## 3 Results and Discussion
### Physicochemical Analysis
Despite the small difference between the alcohol content of the control sample and the aged spirit samples, it proved to be significant in the statistical analysis. The explanation for this increase in alcohol content during ageing may be the occurrence of greater water sorption, as already verified for volatile compounds [37] or a faster penetration of water than ethanol into the wood, which greatly influences the impregnation kinetics in the beginning of ageing [38]. Regarding the methanol content (Table 2), the significantly higher content in AUS aged after six months (between 4239.9 mg/L and 4341.1 mg/L) than after 3 months (between 4225.0 mg/L to 4239.9 mg/L) should be stressed. Methanol is an undesirable compound in all spirits, given its toxicity [39], and its restrictive control is mandatory, but it does not directly affect the flavour of the distillate. All the values found in this work were lower than the legal limit. These results are not in accordance with some authors, who reported a tendency for low amounts of methanol as the ageing time progressed. Such a discrepancy can be assigned to the methanol oxidation and formation of diethoxymethane as consequence of acetalization reaction [26; 40; 41]. Moreover, the cited studies were based on quite a long ageing period (2 to 5 years), which was enough for the aforementioned chemical changes. In our study, based on a six-month period at laboratorial scale, the methanol content in the AUS with six months of ageing was significantly higher than in the other ones, but only by a small percentage, which could be indicative of the wood variability and the usual variability observed in the first stages of the ageing process [19]. Higher alcohols (Table 3) are important compounds related to the quality and aroma of spirit drinks and are considered as part of the aromatic skeleton of fruit distillates [42]. The values of control AUS are similar to those previously reported by other authors [6; 12; 36]. Concerning theutan-1-ol, propan-1-ol, 2-methylpropan-1-ol and butan-2-ol, no significant differences were found between samples. Only for isoamyl alcohols (2+3-methylbutan-1-ol) the values of the aged beverages were lower than those observed in the control. Concerning the aged wood samples, isoamyl alcohols, have significantly lower concentration in the beverages aged with MPT wood. The effect of the ageing time was not significant for the concentration of 2+3-methylbutan-1-ol. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Code**} & **Alcohol** & **Methanol** & **Acetaldehyde** & **Ethyl Acetate** \\  & **(\% v/v)** & **(mg/L)** & **(mg/L)** & **(mg/L)** \\ \hline C & 46.2 \(\pm\) 0.00 * & 4370 \(\pm\) 8.54 * & 115.4 \(\pm\) 0.49 * & 259.4 \(\pm\) 2.12 * \\ LT3 & 47.9 \(\pm\) 0.11 b & 4247.4 \(\pm\) 83.60 a,b & 127.6 \(\pm\) 2.31 b & 245.1 \(\pm\) 46.6 a \\ MT3 & 46.9 \(\pm\) 0.01 a & 4225.0 \(\pm\) 38.76 a & 124.7 \(\pm\) 5.95 b & 258.4 \(\pm\) 9.70 a \\ MPT3 & 46.9 \(\pm\) 0.01 a & 4225.4 \(\pm\) 56.30 a & 122.4 \(\pm\) 5.79 b & 270.0 \(\pm\) 7.41 a \\ LT6 & 46.8 \(\pm\) 0.13 a & 4239.9 \(\pm\) 37.54 b & 124.0 \(\pm\) 4.17 b & 266.6 \(\pm\) 5.98 a \\ MT6 & 47.0 \(\pm\) 0.03 a & 4294.9 \(\pm\) 34.91 a,b & 123.5 \(\pm\) 2.57 b & 269.5 \(\pm\) 3.49 a \\ MPT6 & 47.0 \(\pm\) 0.03 ab & 4301.1 \(\pm\) 63.75 b & 110.2 \(\pm\) 6.52 a & 266.7 \(\pm\) 22.7 a \\ \hline \hline \end{tabular}  
* Control denotes significant differences from the other modalities. Means within the same column followed by different letters are significantly different (\(p<\) 0.05) according to [PERSON]’s test.  
\end{table} 
Table 2: Mean and Standard deviation of alcohol strength, methanol, acetaldehyde, and ethyl acetate of the AUS aged with oak wood. [PERSON] et al. [43] identified evaporation, oxidation, esterification, and sorption as the main sources to explain the decrease in higher alcohols during ageing.