Updated Jan. 2, 2013
You've opened a bottle of wine that's supposed to be outstanding. But when you put your nose to the glass, it smells like something you pulled out from a forgotten corner of a damp basement. What's the problem? Most likely it's TCA.
What is it?
TCA stands for 2,4,6-trichloroanisole, a chemical so powerful that even in infinitesimal amounts it can cause musty aromas and flavors in wines. The compound forms through the interaction of plant phenols, chlorine and mold. It most frequently occurs in natural corks (TCA can even form on tree bark) and is transferred to the wine in bottle--which is why wines with these off-aromas are often called "corky." But the taint can originate elsewhere in wineries, where damp surfaces and chlorine-based cleaning products are commonplace; barrels, wooden pallets, wood beams and cardboard cases are all sources of phenols. If TCA goes undiscovered, it can spread and eventually taint the wines.
How do I recognize it?
Although TCA taint poses no health concerns for wine drinkers, it can ruin a wine. At higher levels, it makes a wine smell moldy or musty, like cardboard, damp cement or wet newspapers. At its worst, the wine is undrinkable. At lower levels, TCA taint merely strips a wine of its flavor, making normally rich, fruity wines taste dull or muted, without imparting a noticeable defect. This can leave drinkers disappointed in a wine without being able to pinpoint why.
Experts say people vary widely in their ability to perceive TCA in wine, depending on their genetics and experience. Some cork producers claim that levels of 6 or even 10 parts per trillion (ppt) are acceptable, as many people won't notice TCA at this level. However, research in Europe and at the University of California, Davis, indicates that some tasters can detect TCA at 1 ppt to 2 ppt, and a rare few can perceive it at even lower levels. People with higher threshold levels may perceive an off characteristic without being able to identify it.
There is no legal standard for acceptable TCA levels in wine.
How common is it?
As with thresholds of perception, estimates of TCA-taint frequency in wines vary widely. In the past, the number cited ranged from 1 percent to as much as 15 percent of all wines, depending on whether the estimate came from closure manufacturers, vintners or another source. Wine Spectator's Napa office has been tracking the number of "corky" bottles in tastings of California wines since 2005, and the percentage of defective corks in that category has dropped from a high of 9.5 percent in 2007 to a low of 3.7 percent in 2012. The cork industry has a different estimate of cork failure: typically 1 percent to 2 percent.
Are there other causes of "corky" wines?
Yes. When repeated bottles of the same wine, multiple wines or multiple vintages from a winery show the same flaws, the problem is not likely due to a few bad corks. There may be widespread cellar taint.
Many cases of taint are caused by other environmental problems at wineries, such as moldy cellars, antifungal treatments and flame-retardant paints. Like TCA, a compound called 2,4,6-tribromoanisole (TBA) gives off musty, papery aromas; it is used in preservatives to treat wood. Contamination from chemically treated woods in renovated cellars plagued many estates in France, particularly in the 1990s. Some properties had to tear down and reconstruct buildings to eradicate the problem.