## 4 Discussion
The plentiful flavor compounds in the fermented grains of the fifth and sixth rounds in the Beijing region were mainly esters, such as ethyl caproate, ethyl acetate, ethyl lactate, ethyl caprylate, and ethyl phenyl acetate, which was similar to that of the third round [13] (Table S3). High concentrations of 1-butanol, 3-methyl-, acetic acid, butyric acid, capric acid, and ammonium acetate were detected in the fermented grains of the fifth and sixth rounds, but there were low concentrations of these flavor compounds or they were not detected in the fermented grains of the third round [13] (Table S3). These results showed that differences in the profile of flavor compounds in fermented grains fromdifferent rounds in the Beijing region were smaller than those in the fermented grains from the Beijing and Guizhou regions. One of the reasons for the differences in the profile of flavor compounds in the fermented grains of sauce-flavor _baijiu_ from the Beijing and Guizhou regions is the difference in microbial community composition. [PERSON] et al. reported that the interactions among _Pichia_, _Zygosaccharomyces_, and _Lactobacillus_ significantly contributed to the formation of 2-acetylpyrrole, tetramethylpyrazine, \(\beta\)-ethylphenethyl alcohol, and benzoic acid during the fermentation process of sauce-flavor _baijiu_ from Guizhou region [15]. In contrast, the concentrations of ethyl acetate, ethyl acrylate, and ethyl phenylacetate, as well as the concentrations of acetic acid and cis-5-dodecenoic acid, were positively correlated with the relative abundance of _Lactobacillus_, _Issatchenkia_, and _Pichia_ during the fermentation process of sauce-flavor _baijiu_ from the Beijing region (Figure 6). Another reason for differences in the profile of flavor compounds is the differences in the fermentation parameters of fermented grains. The generation of tetramethylpyrazine during the brewing process of sauce-flavor _baijiu_ mainly involves two methods: the Maillard reaction and microbial synthesis [29]. The type and concentration of the Maillard reaction substrate in fermented grains have a remarkable effect on the content of tetramethylpyrazine [29]. Dynamic changes in microbial community composition cause changes in the fermentation parameters of fermented grains such as temperature, acidity, and glucose content (Figure 4), which in turn affect the synthesis of flavor compounds such as tetramethylpyrazine [29]. The acidity during the third-round fermentation in Beijing was significantly lower than that in Guizhou, so the concentration of tetramethylpyrazine detected in the fermented grains of sauce-flavor baijiu in the Beijing region was found to be lower than that in the Guizhou region [15]. Therefore, a comprehensive understanding of microbial community succession and fermentation parameter changes during the fermentation process is essential for analyzing the formation of characteristic flavor substances. The microbial community succession in the third round of fermentation of sauce-flavor _baijiu_ in the Beijing region was investigated. With the extension of fermentation time, the relative abundance of _Lactobacillus_ increased, while the relative abundance of _Virgibacillus_, _Oceanobacillus_, and _Kroppenstedtia_ gradually decreased (Figure 2A). The relative abundance of _Byssochlamys_ reached a maximum of 95.16% at the 3 days of heap fermentation and then gradually decreased with time, while _Issatchenkia_ gradually increased, becoming the genus with the highest relative abundance at the end of pit fermentation (Figure 2B). The pattern of microbial community succession during the third round of fermentation was similar to that during the fifth and sixth rounds of fermentation of sauce-flavor _baijiu_ in the Beijing region [13]. The drivers of microbial community succession in fermented grains include microbial interactions and changes in fermentation parameters.