Welcome to the fourth part of the series Terroir & the chemistry of wine. We have looked at soil and climate, at acids and pH, and at the phenols that give colour and structure. Now we turn our nose towards the volatile: the aroma compounds that make a glass smell of citrus, violet, tropical fruit or freshly cut grass.
Aroma is the most complex chapter in the chemistry of wine. Wine contains over 800 different aroma compounds, and the concentrations range from hundreds of mg/L all the way down to ng/L. This means that a compound present in vanishingly small amounts can dominate an entire expression, while another at a far higher concentration is barely noticeable. Here it is about understanding where the compounds come from, and how they become the scent you actually perceive.
What you will learn
- The main groups of aroma compounds in wine and where they originate
- The difference between primary, secondary and tertiary aromas
- Which compounds give typical scents such as floral fruit, tropical fruit and green notes
- How aroma develops from grape to finished wine
Primary, secondary and tertiary aromas
The aroma compounds of wine can be divided according to when and how they arise. The technical literature speaks of four categories of origin, which align neatly with the classic division into primary, secondary and tertiary aroma.
The primary (varietal) aromas come from the grape itself. These are the compounds that give a variety its typicity: monoterpenes, C13-norisoprenoids, methoxypyrazines and thiols. To these also belong the prefermentative aromas, which form enzymatically already at pressing.
The secondary (fermentative) aromas are formed during fermentation by yeast and bacteria. Saccharomyces cerevisiae produces, among other things, ethyl and acetate esters, higher alcohols, fatty acids, lactones, sulphur compounds and benzenoids. It is here that a large part of the wine's fruitiness actually arises, rather than directly from the grape.
The tertiary (postfermentative) aromas arise during ageing and maturation, where the compounds are slowly transformed.
A key concept for understanding which compounds actually matter is OAV (odour active value): the ratio between a compound's concentration and its odour threshold. A compound may be present in ample amounts without contributing, if it lies below the threshold, while a compound with a very low threshold can mark the wine at the ng/L level. The odour threshold is determined via triangle tests with sensory panels at the 50 percent detection level.
The aroma reserve in the grape
An important point: a large part of the grape's aroma compounds is not found as free, fragrant molecules, but as glycosidically bound (odourless) compounds. The bound forms typically exceed the free ones in quantity and function as an aroma reserve. The odorous aglycones can be released enzymatically, by acid or heat, during maturation, processing and ageing. This reserve is a large part of the explanation for why aroma develops over time.
Terpenes and floral fruit
Monoterpenes are some of the most characteristic primary aroma compounds. Linalool, geraniol, nerol and citronellol are the central compounds behind the unmistakable floral-fruity scent of Muscat grapes. They are found in the grape in both free and glycosidically bound (odourless) form, and during fermentation the yeast's glucosidase enzymes release the bound forms, so that the aroma comes forward.
Grape varieties can be divided according to monoterpene content: the intensely aromatic Muscat types with up to 6 mg/L total monoterpenes, the aromatic non-Muscat varieties with 1-4 mg/L, and the neutral varieties, whose expression is independent of monoterpenes.
Closely related are the C13-norisoprenoids, which are formed by oxidative cleavage of carotenoids between C9 and C10, giving 13-carbon compounds that mostly appear as glycosides. They hold some of the most elegant scents in the wine's repertoire: β-ionone (violet), β-damascenone (exotic fruit), β-damascone (rose) and β-ionol (fruit and flowers). Chardonnay is particularly rich in norisoprenoids, which originate from four xanthophyll pigments: lutein, antheraxanthin, violaxanthin and neoxanthin.
Thiols and esters
The thiols are sulphur-containing compounds with strongly tropical-fruity aromas. Compounds such as 3-mercaptohexan-1-ol and 4-methyl-4-mercaptopentan-2-one occur in the must as non-volatile cysteine or glutathione conjugates and are released in free, fragrant form during fermentation. They are central to the bouquet of Sauvignon blanc, with aromas reminiscent of grapefruit or boxwood. An important point for understanding: how much tropical-fruity character comes forward depends to a high degree on the chosen yeast strain, because it is the yeast that cleaves the thiols from their precursors.
