At harvest time, the cut portions of the plant may contain too much green plant matter while the grain may not have reached a uniform degree of maturity and may have too high of a moisture content. Pre-drying is the stage of the post-harvest system during which the harvested product is dried in order to undergo the next operation of threshing or shelling under the best possible conditions. The chief criteria for evaluating the quality of a given batch of grain are:
Drying of the grain is done to lower the moisture content in order to guarantee conditions favourable for storage or for further processing of the product. Drying permits a reduction of losses during storage from causes such as:
This is the phase of the post-harvest system during which the impurities mixed with the grain mass are eliminated. After drying it is important to remove the impurities (earth, straw, small pebbles, plant and insect waste, weed seeds etc.) from the grain. These contaminants hinder drying operations and make them longer and more costly. They lower the quality of the product and are also a focal point for potential infestation during storage. Cleaning Devices Some cleaning devices used are winnowers, pre-cleaners and cleaner-separators.
Whether simple huts that farmers have turned into storehouses or modern well-equipped warehouses, storage buildings must meet the following requirements.
The main products of milling are brown (cargo rice), white rice, and parboiled rice. The by-products include broken, bran and chips. Milling of paddy is carried out by rice factories scattered throughout the five rice growing regions. Milling capacity ranges from 0.5mt/hr to 20mt/hr with the total milling capacity countrywide being 190.95mt/hr. Both multistage and single stage types of mills are used. Parboiled rice is different from milled rice in that the paddy is subjected to a hydro-thermic treatment before milling. Essentially the paddy is steeped in water until it attains a moisture content of between 30 - 35%. The paddy is then steamed, dried down to 12% moisture content and then milled. The traditional parboiling process involves soaking rough rice overnight or longer in water at ambient temperature, followed by boiling or steaming the steeped rice at 100°C to gelatinize the starch, while the grain expands until the hull's lemma and palea start to separate. The parboiled rice is then cooled and sun-dried before storage and milling Modern methods involve the use of a hot-water soak at 60°C (below the starch gelatinisation temperature) for a few hours to reduce the incidence of aflatoxin contamination during the soaking step. Leaching of nutrients during soaking aggravates the contamination, with the practice of recycling the soak water. Soaking sound, rough rice in water inoculated with Aspergillus parasiticus did not result in aflatoxin contamination of parboiled rice (Yap et al, 1987), suggesting that contamination probably has to be present in the grain prior to soaking (Bandara, 1985). Parboiling gelatinizes the starch granules and hardens the endosperm, making it translucent. Chalky grains and those with chalky back, belly or core become completely translucent on parboiling, a white core or centre indicates incomplete parboiling of the grain. Value added rice products may be derived from rough rice, brown rice, milled rice, and brokens. The following are two examples of value added products. Pre-cooked and quick-cooking rice Pre-cooked rice is used for rice-based convenience food products in which non-rice ingredients are packed separately and mixed only during heating. One way this rice is made is by hermetically sealing cooked non-waxy and waxy rice in laminated plastic or aluminum - laminated plastic pouches and pasteurizing at 120 degrees centigrade under pressure . Quick - cooking rice require significantly less cooking time than raw milled rice, (15 to 20 minutes). Various methods are employed to fissure raw rice or to dry cooked rice to produce a porous structure. Dry-heat methods include heating milled and brown rice with 57 to 82 degrees centigrade air for 10 to 30 minutes or with 272 degrees centigrade air for 17.5 seconds to fissure the grain. Parboiled brown rice may be made quick-cooking by scouring about one percent by weight of the pericarp to remove the outer water-impervious layer ( Desikachar, Raghavandra Rao and Ananthachar, 1965). Precooked quick-cooking rice processes include soaking-boil-steam-dry, gelatinize-dry-puff, gelatinize-roll, or bump-dry, freeze-thaw, gun puff, freeze-dry and chemical treatments (Roberts, 1972). Dry precooked rice cereal This is produced by preparing and cooking a cereal slurry, which is then dried in a double-drum atmospheric dryer, flaked and packed ( Brockington and Kelly, 1972). The slurry solids, drum speed and temperature and spacing between drums are carefully controlled. Hydrated precooked and ready to eat infant foods must have the right consistency, soft enough to be swallowed easily but thick enough to feed without spilling. Malt and fungal a-amylase may be added to control the quality of liquid required to reconstitute the dried cereal and to sweeten it by partial hydrolysis of the starch. Other examples of value added products are:
In milling, some of the grains are normally broken, these have been used to make rice flour, rice wines and beer, rice curls and cakes. It is well known that rice makes delicious cakes, pastries, cereals and tasty snacks such as rice crispies. Rice cakes and pastries are produced on a small scale locally. The rice industry is known to yield a number of by-products which could be potentially utilised. The following are some by-products and their uses. Rice Hulls In local manufacture the by-product which consist of a mixture of hulls and chaff is used as fuel for the steam furnaces. In 1961 waste rice hulls were investigated in the ceramic industry and in 1971 in the making of light weight concrete (BUDHU, 1971). Its potential had been investigated by IAST but there have been no follow up publications since. Other uses (not done in Guyana) of rice hulls have been elaborated by Ramgopal (1960). Ground hulls are also to some extent as a filler in mixed feeds. They are also used as supporting medium in hydroponic tank, insulating material, filter in plastic and as a refractory material in the manufacturing of furfural, a product urilised in making synthetic rubber, rayon and similar materials. Rice Bran Rice bran contains oil, the content ranging from 12-22%. After the oil is extracted the bran could be used as a valuable livestock and poultry feed since it would not go rancid and it could have a higher percentage of protein. Polishings After the hull and bran have been removed, a thin coating known as rice polish remains. Besides being an important ingredient of livestock feeds, rice polish has a high food value. Also, it provides nutritious food for fish and the more refined quality is used for human consumption in the form of porridge and as a Vitamin supplement (Ramgopal, 1966). Rice Straw According to Ramgopal (1966), in most rice producing countries large quantities of straw were used as fodder for livestock, as fuel and as raw material for minor industries. He also mentioned that in Korea, rice straw was combined with sodium silicate for the production of board, which had developed unto a thriving industry. Other known uses of rice straw include the making of hats, handbags, ropes, paper and sacks. References
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