# Flavor Chemistry of Virgin Olive Oil: An Overview
## 2 VOO Flavor Perception  
Flavor perception is mainly based on two modalities, i.e., olfaction and taste. The delicate and unique flavor of VOO can be perceived during inhalation, when olive oil odorants released from liquid into the air (headspace) pass through the external nostrils to stimulate the olfactory receptors in the nasal cavity (ortho-nasal route). In this case, the term \"odor\" is used. Subsequently, when the VOO is put into the mouth other sensations take place. Different chemical stimuli are dissolved in the mouth and make contact with several types of sensory receptors on the tongue. These chemical stimuli are responsible for the taste of VOO, particularly bitterness sensation but also sweetness. Other sensations in the mouth occur by the free endings of trigeminal nerve stimulation. In this case, we refer to pungency, astringency and metallic attributes of VOO [19]. Moreover, olive oil odorants can interact with the odor receptors by moving from the mouth to the nasal cavity via the nasopharynx (retro-nasal route). In this case, the term \"aroma\" is used. Olive oil aroma, similarly to other foods and drinks, is significantly affected by oral processing. For this reason, the odor (ortho-nasal odor) and aroma (retro-nasal odor) perception could be different, even though the same olfactory sense is involved [23; 24; 25; 26; 27]. Some compounds such as phenolic compounds (VOO non-volatile matrix) stimulate the tasting receptors and trigeminal nerve while volatile compounds (VOO volatile matrix) stimulate the olfactive receptors and are responsible for VOO odor and aroma. Another taste sensation during VOO taste is the viscosity, i.e., the measure of its resistance to gradual deformation by shear stress or tensile stress. The bitterness taste sensation is more intensely perceived in the back and side of the tongue thanks to the interactions between the polar molecules (polyphenols) and the taste buds present on the tongue [19]. The spicy or pungent sensation resembles a burning feeling; however, it is not generated by high temperatures but to the tactile stimulation of the heat receptors in the oral cavity, in particular on the mucous membranes. Finally, the sensation of astringency, sensation of dryness, roughness and lapping can be perceived not only on the tongue but throughout the oral cavity thanks to the interaction of phenolic compounds with the proline-rich proteins present in the saliva. Several studies have shown that biophenols, in particular the aglycones of the secoiridoids, are the most responsible for the bitter and spicy attributes in the oil [11; 28]. The volatile compounds, of which more than 180 compounds have been identified so far in virgin olive oils, are instead responsible for the different nuances of odor that characterize the different oils, especially in relation to the cultivar. The final flavor perception is eventually codified and interpreted by the human brain (Figure 1). When a stimulus reaches the olfactory or taste or trigeminal receptor, it triggers a cascade of enzymatic reactions that generate second messengers. The intracellular second messenger activates a series of electrical events, when it reaches the ion channels, increasing the cell membrane permeability to certain ions. This sequence of biochemical and electrical events is called transduction, and this is the only mechanism available to the neuroreceptors to report to the brain their successful activation. The number of possible combinations of stimulus-receptor is very large, as the number of neuroreceptors is large, with consequent high number of signals that the brain interprets [30]. [PERSON] and coauthors [31] were the first who applied functional magnetic resonance imaging (fMRI) to evaluate the brain activity of human smelling VOO headspace volatiles. Oil samples of different quality were analyzed by Solid-phase micro extraction coupled with Gas-Chromotography/Mass-Spectrometry approach (SPME-GC/MS) to characterize their volatile composition and verify the differences.