## 3 Results and Discussion
### Composition of the Microbial Community in Three Kinds of Daqu  
There were mainly 139 bacterial genera and 22 fungal genera detected from three kinds of _Daqu_. Most of the bacterial genera belong to Firmicutes, Actinobacteria, and Proteobacteria, and the fungal genera mainly belong to Ascomycota and Mucoromycota. The top 10 dominant bacterial genera and the top 10 dominant fungi genera were shown in (Figure 5a,b). The results showed that the sum of the relative abundance (RA) of the top 10 dominant bacterial genera in YQ. WQ, and BQ was greater than 89.14%, 90.58%, and 98.47%, respectively. The main dominant bacterial genera in YQ (Figure 5a) were _Kroppentseddia_ (37.32%), _Sacharopolyspora_ (12.52%), _Virgibacillus_ (8.92%), _Scopulibacillus_ (8.63%) and _Cupriavidus_ (8.33%). _Kroppentseddia_ (55.84%), _uncultured_Bacillus_ (13.91%), and _Bacillus_ (13.62%) were the main dominant genera in WQ while the dominant bacterial genera in BQ were _Kroppentseddia_ (82.88%), _Sacharopolyspora_ (8.80%), _Pseudonocardiu_ (1.97%), and _Bacillus_ (1.24%) (Figure 5a). Figure 4: Analysis of melanoidin formation pathway in three kinds of _Daqu_. The sum of the RA of the top 10 dominant fungal genera in YQ, WQ, and BQ was more than 99%. The main dominant fungal genera in YQ (Figure 5b) were _Thermomyces_ (48.71%), _Thermascus_ (34.04%), _Aspergillus_ (11.13%), and _Byssochlamys_ (5.09%). _Thermomyces_ (89.13%), _Thermascus_ (3.36%), _Hyhopichia_ (1.72%), _Byssochlamys_ (1.44%), and _Rhizopus_ (1.10%) were dominant fungi in WQ (Figure 5b). The dominant fungi in BQ (Figure 5b) were _Thermascus_ (49.92%), _Thermomyces_ (42.0%), _Monascus_ (4.92%), and _Byssochlamys_ (2.12%). According to the microbial community composition, there were many kinds of high-temperature-resistant microorganisms (_Kroppenstedtia_, _Saccharomycespora_, _Thermascus_, _Thermomyces_ and _Aspergillus_) in YQ and BQ, and the previous research [26] showed that _Saccharomycespora_ is positively correlated with the temperature of _Daqu_, so it is speculated that the temperature of YQ and BQ is higher during fermentation, which intensifies the Maillard reaction and produces more melanoidin, making the appearance of BQ and YQ darker. Meanwhile, the Maillard reaction also provided more flavor substances for high-temperature _Daqu_. Figure 5: Dominant (top10) bacterial genera (**a**) and fungal genera (**b**) in three kinds of _Daqu_; microbial community difference analysis based on PLS-DA (**c**), R\({}^{2}\)X (cum) = 0.639, R\({}^{2}\)Y (cum) = 0.982, Q\({}^{2}\) (cum) = 0.937; microbial genera with a VIP value greater than 1 are defined as microbial markers (**d**) in three kinds of _Daqu_; Spearman correlation analysis between marked microorganisms and Maillard products in BQ (**e**), the blue represents the positive correlation (\(\rho>0.6\)), while the red represents the negative correlation (\(\mid\rho\mid>0.6\), \(\rho<0\)), \({}^{*}\)\(p<0.05\). According to PLS-DA analysis (Figure 5c), the microbial communities of the three kinds of _Daqu_ were significantly different, and the differential microorganisms in the three kinds of _Daqu_ were analyzed by calculating the VIP value. It can be seen that the RA of _Virgibacillus_, _Cupriavidus_, _Staphylococcus_, and _Aspergillus_ in YQ were significantly higher than those in WQ and BQ, and they were the biomarkers in YQ (Figure 5d, VIP \(>\) 1). _Bacillus_, _Thermonyces_, and _Hyphopichia_ were the biomarkers in WQ (Figure 5d, VIP \(>\) 1). _Bacillus_ is an important source of Tetramethylpyrazine in Daqu [27], which is presumably the reason for the high content of Tetramethylpyrazine in WQ. The RA of _Kroppenstedtia_, _Monascus_, and _Thermoascus_ in BQ was significantly higher than that in WQ and YQ, which were the marked microorganisms in BQ. It has been reported [28] that _Thermoascus_, _Virgibacillus_, and _Aspergillus_ are differential functional microorganisms of three kinds of _Daqu_, which can be used as marker microorganisms to distinguish the three kinds of _Daqu_.