Wine aromas : Where do they come from?

Passion fruit, honeysuckle, clove, bell pepper, bacon, burnt tire, or even cat pee… where on earth do all these aromas come from?

Every wine taster has already experienced that describing a wine in front of a non-initiated public is to expose oneself to mockery and sarcasm. How indeed, can it be explained that a fruit with as little smell as grape can give life to a liquid as fragrant as wine? Either tasters are great poets, or winemakers are great magicians!

And yet, as surprising as they are, these aromas are real, modern oenology has demonstrated it. In addition to being real, they are also naturally present. Are you sitting down? At least a thousand volatile[2] compounds have been identified in wines and contribute to their aroma.

A few chemical considerations

Smells are generally made up of one or more molecules. The smell of a rose, for example, is made up of a hundred or so molecules. The same is true for wine, whose smell is the result of the alchemy of thousands of different compounds. When we smell grapefruit in a wine, it is not that there is indeed the smell of grapefruit but rather a compound that enters the composition of grapefruit and therefore reminds us of it.

Family resemblance

For example, the banana-smelling isoamyl acetate is also partly responsible for the smell of strawberry. This explains why, if it is present in the wine, these two aromas can appear to the taster. The smell of cinnamon is close to the smell of mango or lavender. However, neither of these aromas belongs to the same aromatic family. The choice of one over the other depends on one’s olfactory culture. In reality, we all smell the same thing but talk about it differently, which easily explains the differences in comments during a tasting.

A few physiological considerations

Note that beyond these chemical considerations, we are physiologically different from each other. We have a “perception threshold” that can vary according to individuals and according to compounds. There is also phenomenon of anosmia[3] in the population, linked to genetic variations. This is the case for the smell of sulfur released by urine after eating asparagus, which is only perceived by a handful of lucky people. Or the violet-smelling beta-ionone, which is only perceived by 60% of the population, the rest being insensitive to it and therefore unable to smell it in wines.

The smell of a wine depends on its grape variety, its origin, and its age. We often classify the odors of wines by family of aromas: fruity, floral, vegetal, animal, spicy, chemical… But we can also divide them into three categories: the so-called primary aromas, the secondary aromas and finally the tertiary aromas.

Primary aromas

The primary aromas come from the grape. They are associated with the smell of certain grape varieties and contribute to their typicity. This is what allows some professionals to identify a grape variety on a blind tasting or to differentiate it from another. They are also called “varietal aromas”.

For example, the typical varietal aroma of Gewürztraminer is rose or lychee while grapefruit, boxwood or freshly cut grass may be characteristic of Sauvignon Blanc.

Some of these compounds exist in the grape in odorous form (free), others in the form of “precursors” (bound form). It is the alcoholic fermentation, a complex biochemical phenomenon, which allows the transformation of the relatively neutral fruit that is grape, into a sometimes extraordinarily aromatic liquid. Thus, the aroma precursors present in the grape in bound form are released by the yeast enzymes during fermentation.

Aromatic compounds

Primary aromas can be divided into several families of aromatic compounds: monoterpenes, norisoprenoid derivatives, methoxypyrazine, thiols, lactones… Each of these compounds is present in many grape varieties but some have more than others.

Terpenes with their floral smell are characteristic of the Muscat wines and remind of rose, lavender, geranium and lemongrass. These aromatic substances are also used in the composition of the Nivea cream! Other aromatic grape varieties contain them, such as Riesling, Gewürztraminer, Silvaner and Pinot Gris. In 2008, researchers identified a terpene derivative, Rotundone, responsible for the black pepper smell in some wines. Syrah, Gamay and Austrian Grüner Veltliner are very rich in Rotundone.

Cabernet Sauvignon is known for its pyrazines with a green bell pepper smell. The concentration of this compound decreases during the ripening of the grapes and is therefore often linked to a lack of maturity. It also occurs in Cabernet Franc, Merlot and Carmenère.

What about Thiols?

Thiols have characteristic odors, often associated with Sauvignon Blanc. These same thiols are present in grapefruit and give it its typical smell. Nothing more natural, therefore, than to perceive this aroma in Sauvignon blanc wines. Depending on the concentration of thiol, the wines can express a fruity character, such as passion fruit, or more vegetal, broom, boxwood, or even more sulphurous odors such as cat pee or sweat. Wine never ceases to surprise us!

The intensity, the strength of the smell of the primary character can vary according to the maturity of the grape, the finesse of the “terroir” or the wine making process, the yields, etc.  

Secondary aromas

Secondary aromas appear during the wine making process.

Some acids synthesized by the yeast during fermentation can participate to the smell of wine. This is the case of acetic acid and its vinegar smell. In small concentrations, it contributes to the fragrant odor of wine, but if its concentration increases, it is considered a defect. Others can explain cheesy or even smell of stinky feet in some wines! Lactic acid, produced during malolactic fermentation[4], brings a buttery or creamy character typical of some Chardonnay wines.

