## 3 Results and Discussion
### Correlation Analysis Between Flavor Compounds and Microorganisms  
[PERSON]'s correlation analysis was used to investigate the correlations between the volatile components and microorganisms in douchis [49]. In Figure 7, a red hue suggests a positive correlation; the darker the color indicates the stronger the positive correlation (a correlation coefficient closer to +1). A blue hue suggests a negative correlation, and the darker the color indicates the stronger the negative correlation (a correlation coefficient closer to \(-1\)).  
Figure 6: O2 PLS-DA results for different douchi products. (**a**) The O2 PLS-DA scores’ plots of the volatile compounds and amino acids in the douchi products; (**b**) model cross-validation results.  
Among the bacteria, _Bacillus_ showed the highest number of highly positive correlations (\(r>0.7\)) to flavor compounds (4), followed by _Tetragenococcus_ (2), _Pseudomonas_ (2), and _Weissella_ (1), while _Pseudomonas_ was highly negatively correlated with 7 flavor compounds. Furthermore, _Bacillus_ was significantly positively correlated with isovaleraldehyde, phenylacetaldehyde, ethyl caprylate, and 2-pentylfuran, which indicated its important contribution to the metabolism of the volatile compounds in the douchi. Previous studies had shown that _Tetragenococcus_ strongly influences the flavors of fermented foods [50; 51]. In this study, _Tetragenococcus_ showed significantly positive correlations with ethanol and ethyl acetate. On the negative side of the correlation, _Pseudomonas_ was the more prominent bacterial genera, which is a finding similar to those in previous research [49]. Of these, _Pseudomonas_ was strongly negatively correlated with 1-octen-3-ol, isovaleric acid, isoamyl acetate, acetaldehyde, and phenylethyl alcohol.  
In fungi, _Mucor, Meyerozyma_, and _Isatchenkia_ showed the highest number of highly positive correlations (\(r>0.7\)) to flavor compounds (6), followed by _Hyhphichia_ (5), _Penicillium_ (4), and _Aspergillus_ (2). _Mucor_ and _Penicillium_ were strongly positively correlated with 1-octen-3-ol and ethyl isobutyrate. Meanwhile, _Hyhphichia, Meyerozyma_, and _Isatchenkia_ were strongly positively correlated with ethyl benzoate and isobutyraldehyde and negatively with ethyl isobutyrate and 1-octen-3-ol. It is interesting that Glu only had a positive correlation with _Mucor_.  
Previously, studies on flavor-producing bacteria in douchis have focused almost exclusively on _Staphylococcus_, _Escherichia-Shiegella_, and _Puccinia_ [4]. Studies on flavor-producing fungi in douchis have shown that _Aspergillus_ and _Penicillium_ were positively  
Figure 7: [PERSON]’s correlation heatmap between the dominant microbial community and volatile metabolites of the douchi products. The correlation coefficients appear in different colors; the right side of the legend is the color range of the different values. Significant values were shown as *_p_ < 0.1; **_p_ < 0.02, ***_p_ < 0.01.  
correlated with citric acid, ethyl acetate, phenol, and benzaldehyde, and _Debaryomyces_ was positively associated with ethyl butyrate and Glu [4,38], whereas in this study, many other bacteria and fungi were found to correlate strongly with volatile compounds. It is an interesting finding, and some applications deserve to be carried out to verify whether these bacteria do have the ability to improve the flavor of douchis.