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

¹United 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. ²Current address of corresponding author: Michigan State University, Plant Biology, East Lansing, MI 48824. Phone: 517/432-0706. Fax: 517/353-1926. Email: fgoffman@lycos.com.

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.