## 3 Results and Discussion
### Correlation Analysis of Flavor Chemicals and Microbial Communities  
During the manufacturing process of traditional fermented Baijiu, many microbes co-existed in the fermentation system, the abundances dynamically fluctuated, and the microbial metabolic pathways of flavor chemicals were dynamically changed [33]. Analyzing the relationship between microorganisms and flavor compounds was helpful to identify the potential functional microbial groups and control flavor synthesis during fermentation [34]. For the flavor chemicals related to product quality improvement in round 3, the correlation with microbes was analyzed (Figure 8a). The metabolic synthesis of hexanoic acid, ethyl ester was only positively correlated with _Lactobacillus_, while negatively correlated with many bacterial genera, such as _Virgibacillus_, _Weissella_, _Scopuliobacillus_ and _Staphylococcus_ (Figure 8a). Octanoic acid, ethyl ester; phenylacetic acid, ethyl ester; 3-(methylthio)-propionic acid ethyl ester; acetic acid, phenylethyl ester, and dodecanoic acid, ethyl ester showed a positive correlation with _Lactobacillus_. Decanoic acid, ethyl ester, had no correlation with _Lactobacillus_, while hexanoic acid, butyl ester, showed a negative correlation with _Lactobacillus_ (Figure 8a). These results indicated that _Lactobacillus_ had an important contribution to synthesize esters and was an important functional microorganism in the fermentation process of sauce-flavor Baijiu. Compared with esters, other flavor chemicals that contribute to the quality, such as benzaldehyde, 2,3-butanediol, and tetramethylpyrazine, showed no significant correlation with _Lactobacillus_ (Figure 8a). Benzaldehyde showed positive correlations with _Rhabdanerobium_, _Clostridium_ sensu stricto 15, _Novibacillus_, norank-Bacillusaceae, _Thermoactinomyces_, unclassified-Bacillus_, and _Oceanobacillus_. 2,3-Butanediol only showed a positive correlation with unclassified_Bacillusaceae. No significant positive correlation was identified between tetramethylpyrazine and all of the bacterial genera. On the contrary, some bacterial genera showed a negative correlation with the production of tetramethylpyrazine, such as _Ochrobactrum_ (Figure 8a). This indicated that bacterial genera played a positive role in synthesizing flavor chemicals, but for tetramethylpyrazine, an important flavor chemical in Baijiu, bacterial genera mainly showed the function of metabolic degradation (Figure 8a). This might indicate that fungi in the fermented grains promoted the metabolic synthesis of tetramethylpyrazine and investigated in the following paragraph. In addition, 4-ethyl guaiacol, usually recognized as an important flavor substance of sauce-flavor Baijiu, was positively associated with _Pedicocus_, _Staphylococcus_, _Breivhacterium_, _Brachybacterium_, norank_Pseudonocardiaceae_, and unclassified_Lactobacillus, while other bacterial genera showed negative correlation with the synthesis of 4-ethyl guaiacol, such as _Thermoactinomyces_ and _Pseudomonas_ (Figure 8a). Thereafter, the metabolic relationship between fungi and flavor chemicals was analyzed. Figure 8: Correlation analysis of flavor chemicals and microbial community in the fermented grains of sauce-flavor Baijiu. Red represents a positive correlation, and blue represents a negative correlation. (**a**), bacteria with flavor chemicals. (**b**), fungi with flavor chemicals. * 0.01 < \(p\)\(\leq\) 0.05, **0.001 < \(p\)\(\leq\) 0.01, *** \(p\)\(\leq\) 0.001. For tetramethylpyrazine, although the dominant fungal genera showed negative correlation, such as _Issatchenka_ and _Saccharomyces_, many fungal genera showed positive correlation, such as _Thermoascus_, _Thermomyces_, _Aspergillus_, _Leiothecium_, and _Rasamsonia_ (Figure 8b), indicating low abundance fungi played an important role for synthesizing tetramethylpyrazine and provided a basis for regulating flavor chemical production by microbial intervention.