b) Long-stalk cutting by sickle. This is the most widely-used manual method of harvesting. There are many variations in sickle design, depending on the socio-cultural acceptance of the harvesters. It requires from 80 to 180 labour hours to harvest one hectare of rice crop. There are many variations in sickle design, depending upon the socio-cultural acceptance of the harvesters. The stalk is cut about 10-15 cm above the ground or the stalk length is about 60-70 cm for ease of bundling and threshing. The stalks are laid in small bundles on the stubble. In some places in Thailand the bundles are sized such that each one will give about 10 kg of paddy and laid up on the field for a few days to dry up. The reaping efficiency depends upon the various cultural practices, the plant density and variety, degree of lodging, the soil condition and the skill of the harvester. Lodged paddy and saturated soils may reduce the cutting rate by 50 percent. In Bhutan, bringing home the long stalk paddy is related to socio-cultural practices.
Modern mechanical methods. Unless labour in harvesting has become scarce in a locality due to industrialisation or migration to employment-rich areas, rice harvesting will continue to be done with the sickle method in most developing countries. In the Philippines, the income or share in kind (usually 1/6 of the harvested paddy) gained by a manual harvester is high compared with other field operations. In times of calamity as in a typhoon where the rice crop is lodged and soaked, a farmer-owner is sometimes constrained to share up to 1/2 of the harvest to the harvesters rather than lose the crop altogether.
The following mechanised harvesting methods are used in a country depending upon the custom and the suitability of the machine to the soil conditions and the crop being harvested, the local custom, affordability of the machine, and other socio-economic factors.
(b) Reaper-binder. This had once been popular in Japan but is being replaced by the combine. The machine cuts and bundles the stems together and lays them in the field in one operation. Other Asian farmers have never adopted it.
(c) Combine. The small combine has become popular in Japan since the 1960s. The Republic of Korea has also manufactured it commercially since the early 1980s. It is gradually being introduced in other Asian countries but primary resistance to adoption is the high initial cost and adaptability to local conditions. The self-propelled machines have cutting widths of 50 to 150 cm and have capacities of about O.05 hectare per hour (NAPHIRE 1997). Thailand has local versions of large combines popular in developed countries and are being adopted because of the increased costs and scarcity of labour. As a rice-exporting country, Thailand attempts to mechanise rice production and processing operations. Vietnam that has overtaken Thailand in the rice export trade may also adopt mechanised methods because of economies of scale. Although Malaysia is a net importer of rice, it depends on modified large combines imported second-hand mainly from Europe to harvest its basic rice crop. Large combines are being used in commercial rice production in countries like Brazil and Uruguay in Latin America and in Europe and the USA. Their introduction and failed use in some African countries through aid programmes have been the subject of much criticism as to their appropriateness in situations where ready and efficient repair and maintenance facilities and services are not available.
(d) Stripper harvester. This is an innovation from the International Rice Research Institute which adapted the rotary stripping comb principle developed by the Silsoe Research Institute in Silsoe, U. K. The rice stripper-gatherer ideally works with a variety which is non-lodging, of medium stature with erect panicles, and low to medium shattering. A high grain:straw ratio is advantageous in achieving high harvesting productivity.
The IRRI-designed pedestrian stripper-gatherer has undergone several field trials in more than 20 rice-producing countries since 1994 and the reactions to the machines were mostly favourable, except when the machine has to be used in wet or soft fields where traction is a problem. Efforts however, are needed from the national institutions in the various countries to extend the machine to farmers or to harvesting custom operators and to modify the machines to suit local soil and crop conditions. The local manufacturers must first be trained in its fabrication and in the provision of efficient and reliable after-sales services. The attempt to make a small and ride-on combine version of the machine has been beset by traction and floatation problems in wet and soft soils. The design and development activities on it have been discontinued or suspended indefinitely by IRRI.
The following situations hinder the adoption of mechanised harvesting methods:
(a) Low income, inability to raise capital, reluctance to change traditional methods, poor mechanical aptitude, and the desire to save straw for off-farm uses.
(b) Small land holding, very small plot size with high bunds, poor water control, inadequate ground support and poor trafficability for powered harvesting equipment, and lack of access roads to the fields.
(c) Excessive moisture content at harvest time, uneven ripening, severe lodging and entangling of paddy (specially the traditional long-stalked varieties), and high-shattering and low grain-straw ratio varieties.
There is still a lack of functionally and economically suitable equipment for tropical conditions due to inadequate research, development and thorough field testing activities in the developing countries in the area of mechanical harvesting. The high cost of imported equipment and the requirement of good machinery management must compete with relatively low-cost labour.
In most developing and least developed countries that produce rice, transport methods of paddy from the field to processing areas are mainly by means of human and animal power and sometimes mechanical power with the corresponding devices, tools and equipment. In hilly and mountainous areas where paddy fields are terraced, like in Bhutan and Nepal and in some parts of the Philippines and Indonesia, paddy in panicles or in long stalks, are bundled and transported by human or sometimes animal power. Such method of transport, which is related to the method of harvesting and field drying activities often result in high grain losses. Small- and family-sized volume of paddy is transported in bags from the house storage to the small rice mill by foot, bullock carts, bicycles, motorcycles, small-sized vehicles, or public transport vehicles whichever mode is available and affordable.
In Bhutan, the practice is to windrow the cut paddy in the field for drying from 3 to 7 days depending upon the crop and the weather conditions. For the shattering varieties, tremendous grain losses in the field may be incurred specially if harvesting is delayed with respect to crop maturity date. Apart from harvesting losses incurred in cutting, windrowing, sundrying, collecting and bundling the cut crop, further losses are incurred in loading the bundled paddy-in-straw on a person's back for transport to the house. Along the way, grains continue to shatter because of the jarring action while the carrier (oftentimes a woman) walks, leaving a trail of fallen grains. At rest points, the jarring action is more severe than in walking because of the dropping of the bundle on a ledge, self-loading it again and adjusting it for comfort on the carrier's back with jarring action. At the house or destination where the bundle will be stored by stacking, grains will again be shattered and when threshing time comes the handling will yet cause another shattering action. This traditional method is used for cultural and practical reasons, like food security and use of the straw as thatch animal feed and mulch.
In such difficult cases, however, manual transport may be the only feasible and practical means because of the terrain, terraced fields and lack of access roads. In other developing countries in Asia, women carry bundles of harvested panicles on top of their heads. Men do it by means of a carrying pole over their shoulders. Where paddy is field threshed, men usually carry the 40-75-kg bag of paddy on their shoulders or back from the field to the nearest road. Animals such as donkeys and water buffaloes are sometimes used for transport but resources available to the farmer limit owning them. In areas accessible by river, canal and lake, such as in Bangladesh, Vietnam, China and Thailand, paddy is usually transported by boat.
