Improving protein utilization and fermentation quality of soy sauce by adding protease

Soy sauce is the most popular traditional fermented condiment in Asia. However, some proteins are unutilized during fermentation, causing resource waste. Therefore, this study aimed to improve protein utilization and fermentation quality by adding protease in the moromi. Firstly, Aspergillus oryzae (A. oryzae) endogenous proteases were found to play an important role in nitrogen element utilization, including increasing total nitrogen (TN), amino acid nitrogen (AAN), and relative protein expression. Secondly, among six exogenous proteases, neutral protease (NP) performed better both alone and synergistically with A. oryzae crude protease powder (CPP), because of the enhancement of small peptides and enzymatic effect. Importantly, adding NP increased AAN and TN by 7.45 % and 2.21 % respectively, as well as increasing peptides below 500 Da, Maillard peptides and the proportion of lactic to acetic acid, hence improving the umami and richness of soy sauce. Overall, adding NP could increase soy sauce protein utilization and fermentation quality.