Co-design of processing mode and synthetic microbial community for improving the quality of reduced-salt soy sauce

Reducing sodium in soy sauce is constrained by diminished flavor development, increased batch variability, and elevated safety risks, all of which are strongly influenced by fermentation conditions. To address this, we evaluated whether two synthetic microbial communities (SynMC-A and SynMC-B) could steer quality formation under an industrial temperature-controlled (C-mode) versus a traditional ambient (N-mode) process at reduced (13%) and conventional (16%) salt levels. Multi-omics profiling showed that the C-mode enhanced flavor consistency, with volatile compounds 1.4-2.7 times higher than in the N-mode. Reducing salt to 13% increased amino acid nitrogen and key aromas such as 4-ethylguaiacol, but concurrently elevated tyramine accumulation (up to 316 mg/L), highlighting a critical safety-flavor trade-off. Compared to SynMC-B, SynMC-A demonstrated greater environmental robustness and more targeted reprogramming of flavor metabolism. It enhanced the content of esters and phenolic compounds and elevated umami-associated metabolites under both processes. In conclusion, the synergy between process temperature stabilization and functionally tailored SynMCs provides an actionable framework for reduced-salt soy sauce fermentation. This framework effectively enhances the formation of sensory-relevant metabolites while mitigating the inherent safety trade-offs.