## 3 Results and Discussion
### Correlation Analysis between Aroma Compounds and Bacteria at Genus Level  
To illustrate the relationships between aroma profiles of CSBs and bacteria in the corresponding fermented dough, correlation analysis was performed on the content of aroma compounds in CSBs and the relative abundance of three dominant genera including _Lactobacillus_ (_Lactobacillus_ represents 25 genera, including _Companilactobacillus_, _Fructilactobacillus_, _Levilactobacillus_, _Latilactobacillus_, etc.) [34], _Pediococcus_, and _Weissella_ in fermented dough, and the results are shown in Figure 5B. The aroma compounds of CSBs were divided into two clusters using hierarchical clustering. The first cluster included most of the aldehydes (hexanal, heptanal, octanal, (\(Z\))-2-heptenal, nonanal, and (\(E\), \(E\))-2,4-nonnadienal) and ketones (acetone, 2-heptanone, 2-octanone, and 3-octen-2-one), and four kinds of alcohols (1-pentanol, 1-hexanol, 1-heptanol, and 1-nonanol). Moreover, almost all of the short-chain aldehydes (C6-C9) were distributed in this cluster, and they are generally believed to be formed by auto-oxidation of unsaturated fatty acids from wheat flour, with hexanal mainly generated from C18:2, and the others from C18:1 [35]. Hexanal is a typical product in the oxidation of linoleic acid and arachidonic acid and is considered to be negatively correlated with the bread flavor [36]. [PERSON] et al. [35] demonstrated that heptanal, octanal, and nonanal were formed in sponge cake through a common pathway in auto-oxidation of unsaturated oleic acid. Furthermore, aldehydes with oxidative flavors in wheat bread are denoted as off-flavors when present in high concentrations [37]. _Pediococcus_ and _Weissella_ in fermented dough were positively correlated with short-chain aldehydes, while _Lactobacillus_ was negatively correlated with them, suggesting that dough fermented by _Pediococcus_ or _Weissella_ had more lipid-oxidation products than the dough fermented by _Lactobacillus_. Similar results were found by [PERSON] et al. [38], who showed that _P. pentosaceus_ participated in the lipid oxidation of _broccoli_ juice fermentation, affected the content of lipid degradation products including volatile aldehydes and alcohols, and enhanced the sensory properties of broccoli juice. Lipid oxidation in wheat flour and microbial metabolism are the main pathways for the formation of alcohols in fermented dough. In the lipid oxidation pathway, the alcohols are generated by lipoxidase-catalyzed oxidation of fatty acids and conversion of lipid-oxidizing compounds such as aldehydes to the corresponding alcohols [39]. _Pediococcus_ and _Weissella_ in fermented dough were positively correlated with 1-hexanol (green), 1-nonanol (citrus), and 1-heptanol (green), but _Lactobacillus_ was negatively correlated with them. It was indicated that the fermented doughs containing _Pediococcus_ and _Weissella_ showed stronger lipid oxidation reactions, which provided more precursors for alcohol conversion and formed more alcohol flavor compounds than _Lactobacillus_ in dough. Similarly, ketones, which are also characteristic flavor compounds in wheat bread, were principally produced via lipid oxidation [40]. Figure 5: **(A)** Biplot drawn based on the first two principal components extracted by principal component analysis (PCA) of volatile compounds in alkali-free Chinese steamed breads (CSBs) and alkali-added CSBs. The names of the CSB samples and compounds in the biplot refer to Table S3. The numbers 1–16 represent alkali-free CSBs and alkali-added CSBs fermented by CTSs from eight regions of China in the sequence WW-AB, WW-FB, WN-AB, WN-FB, YC-AB, YC-FB, LY-AB, LY-FB, LK-AB, LK-FB, HZ-AB, HZ-FB, BZ-AB, BZ-FB, WH-AB, and WH-FB. Alc, Ald, Aci, Est, Ket, and Fur represent alcohols, aldehydes, acids, esters, ketones, andfuran, respectively. **(B)** Comparison of volatile compounds in _Weissella-_, _Lactobacillus-_, and _Pediococcus_-associated CSBs. A heatmap was constructed using the relative abundance of bacteria in fermented doubles at the genus level.