Common Wine Faults and How to Detect Them

Why Wine Goes Wrong

A fault is an aroma, flavor, or structural characteristic that indicates something has gone wrong in the vineyard, winery, or bottle. Some faults are immediately obvious — a deeply Corked wine smells unmistakably of musty cardboard. Others are subtle, manifesting as a nagging flatness or a slight off-note that diminishes enjoyment without announcing itself clearly.

Learning to identify faults serves two purposes. First, it gives you the confidence to return or exchange a faulty bottle at a restaurant or wine shop — something many wine drinkers hesitate to do. Second, it sharpens your palate by teaching you what wine should not smell or taste like, which makes the experience of a clean, well-made wine all the more vivid.

This guide covers the most common wine faults in order of frequency.

1. Cork Taint (TCA)

What it is: The most common and most famous wine fault. Caused by 2,4,6-trichloroanisole (TCA), a compound produced when certain chlorine compounds interact with specific molds that can colonize cork bark.

What you detect: A damp, musty odor reminiscent of wet cardboard, wet dog, damp cellar, or musty newspaper. In severe cases, the smell is immediately obvious and deeply unpleasant. In mild cases (often called "cork-affected" rather than fully "Corked"), the wine simply smells muted, flat, or stripped of its fruit.

Palate impact: TCA suppresses fruit aroma perception at extremely low concentrations (as low as 2–5 parts per trillion). A mildly affected wine may not smell obviously musty, but its fruit will seem inexplicably absent, leaving a flat, dull wine.

Frequency: Estimates suggest 3–5% of cork-sealed wines are cork-tainted. Natural cork is the primary vector, though TCA can contaminate winery equipment and affect many bottles from a single winery simultaneously.

What to do: Return it. In a restaurant, inform the sommelier or server — they should replace it without question. At retail, most wine shops will exchange a corked bottle. At home, you can use a LDPE (low-density polyethylene) sheet (plastic food-wrap) to absorb TCA from a mildly affected wine.

2. Oxidation

What it is: Excessive exposure to oxygen before the wine is ready for it — through a faulty seal, poor storage, or winemaker error.

What you detect:

• Appearance: White wines turn from their natural gold toward deep amber or brown. Reds show Browning — orange or tawny edges, often with a flat, dull center color.
• Aroma: Flat, Sherry-like, nutty (specifically walnuts and hazelnuts). Dried fruit character — apricot, prune — rather than fresh fruit. In whites, an aldehyde note (acetaldehyde) smells like apple cider or green apple that has been sitting in open air. In reds, the primary fruit disappears, leaving a thin, tired shell.
• Palate: Flat, broad, lacking freshness. The Acidity and fruit seem to have been erased.

Important distinction: Not all oxidative character is a fault. Wines deliberately made in an oxidative style — Tawny Port, Fino Sherry, Madeira, Amontillado — are intentionally exposed to oxygen and develop beautiful nutty, complex character as a result. The fault is unintentional oxidation in a wine not designed for it.

What to do: A mildly oxidized wine may still be drinkable — pair it with food (fat helps) or use it for cooking. A severely oxidized wine cannot be recovered.

3. Reduction

What it is: The opposite of oxidation — wine that has had too little contact with oxygen develops reductive compounds, primarily sulfur-based.

What you detect:

• Mild reduction: Struck flint, gun smoke, matchstick. These can be positive — many wine lovers appreciate flintiness in white wines from cool, mineral-rich regions.
• Moderate reduction: Rubber, sulfur (Hydrogen Sulfide, the smell of hard-boiled eggs or spent matches).
• Severe reduction (Mercaptan): Garlic, onion, skunk, burnt rubber. At this level the wine is seriously compromised.

Frequency: More common in wines sealed with screw caps (which allow even less oxygen ingress than cork) or in wines bottled young without adequate aeration.

What to do: Aeration helps. Decant the wine vigorously, swirl it aggressively, or drop a clean copper coin into the carafe — copper reacts with sulfur compounds and neutralizes them. If the fault is mild, it often resolves with 30–60 minutes in an open decanter. Severe Mercaptan rarely fully resolves.

4. Volatile Acidity (VA)

What it is: Acetic acid (the acid in vinegar) and its ester, ethyl acetate, produced by acetic acid bacteria when wine is exposed to air during fermentation or aging.

What you detect: Nail polish remover or model airplane glue (ethyl acetate) at low levels; sharp, vinegary, biting acidity at higher levels.

Important nuance: Low levels of VA (below about 0.8 g/L) are normal and can contribute to a wine's complexity. Many great wines — particularly long-aged reds and dessert wines — show perceptible but undistracting VA. The question is whether it enhances or detracts from the overall picture.

