## 4 Conclusions  
To our knowledge, this is the first systematic study to investigate the effects of adding alkali on the microbial community in CTSs and the aroma profiles of CSBs, revealing the internal relationship between LAB and aroma characteristics of CSBs. The dominant bacteria at the genus level in CTS were _Pediococcus_, _Companilactobacillus_, and _Weissella_. Adding alkali did not change the types of dominant LAB derived from CTS but changed their relative abundance in fermented dough. Adding alkali decreased the concentration of volatile acids in CSB. _Pediococcus_ and _Weissella_ were positively correlated with lipid oxidation flavor-related compounds in CSBs; _Lactobacillus_ was positively correlated with sourness volatiles, suggesting that _Lactobacillus_ had strong acid-producing capacity. The synthetic microbial community experiment exhibited that CSB fermented by _P. pentosaceus_ had a strong aroma, and adding alkali weakened the flavor. CSB fermented by heterofermative _W. cibaria_ contained abundant acetic acid, and adding alkali decreased its concentration, promoted the formation of esters, and endowed CSB with a pleasant fruity aroma. Overall, the addition of alkali not only neutralizes organic acid in fermented dough, but also affects microbial metabolism, thereby reducing sourness and changing the lipid oxidation flavor and ester aroma of CSB. This study illustrates the mechanism underlying the improvement in the flavor of CSB via alkali addition and will provide a theoretical foundation for quality improvement and industrial production of steamed bread.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods12030617/s1](https://www.mdpi.com/article/10.3390/foods12030617/s1)]([https://www.mdpi.com/article/10.3390/foods12030617/s1](https://www.mdpi.com/article/10.3390/foods12030617/s1)), Table S1: Information about the strains isolated from Chinese traditional starters (CTSs) used to prepare synthetic microbial community starters; Table S2: Diversity indexes of fungi communities and bacterial communities in Chinese traditional starters (CTSs), alkali-free fermented doughs (FDs), and alkali-added fermented doughs (AFDs) from eight regions of China; Table S3: Concentrations of aroma compounds in alkali-free CSBs and alkali-added CSBs fermented by CTSs from eight regions of China; Table S4: Concentrations of aroma compounds in alkali-free CSBs and alkali-added CSBs fermented by synthetic microbial community starters; Figure S1: Relative abundances of microbial communities in CTS, FD, and AFD groups at the phylum level.  
**Author Contributions**: N.T.: Writing--original draft, Writing--review and editing. X.X.: Writing--review and editing. H.L.: Writing--review and editing. S.J.: Formal analysis. H.J.: Methodology. Z.A.: Supervision, Project administration, Funding acquisition. All authors have read and agreed to the published version of the manuscript.  
**Funding**: This study was financially supported by the National Natural Science Foundation of China (Grant number 32072154).  
**Institutional Review Board Statement**: Not applicable.  
**Informed Consent Statement**: Not applicable.  
**Data Availability Statement**: The data presented in this study are available in supplementary material.  
**Conflicts of Interest**: The authors declare that they have no conflict of interest.