## 2 Results and Discussion
### Chemical Changes during the Storage Experiment
The objective was to identify the compounds whose concentration varied in a greater extent. Table 3 shows the compounds whose concentration changed with a RSD% higher than 50% in the four VOOs. These compounds were different in each oil and the total number of compounds with RSD% \(>\) 50% per oil points out the oxidation state of them at the end of the experiment. These compounds show that VOO1 and VOO3 underwent a decrease in the concentration of the majority of the selected compounds, which have pleasant attributes, such as (\(E\))-3-hexen-1-ol, which explains the loss of their initial positive attributes (median of the fruity attribute) during the storage (Figure 2). Sample VOO3 showed a large increase of the concentration of heptanal and  
Figure 2: Changes of the oil category, the median of the fruity attribute (Mf) and the median of defect (Md) for the four virgin olive oils (VOO1–VOO4) during the storage. (\(E\))-2-heptenal, which are related to oily and oxidized aroma descriptors [1; 31; 35]. Finally, VOO2 and VOO4 were in an advanced oxidation state at the end of the storage, as it is pointed out by the increase of the content of the volatiles related to the rancid defect [36], such as nonanal, heptanal and hexanoic acid. In a next step--and in order to gain a better understanding about which compounds have more influence on the virgin olive oil aroma--the odor activity value (OAV) of each volatile compound was determined at every month during the entire storage time for the different oils. This value results from the ratio of the concentration of the compound to its odor threshold [37]. Many of the compounds derived from the lipoxygenase (LOX) pathway, which contribute with a green and fruit aroma sensory descriptors [1], showed an OAV \(>\) 1 in all samples during the entire storage time, such as hexanal, (\(E\))-2-hexenal, hexyl acetate, hexanol and (\(Z\))-3-hexenyl acetate. The OAV of hexanal and (\(E\))-2-hexenal were particularly high: they were in the ranges of 51.10-27.69 and 13.69-2.84, respectively. Other compounds with unpleasant sensory descriptors showed a high OAV from the beginning of the storage, such as 1-octen-3-ol in VOO1 (135.94), VOO2 (47.57) and VOO4 (47.99), heptanoic acid in VOO3 (49.49) and (\(E\))-2-decenal in VOO4 (255.44). 1-octen-3-ol provides moldy odor to the oil, whereas heptanoic acid and (\(E\))-2-decenal are volatile markers of the rancid defect [31; 38]. Other compounds related to the rancid defect also showed OAV \(>\) 1 but at lower extent, such as nonanal and (\(E\))-2-heptenal. They were found at low concentrations although their odor thresholds were low enough (0.005 mg/kg and 0.15 mg/kg for (\(E\))-2-heptenal and nonanal respectively) to have some impact on the sensory characteristics of the oil. Thus, their OAV in the fresh samples were 2.96-3.49 and 1.26-1.63 for (\(E\))-2-heptenal and nonanal, respectively. Once the compounds that underwent the most significant changes were identified (Table 3), all volatile compounds from Table 2 were studied to identify those whose concentration changes were better correlate to sensory changes. For this purpose, a correlation matrix was performed between the OAV of the volatile compounds (Table 2) and the results from the sensory assessment (medians of the fruity attribute and the defect) monthly obtained. Only (\(E\))-2-hexanal, which exceeded its odor threshold in all samples during the storage experiment, showed a high correlation coefficient (0.70-0.96) with the median of the fruity attribute in the studied oils. Particularly, a strong correlation was found between these two variables in VOO1 (R = 0.96). This compound, which is considered as a freshness marker in vegetables oils [31; 39], contributes with green and fruity attributes (Table 2). Figure 3 shows a double-y graph where the median of the fruity attribute and the OAV for (\(E\))-2-hexenal are plotted per each VOO.