## 3 Results and Discussion
### Analysis of Volatile Compounds in CSB  
Flavor is a vital factor affecting the sensory properties and acceptance of steamed bread [3]. To explore the flavor characteristics of CSB, the volatile compounds in the steamed breads were detected by GC-MS. Thirty-seven kinds of aroma compounds were identified (Table S3), including 10 alcohols, 10 aldehydes, eight acids, six ketones, two esters, and one furan, and they were described as specific odors referring to the report of [PERSON] et al. [29] afterwards. Alcohols are typical flavor compounds in fermented wheat flour products.phenylethyl alcohol with flower fragrance [1] and ethanol denoted as alcoholic odor [30] are the most concentrated alcohols in CSB. The phenylethyl alcohol concentration in CSBs from eight regions was in the range of 4.06-40.39 mg/kg, and the highest concentration was detected in alkali-free CSB from WN. The concentration of ethanol in CSBs was between 19.48 mg/kg and 265.38 mg/kg, and its intensity was the highest in alkali-added CSB from BZ. They were reported to be positively correlated with the aroma of wheat bread [31]. That is to say, the higher the concentration of phenylethyl alcohol and ethanol, the higher the acceptance of bread aroma by consumers. In addition, the compound 1-octen-3-ol was demonstrated to give rise to a \"mushroom-like\" odor [32], which was correlated negatively with bread flavor [31]. Adding alkali obviously decreased the concentration of 1-octen-3-ol concentration from 3.05 to 0.07 mg/kg in CSB from BZ, but its intensity in alkali-free CSB (5.10 mg/kg) increased slightly in comparison with alkali-added CSB (5.37 mg/kg) from HZ. Our results were not absolutely in accordance with the study of [PERSON] et al. [10], who found that adding alkali could improve the flavor of type I sourdough CSB by decreasing the concentration of 1-octen-3-ol. That might be related to the more complex composition of LAB in CTS fermented dough than that in type I sourdough, and the different sensitivities of LAB to alkali. The volatile acids are the main source of sourness in traditional CSB, and the primary purpose of adding alkali is to decrease the sour flavor. Butanoic acid (sweaty) was the most concentrated acid, with a concentration of 21.26 mg/kg in CSB from WN, which dropped to 6.07 mg/kg in the corresponding alkali-added CSB. Pentanoic acid (sweaty), hexanoic acid (sweaty), and heptanoic acid (cheese) were identified in all alkali-free CSBs and were decreased or even undetectable in alkali-added CSBs. Similar results were reported by [PERSON] et al. [10], who observed that the addition of alkali could significantly decrease the intensity of butanoic acid in sourdough CSB. The reason could be that adding alkali neutralized the organic acids produced by microbial metabolism in fermented dough. Aldehydes including hexanal (green), heptanal (fatty), octanal (citrus), (Z)-2-heptanal (green), nonanal (citrus), benzaldehyde (almond), (Z)-2-nonenal (fatty), phenylacetaldehyde (honey-like), (E, E)-2,4-nonnadienal (deep fat fried), and (E,E)-2,4-decadienal (deep fat fried) were detected in all CSBs. Hexanal was the most abundant aldehyde and was more abundant in alkali-added CSB than in alkali-free CSB from most regions, with the highest concentration in CSB from WH, at 26.78 mg/kg. This was essentially consistent with the study by [PERSON] et al. [10], which suggested that 0.1% Na\({}_{2}\)CO\({}_{3}\) addition could significantly increase the concentration of hexanal in sourdough CSB. In addition, ethyl acetate (fruity) and 2-pentylfuran (fruity) were found in all the samples, and these volatiles also imparted CSB typical aromas [33]. PCA was performed to analyze the effect of adding alkali on the aroma compounds in CSB (Figure 5A). The first two principal components explained 53.0% of the total variance. All CSB samples were clearly separated from each other, indicating the diversity of aroma characteristics of CSBs fermented using CTSs from different regions. The score plots distinguished between alkali-added CSBs and alkali-free CSBs.