Effects of Commercial Processing on Antioxidants in Rice Bran

Cereal Chem. 77(5):551–555 Rice bran contains high amounts of beneficial antioxidants including tocopherols, tocotrienols, and oryzanols. Current rice milling technology produces rice bran from different layers of the kernel caryopsis. Under current practices, these layers are combined and then steam-extruded to form a stabilized rice bran pellet that is storage-safe prior to oil extraction. Each of these rice bran intermediates can vary in antioxidant content. The objective of this study was to investigate the changes in selected antioxidants in rice bran from both longand medium-grain rice during commercial milling and bran processing. Rice bran collected from various milling breaks of a commercial system had varying antioxidant levels. Bran collected after milling break 2 had the highest levels of tocopherol and tocotrienol. Oryzanol concentration was significantly higher in outer bran layers. Results also indicate that the long-grain rice bran averaged ≈15% more antioxidants than the medium-grain rice bran. Although it has long been considered an excellent source of vitamins and other nutrients, rice bran is an underutilized coproduct from rice milling. Rice bran contains ≈20% lipids (Saunders 1986), which is similar to other oilseeds, but rice bran oil (RBO) contains more unsaponifiable lipids than other common vegetable oil sources (Orthoeffer 1996). RBO contains ≈3–5% unsaponifiable lipids (Sayre 1988), depending on the type of rice (Gaydou et al 1980) and the method used to extract and refine the lipids. The unsaponifiable fraction in RBO contains a unique complex of naturally occurring antioxidant compounds, of which the tocopherols (vitamin E), tocotrienols, and oryzanol compound groups have received the most interest. The antioxidant compounds in rice bran or RBO have purported health benefits as well as antioxidant characteristics for improving the storage stability of foods. Several studies have reported the effects of RBO on metabolic activities including reduced plasma cholesterol in laboratory animals and humans (Yoshino et al 1989, Qureshi et al 1991, Hegsted et al 1990, Kahlon et al 1992, Hegsted and Windhauser 1993). Oryzanol has been studied for its ability to reduce cholesterol absorption (Rong et al 1997). Komiyama et al (1992) and Nesaretnam et al (1998) reported anticancer activity associated with tocotrienols. Thus, rice bran is viewed as a potential source of these high-value antioxidants for use as additives in foods, pharmaceuticals, and cosmetics. RBO contains high concentrations of the antioxidant tocopherol compared with other oil seeds (Kao and Luh 1991). Approximately 1.0% (v/v) of the unsaponifiable fraction of RBO is vitamin E (αtocopherol). Rice bran also contains an analogue to vitamin E known as tocotrienol. Approximately 1.7% (v/v) of the unsaponifiable fraction of RBO is tocotrienol (de Deckere and Korver 1996), where the unsaponifiable fraction is ≈4.2% of the total lipid content (Hui 1996). Both tocopherol and tocotrienol contain α, β, and γ isomers, but tocopherol isolated from rice bran also has a δ isomer (Morton 1975). Both tocopherol and tocotrienol possess strong antioxidant activity. Oryzanol or γ-oryzanol is a mixture of sterol esters of ferulic acid first isolated by Kaneko and Tsuchiya (1955). Initially oryzanol was thought to be a simple compound but was later found to be a mixture of several different esterified sterols, primarily cycloartenol, β-sitosterol, 24-methylene-cycloartanol, cyclobranol (cycloartenol), and campesterol (4-desmethysterols) (Rogers et al 1993). Figure 1 shows one of the esters of oryzanol. The complete oryzanol group is unique to RBO, but the exact composition of oryzanol depends on the rice cultivar. Crude RBO can contain ≤2% (v/v) oryzanol (Norton 1995). Much of the previous research on antioxidants in rice bran and RBO has focused on procedures using different solvent systems to extract the maximum amount of unsaponifiable lipids. Recent studies have also focused on the health benefits of these compounds. However, few of these studies have documented how initial processing steps of the postharvest rice bran feed stock affect antioxidant retention. Martin et al (1993) noted that heat processing of rice bran to stabilize it against oxidation reduced the concentrations of many valuable compounds. Nicolosi et al (1994) reported striking differences in levels of tocotrienols and oryzanols from commercially available RBO. They found that 90% of the oryzanol and tocotrienols were lost during oil refining. They suggested that differences in the stabilization procedure could result in variable recoveries of the unsaponifiable compounds. Diack and Saska (1994) found that when separating vitamin E and oryzanol compounds from RBO, the individual concentrations varied substantially according to the origin of the rice bran. Wells (1993) noted that oxidative degradation was likely due to loss of antioxidant activity in tocopherol and tocotrienol from high temperatures during extrusion stabilization, which indicated that these compounds were heat labile. The overall goal of this study was to establish a better understanding of how commercial milling systems affect the levels of specific antioxidants in rice bran from longand medium-grain rice cultivars. The intent was to help processors assess the best method for retaining these compounds in rice bran before oil extraction to increase the value of the rice bran. 1 Published with the approval of the Director of the Agricultural Experiment Station, University of Arkansas. Mention of a trademark or proprietary products does not constitute a guarantee or warranty by the University of Arkansas and does not imply its approval to the exclusion of other products that may also be suitable. 2 Research assistant, Biological and Agricultural Engineering Department, University of Arkansas, Fayetteville, AR 72701. 3 Professor, Food Science Department, University of Arkansas, Fayetteville, AR 72701. 4 Corresponding author. E-mail: [email protected] Phone: 501/575-2841. Fax: 501/575-6936. 5 Research assistant professor and director of Central Analytical Laboratory, Poultry Science Department, University of Arkansas, Fayetteville, AR 72701. Publication no. C-2000-0808-06R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 2000. Fig. 1. Molecular structure of ferulic acid esterified with 24-methylenecycloartanol, one of five compounds of oryzanol found in rice bran.