Metagenomic insights into the microorganisms responsible for producing amino acid nitrogen during sufu fermentation

Amino acid nitrogen (AAN), a key contributor to umami taste, plays a central role in flavor development during sufu fermentation. Given the critical influence of microbial metabolism on flavor formation, this study employed metagenomic analysis to investigate the mechanisms of AAN generation. During sufu fermentation, the AAN content increased significantly, reaching a final concentration of 0.56 g/100 g. Metagenomic analysis revealed that both bacterial (e.g., Dysgonomonas macrotermitis, Lactococcus lactis) and fungal (e.g., Rhizopus arrhizus) species were the core microbiota driving AAN formation. These microorganisms encoded abundant proteases and amino acid hydrolases essential for AAN metabolism. Functional profiling highlighted carbohydrate and amino acid metabolism as the dominant pathways in flavor formation. Notably, glutamate synthesis was associated with pathways ko00220 and ko00250, mediated by glutamate synthetase (EC 1.4.1.13) and glutamine synthetase (EC 6.3.1.2). These findings elucidate the microbial enzymatic mechanisms-particularly amino acid transformations-underlying sufu's umami taste development during fermentation.