milling

Byproducts and their utilization

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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.

Rice husk

100 kg of paddy rice will generate approx 20 kg of husk.  The bulk density is 100 to 150 kg/m3.  Rice husk contains 16 to 22% ash, which is high in silica.  The ash composition and structure give rice hulls an abrasive character.  Metal surfaces in frequent contact with rice hulls will wear out and eventually puncture.

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, and rice hulls are widely used for grain drying in the larger rice mills in Northern India.  Rice hulls, once ground, are also used as ingredient in animal feeds.

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.

 

Rice bran

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.  In many cases, bran contains tiny fractions of rice hull, which increases the ash content of bran.  Bran composition is largely dependent on the milling process.  In modern rice mills, several different kinds of bran are produced: coarse bran (from the first whitening step),  fine bran (from second  whitening step) and polish (from the polishing step).  Polish consists of part of the endosperm and is often referred to as meal.

rmimage39 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.  The oily nature makes bran an excellent binder for animal feeds.  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.  In recent years however, advances in stabilization techniques have been made which has led to new uses for bran and its derivatives, most notably bran oil for cooking and waxes for cosmetic products. In the developing countries, rice bran is underutilized due to a lack of suitable stabilization techniques.

 

Brewer’s rice

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.

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