## 3 Results and Discussion
### Volatile Aroma Compounds
#### 3.2.6 Esters  
Esters are among the most important contributors to wine aroma with their positive fruity and floral odors. They are mainly formed during alcoholic fermentation by yeast but can also be synthesized by bacteria during malolactic fermentation and via chemically induced esterification during wine aging. Several yeast enzymes are involved in the biosynthesis of esters, and their activity is mainly determined by the expression of corresponding genes [18]. The variability in ester-related enzymatic activities among _Saccharomyces_ yeasts was already described [41] and suggested for non-_Saccharomyces_ yeasts. The final concentration of esters in the wine after alcoholic fermentation is determined by the competing activity of ester-synthesizing enzymes and esterases from yeasts that can synthesize esters but are mostly responsible for their cleavage [42]. It is known that extracellular esterases are present in _S. cerevisiae_[43], while non-_Saccharomyces_ yeasts still need to be investigated more from this aspect [30]. Another factor that strongly influences the content and composition of esters in wine is the composition of the grape juice matrix, especially the availability of substrates generated in carbon, nitrogen, and fatty acid metabolism [18]. Two main classes of esters are formed in wine: the ethyl esters, which are esters of ethanol and fatty acids, and the acetate esters, which are esters of higher alcohols and acetic acid. The concentration of the main wine ester, ethyl acetate, was higher in _S. cerevisiae_ and _T. delbrueckii_ than in _P. kluyveri_, _S. pombe_, and especially _M. pulcherrima_ must, which contained the lowest concentration (Table 1, Figure S3). The concentration of ethyl propanoate differentiated well the early fermentation profiles of the investigated yeasts. The highest concentration was found in _T. delbrueckii_, followed by _S. cerevisiae_, _L. thermotolerans_, and _P. kluyveri_, with the lowest concentrations found in _M. pulcherrima_ and _S. pombe_ musts which did not differ among each other (Table 1, Figure S3). _Torulaspora delbrueckii_ was also relatively abundant in ethyl isobutyrate when compared to the other musts. Such results are in accordance with previous studies that observed an increase in these esters in mixed _T. delbrueckii/S. cerevisiae_ fermentations [13; 16]. Odd-chain and branched-chain fatty acids, which served as precursors to the mentioned ethyl esters, are not formed from acetyl-CoA through the fatty acid synthase (FAS) complex but from the degradation of threonine and valine, respectively, via the Erlich pathway [21], so the results obtained implied particular differences in amino acid metabolism between the investigated yeast. Linear even-chain fatty acid ethyl esters are some of the most important positive contributors to wine aroma due to their low perception thresholds and relatively high concentrations. The highest concentration of ethyl butyrate was found in _S. pombe_, _S. cerevisiae_, and _L. thermotolerans_ musts, while _T. delbrueckii_ must was the least abundant in this, as well as in middle-chain ethyl esters formed through the FAS complex, such as ethyl hexanoate, ethyl octanoate, and ethyl decanoate (Table 1, Figure S3). At the investigated stage of fermentation, _L. thermotolerans_ and _S. pombe_ starters produced the highest amounts of ethyl hexanoate and together with _P. kluyveri_ were the most abundant in ethyl octanoate, the two among the most important fruity esters [26]. The concentration of ethyl 9-decenoate, another carrier of fruity notes, was higher in _L. thermotolerans_ and _S. pombe_ than in most other musts. _Lachancea thermotolerans_ was previously found to enhance the concentration of ethyl esters in sequential fermentation [10].