Comparative Analysis of Total Phenolic Content and Antioxidant Activities of Black Soybeans with Different Seed Weight

The phenolic contents in soybean seeds are affected by genetic and environmental factors. In the present study, 24 black soybeans and a control variety were grown in Korea and grouped as small (<13 g), medium (13 - 24.0 g), and large (>24 g) based on their seed weight. The total phenolic content (TPC) and antioxidant activities were analyzed and compared with their seed weight. The TPC was in the ranges 2.016 - 5.535, 1.992 - 4.679, and 1.829 - 4.621 mg GAE/g in small, medium, and large seeds with an average of 3.824, 3.108, and 3.626 mg GAE/g, respectively. The FRAP and DPPH activities were in the ranges 4.327 - 6.394 and 0.354 - 0.403 mg AAE/g in small seeds, 2.510 - 6.147 and 0.356 - 0.417 mg AAE/g in medium seeds, and 3.024 - 6.389 and 0.344 - 0.405 mg AAE/g in large seeds, respectively. Besides, ABTS activity was in the ranges 4.833 - 4.967, 3.636 - 4.871, and 4.554 - 4.956 mg TE/g in small, medium, and large seeds, respectively. Small seeds had the highest average FRAP (5.498 mg AAE/g) and ABTS (4.902 mg TE/g) activities while large seeds showed the highest average DPPH (0.387 mg AAE/g) activity although the variations were not significant (p > 0.05). TPC showed positive correlation with every antioxidant activity. However, the TPC correlation was significant (p < 0.05) only with DPPH (r = 0.673). Overall, small seeds presented a relatively high TPC and maximum antioxidant activities. Hence, black soybeans with small seeds could be important sources of high phenolic concentration and provide improved health benefits.