World rice production is close to 650 million metric tons (MMT) of paddy rice in 2008 which constitutes more than a quarter of all cereal grains, and rice is the staple food for large portion of the world population. White or polished rice is obtained by first removing hull and abrading away the bran layer. Rice Bran comprises about 8% of paddy. The bran fraction contains almost all of the oil and the majority of protein, ash, vitamins and fiber of the rice caryopsis. The bran contains about 18% of oil.

However, there is about 3% of rice bran is extractable that equal to about 3.5 MMT of crude rice bran oil. Despite the potential of crude rice bran oil, only about 500,000MT of edible and industrial grades of rice bran oil is currently produced. Crude rice bran oil can be refined into a high quality salad oil, and several other commercial products also can be isolated from the crude rice bran oil.

Source: Lipid Technology            .

The crude rice bran oil can be refined through alkali refining or physical refining. Rice bran oil is an excellent salad oil and is more stable under frying condition than any other vegetable oil due to a more balance between linoleic and oleic acid, a very low linolenic and high level of antioxidants (tocopherols and tocotrienols).

The commercial crude rice bran oil extracted with n-hexane typically contains 3-4% waxes and about 4.2% unsaponifiable lipids. The latter in turn consists of sterol (β-sitosterol, campesterol, and stigmasterol), 43%; triterpene alcohols (24-methylene cycloartanol, cycloartanol, and cycloartenol), 28%; 4-methyl sterol, 10%; and less polar compounds (squalene, tocopherol, and tocotrienol), 19%. Unsaponifiable material is higher in rice bran oil than in other vegetable oils.

Plant sterols have long time known to disminish absortion of cholesterol due to competition with cholesterol for incorporation into micelles for transport across the intestinal cell wall, and most vegetable sterols are poorly absorbed. 3 grams β-sitosterol per day may lower cholesterol absorption by 50%.

The tocopherol and tocotrienol contained in rice bran oil is not unusually high in compared with the other vegetable oils. However, the excellent resistance of rice bran oil to oxidation is thought to be due not only to the tocopherol or tocotrienol content but to ester of ferulic acid. Ferulic acid is esterified with triterpene alcohols, sterols, and methanol. A mixture of esters of ferulic acid with sterols and triterpene alcohols, predominantly β-sitosterol, campesterol, cycloartenol, and 24-methylene cycloartanol have been named oryzanol (or gamma oryzanol). Oryzanol has been reported to have various hormone-like affects on the autonomic nervous system. It has been shown to have growth promoting activity, to maintain estrous cycle and to increase capillary development in skin. Oryzanol amounts to 2.0% of crude rice bran oil and 1.7% of degummed oil. These compounds are concentrated in the gum and soap stock from refining.

In a study at the Kobe University School of Medicine in Japan, oryzanol was added to human subject at the rate of 300mts / day for three months. Significant lowering of total serum cholesterol was found, it is due mainly to reduction in the LDL fraction and there was a trend of increasing HDL. This study is consistent with the hypothesis that hypo-cholesterol activity in rice bran resides mainly in the unsaponifiable fraction of the lipid. Various other therapeutic benefits have been attributed to rice bran oil and to oryzanol.

Phospholipids are also produced from the gums removed from crude rice bran oil. These are diacyl glycerol-phosphate compound which are 4-5% of the rice bran lipid. Phosphatidyl choline (lecithin) has been shown decrease cholesterol absorption in humans and is efficient in increasing blood choline. Lecithin preparation from rice bran lipids contains 20-25% phosphatidyl choline.

Wax comprises 3-4% of crude rice bran oil which has been extracted at high temperature. Rice bran wax is composed of esters of long-chain fatty acids (C₁₆-C₂₆) and fatty alcohols (C₂₂-C­₃₀). It can be isolated by crystallizing and settling at low temperature and purified by washing with acetone or ethanol to remove the residual oil. Rice bran wax is approved in the US as a releasing agent for plastic packaging material intended for food contact and is use for coating for fresh food and vegetables to prevent moisture loss. It also used as a base for lipstick and other cosmetics.

Referrence:

1.       Lipid Technology, volume 2 (July 1990)

2.       FAO Newsroom

3.       Special report on Rice Market in the first half of 2008 and upcoming prospects