Improving Anti-α-Glucosidase Activity of Douchi Koji Using a Newly Isolated Strain of Bacillus subtilis B2

In attempt to develop an effective strategy to obtain douchi with anti-alpha-glucosidase functionality, Bacillus subtilis B2 was used as the initial microorganism to produce bacterial-type douchi koji capable of inhibiting alpha-glucosidase activity in the current study. Various fermentation conditions, such as soybean type, fermentation time and humidity, resulted in different anti-alpha-glucosidase activity of the douchi koji. Regardless of soybean or black bean, the douchi koji exhibited a drastic increase in anti-alpha-glucosidase activity during the first 48 h of fermentation and kept a slight increase from 48 h to 96 h. The anti-alpha-glucosidase activities of both soybean and black bean kojis displaying the lowest at the relative humidity of 70 percent, rose up to the highest at the humidity of 90 percent and then showed a slight decrease. Under the same fermentation conditions, it is found that the soybean douchi koji exhibited higher alpha-glucosidase inhibitory activity than that of black bean douchi koji. Maltose, sucrose and glucose were reduced drastically in soybean and black soybean koji after 96 h of fermentation. One of the most effective alpha-glucosidase inhibitors, deoxynojirimycin (DNJ), with promising biological activity was observed both in the soybean koji and black soybean koji, which provide an inspiring strategy to produce DNJ-derived foods with low content of saccharides for the diabetic patients.