Threshing paddy in the field and transporting the grains in bags will minimise grain losses. It is therefore an improved method over transporting paddy on stalk but the straw, if needed in the house, will have to be retrieved later. Sundrying of paddy may be done in the house yard instead of drying paddy on stalk in the field. Surplus paddy is usually sold fresh to traders or directly to rice mill entrepreneurs as dried paddy commands so little added value to make drying attractive to farmers.
In Myanmar, most of the harvested paddy in straw is sundried in the field; spread on bare dry and hard earth; and then threshed by animal trampling. The threshed paddy in bags is then transported using animal-drawn cart or sled. This threshing and transport scenario is fast changing because of the government's drive to mechanise field operations in a bid to increase its capability to export rice.
In the lowland areas, transport of paddy can be partially mechanised, that is, the bags of paddy are brought from the field to the roadside manually or by animal power. They are then transported to the drying area or rice mill from the roadside by means of motor vehicles like tricycles, power tillers with trailers, tractors with trailers, trucks and lorries. The loading and unloading of the bags or sacks of paddy incur extra labour costs which are assumed by the trader or buyer of the paddy on site.
In the developed and advanced developing countries, paddy is harvested by combines and is handled and transported in bulk. The paddy is power unloaded from the combine by means of an auger conveyor into a waiting lorry or tractor-trailer at the field road, which is part of the infrastructure established for mechanised rice production. Paddy is unloaded from the lorry or trailer onto a floor hopper at the rice mill area for conveyance by either auger or belt conveyor to a mechanical dryer. In Malaysia, each lorry or truckload of bulk paddy is weighed at the unloading site for claiming subsidy from the government.
Commercial or traded rice is bagged at the rice mill and is normally transported to the wholesale and retail markets by means of vehicles of the kind and size depending upon the volume of the rice.
The largest disadvantage of manual transport is the large amount of losses incurred. Field threshing and bagging of the threshed paddy may improve the harvest and transport system but this greatly depends on favourable weather conditions and is adversely affected when harvesting coincides with the monsoon season. Threshing should be done as soon as possible after harvest. Quick threshing can be done only with powered threshers. Even then, timing with good weather is essential as the thresher may clog up or huge losses may be incurred when wet grain and straw are put through the machine.
2.4 Threshing
This operation involves the detachment of paddy kernels or grain from the panicle and can be achieved by rubbing action, impact; and stripping. The rubbing action occurs when paddy is threshed by trampling by humans, animals or tractors. The impact method is the most popular method of threshing paddy. Most mechanical threshers primarily utilise the impact principle for threshing, although some stripping action is also involved.
Paddy threshers may either be hold-on or throw-in type of feeding the unthreshed paddy. In the hold-on type, paddy straws are held stationary while threshing is done by the impact on the particle from cylinder bars spikes or wire loops. In the throw-in type of machines, whole paddy stalks are fed into the machine and a major portion of the grain is threshed by the initial impact of the bars or spikes on the cylinder. The initial impact also accelerates the straw and further threshing is accomplished as the moving particles hit the bar and the concave.
The third type, stripping has also been used in paddy threshing. Some impulsive stripping occurs ordinarily with impact threshing in conventional threshing cylinders.
In the throw-in type of thresher, large amounts of straw pass through the machine. Some designs utilise straw walkers to initially separate the loose grain from the bulk of straw and chaff.
Manual threshing. In this method, threshing is accomplished by either treading, beating the panicles on tub, threshing board or rack, or beating the panicles with stick or flail device. The pedal-operated thresher consists of a rotating drum with wire loops which strip the grains from the panicles when fed by hand. It can be operated by women and can be used in hilly or terraced areas because of its portability.
Power threshing. Treading of the harvested crop under tractor tires is a method used in some Asian countries. The popularity of this method can be attributed to its convenience and the lack of suitable tractor PTO-driven threshers. The grain is separated from the straw by hand and then cleaned by winnowing.
Most, if not all powered paddy threshers are equipped with one of the following types of cylinder and concave arrangement: (a) rasp bar with concave (b) spike tooth and concave (c) wire loop with concave (d) wire loop without concave. Tests by IRRI indicated that the spike-tooth cylinders performed well both with the hold-on and the throw-in methods of feeding and its threshing quality is less affected by changes in cylinder speed.
In the axial-flow thresher, the harvested crop is fed at one end of the cylinder/concave and conveyed by rotary action on the spiral ribs to the other end while being threshed and separated at the concave. Paddles at the exit end throw out the straw and the grain is collected at the bottom of the concave after passing through a screen cleaner. Several versions of the original IRRI design of the axial-flow thresher have been developed in most countries to suit the local requirements of capacity and crop conditions. Thus, there are small-sized portable ones and tractor PTO-powered and engine-powered ones. Many custom operators in Asia use the axial flow threshers to satisfy the threshing and grain cleaning requirements of rice farmers.
Paddy is a hygroscopic, living and respiring biological material. It absorbs and gives off moisture depending upon the grain or paddy moisture content (m.c.), air relative humidity (RH) and temperature of the surrounding atmosphere. As a living biological material paddy respires at an increasing rate with m.c. Paddy respiration is manifested by decrease in dry matter weight, utilisation of oxygen, evolution of carbon dioxide and the release of energy in the form of heat. Respiration is negligible at moisture content of about l2-l4 percent.
Paddy is usually harvested at moisture content of about 24-26 percent (wet basis), higher during the rainy season and lower during the dry season. At this moisture content at harvest, paddy has a high respiration rate and is very susceptible to attack by micro-organisms, insects and pests. The heat evolved during the respiration process is retained in the grain and in the bulk because of the insulating effect of the rice husk. This heat increases the temperature of the grain resulting in increased mould growth, fungi, insects and pests infection, which increases the quantitative loss and qualitative deterioration. Grains become rancid, mouldy, yellowish, insect and pest infested. Newly harvested grain with high moisture content must therefore be dried within 24 hours to about l4 percent for safe storage and milling or to at most 18 percent for temporary storage of up to two weeks in case the drying capacity will jeopardise the drying of the rest of the wet paddy and thus get them spoiled. At moisture content of 14 percent or less, wet basis, paddy will be less susceptible to fungal infestations and likely retain its germination potential. Its shelf life will likely be prolonged and its quality preserved.
