Free and bound phenolic profiles of the bran from different rice varieties and their antioxidant activity and inhibitory effects on ɑ-amylose and ɑ-glucosidase

Free and bound phenolic extracts from three varieties of black, indica, and Japonica rice bran were evaluated for their compositions, antioxidant activity and inhibitory effects on ɑ-amylase and ɑ-glucosidase activities. The free and bound phenolics displayed numerous antioxidant activities of ABTS, DPPH and T-AOC. HPLC results revealed that p-coumaric and ferulic acid mainly existed in bound forms, whereas anthocyanins contents were mainly in free phenolics of black rice bran. The bound phenolics in three varieties of rice bran exhibited higher inhibitory rate towards ɑ-amylase than their corresponding free phenolics did. The ɑ-glucosidase inhibitory rate of the bound phenolics in indica, and Japonica rice bran was higher than that of free forms, which showed the similar trend with that on ɑ-amylase. However, lower ɑ-glucosidase inhibition was observed in the bound phenolics from black rice bran, and the IC50 values of free and bound phenolics on ɑ-glucosidase were 8.59 and 15.26 μg/mL, respectively. This may be attributed to the differences of phenolic composition and content in free and bound forms from varieties of bran. These findings may encourage the consumption of rice in forms of rice bran or whole brown rice for promoting health.