Volatile Compounds in Wine: Esters, Aldehydes, and Ketones

What Are Volatile Compounds?

Volatile compounds are molecules with sufficiently high vapor pressure to evaporate from the wine surface into the air above the glass, where they can be detected by the olfactory system. Wine contains over 1,000 identified volatile compounds, though only about 200-300 are present at concentrations above their individual odor detection thresholds.

These compounds collectively create the extraordinarily complex aromatic profile that distinguishes wine from other beverages. They originate from grapes (varietal character), from fermentation (yeast and bacterial metabolism), from aging (chemical transformation), and from winemaking materials (oak, closures). Understanding the major classes of volatile compounds, their chemistry, and their sensory contributions helps winemakers make decisions that shape a wine's aromatic identity.

The Relationship Between Volatility and Aroma

A compound's contribution to wine aroma depends on two factors: its concentration in the wine and its odor activity value (OAV), which is the ratio of its concentration to its detection threshold. Compounds with high OAV values contribute most to the perceived aroma, even if their absolute concentration is low.

Some volatile compounds have extraordinarily low detection thresholds. 2-Isobutyl-3-methoxypyrazine (bell pepper aroma) can be detected at just 1-2 nanograms per liter. Rotundone (black pepper aroma in Syrah) has a threshold of approximately 16 nanograms per liter. At the other extreme, ethanol has a relatively high threshold of about 14,000 mg/L, though it modifies the volatility of other compounds and influences the overall aromatic release from wine.

Esters: The Fruity Heart of Wine Aroma

Acetate Esters

Acetate esters are formed during fermentation by the enzymatic reaction of acetyl-CoA with various alcohols. They are the primary source of fruity aromas in young wines and include some of the most aromatically impactful compounds in winemaking.

Isoamyl acetate is perhaps the most recognizable, producing banana, pear drop, and candy aromas at concentrations typically found in wine (0.5-5 mg/L). Hexyl acetate contributes apple and pear aromas. 2-Phenylethyl acetate adds rose, honey, and floral notes. Ethyl acetate is the most abundant ester, contributing a pleasant fruity character at low concentrations (below 80 mg/L) but becoming nail polish-like and unpleasant above 150 mg/L.

The enzyme primarily responsible for acetate ester synthesis is alcohol acetyltransferase, encoded by the ATF1 and ATF2 genes. Different yeast strains vary significantly in their ATF1/ATF2 expression, which is why yeast selection is one of the most powerful tools for controlling ester production.

Ethyl Esters

Ethyl esters are formed by the esterification of ethanol with fatty acids, either enzymatically during fermentation or through slow chemical esterification during aging. They generally contribute fruity, waxy, and floral aromas.

Key ethyl esters include ethyl hexanoate (green apple, anise), ethyl octanoate (pineapple, tropical fruit), ethyl decanoate (waxy, floral, grape), and ethyl butanoate (strawberry, pineapple). These ethyl esters of medium-chain fatty acids are produced in roughly similar concentrations across yeast strains, though fermentation conditions (particularly temperature) affect their levels.

During aging, ethyl esters of organic acids form slowly through acid-catalyzed esterification. Ethyl lactate (creamy, mild, buttery) increases in wines that have undergone MLF. Diethyl succinate (fruity, melon) accumulates during extended bottle aging.

Ester Dynamics: Formation and Hydrolysis

Ester concentrations in wine are not static. Chemical hydrolysis slowly breaks esters back into their component alcohol and acid, while chemical esterification slowly forms new esters. This dynamic equilibrium means that the ester profile of a wine changes continuously during aging.

In general, acetate esters decrease during aging (hydrolysis exceeds formation), while ethyl esters of acids increase (formation exceeds hydrolysis). This explains why young wines are more overtly fruity (high acetate esters) while aged wines develop subtler, more integrated aromatic complexity.

Aldehydes

Acetaldehyde

Acetaldehyde (ethanal, CH₃CHO) is the most important aldehyde in wine, present at concentrations of 10-75 mg/L in normal wines but potentially exceeding 300 mg/L in oxidized or sherry-style wines. It is an intermediate in alcoholic fermentation (formed by decarboxylation of pyruvate before reduction to ethanol) and is also produced by the oxidation of ethanol in finished wine.

At low concentrations (below 100 mg/L), acetaldehyde contributes to a wine's complexity without being individually recognizable. At moderate concentrations (100-200 mg/L), it produces a green apple, nutty, or bruised apple aroma. At high concentrations, it creates the distinctive oxidized, sherry-like character.

Acetaldehyde plays a critical role in color chemistry: it mediates the polymerization of tannins and the formation of stable tannin-anthocyanin pigments. It also strongly binds SO₂, converting free SO₂ to bound SO₂ and reducing the wine's antioxidant protection. Each mg/L of acetaldehyde binds approximately 1.45 mg/L of SO₂.

Other Aldehydes

Hexanal and trans-2-hexenal are C6 aldehydes that contribute green, grassy, leafy aromas. They are formed by the enzymatic oxidation of unsaturated fatty acids (the lipoxygenase pathway) during grape crushing. Their concentrations decrease during fermentation but can contribute to green character in wines with minimal skin contact.

