## 3 Results and Discussion
### Evolution of Target Volatile Compounds over the Ageing Experiment
However, due to its high reactivity even in mild conditions, it is expected that the presence of acetaldehyde will decrease over time. Its reactivity is due to the fact that it has a trigonal carbon electrophile which is attacked by nucleophiles for the formation of a tetrahedral products or intermediates. The most important trigonal electrophiles found in wine spirits include carbonyl groups, which are classified as follows: aldehydes > ketones > esters > carboxylate ions. This order is easy to understand. The aldehyde \(\Delta\)E\({}_{\tt H}\) has the lowest activation energy for disrupting \(\pi\) stabilization, whereas the carboxylate ion has the highest [39]. Acetic acid derives from the wine distillate, from the oxidation of ethanol via acetaldehyde, and from the acetyl groups found in the wood xylans, an important type of hemicelluloses [40]. Acetic acid is related to vinegar aroma [11; 12] and existed in the unaged wine spirit (Table 1). Its concentration increased over the ageing time in accordance with other works [10; 14; 41; 42]; which is associated with ethanol oxidation during ageing [43]. Nevertheless, significant differences in the acetic acid amounts were verifiedin the wine spirits originated from different ageing modalities. The highest amounts were determined in the samples aged in the wooden barrels while the lowest were determined in samples from nitrogen modality (CN), confirming that its origin is linked to the oxidation of ethanol and acetaldehyde. These results pointed out the oxygen importance in the acetic acid formation and its increase during the ageing process. Indeed, according to previous studies, acetaldehyde rates are high in very old aged spirits [14] or very old aged wiskiskies as result of ethanol oxidation [15], and acetaldehyde oxidation into acetic acid. Esterification between acetic acid and ethanol has been verified by ethyl acetate formation [15]; therefore, ethyl acetate quantification in wine spirits samples at the end of ageing experiment was made (Table 2). The results revealed slightly higher amounts of ethyl acetate in samples aged in wooden barrels and the lowest occurred in the nitrogen modality (CN), while MOX samples presented intermediate values, in accordance with results for acetic acid. However, only the CB modality showed higher concentration of ethyl acetate than the unaged wine spirit, suggesting that ethyl acetate hydrolysis occurred in MOX and CN modalities (Figure S1-Supplementar material). The different amounts of oxygen in the CN and MOX modalities (Figure 1) could explain the differences in acetaldehyde as well. The acetic acid differences between barrels and the other modalities can be related to differences in wood extraction Thus, further research is needed to understand these differences. Syringol, which is associated with odor notes of wood, smoke [11], is absent on unaged spirit (Table 1), as observed in other works [8; 10]. Its presence in aged wine spirits has been assigned to wood lignin degradation [44]. Its amount increased over the time but the kinetics are quite different according to the ageing modalities (Table 1). In fact, its concentration was always higher in wine spirits proceeding from the MOX modalities (CO15, CO30, CO60) and nitrogen modality (CN) than in those resulting from the wooden barrels (CB). Similar results were previously obtained; with high concentration of syringol found in spirits aged with alternative ageing systems without [10] and with micro-oxygenation [25] in comparison with those aged in wooden barrels. These findings lead us to believe that the degradation of wood lignin can increase in the MOX modalities giving rise to higher concentration of syringol in the beverage. Additionally, since lignins are degraded in contact with alcoholic beverages [45; 46], these results suggest a more intense lignin extraction and degradation in MOX and N modalities. Nevertheless, contribution of wood variability and/or the extraction rate of syringol to such a variation should not be excluded [10; 47].