Co-Fermentation of Edible Mushroom By-Products with Soybeans Enhances Nutritional Values, Isoflavone Aglycones, and Antioxidant Capacity of Douchi Koji

Douchi is a traditional salt-fermented soybean food with various bioactivities, such as anti-oxidation, anti-diabetes, and anti-hypertension, which are greatly affected by the activities of protease and β-glucosidase during koji production. Edible mushroom by-products are ideal ingredients for enhancing food flavor and nutritional quality due to their unique nutritional characteristics of high protein, rich amino acids, and low calories. However, there is no research on the preparation of Douchi by the mixed fermentation of edible mushroom by-products and soybeans. In this study, response surface methodology (RSM) was used to optimize the fermentation conditions of edible mushroom by-product Douchi koji (EMDK) with protease and β-glucosidase activities as indicators, and the changes in the main bioactive compounds and antioxidant activities of unfermented raw samples (URS), Douchi koji without edible mushroom by-product (DKWE), and EMDK were compared. The results of single-factor tests and RSM showed that the optimal fermentation conditions of EMDK were the Aspergillus oryzae to Mucor racemosus ratio of 1:1, inoculation amount of 6%, edible mushroom amount of 21%, and fermentation time of 63 h, and the activities of protease and β-glucosidase under these conditions were 796.03 ± 15.01 U/g and 1175.40 ± 36.98 U/g, respectively. Additionally, compared with URS and DKWE, the contents of total isoflavones and β-glucoside isoflavones in EMDK were notably decreased, while the contents of amino nitrogen, total phenolics, total flavonoids, and aglycone isoflavone, as well as the antioxidant capacity were significantly increased. Furthermore, significant correlations were found between the above components and antioxidant capacity. These results showed that edible mushroom by-product could be incorporated into soybeans for co-fermentation, conferring higher nutritional value to and antioxidant capacity of Douchi koji.