What Are Wine Faults?

Scroll the table sideways to see every column

A “corked” wine smells musty and moldy. The main cause is 2,4,6-trichloroanisole, or TCA. Molds make it when they meet trichlorophenol, a biocide, in cork. Tasters can pick it up at around 1.4 to 3 nanograms per liter, a few parts per trillion.

TCA also dulls your sense of smell. At levels too low to smell as must, it still makes a wine seem flat and short on fruit.

A screw cap does not rule it out. TCA can form in oak, and a related compound, TBA, comes from treated wood, walls, floors and plastics inside a winery. A wine can be tainted without ever touching a cork.

Volatile acidity, or VA, is mostly acetic acid, the acid in vinegar. Every wine has some: a sound wine carries 0.1 to 0.4 grams per liter after fermentation. You can smell it from about 0.1 g/L, and above 0.7 g/L it usually hurts the wine. Acetic acid bacteria make it when a wine gets oxygen. Wild yeasts and Brettanomyces make it too.

Ethyl acetate smells of nail polish remover. Yeasts and the same acetic acid bacteria make it, so it often shows up with VA and makes the VA easier to notice. Normal wine carries 30 to 60 mg/L, where it adds fruity notes. At 150 to 200 mg/L it is a fault.

VA is one of the few faults with a legal ceiling:

• European Union: 18 milliequivalents per liter for white and rosé, 20 for red, about 1.08 and 1.20 g/L of acetic acid. Member states may allow more for some aged or high-alcohol wines (Regulation 2019/934).
• United States: 1.4 g/L for red and 1.2 g/L for other grape wine. Wine over the limit is classed as substandard (27 CFR 4.21).

A mousy wine leaves a taste of a mouse cage, crackers or popcorn at the back of your mouth after you swallow. It takes a few seconds to build and can last more than 10 minutes.

You will rarely smell it in the glass. The compounds behind it, 2-acetyltetrahydropyridine and two relatives, do not evaporate at the acidity of wine. Your saliva is less acidic, so they turn volatile in your mouth and reach your nose from the inside.

Lactic acid bacteria cause most mousiness, and Brettanomyces can too. Researchers link it to wine with little sulfur dioxide, high pH and oxygen exposure. A mousy bottle can taste fine when you open it and turn mousy as it sits open. There is no reliable way to remove it.

Sensitivity varies widely. In one study, the thresholds of 23 tasters spanned more than a 1,000-fold range. The Australian Wine Research Institute has said about 30% of people cannot taste mouse at all. Wine writer Jamie Goode found no published study behind that figure.

Brettanomyces is a yeast. It makes 4-ethylphenol, which smells of Band-Aids and horses, and 4-ethylguaiacol, which smells of smoke and clove. Brett is the only microbe that produces these compounds in red wine. It also mutes the fruit and can leave a drying, metallic aftertaste.

The grape changes how Brett shows. Pinot Noir leans to leather and barnyard. Syrah leans to Band-Aid.

Whether a little Brett is a fault depends on who you ask. A 2017 study found it may add complexity at low levels. An Australian study of 104 red wine drinkers found every sample with Brett compounds was liked less, including the lowest level tested.

Too much oxygen produces acetaldehyde and other aldehydes. At low levels the aroma goes dull. At higher levels the wine smells of bruised apple, straw, nuts and sherry. Whites oxidize faster than reds, because the phenolic compounds in red wine, tannins among them, act as antioxidants.

Some wines are oxidative by design. Fino sherry and the vin jaune of the Jura age under a film of yeast called flor, which gives them acetaldehyde and a nutty character.

Reduction is the name winemakers give to a group of sulfur compounds:

• Hydrogen sulfide smells of rotten eggs. Aeration in the cellar removes it.
• Mercaptans smell of cabbage, onion or rubber. Winemakers remove them with a small copper addition.
• Disulfides smell of cooked cabbage, garlic or burnt rubber. Copper cannot remove them, and aerating a wine that has mercaptans can create them.

