Exploring the significance of the 2nd and 4th round fermentations in the brewing process of sauce-flavor Baijiu

The unique style of sauce-flavor Baijiu derives from its traditional and complex multi-round fermentation process. Among them, the 2nd round without additional ingredients and the 4th round of fermentation that produces the core sauce flavor play a crucial role in the fermentation process. However, there is still a lack of understanding and in-depth analysis of the differences the 2nd and 4th round fermentation in the unique brewing process of sauce-flavor Baijiu. In this study, we discovered that Lactobacillus was the predominant bacterial genus during the 2nd and 4th fermentation rounds, where acidity and temperature serve as primary physicochemical factors regulating microbial succession during these rounds. Variations in the abundance of yeasts, specifically Issatchenkia, and filamentous fungi, particularly Thermoascus, led to differences in flavor compounds observed between the 2nd and 4th round. Through GC-MS comparative analysis, the 4th round exhibited a greater variety of unique characteristic flavor compounds compared to the 2nd round (28 > 15), with more ester compounds found in the 4th round. This study contributes to a deeper understanding of the rotational fermentation process in sauce-flavor Baijiu, providing new perspectives and insights.