## 3 Results and Discussion
### Analysis of Volatile Compounds in Three Wines with Different Levels of Infestation
#### 3.2.1 Screening for Characteristic Flavor Substances
During fermentation, _Saccharomyces cerevisiae_ produced higher alcohols via the Ehrlich pathway when synthesis required amino acids [24; 25]. When grape berries were infested with gray mold, cell permeability increased and led to cell death and autolysis; at the same time, the necrotizing gray mold also increased the free amino acid content of grape berries and decreased the quantity of higher alcohols produced by yeast. In this study, grapes from groups B and C had gray mold infestation, which produced free amino acids and resulted in reduced alcohol production by the yeast [26]. In total, 17 higher alcohol aromatic components were detected in the three groups of wine samples, with the majority of these alcohols being below their threshold concentrations. Only phenylethanol and 3-methyl-1-butanol were present at concentrations higher than their threshold values, indicating that the contribution of higher alcohols to the aromas of the tested wine samples would have been small. In addition, the minor Munson grape compound thiol (3-methylthiopropanol) was only detected in group C samples, and its threshold value in wine was measured at 300 \(\mu\)g/L, potentially conferring tropical fruit aromas such as the aroma of cooked meat fat and cooked vegetables. Acids are byproducts of yeast metabolism during wine fermentation, and their concentrations depend mainly on the initial composition of the wine and the fermentation conditions. Fatty acids often give wines an unpleasant aroma [27]. Five acids were detected in the test wine samples, and the concentrations of hexanoic acid and octanoic acid were significantly higher in both group B and C than in group A wine samples (Table 5). [PERSON] et al. concluded that the fatty acid concentration in noble rot wines was lower than in control wine samples. This was because gray mold infestation led to the oxidation of fatty acids and lipid degradation [22]. Aldehydes and ketones, also known as carbonyl compounds, have a fresh floral and fruity aroma. Four, eight, and seven carbonyl compounds were detected in group A-C wine samples, respectively. In addition, the total carbonyl compound content in group C samples was higher than in group A samples, probably due to gray mold infection. [PERSON] et al. and [PERSON] et al. showed that aldehydes have an inhibitory effect on fungi [28; 29]. The increased concentration of aldehydes in noble rot wines represents a stress response of grape berries to gray mold infestation. Furfural and benzaldehyde were only detected in group B and C wine samples. Although their OAVs are <1, the presence of furfural and benzaldehyde only in the infested grape berries is probably due to gray mold. This explanation is consistent with the results described by [PERSON] et al. [30], who concluded that furfural is a typical aromatic component in Fiano noble rot wines. Another stress-response component is 2-nonanone. This fungus-resistant chemical is produced when gray grapevine spores infest fruits such as strawberries and raspberries [31]. We detected 2-nonanone only in group B and C wine samples, indicating that these grape berries were infested with gray mold. Terpenoids can also generate aromatic components. The total terpenoid content in group C was greater than that in group A or B wine samples, while individual terpenes (E1-E4) showed different trends (Table 5). Terpenoids originate in grape skins and are prominent components of varietal aromas. The maceration process strongly influences the type and content of terpenoids present in wines [32]. Terpenoids have low threshold concentrations in wine. Most terpenoids have floral aromas, although a few have citrus aromas [33], and they can interact with lactones to contribute to the aroma of nuclear fruits [34; 35]. Among the aroma compounds in the \"Miscellaneous\" category, Trimethylbenzene may originate from the alteration of grape skin structure or the promotion of its dissolution by noble rot fungus infection. It is detected in Group A but not in Groups B and C. Dodecane is related to the wax of grape skins.