## 2 Results and Discussion
### Effect of US on Volatile Organic Compounds  
The sensorial properties of virgin olive oils are crucial for defining the quality of the product [24], thus the content of the volatile organic compounds (VOCs) present in the headspace of our samples was determined by HS-SPME-GC-MS. Table 3 shows the content of two groups of VOCs in our samples, selected according to the literature: the first group (first 5 rows in Table 3) is constituted of the molecules originating in the LOX pathway, which are typically associated with the green and fruity notes of virgin olive oils (positive attributes); the second one (last 3 rows in Table 3) is represented by the C7 to C10 aldehydes, both saturated and unsaturated, which are linked to oxidative defects of virgin olive oils [27].  
Figure 4: Hydroxytyrosol percentages evaluated after the acidic hydrolysis for the 23 oils and expressed on the sum of tyrosol and hydroxytyrosol.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**VOC**} & \multicolumn{2}{c}{**1N–12N**} & \multicolumn{2}{c}{**P3U–PTU**} & \multicolumn{2}{c}{**C3U–C6U**} & \multicolumn{2}{c}{**OMN–OMU**} \\  & **U** & **N** & **U** & **N** & **U** & **N** & **U** & **N** \\ \hline LOX C5 & 0.25 a & 0.25 a & 0.47 a & 0.53 a & 0.15 a & 0.14 a & 1.61 a & 1.46 a \\ LOX C6 aldehydes & 7.98 a & 8.24 a & 11.98 a & 12.06 a & 5.22 a & 5.16 a & 34.24 a & 34.78 a \\ LOX C6 esters & 2.91 a & 2.89 a & 0.06 a & 0.05 a & 0.32 a & 0.29 a & 0.07 a & 0.07 a \\ LOX C6 alcohols & 0.77 a & 0.76 a & 1.36 a & 1.77 a & 0.70 a & 0.55 a & 1.63 a & 1.77 a \\ Total LOX & 11.92 a & 12.14 a & 13.87 a & 14.41 a & 6.40 a & 6.14 a & 37.55 a & 38.07 a \\ \hline saturated C7–C10 aldehydes & \textless{}0.01 a & \textless{}0.01 a & 0.29 a & 0.30 a & 0.01 a & 0.01 a & 0.15 a & 0.18 a \\ mono-unsaturated C7–C10 aldehydes & 0.02 a & 0.02 a & \textless{}0.01 a & \textless{}0.01 a & 0.05 a & 0.04 a & \textless{}0.01 a & \textless{}0.01 a \\ di-unsaturated C7–C10 aldehydes & \textless{}0.01 a & \textless{}0.01 a & 0.24 a & 0.26 a & 0.03 a & 0.02 a & 0.12 a & 0.13 a \\ \hline \hline \end{tabular}
\end{table}
Table 3: Distribution of the different groups of volatiles produced during the lipoxygenase cascade (first 5 rows) and linked to oxidative defects (last 3 rows) in oils obtained with (U) and without (N) the use of SHE. For each couple of oils, no differences were highlighted as indicated by the same letter (a).  
Overall, the total amount of the molecules originating in the lipoxygenase (LOX) pathway (green and fruity notes of virgin olive oils (VOOs)) is not significantly affected by the application of the ultrasound technology. At the same time, the different classes of VOCs belonging to the LOX cascade are not affected by the application of such technology. Furthermore, no increase of saturated and unsaturated aldehydes was observed. All these data confirm that the ultrasound technology does not affect the volatile profile of the obtained virgin olive oil samples.  
On the contrary, even if our experiments are not aimed at highlighting differences due to the cultivar, data in Table 3 clearly confirm that the cultivar affects the concentration of these molecules in the oils. The monocultivar oils obtained from Coratina show contents of molecules from the LOX pathway (up to 38 ppm) at least three times higher than the other cultivars, with Cellina di Nardo showing the lowest content (approx. 6 ppm). Regarding aldehydes, all cultivars showed negligible amounts of the monounsaturated ones (which are more linked to the oxidative defects), while Ogliarola and Coratina showed amounts of saturated and di-unsaturated aldehydes slightly higher than Peranzana and Cellina di Nardo, with, however, very low amounts.