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Hydrolytic Degradation of Triacylglycerols and Changes in Fatty Acid Composition in Rice Bran During Storage

¹United States Department of Agriculture, Agricultural Research Service, Rice Research Unit, 1509 Aggie Drive, Beaumont, TX 77713. Mention of a trademark or propietary product does not constitute a guarantee or warranty of a product by the U.S. Department of Agriculture, and does not imply its approval to the exclusion of other products that also can be suitable. ²Corresponding author. Phone: 409/752-5221 ext. 2242. Fax: 409/752-5720. E-mail: fgoffman@lycos.com.

Hydrolytic rancidity makes rice bran unsuitable for human consumption, restricting its use to animal feed. To better understand this lipolytic process, bran from rice cultivars ‘Cypress’ and ‘Earl’ differing in oil concentration (23.5 and 18.3 mg of triacylglycerol/100 mg of bran, respectively) was stored at room temperature for five months, and the changes in triacylglycerol content and fatty acid composition, as well as the accumulation of free fatty acids (FFA) were followed. The decomposition of triacylglycerols displayed a decay pattern, with Cypress showing a more elevated degradation rate when compared with Earl. At day 36, both lines reached the same oil concentration, but the triacylglycerol decomposition in Cypress was still higher, indicating that oil concentration may not be a significant factor affecting the intensity of the rancidity process. The higher degradation rate observed in Cypress was apparently caused by higher lipase activity, which was 26% higher (P < 0.001). Fatty acid composition of triacylglycerols changed during storage, the palmitic acid percentage was similarly reduced in both lines to ≈80% of its initial concentration. Oleic and linoleic acids remained almost unmodified or slightly increased. The final content of FFA was ≈58% higher in Cypress than in Earl (P < 0.001). In conclusion, lipase activity appears to be an important factor determining the intensity of the hydrolytic process, but further research is required to confirm this conjecture. If this hypothesis is verified, a selection for lower lipase activity could be useful for increasing rice bran stability.