Artículo #153
The Chemistry of Wine and Sensory Perception
As human beings, we can only associate flavours and aromas with things we have been exposed to before, which is closely related to sensory learning and memory. We cannot associate a scent with something we do not know or have not been exposed to. Therefore, all our information is related to previous interactions or learning.
Texto destacado
It is estimated wine contains, on average, around eight hundred volatile substances translated into aromas. As good perfumes, aromas manifest as a function of time and result from interacting with the environment.
In this context, all those attributes that can be perceived through taste, sight, smell and touch are called organoleptic characteristics. In simple words, they reflect how as human beings, we perceive the tangible and the intangible, how we incorporate it through our receptors and finally, how our brain translates it into a colour, aroma, taste or texture. They are particularly relevant in wine since they are often indicators of quality attributes and allow us to identify a specific type of wine or differentiate it from another.
In the case of the aroma, the wine has chemical compounds that confer these characteristics and depending upon the grape variety, terroir and the aging time; these are unique and unrepeatable in each bottle. It is estimated wine contains, on average, around eight hundred volatile substances translated into aromas. As good perfumes, aromas manifest as a function of time and result from interacting with the environment.
The origin of scents in grapes and wine.
Wine aromas result from a series of complex metabolic processes from molecules intrinsically in the grape. In this sense, each grape variety has characteristic aromas, which are given mainly by the molecular constituents of its main structures. Whether they are perceptible or not, they are considered free or bound. Free, because they are in their volatile form, it is possible to identify them in the grape berry or linked when they are part of other molecular structures and are released after chemical reactions through the manufacturing process. But, initially, they are not noticeable in the chemical form in which they are found. These aromas are known as primary aromas.
Volatile compounds are gaseous substances at room temperature and in the grape berry mainly concentrated in the skin (1). The grape berry, when chewed, releases these volatile compounds that travel through the nasal cavity and come into contact with the cilia of the olfactory epithelium (the mucosa that lines the inside of the nose), triggering a nerve impulse in the endings of the cranial nerves of smell. This impulse then travels to the brain's temporal lobe, where it is interpreted in the center of olfactory memory, which allows us to distinguish and identify aromas acquired throughout life. The environment strongly influences these volatile compounds in the grape berry and later in the wine.
On the vine, the grape berry is subject to environmental factors that often affect the different aromas in the wine. In the grape berry, it is possible to find volatile compounds in the pulp and seeds, mainly in the form of a type of organic compound called terpenes, which for the most part become part of the wine unchanged and preserve its structure and distinctive aroma which are usually associated with floral scents.
On the other hand, by coating the grape berry, we can quickly identify the epicuticular wax. This waxy, crystalline layer is not exclusive to the grape, as it is also present in all higher plants and their fruits. Its chemical structure varies depending on the species and is strongly affected by external agents such as acid rain, ozone and even environmental dust particles. Over time, this crystalline structure (with perfectly organized atoms) can become amorphous (where there is no regular distribution), affecting the system's permeability and, with that, the complete integrity of the fruit.
This wax, in addition to housing the endemic grape yeasts (because they are not part of the components of the whole structure) and plays a fundamental role in protecting against external agents such as pollution, ultraviolet rays and water loss. It would also play an essential role in the formation of the primary aromas of the wine.
Given the lipophilic nature of the epicuticular wax, it would be able to adsorb external volatile compounds present in the environment, both from other plants and from the soil, incorporating them into the wine during the pressing process and then be distinguished in the final product just as it happens with pollen, which travels long distances with the wind in order to reach the female organs of plants and reproduce.
Terpenes play a fundamental role in plants. As volatile compounds, they can act as aromatic agents to attract insects to favour pollination and attract predators of organisms that threaten the integrity of the fruit, or they may even have the function of dissuading frugivores that may feed on them (2). And in the case of grapes, this would not be different.
Terpenes are organic compounds formed by the union of two or more isoprene. Isoprenes are carbon molecules that the plant synthesizes in order to generate terpenes and are typical constituents of the essential oils of higher plants. Isoprene biosynthesis does not occur by polymerization (several monomers chemically bond to form more complex structures) but rather by one of the two pathways already described for higher plants: the mevalonic acid (MVA) pathway or the methylerythritol phosphate (MEP). The selected route will depend upon the type of terpene required to be synthesized (3) by the plant and gene expression.
