## 3 Results and Discussion  
### SPME-GC/MS Results  
The VOCs detected during the fermentation by the different yeasts with SPME-GC/MS are presented in Table 4. _S. cerevisiae_ strains SafAle US-05 and SafAle WB-06 were selected to be measured using PTR-ToF-MS because previous experiments (data not shown) indicated that they were the most different in terms of their VOC composition. A preliminary data exploration was made using a principal component analysis (PCA), each point representing a distinct measurement (Figure 2). The first two principal components accounted for 88.44% of the total variability. Time-dependent evolutions are observed with different colours, representing the time points from day 1 to day 5. The first time point for all samples was similar, irrespective of yeast strain or compound, and clustered together close to the top left quadrant. After day 1, different evolutions were evident when comparing the samples with or without geranoid and separating by yeast strain. The loadings plot (Figure 2) shows the contribution of each VOC to the principal components at different time points and illustrates the differences in VOC evolution between yeast strains. On the other hand, the correlation circle plot (Figure 3) identifies which VOCs are most strongly associated with each principal component. In this study, terpenoid compounds such as geranol (18), geranyl acetate (15), citronellol (16), and citronellyl acetate (13) played a crucial role in differentiating samples with added geraniol from the samples without added geraniol. The time evolution of the detected terpenes (as the area under the curve) is displayed in Figure 4. In the samples with geraniol spiked, regardless of the yeast strain, the peak area of geraniol decreased over the first two days of fermentation and remained constant. This initial loss could result from several factors, including the removal of the compound from the solution due to CO\({}_{2}\) production by the yeast (stripping) during fermentation, as well as loss during sample measurement (purging). The ability of CO\({}_{2}\) to \"blow off\" the linalool during fermentation was investigated by [PERSON] et al. (1996), who observed a reduction of 7.5% after 24 h [40]. When geraniol was not added, no terpenoids were detected. The terpenoids, geraniol, geranyl acetate, citronellol and citronellyl acetate were only detected in samples containing yeast to which geraniol had been added, with the concentration varying greatly depending on the yeast strain. The peak area of geranyl acetate on the second day of fermentation was more than three times higher with SafAle WB-06 than in the samples \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline  & **MW (g/mol)** & \(m/z\) & **81** & **83** & **93** & **95** & **135** & **137** & **139** & **155** & **157** & **199** \\ \hline Geraniol & 154.25 & C\({}_{10}\)H\({}_{18}\)O & 82.03 & & 100 & 51.25 & & 46.06 & & 0.27 & & \\ Citronellol & 156.27 & C\({}_{10}\)H\({}_{20}\)O & 78.52 & & 100 & 64.75 & & 26.60 & & & 20.32 & \\ Geranyl acetate & 196.29 & C\({}_{12}\)H\({}_{22}\)O\({}_{2}\) & 100 & & 10.45 & 1.6 & 42.53 & & & & \\ Citronellyl acetate & 198.30 & C\({}_{12}\)H\({}_{22}\)O\({}_{2}\) & 40.47 & 100 & & 11.10 & & & 41.32 & & 25.51 \\ \hline \end{tabular}
\end{table} 
Table 3: Terpenes, their molecular weight (MW), and formular and relative abundances of their different fragments determined by PTR-ToF-MS.  
US-05. Interestingly, geranyl acetate concentration in SafAle WB-06 decreased dramatically over time but remained relatively constant for SafAle US-05. For the first two days of fermentation, citronellyJ acetate was not detected in samples fermented with SafAle US-05. It then increased and plateaued until the final measurement on day 5. The formation of citronellyJ acetate with SafAle WB-06 was comparable to the pattern for geranyl acetate; a dramatic increase followed by a decline.