Common Wine Faults

Cork taint is the most recognized wine fault worldwide and the reason many producers have shifted to screwcap or synthetic closures. The compound responsible is 2,4,6-trichloroanisole (TCA), formed when naturally occurring fungi in cork bark interact with chlorinated phenolic compounds. TCA can also originate from winery sanitation products, wooden barrels, or even cardboard packaging, leading to occasional taint in screwcapped wines as well. At low concentrations, TCA simply strips the wine of fruit character, making it taste muted and flat without an obvious musty note. At higher concentrations, the classic wet cardboard, damp basement, and musty newspaper aromas become unmistakable. A wine with cork taint is described as 'corked.' TCA is not harmful to health but renders the wine unpleasant. Most retailers and restaurants will accept returns of corked bottles.

• Primary compound: 2,4,6-trichloroanisole (TCA), with a detection threshold of 1.5-3 parts per trillion
• At low levels, TCA mutes fruit without producing obvious mustiness, making it harder to identify and often mistaken for a dull wine
• Natural cork remains the primary source, but TCA can also form on wooden pallets, barrel staves, and winery walls
• The shift to screwcaps in Australia and New Zealand from the early 2000s was driven largely by cork taint concerns

Oxidation occurs when wine has excessive exposure to oxygen, triggering chemical reactions that break down fresh fruit character and color. White wines turn deeper gold or amber, while reds shift toward brown at the rim. Aromas move from fresh fruit toward bruised apple, sherry-like nuttiness, caramel, and stale notes. Some wine styles embrace oxidation intentionally, including Sherry, Vin Jaune from the Jura, and traditional Madeira. For table wines, however, premature oxidation (premox) is a serious fault. White Burgundy from roughly 1996 through 2012 was particularly affected, with bottles from respected domaines browning and losing freshness within 3-5 years of release. The causes remain debated, but contributing factors likely include variable cork quality (especially from the late 1990s), lower SO2 levels at bottling, and possibly changes in viticulture. Premox remains unpredictable, sometimes affecting single bottles within a case while others age beautifully.

• Visual clues: whites turning deep gold or amber, reds showing brown tinge at the rim, both losing vibrancy
• Aromas of bruised apple, sherry-like nuttiness, toffee, or stale honey indicate oxidative damage in table wines
• Intentional oxidation is central to Sherry (flor and oxidative aging), Vin Jaune (sous voile), and Madeira (estufagem)
• White Burgundy premox led many producers to increase SO2 levels and improve cork sourcing from the 2010s onward

Reduction is the chemical opposite of oxidation and refers to faults caused by volatile sulfur compounds (VSCs) formed in low-oxygen winemaking environments. The simplest is hydrogen sulfide (H2S), which smells of rotten eggs or struck match and forms when yeast cannot access enough nitrogen during fermentation or when sulfur-based vineyard sprays remain on grapes. If H2S is not addressed promptly through aeration or copper fining, it can evolve into more persistent and harder-to-remove compounds: mercaptans (thiols) that smell of rubber, garlic, or burnt match, and disulfides that recall cooked cabbage or sewage. At very low levels, some reductive character can add complexity and a flinty, mineral quality, which is sometimes sought in Burgundy Chardonnay and Loire Chenin Blanc. Screwcap closures, which allow less oxygen ingress than cork, can sometimes amplify reductive tendencies during aging, leading to the phenomenon of 'screwcap reduction.'

• Hydrogen sulfide (H2S): rotten eggs aroma, the most common and most easily corrected reductive compound, often resolved by racking or brief copper fining
• Mercaptans/thiols: rubber, garlic, burnt match aromas; form when H2S reacts with other wine compounds; harder to remove than H2S
• Disulfides: cooked cabbage, sewage; form from mercaptan oxidation; very difficult to treat once established
• Prevention: adequate yeast-available nitrogen (YAN), clean fruit, early racking, and avoiding excessive use of SO2 at crush

Brettanomyces (commonly called 'Brett') is a spoilage yeast that produces volatile phenols responsible for aromas variously described as barnyard, horse blanket, Band-Aid, medicinal, smoky, or sweaty saddle. The two primary compounds are 4-ethylphenol (4-EP), associated with barnyard and medicinal notes, and 4-ethylguaiacol (4-EG), which lends smoky and spicy character. Brett thrives in barrels, especially older barrels with higher pH environments, and is notoriously difficult to eradicate from a winery once established. The topic is genuinely polarizing in the wine world. Many traditional producers in the Southern Rhone, parts of Bordeaux, and Barossa Valley have historically considered low levels of Brett a signature element of terroir and complexity. Others, particularly New World winemakers and MW/MS candidates, view any detectable Brett as a fault. WSET and CMS exams generally treat Brett as a fault when it dominates the wine's profile.

