Influence of milling time on the nutritional composition and antioxidant content of Thai rice bran

Abstract

Four regular rice strains (KDML105, PT1, SPT and RD31) and one pigmented rice strain (DSK) were assessed regarding their composition and antioxidant content. Phenolic compounds and -oryzanol were the dominant antioxidants in all rice varieties. The highest total phenolic content was found in the black glutinous rice DSK with 1.04 mg GAE/g. Anthocyanin was also found at a high concentration in DSK (1.13 mg/g). Brown rice samples were abrasively milled for 0-60 seconds to make a degree of milling (DOM) curve. A non-linear relationship between milling time and DOM was observed in all samples. The curve demonstrated that 30 sec of milling removed almost allof the bran fraction. The brans collected after 10, 20 and 30 sec of milling were assessed regarding their compositions and antioxidant content. Longer milling time decreased the content of protein, fiber and ash in the brans, while carbohydrates were increased. The total phenolic content in rice bran was reduced with longer milling time. On average, bran after 10 sec of milling contained 8.9% and 21.5% more total phenolics than the rice bran after 20 and 30 sec of milling, respectively. The -oryzanol and α-tocopherol contents of rice bran were in the range of 1.40-2.60 mg/g and1.24-21.56 μg/g, respectively. The DPPH radical scavenging activity of DSK bran after 10 sec of milling was highest, and was approximately three-fold higher than the other bran samples. However, the DPPH radical scavenging activity of all extracts was lower than that of BHT, a synthetic antioxidant.