When chemicals in a wine hit the nose and mouth, the mind cries “flavor!” Maybe the mind gets lemon, peach, or nectarine, or maybe the flavors are weirder, like barnyard or green pepper, dill or diesel fuel.
Why does wine conjure all these weird descriptors? Is it something in the wine, something in the mind, or both?
The nose knows
Although the tongue does get involved in tasting, the nose is the main actor. The dominant contributor to the wine’s flavor is its smell, and this smell is a cocktail of volatile aroma compounds.
Researchers have identified at least 800 such volatile chemicals in wine. Some are more potent than others, perceptible at concentrations as low as four picograms per liter. That’s the equivalent of dropping merely 0.0004 grams of the stuff into an Olympic-sized swimming pool.
It’s the mind-stirring mix of these aromas and flavors that gives a wine character.
Where do these compounds come from?
The aroma of wine starts in the fruit. These chemicals contribute to so-called primary notes. Fermentation further modulates the wine’s chemistry; these are wine’s secondary notes. Aging, and in particular bottle aging, brings more layers of complexity, the so-called tertiary notes.
Primary aromas derive from the grape’s own biochemical makeup. The compounds vary by grape variety but tend to be fruity, flowery, herbaceous, and freshly spicy.
Occasionally, grapes pick up ambient chemicals that make it through to the bottle, things like wildfire smoke, or oil from nearby eucalyptus, or other resinous scrub. Sometimes these are pleasant, but they can also ruin a wine.
Organic molecules like thiols and terpenes in grapes yield a range of recognizable smells:
- Rose — from nerol and geraniol, present, for example, in Gewürztraminer.
- Lavender — linalool, common in Muscat.
- Green leafiness — methoxypyrazine, found especially in Cabernet and Carménère.
- Passion fruit — a thiol, a sulfur compound present, e.g., in Sauvignon Blanc; thiols are called mercaptans when they skew negative, say, toward cat pee.
The next layers
Secondary aromas are generated by the action of the yeast and malolactic fermentations. They’re commonly esters and alcohols:
- Banana — isoamyl acetate, formed in conjunction with acetic acid.
- Butter — diacetyl, a ketone, produced by malolactic fermentation.
- Solvent — ethyl acetate, which is the same stuff used in nail polish.
Tertiary notes develop during aging, the result of oxidation-reduction reactions and contact with oak barrels of various types and toast levels. Time in the bottle further complicates the chemistry:
- Coconut — a lactone, from wood barrels.
- Dill — another lactone, most common in American oak.
- Soy sauce — oxidized proteins.
- Petrol, diesel — TDN (1,1,6,-trimethyl-1,2-dihydronapthalene) created during the breakdown of carotenoids in the grape. This is most common in aged Riesling, especially from warmer vintages and regions, and generally only occurs after a long period in bottle.
Life’s chemical stew
There’s no actual petroleum product in the Riesling, of course, only compounds that smell like that.
Sometimes the wine does, however, contain chemicals that are identical to those found elsewhere. Eugenol, a flavor that suggests clove, is present in toasty barrel oak and also many other plants, including basil, celery, bay laurel, and — wait for it — clove. Isoamyl acetate, which tastes like banana, is present in bananas.
Butter is another good example. The buttery flavor of diacetyl, which is generated as a byproduct of malolactic fermentation, is present in butter itself, and synthetic diacetyl is used to flavor the oil used on theater popcorn.
Still, much of the chemistry behind the taste of wine isn’t straightforward. What is sensed as flavor is often a response to an interplay of chemicals.
Sometimes it isn’t even settled science. Researchers are still scratching their heads over mousiness, for example, a wine fault that smells like a used rodent cage. This aroma, sensed retro-nasally, seems to be from a class of chemicals called tetrahydro pyrimidines, generated by both Brettanomyces and lactic acid bacteria but only under certain conditions. Thankfully, it’s rare.
Minerality is another tasting term that earns scant support from chemists. The minerals in vineyard soil — chalk, slate, granite — don’t wind up in the glass, and anyway aren’t volatile. Yet tasters use the term freely to describe aromas like wet slate, or the smell of a sidewalk after rain, also called petrichor. Sometimes, a wine with mercaptans, those stinky, reductive sulfur compounds, gets slapped with the descriptor, too.
Vive la différence
Wine descriptors are often metaphorical, terms that evoke similar smells deriving from nature, tradition, and place. It’s never safe to assume these are shared; one person’s yuzu is another person’s lemon, to say nothing of the culturally freighted Christmas spice.
Maybe one day organic chemists will obviate the need for such figures of speech. Until then, we’re stuck with the colorful language of wine.