Wine Faults

The WSET Level 3 Award in Wines specification includes 'Identify common faults found in wines' among the assessment criteria for its Learning Outcome 5 and names cork taint, oxidation, reduction, volatile acidity, Brettanomyces, out of condition and high sulfur dioxide as the faults in scope. The AWRI draws a line between wine faults, such as oxidation-type, reductive, additive-related and Brettanomyces faults, and wine taints, such as cork-type, chlorophenol or plastic-type, earthy-type and smoke taint. The AWRI reports that concentrations of hydrogen sulfide below its detection threshold might play a role in wine complexity, and that diacetyl at low levels (1 to 4 mg/L) can add 'buttery' or 'butterscotch' characters, while at high levels (above 5 mg/L) the wine might be regarded as defective. For some wine styles, such as sherry, oxidation is deliberately encouraged, according to the AWRI.

• WSET notes that some people like Brettanomyces flavours, so depending on the drinker and the intensity of these flavours, brett may not always be seen as a fault
• WSET describes low-level reduction aromas as smoky and reminiscent of struck matches, typically not seen as a fault unless they overpower the wine
• In the WSET Level 3 Systematic Approach to Tasting Wine, clarity runs from clear to hazy and nose condition from clean to unclean, each marked 'faulty?'
• The AWRI has shown that TCA can be formed in oak, so a wine can have a TCA taint without any contact with cork

Cork taint comes mainly from TCA, which the AWRI describes as one of the most odour intense compounds known. TCA is generally formed when moulds growing on cork come into contact with trichlorophenol, and 2,4,6-tribromoanisole (TBA) behaves similarly, with a similar musty, mouldy aroma. The AWRI attributes oxidation flavour to multiple compounds, including a range of aldehydes; its sensory characteristics range from a dulling of the aroma, to 'cardboard', 'straw' and 'hay-like' aromas, to 'sherry-like' and 'madeirised'. Reductive faults come from volatile sulfur compounds: hydrogen sulfide smells of rotten eggs, while sulfhydryls, also known as thiols or mercaptans, have aromas described as 'cabbage', 'garlic', 'onion' and 'rubber'. Volatile acidity is a measure of the low molecular weight, or steam-distillable, fatty acids in wine, with acetic acid by far the major acid at more than 93%.

• 4-ethylguaiacol, another Brettanomyces spoilage compound, has a 'clove', 'spicy' or 'smoky' aroma, with a reported aroma threshold of 110 µg/L and 158 µg/L found in Australian wine styles; 4-ethylcatechol has been described as 'horsey'
• Acetic acid has a reported aroma threshold as low as 0.1 to 0.125 g/L, but the concentration regarded as detrimental is usually greater than 0.7 g/L; ethyl acetate smells of nail polish remover, with a reported sensory threshold of 12 mg/L (AWRI)
• The AWRI says it is thought that ethyl mercaptan is probably formed by the direct chemical reaction between H2S and ethanol, and that it is best to remove H2S before it reacts further to form thiols
• Smoke exposure of vineyards and grapes can result in wines with 'smoky', 'burnt', 'ashy' or 'medicinal' characters, usually described as smoke tainted (AWRI)

The WSET Level 3 Systematic Approach to Tasting Wine builds the check into every tasting: on appearance, clarity runs from clear to hazy, and on the nose, condition runs from clean to unclean, both with a 'faulty?' prompt. For a wine opened in a restaurant, WSET advises giving the glass a gentle swirl to release the aromas, sniffing with the nose over the rim, and taking a sip as another check if a fault is not immediately apparent. Cork taint can usually be detected by aromas and flavours that are musty or reminiscent of damp cardboard, and their pungency can vary from barely noticeable to overpowering. Some faults need the palate. The AWRI says mousy taint, reminiscent of caged mice or sometimes cracker biscuit, is generally perceived late on the palate or after the wine has been swallowed or expectorated, usually takes a few seconds to build, and is rarely detected by sniffing, so moving quickly to the next wine in a line-up can miss it.

• Cork taint: musty or damp-cardboard aromas and flavours that can dull a wine's fruity characteristics (WSET); the AWRI notes the TCA aroma threshold might be lower in dry white wines or higher in full-bodied red wines
• Oxidation: a brown tinge, fading fruit, and aromas and flavours of bruised apples, nuts and honey; WSET treats these as a fault in young wines meant to be fresh and fruity, such as Pinot Grigio and Sauvignon Blanc
• Reduction: struck-match aromas at low levels, and rotten eggs to dirty drains at higher levels; WSET says it is most common to detect this fault when the wine has just been opened
• Brettanomyces and volatile acidity: brett can dull fruit and add medicinal to farmyard-like aromas and is more common in red wines; volatile acidity at higher concentrations can make wine smell like vinegar or nail polish (WSET)

The WSET Level 3 specification places fault identification under Learning Outcome 5, on providing information and advice to customers and staff about wines, alongside making wine recommendations and the storage and service of wine, and its tasting grid places 'faulty' below 'poor' on the quality scale. For drinkers, WSET says most wine shops will happily exchange wines that are corked and recommends returning as full a bottle as possible; in most restaurants and bars, guests are encouraged to try the wine before it is poured for the table. Tasting or drinking a corked wine will not do any actual harm, according to WSET, but it generally makes the wine taste unpleasant, sometimes to the point where it is undrinkable.

