## 3 Results and Discussion
### PTR-ToF-MS Results  
_S. cerevisiae_ strains SafAle US-05 and SafAle WB-06 and _S. pastorianus_ strains SafLager W-34/70 and SafLager S-23 were selected for measurement with PTR-ToF-MS. Of the measured mass range between 15-245 \(m/z\), 345 peaks were observed. After peak extraction and filtering, data calibration and filtration (eliminating isotopologues, water and ethanol clusters), 47 peaks were assigned to a sum formula and tentatively assigned to one or more compounds based on GC/MS identification and literature (Table 4). These tentatively identified compounds belonged to various chemical classes, many derived from yeast metabolism. PTR-ToF-MS measurement of four individual terpenoids produced fragment ions of masses 81 and 95 as non-isotopic ions (only \({}^{12}\)C and \({}^{1}\)H, not \({}^{13}\)C or \({}^{2}\)H) (Table 3). Mass 81 and 95 have previously been reported as terpene fragments [42; 43; 44]. [PERSON] et al. (2000) proposed calculating the total monoterpene concentration from the signal of masses 67, 81, 95, 137 and 156 [42]. Development of Volatiles during Fermentation  
The emission of ethanol and CO\({}_{2}\) during fermentation is directly associated with yeast activity as carbohydrates are converted into CO\({}_{2}\), ethanol, and hundreds of other secondary metabolites. Monitoring the evolution of ethanol (\(m/z\) 47.049) and carbon dioxide (\(m/z\) 44.999) was easily achieved as their protonated molecular ions are the predominant peaks [45]. No significant difference (\(p<\) 0.001) in the concentration of CO\({}_{2}\) between yeast strains was observed during fermentation (Figure 5). In contrast, the ethanol concentration in the samples produced by yeast strain WB-06 was significantly higher in the second, third and fourth measurement. There was no significant difference in the concentration of ethanol between yeast strains for the remainder of fermentation. Secondary metabolites generated by yeast at the same time as CO\({}_{2}\) and ethanol are formed, which can influence the aroma and taste of beer. Variation in the metabolites across different yeast strains is what allows yeast to impart characteristic flavours to beer [46]. The selected 11 peaks were the protonated molecular ions of each terpenoid and their fragments identified from the SPME-GC/MS data: geraniol (\(m/z\) 155.143, 137.132, 95.089, 93.952, 81.073), geraniol acetate (\(m/z\) 135.109), citronellol (\(m/z\) 157.158), citronellyl acetate (\(m/z\) 199.169, 139.141, 83.084) and a fragment which is used to demonstrate total terpene concentration (\(m/z\) 67.056) as mentioned by [PERSON] et al. (2000). The identification of the compound by \(m/z\) was done using standards, literature (if available) and by comparison to SPME-GC/MS data. Figure 6 displays the 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) and measured by PTR-ToF-MS for four commercial yeast strains. The remaining 7 peaks (\(m/z\) 139.141, 137.132, 95.089, 93.952, 83.084 and 81.073) are displayed in the supplementary material. The signal evolution pattern for \(m/z\)\(157.158\) is associated with citronellol as confirmed with pure standards and literature [47]. The gradual increase in citronellol over time was comparable to the results of SPME-GC/MS (Figure 4). The rapid analysis of samples with PTR-ToF-MS enabled the inclusion of additional yeast strains, such as _S. pastorianus_ strains SafLager S-23 and SafLager WB-34/70, in the analysis. This increased sample throughput enabled the identification of species-specific differences that had not previously been observed. A significant increase in citronellol around the last two days of fermentation was identified, with the final concentration being highest in samples produced by _S. pastorianus_ yeast. Species-dependent differences in the final concentration of citronellol from geraniol have been previously reported by [PERSON] et al. (2018).