## 4 Conclusions  
In this work, the flavor chemicals and microbial communities of fermented grains in the first three rounds of the base Baijiu manufacturing process were analyzed and correlated. Results indicated that the increased concentrations of important chemicals such as hexanoic acid, ethyl ester; octanoic acid, ethyl ester; decanoic acid, ethyl ester; dodecanoic acid, ethyl ester; phenylacetic acid, ethyl ester; 3-(methylthio)-propionic acid, ethyl ester; acetic acid, phenylethyl ester; hexanoic acid, butyl ester, 2,3-butanediol, and tetramethylpyrazine were related to improving the quality of base Baijius. One of the dominant bacterial genera, _Lactobacillus_, and many of the fungal genera with low abundances showed positive interaction with the synthesis of esters, while many low-abundance fungal genera showed positive interaction with the synthesis of tetramethylpyrazine. This work also proposed the synthetic pathway of flavor esters and the synthesis and degradation pathways of tetramethylpyrazine. The above results provide basic data references for the quality improvement of base Baijius in different rounds of sauce-flavor Baijiu.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods12010207/s1](https://www.mdpi.com/article/10.3390/foods12010207/s1)]([https://www.mdpi.com/article/10.3390/foods12010207/s1](https://www.mdpi.com/article/10.3390/foods12010207/s1)). Figure S1. Sample collection positions of fermented grains in Jiaochi during the sauce-flavor Baijiu manufacturing process; Figure S2. PCoA analysis of the bacterial genera (a) and fungal genera (b) of the fermented grain samples in rounds 1b, 2, and 3 of sauce-flavor Baijiu fermentation; Figure S3. Evolutionary relationship analysis of tetramethylpyrazine oxygenase with homologous enzymes from different microorganisms by MEGA software with the Maximum likelihood method and a bootstrap value of 1000; Table S1. Volatile compounds of the fermented grain samples from round 1b based on gas chromatography-mass spectrometry analysis (ug/kg); Table S2. Volatile compounds of the fermented grain samples from round 2 based on gas chromatography-mass spectrometry analysis (ug/kg); Table S3. Volatile compounds of the fermented grain samples from round 3 based on gas chromatography-mass spectrometry analysis (ug/kg).  
Data curation, Y.X., M.W., J.N., H.Z. and K.W.; Formal analysis, Y.X.; Funding acquisition, Y.X., X.L. and B.S.; Investigation, Y.X., M.W. and M.L.; Methodology, Y.X., M.W., M.L., H.Z. and K.W.; Software, J.N.; Supervision, Y.X., X.L. and B.S.; Validation, Y.X.; Writing--original draft, Y.X.; Writing--review & editing, X.L. and B.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (No. 31830069, 32072165), Beijing Municipal Natural Science Foundation & Beijing Municipal Education Commission (No. KM202110011003, KZ202010011018).  
Data Availability Statement: Data are contained within the article. All the data generated for this study are available on request to the corresponding author.  
Beijing Huadu Wine Food Limited Liability Company offers all the fermented grain samples analyzed in this work.  
The authors declare no conflict of interest, and they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.