The esters are in many ways the fruit signature of fermentation. Ethyl esters give fruity aromas and are indispensable for the quality of the wine. They are formed by esterification of activated fatty acids such as acyl-CoA, where the fatty acid concentration is the limiting factor. The acetate esters, by contrast, are formed by condensation of alcohol with acetyl-CoA, catalysed by alcohol acetyltransferases, whose expression determines the acetate ester level. The structure governs the scent: ethyl esters give fruit, medium-chain fatty acids give milky notes, and higher alcohols give a fusel character. Among the higher alcohols you find, among others, 2- and 3-methylbutanol, propanol, 2-methylpropanol, 2-phenylethanol and 3-methylthio-1-propanol.
Pyrazines and green notes
The green, vegetal tones have their own chemistry. Methoxypyrazines have extremely low odour thresholds in the range of 1-10 ng/L and give a characteristic green or herbaceous aroma. In small amounts they contribute to typicity, but at higher concentrations they can tip over into outright off-flavour.
3-isobutyl-2-methoxypyrazine (also called 2-methoxy-3-(2-methylpropyl)pyrazine) is found in, among others, Sauvignon blanc and Cabernet Sauvignon and has a detection threshold around 2 ng/L. The level is higher in grapes grown under cool conditions than under warm ones, and it falls markedly during ripening. Therefore vineyard practice and degree of ripeness can to a high degree govern how much green character ends up in the glass.
The volatile C6 notes
Another source of green and grassy is the prefermentative C6 compounds. When the grapes are crushed in air, lipoxygenases oxidise unsaturated fatty acids with cis,cis-1,4-pentadiene systems into hydroperoxides, which hydroperoxide lyases then cleave into C6 aldehydes. These aldehydes have green, grassy notes, but they are reduced to hexanol during fermentation and therefore contribute only to a limited extent in the finished wine. A good example of how an aroma can arise early and disappear again along the way.
The development of aroma in the bottle
Aroma is not static. The glycosidic reserve in the grape is released gradually, so that bound, odourless compounds become fragrant aglycones during maturation and ageing. This explains why a wine's primary fruit can change character over time, as new compounds come forward.
To this is added the malolactic fermentation, where Oenococcus oeni converts malic acid. It reduces the harshness of the acidity, improves microbial stability and contributes to aroma complexity. Barrel ageing too adds aroma compounds: the oak gives off phenolic compounds, including benzoic acid and cinnamic acid derivatives as well as aldehydes from the breakdown of lignin and tannin. Gamma-lactones are found in all wines and contribute significantly to aroma and bouquet, while delta-lactones are often associated with impact aromas.
Finally, it is worth remembering that what you actually experience is the brain's overall processing. Several odorants interact during perception and can act on one another in an integrating, competing or cancelling way. A compound's OAV tells you something about its potential, but the sum is rarely a simple addition.
In short
- Wine holds over 800 aroma compounds in concentrations from mg/L down to ng/L, and OAV determines which ones actually mark the scent.
- Primary aromas come from the grape, secondary from fermentation, tertiary from ageing and maturation.
- Monoterpenes give floral fruit (Muscat), norisoprenoids give violet and exotic fruit, thiols give tropical fruit, and methoxypyrazines give green notes.
- A large part of the aroma lies as glycosidically bound, odourless reserves that are released over time.
- Yeast choice, malolactic fermentation and barrel ageing all shape the finished aroma profile.
Frequently asked questions
Why do two wines from the same grape variety smell so different?
Because the aroma depends on far more than the variety. The glycosidically bound compounds are released to different degrees, the yeast strain determines how much thiol and ester are formed, and growing conditions such as temperature govern, among other things, the level of methoxypyrazines. To this is added maturation and any barrel ageing.
What does it mean that an odour threshold is very low?
It means that the compound can be perceived even in extremely small concentrations. Methoxypyrazines lie at the ng/L level, so a small amount can mark an entire wine. A compound with a low threshold and thus a high OAV weighs heavily in the overall scent, even though it is measurably present in vanishingly small amounts.
Ready for the next step?
Now you have a picture of where the scents of wine come from, and how they develop. In the next part, Sugar, fermentation and alcohol, we go closer to the fermentation process itself, which is the engine behind many of the secondary aromas we have just looked at.
Do take the new knowledge out to the shelf and have a sniff at what is hiding in a glass. And remember that the best pairing is always the wine you like with the food you like. The rest is nerdery that makes the experience more fun.