Effects of microbial succession on the dynamics of flavor metabolites and physicochemical properties during soy sauce koji making

Koji making is a crucial stage of soy sauce production that affects the later fermentation efficiency and final product quality. However, little is understood about the dynamics and roles of microbiota throughout the koji making, especially the functional microbes responsible for flavor formation. In this work, high-throughput sequencing revealed that the top 3 bacteria were Weissella, Staphylococcus, and Bacillus, and the top 2 fungi were Aspergillus and Trichosporon. The fermentation process could divide into 2 stages based on microbiota succession: 0 h and12-44 h. Additionally, 6 organic acids, 15 amino acids, and 30 volatile flavors were detected, of them, 1-octen-3-ol and 3-octanone were the two characteristic volatile flavors of koji. Analysis of the correlation between microbiota, physicochemical properties, and flavors indicated that Weissella, Staphylococcus, Bacillus, and Trichosporon were the key microorganisms responsible for promoting environmental development and the formation of flavors. What's more, Staphylococcus, Leuconostoc, and Pichia were identified strongly positively correlated with 1-octen-3-ol and 3-octanone, which probably play pivotal roles in the unique aroma formation of koji. These findings provide insight into the potential for developing profitable starter cultures for the manufacture of high-quality soy sauce products.