Effects of Storage and Roasting Condition on the Antioxidant Activity of Soybeans with Different Colors of Seed Coat

The DPPH radical scavenging activity and ORAC value of soybeans (yellow soybean, blue soybean, and black soybean) were increased by roasting at above 190 °C. Concerning raw beans, black soybeans with the darkest seed coat color had the strongest antioxidant activity, indicating the effect of the coat pigment. However, the degree of increased antioxidant activity by roasting was almost similar regardless of seed coat color, suggesting that coat color is independent of the increased antioxidant activity. Concerning aged beans stored at 37 °C/75% RH for 60 days, the antioxidant activity increased in yellow soybean and decreased in blue and black soybean compared to before storage. Conversely, when roasted at 190 °C for 20 min, the DPPH values of all the aged beans were significantly increased. Other analyses of roasted beans with and without seed coat showed that changes in the components of cotyledons during storage may have contributed to the increased antioxidant activity of aged beans, regardless of seed coat color. These results revealed that roasting effectively improves the antioxidant activity of aged soybeans, regardless of seed coat color. We concluded that roasting is recommended for antioxidant properties, particularly regarding the effective use of aged beans.