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

Adzuki beans (Vigna angularis) are major legumes in East Asia, commonly used in traditional Japanese confections in the form of a paste. Although various types of polyphenols are present in seed coats, there are no reports on the diffusion of polyphenols to cotyledons during boiling. This study revealed that oligomeric polyphenols-such as dimeric and more proanthocyanidins-diffused more easily from seed coats to cotyledons than monomeric polyphenols-such as (+)-catechin and (-)-epicatechin-during heating. Microscopy suggested that diffusing polyphenols were bound to cotyledon proteins. Furthermore, bound polyphenols, poorly extracted by ethanol and acetone, contributed to the 1,1- diphenyl-2-picrylhydrazyl radical-scavenging activity of cooked cotyledons. Thus, smooth adzuki bean paste, made by boiling adzuki beans and removing the seed coats, contains oligomeric polyphenols that contribute to its antioxidant activity. By clarifying further physiological activities of polyphenols diffused through cotyledons, adzuki bean paste can be a potential functional ingredient.