Furfural and 5-methylfurfural are produced by the thermal degradation of wood sugars during barrel toasting and contribute caramel, butterscotch, and almond aromas to oak-aged wines. Vanillin (technically an aldehyde) is extracted from oak and provides the familiar vanilla aroma.

Ketones

Diacetyl

Diacetyl (2,3-butanedione) is the most important ketone in wine, contributing a distinctive buttery, butterscotch aroma. Its detection threshold is approximately 0.2 mg/L in white wine and 2.8 mg/L in red wine (the higher threshold in reds reflects masking by other compounds).

Diacetyl is produced primarily during malolactic fermentation as a byproduct of citric acid metabolism by lactic acid bacteria. Its concentration peaks during active MLF and then decreases as yeast (if present on lees) reduce it to the less aromatic acetoin and 2,3-butanediol through enzymatic reduction.

Managing diacetyl levels involves timing: racking wine off lees immediately after MLF preserves higher diacetyl (for a butterier character), while extended lees contact after MLF reduces diacetyl (for a cleaner, less buttery character).

Beta-Damascenone

Beta-damascenone is a norisoprenoid ketone with an extremely low detection threshold (approximately 0.05 micrograms per liter) and a complex aroma described as apple, rose, exotic fruit, and honey. It is formed by the acid-catalyzed hydrolysis of carotenoid precursors in grapes and accumulates during bottle aging.

Beta-damascenone is present in virtually all wines and is considered a key contributor to overall wine fruitiness, even though it may not be individually recognizable. Its OAV is typically among the highest of any compound in wine.

Rotundone

Rotundone is a sesquiterpene ketone responsible for the black pepper aroma found in Syrah/Shiraz, Gruner Veltliner, and several other varieties. Its detection threshold is approximately 16 nanograms per liter, making it one of the most potent aroma compounds in wine. Notably, about 20% of the population is anosmic to rotundone (unable to smell it), which explains why some tasters perceive peppery character in Syrah while others do not.

Volatile Sulfur Compounds

Positive and Negative Sulfur Volatiles

Volatile sulfur compounds (VSCs) occupy a unique position in wine chemistry: at very low concentrations they can add complexity, but at slightly higher levels they become serious off-aromas.

Hydrogen sulfide (H₂S, rotten eggs) is the most common VSC problem. Methanethiol and ethanethiol produce onion, rubber, and cooked cabbage aromas. Dimethyl sulfide (DMS) contributes truffle, blackcurrant, and asparagus aromas at low levels but becomes unpleasant at higher concentrations.

On the positive side, certain varietal thiols are highly prized. 3-Mercaptohexanol (passionfruit, grapefruit) and 4-Mercapto-4-methylpentan-2-one (boxwood, blackcurrant) are signature aromas in quality Sauvignon Blanc.

Practical Applications for Winemakers

Managing volatile compound profiles requires attention at every stage. Choose a yeast strain appropriate for the desired aromatic profile. Ferment at the right temperature (cool for ester preservation, warm for complexity). Ensure adequate yeast nutrition to prevent VSC formation. Make deliberate decisions about MLF and lees contact based on the desired diacetyl level.

During aging, understand that the volatile profile will evolve: acetate esters will diminish, ethyl esters of acids will accumulate, and new compounds will form from precursor hydrolysis. This evolution is natural and often beneficial, but it means that the wine you bottle will taste different from the wine you age.

Frequently Asked Questions

Why do young wines smell fruitier than old wines?

Young wines contain higher concentrations of acetate esters (banana, pear, apple aromas), which are produced during fermentation and slowly hydrolyze during aging. As these esters break down, the overt fruitiness diminishes. Aged wines develop different volatile compounds (ethyl esters of acids, norisoprenoids, and other aging products) that contribute subtler, more integrated complexity rather than primary fruit aromas.

What causes the "nail polish" smell in wine?

The nail polish smell is caused by excessive ethyl acetate, the most abundant ester in wine. At low concentrations (below 80 mg/L), it contributes pleasant fruitiness. Above 150 mg/L, it becomes a defect. Elevated ethyl acetate is often caused by acetic acid bacteria activity or fermentation at excessively high temperatures. It frequently accompanies elevated volatile acidity.

How does oak aging affect volatile compounds?

Oak aging introduces several categories of volatiles including vanillin (vanilla), eugenol (clove), guaiacol (smoke), furfural (caramel), and oak lactones (coconut). The specific profile depends on oak species, toast level, barrel age, and contact time. Oak aging also promotes oxidative reactions that generate acetaldehyde and modify the existing volatile profile through ester formation and hydrolysis.

Why does my wine smell like butter?

The buttery aroma comes from diacetyl, a ketone produced during malolactic fermentation by lactic acid bacteria. Diacetyl is a normal byproduct of MLF and is desirable in some wine styles (oaked Chardonnay) but unwanted in others. To reduce butteriness, keep the wine on yeast lees after MLF, as yeast enzymes reduce diacetyl to less aromatic compounds. To preserve butteriness, rack off lees promptly after MLF completion.

Can volatile compounds be measured at home?

Most volatile compounds require gas chromatography for precise measurement, which is beyond home winemaking capability. However, trained sensory evaluation is remarkably effective for detecting volatiles above their thresholds. Regular smelling and tasting during fermentation and aging, using a standardized approach and clean glassware, is the best tool available to home winemakers for monitoring volatile compound development.