## 3 Results and Discussion
### Volatile Composition
#### 3.3.1 Free Volatile Compounds
In this study, it was only \(\alpha\)-calacorene in 'Caino Tinto' that was detected in the free fraction. Nevertheless, it showed a high interannual variability as it was only detected in 2017. This terpene was also previously identified in 'Baga' ripe grapes by [PERSON] et al. [50] and in the skin of 'Bual' and 'Bastardo' grapes by [PERSON] et al. [51], reported for exhibiting a woody aroma. Organic acids, together with terpenoids, precursors of aromatic esters, aldehydes and thiols are some of the most important families of volatile compounds. Organic acids could provide sourness to the wines or even rancid or pungent aromas [52]. In this study, 'Verdello Blanco' stood out for its high concentration, facing 'Brancellao Blanco' with the lowest value. However, it should be noted the high standard deviation shown in 'Verdello Blanco' could point out an outlier value that stands out over the other two vintages. Hexanoic acid, with rancid, grass and fruity descriptors, was the major acid identified in almost every variety studied, such as 'Verdello Blanco' among the white varieties (Table 6) and 'Souson' among the red ones (Table 7), coinciding these last results with those previously obtained by [PERSON] et al. [53]. Octaonic and nonanoic acid were two other acids identified in all grape varieties studied. C\({}_{6}\) compounds and alcohols were the volatile compounds with the higher total concentration in free form. Alcohols showed their lowest values in 'Caino Tinto' with a total content of 651.62 \(\upmu\)g\(\cdot\)L\({}^{-1}\), with 'Verdello Blanco' once again being the variety with the highest mean concentration, 1615 \(\upmu\)g\(\cdot\)L\({}^{-1}\), highlighting its high standard deviation. Higher alcohols, being one of the most quantitatively important family compounds in grapes, allow us to explain some aromatic part of the future wines, such as structure and vinous characteristics [54]. They generally have solvent odor descriptors with some exceptions, such as 2-phenylethanol which is characterized by a floral, rose-like odor. Its level in wine is associated with the type of grape variety used, as it is one of the main derivatives of the phenylpropanol metabolism [55]. In this study, 1-hexanol, 2-ethyl was the most abundant volatile in all varieties, except for 'Castafial' with _cis_-2-ethyl-2-hexen-1-ol (Tables 6 and 7). These compounds could be responsible for herbaceous or green aromatic notes. Regarding C\({}_{6}\) compounds, the lowest values were observed in 'Caino Blanco' with 1305 \(\upmu\)g\(\cdot\)L\({}^{-1}\) compared to 'Brancellao' which showed the highest value of 5876 \(\upmu\)g\(\cdot\)L\({}^{-1}\). C\({}_{6}\) compounds include alcohols and aldehydes that are derived from the lipidic membrane of grapes. As those compounds are derived from varietal precursors, they have been tested as varietal markers with monovarietal wines with some preliminary successful results [24]. They generally receive green and grass descriptors, including those in the herbaceous odor series [56]. 'Brancellao' showed the highest C\({}_{6}\) compound concentration with 2-hexenal, (\(E\)) as its major compound, being possibly the most herbaceous-like variety in this study. Alternatively, 'Castanal' and 'Souson' showed 1-hexanol as their main C\({}_{6}\) compound, with the 'Souson' results coinciding with those obtained by [PERSON] et al. [53] (Table 7). Among the white varieties, high values were also shown in 'Albarino' by [PERSON] et al. [57], which specially relates C\({}_{6}\) compounds such as hexanal, (\(E\))-hex-3-en-1-ol and (\(E\))-hex-2-en-1-al with herbaceous notes, usually related with the maturation index of the grapes. Generally, grape varieties with higher acidity levels show a higher concentration of these volatile compounds [57]. Aldehydes were found among a narrow range of values in the free fraction in almost every grape variety studied, with the exception of 'Brancellao' with an outlier value of 629.34 \(\upmu\)g\(\cdot\)L\({}^{-1}\).