The ester family is created by the alcohols and acids in the wine which, during fermentation, form aromatic molecules: green apple, pear, banana, English candy, pineapple, kiwi… This very fruity profile is stimulated by cool fermentation temperatures of very clear musts and often with the help of selected yeasts that favor certain aromatic profiles. It has been shown that the aromatic contribution of these esters is no longer evident after 2 or 3 years (chemical hydrolysis), which is why these wines should be drunk very young. Pinot grigio or Beaujolais nouveau, for example, are both very rich in esters.

The lees, dead yeast residuals, can contribute to the secondary character of wines left in contact with them for a prolonged period.  They bring discreet aromas, with a pastry character: brioche, bread, toast, cookie dough or even cheese rind, popcorn…  

Tertiary aromas

Another fascinating subject is the development of the “bouquet” of the wine, its tertiary character, linked to its evolution. The primary aromas that were previously mentioned will gradually fade in the wine after a few years of ageing. This is why the vast majority of wines marketed today rarely benefit from prolonged aging. Only the greatest wines can reach a “bouquet”, i.e. acquire a complex tertiary character.

How can a blackcurrant smell, over the years spent in bottle, turn into a smell of undergrowth or leather? This is something that science does not yet fully understand.

Oxidation VS Reduction

We must distinguish the oxidation bouquet from the reduction bouquet. The oxidation bouquet occurs when the wine is placed in an oxidative environment, in contact with oxygen, which will slowly play its role. Characteristic smells of nuts, oxidized apple, caramel, toffee, coffee, curry, or prune will then appear… This aromatic profile is sometimes called rancio. These aromas, sought after in certain wines such as Port, Sherry or even Banyuls and Rivesaltes, are oxidation defects in other wines which by nature should be fresh.

The reduction bouquet occurs in wines aged in airtight containers such as a bottle, almost completely sealed. Great red wines, great dry and sweet white wines can offer bouquets of reduction with time. Such wines will display surprising aromas of leather, undergrowth, humus, cigar box, truffle for red wines or beeswax, grilled hazelnut, toast, nougat, or quince for whites. Tannins in red wines, which diminish with time, most likely also contribute to the bouquet of old red wines, both in taste and smell.

One compound is often associated with the development of the bouquet: dimethyl sulfide (DMS) with its smell of truffle, hay and green olive, whose concentration increases during ageing.

TDN (trimethyl-dihydronaphthalene), responsible for the so-called “petrol” or “tar” odor in some Riesling wines, can also increase as the wines age.  

What about oak?

There are exogenous aromas in wines, conferred to some wines that have been placed in contact with oak. Their influence in wines depends on many factors: the origin of the wood, its specie, the drying process and maturation of the wood, the degree and time of toasting, but also the age and size of the barrel. European oak is mostly used in cooperage and to a lesser extent, American oak. The aromas imparted by a barrel can be very diverse, but they often have a very “gourmand” character: vanilla, coconut, clove, cinnamon to name just a few. These are aromas that tasters sometimes describe as “sweet” although this is only an aromatic association. Their formation in wine is linked to the so-called “Maillard reaction” which occurs when a resource is heated which transforms its color, smell and flavor. This same reaction takes place when a steak is cooked!

A young barrel will have much more effect on the wine than a barrel of several wines and so will its size: the smaller it is, the greater the effect on the wine. Great wines can withstand 100% new oak because they will still retain their character and personality. New oak should be used in the same way as salt or spices in cooking, as a flavor enhancer, to sublimate and not mask. We could conclude by adding to all this information an additional factor: the notion of suggestion in tasting. When I feel like using of my authority as a teacher or when my students have been testing my patience, I have them experience a humbling exercise: tasting a white wine that I have ignominiously tinted with red. The exercise is always successful since they never fail to attribute to the wine the aromatic descriptors often associated with red wines. It remains, therefore, that a significant part of subjectivity, of external influence, co

Find all of these explanations on the genesis of aromas during our wine courses in Toulouse and Bordeaux.

Article written by Annabelle Mispelblom Beijer, wine expert at WiSP

[1] Often associated with the smell of the South African Pinotage. Its origin is yet to be identified.

[2] That can take a gaseous form and therefore be smelled.

[3] Anosmia is the loss of sense of smell which can be temporary, as in the case of COVID, but also permanent.

[4] Malolactic fermentation often occurs after alcoholic fermentation and is caused by lactic bacteria that transform malic acid, naturally present in the grapes, into lactic acid, which is rounder and has a milky smell.

[5] The “woody” character linked to the use of oak is sometimes cited among the secondary aromas, sometimes among the tertiary aromas depending on the pedagogical approach.