food metagenome Raw sequence reads

Koji making is a pre-fermentation stage for soy sauce manufacturing and impacts the quality of the final product. However, the changes in microbial community and metabolites during koji making have not been well studied. In this study, Firmicutes and Ascomycota were identified as the most predominant phyla in the beginning of koji making (0-12h). The fungi genus Aspergillus and bacterial genus Weissella increased remarkable, from 0.38% to 35.77%, and from 0.12% to 28.38% after 48h fermentation, respectively. Metabolite analysis revealed that the concentrations of aspartic acid, lysine, methyl acetate, isovaleraldehyd, and isoamyl alcohol increased approximately 7, 9, 5, 49, and 10 times from 0h to 48h-fermentation. Functional profiling of metagenomes showed that koji samples were predominated by genes related to carbohydrate metabolism and amino acid metabolism, and koji microbiome exhibited a drastic shift of functional potential after 24h fermentation. In carbohydrate and amino acid metabolism, all functional potential increased during koji making, except for pyruvate metabolism, glycolysis/gluconeogenesis and citrate cycle. Correlation analyses of microbial composition and metabolites showed that Aspergillus, Lactococcus, Enterococcus, Corynebacterium and Pediococcus were mostly correlated positively with amino acids (all P < 0.05), while Weissella associated positively with volatile flavor compounds, such as phenylacetaldehyde, acetic acid, and 2,3-butanediol, ethyl acetate (all P < 0.05). The results of this study provide important information for understanding the microbial function and mechanism of flavor formation during koji making.