1. Introduction

Virgin olive oil (VOO) is a staple ingredient in the Mediterranean diet [1,2,3], and it is a food providing great nutritional properties due to its balanced fatty acid composition and presence of phenolic compounds, as well as unique sensory quality [4,5,6,7].

VOO is obtained from the fruit of the olive tree solely by mechanical or other physical means under conditions, particularly thermal conditions, that do not lead to alterations in the oil, and which have not undergone any treatment other than washing, decantation, centrifugation and filtration [8]. Among the many commercial categories existing for olive oil, extra-virgin olive oil (EVOO) has particularly high standards in terms of composition as well as sensory characteristics assessed by recognized panels [8]. From a sensory and chemical point of view, it is a complex food mainly composed by triacylglycerols, which account to 98% of the total composition, and a series of minor constituents that are of paramount importance for its health significance and sensory implication [9]. This minor fraction comprises free fatty acids, phenols, tocopherols, sterols, phospholipids, waxes, squalene, other hydrocarbons and volatile compounds. VOO fatty acid composition is known to vary according to environmental and agronomical conditions, with oleic acid (C_18:1) being the most abundant fatty acid, representing usually 60-80% total fatty acid composition [10].

Several polyphenols in VOO are hydrolysis products of oleuropein and ligstroside and at least 30 different compounds have been identified so far [11]. They affect VOO taste in terms of its bitterness and pungency [12], the antioxidant properties of VOO [13], and they are known to play positive roles on human health [14]. Olive oil phenolics may inhibit oxidation of low-density lipoproteins, which are the most atherogenic ones, and several other positive health effects have been associated to their consumption [15,16,17]. Volatile compounds with 5 and 6 carbon atoms are the most abundant classes in VOO aroma, contributing to its typical “green” fruity odor [5]. Aldehydes such as hexanal, trans-2-hexenal, cis-3-hexenal, and trans-2-pentenal, both with alcohols (cis-3-hexen-1-ol, trans-2-hexen-1-ol, and 1-hexanol) and 1-penten-3-one contribute strongly to the typical green notes. The different nuances of VOO are related to the level and composition of the volatile fraction, which is affected by wide possible variations in olive oil production techniques and the starting raw material [4,18,19,20]. In addition to the lipoxygenase pathway, various other reactions can lead to the formation of other volatile compounds which thus increase the olfactory complexity of the oil. However, when these reactions prevail, they can lead to the appearance of odor defects (off-flavor) [4].

VOO is highly appreciated for its sensory properties, and sensory assessment is compulsory for every lot of VOO that is commercialized, in accordance with the national legislations of many countries and the standards of the International Olive Council (IOC) regulations on the trade standard of this product and its sensory assessment guide. In fact, olive oil that is to be classified as “extra-virgin” must not have any presence of unpleasant aroma defined as off-flavor, while lower categories can present a slight level of sensory defects which is categorized and quantified. The presence of off-flavors or absence of fruity aroma can therefore change the commercial category of a VOO, and when a defect is too strong, even if the chemical parameters are within acceptable ranges, it will result in a product that is not-marketable, unless refined [21]. VOO is therefore a highly regulated food both at national or supranational level, e.g., by the EU, and international level. In addition, its quality may be defined by a number of parameters assessed by analytical tests, but the sensory perception of odor is the ultimate determinant [22]. Sensory analysis is still the most effective tool to evaluate VOO quality. The method established by the current EU legislation to assess the organoleptic characteristics of VOO is through sensory analysis, specifically the so-called “panel test” conducted according to the IOC method [8].

Tasting virgin olive oils is a multisensory experience that involves the visual, olfactory, gustatory and tactile senses, whereas the latter is less relevant in VOO tasting. Smell, taste and mouth-feel sensations are defined with only word “flavor”. Investigating the factors that influence VOO flavor is of importance for the production and appreciation of a successful product.

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).

