When harvested from the field, rice is in the form of paddy or rough rice, where the kernel is enveloped by a rice hull. After being dried, rice for human consumption is first milled to remove the hull, yielding brown rice. In a second stage of milling, the outer brown layer is removed from the rice kernel to yield polished or white rice. Depending on the milling techniques, rice bran may include part of the germ and it may also be mixed with part of the hull.
The composition of rice bran (in percent by weight) is generally 11-13% of water, 18-21% of crude fat and oil, 14-16% crude protein, 8-10% of crude fiber, 9-12% of ash and 33-36% of carbohydrate. Rice bran has naturally occurring lipases that hydrolyze the oil into glycerol and free fatty acids which give the product a rancid smell and taste. Under normal milling conditions (ambient temperatures above freezing), rice bran will degrade in approximately six hours into an unpalatable material which is not suitable as a human food. Because of the problem with rancidity, most rice bran is used as feed for animals or as fertilizer or fuel.
The oil from rice bran is sometimes extracted for use as human food. Because of the lipases, most extractions are carried out close to the growing areas in small capacity rice mills. To obviate this problem, others have found that naturally occurring lipases can be deactivated by heating the rice bran for a short period of time, as for example by passing it through a high temperature high pressure extruder. The heat stabilizes the rice bran such that it can be transported to a central operation for extraction of the oil but the protein is denatured by the heat such that the rice bran is unfit for use in the manner of the present invention.