In wet grain, vapour pressure is high because of the high moisture content. When this grain is subjected to an atmosphere where vapour pressure is low, vapour transfer or movement will occur from high to low until such time that the vapour pressure is the same or the grain is in equilibrium with the atmosphere. Drying therefore is subjecting the grain to an atmosphere of low vapour pressure and providing the necessary heat to vaporise and means to remove the evaporated moisture from the grain. The same is true to moisture movement within the grain. Moisture from the outer surface of the grain is evaporated during drying. Moisture transfer from the core to the grain surface occur during and after drying until such time that moisture is evenly distributed within the grain. Thus, in sun or solar drying, energy from the sun heat the grain evaporating the moisture and the natural air movement on top of the grain removes the evaporated moisture. Also, in heated air drying, the heat from the drying air vaporise the moisture from the grain and the same drying air removes the evaporated moisture away from the grain. The higher the temperature of the drying air the faster is the drying rate.
Delayed drying may result in stackburning of wet grain due to non-enzymatic browning and microbial growth and mycotoxin production in parboiled rice. Yellow or discoloured grains result from a non-enzymatic browning type reaction (NRI 1991) and all varieties are affected.
Caution however, should be made in paddy drying. Slow drying is recommended to preserve the viability and wholeness of the grain. A heated air temperature of 43oC is recommended in drying paddy for seeds or for food grain milling. High drying air temperature will affect grain viability and the quantity and quality of milled rice during milling. High drying air temperature will not only expose the germ to high temperature but also dry the outer surface of the grain faster than the moisture can move from the core to the grain surface. This uneven dryness of the grain results in internal stresses that cause the grain to crack. The same is true when water is poured on a dry grain as rain on g rain during sundrying. These cracks on the grain are not externally visible but manifest during milling as low grain recovery and high percentage of brokens. The table below shows the different methods and equipment usually used in drying paddy.
The magnitude of losses attributed to drying ranges from 1 to 5 percent. Considering the volume of production in a country, losses due to improper drying and inadequate drying facilities alone would be tremendous considering the equivalent monetary value.
Drying is the most important method in minimising post-harvest losses, since it directly affects safe storage, transportation, distribution and processing quality. Currently, considerable losses are incurred annually during storage and transportation of grain, as a result of inadequate drying.
Specific properties relevant in drying are moisture content (water activity) and both critical and equilibrium moisture contents, and hull of husk tightness. Drying should consider the varietal differences in critical moisture content (11-16 percent), below which the grain fissures upon moisture adsorption (Juliano et al., 1993).
Improper and over-drying as it normally happens in sundrying, which is difficult to control, may reduce head rice yield and aroma. Low temperature drying preserves the rice aroma principle, 2-acetyl-1-pyrroline (Itani and Fushimi 1996).
Hot sand flash drying results in parboiling when done in the wet season because of the high moisture content at harvest. However, when it is done in the dry season and under control, it results in the improvement in grain translucency and milling quality (Arboleda, 1983).
Table 3.5.1 and Table 3.5.2, Annex 3.5 T describes the features of different methods of drying paddy. The dryers listed in Table 3.5.1, Annex 3.5 T are used by farmers, co-operatives, and the private and public sectors involved in post-harvest handling and processing of paddy. The kind of machine and process used depends upon the season, quantity and moisture contents of paddy handled.
Field drying is practised solely by the farmers. This method is resorted to during rainy season or harvesting immediately after the rain to remove surface moisture on the cut panicles, grains, reduce heating when harvested stalks are piled for threshing and to reduce the weight for easier handling in the field.
Shade drying is also practised by farmers particularly for grains intended for seeds. This method of drying is also used in cooling grains, which heat up in storage.
Sun or solar drying of threshed grain, being the cheapest method, is practised by all sectors (farmers, co-operatives, commercial millers, and the government grain agency) in most developing countries. Almost all the 70-90 percent of field harvest retained in the farm is sundried. Women and children in the family usually do sundrying. Co-operatives, private and public sectors handling and processing paddy use this method extensively as shown by the big drying pavements adjacent to the warehouses and rice mills. These pavements are usually undulating and slightly sloping. The surface profile provides for water drainage to the furrow portion and for piling and covering of the grain on the crested portion during drying in the rainy season. Sundrying labour is usually contracted to a crew on per bag or quantity basis. To augment their drying capacities, rice processing co-operatives, private and public sectors have heated air dryers of suitable size, process and system and according to their available resources and other factors.
Shallow bed batch dryers are sometimes used to supplement the sun drying method when processing requirements are comparatively small. The University of the Philippines, Los Baños (UPLB) designed and developed during the late 1950s the one ton capacity shallow bed batch dryer primarily for use by farmers with about 2 to l0 hectares of rice fields. IRRI and other national agricultural engineering research and educational institutions in developing countries modified the dryer as to construction, fuel used and other technological improvements and tried to promote the dryer among farmers and farmer groups. While the design was technically sound in that drying was accomplished to the desired degree and quality, the dryer has not been adopted by small rice farmers because of certain industry constraints, notably the unsound economics involved. There was not enough incentive for farmers to dry their paddy intended for sale. Small volume of rice production by individual farmers, high cost of dryer and drying, new dryer technology requirements, and lack of industrial promotion and after sales services to end users are some of the other constraints enumerated by Andales (l996) in drying paddy. The size of the dryer was not suitable for co-operatives and private sector millers because they require larger capacity units.
Studies have been conducted that high moisture newly harvested paddy could be dried faster without any detrimental effects on the quality and quantity of the milled rice if subjected to a very high temperature for a very short time before final solar or heated air or shade drying is done. Khan et al. (1973) subjected very wet paddy to sand heated to 150 - l80oC and another to direct flame for less than one minute before final drying was done. Results showed that total drying time was reduced by about 50 percent and the quality and quantity of milled rice was improved because of the gelatinization of the grain in the process. Bulaong et al. (1996) reported that subjecting the wet harvested paddy to drying air temperature of 80-90oC for 15-20 minutes reduced the moisture content to about l8 percent which could be further dried in storage or other methods. This same principle is being utilised by the big recirculating mixing or non-mixing dryer where wet grain is subjected to heated air temperature of about 70-90oC for about l0-l5 minutes and placed in an aerated tempering bin before it is recycled to the dryer until the grain is dried. In this method, total drying time is drastically reduced.
