CORK TAINT is variously described as "moldy" or "musty" or "earthy" or sometimes "medicinal" smell that, regardless of its description (which in fact has no established definition), either masks or dominates fruit aroma in wine and reduces overall wine appeal. Infected wines are said to be "corked" or "corky" and the contaminant often referred to as "cork taint", although there are many other possible origins besides corks for its presence in wine.
Cork Taint can impart a very unpleasant smell that, depending upon its severity, tends to dominate all other aroma characteristics of any wine it contaminates. The least offensive and most subtle sign of TCA is wine that has very little aroma at all. The Australian Wine Research Institute conducted experiments in early 2003, that demonstrated even a very low level of contamination, as little as one or two nanograms (billionth of a gram) per liter, suppresses positive fruit aroma character in wine by as much as 50%.
As with all aromas, individuals vary widely in their threshold ability to detect the presence and strength of Cork Taint (as well as their tolerance for it). Experience can increase sensitivity. Canadian wine researcher Dr. Ronald S. Jackson estimates that 99% of the population can detect TCA when it reaches a concentration of 200-300 parts per million. Individual human thresholds for detection may be magnitudes lower, but are generally considered to be above 5 parts per billion.
The source of this odor may be one or more of several particular chemical compounds formed by a reaction between naturally occurring molds and chemicals used in the treatment of wood products.
Molds may be originally present in raw cork bark or in wood used for barrels or barrel racks, tanks, scaffolding, walls, stairs, pallets, cardboard boxes, or other many other types of winery equipment or facilities. TCA can also can infect cork or wood that is in storage.
Ironically, chemicals used for keeping the production environment sterile and safe from contamination become degraded by fungi or molds indigenous to wood products. The main culprit is thought to be chlorine bleach used in cork processing and also as a routine disinfectant in wineries. Common treatments such as insecticides, pesticides, herbicides, or flame-retardants, although no longer used (or, in some cases, even legal to use) on vines are also identified sources. Another possibility is atmospheric pollution by off-gassing from plastic equipment, or simply from old wooden buildings that have absorbed any of these chemical pollutants over time.
TCA is the common abbreviation for the chemical compound 2,4,6-trichloroanisole, first identified by a Swiss chemist in 1981, and thought to be the primary cause of cork taint. Other chloroanisole contaminants of wine may include 2,3,4,6-tetrachloroanisole (TeCA) and pentachloroanisole (PCA). Scientists in Bordeaux, France, using gas chromatography and mass spectrometry, have recently isolated another compound, 2,4,6-tribromoanisole (TBA), that similarly ruins wine aromas.
Damage to the wine industry annually from Cork Taint is estimated to be $10-billion worldwide. A reliable process has been developed to detect and remove TCA from wine before it is bottled.
The cork industry has been working diligently since the 1990s to prevent TCA and related compounds by eliminating any chlorine and bromine treatments from their forests and processing, but cork is harvested only once every eight to ten years, so it may take decades to show measurable improvement.
While TCA can be detected in corks, however, there is of yet no proven method to remove it from this source. This presents both a complex challenge to wine science and mutual frustrations to the wine and cork industries. Experiments are ongoing to prevent or purge TCA from corks, using alternative chemical treatments, steam, gas emersion, microwaving, etc.
For a more detailed explanation of TCA-related wine spoilage, see Cork Taint: TCA and Related Compounds, a web project of Professor Andrew Waterhouse's Students of Natural Products of Wine at the University of California at Davis. In terms of providing a thorough explanation of wine chemistry that can be easily understood by a layman, this is one of the very best articles on the Internet.