## Results and Discussion
### Volatile Compounds (VOCs) and Sensory Attributes  
#### 2.2.1 GC-MS and Sensory Attributes  
The characteristic and unique flavor of EVOO, in particular its green and fruity attributes depend on many volatile compounds [10; 11]. The identification and quantification of the compounds, causing both positive odors and off-flavors, is considered to be crucial for EVOO quality control. The list of selected identified and confirmed compounds, sorted by decreasing \(F\)-value, is shown in Table 1. Two groups of samples were successfully differentiated by one-way ANOVA. The concentrations of the majority of C6 and C5 aldehydes, which are regularly listed among the key ones that are responsible for positive green and fruity odors [18; 19], were clearly higher in monovarietal PDO EVOOs than in the commercial-blended ones. The most abundant volatile compound in all of the investigated samples, (\(E\))-2-hexanal, was also found in higher amounts in monovarietal PDO EVOOs, although it was not among the ones with the highest discriminative power judging from the \(F\)-values. However, (\(Z\))-2-hexanal and (\(Z\))-3-hexanal, carriers of major positive fruity and green notes, together with the isomers of 3-ethyl-1,5-octadiene and 4,8-dimethyl-1,3,7-nonatriene with unknown sensory relevance [20], turned out to be decisive for the differentiation of monovarietal PDO from commercial blended EVOOs. Among other possible causes of the lower amounts of these compounds in commercial blended EVOOs were possibly the changes induced by EVOO aging during storage, as it was shown in earlier studies [21]. In contrast to monovarietal PDO EVOOs that were analyzed relatively fresh, the age of commercial blended EVOO was not declared by the producers/sellers and it was practically unknown, and it was possible that these samples were produced or partially composed from olive oils that were obtained in harvests prior to 2016. Nevertheless, it must be kept in mind that all of the investigated EVOOs were carefully selected and sampled at the same time and they were therefore valid and authentic representatives of both groups of EVOOs offered on the market at that given moment [21]. The commercial-blended EVOOs from supermarkets were mostly characterized by the higher concentration of saturated esters, aldehydes, and alcohols (Table 1). Such compounds do not originate from the LOX pathway and they are mostly the result of other, mostly undesirable processes [21; 22; 23]. Ethyl and methyl acetate, which are responsible for winey-vinegar defect and, together with 2-methylbutanol and 2-phenylethanol, clearly indicated that olives underwent fermentation [12], were the major differentiators of the commercial-blended from the monovarietal PDO EVOOs. Non-LOX C4 and C5 branched compounds are known to derive from the conversion of certain amino acids, while linear acids, esters, and ketones originate from fatty acid metabolism [24]. All of the mentioned processes are commonly linked to more or less degraded raw olive fruit material, due to physical damage, inadequate sanitary conditions, or unsuitable storage of fruit before processing, and they are often found in VOOs with sensory defects [24; 25; 26; 27]. The possibility that particular non-LOX volatile compounds were formed and/or increased in concentration as a result of various oxidative processes during an (unknown) storage period of a number of commercial blended EVOOs, as shown previously by other authors [21], should not be neglected. PCA analysis clearly divided the samples in two groups (Figure 2). The majority of the investigated aldehydes and ketones that derive from the LOX pathway, including the major ones, such as (_E_)-2-, (_Z_)-2-, and (_Z_)-3-hexenal, as well as 1-penten-3-one and unsaturated hydrocarbons, were characteristic for monovarietal PDO EVOO samples, and they could have contributed to generating positive green and fruity notes [19] since a positive correlation between their concentrations and the intensities of such sensory attributes was evident (Figure 2b).