Field drying is commonly done by farmers world-wide, particularly in the least developed countries. By definition, natural or field drying involves the reduction of paddy moisture while the grain is still attached to the panicle. This is achieved by letting the moisture of the field crop to decrease or by cutting the stalk and leaving it in the field to dry.
Farmers resort to field drying out of tradition and necessity because threshing equipment or labour is not always available on time. The introduction of mechanical thresher has reduced the time interval between harvesting and threshing. Farmers field dry their crops by laying the cut stalks in bundles on the stubbles, either horizontally or upright with panicles on top. The latter is a better method because of better aeration and exposure to the sun and avoidance of soaking in the undrained field, especially when it rains. However, it is laborious. Stacking the stalks in rectangular or conical piles for about a week until a mechanical thresher becomes available often results in stackburning, especially when the paddy is wet or has very high moisture content. Loss in quality due to yellowing of the grain is attributed to inappropriate field drying particularly, stacking when paddy is wet. Field stacks are however, subjected to re-wetting when it rains and should therefore be amply protected during rainy season harvest.
Majority of farmers rely on sundrying of threshed paddy. The process is normally cheaper than artificial drying and requires no special skills. Essentially, it is a natural method of drying since it relies mainly on solar energy and natural air movement. It is different from natural field drying since conventional sundrying involves drying of threshed grains and requires a drying floor and occasional mixing or turning of the grain to avoid uneven drying or subjecting the grain to excessive temperatures which induce cracking or fissuring.
For small-scale drying operations, concrete pavements such as sports game and open-air courts as well as roads are popularly used when available and tolerated as in some places. Some well-to-do farmers construct their own paved surfaces for drying while smallholders use drying sheets such as nylon nets, gunny sacks, canvass, plastic films, bamboo mats and tarpaulins in lieu of paved floors. Depending on the volume of paddy to be dried at one time, the stirring or turning devices consists of hands or feet, hand rakes and motorised rakes. Most small farmers occasionally sell their marketable surplus paddy either wet as harvested or partially dried.
Sundrying is a labour-intensive operation. Rice mill owners hire labour groups to perform drying operations in the rice mill yard. A commercial large-scale rice mill owner in Thailand innovated a partially mechanised sundrying system on about 2000 square meters of concrete pavement. The equipment consisted of a payloader, a dump truck and a vehicle-drawn rake/spreader. It has mechanical dryers heated by rice hull furnace as back-up.
Harvesting early when paddy is still at high moisture content will minimise shattering losses in the field. The increased volume of paddy production of high yielding rice varieties and the adoption of advanced production technology, have created a problem of drying large quantities of wet grain at the shortest possible time to minimise the risk of spoilage. Fast drying can only be achieved by means of artificial or mechanical dryers. Sundrying is no longer adequate to meet the drying needs especially during the peak harvest of the wet season crop.
The use of mechanical dryers eliminates the problems associated to sundrying. Mechanical drying offers the advantage of timeliness in drying, reducing handling losses, maintaining grain quality, and better control over the drying process.
Mechanical drying of paddy basically involves the heating of ambient air to increase its water holding capacity and then forcing the heated air through the wet grain mass. The basic components of a mechanical dryers consist of a drying bin to hold the grain, a power driven fan or blower to force the air through the grain mass, and an air heating system to raise the temperature of the drying air and increase its moisture absorbing capacity. The forced heated air drying system accelerates the drying process, thus, reducing the drying time.
The choice of the dryer for a particular drying operation depends on several factors such as the drying capacity needed, ease in installation and operation, portability, fuel heat source and the initial cost of acquisition. The most common type of dryers are batch-in-bin, recirculating batch, and continuous-flow.
Batch dryers. In this type, paddy is placed in a drying bin and hot air is forced through the stationary grain mass until the desired moisture level is reached. This can be flat-bed or circular bin type. Specifications of the dryer include an axial-flow fan, internal combustion engine or its equivalent electric motor to drive the fan. Temperature for drying paddy should not exceed 43.3 oC for seed and 54.4oC for food grain.
Flat-bed dryers are classified as shallow bed drying system. Deep-bed batch dryers have grain layer thickness of 2.5-3.0 meters. The recommended airflow rates for this type of dryers range from 3 to 4 m3/min per ton of paddy. The grain is cooled in the same unit for 2 to 4 hours using ambient air.
Continuous flow dryers. Mostly used for large-scale commercial rice mills, the continuous flow drying process has large capacity requirement. The system has advantages of shorter drying time, larger volume of paddy handled, and more uniform drying of the grains over the batch-type drying system.
This system involves the movement of both the drying air and the grains in either cross-flow in counter-flow manner. The LSU and the columnar dryers which may be of the mixing and non-mixing are the most common types used.
Columnar dryers use large airflow rates/ton of paddy. It is a recirculation batch dryer with high capacity and drying air temperature of 60-88oC. The column has perforated metal sheets on both sides allowing a grain layer thickness of 50 cm across through which air flows. The other features are the presence of a holding bin, an elevating device at the discharge side, and a return conveying system.
The LSU dryer offers the largest drying capacity of 1-10 tons per hour. Drying of large volumes of wet paddy is accomplished by exposing the paddy to a high temperature of about 66oc. Several passes of 15-30 minute-exposure per pass are required for complete drying. After each pass, paddy is transferred to tempering bins to allowed cooling and migration of moisture to the grain surface before the next pass. Tempering period ranges from 4-8 hours. Depending on the initial moisture content, drying may be completed in 2-5 passes. Tempering bins and conveyors are integral parts of the system.
The following are some of the constraints to adoption of artificial drying technologies for rice:
(a) Mechanical drying suffers from high fuel cost;
(b) There are farmers with small land-holdings and the volume of paddy produced is small and can easily be sundried;
(c) Some believe that because of the bleaching effect from the sun sundried paddy results in whiter grains than artificially dried paddy;
(d) There is lack of capital to invest in mechanical dryers;
(e) There is lack of knowledge about the technology.
Some of the farmers' perceptions about mechanical drying are as follows:
(a) Mechanical drying entails high fuel costs;
(b) Mechanical dryers are only used when sundrying is not possible due to bad weather;
(c) Utilisation period of the equipment is short;
(d) There is limited volume of production to justify dryers;
(e) Poor quality is produced from mechanical dryers;
(f) There is lack of training on the part of the mechanical dryer operator.
Rice millers have the following perceptions about mechanical dryers:
(a) Mechanical drying entails high fuel costs;
(b) Mechanically dried paddy is of poor quality;
(c) Mechanical drying operation requires more attention;
(d) Flat-bed dryers have limited capacity especially during peak wet harvest season.
