Rice bran lipase

Lipase has a molecular weight of about 40,000 Da and an isoelectric point (pi) of 8.56 (32). It is activated by calcium and inhibited by heavy metals. The optimum pH is 7.5-8.0, and the optimum temperature is 37°C. It is inactivated by heating at 60°C for 15 minutes. Rice bran lipase preferentially hydrolyzes fatty acids from the... 

A second rice bran lipase has a pi of 9.1 and an optimum temperature of 27°C (33). It has a high specificity for triacylglycerols haviug short-chaiu fatty acids. 

Rice bran oil with a high FFA content is often used as an industrial oil, because FFA removal often leads to a reduction in the content of desirable antioxidant/ bioactive components. A short discussion on methods to control FFA formation is therefore warranted. Lipase inactivation becomes significant when rice bran oil cannot be extracted from rice bran immediately after the milling process. As mentioned above, the rate of FFA formation within bran lipids can reach 5-7% per day (Nasirullah et al., 1989), with consequent reductions in oil yield and quality. Pretreatment of the rice bran by physical methods has been the primary means to inactivate rice bran lipase prior to oil extraction. 

Mulyantini, N.G.A., Choct, M., Li, X. and Lole, U.R. (2005) The effect of xylanase, phytase and lipase supplementation on the performance of broiler chickens fed a diet with a high level of rice bran. In Scott, T.A. (ed.) Proceedings of the 17th Australian Poultry Science Symposium, Sydney. New South Wales, Australia, 7-9 February 2005, pp. 305-307. 

Abrasive milling removes the outer bran layer to produce partially polished rice or, after polishing to remove the entire bran layer, white rice. Rice bran or polish may be subsequently stabilized by heat treatment to inactivate lipases. Stabilized rice bran has found use as an ingredient in human-grade processed foods. 

Unstabilized bran and polish have been used almost exclusively for animal feed, due to the bitter flavor that develops from the lipolytic action of enzymes on the oil found in them. However, development of a thermal process that inactivates the lipases has resulted in a stabilized rice bran product that is suitable for the food industry. The impressive nutritional qualities of the oil, fiber, carbohydrate and proteins of rice bran have made it a valuable food material. Removal of fiber from the bran by physical K,J7or enzymic1819 processes produces a milk-like product having desirable nutritional and functional properties. The nutritional composition of the rice bran milk product described by California Natural Products has been shown to match the nutritional requirements of an infant formula. Originally, the anti-nutritional factor of the residual phytates was of concern. However, as of 2005, phytase enzymes are suitable for use to break down these phytates. 

Lipases liberated from the testa and the cross cells promote rapid hydrolysis of the oil, and therefore, it should be extracted within hours of milling. Attempts have been made to upgrade oil with 30% free acid by reaction with glycerol and the enzyme Lipozyme Mucor miehei lipase) followed by neutralization. The major acids in rice bran oil are palmitic (12-18%, typically 16%) oleic (40-50%, typically 42%), and linoleic acid (29 2%, typically 37%). The oil contains phospholipids ( 5%), a wax that may be removed and finds industrial use, and unsaponifiable matter including sterols, 4-methylsterols, triterpene alcohols, tocopherols, and squalene among others. 

Rice bran contains active enzymes (30). Germ and the outer layers of the caryopsis have higher enzyme activities. Some enzymes that are present include a-amylase, p-amylase, ascorbic acid oxidase, catalase, cytochrome oxidase, dehydrogenase, deoxyribonuclease, esterase, flavin oxidase, a and p-glycosidase, invertase, lecithi-nase, lipase, lipoxygenase, pectinase, peroxidase, phosphatase, phytase, proteinase, and succinate dehydrogenase. 

Particularly lipase, but also lipoxygenase and peroxidase, are probably most important commercially because they affect the keeping quality and shelf life of rice bran. 

The instability of rice bran has long been associated with lipase activity (35). As long as the kernel is intact, lipase is physically isolated from the lipids (29). Even dehulling disturbs the surface structure allowiug lipase and oil to mix. Oil in intact bran contains 2—4-% free fatty acids (2). Once bran is milled from the kernel, a rapid increase in the FFA occurs, lu high huuiidity storage, the rate of hydrolysis is 5-10% per day and about 70% in a month as shown earlier. The objectives of rice bran stabilization are as follows ... 

Lipase activity results in hydrolytic rancidity. There is little or no change in flavor of the bran with an increase in FFA (5). Lipoxygenase activity, however, increases with the presence of FFA resulting in oxidative rancidity (36). It is oxidative deterioration that is responsible for the flavor and odor of rancid rice bran. 