When it becomes a fault: When VA is the dominant impression on the nose or palate, or when it makes the wine sharp and unpleasant. Some highly acclaimed, expensive bottles from prestigious producers show "controversial" levels of Volatile Acidity — wine critics debate whether this represents character or flaw in individual cases.

What to do: VA cannot be fixed once bottled. Pair affected wines with rich foods to minimize the perception of acidity.

5. Mousy Taint

What it is: Mousy character is caused by several compounds (primarily acetamides) produced by Brettanomyces yeast or lactic acid bacteria. Unlike most faults, mousy compounds are not volatile at the wine's normal pH — they only become apparent after the wine mixes with your saliva.

What you detect: A distinctive, persistent aftertaste of mouse cage droppings, popcorn, or cracker — only detectable after swallowing (or spitting). To check for it: rub a small amount of wine on the back of your hand, allow it to dry slightly, then smell. The elevated pH of skin makes the fault apparent immediately.

Why it is distinctive: Because it only manifests post-swallowing, it is possible to enjoy a wine's aroma and even its initial palate impression before the fault reveals itself on the finish. This makes it particularly disorienting.

Frequency: More common in natural wines where winemakers use minimal sulfur additions and allow ambient yeast fermentation. Brett (Brettanomyces) can also produce mousy character alongside its better-known barnyard aromas.

6. Brett (Brettanomyces)

What it is: A wild yeast species (Brettanomyces / Dekkera) that can colonize wineries and barrels, producing a family of volatile phenol compounds.

What you detect: The signature character varies by compound: - 4-EP and 4-EG produce: barnyard, saddle leather, horse stable, Band-Aid - Isovaleric acid: cheesy, rancid butter, sweaty socks

The controversy: Brett is one of wine's most debated topics. At low levels, particularly in certain regional styles (some aged Bold Red wines from Châteauneuf-du-Pape, some Barolo and Barbaresco), Brett character has been historically accepted — even celebrated — as part of regional typicity. At higher levels, it overwhelms the wine's fruit and structure.

Modern winemaking has moved toward stricter Brett control through improved hygiene, controlled fermentation environments, and judicious use of Sulfites (sulfur dioxide, which inhibits yeast and bacteria).

What to do: Accept it if the level enhances complexity; return it if it overwhelms the wine's character.

7. Heat Damage (Maderization)

What it is: Premature oxidation caused by exposure to excessive heat, which accelerates the oxidation process dramatically.

What you detect: Stewed, cooked fruit (jammy, pruny, caramelized). In whites: a distinctly Madeira-like quality — baked apple, caramelized sugar, Browning. The term "maderization" comes from the similarity to Madeira wine, which is intentionally exposed to heat during production.

Causes: Improper transport or storage — wines left in a hot car, shipped through summer heat without temperature control, or stored above 24°C for extended periods.

Prevention: Store wines on their side (for corks) at 10–15°C, away from heat sources and direct sunlight. Inspect bottles at purchase for signs of seepage around the cork (suggesting the wine expanded from heat) or a pushed-up cork.

8. Sulfur Dioxide Excess

What it is: Winemakers add Sulfites (SO₂) as an antioxidant and antimicrobial agent. Too much produces detectable and unpleasant character.

What you detect: A sharp, acrid, choking sensation at the back of the throat, sometimes described as a burning match or sulfurous gas. This is distinct from hydrogen sulfide (rotten eggs) — excess SO₂ feels like an irritant rather than smelling like a specific compound.

Frequency: Less common in quality-focused wineries. More likely in industrially produced wines or wines that were not well-integrated before release.

What to do: Decant the wine and swirl vigorously. Excess SO₂ is volatile and often dissipates with aeration within 15–30 minutes.

Sending a Bottle Back

In a restaurant, you have the right to return a bottle for a verifiable fault: cork taint, oxidation, severe reduction, or mousy taint. You do not have the right to return a bottle simply because you do not like the style or because it tastes different from what you expected.

A confident return: "This wine is corked — I can smell TCA clearly on the nose. Could you please check the bottle?"

An inappropriate return: "This is too tannic / too acidic / not what I expected."

Most sommeliers and servers are trained to handle fault returns professionally. A well-run restaurant will immediately check the bottle (sometimes tasting it themselves to confirm) and replace it. If they push back on a clearly faulty bottle, that is a service failure on their part.

Knowing your wine faults gives you the vocabulary to communicate clearly and the confidence to act on your assessment. It is one of the most practically valuable skills a wine lover can develop.