The traditional sundrying of rice is carried out on any of the following surfaces by small landholders and landless workers who are paid in kind for their harvest labour: concrete pavements, earthen yards surfaced with cattle dung and clay mortar, plastic sheets, woven bamboo or palm leaf mats and fine mesh net overlays on hardened earth or grass area. Sundrying is low cost, utilises free heat energy and does not need any machine except for simple and home-made stirring devices and scoops. However, this method is adapted for small quantities of paddy at a time and even then, often brings problems of spoilage during the monsoon season harvest due to inadequate sunshine hours and rain interruptions.
Women usually do the stirring and tending of the paddy being sundried to shoo away birds, chickens and sometimes livestock. Men assist them in carrying the bagged paddy to and from the drying area. Commercial rice mills generally have large-area concrete pavements and a core or contracted drying crew for the purpose. In one large-scale rice mill in Thailand, sundrying is partially mechanised with a tractor-drawn rake for spreading and stirring, a dump truck for unloading the paddy on the pavement and a payloader for loading the paddy onto the dump truck.
Artificial or mechanical dryers are generally expensive to establish even the small one-ton capacity due to the structure, equipment, storage space and labour required. Attempts to popularise these small-scale dryers among farmers or groups of farmers have met only partial success and then only in exceptional cases such as viable co-operatives. There is very little, if any incentive for the farmer to dry paddy and sell it as such.
Commercial-scale dryers normally operate in conjunction with rice mill operation or enterprise because of the low appraisal for value added in drying in spite of the necessity for proper drying. The drying operation is usually not a stand-alone enterprise as it may not be economical to do so especially on a small scale. The heating of air and forcing it through the grain mass as well as the moving of the grain mass as in the circulating types entail large amounts of heat and mechanical energy which may only be feasible in commercial scale drying and milling operations. The labour usually used in such mills are males as manual lifting and moving are required in short-run moving, loading and unloading of the grains within the premises of the rice mill. Table 3.5.2, Annex 3.5 T gives information on types, capacities, source of supply and prices of grain dryers in the Philippines as of October 1995.
The University of Agriculture and Forestry (UAF) in Vietnam has developed a so-called very low cost dryer (SRR) claimed to be suitable for small holders who may want to store only about one ton of paddy for own family consumption. It is made of bamboo mat cylindrical container and an electric powered heater and blower for slow and low temperature in-store drying. The technology is attaining wider acceptability among farmers in Vietnam through the extension efforts primarily by UAF. Figure 3.5.1, Annex 3.5 F shows the design of the SRR.
Sundrying, the method commonly practised for rural family-consumption paddy in developing countries, is mostly done by women. Since the drying space available is usually limited, the drying batch is limited. When the paddy is harvested very wet as in the case of rainy weather, the paddy is aerated at the same time to prevent spoilage. Sundrying has become to some extent, a family activity during the harvest season with the smaller children assigned as watchers to shoo away birds and domestic fowls from the drying area.
In introducing new drying technologies either to the farmers or to commercial rice mill operators, the aspect of economics of operation is a major factor in decision-making. If the eventual rice product is for the high-end market, that is, for high quality and high price, investments in more sophisticated mills may come as an easier decision. However, since rice is often a political commodity, the government may have a direct or indirect control on pricing of rice. Usually, the high-grade rice is exempted from such regulations but the quantity may be restricted to avoid politically damaging shortages of the staple for the masses that cannot afford the high quality rice.
A new technology replacing sundrying of household-scale paddy for own consumption with artificial drying may meet resistance unless the economic advantages of the new technology are obvious. The Vietnam SRR dryer mentioned above may show some promise but the additional investments, even if small, may hinder the adoption of the technology under most circumstances. The effect on the traditional role of women in the drying operation may change as the technology changes. In the conservative society of the rural area, the change in gender roles demanded by the new technology may also hinder the adoption of the technology even if it is recognised as superior over the traditional one.
Drying of paddy is done on the farm as well as off the farm. Drying could be accomplished by field drying, conventional sundrying or by artificial drying. Drying methods or techniques vary from place to place and are dictated by the farmers' socio-economic condition or the degree of awareness of post-harvest technology.
The dryers listed in Table 3.5.1, Annex 3.5 T are used by farmers, co-operatives, and the private and public sector users involved in post-harvest handling and processing of paddy. The kind of machine and process used depends upon the season, quantity and moisture contents of paddy handled.
Field drying is practised solely by the farmers. This method is resorted to during rainy season or harvesting immediately after the rain to remove surface moisture on the cut panicles, grains, reduce heating when harvested stalks are piled for threshing and to reduce the weight for easier handling in the field.
Shade drying is also practised by farmers particularly for grains intended for seeds. This method of drying is also used in cooling grains, which heat up in storage.
Sun or solar drying of threshed grain, being the cheapest method, is practised by all sectors (farmers, co-operatives, commercial millers, and the government grain agency) in most developing countries. Almost all the 70-90 percent of field harvest retained in the farm is sundried. Women and children of the family usually do sundrying. Co-operatives, private and public sectors handling and processing paddy use this method extensively as shown by the big drying pavements adjacent to the warehouses and rice mills. These pavements are usually undulating and slightly sloping. The surface profile provides for water drainage to the furrow portion and for piling and covering of the grain on the crested portion during drying in the rainy season. Sundrying labour is usually contracted to a crew on per bag or quantity basis. To augment their drying capacities, rice processing co-operatives, private and public sectors have heated air dryers of suitable size, process and system and according to their available resources and other factors.
Shallow bed batch dryers are sometimes used to supplement the sun drying method when processing requirements are comparatively small. The University of the Philippines, Los Baños (UPLB) designed and developed during the late 1950s the one ton capacity shallow bed batch dryer primarily for use by farmers with about 2 to l0 hectares of rice fields. IRRI and other national agricultural engineering research and educational institutions in developing countries modified the dryer as to construction, fuel used and other technological improvements and tried to promote the dryer among farmers and farmer groups. While the design was technically sound in that drying was accomplished to the desired degree and quality, the dryer has not been adopted by small rice farmers because of certain industry constraints, notably the unsound economics involved. There was not enough incentive for farmers to dry their paddy intended for sale. Small volume of rice production by individual farmers, high cost of dryer and drying, new dryer technology requirements, and lack of industrial promotion and after sales services to end users are some of the other constraints enumerated by Andales (l996) in drying paddy. The size of the dryer was not suitable for co-operatives and private sector millers because they require larger capacity units.
