## 3 Results and Discussion
### Volatile Aroma Compounds
#### 3.2.6 Esters
It is known that during fermentation the production of linear ethyl esters and other volatile compounds fluctuates, often peaking at the end of the growth phase and decreasing during the stationary phase, sometimes with the second peak corresponding to the start of the decline phase and release of intracellular volatiles after yeast cell autolysis [21]. Various non-_Saccharomyces_ yeasts exhibit diverse performance and dynamics of the production of volatile compounds during fermentation, which is among the probable reasons for the differences observed in this study at the monitored point of fermentation. Compared to ethyl esters, the concentrations of acetates formed during alcoholic fermentation depend more on yeast enzymatic activity, especially that of acetyltransferases, and less on substrate availability [21]. The highest concentrations of propyl and isobutyl acetate, esters exerting fruity notes, were found in the control treatment inoculated with _S. cerevisiae_, while _M. pulcherrina_ must contained the lowest amounts (Table 1, Figure S4). In general, the must inoculated with _P. kluyveri_ contained elevated levels of isoamyl and hexyl acetate, two major acetate esters responsible for fruity and flowery notes [26], respectively. _Pichia kluyveri_ must was the most abundant in both 3-hexen-1-yl acetate isomers and also the sole treatment with the ratio of _trans_ to _cis_ form higher than one (Table 1, Figure S4), suggesting a possible activity of particular invertases. _Metschnikowia pulcherrina_ and especially _T. delbrueckii_ starters produced the lowest quantities of the abovementioned fruity acetates. Despite its positive influence on the concentration of particular esters, _M. pulcherrina_ was previously shown to be able to reduce the content of acetates [10]. The results obtained for _T. delbrueckii_ were partly in agreement with previous studies that observed both a decrease [11; 12] and an increase [13; 16] in isoamyl acetate concentration after inoculation with this yeast in co-fermentation. The largest difference among the acetates was observed for 2-phenethyl acetate, an ester imparting pleasant floral notes to wine. It was produced in the highest concentration by _P. kluyveri_, with a two- to three-fold increase relative to the other musts. Such an outcome at this stage corresponded significantly to previous findings showing an increase in the concentration of this ester in finished wines after sequential or mixed inoculation with this non-_Saccharomyces_ yeast [10; 15; 16], suggesting this effect remains significant throughout fermentation. _Schizosaccharomyces pombe_ fermented must contained the highest concentration of isobornyl acetate. A number of esters of methanol and higher alcohols with various fatty acids were identified (Table 1, Figure S5). While the odor of methyl hexanoate is generally described as fruity, methyl esters of higher molecular weights are perceived as fatty and waxy. _Saccharomyces cerevisiae_ and especially _T. delbrueckii_ musts contained the lowest concentration of the majority of these esters at this stage of fermentation. Interestingly, _T. delbrueckii_ must,previously found to contain an elevated level of ethyl propanoate, was characterized by the highest concentrations of other esters of propanoic acid, such as isoamyl propanoate (fruity) and 2-phenethyl propanoate (floral), the latter not commonly synthesized by _S. cerevisiae_ (Table 1, Figure S5). The observed highest concentration of 2-phenethyl propanoate was in accordance with a previous investigation reporting high concentration in _T. delbrueckii_ inoculated must [22]. [PERSON] et al. [22] found this ester in grape musts fermented by some other non-_Saccharomyces_ yeasts as well, such as _M. pulcherrina_, _L. thermotolerans_, and especially _Kazachstania gamospora_. In this study, _M. pulcherrina_ also produced a notable concentration of 2-phenethyl propanoate, although lower than that found in _T.