• Key compounds: 4-ethylphenol (barnyard, Band-Aid) and 4-ethylguaiacol (smoky, spicy, clove)
• Brett sensitivity varies widely among tasters; some detect it at very low thresholds while others require higher concentrations
• Prevention: strict barrel hygiene, SO2 management, sterile filtration, and maintaining lower pH
• Cultural divide: traditional Rhone and Bordeaux producers may accept low Brett; most New World producers consider any level a fault

Volatile acidity (VA) is the measure of steam-distillable acids in wine, primarily acetic acid (vinegar) and its ester ethyl acetate (nail polish remover). Low levels of VA are a natural byproduct of fermentation, and trace amounts can lift and brighten a wine's aromatic profile. Problems arise when acetic acid bacteria (particularly Acetobacter) proliferate in the presence of oxygen, converting ethanol into acetic acid. This is accelerated by warm cellar temperatures, ullaged (partially empty) barrels, and fruit flies carrying bacteria into the winery. At high concentrations, VA makes wine smell like vinegar with a sharp, pungent bite on the palate. Ethyl acetate is often more detectable than acetic acid at moderate levels, presenting as nail polish or solvent. Some natural wine producers deliberately work with elevated VA as a stylistic choice, though this remains controversial. In dessert wines such as Sauternes and Tokaji, slightly higher VA is tolerated because residual sugar balances the perception of acidity.

• The OIV sets the maximum volatile acidity at 1.2 g/L for red wine and 1.08 g/L for white wine, measured as acetic acid
• Ethyl acetate becomes noticeable at around 150 mg/L, adding a nail polish or solvent note before acetic acid becomes apparent
• Prevention: minimizing headspace in barrels and tanks, maintaining free SO2, controlling cellar temperature, and fruit fly management
• Sweet wines tolerate higher VA because residual sugar counterbalances the vinegary perception

Heat damage (cooked wine) occurs when wine is exposed to temperatures above roughly 26-30C (80-86F) for extended periods, such as during shipping, storage in hot warehouses, or sitting on a loading dock in summer. Signs include a raised or leaking cork (the wine expanded inside the bottle), a dulled and 'stewed' fruit character, and a loss of freshness. Heat-damaged wines often taste jammy, flat, and lifeless. Light strike (gout de lumiere) is a photochemical reaction where riboflavin (vitamin B2) in wine reacts with sulfur-containing amino acids under exposure to UV or fluorescent light. The resulting compounds, including dimethyl disulfide, produce aromas of wet wool, cooked cabbage, or struck flint. Light strike most commonly affects wines in clear (flint) glass bottles, particularly Champagne, Provencal rose, and white wines displayed under fluorescent lighting. Green and brown glass filter significantly more UV, which is why they remain the standard for most wine packaging. Some Champagne houses have begun using UV-protective film on their clear bottles to combat this issue.

• Heat damage indicators: pushed or leaking cork, dulled 'stewed' fruit, loss of acidity and freshness, sometimes an acetaldehyde-like character
• Light strike primarily affects wines in clear glass; green glass blocks roughly 50% of UV, brown glass blocks over 90%
• Champagne is particularly susceptible to light strike due to clear glass, riboflavin content, and sulfur-containing amino acids from lees aging
• Prevention: proper temperature-controlled shipping, dark storage, avoiding fluorescent retail displays for extended periods

Refermentation is the unintended restart of fermentation in bottle, caused by residual sugar and active yeast that survived bottling. It produces unwanted bubbles, haziness, and sometimes off-aromas. This is distinct from intentional secondary fermentation in sparkling wine production. Refermentation is most common in wines bottled with slight residual sugar and insufficient SO2 or filtration. Haze and sediment, while not always true faults, can indicate problems. Protein haze in white wines forms when unstable proteins denature and precipitate, producing cloudiness. This is prevented by bentonite fining before bottling. Tartrate crystals (sometimes called 'wine diamonds') form when tartaric acid combines with potassium at cold temperatures; these are harmless and not considered a fault by professionals, though consumers sometimes mistake them for glass shards or sugar. Mousiness is a bacterial fault associated with certain lactic acid bacteria and Brettanomyces. It is unusual because the responsible compounds (tetrahydropyridines) are only perceived after the wine warms in the mouth, producing a persistent mousey, cereal, or cracker-like aftertaste that can linger for minutes.

• Refermentation: unexpected bubbles and spritz in still wine, caused by residual yeast and sugar reactivating after bottling
• Protein haze: cloudiness in white wines from heat-unstable proteins; prevented by bentonite fining prior to bottling
• Tartrate crystals: harmless precipitates of potassium bitartrate that form at cold temperatures; prevented by cold stabilization
• Mousiness: a bacterial fault perceived as a persistent cereal or cracker aftertaste; not detectable by smell, only on the palate after swallowing