## 2 Materials and Methods
### Solvents and Chemicals
#### 2.2.1 Wines for Sensory Analysis  
Two wine models were generated mimicking: (a) a young wine and (b) an oaked wine. They were prepared by mixing a pool of common non-volatile and volatile wine components, detailed in Table 1, corresponding to the average concentration of aroma compounds found in a previous work [22].  
For studying the sensory relevance derived from the release of Strecker aldehydes from hydroxyalkylsulfonate adducts during wine storage for each of the two wine models, five aldehydes (isobutyraldehyde, 2-methylbutanal, 3-methylbutanal, methtional and phenylacetaldehyde) were added at six concentration levels based on previous studies [8]. These levels are detailed in Table 2.  
These levels were selected in order to simulate the concentration of free aldehydes during different stages of wine ageing. Levels 4, 5 and 6 (6 being the highest level, Table 2) mimic the maximum concentrations of aldehydes that can be released after the breakdown of the hydroxyalkylsulfonates by oxidation processes (in other words, maximal levels of aldehydes in the free forms). Differently, levels 1, 2, and 3 correspond approximately to 10 or 30% (depending on the compound) present in levels 4, 5 and 6. The rationale behind the selection of these two groups of concentration of the free aldehydes is that as stated in the mentioned reference, in a recently opened commercial wine with an adequate SO2 level for its preservation, the free forms present in the headspace are 30% of the total amount of isobutyraldehyde and 2-methylbutanal, whereas only the 10% for 3-methylbutanal,  
\begin{table}
\begin{tabular}{c c} \hline \hline \multirow{2}{*}{**Compounds**} & **Concentration (mg/L)** \\  & **Young Wine** & **Oaked Wine** \\ \hline Isoamyl alcohol & 180 \\ \(\beta\)-Phenylethanol & 30 \\ Acetic acid & 150 \\ Ethyl acetate & 50 \\ Hexanoic acid & 2.0 \\
3-Methylbutanoic acid & 0.30 \\
2,3-Butanedione & 0.40 \\ Ethyl hexanoate & 1.0 \\ Isoamyl acetate & 1.0 \\ Ethyl 2- methylbutanoate & 0.12 \\ Ethyl vanillate & 0.55 \\ \(\beta\)-Damascenone & 0.0040 \\ \(\beta\)-Ionone & 0.00030 \\ \(\gamma\)-Nonalactone & 0.020 \\ \hline Guaiacol & 0.010 & 0.030 \\ Vanillin & 0.070 & 0.17 \\ Whisky lactone & 0 & 0.20 \\ Eugenol & 0 & 0.020 \\
4-Hydroxy-2,5-dimethyl-3(2 H)-furanone & 0 & 0.10 \\ Acetovanillone & 0 & 0.20 \\ \hline pH & 3.5 \\ Glicerine (g/L) & 10 \\ Quinine (mg/L) & 7.0 \\ Arabic gum (mg/L) & 75 \\ Ethanol (\%) & 12 \\ \hline Tannic acid (mg/L) & 50 & 100 \\ Tartaric acid (g/L) & 5.0 & 4.0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Wine models composition.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Level 1** & **Level 2** & **Level 3** & **Level 4** & **Level 5** & **Level 6** \\ \hline Isobutyraldehyde & 4.30 & 8.50 & 14.2 & 16.6 & 33.2 & 55.3 \\
2-Methylbutanal & 3.70 & 7.30 & 12.2 & 11.7 & 23.4 & 38.9 \\
3-Methylbutanal & 2.50 & 5.00 & 8.50 & 24.3 & 48.6 & 82.6 \\ Methional & 1.30 & 2.40 & 4.00 & 13.5 & 25.6 & 43.1 \\ Phenylacetaldehyde & 4.70 & 8.50 & 14.4 & 49.2 & 88.6 & 149 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Strecker aldehydes concentrations (\(\mu\)g/L) in wine models without sulphur dioxide.  
methional and phenylacetaldehyde, while the same wine under oxidative conditions, would present most of its aldehydes as free form. Accordingly, these six levels would correspond to three different wines with the same levels of total aldehydes: low category for levels 1 and 4, intermediate category for levels 2 and 5 and high category for levels 3 and 6.