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Stabilising rice bran through high shear extrusion
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by Dr Nabil W. Said, VP Nutrition & Extrusion Technologies, Insta-pro International
Figure 1
ice is considered one of the world’s most important crops and is a major part of the food culture in Asia, Latin America, Africa and other parts of the world. Global rice production is steadily increasing to meet the growing demand for food from a rapidly rising global population. The Food and Agriculture Organisation (FAO) – United Nations estimated that 2015 production of rice is around 500 million metric ton. Production of grains including rice is currently meeting and exceeding consumption. As more rice is being produced and processed, another part of the paddy, rice bran, is being generated. Rough rice or paddy (see diagram) consists of the white starchy rice kernel, surrounded by a tightly adhering brown coating of bran and enclosed within a loose outer hull. During the rice milling process, the hull and the bran along with the rice germ are removed mechanically to access and polish the white rice, which is the principle sustenance for the majority of world’s population. Due to the lack of a proper method for rice bran stabilisation in under developed and developing countries, rice bran was underutilised until low cost extruders were introduced in the market.
from raw or parboiled paddy. Besides the protein and energy, stabilised rice bran is an excellent source of vitamins B and E and some antioxidants. It has been used as an ingredient in poultry, pet food, and ruminant animal diets.
The urgency for stabilisation of rice bran
Rice bran has a powerful enzyme system which is activated during the milling process and causes rapid deterioration of the oil by exposure to the enzyme lipase and, to a certain extent, oxidase. Enzymatic hydrolysis of the oil into free fatty acids and glycerol will start and the acidity increases (reduced PH) resulting in the development of a soapy off flavor. The oil in the un-milled rice paddy is stable, as the Lipolytic enzymes are located primarily in the seed coat. Whereas most of the oil in the un-milled paddy or brown rice is stored in the germ and the Aleurone cells (the protein rich outmost layer of the endosperm), upon milling, the oil is subject to the activity of the powerful lipase enzyme causing the accelerated break down of the oil into free fatty acids and glycerol. The free fatty acids become susceptible to further decomposition through oxidative rancidity that will produce free radicals, cause soapy flavour and
General steps in the production and utilisation of stabilised rice bran
The separation of the hulls and the bran can be through one, or more stages. If the dehulling took place in one stage where both the hulls and the bran are mixed, the oil content will be low (below 10 percent), an economical separation of the oil is not possible. However, the use of two stage rice mills, in which the bran and the hulls are recovered separately, allows for an economical extraction of oil. The hulls (about 20 percent of the rice paddy by weight) have no significant nutritional value as they consist mainly of cellulose, lignin and select minerals. On the other hand, rice bran (approximately 5-10 percent of the rice paddy by weight) is rich in protein (14-18 percent) and energy; mainly in the form of the oil it contains (10-20 percent). The percentage of oil in the bran depends on the milling process, the contamination of the bran with hulls and broken kernels and whether the bran is obtained 30 | Milling and Grain
Figure 2