## 3 Results
### Changes in the Volatile Compounds of Co-Fermented Steamed Bread during the Steaming Process
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CAS** & **Metabolites** & **Sub Class** & **FC(SB/D)** \\ \hline
109-97-7 & Pyrrole & Heterocycle & 0.10 \\
106-33-2 & Dodecanoic Acid, Ethyl Ester & Esters & 0.10 \\
124-06-1 & Tetradecanoic Acid, Ethyl Ester & Esters & 0.09 \\
4411-89-6 & \(\alpha\)-Ethylidene-Benzeneacetaldehyde & Aldehydes & 0.08 \\
544-35-4 & Linoleic Acid Ethyl Ester & Esters & 0.02 \\
628-97-7 & Hexadecanoic Acid, Ethyl Ester & Esters & 0.00 \\
1450-72-2 & 1-(2-Hydroxy-5-Methylphenyl)-Ethanone & Ketones & 9.56 \\
6a-Ethyl-2,5-Dioxohexahydrouro[2,3-B]Furan & Heterocycle & 9.25 \\
31081-18-2 & 3-Methyl-5-Propyl-Nonane & Hydrocarbons & 9.12 \\
128-37-0 & Butylated Hydroxytoluene & Other & 8.70 \\
98-01-1 & Furfural & Aldehydes & 7.35 \\
16747-26-5 & 2,2,4-Trimethyl-Hexane & Hydrocarbons & 7.16 \\ Carbonic Acid, Decyl Nonyl Ester & Esters & 6.74 \\
1636-43-7 & 5,6-Dimethyl-Decane, & Hydrocarbons & 6.56 \\
17312-53-7 & 3,6-Dimethyl-Decane, & Hydrocarbons & 6.05 \\
123-92-2 & 3-Methyl-1-Butanol Acetate & Esters & 5.69 \\
20905-05-9 & 4,5-Dipropyl-Octane & Hydrocarbons & 4.13 \\
17312-62-8 & 5-Propyl-Decane & Hydrocarbons & 3.84 \\
103-45-7 & Acetic Acid, 2-Phenylethyl Ester & Esters & 3.78 \\
120-72-9 & Indole & Heterocycle & 3.70 \\
590-90-9 & Ch3c(O)Ch2 ch2 oh & Other & 3.23 \\ \hline \hline \end{tabular} Note: FC (SB/D): Fold Change (Steamed bread/Dough).  
\end{table} 
Table 3: _Cont._Remarkably, aldehydes made important contributions to the aroma of steamed bread, including Hexanal, Heptanal, Nonanal, Decanal, (E,E)-2,4-Decadienal, (E,E)-2,4-Nonadienal, and Phenylacetaldehyde. Among them, (E,E)-2,4-Decadienal was the volatile aroma substance with the largest ROAV value (ROAV = 100) in steamed bread fermented by _S. cerevisiae_ SQJ20 with or without _W. anomalous_ GZJ2, contributing to the aroma of fried, waxy, and fat, which was related to the lipid oxidation reaction of flour [2; 24]. Compared with those of steamed bread fermented by _S. cerevisiae_ SQJ20, the ROAVs of (E,E)-2,4-Nonadienal (ROAV = 22.85) and Nonanal (ROAV = 12.16) in the co-fermented steamed bread increased, indicating that the addition of _W. anomalous_ GZJ2 improved the aroma of fat, citrus, green, and wax. Meanwhile, the ROAV of Decanal in co-fermented steamed bread decreased compared with that of steamed bread fermented by _S. cerevisiae_ SQJ20, and the ROAVs of Hexanal and Heptanal had similar values, which were the common flavor substances in steamed bread that come from the flour itself [26]. A further correlation analysis between aroma substances and aroma characteristics in the co-fermented steamed bread (Figure 7) confirmed that fat aroma was prominent and associated with five aldehydes, indicating that aldehydes were important sources in complex fermented steamed bread. Alcohols also occupied an important position in the aroma of steamed bread, including Phenylethyl alcohol, 1-Octen-3-ol, and 1-Heptanol. 1-Octen-3-OI, with mushroom fragrance, and had higher ROAVs in both kinds of steamed bread (49.31, 50.07), indicating that 1-Octen-3-OI was an important volatile aroma component in steamed bread. It is worth noting that the ROAV of Phenylethyl alcohol (ROAV = 4.84) in co-f  
Figure 7: Correlation network diagram of sensory flavor characteristics and flavor substances in co-fermented steamed bread. was higher than that of steamed bread fermented by _S. cerevisiae_ SQJ20 (ROAV = 3.58). In fact, the relative content of Phenylethyl alcohol in steamed bread fermented by _S. cerevisiae_ SQJ20 with or without _W. anomalies_ GZJ2 was relatively higher, which was 39.12% and 46.01%, respectively. The smaller ROAV of Phenylethyl alcohol was due to its higher threshold (750 \(\upmu\)g/L). Phenylethyl alcohol has mixed aroma characteristics of honey, spice, rose, and iliac, and it is a typical alcohol produced by the yeast Ehrlich pathway, which is derived from phenylalanine in flour and converted into Phenylacetaldehyde and finally into Phenylethyl alcohol [24; 33; 34]. After the addition of _W.