## 3 Results and Discussion
### PTR-ToF-MS Results
Unhopped wort with 70 \(\mu\)g/L of geraniol produced between 0.7-0.9 \(\mu\)g/L and 0.4 ug/L of citronellol by _S. cerevisiae_ and _S. pastorianus_, respectively [48]. The old yellow enzyme (OYE) has been postulated as the enzyme responsible for this reduction of geraniol into citronellol [14]. The authors demonstrated this by fermenting using strains with the OYE2 gene either overexpressed or deleted. Deletion of the gene resulted in considerably less citronellol, and overexpression of the gene resulted in considerably more citronellol. The reduction of geraniol to citronellol may change the floral character of the beer to a more citrus-like character [4]. Figure 5: Mean concentration (ppbV) of ethanol (\(m/z\)\(47.049\)) and carbon dioxide (\(m/z\)\(44.999\)) during fermentation by commercially available yeast: _Saccharomyces cerevisiae_ strain SafAle US-05, _Saccharomyces cerevisiae var Diastaticus_ strain SafAle WB-06 and _Saccharomyces pastoristimus_ strains SafLager S-23 and SafLager W-34/70. Data presented as mean \(\pm\) standard deviation of seven independent measurements. Asterisk (*) reflects statistically significant differences between strains with a \(p\)-value \(<\) 0.001, one-way ANOVA. The unique signals of \(m/z\) _83.084_, \(m/z\) _139.141_ and \(m/z\) _199.169_ were associated with citronellyl acetate. Monitoring the protonated molecule (\(m/z\) _199.169_) showed increased concentration throughout fermentation by all yeast strains. Consistent with the SPMS GC/MS results,  
Figure 6: Mean concentration (ppbV) of geranyl acetate (\(m/z\) _135.109_), geraniol (\(m/z\) _155.143_), citronellol (\(m/z\) _157.158_) and citronellyl acetate (\(m/z\) _199.169_) during fermentation by commercially available yeast: _Saccharomyces cerevisiae_ strain SafAle US-05, _Saccharomyces cerevisiae_ our _Dustaticus_ strain SafAle WB-06 and _Saccharomyces_ _pustorianus_ strains SafLager S-23 and SafLager W-34/70. Data presented as mean \(\pm\) standard deviation of seven independent measurements. Asterisk (*) reflects statistically significant differences between strains with a \(p\)-value \(<\) 0.001, one-way ANOVA. an initial lag in the concentration of citronellyl acetate by SafAle US-05 was observed. All four yeast strains produced similar concentrations throughout fermentation until the end of day four, where a significantly higher concentration was produced by both _S. cerevisiae_ strains. The unique signal of \(m/z\) 135.109 was associated with geranyl acetate, which was determined using a pure reference standard. A significant difference in the concentration began halfway through day 3 and continued until the end of fermentation, with the highest concentration from samples produced by SafAle WB-06. The lowest concentration was from samples produced by SafAle US-05. SafLager S-23 and W 34/70 had a similar formation pattern throughout fermentation. Alcohol acetyltransferase(ATF) is the enzyme involved in the esterification of geraniol to citronellyl- and geranyl acetate [49; 50]. [PERSON] et al. (2013) used a BY4741 strain with ATF1, or ATF2 genes deleted derived from _S. cerevisiae_ S288C to display the involvement of ATF1 and ATF2 in the formation of terpenyl acetates through overexpression and deletion. When ATF1 and ATF2 were deleted, there was a drastic reduction in the formation of geranyl- and citronelly acetate from geraniol. When the same gene was overexpressed, an increase in formation was observed. In the current study, the level of OYE and ATF expression in the different strains is likely to explain the differences observed. The loss of geraniol by bitotransformation is mainly due to its reduction to citronellol catalyzed by OYE and acetylation to citronelly acetate and geranyl acetate catalyzed by ATF. Brewers could utilize this knowledge to choose yeast strains that express these genes at desired levels, thereby achieving the desired concentrations of citronellol, citronelly acetate and geranyl acetate in their beer.