Soluble and bound phenolics of two different millet genera and their milled fractions: Comparative evaluation of antioxidant properties and inhibitory effects on starch hydrolysing enzyme activities

Abstract

Soluble and bound phenolic extracts of foxtail and little millet whole grains and their milled fractions (dehulled, hull, bran and pearled) were comparatively evaluated for their phenolic contents, antioxidant properties and inhibition of α-amylase and α-glucosidase activities. Although whole grain foxtail millet had higher total phenolic content than little millet, both the millets possess similar total flavonoid contents. Phenolic contents decreased from outer layers to the inner layers in the order hull>bran>pearled grains in millets. Thus, hull and bran fractions possess higher phenolic content compared to other fractions. Hydroxycinnamic acid derivatives accounted for more than 83% to the total phenolic content in soluble and bound extracts of milled fractions. Caffeic, ferulic and sinapic acids were the major phenolic acids in soluble fractions of whole grain millets, while ferulic and p-coumaric acids were present in higher levels in bound fractions. Quercetin was the most abundant flavonoid detected in all the fractions. However, a remarkably high amount of daidzein was found in bran and pearled grain fractions. Milled fractions of millets exhibited various antioxidant activities. Bound extracts of pearled grain fractions with lower phenolic content presented very high metal chelating activity than other fractions. Furthermore, soluble and bound extracts of both the millets inhibited α-amylase and α-glucosidase activities in a dose-dependent manner. These results suggest the potential utility of millets and their milled fractions as ingredients in functional foods for the management of diabetes by controlling oxidative stress and inhibiting α-glucosidase and α-amylase activities.