Relationship Between Hydrolytic Rancidity, Oil Concentration, and Esterase Activity in Rice Bran

Cereal Chem. 80(6):689–692 Hydrolytic rancidity restricts the utilization of rice bran, reducing its potential value. In the present study, three groups of eight rice cultivars each displaying different levels of oil concentration (high, medium, and low) were cultivated in 1999 and 2000 under field conditions and evaluated for oil content, hydrolytic rancidity, and esterase activity in the bran fraction. Genotype effects were statistically significant for all measured traits (P < 0.05), whereas environment (year) was nonsignificant. Hydrolytic rancidity was strongly correlated with esterase activity (r = 0.89***), but not with oil concentration (r = –0.01). A wide variation was found for both hydrolytic rancidity and esterase activity, which ranged from 6.8 to 56.0 mg of C8:0/g of bran (CV = 49.1%) and from 4.3 to 22.8 mg of C8:0/g of bran (CV = 34.3%), respectively. Red bran displayed the lowest values for both hydrolytic rancidity (mean = 10.2 mg of C8:0/g of bran) and esterase activity (mean = 5.4 mg of C8:0/g of bran). Apparently, the low values for hydrolytic rancidity were related to the inhibition effect of bran tannins on lipase activity. In conclusion, cultivar variation was detected for both hydrolytic rancidity and esterase activity in the studied genotypes, esterase activity being the principal factor explaining the variation found for the former trait. Therefore, it may be possible to create new cultivars with increased stability against hydrolytic rancidity by selecting for lower esterase activity. Rice (Oryza sativa L.) bran is a by-product of the rice milling process and a valuable source of various phytochemicals such as phenolics, tocotrienols, and -oryzanol. However, as a food ingredient, its use is severely limited by its high susceptibility to developing hydrolytic rancidity. During the milling process, rice bran lipids come into contact with lipases that rapidly hydrolyze the ester bonds of triacylglycerol (esterase activity), releasing fatty acids (known as free fatty acids), and glycerol (Ramezanzadeh et al 1999). The free fatty acids increase acidity, generate unacceptable functional properties, and produce undesirable organoleptic characteristics. As a consequence, the bran becomes unsuitable for human consumption or for production of edible oil with acceptable quality (Barnes and Galliard 1991). Although the lipolytic process can be inhibited by deactivating lipases through the use of various stabilization methods (Malekian et al 2000), such procedures are only economically justifiable for large-scale operations, which are not common in rice processing (McCaskill and Zhang 1999). Therefore, a cost-effective alternative is desired to reduce the susceptibility of rice bran to hydrolytic rancidity. The use of breeding techniques could be effective in increasing the stability of rice bran against lipid hydrolysis if genetic differences between cultivars exist for this trait. Tsuzuki et al (1994) observed cultivar variation for esterase activity in rice bran. Significant variation (17.3–27.4%) in rice bran oil content has been reported by Goffman et al (2002). By monitoring the hydrolytic degradation of bran lipids during storage in two rice cultivars, Goffman and Bergman (2003) also found significant differences in esterase activity, suggesting that lipase (esterase) activity may be an important factor determining the intensity of the hydrolytic degradation of bran lipids, whereas bran oil concentration may not be as significant as lipase activity in this process. To determine the relative significance of lipase activity and oil concentration to the intensity of hydrolytic process in rice bran, an experiment including genotypes showing different oil concentrations in the bran is required. In the present study, 24 rice cultivars differing in oil content were evaluated for hydrolytic rancidity and esterase activity in the bran. MATERIALS AND METHODS Plant Material Three groups of eight rice (Oryza sativa L.) cultivars displaying low (16–20%), medium (20–24%), and high (24–28%) bran oil content were used in this study. The genotypes were selected from a previous study which included more than 200 rice accessions (Goffman et al 2002). The cultivars were grown under field conditions in Beaumont, TX, using cultural management practices common for the region. The plants were cultivated in single plots, arranged in a completely randomized design. The plots consisted of six rows, 3.5 m long, spaced 15 cm apart. The within-row spacing was 10 cm. The plots were kept continuously flooded at 10 cm of standing water. At maturity, the plants were threshed by hand, the grains were dehulled, and all broken, diseased, and immature kernels were removed. Dehulled kernels ( 50 g) were milled using a McGill mill #1 for 30 sec with an 858 g weight in position 12 and 6 for long and medium grain types, respectively. The bran fraction was collected and sieved through a 840-μm sieve. Bran samples were conserved in a freezer (–20°C) under nitrogen until analysis. Surface lipid content was determined by refluxing 5 g of milled rice with petroleum ether in a Goldfish extraction apparatus for 30 min. The solvent was collected and evaporated, percent surface lipid content was calculated as the mass of the extracted lipid divided by the beginning total milled rice mass. This measurement was used to ensure that all samples were milled within a similar range in degree of milling (i.e., <0.5% surface lipid content). Determination of Hydrolytic Rancidity The hydrolytic deterioration of rice bran lipids was estimated as the accumulation of free fatty acids (FFA) in the bran after 48 hr of storage. Preliminary analysis (not reported) indicated this storage period was optimal for detecting differences in the hydrolytic rancidity of bran lipids between cultivars. Fresh rice bran ( 200 mg) was stored in an incubator for 48 hr at 35°C in 5-mL sealed polypropylene test tubes. After incubation, the accumulation of FFA was measured according to the method of Kwon and Rhee (1986) with minor modifications and using a caprylic acid 1 United States Department of Agriculture, Agricultural Research Service, Rice Research Unit, 1509 Aggie Drive, Beaumont, TX 77713. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Current address of corresponding author: Michigan State University, Plant Biology, East Lansing, MI 48824. Phone: 517/432-0706. Fax: 517/353-1926. Email: [email protected]. Publication no. C-2003-1002-02R. 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., 2003.