Diffusion of adzuki bean (Vigna angularis) oligomeric polyphenols from seed coats to cotyledons during boiling.

This article investigates the diffusion and binding behavior of polyphenols from the seed coats of adzuki beans (Vigna angularis) into the cotyledons during boiling, a process relevant to the production of traditional Japanese adzuki bean paste. The study found that oligomeric polyphenols, including dimeric and larger proanthocyanidins, diffuse more readily into the cotyledons than monomeric polyphenols such as (+)-catechin and (−)-epicatechin. Microscopic analysis suggested these diffused polyphenols bind to cotyledon proteins, forming complexes that are poorly extracted by ethanol and acetone but contribute significantly to the antioxidant activity measured by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging assays. These findings indicate that the antioxidant properties of smooth adzuki bean paste derive in part from oligomeric polyphenols bound within the cotyledons, highlighting the potential of adzuki bean paste as a functional food ingredient.