Antioxidant capacities, total phenolics and flavonoids in black and yellow soybeans fermented by Bacillus subtilis: A comparative study of Thai fermented soybeans (thua nao)

B. subtilis TN51 was inoculated in cooked black and yellow soybeans, after incubating at 42°C and 4°C for 48 h, the fermented thua nao were produced. The antioxidant capacities, total pehenolics and flavonoids of the fermented black (TN-BS) and yellow (TN-YS) soybeans were then examined as compared to their cooked non-fermented (CNF) soybeans. It was found that fermentation enhanced the increase of total phenolic and flavonoid contents (except yellow soybean fermentation) as well as antioxidant activities of anti-DPPH radicals and ferric reducing antioxidant power (FRAP) of the soybean extracts. The extracts of CNF black soybean and TN-BS, showed higher total phenolic content, total flavonoids, anti-DPPH radicals and lipid peroxidation inhibition than those of yellow soybean extracts. While, FRAP value was found higher in the extract of TN-YS than that in TN-BS extract. Correlation studies indicated significant (P ≤ 0.01) positive correlation between the total phenolic contents and the values of FRAP in soybean extracts (r = 0.889). Whereas, the contents of total phenolics (r = -0.731) and flavonoids (r = -0.709) were negatively correlated (P ≤ 0.01) with IC50 of DPPH radical-scavenging activity. Also, a significant negative correlation (P ≤ 0.01, r = -7.39) was demonstrated between the total flavonoid contents and IC50 of lipid peroxidation inhibition. These results show that black soybean thua nao produced by powder culture B. subtilis TN51 could be used for possible commercial production of functional food to alleviate oxidative stress.