Figure 1. Schematic diagram representing virgin olive oil flavor perception. Legend: (1) extra virgin olive oil glass, (2) odor (through ortho-nasal route), (3) aroma (through retro-nasal route), (4) nasal cavity, (5) olfactory bulb, (6) olfactory epithelium (sense of smell), (7) tongue (sense of taste), (8) taste buds (bitterness perception), (9) oral cavity, (10) trigeminal nerve (chemesthesis perception: pungency), and (11) signal in the brain and the recognition of sensory perception (adapted from Genovese and Sacchi [29]).

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].

Garcia-Gonzalez 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-Chromatography/Mass-Spectrometry approach (SPME–GC/MS) to characterize their volatile composition and verify the differences. The results of the fMRI obtained showed different hedonistic values of the olfactory perception of a related oil in relation to its quality and therefore its pleasure for every single individual (Figure 2). The zones with the highest activity were the orbitofrontal, the frontal and the temporal lobes which correspond to Brodmann areas 6, 10, 11, 20 and 47. The bilateral activations BA 10, 11 and 47 were associated to the olfactory process which explains their activation in response to both pleasant and unpleasant samples. An increase in cerebral blood flow in the BA 11 is also associated with the familiarity of odors, which would explain the high activation area seen in subjects who were regular users of virgin olive oils. Details of the application of functional magnetic resonance imaging (fMRI) and the aspects related to experimental design, data acquisition and data processing are described in Marciani and coauthors [32].

Figure 2. Results of functional Magnetic Resonance Imagining (fMRI) on the human brains of subjects exposed to rancid olive oil and extra virgin olive oil. Brodmann areas (BA) are marked with circles (Reproduced with permission from García-González, D.L.; Vivancos, J.; Aparicio, R., J. Agric. Food Chem.; published by ACS Publications, 2011).

One can hypothesize that some olive oil odorants might be able to mislead the brain. It can also be hypothesized that the “green” flavor of VOO exerts a positive effect on the food intake and satiation. In fact, German scientists reported that yogurt containing an aromatic extract of VOO modulates the cerebral blood in frontal operculum, inducing brain activation more similar to that induced by the high-fat yogurt [33]. Using a fat-free yogurt, two groups of assessors tasted a product added with an olive oil aroma extract in comparison to a control product. The group who ate the plain yogurt showed a drop in serotonin levels a hormone associated with satiety. Thus, this group reported less satiation after eating it. They also did not cut back on other calories to compensate; instead, their intake increased an average of 176 calories a day. On the contrary, the group eating the olive-oil flavored yogurt reduced their calories from other foods and showed better responses when given glucose tolerance tests, which measures the blood sugar control. Abrupt swings in blood sugar are part of what drives hunger and satiety. The sense of gratification and/or reward linked to VOO intake might be attributed to the herbaceous odor of molecules such as hexanal, trans–2–hexenal, and cis–3–hexenol.

These molecules are the most abundant volatiles in VOO, and previous research has shown that they are able to stimulate the release of dopamine in the brain [34]. This was also confirmed by the study by Kobayashi and coauthors [35], which verified an increase in the release of dopamine in the presence of herbaceous odors such as that of hexanal, explaining that this molecule could induce an increase in the intracellular concentration of Ca²⁺ and the dephosphorylation of phosphorylated proteins, phenomena required for a greater release of dopamine. The herbaceous smell of VOO, therefore, could influence and regulate the stimulation of dopamine release and thus increase the sense of gratification of the food consumed.

Different agronomical and technological factors influence the initial composition of the VOO volatile and non‐volatile compounds [7,20] as well as its flavor perception. During olive oil tasting, factors such as salivation, mouth size, breathing and temperature are also able to change the volatility of olive oil odorants and consequently VOO aroma. In addition, another mechanism that can happen is the potential interaction between the matrix components (such as VOO phenolic compounds) and the volatile compounds that could reduce the aroma release and change the odor perception of olive oil.

This entry is adapted from the peer-reviewed paper 10.3390/app11041639