## 3 Results
### Dynamic Changes of Volatile Compounds during the Making of Steamed Bread Using HS-SPME-GC-MS  
The volatile compounds of the different dough and steamed bread were detected using HS-SPME-GC-MS. The volatile compounds with scores > 70 were qualitatively analyzed using the NIST database, and the results are shown in Figure 2A. From five samples of dough or steamed bread fermented by _S. cerevisiae_ SQJ20 with or without _W. anomalous_ GZJ2, a total of 236 substances in eight categories were detected, including alcohols, aldehydes, ketones, esters, acids, alkanes, heterocycles, and others. Of all the substances, hydrocarbons, alcohols, and esters accounted for 28.8%, 15.3%, and 15.3%, respectively, accounting for 59.4% of the total. The relative abundance stack diagram was drawn according to the relative contents of the various substances, and the dynamic changes in the volatile substances in the process of making steamed bread were observed through the changes in the relative contents of the volatile substances (Figure 2B). Significantly, compared with the control, alcohols had the greatest increase in the fermented dough and steamed bread. Moreover, the content of alcohols in the mixed-fermented dough (56%) or steamed bread (53%) was higher than the content in the single-fermented dough (45%) or steamed bread (51%), respectively, which may be caused by the degradation, decarboxylation, and deamination of amino acids in flour by different yeast strains. Among them, the content of Phenylethyl alcohol changed most significantly after fermentation  
Figure 1: E-nose analysis of different dough and steamed bread fermented by _S. cerevisiae_ SQJ20 with or without _W. anomalous_ GZJ2. (**A**) Principal component diagram; (**B**) Radar map.  
or steaming. In the control, the content of Phenylethyl alcohol was only 0.59%; however, it reached 31.51% in fermented dough by _S. cerevisiae_ SQJ20, and it reached 35.29% in co-fermented dough by _S. cerevisiae_ SQJ20 with _W. anomalous_ GZJ2. After steaming, the content of Phenylethyl alcohol increased to 39.12% and 46.01% in steamed bread fermented by _S. cerevisiae_ SQJ20 alone and with _W. anomalous_ GZJ2, respectively. Phenylethyl alcohol has been proven to have a sweet fragrance of roses and honey, which contributes a lot to the flavor of steamed bread [23; 24]. In this study, both the fermentation and steaming processes were conducive to the increase in the Phenylethyl alcohol content, and the addition of _W. anomalous_ GZJ2 further promoted the increase in the Phenylethyl alcohol content, which enhanced the flavor of steamed bread.  
In addition, the content of the hydrocarbons also increased significantly after steaming, possibly because the heating promoted the oxidation and degradation of fatty acids [25]. Although the relative content of the hydrocarbons in the steamed bread fermented by _S. cerevisiae_ SQJ20 (18%) was higher than that in the co-fermented steamed bread (14%), most hydrocarbons had a higher aroma threshold and thus contri  
Figure 2: Dynamic changes in volatile substances and their contents in the dough or steamed bread fermented by _S. cerevisiae_ SQJ20 with or without _W. anomalous_ GZJ2. (**A**) The amount of various volatile substances; (**B**) Stack diagram of the relative abundance of volatile substances.  
flavor of steamed bread [26]. It is worth noting that the relative content of the acids in dough and steamed bread by mixed fermentation was higher than in those fermented by _S. cerevisiae_ SQJ20. Acid substances not only provide some specific flavors, but also activate people's taste receptors, thus affecting people's perception of flavor substances and obtaining a diversified flavor [27].  
Effect of Different Yeast Strains on the Formation of Volatile Compounds in the Fermentation of Dough