## 3 Results and Discussion
### GC-MS Volatile Metabolome
Judging from its high average OAV (Table S2), \(\upbeta\)-damascenone most probably significantly impacted the aroma of the two investigated monovarietal wines. However, certain authors determined much higher thresholds in experiments with real red wines, implying \(\upbeta\)-damascenone has more an indirect than a direct impact on red wine aroma [39]. The odor of \(\upbeta\)-ionone is commonly described as reminiscent of violets, whereas \(\upbeta\)-damascenone is responsible for odors of stewed apple, dried plum, and honey. A fraction of C\({}_{6}\)-alcohols derives from enzymatic cleavage of unsaturated long-chain fatty acids, whereas another portion of these compounds is released into must and wine from the corresponding glycosides from grapes. Both processes take place predominantly during pre-fermentation steps, including harvest, transport, and grape processing, when physical damage to the berries induces the activity of the enzymes from the so-called lipoxygenase pathway. The odors of C\({}_{6}\)-alcohols are commonly described as green, vegetable, and herbal. They rarely directly participate in wine aroma due to relatively high odor perception thresholds, such as 8000 \(\upmu\)g/L for 1-hexanol [34] (Table S2), although there is a general opinion that higher concentrations may impart wine aroma with negative odors. Although some authors found that wines can be differentiated among each other according to variety based on the levels of these compounds [40], no significant differences were found between Plavac mali and Teran wines (Table 2). The same was observed for 2-phenylethanol, an alcohol mostly a product of alcoholic fermentation, although its smaller fraction also derives from grapes. Its average concentration found in both investigated monovarietal wines was similar and almost five times its odor perception threshold of 10,000 \(\upmu\)g/L [34] (Table 2 and Table S1). This implies 2-phenylethanol significantly contributed to the aroma of both Plavac mali and Teran wines with its odor reminiscent of roses. Acids and esters identified by GC-MS are formed in alcoholic and/or malolactic fermentation thus depend mostly on fermentation conditions, such as yeast strain and fermentation temperature, maceration duration, cap management, degree of pressing, etc. Although much less dependent on the variety than those originating from grapes,ferentation compounds have also been previously shown to be useful in differentiating monovarietal wines [27; 28; 41]. In our previous report on the varietal differentiation of white wines, the effect of variety was significant for many fermentation aroma compounds determined by both GC-MS and GC\(\times\)GC-TOF-MS [5]. In this study, among the fermentation compounds identified by GC-MS, a significant difference was determined only for a minor, often neglected ester with unknown sensory relevance, isoamyl lactate. Higher concentration was found in Teran than in Plavac mali red wine (Table 2). Nevertheless, the major fermentation acids and especially esters determined by GC-MS in this study generally belong to a group of volatile compounds with the strongest impact on wine aroma. The concentrations of major volatile acids, i.e., butyric, hexanoic and octanoic acid, were higher than their corresponding odor thresholds of 173, 420 and 500 \(\upmu\)g/L, respectively [42], meaning they directly contributed to the aroma of wines of both varieties with odors reminiscent of fats and cheese (Table S2). Ethyl octanoate, ethyl hexanoate and ethyl butyrate among ethyl esters, and isoamyl acetate and 2-phenethyl acetate among acetates, with the corresponding thresholds of 2, 5, 20, 30 and 250 \(\upmu\)g/L and OAVs multiply exceeding the unit value [34] (Table S2), certainly defined the fruity-flowery component of the aroma profile of both Plavac mali and Teran monovarietal red wines. The average concentrations of ethyl 2-methylbutyrate and ethyl 3-methylbutyrate significantly surpassed their corresponding odor thresholds of 1 and 3 \(\upmu\)g/L [34] as well, respectively.