Studies have been conducted that high moisture newly harvested paddy could be dried faster without any detrimental effects on the quality and quantity of the milled rice if subjected to a very high temperature for a very short time before final solar or heated air or shade drying is done. Khan et al. (1973) subjected very wet paddy to sand heated to 150 - l80oC and another to direct flame for less than one minute before final drying was done. Results showed that total drying time was reduced by about 50 percent and the quality and quantity of milled rice was improved because of the gelatinization of the grain in the process. Bulaong et al. (1996) reported that subjecting the wet harvested paddy to drying air temperature of 80-90oC for 15-20 minutes reduced the moisture content to about l8 percent which could be further dried in storage or other methods. This same principle is being utilised by the big recirculating mixing or non-mixing dryer where wet grain is subjected to heated air temperature of about 70-90oC for about l0-l5 minutes and placed in an aerated tempering bin before it is recycled to the dryer until the grain is dried. In this method, total drying time is drastically reduced.
Cleaning of paddy involves the separation of undesirable foreign matter or materials other than grain and leaving a cleaned paddy for storage and processing. Depending on the production management, harvesting, threshing and handling methods used, the field-processed paddy may contain various other crop and weed seeds, straw, chaff, panicle stems, as well as empty, immature and damaged grains. Sand, rocks, stones, dust, plastic bits and even metal and glass particles can contaminate the grain bulk due to careless handling after threshing of the paddy. The cleaning or separation process utilises the differences in aerodynamic and other physical properties of the paddy grain from the other materials.
In most developing countries where threshing is done manually, paddy is usually contaminated by a large percentage of foreign matter. Rough paddy cleaning is accomplished in the field right after the threshing operation. It consists of hand raking and sifting the bits of straw, chaff and other large and dense materials from small piles of paddy followed by hand winnowing against the breeze. In some places, a hand/pedal-operated blower or an engine-powered fan is used to remove the chaff, dust, weed seeds and other light materials from the paddy. Cleaned paddy commands higher price than the non-cleaned paddy and therefore there is incentive in cleaning it. If the harvesting and threshing labour is paid for in kind, the farmer will demand that the paddy be cleaned. Rice for home consumption is desired to be clean. Women normally do the cleaning process of the paddy in the field and finally, of milled rice in the kitchen prior to cooking.
Cleaning of grain involves the separation of bulk straw, chaff, empty kernels, and very light and fine impurities from the grain. In the simplest form, and chaff is manually separated and the grain is dropped through a crosswind to remove the light impurities. Air can only remove impurities that have different aerodynamic properties from the grain. In the hold-on type of thresher, a major portion of the straw does not pass through the machine, and only the removal of chaff and light impurities from the grain is necessary. This requires pneumatic means and in some cases the combination of screen and air is required.
During sundrying, paddy may be further contaminated with sand, soil, stones, animal excreta, fowl droppings and bits of other biological materials depending upon the place, the activities involved, and the techniques used.
In small-scale and village rice mills, no prior cleaning is usually done on the paddy except perhaps using a screen or sieve on top of the hopper of the paddy husker. The absence of effective cleaning devices often result in poor quality milled rice in terms of contaminants. Stones and other hard object admixtures in the paddy also shorten the life of the milling machinery. The milling recovery is usually low.
In commercial large- and medium-scale rice mills, scalping is the first stage in the industrial process where most foreign matter is removed to reduce drying cost, eliminate clogging or damage to conveying equipment and prevent paddy deterioration during storage due to high-moisture spots of non-grain organic matter. The second cleaning stage occurs after storage prior to milling process to remove the remaining foreign matter that could damage the milling machinery and affect the grain quality or grade of milled rice and therefore, its market value. Cleaning devices may include vibrating or rotating sieves, aspirators, destoners and magnetic separators.
In small-scale production and processing, field-threshed and partially cleaned paddy is bagged in jute or propylene sacks for handling purposes in transporting paddy from the field to the roadside or to the house. The weight of each bag ranges from 30 to 100 kg depending upon the trading practice in the locality or country.
In large-scale and mechanised rice production operations where the combine is used, the paddy is not packaged but instead delivered in bulk to the rice mill or drying compound. From the combine hopper, paddy is transferred by means of an auger conveyor to a waiting lorry or wagon at the roadside or alongside the combine depending upon the trafficability of the field.
Milled rice, the final product for marketing, is packaged in polyethylene, propylene or jute sacks in weights ranging from 1 kg to 1000 kg depending upon whether the market is for retail or wholesale or for export. Higher quality rice normally retailed in speciality groceries and in supermarkets is packed in attractively labelled packages made of polyethylene, propylene, jute and paper bags or cardboard boxes. Brown rice, which has a special market, is packed in sealed polyethylene bag inside the cardboard box or the outer bag. This is to increase the shelf life of the grain, which is prone to rapid rancidity due to the free fatty acid in the bran.
Rice is retailed in small village stores and displayed in their original large sized sacks or in wooden bins and labelled as to variety and price per unit weight or volume as may be required by law in some developing countries. In this case, purchased rice is weighed or measured and packed in plastic bags or other container brought in by the customer.
Paddy is a seasonal crop generally harvested once a year. In fully irrigated, well-developed farming systems in the tropics, paddy can be planted and harvested throughout the year. As a staple food in most of the rice producing countries, harvested paddy must be dried, cleaned and stored as a source of food supply until the next harvest. Not all of the paddy produced are retained by the farmers. Decisions on how much to retain is influenced by previous cash commitments, labour scarcity at harvest, transport difficulty, weather conditions, and lack of handling and storage facilities, current prices and immediate source of cash during emergencies. Up to about 70-90 percent of farm produced in the Asian region is retained in the farms. The rest is deposited/sold to agricultural co-operatives and/or sold to the private and public sectors.
Rice in either milled or paddy form, is stored to provide a buffer stock of the staple, the amount, form, and sophistication of which depend upon whether the level of storage is household or own consumption, rice mill, wholesaler, retailer or distributor and government logistics. At any level and scale of storage, drying of paddy to the moisture content level of about 14 percent is a basic requirement to prevent spoilage. Uniform drying and prevention of moisture spots and moisture migration inside the grain mass by means of aeration is essential especially in large-scale storage where metal or concrete silos are used. Proper storage for seed purposes is necessary to maintain its viability. Hence, the storage structure must protect the paddy from extreme heat or cold, moisture levels at which the seed will spoil and be subjected to microbial or fungal attacks, insect pests and rodent consumption or damage. The place of storage is as important as the storage structure itself as the storage container should be protected from the weather elements and other stresses such as heat from fire and possible damage or structural failure due heavy loads.