The most significant proportion of terpenes in the grape berry is monoterpenes, mainly within the fruit. However, it is also possible to identify sesquiterpenes in the grape berry. The term comes from chemical nomenclature, where "sesqui" means one and a half; three bonded isoprenes and fifteen carbons total. Sesquiterpenes have been well studied in other essential oil-producing plants, such as conifers, where specific structures secrete them that grape berries lack, such as oil ducts.
Once sesquiterpenes are synthesized in the fruit, they migrate to the epicuticular wax. Its chemical properties and affinity for lipophilic structures could explain this phenomenon. However, it is still unknown how sesquiterpenes, being hydrophobic structures, cross the hydrophilic cell wall of the grape berry to remain in the lipophilic epicuticular wax.
In grapes, monoterpenes are stored mainly in glycosylated form. Through a chemical reaction, the molecule binds to carbohydrates in the medium, which is highly beneficial for alcoholic fermentation. The pyruvic acid pathway breaks down the glucose molecule to form ethanol and carbon dioxide. In addition, it seems to be the most stable molecular form for monoterpenes since it has been shown that terpineol (terpenes with alcohol -OH functional group) could act as inducers of cell apoptosis and, therefore, be detrimental to the proper development of the fruit (4). On the other hand, there is no evidence that sesquiterpenes exist in their glycosylated form in the grape because they have been found unchanged (non-glycosylated form) as a complement of the epicuticular wax.
New aromas are incorporated from this initial aromatic development during the various oenological processes that wine undergoes throughout the different stages of its elaboration. These, commonly identified as secondary aromas, originate from the action of yeasts and bacteria in the case of alcoholic fermentation. Additionally, the incorporation of empyreumatic aromas arises from the aging process in the barrels. Barrels with different levels of toasting or roasting incorporate a set of new scents into the wine, identifiable as "spicy" since they evoke the presence of various spices and other vegetable substances. Each wine aroma is a descriptor to identify a wine variety. They originate from the two main raw materials of the grape and the primary aromas.
Taste and smell go hand in hand.
Senses of taste and smell are closely related. In fact, what we call "wine flavour" is actually the aroma. Primary flavours such as acid, salty, sweet, bitter and savoury or umami can be easily identified by the taste buds without the intervention of smell. Yet, the blueberry flavour, classic of Cabernet Sauvignon, requires the integration of signals received by the sense of taste to be able to be identified and from a trained sense of smell to isolate one from another.
Thus, when we commonly point to a particular food flavour, we refer to the integrated set of its aromas and how they interact with our senses. In order to distinguish flavours, the brain needs information from smell and taste. Both are chemical receptors; they respond to a stimulus that comes into physical contact with them to generate a biological response. Taste and smell are intimately linked to the emotional and behavioural functions of our primitive nervous system.
The smell is one of the least known senses, given its degree of subjectivity and how little developed it is in humans compared to other animals. But this does not mean we are exempt from using it to our fullest potential. There are several techniques widely used to train both senses in order to enhance the performance of the organoleptic analysis in wine tasting. These techniques are cultivated both by enology and by sommeliers, with particular emphasis on the latter, for presentation and appropriate service of wine in various instances of consumption.
Generation of olfactory memory.
There is a relationship between smell, memory and emotion. It is said that the sense of smell also has an affective nature. The scent of perfume, food or, in our case, a wine can evoke memories, create a wave of nostalgia for the time, place or people associated with said aroma and be a powerful stimulant in human emotions.
In a way that is not thoroughly described, the processing of odours through the limbic system creates olfactory memories. Particular combinations of olfactory receptors connect with other patterns of sensory experience, such as taste, touch or sight, so the stimulation of a single one stimulates all of them (5,6). Thus, it is necessary to consider that the stimuli generated by certain odours in our memory transport us, figuratively, to different significant moments of our lives, including childhood.
