Zhaohong Ci^{1, 2}, Michiyuki Kojima^{2, 3,}

Scarlet runner beans (SRB) are a valuable source of many nutrients, including proteins, starch, dietary fiber, and oligosaccharides, and are used in various foods in Japan. To extend our knowledge of the effects of SRB on human health, we analyzed the color, polyphenol and procyanidin contents, DPPH radical scavenging activity, and reducing power of various SRB. The L* and C values were highest for SRB (white) and lowest for SRB (black). SRB (purple) and SRB (brown) showed higher polyphenol and procyanidin contents than those of SRB (white) and SRB (mixed). SRB (brown) and SRB (mixed) showed the highest DPPH radical scavenging activity and reducing power. SRB (white) had the lowest ratio of oligomeric and polymeric polyphenols and the lowest DPPH radical scavenging activity and reducing power. We found a positive correlation between polyphenol content and both DPPH radical scavenging activity and reducing power. Moreover, polyphenols from SRB inhibited the activity of α-glucosidase in a dose-dependent manner. The polyphenols (50 µg/mL) of SRB (black) showed the highest α-glucosidase inhibitory activity (85.7%), and those of SRB (white) showed the lowest inhibitory activity (53.8%). SRB (black) had a lower IC₅₀ value (26.4 µg/mL) and SRB (white) had a higher IC₅₀ value (58.4 µg/mL) than those of other SRB. We speculate that the degree of polymerization for polyphenols affects antioxidant activity and α-glucosidase inhibitory activity for SRB. These results suggest that SRB has antioxidant and enzyme inhibitory effects via its polyphenols and provide a basis for selecting SRB cultivars and for developing SRB-based functional foods with improved health benefits.

scarlet runner bean, polyphenol, DPPH radical scavenging activity, reducing power, α-glucosidase inhibitory activity