Comparison of milling fractions of tartary buckwheat for their phenolics and antioxidant properties

Tartary buckwheat hull, coarse bran, fine bran and light flour were examined and compared for their free and bound phenolics, flavonoids, phenolic acid composition and antioxidant activity. The results showed that free phenolic contents were much higher than bound phenolics for each sample, and four samples significantly differed in their phenolic and flavonoid contents, phenolic acid composition and antioxidant activities. The fine bran had the greatest free phenolic and flavonoid contents, concentrations of all identified phenolic acids, ABTS+ scavenging activity and total reducing power. The hull had the greatest bound phenolic and flavonoid contents, concentrations of all identified phenolic acids, DPPH scavenging activity and total reducing power. The results suggest the potential of postharvesting treatment on antioxidant activities and availability of phenolics. Rutin, p-hydroxybenzoic, protocatechuic, caffeic, chlorogenic, gallic, ferulic, p-coumaric, syringic and vanillic acids were detected in all fractions. The fine bran had the greatest rutin content and up to 7.43% on a weight basis. Furthermore, protocatechuic acid was the dominant phenolic acid of the hull accounting for 65.54%, while p-hydroxybenzoic acid was the dominant phenolic acid of the bran and the light flour and accounted for 58.61–78.82%, calculated on respective total phenolic acid content. Rutin and p-hydroxybenzoic acid showed significantly positive correlations with antioxidant activities (r ≥ 0.98, P < 0.01), suggesting the potential use of rutin and p-hydroxybenzoic acid as markers of tartary buckwheat antioxidants. These data suggest the possibility to improve the antioxidant release from tartary buckwheat-based food ingredients through postharvesting treatment or processing. Tartary buckwheat hull and bran with higher amounts of phenolic compounds are considered to be excellent materials for cereal-based food processing with significant health benefits.