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The Science Behind the Main Wine Aromas, Explained

Wine descriptions often come under fire for being fanciful. Can you really smell grass and grapefruit, rose petals and pepper in a wine?

The answer is yes, you can. Here’s the science behind it.

It’s all down to organic chemistry. Wine is made from grapes, and grapes draw on the same set of elements as all other fruits and plants. In unfermented grapes, most aroma molecules are bound up with sugar, so you cannot smell them. However, once fermentation turns sugar into alcohol, those volatile flavor compounds are set free and can be detected by our sense of smell.

Some aromas in wine come from the grape itself and are the same compounds that occur elsewhere in nature. A type of chemical compounds found in Riesling, called terpenes, are also in citrus peel.

Other aromas, like esters, are created by fermentation, while even more develop over time as wine’s constituents react with each other. Then there’s the influence of oak and winemaking practices, which can impart compounds that convey vanilla, caramel or coconut.

All these factors contribute to the complex array of aromas we get every time we smell wine. Our brains can decipher many of them, depending on our sensitivity and the concentration of the compounds.

Here’s a list of noteworthy compounds found in wine, and what common aromas and flavors you may not realize come from them.

Illustration of lemon, eucalyptus leaves and rose
Citrus, rose petals and eucalyptus / Illustration by Ryan McAmis

Terpenes

If you smell rose petals or citrus in your wine, it’s due to terpenes, which reside primarily in grape skins and are abundant elsewhere in nature: in blossoms, fruits and leaves of myriad plants.

Terpenes are a class of compound that make wines like Muscat and Gewürztraminer fragrant and give characteristic citrus flavours to Riesling. Here are some you will recognize.

Linalool: This terpene, in connection with others, causes the perception of lavender, orange blossom, lily and even bay leaves.

Geraniol: The compound behind the tell-tale smell of rose petals.

Nerol and citronellol: Both these compounds create the floral, citrusy scents that occur in numerous flowers and fruits, and wine.

Limonene and citral: These zesty flavors in wine are derived from the same compound found in citrus peel.

Hotrienol: The smell of linden blossom is also a constituent of that heady elderflower smell in Sauvignon Blanc.

1,8-cineole and alpha-pinene: If you regularly smell eucalyptus in Australian red wines, or the scent of garrigue scrubland in the red wines of Southern France, you’re not fantasizing. The compound 1,8-cineole gives eucalyptus trees their distinctive smell, while alpha-pinene is found in aromatic shrubs like juniper and rosemary. Both are airborne, highly volatile aroma compounds that can cling to the bloom on the skin of grapes.

Rotundone: Since red wines are fermented on their skins, highly aromatic and stable compounds can make it into the wine. As a sesquiterpene, rotundone is better known for the pungent aroma it gives peppercorns, particularly white pepper. If your Shiraz, Syrah or Grüner Veltliner gives you a whiff of a recently used pepper mill, you are smelling this compound.

Illustration of vanilla bean and flower
Vanilla / Illustration by Ryan McAmis

Aldehydes

Hexanal and hexenal: These two aldehydes are responsible for scents of freshly cut grass and tomato leaf in Sauvignon Blanc.

Vanillin: Another famous aldehyde, this is the chief flavor of vanilla beans. In wine, it derives from fermenting or aging in oak barrels. American oak (Quercus alba) tends to have more vanillin than French oak (Quercus robur), so you’re not mistaken if that oaked California Zinfandel seems to particularly smell of vanilla.

Benzaldehyde: If you come across the scent of bitter almond or marzipan in aged Pinot Gris or some Italian white wines, you are smelling this compound.

Furfural: Reminiscent of dried wood, caramel and wheat bran, this chemical often occurs in oak-aged wines.

Illustration of slices of green bell pepper
Green bell pepper / Illustration by Ryan McAmis

Pyrazines

Methoxypyrazines: Ever smelled green bell pepper in Cabernet Sauvignon or leafiness in Carmenère? What you’re sniffing are methoxypyrazines. These incredibly stable compounds form part of the varietal character of some grapes, especially those in the Sauvignon family.

Methoxypyrazines can also be a sign of underripeness in other grape varieties, where they present as an almost acrid herbaceousness. After all, green bell pepper is just the unripe version of red and yellow bell peppers.

Illustration of sliced apple and banana
Apple and banana notes / Illustration by Ryan McAmis

Esters

Esters are volatile flavor compounds created by reactions between alcohols and acids. Esters are responsible for primary fruit flavors we get in very young wine.

Isoamyl acetate and ethyl acetate: Very young, white wines often smell of pear-drop candy, or a mix of banana and pear flavor. This is the result of these two very common esters.

Octyl acetate: Expect aromas of oranges and citrus fruits.

Butyl acetate: This ester smells of Red Delicious apples.

Illustration of buttered popcorn and violets
Buttered popcorn and violets / Illustration by Ryan McAmis

Ketones and diketones

Beta-ionone: This compound creates the haunting scent of violets in Pinot Noir or Syrah.

Beta-damascenone: Floral aromas with an undertow of red fruit come from this ketone.

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Diacetyl: The most obvious diketone, diacetyl smells like melted butter and can also come across as creaminess in wine. Diacetyl is a byproduct of malolactic fermentation where bacteria transforms sharp malic acid in wine into much softer lactic acid. That is why many Chardonnays smell buttery and creamy.

When Chardonnay that went through malolactic fermentation is aged in new American oak, which imparts vanilla and nutty aromas, it can easily be reminiscent of buttered popcorn.

Illustration of gooseberries and black currants
Gooseberries and black currants / Illustration by Ryan McAmis

Mercaptans

These volatile sulfur compounds occur in grapes and are released by fermentation.

3MH (3-mercaptohexan-1-ol): The 3MH mercaptan gives intense passion fruit notes to Sauvignon Blanc.

3MHA (3-mercaptohexyl acetate): Guava and gooseberry aromas as the result of fermentation are signatures of this compound.

4MMP (4-mercapto-4-methylpentan-2-one): The leafy, fruity notes of blackcurrant, so often encountered in Cabernet Sauvignon, are the result of 4MMP.

Illustration of coconuts
Coconut notes tend to come from aging / Illustration by Ryan McAmis

Lactones

Sotolon: This lactone is formed in wines that have aged. Sotolon is associated with botrytised wines like Sauternes and long-aged wines like Madeira. Depending on its concentration, it can smell of curry spice, nuts, toast or maple syrup.

Octalactone: This compound comes from aging on oak, and smells of coconut.

Illustration of cloves and kerosene/petrol
Cloves and kerosene (also referred to as petrol, in wine) / Illustration by Ryan McAmis

Other common wine aroma compounds

The famous petrol or kerosene smell in mature Riesling is known as TDN (1,1,6-trimethyl-1,2-dihydronaphthalene—now you know why it’s abbreviated) and relates to sun-exposure in the grape skins. Phenols are derived from oak aging: guaiacol gives smoky, toasty and roasted aromas to wines while eugenol is responsible for spicy notes of clove.



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