## 3 Results
### Dynamic Changes of Volatile Compounds during the Making of Steamed Bread Using HS-SPME-GC-MS
#### 3.3.3 Effects of _W. anomalous_ GZJ2 on the Formation of Volatile Compounds  
To analyze the effects of _W. anomalous_ GZJ2 on the formation of volatile compounds during fermentation, Table 2 shows the upregulated volatile metabolites in the fermented dough by _S. cerevisiae_ SQJ20 alone or with _W. anomalous_ GZJ2 compared with the control. Among the volatile metabolites with FC > 30, the increase in two alcohols and two esters was higher in the co-fermented dough, including Phenylethyl alcohol (90.40), 3-(methylthio)-1-Propanol (34.28), Hexadecanoic acid ethyl ester (83.09), and Linoleic acid ethyl ester (58.16). However, only Phenylethyl alcohol (75.69) and Phenylacetaldehyde (43.59) were found in the fermented dough by _S. cerevisiae_ SQJ20. Among them, Phenylethyl alcohol was the volatile metabolite with the largest difference in both fermented dough by _S. cerevisiae_ SQJ20 alone or with _W. anomalous_ GZJ2, with the highest difference ratio in the mixed fermentation. Phenylethyl alcohol, with rose aroma and honey sweetness, was a typical product in the Ehrlich metabolic pathway of yeast, which was converted from phenylalanine in flour to Phenylacetaldehyde and then to Phenylethyl alcohol. In this study, the upregulation ratio of Phenylacetaldehyde in the fermented dough by _S. cerevisiae_ SQJ20 (43.59) was higher than that in the co-fermented dough (9.10), which was likely because _W. anomalous_ GZJ2 promoted the conversion of Phenylacetaldehyde to more Phenylethyl alcohol during the mixed fermentation process, thus improving the flavor of the co-fermented steamed bread. In addition, Hexadecanoic acid ethyl ester, with cream aroma, was upregulated by 83.09 FCs in the mixed fermented dough, but by only 29.93 FCs in the fermented dough by _S. cerevisiae_ SQJ20 alone. 3-(methyl thioyl)-1-Propanol, with the aroma of baked potatoes, was upregulated by 34.28 FCs in the co-fermented dough, but only by 14.85 FCs in the fermented dough by _S. cerevisiae_ SQJ20 alone. \begin{table}
\begin{tabular}{c c c c}
**Metabolites** & **Sub Class** & \
\begin{tabular}{c} **FC** \\ **(SQJ20/Control)** \\ \end{tabular} &
\begin{tabular}{c} **FC** \\ **(SQJ20 + GZJ2/Control)** \\ \end{tabular} \\ \hline Phenylethyl Alcohol & Alcohols & 75.69 & 96.40 \\ Hexadecanoic Acid, Ethyl Ester & Esters & 29.93 & 83.09 \\ Linoleic Acid Ethyl Ester & Esters & 21.70 & 58.16 \\
3-(Methylthio)-1-Propanol & Alcohols & 14.85 & 34.28 \\
2-Methyl-Propanoic Acid & Acid & / & 22.69 \\ Dodecanoic Acid, Ethyl Ester & Esters & / & 23.22 \\
2-Phenylethyl-Acctic Acid, Ester & Esters & 9.25 & 14.39 \\
3-Hydroxy-Butanoic Acid, Ethyl Ester & Esters & / & 13.61 \\ \(\alpha\)-Ethylidene-Benzeneacetaldehyde & Aldehydes & 22.92 & 12.25 \\ (E)-5-Decen-1-Ol & Alcohols & 27.17 & 12.39 \\ Tetradecanoic Acid, Ethyl Ester & Esters & / & 10.74 \\
3-Methyl-Butanoic Acid & Acid & 4.45 & 9.39 \\ Phenylacetaldehyde & Aldehydes & 43.59 & 9.10 \\
5-Hepptyldihydro-2(3h)-Furanone & Heterocycle & 28.71 & / \\
4,8-Dimethyl-Undecane & Hydrocarbons & 4.46 & / \\ Octanoic Acid & Acid & 3.98 & / \\ Octadecanoic Acid & Acid & 2.29 & / \\
2-Methyl-1-Propanol & Alcohols & 8.56 & 8.58 \\ (Z)-3-Nonen-1-Ol & Alcohols & 6.93 & 8.90 \\
2-Methyl-Butanoic Acid & Acid & 5.67 & 8.39 \\ \end{tabular}
\end{table} 
Table 2: Upregulated differential volatile metabolites in fermented dough by _S. cerevisiae_ SQJ20 with or without _W. anomalous_ GZJ2 compared with unfermented dough. In addition, several volatile metabolites were upregulated only in co-fermented dough, including Dodecanoic acid ethyl ester (23.22), 2-Methyl-propanoic acid (22.69), 3-Hydroxy-butanoic acid ethyl ester (13.61), Tetradecanoic acid ethyl ester (10.74), Benzaldehyde (7.47), Benzoic acid ethyl ester (4.67), 3-Methylene-tridecane (4.33), 1-Butanol (3.41), and 1-Heptanol (2.66).