Rice 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.
The global rice production is steadily increasing to meet the growing demand for food from a rapidly rising global population. The Food & Agriculture Organization (FAO) – United Nations estimated that 2015 production of rice is around 500 million metric ton.
Production of grains including rice 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 below) 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 stabilization in under developed and developing countries, rice bran was underutilized until low cost extruders were introduced in the market.
General steps in the production and utilization of stabilized 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%), 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% 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% of the rice paddy by weight) is rich in protein (14-18%) and energy; mainly in the form of the oil it contains (10-20%). The percent 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 from raw or parboiled paddy.
Besides the protein and energy, stabilized 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 stabilization 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 flavor and a reduction in the nutritional value. For this reason, it is of utmost importance to immediately stabilize the bran to prevent the deterioration and the rancidity of the oil.
Inactivation of enzymes through extrusion:
High shear dry extrusion was developed by Insta-Pro International in the late 1960’s to inactivate the anti-nutritional factors present in soybeans. Those anti-nutritional factors are enzymes such as the trypsin Inhibitors, hemagglutinins, lipase, lipoxygenases, urease and other anti-nutritional factors. If ingested in their intact state, the anti-nutritional factors will adversely affect the digestion of nutrients through their inhibitory effects on the pancreatic enzymes responsible for digesting the protein and carbohydrates. The high dry shear extruder generates heat through friction (mechanical energy) along with pressure. The barrel of the extruder consists of segmented chambers with a shaft that rotates at constant speed.
Screws are fitted on the shaft separated by steam locks or shear locks designed to disrupt the conveying of the material and force some of it to revert back through channels in the wall of the barrel thus generating shear and friction that raises the temperature of the extrudate in a controlled manner (see the following figures).
Upon exiting the extruder, a sudden drop in the pressure takes place that results in rupturing cell walls, releasing natural anti-oxidants (tocopherols), partially dehydrating the product as some of the moisture will be flashed off in the form of steam. This sudden drop in pressure also results into sterilization of the product. As concluded from numerous scientific studies, viruses, bacteria, coccidia, mold and yeast cannot survive the extrusion conditions. The inactivation of the enzymes takes place through altering the tertiary structure by breaking the disulfide bonds holding the amino acids strands together. The proper optimization of the extrusion parameters allows for the denaturing of those enzymes without the adverse effect on the primary structure of the amino acids.
The cooking time takes only a few seconds and under the proper conditions of operation, the digestibility of nutrients in the extrudate would be at its highest level. The extruder has been used as one of the most effective tools to stabilize products through enzymes inactivation thus in the case of rice bran, stabilizing it immediately after milling.
Fresh milled rice bran oil normally contains 1.5-2.0 % free fatty acids. Free fatty acids level of less than 5% is desirable. If not stabilized within few hours after milling, the free fatty acids in the oil will exceed the 5% level. Exceeding 10% free fatty acids in the oil would indicate less recoverable edible oil.
Utilization & benefits of stabilized rice bran
Stabilized and extracted rice bran oil is very popular as a cooking vegetable oil in China, Japan, Taiwan and other Asian countries. In the late 1800’s, it was discovered that East Indies sailors suffered from what thought to be toxins or infection generated from the white rice that they consumed. The disease is called Beriberi. It was later discovered that when fed meat, vegetables and dry milk, the symptoms subsided and disappeared. It was then recognized that it was a nutritional deficiency in the polished rice that lead to the discovery of vitamin B1, thiamin. This vitamin stays in the bran upon polishing the rice. By adding rice polishing to the Beriberi inflicted chicken’s diet, the fowl were cured of the disease.
Because of its content of decent protein level, oil, fiber and antioxidants, stabilized rice bran can be used as an excellent source of energy, protein and fiber in animal feeds, human food and as a functional food or nutraceutical ingredient. Many claims have been made as to its benefit in lowering the cholesterol through its content of tocotrienols tocopherols, ß-sitosterol and γ-oryzanol. It is also been used as a stool bulking agent.
In the feed industry, stabilized rice bran is being used as a palatable ingredient and excellent source of protein in pet food; energy and fiber in equine feed, poultry and ruminant animals.
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