At the farm household level, storage is usually equated to food security or as commodity bank for conversion to cash when the need for it arises. As the small-scale or marginal farmers do not have the resources to store large amounts of grain or to have large storage facilities, they usually sell out their paddy to traders or buyers immediately after harvest. No further processing such as drying, cleaning, and grading is done because of the immediate need for cash and the lack of incentive to dry the paddy as the price differential between wet and dried paddy is usually marginal. Therefore, they only dry for safe storage, the amount of paddy for their own consumption until the next harvest or a little more for cash conversion in time of need for cash or for a better price. Otherwise they sell the surplus paddy. These small farmers as well as the landless rural workers who earn their harvesting labour in kind store their paddy in bamboo, wooden or metal bins of about one ton capacity. In some countries the common storage method is by using jute or propylene sacks. These storage sacks or bins are secured against theft and are protected from rain or moisture, insect pests, and rodents.
Commercial-scale rice mills have drying and storage facilities for paddy and for milled rice. Paddy needs to be dried to about 14 percent moisture content for safe storage. As the mill has to procure large stocks of paddy for year-round milling, the drying and storage capacities of the facilities are balanced with the milling and the distribution capacities or outputs. Paddy is sometimes stored in bulk on floor or platform with built-in ducts for heated air drying and subsequent aeration. The warehouse is within the rice mill compound which invariably has a sundrying pavement also even if mechanical drying is the main drying system.
Milled rice wholesalers usually store their product in sacks which are stacked in secured warehouses using one or a combination of sandwich, window or block stacking technique to permit maximum air flow through the spaces and maintain aeration. Where space permit and there is need for longer storage period either by design to wait for better prices or there is an overstocking, retailers also stack their rice bags in such a way that aeration is maintained. This is to prevent rice from deteriorating in quality due to moisture absorption. Rodent traps and other means of controlling them and insect pests are also instituted by necessity. Keeping only enough inventory is also a way of avoiding storage problems by retailers.
In the mountain areas in the Philippines, special rice varieties are for ceremonial or special purposes, such as wedding, other celebrations and rice wine making. Dried paddy in panicles, are usually stored on a platform above the kitchen to protect the grains from insects and varmints.
Government storage systems in countries where rice is a regulated commodity usually have standards following international practice or adaptations from those of developed countries. Their establishment usually has benefited from foreign expertise assigned by international aid or financial organisations or by the suppliers of storage equipment and facilities. Nevertheless, operational problems leading to quality deterioration or spoilage losses of paddy or milled rice do occur because of lack of sustained training of personnel in technical requirements, occasional lapses in management and other causes.
In India, Nepal, and other developing countries, sheet metal storage bins were promoted among rural households by the governments in line with the "Save grain" program of FAO. One of the major purposes is to protect the rice or paddy from attacks of insects and rodents. This is aimed at improving the traditional wooden bin or jute sack storage method in terms of avoiding or minimising losses of grain while in storage.
The traditional storage structure is usually a crude container made of woven bamboo or palm leaves or wood. The design is simple but the maintenance of the storage integrity is usually not ideal as spoilage due to high grain moisture, wetting by rain due to inadequate protection from rain, storm or flood, dirt contamination, insects, rodents and losses due to theft or pilferage and grain retrieval, collapse of the storage structure, and complete loss due to flood and fire.
The commercial grain storage silo is made of either sheet metal or concrete. In the developing countries, the metal silos are usually imports from developed countries but due to storage problems brought about by the humid conditions and also of their doubtful sustainability as shown by past experiences with them, such storage structures have never become popular in the humid tropics. However, concrete silos are common in these countries but the problem is the over capacity or inadequate rice production except in the rice-exporting countries. Such silos must have aeration and stirring systems to maintain the ideal storage moisture of the grain and to minimise moisture migration and concentration. They must also have the necessary loading and unloading equipment integrated with the grain receiving, drying and milling operations.
(a) Own consumption. Rural households set aside a portion of the rice production or acquisition during harvest for food security. For this purpose the paddy or milled rice must sustain its integrity in terms of quality and quantity by preventing spoilage and other losses;
(b) Commodity bank or financial security. In rural households, extra supply of rice is stored together with that for own consumption for conversion into cash in time of need. This is in lieu of selling the paddy outright and putting the money in the bank which may be strange to the farm household;
(c) Seeds for the next season. A rice farmer would normally select seeds from the standing crop and store it properly and sometimes differently from the method used for paddy for own consumption. This additional or special care is aimed at maintaining seed viability or high percentage of germination;
(d) Expecting a better price for paddy or rice. For a marginal farmer, the timing of sale of stored paddy or rice is not as much for a better price as for the need for money. For a merchant such as a wholesaler, trader, retailer, or rice mill owner, some rice stocks may be earmarked for sale during lean months (during low supply of staple as a certain period before harvest time) to get a better price. However, when a rice supply crisis occurs, such merchant may not be allowed to hoard the stocks and may be compelled by the government to sell not at the ideal time and at a certain regulated price;
Table 3.7.1, Annex 3.7 describes the different storage facilities.
Farmers store their paddy in traditional and non-traditional structures primarily for food security until the next harvest. Also, as a source of cash during emergencies, for seeds, for future increase in price of paddy during the lean months, and for anticipated future festivities. Paddy retained for storage are sun-dried several times and cleaned before loading to the storage containers. Although farmers do not have moisture meters, they know by experience the dryness of the grain appropriate for storage. Grain dryness is determined by pressing hard a bunch of grains on the hand and/or biting several grains to determine hardness. Usually, a fully dried grain is hard. Paddy is usually stored with 14 percent moisture content or lower. Storage containers are checked, cleaned and repaired if necessary, before loading the grain. Paddy is stored until the next harvest season or for 6-12 months.
Very few farmers apply insecticide on food grains before or during storage, probably due to small quantity stored and relatively short storage duration. Grain used for seed requires different treatments. It must be well-dried to about 12 percent m.c.; cleaned to remove all immature and empty grains; mixed with insecticide powder; and stored preferably in a sealed container. Any available insecticide dust applied at the proper dosage rate will be satisfactory. The main causes of loss in storage are rodents, moulds due to moisture, mites, insects, spillage, and sometimes theft. Losses in farm storage have been estimated to reach up to 6.2 percent.
