Variation in antioxidant capacity of the seven azuki bean (Vigna angularis) varieties with different seed coat color

Abstract

ABSTRACT Azuki bean (Vigna angularis) is a small red bean commonly used in Japanese cuisine from ancient time. The most common azuki bean seed is red, but there are black, grey, brown, green, straw-colored, white, and various flecked varieties. We evaluated the total antioxidant capacity and phytochemical phenolics profiles of seven azuki bean varieties in relation to their seed coat color according to the L*a*b* system using a color-difference meter. Not the cotyledons but the seed coat of all the varieties, except the white variety, showed antioxidant capacity (DPPH radical-scavenging activity). In the extracts obtained with 0.4% trifluoroacetic acid using the whole seeds without grinding, a wide range of antioxidant capacity from 36.2 to 2.8 μmol Trolox eq./g DW was obtained. The antioxidant capacity in the azuki bean was localized at the seed coat, and correlated positively with the total phenolics content and negatively with the color lightness (L*). Anthocyanin was present in only three darker varieties, Kuro-azuki, Shimofuri-azuki and Nezumi-azuki. Tamba-dainagon, the most popular red colored variety, and other three varieties contained no anthocyanin. However, these varieties without anthocyanin, except for one white variety, clearly showed antioxidant capacity. The antioxidative capacity of the deep colored azuki bean was much higher than that of light colored azuki bean. The azuki seed coat is revealed as an effective potential source of natural antioxidants. GRAPHICAL ABSTRACT