## 3 Results and Discussion
### Analysis of VOMs  
#### 3.5.1 Effect of \(S\). _aureus_ Contamination on VOMs in Sourdough  
In total, 40 VOMs were identified, including 14 aldehydes, 11 alcohols, 7 acids, and 3 esters (Figure 4). They were found to be the main flavor compounds of sourdough [28]. The dominant aldehyde, acid, and alcohol were hexanal, butanoic acid, and 1-hexanol, respectively, which is consistent with the data reported in the literature [28,36]. The heptanoic acid, 3-methyl-1-butanol, 2-methyl-1-butanol, 3-methyl-hepta-1,6-dien-3-ol, and 2-octen-1-ol were detected only in the non-contaminated control sourdough.  
Fresh dough contained the highest amounts of 3,4-dimethyl-2-en-1-ol, tetrahydro-6-(2-pentenyl)-2H-pyran-2-one (also known as jasmine lactone), n-hexane, and butyl formate. However, their contents were significantly decreased during the fermentation process. The decrease in the contaminated sourdough was less, indicating that \(S\). _aureus_ could inhibit the fermentation of sourdough, which is consistent with the TTA (Figure 3). The content of n-hexane and 3,4-dimethyl-2-en-1-ol in the non-contaminated control sourdough decreased to below the detectable limit after 4 h of fermentation. However, n-hexane could be detected throughout the fermentation process of contaminated sourdough.  
Aldehydes provide sourdough with oil, fruit, and almond aromas. \(S\). _aureus_ significantly reduced the content of hexanal, 2-hexenal, 2-ethylhexanal, 2-methylheptanal, and octanal but significantly increased the content of 2,4-decadienal, and 2-octenal (\(p<0.05\)). The threshold of 2,4-decadienal was very low (0.007 ppb), and its off odor [37] might have a negative effect on the flavor of sourdough.  
Alcohols, such as 1-hexanol, 1-pentanol, 1-octen-3-ol, 3-methyl-1-butanol, etc., are the main flavor compounds of sourdough that provide sourdough with fruit, wine, and fermented aroma [38]. \(S\). _aureus_ reduced the content of 2-methyl-1-butanol, 3-methyl-1-butanol, 2-octen-1-ol, 3-methyl-hepta-1,6-dien-3-ol, 1-hexanol, and 1-octen-3-ol while significantly increasing (\(p<0.05\)) the content of 1-pentanol.  
The content of acetic acid, butanoic acid, 2-methylbutanoic acid, hexanoic acid, 3-methyl-4-oxo-pentanoic acid, and heptanoic acid in the non-contaminated control sourdough were higher than those in contaminated sourdough, which is in agreement with the TTA (Figure 3). However, the content of 3-methylbutanoic acid in S16 was higher than that in Co16 (\(p<0.05\)).  
Esters provide sourdough with \"fermented aroma\". Esters identified in sourdough were methyl formate, hexyl acetate, and butyl formate, and the presence of \(S\). _aureus_ reduced the content of these esters. Among these esters, hexyl acetate had a low content of 2 ppb, and it contributes to fruity and sweet flavors.  
Figure 4: Heatmap of VOC in sourdough. Flo indicating fresh dough. Co4, Co8 and Co16 indicating wholewheat sourdough fermented for 4, 8 and 16 h, respectively. S4, S8 and S16 indicating wholewheat sourdough artificial contaminated with _S. aureus_ (\(10^{2}\) cfu/g) and cultured for 4, 8 and 16 h, respectively (\(n=6\)).  
Furthermore, _S. aureus_ significantly increased the content of furfural (\(p<0.05\)); its threshold was higher than 3000 ppb and thus perhaps has no great influence on the flavor of sourdough. On the other hand, _S. aureus_ decreased the content of 2-pentylfuran significantly (\(p<0.05\)). The 2-pentylfuran can provide \"fruit aroma\" at a threshold of only 6 ppb and is the characteristic VOMs in sourdough [38]. [PERSON] et al. [39] reported that the 2-pentylfuran transferred from 2,4-decadienal, which is consistent with our results that the content of 2,4-decadienal in contaminated sourdough was higher (\(p<0.05\)) than that in the non-contaminated control sourdough.