How can you tell if a wine has gone off?

Cork taint smells of damp cardboard, a wet cellar or musty wood and takes the fruit out of the wine.

The cause is 2,4,6-trichloroanisole, TCA for short. The Australian Wine Research Institute cites a threshold of about 1.4 nanograms per litre for a pinot noir; in dry white wine it can be even lower. Airing or decanting does not help, because the substance does not disappear from the wine.

An oxidised wine is dull in colour and smells of overripe apple, nuts or sherry.

The substance behind it is acetaldehyde, which according to the Australian Wine Research Institute becomes noticeable from 100 to 125 milligrams per litre. Oxidation arises through contact with oxygen: a cork that does not close properly, a bottle that has stood too warm or an opened bottle that was left open too long. In sherry and old tawny port the same smell is actually desired.

Bubbles in a wine that is not supposed to be sparkling point to refermentation in the bottle.

Sugar and live yeast are then left behind and fermentation continues in the closed bottle. The wine turns cloudy, tingles on the tongue and the cork may push upwards. If nothing about sparkling or lightly sparkling is stated on the label, such a bottle belongs back with the seller.

A wine that is too old is not spoiled but exhausted: the fruit has disappeared and mostly acidity and alcohol remain.

There is no legal expiry date on the bottle, because drinks of 10 percent alcohol by volume and above are exempt from the best-before date. Colour is the first clue: white wine gets darker and browner, red wine paler and orange at the rim. Storage temperature, light and the position of the bottle determine how fast that happens.

Take a bottle with a clear defect back to the shop where you bought it, with the cork and most of the wine still in it.

From the cork and the wine you can usually see whether it is cork taint, oxidation or re-fermentation. For a demonstrable fault, returning the bottle is the normal course of events. If you simply do not like a wine, that is not a fault; asking for advice at the next choice is the better route.