Formation of polyphenol-protein complexes in boiled adzuki beans with digestive resistance

Reddish-purple adzuki beans are widely consumed in East Asia in boiled and paste forms. Polyphenols prevalent in the seed coats exhibit various bioactivities. Oligomeric polyphenols diffuse from the seed coats into the cotyledons during boiling and preferentially bind to proteins than to starch. In this study, the proteins were fractionated from adzuki-bean cotyledons according to their solubility, each protein fraction was boiled with seed-coat polyphenols, and the properties of polyphenol-protein complexes were evaluated. The denaturation temperature of albumin was lower than that of globulin and glutelin, and the hydrophobicity of boiled albumin and glutelin was higher than that of boiled globulin. Seed-coat polyphenols were bound to cotyledon proteins to produce high-molecular-mass polyphenol-protein complexes, with albumin and glutelin exhibiting high binding capacity. These complexes were resistant to proteolytic digestion. Further elucidation of their bioactivities may expand the use of processed adzuki beans as functional ingredients.