In some countries, agricultural co-operatives are formed by farmers to handle their marketing problems and make available loans in cash or kind intended for farm inputs including household supplies. Paddy acquired from co-operative members (as deposit or purchase) and purchased from non-members are processed, stored, and marketed as paddy or milled rice. Facilities such as rice mills, drying pavements, mechanical dryers, warehouses, transport, and other accessories are made available. Because of their wide community operations and established operating systems dealing directly with the farmers, co-operatives are usually effective and beneficial as a purchasing and marketing arm.
Paddy is usually stored in bags which are stacked inside warehouses. Some are stored in bulk using bins or silo associated with the drying and milling operations. Fumigation, aeration and the maintenance of clean warehouses are considered good warehouse management practices.
Co-operatives help the public sector in making grains available to the consumers and in maintaining a stable price of the commodity between the producers and the consumers.
Private traders operate mainly for profit. They have drying facilities such as concrete pavements and mechanical dryers, rice mills, warehouses, and transport facilities. Paddy is purchased dried, stored, processed and marketed as paddy or milled rice. Paddy/milled rice are stored in bags stacked inside a warehouse. Storage is usually less than six months and depends upon rapid turnover for more profit or, in some cases to pass on the problems leading to losses and costs to the consumers. With this kind of operation, they do not have the incentives or facilities for disinfestation. Warehouses of poor design and low construction standards built at minimum cost are often not suitable for proper management and storage of food grain for food security.
Public agencies are involved in processing up to about 30 percent of paddy produced in the country. The storage structures used vary from bulk pile to fairly advanced systems of modern structures conforming to good warehousing practices. These practices include monitoring and maintaining temperature of the grain, waterproofing, insulation, rat and bird proofing and regular pest control. Causes of losses on the public sector storage are insect, rodents, birds, moisture (moulds), theft and pilferage. Except for theft and pilferage, losses of up to 6.6 percent was assessed due to these factors.
The methods and storage structures used in the public sector vary and are adapted to existing conditions, available resources and technology adopted in the country. Grains are stored in bags and bulk with each having comparative advantages and disadvantages.
The Indian Grain Storage Institute having developed 95 designs of improved farm level storage structures with various capacities. The Institute also devised a code of scientific storage practices suited to farm level storage and measures to improve on traditional storage structures used by Indian farmers. The propagation of scientific storage practices was being undertaken by the Central Save Grain Teams in collaboration with the State Governments (Bansal, 1986).
Storage structures used for paddy are listed in Table 3.7.2, Annex 3.7. Storage structures used by farmers in the different rice producing countries vary depending upon materials available, technology accepted and practised, weather conditions and availability of non-traditional materials in the area.
Recent advances in long-term cereal storage include the hermetic or sealed storage and controlled atmosphere storage (CAST). As the name implies, the containers in hermetic or sealed storage are made airtight after storing the cleaned and dried grain. Oxygen will be depleted inside the storage container to the point where growth of insects, moulds, and fungi are restricted. In CAST, the air inside the sealed storage containers is replaced by modified atmosphere that is insecticides, prevents mould growth and quality deterioration of the commodity. The modified atmosphere inside the storage containers can either be: (a) low- oxygen atmosphere generated by purging with nitrogen; (b) low-oxygen atmosphere produced and maintained by the combustion of hydrocarbon fuels or (c) high-carbon dioxide atmosphere (35-80 percent). Although hermetic or sealed storage and CAST are effective in long term preservation of cereals, it is not popular in developing countries because of its cost, technology required and associated equipment needed to use the method.
Stored paddy is lost due to moisture (moulds and fungi), vertebrate pests (rodents and birds), insects, mites, spillage and possibly theft or pilferage.
Paddy is a living, hygroscopic, and biological food materials that respire at a rate dependent upon its moisture content and surrounding conditions. Respiration rate increases with moisture content of the grain. Heat, a by-product of respiration, is released in the process and retained in the grain mass because of the insulating effect of the rice husk. High moisture content will increase respiration rate, heat evolved and ultimately grain temperature which leads to deterioration. Also, high moisture makes the grain soft and susceptible to insect attack and mould growth resulting in increased grain loss. Loss due to vertebrate pests (rodents and birds) is possible only when their access to the grain is possible. Insect eggs or adults already in the grain prior to storage, adults in the crevices inside the container or openings in the container are the means by which insects gain access to the stored grain.
The above factors must be carefully considered in storing paddy. Grain for storage must be well cleaned and appropriately dried. Moisture content of 14 percent can be stored up to six months depending upon the container and surrounding conditions. Lower grain moisture content than 12 percent is appropriate for storage of more than six months. Storage containers must be free from large and small openings to prevent vertebrate pests and insects to gain access to the stored grain. It must be located in a cool dry place with minimal temperature variation within the container to prevent moisture migration within the stored grains. Paddy exposed to damp or very humid conditions absorbs moisture increasing its moisture content and resulting in shortened storage life.
Majority of the storage containers used by farmers (open top woven bamboo or straw baskets with or without mud/dung plaster and jute or propylene bags) do not conform with the above criteria for good storage because moisture, pests and insects can easily gain access to the stored grain. Farmers do not usually consider storage losses significant because of the short duration of storage. Grain is consumed before infestation is discernible. Woven bamboo or straw basket with mud/dung plaster on both sides provided with cover to seal insect passage, and metallic drums or GI bins, if available, is recommended for on-farm storage. Any other structure that can satisfy the above criteria can be used particularly if materials and technology is locally available.
Bag storage is used by all sectors in storing paddy for both short and long duration although it does not conform to the above criteria. It is recommended that this method be used for short duration of up to three months and when grain is stored longer, frequent inspection for any sign of rodent, insect, heating and mould on the stored grain. Necessary steps should be applied to remove the cause of grain loss, that is, plugging possible entries of vertebrate pests, fumigation to kill insect infestation and aeration or re-drying of grain. In addition to cleaning and drying of the grain to about 12 percent, bags must be cleaned and treated to kill hiding insects. Bags must be piled on dunnage with provision for aeration or free air circulation.
Bulk storage in silos made of bricks, concrete, metal or mud straw (China) is commonly practised by the public sector and sometimes by the co-operatives and commercial millers. It is recommended that when this system is used for any length of storage, associated auxiliary equipment to dry, aerate, transfer, transport and monitor the temperature of the grain inside the silos must be made available. Regular inspection of the grain and fumigation if insect infestation is observed must be done.
In addition to the above recommendations, the surrounding and the inside areas of the warehouse must be regularly cleaned. Also, the warehouse and storage containers must be fumigated before the seasons' harvest is stored.