# Screening of Sourdough Starter Strains and Improvements in the Quality of Whole Wheat Steamed Bread
## 2 Results and Discussion
### Determination of Volatile Compounds of Whole Wheat Steamed Bread  
Flavor plays an important role in whole wheat steamed bread quality, and is formed by a number of volatiles, including alcohols, aldehydes, ketones, ethers, acids, esters, hydrocarbons, and aromatics. The volatiles of steamed bread are affected by many factors, such as raw materials, leavening agents, microbial strains, and fermentation conditions [30; 31; 32; 33]. In this study, GC-MS was applied to analyze the volatile compounds. A total of 38 volatile compounds were detected in whole wheat steamed bread with the three treatments (Table 6), comprising 6 alcohols, 7 aldehydes, 2 ketones, 5 acids, 14 esters, 2 hydrocarbons, and 2 other compounds. Moreover, 33 volatile compounds were detected in CS-WB, 14 more than in DY-WB, while 31 volatile compounds were detected in SY-WB. Figure 5: Photographs of whole wheat steamed bread. Note: CS-WB—whole wheat steamed bread fermented by compound starter; SY-WB—whole wheat steamed bread fermented by _Saccharomyces cerevisiae_; DY-WB—whole wheat steamed bread fermented by commercial dry yeast. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Treatment** & **Height (cm)** & **Volume (cm3)** &
\begin{tabular}{c} **Specific Volume** \\ **(cm3/g)** \\ \end{tabular} & **Hardness (g)** & **Chewiness** & **Springiness** \\ \hline DY-WB & 4.03 \(\pm\) 0.09 a & 80.35 \(\pm\) 2.07 a & 1.62 \(\pm\) 0.02 a & 901.91 \(\pm\) 18.2 c & 700.34 \(\pm\) 20.3 c & 0.83 \(\pm\) 0.008 a \\ SY-WB & 4.22 \(\pm\) 0.12 b & 85.42 \(\pm\) 1.50 b & 1.73 \(\pm\) 0.02 b & 716.05 \(\pm\) 16.1 b & 585.21 \(\pm\) 18.9 b & 0.88 \(\pm\) 0.008 b \\ CS-WB & 4.35 \(\pm\) 0.08 b & 92.01 \(\pm\) 1.71 c & 1.83 \(\pm\) 0.03 c & 621.23 \(\pm\) 15.1 a & 463.95 \(\pm\) 19.5 a & 0.92 \(\pm\) 0006 c \\ \hline \hline \end{tabular} Note: DY-WB—whole wheat steamed bread fermented by commercial dry yeast; SY-WB—whole wheat steamed bread fermented by _Saccharomyces cerevisiae_; CS-WB—whole wheat steamed bread fermented by compound starter. Means in the same column with different superscript lowercase letters of the same modulus indicate a significant difference (\(p\) < 0.05).  
\end{table} 
Table 5: Textural properties of whole wheat steamed bread. Alcohols are the hydroxyl compounds produced in microbial metabolism. The contents of alcohols were higher in all samples, especially phenylethanol. Phenylethanol mostly exists in natural products such as apple, strawberry, and honey, and has the fragrance of rose, which has a positive correlation with bread aroma [34]. It is worth noting that the content of phenylethanol in CS-WB was the lowest among the three treatments, comprising only 10.51% of its total MS peak area, which may be because the presence of _Lactobacillus johnsonii_ and _Acetobacter pasteurii_ enhanced the metabolism of phenylethanol by _Saccharomyces cerevisiae_. The molecule 3-methyl-1-butanol has a positive correlation with aroma, and it is the most-cited compound in bread flavor research, with a high odor activity value, and gives a \"balsamic, alcoholic, malty\" flavor [34]. Interestingly, 3-methyl-1-butanol and phenylethanol showed the same trend of change, with their content decreasing to 1.58% when fermented by the compound starter. The content of four acids increased after compound starter leaving, especially with the acetic acid, reaching 21.37% of its