Effects of Variable Temperature Fermentation Technology on the Flavor Profile in Low-Salt Solid-State Soy Sauce Brewing

In order to clarify the impact of variable temperature fermentation technology on the flavor of low-salt solid-state soy sauce during brewing, two fermentation technology, namely natural temperature-controlled fermentation (LSF_S) and gradient temperature-controlled fermentation (LSF_C), were employed using non-targeted metabolomics to reveal the mechanism by which variable temperature fermentation technology drives flavor formation through regulation of core metabolic pathways. The results demonstrated that the Maillard reaction and Strecker degradation during the high-temperature phase of the LSF_S process, employing an initial high temperature (42~45 ℃) followed by a lower temperature (32~35 ℃), were intensely promoted. The rapid accumulation of 3-(Methylthio)propanal (contributing a baked potato aroma) and guaiacol (smoky aroma) were then rapidly accumulated. The synthesis of alcohols such as phenylethyl alcohol was enhanced at the subsequent lower-temperature stage, ultimately yielding a rich flavor profile characterized by distinct sauce-flavor, smoky, and floral notes. In contrast, during the LSF_C process, featuring a gradient increase from an initial 15 ℃ to 45 ℃ at the later stage, yeast enrichment was facilitated at the low-temperature phase, which the synthesis of ethyl acetate (imparting a fruity aroma) was significantly boosted. The accumulation of aldehydes (Isovaleraldehyde, Phenylacetaldehyde) and ketones was promoted at the subsequent high-temperature phase, though phenol synthesis was limited. Consequently, a mellow flavor dominated by caramel, roasted, and floral notes was produced during LSF_C process, with only a subtle fruity character. Metabolomic analysis revealed that metabolic activity was concentrated at the early stage for LSF_S, whereas the metabolic network in LSF_C became more active during the later high-temperature period, enabling sustained conversion of flavor precursors. Further analysis confirmed that a strong association between specific peptides and the formation of key aldehydic flavor compounds was correlated. In conclusion, this study establishes that tailoring the fermentation temperature profile is an effective strategy for restructuring the metabolic network of soy sauce fermentation and achieving targeted modulation of its flavor spectrum.