Study and careerTasting & aromas

Wine faults: TCA, H2S, VA, Brett and oxidation

How to spot the five faults on every sommelier's tasting grid, what causes them, the other faults to know, and what to do at the table when a bottle is off.

Alper Billik · Advanced Sommelier

Updated October 2026 · 5 min read · Edited with AI

This article may contain affiliate links.

On this page

The five faults at a glance
TCA: cork taint
H2S and reduction
VA: volatile acidity
Brett
Oxidation
Other faults to know
What to do at the table
Quick quiz: name the fault
Questions people ask

The short answer: the five faults every sommelier checks first are TCA (cork taint: wet cardboard, damp cellar), H2S (rotten egg), VA (vinegar, nail polish), Brett (farmyard, sticking plaster) and oxidation (bruised apple, flat and brown). On the Court of Master Sommeliers' tasting grid, "clean or faulty" is the very first question on the nose.

The five faults at a glance

Fault	Smells like	Cause
TCA (2,4,6-trichloroanisole)	Wet cardboard, damp cellar, mouldy newspaper; fruit muted	Moulds meeting chlorophenols, in cork or in winery wood and materials
H2S (hydrogen sulfide)	Rotten egg, struck match, drains	Yeast under stress during fermentation
VA (volatile acidity)	Vinegar; nail polish remover	Acetic acid bacteria: acetic acid, often with ethyl acetate
Brett (Brettanomyces)	Farmyard, horse, sticking plaster, smoky bacon	A spoilage yeast making 4-ethylphenol and 4-ethylguaiacol
Oxidation	Bruised apple, sherry-like, nutty; brown colour	Too much air: acetaldehyde, loss of fresh fruit

TCA (2,4,6-trichloroanisole): Smells likeWet cardboard, damp cellar, mouldy newspaper; fruit muted; CauseMoulds meeting chlorophenols, in cork or in winery wood and materials

H2S (hydrogen sulfide): Smells likeRotten egg, struck match, drains; CauseYeast under stress during fermentation

VA (volatile acidity): Smells likeVinegar; nail polish remover; CauseAcetic acid bacteria: acetic acid, often with ethyl acetate

Brett (Brettanomyces): Smells likeFarmyard, horse, sticking plaster, smoky bacon; CauseA spoilage yeast making 4-ethylphenol and 4-ethylguaiacol

Oxidation: Smells likeBruised apple, sherry-like, nutty; brown colour; CauseToo much air: acetaldehyde, loss of fresh fruit

TCA: cork taint

A "corked" wine smells musty and, just as important, flat: the fruit seems hidden. TCA forms when moulds meet chlorinated compounds, most famously in natural cork, but also in barrels, pallets and winery buildings, so a whole batch can be tainted, even under screwcap. People notice it at tiny amounts, a few nanograms per litre, though sensitivity varies a lot between tasters. It is unpleasant, not dangerous. More for guests in can wine go bad?.

H2S and reduction

Hydrogen sulfide smells of rotten egg. Yeasts make it during fermentation, especially when short of nutrients. Left alone, it can turn into heavier sulfur compounds, the mercaptans (cooked cabbage, onion, garlic, rubber) and then disulfides. Winemakers remove H2S and mercaptans with a careful copper treatment; disulfides don't react with copper, so prevention matters. A faint struck-match note in a young wine is often called "reductive" and can be a deliberate style. At the table, decanting can soften a light reductive note; real rotten egg or cabbage is a fault.

VA: volatile acidity

Most VA is acetic acid, the acid in vinegar, made by bacteria when wine meets air. It often comes with ethyl acetate, which smells of nail polish remover. A trace can lift the aroma of some big reds, and some sweet botrytis wines naturally carry more. When it smells of vinegar or solvent and the finish is sharp and thin, it is a fault.

Brett

Brettanomyces is a yeast that lives in cellars and old barrels. Its compounds 4-ethylphenol (sticking plaster, farmyard) and 4-ethylguaiacol (smoky, clove) build up over time and strip away fruit. Some drinkers enjoy a hint of it in a rustic red; in excess, every wine starts to smell the same, and that is the real problem.

Oxidation

Air is part of winemaking, but too much of it, through a failing cork, warm storage or an open bottle, makes acetaldehyde: bruised apple, sherry and nuts. The colour browns, the fruit fades and the wine tastes flat. Remember that some wines are oxidative on purpose: fino and oloroso sherry, tawny port, vin jaune. In those, these notes are the style, not a fault.

Other faults to know

Light strike: wine in clear glass exposed to light can develop cooked cabbage or wet wool notes. Keep sparkling and white wines in the dark.
Heat damage (cooked wine): stewed, jammy fruit, a pushed-up cork or a sticky capsule after warm storage or transport.
Smoke taint: ashy, smoky, medicinal notes when grapes are exposed to bushfire smoke; from volatile phenols such as guaiacol.
Mousiness: a caged-mouse taste you notice on the finish, after swallowing, not on the nose. Linked to bacteria, and most often found in wines made with little or no sulfur.
Refermentation: unwanted spritz and cloudiness in a still wine with sugar left in it.

What to do at the table

Smell every bottle first, before the host tastes. A faulty bottle should never reach the guest.
Replace it calmly with a second bottle of the same wine, and taste that one too.
Never argue about taste. If a guest finds a sound wine faulty, offer an alternative. A fault is about the bottle; a dislike is about the wine. See 4 common guest complaints.
Keep the faulty bottle with its cork for the supplier: many will credit or replace it.