## 2 Results and Discussion
### Identification of VOCs by HS-SPME/GC-MS Analysis  
Olive oil volatile compounds are primarily derived from the enzymatic oxidation of the fatty acids linoleic and linoleic. Positive aroma perceptions in olive oil are produced by endogenous plant enzymes, through the LOX, while aroma defects are linked with chemical oxidation and exogenous enzymes [5]. The volatile fraction of VOOs mainly consists of aldehydes, ketones, aliphatic and aromatic hydrocarbons, ethers, esters, aliphatic and triterpen alcohols, and furan derivatives. Aroma compounds responsible for green desirable notes, found in high-quality VOOs, are produced enzymatically from polyunsaturated fatty acids through the LOX pathway. It has been observed that in the aroma of these oils, C6 aldehydes and alcohols and their corresponding esters are the most abundant accumulation products [26]. Both major and minor VOCs are important to olive oil aroma, since minor compounds with low odor threshold values are frequently considered significant contributors. Even those whose levels are below their olfactory threshold can still provide useful quality marker information [27]. In all samples, C6 and C5 VOCs derived from the LOX pathway were the most abundant (Figure 4a,b). This is in accordance with the organoleptic results obtained by the tasting panel, where samples were identified as EVOOs with medium fruitiness attributes (3 < x < 6). C6 and C5 VOCs contribute to the delicate green aroma of VOO, which enhances the \"green\" fruitiness attribute and is highly valued by consumers. The formation of these compounds is mainly influenced by the cultivar, the degree of ripeness of the fruit, the storage period of fruits before oil extraction, and the processing method [28]. C6 aldehydes produced from the linolenic acid, _E_-2-hexanal, 3-hexanal, and _Z_-2-hexanal, were found in considerable amounts. In agreement with data reported in other  
Figure 4: Natural variation of VOCs derived from LOX pathway in a (**a**) Koroneiki and (**b**) Tsounati variety sample. studies examining European olive oil cultivars, (\(2E\))-hexenal, responsible for the \"green\" attributes, was the most prevalent aroma compound in all VOO samples [20]. More specifically, concerning the Koroneiki cultivar, (\(2E\))-hexenal ranged from 7.64 mg kg\({}^{-1}\) in a conventional sample to 51.25 mg kg\({}^{-1}\) in an organic sample from the Heraklion region unit. In a similar manner, \(Z\)-3-hexanal, which is positively correlated with fruity, leave/grass, and almond notes [29], was found in high levels in VOO samples, reaching 8.77 mg kg\({}^{-1}\) in an organic sample from Chania. High amounts of hexanal, produced by breaking 13- hydroperoxide from linoleic acid, are frequently related to sensory notes sweet, apple, and green [30]. In the studied samples, hexanal reached 8.04 mg kg\({}^{-1}\) in an organic sample. C6 alcohols such as \(Z\)-3-hexen-1-ol, \(E\)-2-hexen-1-ol, and hexan-1-ol, related to sweetness, were also identified [5]. \(Z\)-3-hexen-1-ol and hexan-1-ol were then esterified by the alcohol acetyltransferase to produce the corresponding esters (\(3Z\))-hex-3-en-1-yl acetate and hexyl acetate, which also were found in considerable amounts in the studied samples ranging from 2.13 to 40.08 mg kg\({}^{-1}\) and from 0.5 to 9.75 mg kg\({}^{-1}\), respectively. The contribution of these volatiles is quite significant, as they complement aromas with sweet and pleasant notes [31]. Aside from C6 molecules, the headspace of VOO contains a reasonable amount of C5 alcohols and C5 carbonyl compounds [32]. The existence of these VOCs indicates the development of a new branch of the LOX pathway that produces C5 compounds. This extra branch is active during the biogeneration of olive oil aroma. C5 aldehydes and alcohols contribute to the positive flavor attributes of olive oil, providing pungent sensations, and are correlated to bitterness [5].