Assisted Fermentation by a Modified Bacillus subtilis Strain Producing Protease Improved the Quality of Sufu

Traditionally fermented sufu is popular because of its flavor, abundance of nutrients, and long shelf life. However, traditional sufu is difficult to produce via industrial processes because of dominant microorganism attenuation during fermentation. Herein, specific protease-producing strains were isolated from traditional sufu. After strain identification, mutation, and domestication, the strains were applied in fermentation. The taste, texture, and nutrient and flavor components of the fermentation products were investigated via organoleptic, textural HPLC and HS-GC-IMS analyses. Results revealed that Bacillus subtilis (DF1) and the derived strains DF1v and DF1vd had increased protease activity relative to other strains. When these strains were applied for sufu fermentation, the production period significantly shortened to 6-8 days for pehtzes and to 20-26 days for postripening. The nutrient and flavor compound composition of both sufu pehtzes and products improved, including increases in water-soluble proteins, amino acids, and substances with beany and umami aromas and decreases in nonbeneficial biogenic amines and moldy odor-imparting substances. Among the strains, DF1vd showed the greatest benefits in sufu-assisted fermentation. In summary, a modified Bacillus subtilis strain (DF1vd) producing protease was isolated, which improved the nutrient profile and flavor of sufu and shortened the production period.