• Low-level taint still costs quality: the AWRI found TCA at a concentration as low as 1 ng/L suppressed the ratings for overall aroma intensity and positive fruit-derived characters of a Semillon wine
• Sensitivity varies: WSET notes some people are more sensitive to cork taint than others, and the AWRI reports considerable variation between individuals in sensitivity to mousy taint
• Storage matters: WSET says wine left for prolonged periods in hot environments can lose its fresh fruit characteristics and taste dull and stale, and heat can force the cork out, after which the wine will become oxidised
• Regulation: the AWRI notes that volatile acidity is still prominent in the wine regulations of most countries, even though its components represent no threat to health

The AWRI writes that until Pasteur's work in the mid-19th century, a high proportion of wines spoiled through acetic acid bacteria converting alcohol to acetic acid before they could be consumed. It also describes oxidation as a common fault in Australian white wines 40 years ago, now much less common with refrigeration, inert gas blanketing during production and packaging, and effective sulfur dioxide management. The AWRI observed an increased incidence of mousy wines during the 1990s, when winemakers moved to a lower sulfur dioxide regime for red wines in particular, and another increase in the 2010s, when some winemakers experimented with extended lees ageing, high pH with minimal SO2, oxidative ageing and minimal clarification or filtration in white wines. Premature oxidation, or premox, was first noticed in the early years of the new millennium and seemed particularly, though not exclusively, to have affected white Burgundies produced in 1995/96 and on, according to Decanter.

• Decanter describes affected wines as showing colours dulling or fading to brown, with fresh fruity aromas and tastes replaced by bruised apple, honeyed, waxy or stewed fruit characters
• Decanter (2021) says it is still not really known for sure what combination of factors caused premox; possible contributors it names include lower sulphur dioxide additions, a possible decline in cork consistency allowing excessive oxygen ingress, climate change, moves towards organics and changes in chemical treatment regimes in vineyards, shipment and storage, and batch and bottle variation
• In a 2007 blog post on winespectator.com, Rajat Parr said the premox problem apparently began with the 1995 and 1996 vintages but was not really recognized as systemic until around 2002; he pointed to soft pneumatic pressing, excessive bâtonnage and lower sulfuring, and above all to a change in cork coatings from paraffin to silicone
• Decanter reports that the premox issue seems to have abated over time and that the debate in the trade subsided

The AWRI outlines cellar controls for the main faults. Winemakers minimise excess H2S in white wines by settling, centrifuging or filtering the must before fermentation, which removes solids that might contain elemental sulfur; some remove excess H2S from red wines by aerating at the first racking, and many fine with copper sulfate, which reacts with H2S to form highly insoluble copper sulfide. Careful laboratory trials should precede any copper sulfate addition, as an instability can result if the copper concentration exceeds approximately 0.5 mg/L. Disulfides formed when mercaptans are aerated do not react with copper, so their removal first requires reducing conditions created with ascorbic acid and SO2. Against mousiness, the AWRI has recommended working at between 50 and 80 mg/L total sulfur dioxide for red wine production, more for wines of high pH, where cellar sanitation is in doubt. Geranium taint, formed when lactic acid bacteria metabolise sorbic acid, is practically impossible to remove, so the AWRI says sorbic acid should not be used to treat red wines.

• Causes of excess H2S listed by the AWRI include elemental sulfur on grape skins from sulfur sprays, inadequate free amino nitrogen, added SO2, a deficiency of B-complex vitamins, unusually high cysteine and a high concentration of metal ions
• Spoilage from acetic acid bacteria only occurs when wines are exposed to air (AWRI)
• Dimethyl sulfide does not bind to copper, but the AWRI says removal might be possible by sparging with nitrogen or by using reverse osmosis
• Reduction aromas start to dissipate with exposure to air, and techniques that increase aeration, such as decanting, can help to rectify the fault (WSET)

Primary sources: https://www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/recognition-of-wine-faults-and-taints/wine_faults/, https://www.wsetglobal.com/media/11731/wset_l3wines_specification_en_highres_may2022_issue2.pdf, https://www.wsetglobal.com/knowledge-centre/blog/2023/august/24/common-wine-faults-and-how-to-spot-them, https://www.oiv.int/standards/international-code-of-oenological-practices/annexes/maximum-acceptable-limits, https://www.ecfr.gov/api/renderer/v1/content/enhanced/current/title-27?part=4&section=4.21. Secondary sources: https://www.decanter.com/learn/what-is-premature-oxidation-245693/, https://www.winespectator.com/articles/a-warning-on-white-burgundies-15102. Last verified 2026-09-29