Effects of rice bran rancidity on the oxidation and structural characteristics of rice bran protein

Rice bran rancidity may affect rice bran protein through protein oxidation. However, little is known about the relationship between rice bran rancidity and rice bran protein oxidation. The effects of rice bran rancidity on the oxidation extent and structural characteristics of rice bran protein were investigated. As storage time of rice bran increased, the acid value, peroxide value, and value of thiobarbituric acid reactive substances in crude rice bran oil increased from 4.31 mg KOH/g, 2.84 Meq/kg, and 6.22 mu g MDA/g to 38.72 mg KOH/g, 15.58 Meq/kg, and 28.99 mu g MDA/g, respectively, which indicated that hydrolytic rancidity and oxidative rancidity of rice bran occurred simultaneously. The gradual increase in protein carbonyl and dityrosine content from 2.12 nmol/mg and 88.61 A.U. to 13.8 nmol/mg and 159.37 A.U. was accompanied by a steady decrease in free sulfhydryl content of rice bran protein from 22.6 to 9.6 nmol/mg, which implied that the products of rice bran rancidity induced rice bran protein oxidation. The rice bran protein oxidation subsequently resulted in a loss of the ordered state of secondary structure and the formation of aggregates as well as cross-link, when both disulfide bonds and non-disulfide covalent bonds participated in cross-link formation.