Definition: Aroma

FROM SMELL TO FLAVOUR

Considerations relating to the act of tasting and the enjoyment of wine

This text draws on advances in neuroscience to provide a better understanding of how our sensory apparatus works. A basic understanding of anatomy and physiology is necessary to explain, in particular, the remarkable performance of the sense of smell. Nevertheless, we become aware of the limits of our performance and the relative objectivity of taste judgement. In fact, humility must prevail in proper tasting, as well as in sharing with others.

Taste is a polysemic term. On the one hand, it represents the sensory organ itself, upstream of the digestive tract, in the mouth and nasopharynx, and on the other hand, it represents the consequence of its functioning: sour taste, for example. A second ambivalence stems from its usual meaning, which associates smell with true taste (flavour), both chemical (sour, sweet, salty, bitter) and somesthetic (touch, pressure, hot, cold, pain). The pleasure of taste in the broad sense comes from the flavour that combines aroma and taste in the mouth.

Beyond its primary function of expertise and prevention, the sense of taste has risen to the rank of a true culture. Unlike sight and hearing, it proceeds through direct contact with matter, whose molecules, rather than waves (painting, music, etc.), are the vectors of sensitivity. By virtue of this functioning and the remarkable performance of the sense of smell, food taste is recognised as having a certain objectivity.

Below, we will describe the anatomical structure of the receptor mucous membranes and follow the path of stimuli to consciousness. Then, based on the 'intelligent' synthesis of flavour, we will attempt to show how taste awareness arises and contrast global reading and discrimination in the act of tasting.

We will then discuss the differences in sensory equipment between individuals and the reasons for the difficulties encountered in verbalising our perceptions. Finally, we will briefly address the question of whether each person's taste and smell abilities are acquired or innate.

TRUE TASTE (flavour)

The four chemical tastes (sour, sweet, salty and bitter) correspond to relatively specific taste buds scattered throughout the tongue's mucous membrane. Each of these, in its lateral parts, carries taste buds, consisting of a cluster of epithelial cells equipped with a sensitive rod opening into the lumen of a narrow canal, where it encounters sapid molecules. They are connected to the dendritic endings of the first taste neuron, whose cell body is located, according to the topography of the tongue, in one of the ganglia of the three cranial nerves (VII bis, IX, X).

The lingual branch of the trigeminal nerve collects, on the one hand, thanks to mechanoreceptors, superficial somesthesia, which includes coarse touch for general consistency (protopathic somesthesia) and fine touch for texture fineness (epicritic somesthesia), and, on the other hand, through essentially thermal sensors, thermo-algesic sensitivity. Finally, deep or proprioceptive somesthesia provides information about the position of the wine in the mouth, its volume and its degree of suppleness (consistency) through the movements of the muscles of the tongue, cheeks and jaws.

Flavour combines chemical, mechanical (touch), thermal, algic and spatial sensations.

SMELL

The smell of food is often perceived from a distance and can be examined separately, but it is in the mouth, combined with sapid sensations (true taste), that it forms taste in the broadest sense: flavour. When you have a cold, only the true taste remains, as the sense of smell has disappeared due to the blockage of the access routes to the olfactory mucosa and the dilution of molecules in excessive mucus.

The pleasure of taste comes mainly from the performance of the sense of smell, whose complex anatomy is concentrated in five square centimetres (25 million cells) of a highly specialised receptor mucosa or 'yellow spot' located on the roof of each nasal cavity, on either side of the septum. Fifty million sensory cells, equipped with cilia bathed in mucus, form a surface area comparable to that of a sheet of notebook paper.

Each neuron has a single type of molecular receptor in the form of a specific protein present in the ciliary membrane, providing a three-dimensional binding site. Its configuration is such that only a small number of odour molecules, out of the thousands of possible ones, will have a compatible spatial structure. Due to the weakness of the bonds (non-covalent), the chemical nature of the molecules does not predict the odour. Moreover, we do not know why an odour is pleasant or unpleasant. A thousand different proteins, i.e. specific receptors, have been identified, each of which can detect several types of molecules that share a recognisable geometric property. Molecules of different chemical nature but similar architecture offer spatial compatibility with the receptor and produce the same odour, such as oak gamma-lactone and vanillin.

Odour also depends on the size of the molecule (the molecular weight of odorous substances varies between 30 and 300), its solubility (water and fat) and its concentration. The same molecule, isobutylmethylpyrazine in Sauvignon Blanc, produces the aroma of passion fruit in low doses and that of cat urine in high doses, with all kinds of intermediates in between.

The threshold of perception, from nanograms (ng = 10 to the power of minus nine grams) to mg/l, varies depending on the molecules and individuals. Much greater than that of taste, olfactory sensitivity exceeds that of any device. A few molecules in the air (trichloranisole, skatole, etc.) are enough to cause a sensation.

In the case of the perception of natural odours (fruit, flowers, wine, etc.) or even synthetic odours (perfumes), composed of a subtle mixture of volatile molecules, the physiology of the receptors becomes extremely complicated. The sensation varies according to their relative concentration and type. The field of perception is as infinite as that of odours. Gil MORROT translates this power of smell into images by considering the thousand different types of sensors (specific receptors), which function in binary mode (activated/non-activated) and are capable of encoding two to the power of a thousand (i.e. ten to the power of three hundred) different odours. Furthermore, as each sensor is capable of varying its response according to the criteria mentioned above, the number of combinations is beyond imagination. Finally, there is great inter-individual diversity in odour detection. Each protein is encoded by a gene that can take different forms (alleles). For example, the perception threshold for pyridine (the smell of truffles) varies between 0.04 and 40 ppm (parts per million) depending on the individual, a factor of 1000!