A major problem with rice bran oU extraction is the high lipase activity, which results in FFA formation within a few days of milling particularly at high temperature and humidity. Free fatty acid content in rice bran increases during storage, i.e., 2-A% in a fresh crop, 5-8% in 1-year grain, and >10% in a 2-year-old crop (105). Thus, lipase is inactivated to stabihze rice bran prior to oil extraction (88). Heat-stabilized bran may be stored up to three months. However, oil extraction should be carried out within the first month to obtain better efficiency and higher quality oil. 

As expanders cook within 20 seconds, they counteract the activity of troublesome enzymes, such as lipase in rice bran (40) and urease in soybean (41). The short time between enzyme activation and inactivation destroys the enzyme before it has time to cause damage. An expander is even more effective than the horizontal, atmospheric pressure cookers described earlier. 

Lipase of castor beans has been studied most extensively (139-146). In addition, lipases have been shown to be present in the seeds or fruit of oil palm (147), lettuce (148), rice bran (149), barley and malt (150, 151), wheat (150, 152-155), oats and oat flour ( 138, 153, 156), cotton (157), tung kernels (158, 159), com (160, 161), millet (162), coconuts (163), walnuts (164), fusarium (165, 166), Cannabis and Cucurbita (167). Lipase aetivitites of various seeds have been eom-pared (140,168). 

Comparative investigations of the rates of hydrolysis of various natural triglycerides by pancreatic lipase have been carried out by several workers (257-260), As a general rule it is found that vegetable fats, such as coconut oil, palm oil, peanut oil, and rice bran oil, are hydrolyzed more rapidly than animal fats, such as beef fat or whale oil. These studies do not shed much light on the mode of action of lipase, but tend to support the often quoted view that unsaturated fatty acids are split off more readily than saturated acids they are useful when the nutritional values of natural fats are under consideration. Castor bean lipase hydrolyzes coconut oil more rapidly than beef fat and certain other fats (261). 

Lipase activity is found mainly in the bran components, which readily accumulate free fatty acids during ambient storage. Bran lipase is most active at 17% moisture, in finely divided milled products it is rapidly inactivated by heating at 100°C for 10 min. Lipase activity is high in other cereal grains including oats and rice. 

Lai, C.-C., S. Zullaikah, S. R. Vali, and Y.-H. Ju. 2005. Lipase-Catalyzed Production of Biodiesel from Rice Bran Oil. Journal of Chemical Technology Biotechnology 80 (3) 331-337. 

Lai CC, Zullaikah S, VaU SR, Ju YH. Lipase-catalyzed production of biodiesel from rice bran oil. J Chem Technol Biotechnol 2005 80 331-337. 

Lipase, phospholipase and lipoxygenase are the enzymes primarily responsible for poor-quality rice bran oil. They are activated during the bran removal process (Vetrimani et al., 1992 Takano, 1993), and can cause the rate of free fatty acid (FFA) formation to be as high as 5-7% per day (Nasirullah et al., 1989). Thus, inactivation of lipases is important for producing high-quality rice bran oil. The quality of rice bran oil is inversely related to the level of FFA, and this must be kept low if the oil is to be edible and acceptable in frying applications. However, the removal of FFA is not a simple process and is accompanied by the loss of important antioxidants (Krishna et ai, 2001). This loss must be kept to a minimum if rice bran oil is to be used as a functional food component. 

Stabilized whole rice bran is rich in protein, lipids, dietary fiber, vitamins, essential minerals, and important nutraceuticals such as phospholipids, choline, inositol, phytosterols, tocols, and tocotrienols. Tocols and tocotrienols are potent antioxidants that reduce the risk of cancers and CVD. The whole rice bran is stabUized in order to denature lipases that cause oxidative rancidity and to protect the intrinsic nutraceutical compounds. The stabilization of the rice bran is usually done by applying heat in an extrnder. The preservation of the oryzanol is of utmost importance because this compound is very effective in promoting cardiovascular health (Hoffpauer 2005). 

Butsat, S. Siriamompun, S. 2010. Antioxidant capacities and phenolic compounds of the husk, bran and endosperm of Thai rice. Food Chem. 119 606 613. Chigorimbo-Murefu, N.T.L. Riva, S. Burton, S.G. 2009. Lipase-catalyzed synthesis of esters of ferulic acid with natural compounds and evaluation of their antioxidant properties. J. Molec. Cat. B Enzym. 56 277-282. 

Ideally, bran should be stabilized within a few minutes after removal from the kernel. Stabilization process inactivates enzyme lipase that causes rapid hydrolysis of TAG. Three methods developed for brown rice stabilization are (1) heat dena-turation and inactivation of lipases, (2) extraction with an organic solvent to remove... 

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