## 3 Results and Discussion
### GC\(\times\)GC-TOF-MS Volatile Metabolome
On the other hand, 2,3-butanediol was a subject of many studies since it is a precursor and a major product of yeasts and lactic acid bacteria in alcoholic and malalactic fermentation [53]. No significant differences were found for volatile fatty acids. Esters generally contribute to wine aroma by fruity and flowery odors [34; 42]. Many minor esters were identified by GC\(\times\)GC-TOF-MS analysis, including those of ethanol and various acids (ethyl esters), esters of various alcohols and acetic acid (acetates), as well as esters of minor alcohols and minor acids. Several esters of ethanol and other alcohols with dicarboxylic acids were also found, such as diethyl oxalate, diethyl fumarate, diethyl malate, diethyl glutamate, diethyl 2-methylsuccinate, and diethyl malonate, as well as various esters of different combinations of alcohols and succinic acid (Table 3). The concentrations of seven esters, namely ethyl methylhexanoate, diethyl oxalate, diethyl fumarate, isoamyl lactate, butyl ethyl succinate, diethyl malate, and isoamyl isobutyrate were higher in Teran, whereas the concentrations of ethyl pentanoate and isoamyl 2-methyl-2-propenoate were higher in Plavac mali wines. The higher concentration of isoamyl lactate in Teran wines was determined by both GC-MS and GC\(\times\)GC-TOF-MS analysis. Most of the listed above were ethyl esters, which is in line with a thesis that their concentration depends more on the amount of acid precursors, possibly partly predetermined by variety, than it is the case for acetates whose production is influenced by the activity of alcohol acetyltransferases in addition to the availability of nitrogen [21; 54; 55]. This implied the possibility that variety and grape growing conditions indeed exhibited a particular effect on these particular esters that prevailed over the effects of fermentation parameters. The contribution of the mentioned esters to the aroma of the investigated wines was probably not large, either because of their low concentrations or low volatility. Furanoids found in the highest concentrations were ethyl 2-fluoroate and furfural, whereas among lactones \(\gamma\)-butylrolactone was the most abundant (Table 3). However, significant differences were determined for other, minor lactones, with pantolactone, \(\gamma\)-ethoxybutryrolactone, and \(\gamma\)-octadecone found in higher concentration in Teran, and \(\alpha\)-methyl-\(\gamma\)-crotonolactone in Plavac mali wines. Lactones from the \(\gamma\)-series, formed by cyclisation of the corresponding \(\gamma\)-hydroxycarboxylic acids, were the most numerous. Their odors are usually described as fruity, sweet, milky, and coconut-like [56]. Three sulfur containing compounds were found to be significantly different between the investigated wines. Ethyl 3-(methylsulfonyl)propanoate emerged as a strong varietal differentiation with a relatively high \(F\)-ratio value, with higher concentration found in Teran wines. Teran was also more abundant in dihydro-2-methylthiophen-3-one, whereas Plavac mali wines contained more ethyl methanesulfonate. Sulfur containing volatile compounds are formed during fermentation by the degradation of sulfur containing amino acids or by the reduction of elementary sulfur, sulfite, or sulfate [21], whereas their content and composition depends strongly on the yeast strain used. These compounds are usually associated with negative odors reminiscent of rotten egg, cabbage, garlic, onion, cheese, and burnt rubber and are often referred to as reduced or reductive aromas [57; 58]. A sulfur-containing compound found in the highest relative amount was methanol, often found in white and red wines. Its odor perception threshold ranges between 500 to 4500 \(\upmu\)g/L, as determined by various authors [59]; therefore, it most probably did not have a significant contribution to the aroma of Plavac mali and Teran wines. Among other compounds, tridecane showed a significant potential to differentiate the two investigated monovarietal wines.