Common Wine Faults and Flaws: How to Identify and Understand Them

Wine faults are chemical or microbial defects that compromise a wine's quality in measurable, reproducible ways — and knowing how to identify them separates frustrating experiences from informed ones. This page covers the most significant faults encountered in wine evaluation, explaining their causes, sensory signatures, and the threshold questions that distinguish a flaw from a stylistic choice. Whether a bottle smells like a wet dog or tastes like vinegar, there is almost always a precise explanation.

Definition and scope

A wine fault is a deviation from the expected sensory baseline caused by a specific, identifiable compound or process — not by stylistic difference or personal preference. The distinction matters enormously. Tasters new to wine tasting techniques sometimes mistake an unfamiliar style for a broken wine, and occasionally make the reverse error, accepting a truly defective bottle as "rustic character."

The Wine Aroma Wheel, developed at UC Davis by Ann C. Noble, categorizes off-aromas systematically and remains a foundational reference tool for fault identification. Institutions including the Court of Master Sommeliers and the Wine & Spirit Education Trust (WSET) use fault recognition as a graded competency in their formal examinations — evidence that this is treated as a serious analytical skill, not a parlor trick.

Faults fall into two broad categories:

How it works

Most wine faults operate at the molecular level, producing compounds detectable by the human nose at concentrations measured in parts per billion or parts per trillion. The sensory threshold varies by compound and by individual — which is why one taster dismisses a bottle as fine while another correctly identifies a problem.

The five most significant faults in commercial and fine wine contexts:

  1. Trichloroanisole (TCA) / Cork taint — produced when certain chlorine compounds interact with naturally occurring fungi in cork. The sensory profile is musty, damp cardboard, or wet basement. Detection thresholds for TCA can be as low as 2–3 parts per trillion (Cork Quality Council). Estimates of cork taint incidence have varied across studies, but figures cited by the Cork Quality Council have ranged from 1% to 3% of cork-sealed bottles.

  2. Volatile acidity (VA) — primarily acetic acid, the same compound in vinegar. At low concentrations VA can add complexity; above roughly 1.2 g/L in red wines (the legal limit under TTB regulations, 27 CFR Part 4) it becomes a fault. It signals bacterial spoilage, often from acetobacter activity.

  3. Reduction — the presence of hydrogen sulfide (H₂S) or mercaptans from sulfur compounds produced under fermentation stress. The smell ranges from struck match (mercaptans) to rotten egg (H₂S). Unlike cork taint, reduction is sometimes reversible: aeration or a copper coin in the glass can dissipate lighter expressions of it.

  4. Oxidation — excessive oxygen exposure producing acetaldehyde, turning wine flat and nutty in a way that lacks the intentional complexity of a Sherry-style wine. Fortified wines (fortified wine types and production) are intentionally oxidized within controlled parameters — the same aromatic signature in an unfortified table wine is a defect.

  5. Brett (Brettanomyces) — a genus of wild yeast producing 4-ethylphenol and 4-ethylguaiacol. Descriptors include barnyard, Band-Aid, smoked meat, or leather. Brett is the single most contested fault in wine: at low concentrations, it contributes complexity in certain Old World styles; at high concentrations, it overwhelms fruit entirely.

Common scenarios

Heat damage — sometimes called "maderization" — occurs when wine is stored or shipped above approximately 75°F (24°C) for extended periods. The wine takes on a cooked, jammy, or prune-like character, and the cork may push slightly from the bottle. This is irreversible, and proper wine cellar and storage conditions (typically 55°F / 13°C with 70% humidity) exist specifically to prevent it.

Light strike is common in white and sparkling wines bottled in clear glass. UV exposure triggers a reaction between riboflavin and sulfur-containing amino acids, producing dimethyl disulfide. Champagne houses documented this effect systematically, which is why most quality sparkling wines appear in dark glass.

A third scenario worth naming: re-fermentation in bottle, producing unexpected carbonation in still wines. This results from residual sugar and active yeast surviving into the bottle — either from premature bottling or contamination. The wine may appear fizzy, slightly cloudy, and sour.

Decision boundaries

The fault-versus-flaw-versus-style question has no universal answer, but there are reliable frameworks. A fault is objective when a specific compound is present at or above its sensory detection threshold and produces a negative deviation from the wine's type. A flaw is a lower-level expression of the same compound that some tasters find acceptable. A style is an intentional winemaking choice made in full control.

Brett is the clearest case study in the fault/flaw/style continuum. A Pomerol with modest barnyard notes that frame its black fruit is a stylistic outcome; a Napa Cabernet that smells only of Band-Aid failed either sanitation or temperature management. Context — region, producer intent, grape variety — informs the judgment.

Reduction occupies similar contested territory. Natural winemakers (natural fermentation and winemaking trends) working without added sulfur dioxide accept higher reduction risk as a trade-off; critics of this approach argue that consumer bottles should not require 20 minutes of aeration to become drinkable.

The foundational resource on global wine quality evaluation consistently returns to one principle: identifying a fault requires knowing what the wine was supposed to taste like in the first place. That knowledge is the harder skill.

References

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