# Natural Variation of Volatile Compounds in Virgin Olive Oil Analyzed by HS-SPME/GC-MS-FID
## 3 Results and Discussion  
The extensive genetic patrimony of the olive tree is currently used in different olive breeding programs, but only recently includes the sensory properties of VOO as main traits, in addition to the agronomic traits [25; 27; 30]. Regarding these sensory properties, the natural variation of aroma compounds in VOO was studied using the WOGC olive cultivar collection. For this purpose, around 15% of this germplasm collection was considered (Table 1) [24]. Only healthy fruits were picked at the turning stage, and mild operating conditions were employed for the oil extraction process, to minimize the production of compounds that cause sensory defects in the oils. This subset of the olive germplasm collection showed a high degree of variability, in terms of the content of volatile compounds (Figure 1 and Table 2). As mentioned, most of the volatile compounds found in the oils were synthetized by the enzymes included in the so-called LOX pathway [7], and can be grouped according to the length of the chain, the fatty acid substrate (C6/LnA, C6/LA, C5/LnA, C5/LA), and the origin of esters (LOX esters). Moreover, compounds derived from amino acid metabolism, which has a branched-chain chemical structure (BC), and a terpene also contributed quantitatively to the volatile fraction of VOO. C6 compounds derived from linolenic acid (C6/LnA) represented, on average, about 70% of total volatile fraction in the oils (Figure 1). The C6/LnA compounds varied from 5.53 to 67.94 \(\upmu\)g/g oil, a variability higher than that found for oils produced from a collection of 39 olive cultivars, cultivated under the same edaphoclimatic conditions (2.52-18.11 \(\upmu\)g/g oil) [31]. The C6/LnA aldehyde group was the most abundant, of which (E)-hex-2-enal was the main compound (91% of total C6/LnA compounds on average, Figure 1). The mean content of (E)-hex-2-enal in the oils was 27.19 \(\upmu\)g/g oil, ranging from 4.70 to 65.03 \(\upmu\)g/g oil. The large amount, and the relatively low odor threshold [32], of this C6/LnA compound make it one of the main contributors to the aromas of the oils, which seems to be typical of oils from the olive species [29; 31]. C5/LnA compounds showed contents close to those of the C6/LnA compounds. Quantitatively, the pentene dimers are the major compounds in this group. They are thought to be produced in the same branch of the LOX pathway as the rest of the C5 compounds [33]. The pentene dimers represented 82% of the C5/LnA content on average (Figure 1), but seem to have a quite low or negligible sensory contribution to the VOO aroma, according to the estimated odor thresholds for these compounds [29]. The rest of the C5/LnA compounds seem to contribute to VOO aroma according to their OAVs (Table 2). Pent-1-en-3-one and the pent-2-en-1-ols are especially remarkable. All the cultivars showed to have contents of pent-1-en-3-one above its odor threshold (0.73 ng/g oil). The odor of this compound is considered unpleasant [9], while the aroma of pent-2-en-1-ol is pleasant, green-fruity. Most of the olive cultivars (90%) presented (E)-pent-2-en-1-ol contents above its odor threshold [29] (Table 2). On the contrary, (Z)-pent-2-en-1-ol contents suggest that this compound is of little relevance to VOO aroma, as it was present below its threshold concentration in the oils. Fruity odor notes are positive attributes of the oil aroma. Volatile esters are the main compounds responsible for these odor notes, especially LOX esters. Their content in the olive collection subset was 1844 ng/g oil on average, and is present in a range of 14-15,369 ng/g oil, much higher than those values found in a progeny of the cross of cultivars Picual and Arbequina [29]. Only a few accessions (3%) had (E)-hex-2-en-1-yl acetate contents that contribute to their aroma (OAV > 1, Table 2). However, (Z)-hex-3-en-1-yl acetate contents suggest this compound to be an important contributor to VOO aroma in more than half of the olive collection subset (Table 2).