India is the second largest producer of rice in the world. The mechanized sector of rice milling industry handles more than 45 million tonnes of paddy annually. About 10-15 million tonnes are processed through large scale modern mills involved in producing fine quality of rice for export market.
Milling is a crucial step in post-production of rice. The basic objective of a rice milling system is to remove the husk and the bran layers, and produce an edible, white rice kernel that is sufficiently milled and free of impurities. Depending on the requirements of the customer, the rice should have a minimum of broken kernels.
Many crop management factors have an impact on the quality of paddy. A sound paddy kernel, one that is fully matured and not subjected to physiological stresses during its grain formation stage.
Timely harvesting, threshing, drying, and stored properly can result in the production of good quality milled rice. Mixtures of chalky and immature kernels, mechanically stressed grain during harvesting threshing, delays in drying, and moisture migration in storage can result in broken and discolored milled rice. Blending/mixing different varieties with different physico-chemical properties during the post-harvest operations contribute to a large extent in the lowering of the milled rice quality produced. Purity is related to the presence of dockage in the grain. Dockage refers to material other than paddy and includes chaff, stones, weed seeds, soil, rice straw, stalks, etc. These impurities generally come from the field or from the drying floor. Unclean paddy increases the time taken to clean and process the grain. Foreign matter in the grain reduces milling recoveries and the quality of rice and increases the wear and tear on milling machinery.
Modern rice milling processes consist of :
Pre-cleaning: removing all impurities and unfilled grains from the paddy
Husking: removing the husk from the paddy
Husk aspiration: separating the husk from the brown rice/unhusked paddy
Paddy separation: separating the unhusked paddy from the brown rice
De-stoning: separating small stones from the brown rice
Whitening: removing all or part of the branlayer and germ from the brown rice
Polishing: improving the appearance of milled rice by removing remaining ran particles and by polishing the exterior of the milled kernel
Sifting: separating small impurities or chips from the milled rice
Length grading: separating small and large brokens from the head rice
Blending: mix head rice with predetermined amount of brokens, as required by the customer
Weighing and bagging: preparing milled rice for transport to the customer
The major portion of paddy is still processed through hullers which are usually low capacity mills and result in very high percentage of brokens. In these hullers both shelling and polishing operations are carried out simultaneously and there is no control on the polishing of rice. As a result impure bran mixed with husk is obtained and a higher breakage of rice of rice results in loss of revenue. To overcome these problems it is necessary to carry out the shelling and polishing in two separate units.Mini Rice mill
A mini rice mill has been developed by CFTRI, Mysore and many manufacturers have sprung up having their own versions. The sheller is a compact unit designed on the densimetric classification principle; the polisher could be light either vertical cone polisher or a horizantal roller polisher.
The main byproducts of rice milling are rice hulls or husk, rice bran, and brewer's rice. Rice hulls are generated during the first stage of rice milling, when rough rice or paddy rice is husked. Rice bran is generated when brown rice moves through the whiteners and polishers. When paddy is hand-pounded or milled in a one-pass Engleberg steel huller, rice bran is not produced separately but mixed with rice hulls. Brewer's rice is separated produced when milled rice is sifted.
100 kg of paddy rice will generate approx 20 kg of husk. Rice husk contains 16 to 22% ash, which is high in silica. Common use of rice husks are as bedding materials, and as source of energy. In the modern rice milling industry, rice husks are increasingly being used as a fuel source for grain drying and parboiling, and for electricity generation. In Bangladesh, rice hulls are the preferred fuel for parboiling.
Using rice husk in gasifyers or furnaces that are used in small and medium size plants Asia produces black ash which still contains around 20% carbon. This so called carbonated rice husk is often used as soil conditioner for poor soils, as an ingredient for bio fertilizers and currently research is elaborating it's potential for carbon sequestration in soils to reduce greenhouse gas emissions.
- 100 kg of paddy rice will generate approx 5 to 10 kg of bran. Rice bran is a mixture of substances, including protein, fat, ash, and crude fiber.
- Rice bran has a high nutritive value. Besides proteins, rice bran is an excellent source of vitamins B and E. Bran also contains small amounts of anti-oxydants, which are considered to low cholesterol in humans. Rice bran contains 10-23% bran oil.
- Bran oil, once stabilized and extracted, is a high quality vegetable oil for cooking or eating. The conventional use of rice bran is as ingredient for animal feeds, in particular ruminants and poultry.
Brewer's rice is often used as ingredient for beer brewing, hence the name. In rural areas, brewer's rice has a variety of uses including ingredient for rice flour and rice noodles.