Comparison of Anthocyanins, Proanthocyanidin Oligomers and Antioxidant Capacity between Cowpea and Grain Legumes with Colored Seed Coat

The aim of this study was to investigate the composition and contents of anthocyanins and proanthocyanidins, and the antioxidant capacity of black and red cowpeas grown in Japan, with comparison to grain legumes (black kidney beans, black soybeans, and azuki beans) commonly consumed in Japan. This study revealed that black cowpeas contained seven anthocyanins, the 3–O–galactosides and 3–O–glucosides of cyanidin and delphinidin, and the 3–O–glucosides of malvidin, peonidin, and petunidin, at higher levels than other grain legumes. In addition, black and red cowpeas were rich in proanthocyanidin oligomers (monomers to hexamers) when compared with other grain legumes. Among these, black and red cowpeas showed potent hydrophilic oxygen radical absorbance capacity (H–ORAC), reaching 540.6 and 367.5 µmol–Trolox equivalents/g–dry matter, respectively. The contributions of anthocyanins and proanthocyanidin oligomers to H–ORAC values were 45.8% and 17.5% in black and red cowpeas, respectively.