Isoflavone aglycone contents of commercial fermented soybean products in Korea and the isoflavone aglycone-producing ability of Bacillus subtilis isolated from the products

The isoflavone aglycone (IA) contents of commercial fermented soybean products in Korea, such as cheonggukjang and doenjang , and the IA -producing ability of the Bacillus subtilis strains isolated from the products were examined. Among three commercial products, product C ( doenjang ), which is produced using a long fermentation method, showed the highest conversion rate of isoflavone glucosides to aglycones. Of all eight B. subtilis strains isolated from the products, four strains had the same or higher level of IA production than B. subtilis (natto), with the C-3 strain showing the highest level. Hence, the difference in the conversion rate among the products might be attributed not only to fermentation time during the manufacturing process, but also the IA -producing ability of the B. subtilis strains. Moreover, the C-3 strain produced IA during the early stage of culture with soybean liquid medium, and the IA -producing ability of the C-3 strain in the fermentation with soybean solid medium was higher than that of B. subtilis (natto). Thus, the rapid IA production by the B. subtilis C-3 strain might contribute to the high IA content of product C.