# Correlation Analysis of Microbiota and Volatile Flavor Compounds of Caishiji Soybean Paste
## 4 Discussion  
VFCs and microbiota strongly affect the flavor of soybean paste. In this study, VFCs and microbiota in CSP at different fermentation stages (CG0, CG1, CG2, CG3, CG6, CG8) were detected to identify potential correlations between them. More than 70 types of VFCs, including 12 types of characteristic VFCs (based on OAV), were detected. Among them, esters, alcohols, and aldehydes were the most prevalent in the CSP fermentation process. The contributions of VFCs to CSP flavor depend not only on their concentrations but also on their odor threshold values [22]. Esters (especially short-chain esters), which are  
Figure 4: Heat-map (**a**,**b**) and RDA (**c**,**d**) correlation analysis of microbiota and characteristic VFCs of Caishiji bean paste in different fermentation stages. (\(R\) value was displayed in different colors in the heat-map. If the \(p\) value was less than 0.05, it is marked with *, the legend on the right side was the color range of different R values. * 0.01 \(<p\leq\) 0.05, ** 0.001 \(<p\leq\) 0.01, *** \(p\leq\) 0.001. In RDA, the red arrow indicates the flavor substance, and the length of the arrow indicates the impact degree of the flavor substance on the species; the angle between the environmental factors arrows represents positive and negative correlation (acute angle: positive correlation; obtuse angle: negative correlation; right angle: no correlation); the distance from the projection point to the origin represents the relative impact of the flavor substance on the distribution of the sample community). formed by a series of esterification reactions, are vital aromatic compounds in fermented foods, and it can endow fermented foods with sweetness and fruitiness. In the present study, 15 esters were detected during the CSP fermentation process. Among them, amyl acetate, \(\alpha\)-pentyl-\(\gamma\)-butyrolactone, and ethyl palmitate, which have also been identified in other fermented soybean products, were common to CSP at differing fermentation stages. Based on the results of OAV analysis, ethyl butyrate, ethyl caproate, and hexyl caproate were the esters that most strongly affected the flavor of CSP. Ethyl butyrate, which produces a beamy flavor [28], was detected only in CG3, and ethyl hexanoate and hexyl caprylate, which produce fruity flavors [28], were detected in CG1 and CG3. Most of the alcohols, which can endow CSP with pleasantly sweet and floral flavors, were formed in the later fermentation period. Isoamyl alcohol, 2-methyl-1-butanol, 1-pentanol, 2,3-butanediol, and 1-nonyl-3-ol were detected in CG6 and CG8. Phenethyl alcohol, which produces aromatic flavors [29], most strongly affected the flavor of CG3 according to the results of the OAV analysis. Aldehydes, including monoolefinic aldehyde, dialdehyde, and saturated aldehyde, which are mainly produced by the lipoxygenase-catalyzed decomposition of linoleic and linoleic acids, can endow fermented foods with sweet, fruity, nutty, and caramel flavors [30]. Fourteen aldehydes were detected in CSP, most of which were formed during the late stages of the fermentation process. The results of the OAV analysis indicated that seven characteristic aldehydes affected the flavor of CSP. Isovaleraldehyde, 2-methylbutylraldehyde, and benzaldehyde, which produce fruity flavors, stimulating oders, and cherry like flavors, respectively [29], were detected in each fermentation stage; 3-methylthiopropionaldehyde, which had the highest OAV value, can endow CSP with a strong roasted-potato flavor [29]. Although the other VFCs had lower thresholds, they may also endow CSP with strong flavors. Microbiotas are the main contributors to soybean paste fermentation [31; 32]. The fermentation of soybean paste involves a variety of bacteria and fungi. In addition, soybean pastes with distinct microbiota exhibit distinct flavors and qualities. In some previous studies, _Bacillus_, _Aspergillus_, _Tetragonococcus_, _Salinivibrio_, _Halomonas_, and _Staphylococcus_ were the most abundant genera in CSP (Figure 3).