## 3 Results and Discussion
### Odonant Compounds of Aged Wine Spirits
The presence of these substrates, particularly at higher concentrations, can contribute to the disappearance of isobutyl acetate and other acetates, due to: (i) nucleophilic acyl substitution; (ii) deacetylation processes; (iii) competition with its formation by consumption of direct or indirect building blocks required for the formation of isobutyl acetate (acetic acid, isobutyl alcohol, or pyruvate acid) by reactions with lower activation energies in order to form other, more energetically viable species. In the case of ethyl hexanoate, which is also related with fruity notes [48], the highest concentrations were found in wine spirits from the barrels (CB) and the lowest were found in wine spirits from CN modality. The MOX modalities provided intermediate values. In a previous work [25], the amount of this ester was not influenced by the ageing system, likely due to the higher flow rate of oxygen used. Guaiaicol and 4-methylguaiacol, which are usually related to smoky odor notes [49], behaved similarly to syringol, showing the lowest concentrations in the barrel samples and the highest levels in the other samples (MOX modalities and nitrogen modality). Also for 4-methylsyringol, which is associated with smoke and burned odor notes [11], the lowest concentrations were found in barrel samples, and the highest values were determined in the samples from MOX modalities. The modality with nitrogen (CN) showed an intermediate concentration of 4-methylsyringol. The behavior of eugenol (4-allylguaiacol), which is well correlated with spicy odor notes [11], was rather different with high concentrations occurring in wine spirit samples from barrels and low concentrations in those obtained under nitrogen modality (CN). The wine spirit samples proceeding from MOX modalities exhibited intermediate values of eugenol. Some of these results are in agreement with those obtained in previous research work [25] which found significantly higher concentrations of guaiacol, methylguaiacol, 4-methylsiringol, and syringol in wine spirits produced using alternative ageing system (staves and micro-oxygenation) in comparison with those produced by the traditional process (wooden barrels). The vanillin amounts, generally well correlated with vanilla attribute in aged wine spirits [11], were significantly influenced by the ageing modalities (Table 2) with the lowest amounts on the samples produced under nitrogen (CN). These results confirmed the importance of oxygen in the pathway of vanillin formation during the wood contact. The vanillin is usually available in the wood as result of thermal lignin degradation during the cooperative manufacturing of the barrels or staves, but their presence in wine spirits results from wood extraction, lignin degradation by hydroalcoholysis and by oxidation of coniferaldehyde [4; 14]. The acetovanillon, which also results from lignin degradation [4] and also presents vanilla-like odor notes [50] was also significantly influenced by the ageing modalities (Table 2) with the lowest amount in wine spirits aged in wooden barrels and the highest amounts were observed in wine spirits from CO60 modality. The samples from other modalities exhibited intermediate values of this compound. Although the MOX modalities (CO15, CO30 and CO60) were designed to reproduce and accelerate reactions occurring in wooden barrels (CB), the obtained results suggest that the reactions were not the same. These differences may be due to several factors that will need to be further explored. As predicted, the dissolved oxygen levels in MOX modalities were higher than those observed in the wooden barrel (Figure 1). Given that the oxygen dissolved in a wooden barrel is always the result of a balance between the oxygen that passes through the wood and the oxygen that is consumed in the reactions [51], lower levels of dissolved oxygen in the CB than in MOX modalities may indicate a higher consumption or a lower oxygen entry through the barrel.