# Flavor Chemistry of Virgin Olive Oil: An Overview
## 2 VOO Flavor Perception
### Volatile Compounds  
VOO volatile compounds were reviewed in the last years mainly in relation to factors affecting their production [20], oil quality [4; 5; 19], and to explain sensory perception of virgin olive oil [52; 53]. Overall, about 180 volatile compounds belonging to the chemical classes of aldehydes, alcohols, esters, hydrocarbons, ketones, furans and terpen have been identified in VOO. Among these, 5 or 6 carbon atoms, volatile compounds generated during a complex enzymatic process named \"lipoxygenase pathway\" [54] generate the majority of volatile compounds in VOO (Figure 3). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Chemical Compound** & **Chemical Structure** & **Sensory Property** \\ \hline
3,4–DHPEA–EDA (oleacein) & & bitter, astringent and burning (mostly on tongue) \\ \hline
3,4–DHPEA–EA (oleuropein aglycon) & & very bitter, very astringent \\ \hline p–HPEA–EDA (oleocanthal) & & strong burning/pungent (mostly at the back of throat); \\ p–HEPA–EA (ligtsroside aglycon) & & slightly bitter and astringent \\ \hline \hline \end{tabular} Source: [PERSON] and co-workers [12]; [PERSON] and co-workers [40]. Abbreviations: 3,4–DHPEA–EDA, dialdehyde form of decarboxymethyl elenoic acid linked to hydroxytrosol; 3,4–DHPEA–EA, aldehyde form of elenoic acid linked to hydroxytrosol; p–HPEA–EDA, dialdehyde form of decarboxymethyl elenoic acid linked to tyrosol; p–HEPA–EA, deacetoxy–ligtsroside aglycon.  
\end{table} 
Table 1: Some of the main phenolic compounds responsible for the bitterness, astringent, and pungent sensation of VOO. These enzymatic reactions start from polyunsaturated fatty acids and in the presence of enzymes such as lipoxygenases (LOX), hydroperoxyledylase (HPL) and isomerase. These enzymes are naturally found in the olive fruit and are active since the fruit is crushed during the olive milling phases. LOX is a non-heme enzyme containing iron, which has the function of catalyzing the oxidations of the 1,4-pentadiene sequence of polyunsaturated fatty acids to produce the corresponding hydroperoxides. From the sequential action of lipases and lipoxygenases and together with the combined action of HPL, aldehyde compounds with six atoms and nine carbon atoms are formed. Subsequently, by reduction of the aldehydes through alcohol-dehydrogenase (ADH) activity, the respective alcohols are obtained. From these, in turn, the respective esters can be formed by the action of alcohol acetyl transferase. The fatty acids most involved in the lipoxygenase cascade are linoleic acid (LA) and linolenic acid (LnA). The LOXs lead to the formation, preferentially, of hydroperoxides in position 13, whose decomposition, catalyzed by hydroperoxide-lyase, determines the formation of aldehydes with six carbon atoms, including hexanal (green notes) and _cis_-3-hexenal (notes of tomato leaf) [55]. Subsequently, the _cis_-3-hexenal isomerizes, giving rise to _trans_-2-hexenal (notes of grass, almond), which usually represents the most abundant volatile compound of good quality EVOOs, giving the oil the herbaceous and pleasant aroma. The ADH subsequently have the function of transforming the aldehydes into the corresponding alcohols, making the herbaceous notes less aggressive. The subsequent esterification by alcohol-acetyl-transferase (AAT), gives rise to the production of esters with a sweet odor impact, typical of rip fruits [19]. An additional pathway to the lipoxygenase pathway is activated when the substrate is linolenic acid. LOX could catalyze, in addition to the formation of hydroperoxides, also their cleavage through a donation of an alkoxy radical group for the stabilization of the 1,3-pentene radical. These latter compounds can dimerize leading to the formation of ten-carbon hydrocarbons (C\({}_{10}\)), known as pentenes dimers, or they can join with hydroxy radicals present in the medium, producing five-carbon alcohols, which can then be enzymatically oxidized to the corresponding compounds with five carbon atoms [19].