## 4 Conclusions  
The fate of geraniol during beer fermentation by _S. cerevisiae_ or _S. pastorianus_ was investigated as an example of biotransformation of hop flavour compounds occurring during beer fermentation. The reduction in the concentration of geraniol was closely followed by the detection of citronellol, citronellyl acetate and geranyl acetate. The ability of yeast to transform geraniol into these compounds has previously been identified. However, this is the first study to monitor the changes in real-time by direct injection mass spectrometry. The use of PTR-ToF-MS enabled differences between yeast strains to be identified during fermentation with high temporal resolution. The implementation of two separate techniques allowed for the identification of compounds (GC/MS), online monitoring and quantitative determination (PTR-MS). The results show that the concentrations of terpenoids detected throughout fermentation were impacted by the yeast species and strain. In general, a higher concentration of the detected terpenoids was produced by _S. pastorianus_. An example of strain-dependent differences was shown on the initial day of fermentation, where the increase in the concentration of citronellyl acetate (_m/z 199.169_) was slower in _S. cerevisiae_ SafAle US-05 and WB-06 when compared to _S. pastorianus_ SafLager S-23 and W-34/74. Biotransformation terpenoids increased in concentration at a similar rate for _S. pastorianus_ strains, whereas _S. cerevisiae_ strains SafAle- US-05 and WB-06 differed greatly. The difference between yeasts may be due to the diverse level of OYE and ATF expression, impacting the concentration of citronellol, citronellyl acetate and geranyl acetate, respectively. The concentration of some of the VOCs detected in the micro-fermentations (3 mL) may have different magnitudes when compared to industrial-sized fermentation. Still, the yeast differences and proposed pathways are expected to be comparable. Using this developed method to investigate terpenoids important for beer aroma is essential and might be used to investigate the effect of biological and technological parameters. To expand the current understanding of aroma generation during fermentation, analyzing more terpenoids with a similar experimental design will provide more valuable information on yeast production and transformation reactions. There is also a need to analyze different yeast strains, which would give the brewers additional information to manage and change the aroma of the beer to meet consumers' preferences.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/](https://www.mdpi.com/article/10.3390/)]([https://www.mdpi.com/article/10.3390/](https://www.mdpi.com/article/10.3390/)) fermentation9030294/s1, Figure S1: Mean concentration (ppbV) of _m/z 139.141_, _m/z 137.132_, _m/z 95.089_, _m/z 93.952_, _m/z 83.084 and _m/z 81.073_ during fermentation by commercially available yeast: _Saccharomyces cerevisiae_ strain SafAle US-05, _Saccharomyces cerevisiae_ _m/z Distaticus_ strain SafAle WB-06 and _Saccharomyces pastorisianus_ strains SafLager S-23 and SafLager W-34/70. Data presented as mean \(\pm\) standard deviation of seven independent measurements.  
Conceptualization G.T.E., P.S., P.B. and F.B.; Methodology, R.R., G.T.E., P.S., P.B., I.K. and F.B.; Investigation, R.R.; Formal analysis, R.R., I.K. and E.B.; Data Curation, R.R., I.K. and E.B.; Writing--original draft, R.R.; Writing--review and editing, F.B., G.T.E., P.S., P.B. and I.K.; Supervision, G.T.E., P.S., P.B. and F.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Provincia Autonoma di Trento (ADP FEM 2022).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
[PERSON] is grateful to receive funding from the University of Otago (Doctorial Scholarship) and Fondazione [PERSON].  
The authors declare no conflict of interest.