Revealing salt concentration for microbial balance and metabolite enrichment in secondary fortified fermented soy sauce: A multi-omics perspective

This study examined the impact of varying salt concentrations on microbiota, physicochemical properties, and metabolites in a secondary fortified fermentation process using multi-omics techniques. It aimed to determine the influence of salt stress on microbiota shifts and metabolic activities. The findings demonstrated that moderate salt reduction (MS) was found to enhance moromi's flavor and quality, while mitigating the negative effects of excessive low salt (LS). MS samples had 1.22, 1.13, and 2.92 times more amino acid nitrogen (AAN), non-volatiles, and volatiles, respectively, than high salt (HS) samples. In contrast, lactic acid and biogenic amines in LS samples were 1.56 g/100 g and 4115.11 mg/kg, respectively, decreasing to 0.15 g/100 g and 176.76 mg/kg in MS samples. Additionally, the contents of ethanol and small peptides increased in MS due to the growth of specific functional microorganisms such as Staphylococcus gallinarum, Weissella confusa, and Zygosaccharomyces rouxii, while food-borne pathogens were inhibited. Network analysis revealed that the core microbial interactions were enhanced in MS samples, promoting a balanced fermentation environment. Redundancy analysis (RDA) and correlation analyses underscored that the physicochemical properties significantly impacted bacterial community structure and the correlations between key microbes and flavor compounds. These findings provided a theoretical foundation for developing innovative reduced-salt fermentation techniques, contributing to the sustainable production of high-quality soy sauce.