True cork taint is due to 2,4,6-trichloroanisole (TCA), formed when certain phenolic compounds react with chlorine-containing compounds used as disinfectants. This need not result from the cork, as TCA is also found in barrels and other winery equipment. If there is a high degree of cork taint, the wine smells musty (‘like wet cardboard’) and falls flat on the palate, without fruit or vibrancy. Some people are very sensitive to cork taint, others much less so.
The benefits or otherwise of Brettanomyces in a wine is a controversial topic. The yeast is encouraged by a lack of hygiene in the winery. Some claim that it lends added complexity to a wine in the form of horsey or farmyard notes. Others claim that it is a taint that overlies the natural fruit character of the wine with ‘unclean’ notes of sticking plaster or rancid cheese. Some of the world’s finest wines, most notably older vintages of Château de Beaucastel in Châteauneuf-du-Pape, carry an unmistakeable trace of Brett. Whether this is a benefit or a fault is ultimately a matter of personal preference; the least that can be said is that Brett should never dominate a wine.
If active yeasts and fermentable sugars remain in a wine, it will continue to ferment in bottle. This second fermentation and the carbon dioxide released is exactly what is wanted in traditional method sparking wine (see Chapter X) but not in other wines which will become inappropriately cloudy and fizzy. In severe cases, the bottle may even burst. Off-dry and semi-sweet wines are particularly susceptible to a second fermentation, and care must be taken to remove or kill all yeast cells prior to bottling.
Similarly, bacteria within the bottle can cause a wine to go off. Lactic acid bacteria make a wine smell like mouse droppings, while acetic acid bacteria convert ethanol to acetic acid, the acid ingredient in vinegar. Acetic acid reacts with ethanol to form ethyl acetate, an ester that makes the wine smell like nail varnish. A small amount of this so-called volatile acidity can be tolerable, but, given time, the wine will go to vinegar.
Oxidation and reduction
Oxidation is probably the source of most wine faults, although consumers often mistake the musty note of oxidation for cork taint. For both red and white wines, excess exposure to oxygen (beyond the small amount required for the development of tertiary notes) gradually leads to a brownish colour. The wine loses its fruit character, which is replaced by a musty, dusty, ‘flat’ note reminiscent of beef stock. Oxidation may result from careless winemaking, an inadequately sealed bottle, or inadequate packaging. Corks degrade over time, especially if the bottle is stored upright so that the liquid is no longer in contact with the surface of the cork, and most cellaring companies offer a re-corking service for old bottles. Note that some wines, such as madeira or tawny port, are made in a deliberately oxidative style that is part of their character and appeal.
Reduction is the opposite chemical reaction to oxidation and occurs when the wine is deprived of oxygen. Oxygen prevents the conversion of sulphur dioxide dissolved in the wine to hydrogen sulphide, a gas with an unpleasant smell of rotten eggs or drains. Reductive taint is most common in bottles sealed by a screw cap, which forms an airtight seal and prevents the wine from ‘breathing’. Mild reductive taint ought to disappear as soon as the wine is swirled around in the glass and exposed to air. Severe reductive taint can be treated with a copper coin, which leads the sulphide ions to precipitate out of the wine.