## 3 Results and Discussion
### Volatile Compounds (VCs)
commonly referred as ethyl caprate, is created when ethanol reacts with capric acid (a saturated fatty acid labeled C10:0). Ethyl decanoate was detected only in oil produced from CO\({}_{2}\)-processed olives in a low concentration (0.26 \(\pm\) 0.07%), as well. These compounds were also detected earlier in virgin olive oil [16; 47; 52]. Ethenylbenzene, also known as styrene, is a volatile aromatic hydrocarbon that can be produced artificially (industrial paints, adhesives, packages, combustion products) or naturally (plant wax, amino acid fermentation). Ethenylbenzene was detected only in oil produced from CO\({}_{2}\)-processed olives at a low concentration (4.10 \(\pm\) 0.42%). It has also been reported by other authors as a component of virgin olive oil [38]. Another aromatic heterocyclic compound, 1,3-benzothazole, was detected in all oils examined in this study at a low percentage (0.42-0.31%). However, no reports on its appearance in olive oils exists in the literature. 1,3-Benzothiazole belongs to benzothiazoles which are a class of high-production volume chemicals with various applications in the industry [55; 56]. Benzothiazoles are used in agriculture to prevent and control soil-borne phytopathogenic fungi which affect crops [57]. The measurement of nonanal may be a suitable technique to identify the onset of oxidation. Even if hexanal is present in the original flavor, it was discovered that the hexanal/nonanal ratio is a suitable indicator to identify the start of oxidation and follow its progress [38]. This happened in oil produced from control olives (hexanal/nonanal, 24.09%/1.06%). However, in oil produced from CO\({}_{2}\)-processed olives nonanal was detected at 0.58 \(\pm\) 0.10%, while hexanal was not detected. The major compounds formed in oxidized olive oil are saturated carbonyl compounds, including pentanal, hexanal, octanal, and nonanal [12], and were not detected in commercial oil. The most frequently appearing volatile markers of oxidation of virgin olive oil during storage are nonanal and \(E\)-2-decenal [49]. The autoxidation of oleic acid can be exclusively responsible for the appearance of nonanal [58]. Vegetable oil hydrocarbon fraction composition exhibits significant variations that may be used to characterize the oil. Despite their low concentration in olive oil, terpenic hydrocarbons (mono- and sesquiterpenes) exhibit significant variation depending on the variety and the region of production [40]. Limonene is a hydrocarbon classified as a monocyclic monoterpene (C\({}_{10}\)) and generated by alcohol \(\alpha\)-terpineol with a loss of a proton. (+)-Limonene smells of oranges, whilst (\(-\))-limonene resembles the smell of lemons [59]. (+)-Limonene was detected in all samples. The highest quantity appeared in oil produced from control olives (13.11 \(\pm\) 0.70%), followed by commercial oil (1.60 \(\pm\) 0.06%), while the lowest concentration was detected in the oil produced from CO\({}_{2}\)-processed olives (0.90 \(\pm\) 0.04%). The presence of (+)-limonene was previously reported in virgin olive oils [21; 35; 36; 37; 39; 41; 52]. \(\alpha\)-Myrcene is a structural isomer of \(\beta\)-myrcene and during our study was detected both in oil produced from control olives and commercial oil at the concentration of 0.98 \(\pm\) 0.12% and 0.50 \(\pm\) 0.11%, respectively. Virgin olive oil has previously been reported to contain this compound [39; 40]. The acyclic monoterpene \(\beta\)-myrcene (C\({}_{10}\)) and \(E\)-\(\beta\)-ocimene (C\({}_{10}\)) are generated through the monoterpene synthase enzyme under the geranyl pyrophosphate [59]. \(\beta\)-Guaiene is a bicyclic sesquiterpene (C\({}_{15}\))--a compound from the azulene group. It was detected only in oil produced from CO\({}_{2}\)-processed olives, in low concentrations (0.12 \(\pm\) 0.01%). [PERSON] et al. [40] have identified a different natural chemical compound, \(\delta\)-guaiene.