## 4 Discussion  
The flavor of sauce-flavor _baijiu_ produced in Beijing and Guizhou is different, but the reason is unclear. The current study investigated the temporal profile of the microbial community and flavor compounds in the fermented grains of sauce-flavor _baijiu_ produced in the Beijing region during the third round of fermentation; among them, the dominant fungi included _Byssochlamys_, _Issatchenkia_, _Monascus_, _Thermoascus_, _Aspergillus_, and _Thermonysces_, while the dominant bacteria included _Lactobacillus_, _Virgibacillus_, _Oceanobacillus_, _Kroppenstedtia_, and _Bacillus_ (Figure 2). In contrast, the dominant fungi in third-round fermented grains of sauce-flavor _baijiu_ in the Guizhou region included _Pichia_, _Zygosaccharomyces_, _Monascus_, _Saccharomyces_, _Thermoascus_, _Aspergillus_, and _Thermonysces_, while the dominant bacteria included _Lactobacillus_, _Virgibacillus_, _Oceanobacillus_, _Kroppenstedtia_, _Sphingobacterium_, _Enterobacter_, _Pantoea_, and _Bacillus_[27; 28]. This study was the first to compare and analyze the microbial community composition in the fermented grains of sauce-flavor _baijiu_ in the Beijing and Guizhou regions during the third round of fermentation. The results showed that the difference in fungi in the fermented grains of sauce-flavor _baijiu_ between the Beijing area and Guizhou area was more significant than that of the bacteria. To better understand the differences in the microbial community composition of sauce-flavor _baijiu_ in different geographic areas, we compared the microbial community structure in fermented grains from the third, fifth, and sixth rounds of fermentation in the Beijing region. We found that the dominant bacterial genera were approximately the same among the three rounds, with _Lactobacillus_, _Virgibacillus_, and _Oceanobacillus_ as the dominant bacterial genera. However, there were slight differences in the composition of the fungal genera, with the dominant fungal genera in the third round being _Byssochlamys_, _Issatchenkia_, and _Monascus_; the dominant fungal genera in the fifth round being _Issatchenkia_, _Thermoascus_, and _Thermonysces_; and the dominant fungal genera in the sixth round being _Issatchenkia_, _Thermoascus_, and _Byssochlamys_[16]. Our previous study found that differences in the abundance of _Pichia_ and _Issatchenkia_ in the brewing environment partially contributed to the differences in microbial composition of the fermented grains in the Guizhou and Beijing regions [16]. The differences in the microbial community structure of fermented grains from different rounds in the Beijing region were smaller than those from the Beijing and Guizhou regions, suggesting that the sauce-flavor _baijiu_ microbiome is geography dependent [15]. There is an inevitable relationship between the flavor of sauce-flavor _baijiu_ and its microorganisms [15; 29]. The volatile compounds analysis showed that esters, such as ethyl caproate, ethyl acetate, ethyl lactate, ethyl caprylate, ethyl phenyl acetate, phenethyl acetate, ethyl myristate, and ethyl palmitate, and alcohols, such as ethanol and phenylethanol, were abundant in the fermented grains (Table S3). In contrast, the main flavor compounds in the fermented grains of the Guizhou region included ethyl lactate, ethyl phenylacetate, ethyl propionate, 2-pentyl-furan, 3-hydroxy-2-butanone, 4-ethyl-2-methoxyphenol, and (2r,3r)-butanediol [15]. Moreover, butanoic acid, 3-methylbutanoic acid, tetramethylpyrazine, and other substances were considered to be the chemical markers and flavor-related chemicals in the fermented grains of sauce-flavor _baijiu_ in the Guizhou region; however, only a relatively low concentration of tetramethylpyrazine was detected in samples from the Beijing region [15] (Table S3), suggesting that the profile of flavor compounds in the fermented grains of sauce-flavor _baijiu_ in the Beijing and Guizhou regions had significant differences.