Changes in enzymatic activities during “koji” incubation and natural fermentation of soybean paste

Abstract

Soybean paste production requires two steps “koji” incubation and natural fermentation. During the incubation of the koji, the protease activity was gradually increased and the highest activity was obtained on 30th day. Neutral protease was the predominant protease and it was significantly increased during the incubation of the koji. The nitrogen solubility index, nonprotein nitrogen (NPN), aminophenol nitrogen, and peptides concentration were gradually increased and they were 69.07%, 42.07%, 3.73%, and 38.34%, respectively, at the end of the fermentation process. The activities of α-amylase, glucoamylase, and cellulase have different changing periods. All enzymes showed a significant increase followed by a gradual decrease to reach a linear shape at later koji incubation times. During the moromi fermentation, all enzymes showed a gradual decrease. This result is of immense importance for understanding the enzymatic mechanism involved and improving the final product by acting at different stages along the fermentation process. Practical applications Soybean paste production requires two steps “koji” incubation and natural fermentation. This study aims to understand changes in enzymatic activities during soybean paste production. This result is of immense importance for understanding the enzymatic mechanism involved in the fermentation process and supposed to improve the quality of final products. It may help to improve the final product by acting at different stages along the fermentation process.