## 3 Results and Discussion
### Effect of BZ25 Addition on the VOCs of Natto
[41] found that the co-fermentation of two strains during mango wine fermentation may decrease volatile acid and improve flavor. Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA)  
To visualize a total picture of the distributions of 73 odor-active compounds (OAV > 1) in all samples, PCA was applied (Figure 2A). PCA showed the major differences among the volatile profiles of four samples. The first principal component (PC1) of the PCA accounted for 50.1%, and PC2 explained an additional 29.7% of the total variability. And the cumulative contribution rate reached 79.8%. PCA Score Plot (Figure 2A) shows that NMB and SFB were located in the first and second quadrants and positively correlated with PC2. SS and NMBB were located in the third and fourth quadrants and negatively correlated with PC2. Four samples were distributed in different quadrants, indicating that there were significant differences in the flavor substances between samples. The characteristic components of SS were 2-methyl-propanoic acid ethyl ester (B4), 3-octanone (D8), 3-methyl-butanol (F3), 6-methyl-5-hepten-2-one (D19), phenylacetaldehyde (F6), eucalypttol (G20), 2-pently-furan (I1), 2-ethyl-furan (I3); those of SFB were 1-hexanol (G4), 4-methyl-3-penten-2-one (D15), and 2-methoxy-phenol (H1); those of NMB was \(N\),_N_-dimethyl-methylamine (A1), 2-ethyl-3,5-dimethyl-pyrazine (C3), acetic acid (E1), 2-methyl-butanol (F4), and those of NMBB were hexanoic acid ethyl ester (B1), trimethyl-pyrazine (C1), 2-ethenyl-6-methyl-pyrazine (C8), 2-hexadecanone (D3), 2,3-butanedione (D4), nonanal (F2), 3-octanol (G3), biphenyl (H2), styrene (J3), and naphthalene (J6). Therefore, the characteristic flavor components of natto can be changed by mixed fermentation of two bacteria. Figure 2B shows hierarchical clustering and heat map visualization of volatile compounds of four samples in different treatments (OAV > 10). The gallery plot of four samples was applied to visualize the differences in aroma compounds. The aroma components of natto produced by co-fermentation were different from those produced by single strain GUTU09. Next, we used clustering analysis for volatile compounds content and found that the four groups could be divided into three categories. Class I (SFB and NMB) included high content of acetic acid (E1), 2-ethyl-3,5-dimethyl-pyrazine (C3), 2-methyl-butanal (F4), 1-hexanol (G4), 2-methyl-3-hexanol (G9) and 2-methoxy-phenol (H1). Class II (SS) contained high content of 2-pently-furan (I1), 2-ethyl-furan (I3), hexanal (F9), 6-methyl-5-hepten-2-one (D19), eucalypttol (G20), 3-methyl-butanal (F3), 2-methyl-propanoic acid ethyl ester (B4), octanal (F5), 3-octanone (D8) and phenylacetaldehyde (F6). Class III (NMBB) included high content of 2,3-hexadecanone (D3), 2,5-dimethyl-pyrazine (C2), trimethyl-pyrazine (C1), hexanoic acid ethyl ester (B1), 1-octen-3-ol (G1), styrene (J3), biphenyl (H2), 3-octanol (G3), naphthalene (J6), 2,3-butanedione (D4) and 2,3-dihydro-benzofuran (I2). Figure 2B shows that the VOCs of NMBB accumulated more than that of NMB, and the flavor substances that contribute more to the whole are very different from NMB. PCA plot and gallery map show that mixed fermentation can greatly change the composition of VOCs of natto, thus affecting the sensory score and increasing people's acceptance of natto.