Microecology succession process from soy sauce koji to mash and its contribution to soy sauce flavor

To investigate the importance of soy sauce koji preparation, the microecology succession process from soy sauce koji to mash and its effect on soy sauce flavor were analyzed using high throughput sequencing technology combined with electronic nose, sensory quantitative description analysis (QDA) and gas chromatography-mass spectrometry (GC-MS). The results showed that bacterial genera such as Weissella, Tetragenococcus, Bacillus and Enterococcus in soy sauce koji persisted throughout the entire fermentation period. Among them, the relative abundance of Weissella was the highest in Quchi (68.7%), decreased to over 40% during the koji-making period, remained at 10%-20% in the early stage of brewing, and dropped to the lowest in the later stage of brewing (7.0%). In contrast, the relative abundance of Tetragenococcus increased progressively with brewing time, peaking at 41.3% in the later stage of brewing. The relative abundance of Bacillus and Enterococcus remained stable (10%-20%). Aspergillus played a crucial role during both the koji-making and brewing stages of soy sauce, with an average relative abundance of 74.9%. However, the relative abundance of Aspergillus decreased in the later stage of brewing, while that of Zygosaccharomyces increased, reaching the highest (77.4%) at brewing 90 d. The connectivity of the microbiota co-occurrence network in the later stage of brewing was 2.6 times that of the koji-making stage, indicating that the microecology in koji evolved to the brewing stage and showed one-to-many microbiota interactions. Bacteria served as the source for the conversion of aldehydes into acids, while Tetragenococcus and Zygosaccharomyces were responsible for the production of alcohols and esters. In summary, the microecology in koji played a crucial role in soy sauce brewing, laying a foundation for the shaping of the microbial community and the targeted regulation of flavor in soy sauce brewing.