## 3 Results and Discussion
### Volatile Composition
#### 3.3.2 Aromatic Precursor Fraction
Regarding the organic acids, 'Albarinfo' was the grape variety with higher contribution of this volatile family in bound form. It showed an average concentration value of 3040 ug\(\cdot\)L\({}^{-1}\) compared to the red variety 'Caino Bravo' with 15.08 ug\(\cdot\)L\({}^{-1}\), which additionally showed a high standard deviation. Acids in bound form showed higher contents in white grapes, except for 'Verdello Blanco' (Table 4), while different behaviors were appreciated among the red grape varieties studied (Table 5). 'Brancellao', 'Caino Bravo', 'Caino Tinto' and 'Souson' showed lower content than in the free fraction, whereas 'Caino Longo 1', 'Caino Longo 2' and 'Castanal' increased their corresponding values. Octanoic and nonanoic acid were the two major acids found, both also being abundant in the free fraction (Tables 6 and 7). As it was previously mentioned, the octanoic acid has fatty acid, dry and dairy descriptors [76]. C\({}_{6}\) compounds generally showed higher concentrations in the bound fraction of red varieties, highlighting 'Souson' for its quite high concentration of 6381 ug\(\cdot\)L\({}^{-1}\). It was identified as the lowest concentration in 'Caino Blanco' with 1156 ug\(\cdot\)L\({}^{-1}\). 1-hexanol was the compound that showed the highest concentration in all varieties in the bound fraction (Tables 6 and 7). All red and white varieties showed a higher concentration of bound alcohol compounds, 'Loureira' being the one showing the highest concentration with 21,216 ug\(\cdot\)L\({}^{-1}\), compared to 'Brancellao' with an average concentration of 8127 ug\(\cdot\)L\({}^{-1}\). A high concentration of alcohols is desirable because they have a great trend of producing fruits esters with the presence of carboxylic acids along the unification process [52]. It should be highlighted that the major alcohols identified were: 2-phenyl ethanol and benzyl alcohol. Both benzyl and phenyl derivatives were found to be the major compounds in the bound fraction of numerous neutral grape varieties [2; 26]. 2-phenyl ethanol is characterized by rose [77; 78; 79; 80], honey [78; 80] and floral [82] descriptors and benzyl alcohol is characterized by roasted, toasted [78; 80], sweet and fruity descriptors [82]. Among the white varieties, benzyl alcohol was the major alcohol in 'Brancellao Blanco' and in 'Albarino', with a similar content of 2-phenyl ethanol in accordance with the results obtained by [PERSON] et al. [34] in 1997 (Table 6). Benzyl alcohol was also the major alcohol in 'Brancellao', 'Caino Bravo' and 'Caino Tinto' among the red ones (Table 7). [PERSON] et al. [36] highlighted its concentration in 'Caino Bravo' wines as it could provide blackberry notes to this variety [83]. 2-phenyl ethanol was the major alcohol in 'Caino Blanco', 'Loureira' and 'Verdello Blanco' among the white ones (Table 6) and in 'Caino Longo 1', 'Caino Longo 2', 'Castanal' and 'Souson' among the red ones (Table 7). Only four aldehydes were detected in bound form, lower than in free form but in significantly higher content mainly in red grape varieties (Table 5). Benzaldehyde was the major aldehyde detected, with an almond-like and bitter volatile compound descriptors [84] (Tables 6 and 7). Compared to what happened in the free fraction with esters and phenols, in the bound fraction, a higher number of different esters were identified and in a higher concentration. It should be noted that 'Albarino' and 'Caino Longo 2' are those varieties with the highest values, with nonanoic acid ethyl ester and nonanoic acid methyl ester, respectively, compared to 'Castanal' showing the lowest concentration. Methyl salic, with mint and green-like flavored odors and its wintergreen characteristic aroma [85], was the major ester in 'Brancellao', 'Caino Blanco', 'Caino Bravo', 'Caino Longo 1', 'Caino Tinto', 'Souson' and 'Verdello Blanco' (Tables 6 and 7). This compound, together with acetaldehyde, benzyl alcohol or (\(E\))-2-octanel, was reported as important in Noble muscadine wine flavor [29].