## 3 Results and Discussion
### Volatile Compounds
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Volatile Compounds** & **\% of Samples** & **Concentration (\(\mu\)g/g)** & **Identification a** \\ \hline \multirow{2}{*}{Dodecane} & \multirow{2}{*}{67} & 0.4 \(\pm\) 0.5 & \multirow{2}{*}{MS/IK.DB} \\  & & & (0.1–3.2) & \\ \hline (1S-Z)-Naphthalene, & & & \\
1,2,3,5,6,8a-hexah dro-4,7- & & & \\ dimethyl-1-(1- & & 0.6 \(\pm\) 0.3 & \multirow{2}{*}{MS/IK.DB} \\ methylethyl) & & & \\ \hline
2,2,6-Trimethyl-octane & 5 & 0.5 \(\pm\) 0.3 & \multirow{2}{*}{MS/IK.DB} \\  & & (0.2–0.9) & \\ \hline \multirow{2}{*}{o-Cymene} & \multirow{2}{*}{9} & 1.0 \(\pm\) 0.6 & \multirow{2}{*}{MS/IK.DB} \\  & & (0.4–1.9) & \\ \hline Tetradecane & 5 & 0.3 \(\pm\) 0.1 & \multirow{2}{*}{MS/IK.DB} \\  & & (0.2–0.5) & \\ \hline
3,6-Diethyl-3,6- & & & \\ dimethyltricylo[3.1.0.02\({}^{A}\)4] & 21 & 0.2 \(\pm\) 0.1 & \multirow{2}{*}{MS} \\ hexane & & (0.1–0.4) & \\ \hline Undecane & 39 & 0.5 \(\pm\) 0.1 & \multirow{2}{*}{MS/IK.DB} \\  & & (0.3–0.8) & \\ \hline \multicolumn{4}{c}{**Ketones**} \\ \hline
3,3,6-Trimethyl-1,5- & & 1.3 \(\pm\) 0.6 & \multirow{2}{*}{MS/IK.DB} \\ heptadien-4-one & & (0.3–3.5) & \\ \hline
1-Penten-3-one & 68 & 3.9 \(\pm\) 3.3 & \multirow{2}{*}{MS/IK.DB} \\  & & (0.9–16.8) & \\ \hline
2-Heptanone & 5 & 0.6 \(\pm\) 0.1 & \multirow{2}{*}{MS/IK.DB} \\  & & (0.4–0.8) & \\ \hline
3-Pentanone & 56 & \(\begin{array}{c}5.5\pm 2.7\\ (1.8–17.0)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline
6-Methyl-5-hepten-2-one & 95 & \(\begin{array}{c}1.6\pm 0.8\\ (0.2–4.2)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline \multicolumn{4}{c}{**Nitriles**} \\ \hline Neryl nitrile & 100 & \(\begin{array}{c}16.7\pm 9.9\\ (1.2–48.2)\end{array}\) & \multirow{2}{*}{MS} \\ \hline \multicolumn{4}{c}{**Terpenes**} \\ \hline (+)-Eremophilene & 93 & \(\begin{array}{c}0.8\pm 0.6\\ (0.1–2.6)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline (\(E\))-\(\alpha\)-Bergamotene & 32 & \(\begin{array}{c}0.4\pm 0.4\\ (0.1–1.9)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline (E)-\(\beta\)-Ocimene & 51 & \(\begin{array}{c}0.3\pm 0.1\\ (0.1–0.7)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline (Z)-3,7-Dimethyl-1,3,6- & & 100 & \(\begin{array}{c}6.7\pm 3.7\\ (1.2–18.1)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\  & & (0.1–1.5) & \\ \hline \(\alpha\)-Bergamotene & 47 & \(\begin{array}{c}0.6\pm 0.4\\ (0.2–1.8)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline \(\alpha\)-Copaene & 100 & \(\begin{array}{c}4.5\pm 3.5\\ (0.8–19.7)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline \(\alpha\)-Cubebene & 75 & \(\begin{array}{c}0.7\pm 0.4\\ (0.2–1.8)\end{array}\) & \multirow{2}{*}{MS/IK.DB} \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._Although hydrocarbons were present in greater numbers, these were not the compounds that most contributed to the volatile profile (Figure 1). Aldehydes were the compounds that presented the highest concentrations, representing 40% of the concentration of volatile compounds, corresponding approximately to 324 ug/g. Identically to the previous class, and very close to it, the second class that most contributed to the total concentration of volatile compounds were alcohols, representing 39%, corresponding to an average concentration of 316 ug/g. The major compounds detected in the Douro olive oils were the following: (_E_)-2-hexenal at 259 \(\pm\) 187 ug/g, (_Z_)-3-hexen-1-ol at 157 \(\pm\) 109 ug/g, (_E_)-2-hexen-1-ol at 105 \(\pm\) 148 ug/g, (_Z_)-hexen-1-ol acetate at 45.0 \(\pm\) 78.2 ug/g, 1-hexanol at 44.4 \(\pm\) 71.8 ug/g, and 2-methyl-4-pentenal at 37.7 \(\pm\) 21.7 ug/g. These compounds were identified in a significant number of samples except for (_E_)-2-hexen-1-ol and (_Z_)-3-hexen-1-ol, which were identified only in 19 and 16% of samples, respectively. This may be because these compounds have a retention time close to that of (_E_)-2-hexenal, making their presence and identification difficult in certain situations. The six compounds mentioned above were identified as the major compounds in oils produced in the Douro Valley [23]. These major