The distribution of endogenous polyphenols in black rice bran and their effects on starch digestion

This study investigated the distribution of endogenous polyphenols across different layers of black rice bran and their impact on starch structure and digestibility. Non-targeted metabolomics showed that the outer black rice bran layer was enriched in hydrophilic polyphenols (cyanidin-3-O-galactoside, tricin), whereas the inner layer accumulated hydrophobic flavonoids (nobiletin). XRD confirmed the formation of V-type crystalline structures with crystallinity reaching 26.5% - 36.5%. FTIR analysis showed enhanced short-range ordered structure and increased helical content. Rheological measurements revealed that black rice bran polyphenols (BRBPs) reinforced the gel network, with the crossover modulus increasing by up to 280.4%. Notably, BRBP1 from the outer layer exhibited the strongest anti-digestive effect, which was suggested to be attributed to its unique polyphenolic composition enabling dual hydrogen-bonding and hydrophobic interactions. In vitro digestion demonstrated that BRBPs significantly increased resistant starch content (from 19.5% to 24.6% - 32.6%) while reducing overall digestibility. These findings highlight the layer-specific functionality of black rice bran polyphenols and their potential as functional ingredients for modulating starch digestibility.