At low levels the same family of compounds gives the flint, wet stone and struck-match smell that many producers want in Chardonnay and Loire Sauvignon Blanc.

Research cited by the Australian Wine Research Institute found faulty flavors after 12 months at 30°C (86°F). Above 40°C (104°F), wine changes in a matter of days. Storage above 25°C (77°F) for long periods can affect quality.

A pushed cork or a sticky neck shows the wine expanded in heat. It does not prove the wine is damaged. The only sure test is to taste it next to a bottle of the same wine that stayed cool.

Store wine at 15 to 20°C (59 to 68°F), with the cork touching the wine and no big swings between day and night.

• Lightstrike: light breaks down riboflavin and an amino acid in wine into sulfur compounds that smell of cooked cabbage and wet wool. In one test, wine in clear glass showed it after about 3 hours under fluorescent light. Wine in green glass took 18 to 31 hours.
• Refermentation in the bottle: yeast left in a bottled wine with sugar starts fermenting again. The wine turns cloudy and fizzy and smells of yeast. Above 10 g/L of sugar, the bottle can burst.
• Geranium taint: lactic acid bacteria turn sorbic acid, a preservative added to some sweet wines, into a compound that smells of crushed geranium leaves. It cannot be removed.

• Tartrate crystals: glassy flakes on the cork or in the base of the bottle. They are potassium bitartrate from the grapes and have no effect on the wine’s quality. In red wine they are stained dark red.
• Sediment in red wine: phenolic compounds bind together as the wine ages until they are too large to stay dissolved, and they settle.

• Keep the wine, the bottle and the cork, and tell the shop or restaurant where you bought it.
• If you suspect heat damage, taste it next to a second bottle of the same wine.
• If a wine smells of rotten egg, try decanting it. Aeration is how winemakers remove hydrogen sulfide.
• We found no source showing air removes cork taint or volatile acidity. A mousy wine can get worse once opened.

What does corked wine smell like?

Musty and moldy. At low levels TCA, the compound behind it, just makes the wine smell flat and short on fruit.

Can a wine with a screw cap be corked?

Yes. TCA can form in oak, and TBA, a related compound, comes from treated wood, walls and plastics in a winery. Neither needs a cork.

Why can I taste mouse but not smell it?

The mousy compounds do not evaporate at the acidity of wine. Saliva is less acidic, so they become volatile in your mouth after you swallow.

Is wine with volatile acidity safe to drink?

The Australian Wine Research Institute says the compounds behind volatile acidity pose no threat to health.

Is there a legal limit for volatile acidity?

Yes. The EU allows 18 milliequivalents per liter for white and rosé and 20 for red. The US allows 1.2 g/L for white and 1.4 g/L for red.

Are tartrate crystals a fault?

No. They are potassium bitartrate from the grapes, and they have no effect on the wine’s quality.

• Australian Wine Research Institute: wine flavours, faults and taints
• Australian Wine Research Institute: Brettanomyces FAQ
• Australian Wine Research Institute: avoid mousy off-flavours (2015)
• Australian Wine Research Institute: the effects of heat and light on wine during storage
• Takeuchi et al. (2013): TCA suppresses olfactory transduction
• Chatonnet et al. (2004): TBA contamination in wineries
• Tempère et al. (2019): sensory methods for detecting mousy off-flavor
• Moulis et al. (2023): microorganisms behind mousy off-flavor
• Jamie Goode (2020): the latest on mouse
• Schumaker et al. (2017): consumer perception of Brettanomyces
• IVES: chemical drivers of positive reduction in Chardonnay
• Fracassetti et al. (2021): light-struck taste in wine
• EU: Delegated Regulation (EU) 2019/934, Annex I, Part C
• US: 27 CFR 4.21, standards of identity for wine

Icons drawn from photos on Wikimedia Commons by Hudson C. S. de Souza, Andy Mabbett, Köttbulleledaren, Sandy Horvath-Dori, Ivar Leidus, Осадчая Екатерина and TCdeOLiveira (CC BY-SA 4.0 and CC BY 2.0). The mouse is from a public domain photo.