Effect of Gradient Heating Method on the Microbial Changes and Flavor Formation of Liquid-State Soy Sauce Fermented with Different Salt Concentrations

In this paper, liquid-state fermented sauce mash with salt concentrations of 12% and 18% (w/v) was used as the research object, to explore the effect of gradient heating (15 ℃-20 ℃-25 ℃-30 ℃) on the microbial counting and flavor formation of fermented sauce mash. The results showed that the yeast counts of the low-salt sauce mash sample (TC12) using the gradient heating method was about 1.0 log CFU/g higher than that of the constant temperature (30 ℃) control group (C12) at the end of fermentation, which indicated that the yeasts maintained growth and metabolism for a long time in the gradient heating group. This might be related to the fact that the lower temperature in the early stage of fermentation limited the rapid propagation of yeasts. Among them, the amino nitrogen of the fermented soy sauce from TC12 was as high as 1.14 g/100 g, which met the standard of premium-grade brewed soy sauce in China. At the same time, the content of umami (19.03 g/L) and sweet amino acids (19.32 g/L) in TC12 was high, accounting for about 60% of the total amino acids, presenting a better sweet and savory taste. The volatile compound detection data showed that the content of ethanol, ethyl phenylacetate, ethyl palmitate, 3-(methylthiopropionaldehyde) and benzaldehyde in TC12 was relatively high, and the sample showed an overall sweet-fruity aroma and ester-like aroma. On the other hand, the gradient heating sample of the high-salt group (TC18) contained more guaiacol and methyl maltol, showing a smoky incense and saucy aroma. The sensory evaluation results showed that the low-salt gradient heating fermentation samples TC12 had higher aroma and taste scores, as well as better comprehensive tastes than the constant temperature fermentation samples C12. In summary, the gradient heating method is beneficial to limit the excessive growth of yeast in low-salt (12%) fermented sauce mash in the early stage and maintaining a considerable cell count of yeast in the late stage of fermentation, so that the yeast can continuously ferment and generate more flavor substances and improve the overall flavor of low-salt fermented soy sauce.