## 2 Results and Discussion
### Headspace Composition  
Olive oil, compared to other vegetable oils, is distinguished by a characteristic aroma. The sensory characteristics, together with high nutritional value are the main features that have resulted in the increase of VOO consumption in recent years [14]. Aroma is an important criterion for VOOs. Consequently, the identification of the compounds contributing to the aroma is considered as a key for quality and authentication control [15]. The headspace profile of tested monovarietal oils was dominated by C\({}_{6}\) volatile organic compounds, mostly aldehydes (Table 2). The C\({}_{6}\) compounds responsible for green and fruity VOO perception are produced through lipoxygenase pathway during the olive fruit crushing and malaxation and incorporated into resulting oil. The most abundant was (_E_)-hex-2-enol (up to 63.8%, 69.8% and 63.6% respectively). (_E_)-Hex-2-enol was the most important positive contributor of lawn perception [9]. Considering other C\({}_{6}\) compounds it can be seen that (_Z_)-hex-3-enol was not found in _Lastovka_ oil, while _Levantinka_ oil did not contain hexanal. Different values of identified C\({}_{6}\) aldehydes in the samples could be due to different acyl hydrolase activity and consequently good or poor availability of free polyunsaturated fatty acids [16]. The results are in accordance with previous research [17] confirming that the cleavage by heterolytic hydroxydeperoxide lyase was the most important process \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Compounds**} & \multirow{2}{*}{**RI**} & \multicolumn{3}{c}{_Obtica_ **Area (\%)**} & \multicolumn{3}{c}{_Lastovka_ **Area (\%)**} & \multicolumn{3}{c}{_Levantinka_ **Area (\%)**} \\ \cline{3-10}  & & & **Min** & **Max** & **Mean \(\pm\) SD** & **Min** & **Max** & **Mean \(\pm\) SD** & **Min** & **Max** & **Mean \(\pm\) SD** \\ \hline
1. & Isoprene & \(<\)900 & 1.1 & 1.5 & 1.31 \(\pm\) 0.14a & 1.9 & 2.3 & 2.16 \(\pm\) 0.14b & 1.6 & 2.1 & 1.78 \(\pm\) 0.19c \\
2. & Ethyl acetate & \(<\)900 & 0.5 & 0.8 & 0.66 \(\pm\) 0.14a & 1.6 & 1.7 & 1.65 \(\pm\) 0.05b & 0.3 & 0.4 & 0.36\(\pm\)0.05c \\
3. & Pent-1-en-3-one * & \(<\)900 & 1.5 & 1.9 & 1.67 \(\pm\) 0.13a & 3.2 & 4.1 & 3.57 \(\pm\) 0.24b & 2.1 & 2.3 & 2.19 \(\pm\) 0.07c \\
4. & Pentan-3-one & \(<\)900 & 2.3 & 3.1 & 2.52 \(\pm\) 0.24a & 3.9 & 6.1 & 5.30 \(\pm\) 0.75b & 1.7 & 2.1 & 1.87 \(\pm\) 0.16c \\
5. & (_E_)-Pent-2-enol & \(<\)900 & 0.0 & 0.0 & 0.00 \(\pm\) 0.00a & 0.0 & 0.0 & 0.00 \(\pm\) 0.00a & 0.3 & 0.3 & 0.30 \(\pm\) 0.00b \\
6. & (_Z_)-Hex-3-enol & \(<\)900 & 6.2 & 6.9 & 6.48 \(\pm\) 0.27a & 0.0 & 0.0 & 0.00 \(\pm\) 0.00b & 16.2 & 17.0 & 16.64 \(\pm\) 0.28c \\
7. & Hexanal & \(<\)900 & 4.1 & 4.5 & 4.40 \(\pm\) 0.14a & 4.8 & 6.2 & 5.31 \(\pm\) 0.47b & 0.0 & 0.0 & 0.00 \(\pm\) 0.00c \\
8. & (_E_)-Hex-2-enol & \(<\)900 & 63.2 & 63.8 & 63.50 \(\pm\) 0.24a & 69.2 & 69.8 & 69.60 \(\pm\) 0.19b & 62.0 & 63.6 & 62.60 \(\pm\) 0.49c \\
9. & 3-Ethyloct-1,5-diene & 939 & 5.0 & 8.1 & 7.10 \(\pm\) 0.88a & 3.9 & 5.3 & 4.51 \(\pm\) 0.53b & 3.9 & 4.0 & 3.96 \(\pm\) 0.05c \\
10. & 3-Ethyloct-1,5-diene & 997 & 4.5 & 6.3 & 5.55 \(\pm\) 0.67a & 2.3 & 2.7 & 2.51 \(\pm\) 0.11b & 3.7 & 4.2 & 3.97 \(\pm\) 0.16c \\
11. & _trans_-\(\beta\)-Ocineme & 1054 & 1.0 & 1.1 & 1.06 \(\pm\) 0.05a & 0.1 & 0.4 & 0.21 \(\pm\) 0.09b & 0.0 & 0.0 & 0.00 \(\pm\) 0.00c \\
12. & _a_-Copane & 1380 & 0.9 & 1.3 & 1.11 \(\pm\) 0.11a & 0.0 & 0.0 & 0.00 \(\pm\) 0.00b & 1.2 & 1.4 & 1.31 \(\pm\) 0.09c \\ \hline \multicolumn{10}{c}{Total identified} & \multicolumn{3}{c}{(91.0\%–98.2\%)} & \multicolumn{3}{c}{(90.9\%–96.1\%)} & \multicolumn{3}{c}{(90.3\%–95.7\%)} \\ \hline \multicolumn{10}{c}{RI = retention indices on HP-5 MS column; Min = minimal percentage; Max = maximal percentage; mean = average percentage; SD = standard deviation; the mean values within each row labeled with different letters are significantly different (Tukey’s test, \(p\) \(<\) 0.05); * - tentatively identified.} \\ \end{tabular}
\end{table}