It has been proven that olfactory memory can be trained. And wine experts provide essential evidence to test skill acquisition theories because these are thought to be based primarily on advanced perceptual skills rather than cognitive skills, such as categorical knowledge or episodic memory.
In a study with sommeliers, when compared with untrained people undergoing functional Magnetic Resonance, it was found that both brain areas involved in sensory integration were activated. However, the group of experts was characterized by a more immediate reaction, directed to the wine stimulation, with a mechanism that reduced effort compared to untrained people. The group of experts showed activations in the brainstem and the left hemisphere, while the group of untrained people showed activations in different associative cortices, predominantly in the right hemisphere. These results confirm that wine experts work simultaneously on quality assessment and label recognition (7).
In this context, olfactory memory can be made up of several layers, from the most recent to the most remote memories and from the simplest to the most complex. As a whole, they constitute what we could call a "database" of our scent records, which are related to specific memories from our life trajectory. It is precisely these memories that generate the emotions that we ultimately associate with particular scents.
Finally, it must be taken into account that olfactory memory is closely related to the cultural context in which individual existence develops. The relationship between smells, memories and emotions is associated with the various characteristics that define a given culture. Just as the same smells do not exist on each continent or in different cultures, these smells do not evoke the same significant memories in each place or person. These cultural differences will depend on countless individual and social factors, shaped mainly by the culture in which they are immersed.
Relationship between terroir and wine aromas
If we consider that terroir is a dynamic system in which the biology of the vine interacts with environmental factors (mainly edaphoclimatic), we can perceive it as a set of differentiating elements of a given wine, colloquially known as "sense of place." Therefore, the terroir is the product of a group of adaptive mechanisms of the vine and subsequently of the fruit basically because primary wine aromas are the most influenced by the terroir.
These aromas are especially relevant because they are connatural to each variety and establish a palette of easily identifiable basic descriptors in each strain. Terpenes (mono and sesquiterpenes) are responsible for granting the primary wine aromas, and as discussed, their synthesis is strongly influenced by external factors of environmental stress specific to the climatology of the place, the characteristics of the soil and any other external stimulus, such as pests or predators.
It can be considered that the sense of place of a given wine is the reflection of all those adaptive mechanisms, which are perceived both in the fruit and later in the wine, either in the form of certain concentrations of acids, minerals, terpenes, flavonoids and sugars, as well as in the resulting wine, through the taste, smell and touch, as certain aromas, flavours and tactile sensations that we can distinctively identify in certain wines and not on others.
For this reason, the sense of place or terroir not only exists as a material reality in wine, but it is also possible to identify its components and analyze them with the proper technology.
For example, in vitro studies have shown that the synthesis of monoterpenes and sesquiterpenes can be induced by external stimuli outside of their natural occurrence and that treatment of grapes with methyl jasmonate results in the rapid emission of volatile compounds and de novo synthesis of terpenes that were not initially present in the variety (8).
The methyl jasmonate route has been widely studied in other higher plants in such a way that can explain the reason why the synthesis of terpenes in the grape berry is closely linked to the terroir since the primary function of jasmonic acid and its metabolites are the regulation of metabolic stress before environmental, biotic and abiotic stimuli, and how these can induce the expression of genes that help vine survival through the transcriptional reprogramming of the transcription factor WRKY and, therefore, of adaptive mechanisms. These transcription factors, called WRKY (which are exclusive to plants), have various biological functions in resistance to diseases, responses to abiotic stress, nutrient deprivation, and senescence, among others (9). All these aspects help to understand how environmental stimuli can affect the expression of genes and enzymes of known metabolic pathways relevant to the identifiable aromas of wines.
Finally, we should note that all the processes occurring within the grape berry that will result in wine are closely related and are of a complexity that it is not yet possible to elucidate fully. Therefore, we believe these processes need to be the object of a holistic vision and should not be treated as isolated events because they are essentially a type of primary response to environmental factors and are related to the biology of the vine in a particular place.
Cristina Nunez Olivares is a Pharmaceutical Chemist, with a diploma in Strategic Marketing from Universidad Adolfo Ibáñez. She has a postgraduate degree in Brand Management from Seneca College in Toronto, Canada.