## 3 Results and Discussion
### Correlation Analysis of Flavor Compounds and Microorganisms
However, in sample 4, some fungal genera showed an inhibitory correlation with flavor compounds, such as _Wallemia_, _Thermomyces_, _Papilitorema_, and _Rasamsonia_. The abundances of these fungal genera were not high during the fermentation process. Esters are the most important flavor compounds in Baijiu, among which hexanoic acid, ethyl ester; butanoic acid, ethyl ester; acetic acid ethyl ester; lactic acid, ethyl ester; and octanoic acid, ethyl ester are the main flavor compounds in Strong-flavor baijiu, with fruity, pineapple-flavor, sweet, and floral aromas, and significantly affect the quality of Strong-flavor Baijiu [2,45]. The results of redundancy analysis (RDA) show the influence of the common microorganisms in the four fermentation systems on the main flavor ester compounds, and also reflect the main divergences between different samples. Therefore, an analysis of functional microorganisms and major ethyl esters was performed using RDA. The bacterial-microbial RDA found that the interpretation rate of the two axes was 86.51%, which indicated that microorganisms were closely related with the production of important esters (Figure 9a). The results showed that in sample 2, _Lactobacillus_, _Cloacibacterium_, and _Delftia_ displayed only weakly positive correlations with acetic acid, ethyl ester and lactic acid, ethyl ester. In sample 3, there was a strong positive correlation between butyric acid, butanoic acid, ethyl ester; hexanoic acid, ethyl ester; octanoic acid, ethyl ester and the bacteria _Sporolactobacillus_, _Closridium sensu strictio 1_, _Closridium sensu strictio 13_, _Sarcina_, and _Anaerosporobacter_. In sample 4, _Lactobacillus_, _Cloacibacterium_, _Delftia_, and _Anaerosporobacter_ showed strongly positive correlations with butanoic acid, ethyl ester; hexanoic acid, ethyl ester; octanoic acid, ethyl ester; acetic acid ethyl ester; and lactic acid, ethyl ester. This indicated that the fermentation by the combination of functional microorganisms made good contributions to the medium-chain fatty acids and ethyl esters, while the microorganisms in Daqu showed contributions to the short chain fatty acid ethyl esters, and the use of the functional microbial combination to replace part of the Daqu could combine the advantages of the two fermentation systems, showing a strongly positive correlation with the synthesis of both short- and medium-chain fatty acid esters. Figure 7: Correlation analysis of bacteria and flavor compounds. (a) Sample 2, (b) Sample 3, (c) Sample 4. (\(P<0.05\), \(m>0.01\), \(m>0.001\). Each number represents a flavor compound, and the corresponding data are in Table 3.  
Figure 8: Correlation analysis of fungi and flavor compounds. (**a**) Sample 2, (**b**) Sample 3, (**c**) Sample 4, (**c** \(p\) -0.05, **\(p\) -0.01, **\(p\) -0.001. Each number represented a flavor compound, and the related data are listed in Table 3.  
The RDA of fungal microorganisms found that the explanation rate of the two axes reached 94.9%, which indicates that fungi show close correlations with the production of flavor esters (Figure 9b). The results show that all of the samples are clustered together during the early and middle stages of fermentation, while there are significant differences in the middle and late stages of fermentation. Positive correlations are shown between _Saccharomyces_, _Cephalotrichum_, and _Chaenothecopsis_ and hexanoic acid, ethyl ester; butanoic acid, butyl ester; lactic acid, ethyl ester; acetic acid, ethyl ester; and octanoic acid, ethyl ester in sample 4. The abundance of _Saccharomyces_ in all samples was high, and it showed a strongly positive correlation with several ethyl esters, which might be due to the fact that _Saccharomyces_ mainly produced ethanol during fermentation, and ethanol served as the precursor for synthesizing the ethyl esters. In addition, there were positive correlations between the fungi _Saccharomyces_, _Colacologoa_, _Chaenothecopsis_, and _Trichosporon_ and the flavor compounds butyric acid and butanoic acid, butyl ester.