Ahead of my upcoming masterclass on Flavour chemistry: identifying faults in wine on 26 May 2026, I’d like to share a brief overview on the subject.

There are still last few spots available if you'd like to attend!

Wine consists of hundreds of different compounds, or molecules, that determine its characteristic flavour. In this post we take a close look at 10 compounds that are typically undesirable in wine. They have unpleasant flavours and detract from wine quality.

Acetaldehyde

Acetaldehyde smells of bruised apple, carved pumpkin and paint. It is produced by yeast during fermentation, as well as being an oxidation product.

Origins

Acetaldehyde is an intermediate product in the formation of alcohol during fermentation. Put simply, yeast converts sugar into acetaldehyde, then acetaldehyde into alcohol. In early stages of fermentation, wine has very high levels of acetaldehyde. When fermentation finishes, acetaldehyde levels are low.

The aroma of bruised apple is therefore an indication that fermentation is not yet completed, and perhaps the winemaker was in a rush to release the wine onto the market as soon as possible.

Acetaldehyde is also an oxidation product of alcohol. A wine bottle that has been opened for too long contains high levels of acetaldehyde.

Acetic acid

Acetic acid smells of vinegar or pickles, and is perceived as stinging in the nose. It is the main component of “volatile acidity”.

Origins

Volatile acidity is caused by certain bacteria, conveniently called acetic acid bacteria, as they produce acetic acid. These bacteria need oxygen to live. Volatile acidity is therefore both a hygiene and an oxidation problem. Damaged or dried grapes, skin contact, pumping over, barrel ageing lead to elevated levels of volatile acidity.

ATHP

ATHP has a flavour of stale popcorn, sweat and basmati rice. It is typically referred to as “mousiness”. ATHP is only detectable on the palate, not on the nose.

Origins

This fault is produced by lactic acid bacteria. These bacteria are very sensitive to SO₂. The fault typically only appears in wine where no SO₂ was added at all: think of natural/low intervention wines.

Ethyl acetate

Ethyl acetate smells of nail varnish remover or glue. It is another component of “volatile acidity”.

Origins

When alcohol and acetic acid are present at the same time, yeast or bacteria can combine those to form ethyl acetate. Therefore ethyl acetate is usually present in wine when acetic acid is also present.

4-EP

4-EP smells of band-aid, horse blanket or blue cheese. It is produced by wild yeast “Brettanomyces”.

Origins

Brettanomyces, or Brett for short, is a type of yeast often thought of as a wild yeast. It is able to produce some compounds, such as 4-EP, which are not produced by the type of yeast that normally carries out alcoholic fermentation (Saccharomyces Cerevisiae).

Brett contamination is more likely when fermentation is spontaneous, when the wine is barrel aged, when residual sugar is high and when sulphur dioxide levels are low. Red wines are at higher risk than white wines.

Guaiacol

Guaiacol smells of smoked meat, smoked fish or smoked cheese. It is related to the fault “smoke taint”.

Origins

In the event of a wildfire, smoke from the fire may travel into the vineyard and cover the vines in smoke. The smoke aroma compound guaiacol attaches to the ripening grapes. When these grapes are used in winemaking, the smoke aroma may end up in the finished wine. It is mostly a problem in red winemaking, as the smoke aroma is found in the grape skins.

3-Isopropyl-2-methoxypyrazine

This compound smells of potato skin, dug soil or quinoa. It causes “ladybird taint”.

Origins

When populations of ladybird beetles are unusually high in the vineyard, they can impact wine flavour when the beetles end up in the grape crusher after harvest.

The ladybird beetle was artificially introduced in North America to help with soybean production. Ladybirds are a natural predator of aphids, which are a problem in soybean plantations. As soybean harvest is somewhat earlier than grape harvest, ladybird beetles may migrate to vineyards afterwards. Grape sorting is required to remove the ladybirds.

Methyl mercaptan

Methyl mercaptan smells of cabbage, camembert, drains and rubbish bin. It is related to the faults “reduction” and “light strike”.

Origins

Reduction is a condition caused by stressed yeast. Yeast becomes stressed when it is lacking certain nutrients, particularly nitrogen. Under those circumstances, yeast produces excessive sulphur compounds, of which methyl mercaptan is one. When yeast is lacking oxygen, the problem is exacerbated. Some grape varieties are at higher risk of reduction. Reduction can be partly remediated by decanting wine.

Wine exposed to daylight may also have higher levels of methyl mercaptan. This is a common problem in wines packaged in clear glass, like most rosés and many whites.

Sotolon

Sotolon smells of caramel, brown sugar, curry powder and Maggi sauce. In faults terminology, we call it “oxidation”.

Origins

During barrel ageing, bottle ageing and after opening the bottle, sotolon levels can increase. Heat exposure accelerates the formation of this compound, and wines that have been excessively affected by heat are referred to as “maderised”. Wines made with botrytis-affected grapes also have high sotolon levels.

TCA

TCA smells of wet cardboard, damp cellar and swimming pool. It causes the fault “cork taint”.

Origins

Around 1 in a 100 bottles fitted with a natural cork can develop this taint, which is produced by moulds. These moulds do not grow in wine (they cannot tolerate alcohol), therefore they grow in the cork. They form TCA, which is subsequently released into the wine. Cork-tainted wine is safe to drink. Broken corks are unrelated to the fault. Screw-cap bottles have a very low chance of containing this fault.

TCA is a chlorine-containing compound. It can be traced back all the way to chlorine-containing herbicides and pesticides used in the growing environment of cork trees.