## 3 Results and Discussion
### Analysis of VOMs
#### 3.5.3 Identification of Characteristic VOMs in Sourdough Contaminated by _S. aureus_  
Comparing VOMs in S4 and Co4 showed that the content of n-hexane, 3,4-dimethylpent-2-en-1-ol, and butyl formate in S4 were higher (\(p<0.05\)) than those in Co4. However, because these compounds are derived from flour, it appears that the characteristic VOMs produced by _S. aureus_ were not found in S4.  
Comparing VOMs in S8 and Co8 indicated that the content of 1-penten-3-ol, benzaldehyde, heptanal, 3,4-dimethylpent-2-en-1-ol, 2,3-dimethylundecane, furfural, and n-hexane in S8 were higher (\(p<0.05\)) than those in Co8. Comparing VOMs in S16 and Co16 showed that the content of 1-pentanol, benzaldehyde, heptanal, 2,4-decadienal, 2,3-dimethylundecane, 2-hepten-1-ol, and 3-methylbutanoic acid in S16 were higher (\(p<0.05\)) than those in Co16. Among those VOMs, n-hexane and 3,4-dimethylpent-2-en-1-ol were  
Figure 5: PCA analysis of VOC in sourdough. Flo indicates fresh dough without fermentation. Co4 and Co8 indicate wholewheat soundough fermented for 4 and 8 h, respectively. S4 and S8 indicate wholewheat soundough artificially contaminated with _S. aureus_ (\(10^{2}\) cfu/g) and cultured for 4 and 8 h, respectively (\(n=6\)).  
from flour, whereas benzaldehyde, heptanal, and furfurral are the characteristic VOMs in sourdough. In addition, the content of 1-penten-3-ol in contaminated sourdough gradually decreased after 8 h, which was conversely correlated with the cell count of _S. aureus_. Thus, we might conclude that 1-pentanol, 2,3-dimethylundecane, and 3-methylbutanoic acid are the characteristic VOMs in _S. aureus_-contaminated sourdough.  
The 3-methylbutanoic acid has also been reported to be the characteristic VOMs of _S. aureus_ in various cultures and contaminated food matrixes. [PERSON] et al. [40] identified that _S. aureus_ specifically produces 3-methylbutanoic acid in brain heart infusion broth, tryptic soy broth, and 1% glucose enteric fermentation-based broth. [PERSON] et al. [41] described that 3-methylbutanoic acid and 3-methylbutanal are the volatile biomarkers for _S. aureus_ in tryptic soy broth. [PERSON] et al. [10] reported that 3-methyl-butyric acid is the characteristic compound of _S. aureus_ in the culture medium or in pork. A rapid detection method of _S. aureus_ contamination in milk and pork based on 3-methylbutanoic acid content has also been reported [10; 11]. 3-methylbutanoic acid could be detected when the population of _S. aureus_ in pasteurized milk reached \(10^{6}\) to \(10^{7}\) cfu/mL, when that in raw milk reached \(10^{5}\) to \(10^{6}\) cfu/mL [11], and when that in pork reached \(10^{6}\) to \(10^{7}\) cfu/g [10]. In the present study, 3-methylbutanoic acid was also detected till the number of _S. aureus_ cells reached \(10^{6}\) cfu/g.  
Similar to that of 3-methylbutanoic acid, the content of 1-pentanol and 2,3- dimethylundecane were positively correlated with the number of _S. aureus_ cells in contaminated sourdough. Furthermore, the ratios of 1-pentanol and 2,3- dimethylundecane in the contaminated sourdough and the non-contaminated control sourdough were greater than that of 3-methylbutanoic acid. It appears that 1-pentanol and 2,3-dimethylundecane have higher sensitivity than 3-methylbutanoic acid, especially for 2,3-dimethylundecane, which was detected only in _S. aureus_-contaminated sourdough (S8 and S16). Despite all of the above, more studies are needed to confirm and to fully understand this finding.