INTELLIGENT SYNTHESIS OF FLAVOUR

Olfactory information travels along a direct, archaic pathway involving two neurons. Smell is a primitive sense that appeared very early in the evolution of living beings. The axons of the receptor cells in the "yellow spot" pass through the base of the skull via the cribriform plate (ethmoid bone) and connect with the mitral cells of the olfactory bulb, whose axons form the olfactory nerve. Thanks to an astonishing process of neurogenesis, the axons of the first neuron group together by affinity, resulting in a thousand axons reaching the mitral cell. Fifty thousand mitral cells amplify and transmit the specific message of the odour that concerns them to the cortex, with numerous subcortical projections.

The gustatory pathway, which is phylogenetically more recent, is composed of three successive neurons: the first, already seen in the ganglion of the cranial nerve concerned; the second at the level of the bulb, extending towards the subcortical nuclei. Access to consciousness is via a third thalamocortical neuron.

The abundance of horizontal connections and feedback loops with other subcortical structures governing vegetative or humoral functions testify to the emotional power of taste.

After this complex journey, all the gusto-olfactory neurons end up in the orbitofrontal cortex, where the "intelligent" synthesis of flavour takes place, tested by memory and coloured by affective tone. This is how "sensory thinking" is formed, between pure perception and memorised consciousness. The sense of smell, often composed of several molecules, corresponds to the activation map of the receptors, like a dashboard where some points light up and others do not; the perceived quality depends on the outline, on the image thus drawn. "Sensory thought" becomes more complex when the actual taste (chemical and somesthetic) is added to the smell in the mouth to form a composite image: flavour.

The degree of satisfaction (pleasure/displeasure) arises from this cortical process, which merges sensations, relays them to memory and imprints them with emotional content.

GLOBAL READING AND DISCRIMINATION

From spontaneous pleasure to the most detailed analysis of the sensory image presented, wine tasting takes many forms.

Great wines provoke the most elaborate intellectualisation of taste. In this tangle of perceptions, especially olfactory ones, the cortex seeks to see more clearly. It focuses on each one in turn to appreciate its contribution.

Through a process of separation, which is more or less automatic in the experienced taster, the mental image is nourished and refined. Oenological knowledge and habit facilitate the exercise and resolve, at least partially, the neurological conflict between the overall vision and the necessary obligation to analyse the parts separately. Oscillating between an overall awareness of the wine, reflecting its hedonistic value, and an examination of its components, the taster assesses their pleasure (or displeasure), compares and evaluates it.

Sensory analysis finds its full definition here with the constant and almost reflexive concern to limit one's investigations. The act of tasting takes place in a relatively short time; awareness of the wine as a whole should not suffer from overly detailed discrimination.

The frustrating difficulties in verbalising sensations and the subjectivity of overly detailed descriptions stem, on the one hand, from the global integration of information and, on the other hand, from a differentiation between the two cerebral hemispheres in their cognitive function: the right hemisphere (the opposite in true left-handed people) is the seat of taste, emotions and feelings, known as the 'artistic brain', while the left hemisphere is devoted to language, reason, calculation, action and writing. The verbalisation of sensations comes up against the separation of powers. In order to identify a scent that is part of a bouquet, at the same moment it is perceived "on one side", a term must be found "on the other" to designate it. Even with training, this memory is poor: out of twelve common scents (flowers, fruits, etc.), the average subject recognises only three. It is clear that the poverty of language is matched only by the complexity of great wines.

Joint research in neurophysiology and sensory analysis tends to show that training has no effect on performance. Each individual, depending on their own equipment, has a genetically determined detection potential; the thresholds are innate. However, this idea that function does not create the organ has its limits, since stimuli undergo cognitive processing under the control of memory before becoming conscious representations. Practice does not alter sensitivity, but increases the number of memorised "oenocultural" references and the frequency with which they are accessed by the conscious mind.

The experienced taster detects olfactory defects that are harmful to the aroma and the wine as a whole with greater acuity because they are often confronted with them and recognise them more easily. The subliminal perception of a deviant aroma (mouldy, etc.), detected but not recognised, is compared with its memorised image. Any match confirms the 'sensory intuition'. Doubt reverses the process: we start with the memorised reference and compare it with the perceived signal on which our attention is focused. The ideal is an 'honest' encounter, rather than a misleading influence of intuition on memory or suggestion on perception. A good taster does not necessarily have a better sensory system than a random individual. Experience, training and a personal ethic to counteract the influence of the imagination can, in some cases, optimise performance. But when faced with the perilous exercise of blind tasting, these will always remain very limited.

Despite the sophistication of our taste and smell apparatus, wine, especially fine wine, will always retain the mystery of its complexity. This is what gives it its irresistible power of attraction and seduction.

Professional experience:

1967: executive at a Bordeaux wine merchant (Barton et Guestier).

1970 Co-manager of family estate in Barsac.

1980: creation of a wine merchant business with cultural site (tasting school, museum), sales and tastings, group receptions with restaurant service in vaulted cellars (600 m²) in the centre of Bordeaux.

1989 Sales executive at Ets. Duclot in Bordeaux (pre Pétrus), specialising in Grands Crus and en primeur sales.