AbstractBACKGROUND: To make nutrients more accessible and further increase biological activity, cooked black soybeans were inoculatedwith Bacillus natto and fermented at 37 °C for 48 h. The changes in physiochemical properties of fermented black soybean natto were investigated.RESULTS: The inoculation procedure significantly increased moisture, viscosity, color, polyphenol compounds and anthocyanin, and significantly decreased hardness after 48 h fermentation. Fibrinolytic and caseinolytic protease, β‐glucosidase activities, TCA‐soluble nitrogen, and ammonia nitrogen contents in the inoculated samples significantly increased as fermentation time increased. Genistin and daidzin concentrations gradually decreased with increased fermentation time. However, genistein and daidzein increased with fermentation time, which reached 316.8 and 305.2 µg g−1 during 48 h fermentation, respectively. DPPH radical scavenging activities of the fermented black soybeans increased linearly with fermentation time and concentration. Compared with the soaked black soybeans and cooked black soybeans, the fermented black soybeans with B. natto resulted in higher scavenging activity towards DPPH radicals, which correlated well with the content of total phenols (r = 0.9254, P < 0.05) and aglycone isoflavone (r = 0.9861, P < 0.05).CONCLUSION: Black soybean natto fermented by B. natto has the potential to become a functional food because of its high antioxidant activity. Copyright © 2010 Society of Chemical Industry