Abstract: To investigate the correlation between the microbial community structure and composition and physicochemical indicators as well as the sources of microorganisms in fermented grains during the stacking and pit fermentation processes in the first round of jiang-flavor baijiu production, traditional culture methods and Illumina MiSeq sequencing were used to study the composition and succession of microbial communities in the fermented grains and the brewing environment, and their correlation with physicochemical factors was analyzed. Furthermore, the SourceTracker software was employed to determine the sources and proportions of dominant microorganisms in the fermented grains. The results showed that on the 6th day of stacking fermentation, both the total bacteria and yeast counts reached maximum values of (3.45 ± 0.15) × 106 CFU/g and (40.5 ± 2.50) × 104 CFU/g, respectively. Unclassified_Lactobacillaceae (91.23%–91.77%) and Pichia (32.84%–91.00%) were the absolute dominant bacterial and fungal genera. Unclassified_Lactobacillaceae showed a significant positive correlation with the moisture content and acidity of the fermented grains, and Pichia exhibited a significant positive correlation with the moisture content and reducing sugar content. The microbial source tracing analysis indicated that the dominant microorganisms on the 0th and 3rd day of stacking fermentation primarily originated from the ground (32.44%–57.37%) and production utensils (26.89%–50.74%). From the 6th day of stacking fermentation to 43rd day of pit fermentation, the dominant microorganisms mainly originated from surfaces such as window sills and walls (86.41%–87.31%). Specifically, unclassified_Lactobacillaceae mainly came from the ground (85.79%) across the stacking and pit fermentation processes and from windowsill and wall surfaces (91.96%–94.91%). There were more diverse sources of Pichia (1.32%–87.06%) in the brewing environment. Before the 6th day of stacking fermentation, its major sources were the air inside the workshop and the surface of production utensils, windowsills, and walls. During pit fermentation, its major sources were the inner wall of the pit, accounting for 18.21%–30.79%. This study provides a theoretical basis for elucidating the brewing mechanism of jiang-flavor baijiu.

Key words: jiang-flavor Baijiu; stacking and pit fermentation; correlation; microbial community structure; microbial source tracing