Enhancements of isoflavone aglycones, total phenolic content, and antioxidant activity of black soybean by solid-state fermentation with Rhizopus spp.

Abstract

In the present study, a solid-state fermentation process of black soybean was developed. Three filamentous fungi including Rhizopus oligosporus (BCRC 31996), R. oligosporus (NTU-5), and Rhizopus oryzae (BCRC 30894) were cultivated with steam-cooked intact and dehulled black soybeans. It was found that fermentation enhanced the total phenolic and isoflavone aglycone content as well as antioxidant activity of the black soybean methanol [80 % (v/v)] extracts. Among the three candidates, R. oligosporus BCRC 31996 was chosen as the working strain, and dehulled Tainan NO.3 black soybeans were used as the substrate based on the relatively high total phenolic content, isoflavone aglycone accumulation, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging effect of the fermented extracts. In a 6-day fermentation, extracts of the fermented dehulled black soybeans yielded higher total phenolic content (2,876.2 μg/g) which was 1.16 times of the result from the intact soybean extracts. The DPPH scavenging effect of the fermented dehulled black soybean extract reached its maximum on the third day. The activity of β-glucosidase of dehulled black soybean samples increased with time and resulted in accumulation of isoflavone aglycones. The product of solid-state black soybean fermentation serves as a functional ingredient in the products of nutritional supplements and health foods.