## 3 Results and Discussion
### Volatile Composition
#### 3.3.2 Aromatic Precursor Fraction  
Tables 4 and 5 show, for the white and red varieties, respectively, the volatile composition (2015-2017) for each volatile family in bound form. Tables 6 and 7 list which compound is the most abundant in each grape variety and also if there are common compounds among them. The contribution (%) of each family of volatile compounds in bound form to the aromatic profile of every grape variety is shown in Tables S4 and S5. Despite the fact that numerous grape berries, specially _Vitis vinifera_ L., have no odor, their wines still present a characteristic varietal aroma [71]. This is due to the aroma glycosides contained in grapes, above all in those non-aromatic varieties, in which glycosidic aromatic compounds are an important source of potential aroma volatiles that would contribute to the wine aroma profile [72]. A broad range of concentration values were also identified among the different aromatic precursor families. 'Brancellao' was the variety with the lowest total concentration, 16,199 \(\mathrm{\SIUnitSymbolMicro L}^{-1}\) compared to 'Caino Longo 2', with 80,736 \(\mathrm{\SIUnitSymbolMicro L}^{-1}\), which showed the highest total concentration value. The most abundant volatile families in bound fraction were alcohols, phenols, terpenes and esters. C\({}_{13}\)-norisoprenoids were more abundant in the free fraction than under bound form. Among the white varieties, 'Albarino' showed the highest total concentration of C\({}_{13}\)-norisoprenoids followed by 'Loureira' and 'Caino Blanco' (Table 4). Whereas in the red varieties, C\({}_{13}\)-norisoprenoids in 'Caino Longo 2' were only identified (Table 5). This grape variety also showed the highest concentration of C\({}_{13}\)-norisoprenoids in free form. \(\alpha\)-damascenone was identified in 'Albarino' and 'Caino Longo 2' while dihydro-\(\beta\)-ionol was detected in 'Loureira' and 'Caino Blanco', with a woody, flowery and camphoraceous odor [46]. Results in Tables 4 and 5 also showed that there was an important increase in the concentration and number of terpen in all varieties with respect to their corresponding free fraction. This is very interesting to show the notable importance of the hidden compounds in the aromatic potential of certain grapes. Results obtained in previous research by [PERSON] et al. [44] showed that terpene glycoside concentration exceeds the free terpenes one being the bound: free terpene ratio higher than 1 in all grapevine varieties studied [73]. Unfortunately, this glucoside aromatic potential usually keeps rather stable during the winemaking process [74]. In general, white varieties show higher terpene levels than the red ones [26]. Similar to the free fraction, in bound form, 'Loureira', 'Verdello Blanco', 'Albarino' and 'Caino Longo 2' were those varieties with a higher number and total terpene concentration, linalool being the major terpene in all of them (Tables 6 and 7). [PERSON] et al. [34] also identified linalool as the major terpene in 'Albarino' in its bound form and [PERSON] et al. [45] in 'Loureira' and as the second major one in 'Albarino'. In 'Verdello Blanco' and 'Loureira', the second highest terpene was tetrahydro linalool with sweet, oily and floral odor [46]. In 'Albarino', the presence of 3-octanol, 3,6-dimethyl- was very important, also known as AR1. In 'Caino Longo 2', the presence of \(\alpha\)-terpineol is notable, with anise notes, and was also the major terpene in the free fraction of this variety. Thymol was the major terpene in 'Brancellao', 'Caino Bravo', 'Caino Longo 1' and 'Souson' (Table 7). Three sesquiterpenes were detected in the bound form. Patchoulane, a tricyclic structure sesquiterpene, was detected in 'Albarino', 'Brancellao', 'Brancellao Blanco', 'Caino Blanco', 'Caino Longo 2' and 'Verdello Blanco'. This compound was also identified in Merlot wines by [PERSON] et al. [75]. Ledene oxide-(II) and isopatchoulane were also identified in 'Caino Longo 2'.