Anti-α-glucosidase activity of Chinese traditionally fermented soybean (douchi)

Anti-α-glucosidase activity of aqueous douchi extracts was investigated in this study. Thirty-one douchi samples collected from different parts of China exerted various degree of inhibitory activity against rat intestinal α-glucosidase. Among them, three samples, sourcing from Hunan, Sichuan and Jiangxi province, respectively, showed a significant higher anti-α-glucosidase activities than other samples (p < 0.05). Moreover, three fungal strains, namely Aspergillus oryzae, Actinomucor elegans and Rhizopus arrhizus were then used to prepare douchi in our laboratory. The α-glucosidase inhibitory activities of all soybeans increased slightly and no apparent differences were found in anti-α-glucosidase activity among the soybeans at the end of pre-fermentation. For maturation, different salt levels (5.0%, 7.5%, 10.0% and 12.5%) were then added to the douchi qu resulted from pre-fermentation. The anti-α-glucosidase activity of douchi qu fermented with A. oryzae were higher than those of A. elegans and R. arrhizus and the highest anti-α-glucosidase activities was observed in douchi qu fermented with A. orzyzae at 5.0% and 7.5% salt levels. The results indicated that A. oryzae could utilize cooked black soybean to generate certain α-glucosidase inhibitor more effectively than A. elegans and R. arrhizus.