The Academy of Wine

Many people have heard a wine described as “corked”, but most do not know what that means. Most wine lovers know that it is a flaw in wine that creates musty and moldy aromas. But what causes it? Some say it is caused by bacteria; others say mold. Some believe its sole source is the cork in the bottle; others say it can also be caused by contamination in the winery cellar. Some think that using screw caps or synthetic corks instead of natural corks will totally eliminate the problem; others say that simply is not true. It’s time to do a little research to help put an end to all of the confusion.

A "corked" wine has been affected by a compound called 2,4,6-trichloroanisol which is commonly referred to as TCA in the wine industry. It causes musty, moldy aromas in the wine similar to the smell of wet cardboard, damp cement, old books, damp magazines or newspapers. These musty aromas will mask or dominate the fruit aromas of the wine and will reduce the overall quality. TCA is created by an interaction of mold, chlorine and phenols which are organic compounds found in all plant matter, and can only occur when these three ingredients are present. The compound can develop in the corks themselves, which is why corks are often linked to TCA. However, it isn’t always easy to pinpoint the source of TCA, because it can also originate in cardboard cases and wooden pallets, racks, barrels, tanks, walls or stairs since they all contain phenols. The main culprit seems to be chlorine bleach used for cork processing and as a routine disinfectant in wineries. These chemicals used to keep the production environment sterile, ironically become degraded by molds that are indigenous to wood products. The mold consumes the plant matter containing the chlorine converting it to the TCA compound. Entire wineries have been contaminated with TCA, meaning that a wine can be “corked” before a cork even goes into the bottle!

Regardless of the source, cork taint can impart a very unpleasant smell that can dominate all other aromas, but there are many different levels of TCA. The lowest level of TCA is the most subtle and least offensive. At its lowest levels, TCA will actually subdue the aromas of the wine causing it to have little or no aroma at all. TCA can be detected in dry white wine and sparkling wine at levels as low as 2 parts per trillion (0.000000000002 grams in a liter of wine), and in red wine and port at about five parts per trillion. In simple terms, that can be compared to about 1 teaspoon of TCA in a couple thousand Olympic sized swimming pools or one second in 32,000 years. A single gram of TCA could badly taint all of the wine produced in Australia in a single vintage year! Potent stuff!

Even experienced tasters may argue over whether or not a particular wine is corked. The first reason for their differing opinions is that people vary greatly in their sensitivity to aromas including taint. Generally, the top 5% of the population having the highest level of perception is about 200 times more sensitive than the bottom 5%. That means that at low levels many will not even detect cork taint. Secondly, at low levels TCA has no aroma but will suppress the wine’s aroma and flavor and can only be detected by comparison to another bottle. Detection is also complicated because our sense of smell quickly becomes adapted to TCA. Continued sniffing of a wine with TCA contamination results in a rapid reduction in perception of the musty aroma.

In an effort to reduce the occurrence of TCA, the cork industry has begun to substitute peroxide bleaching for chlorine bleaching of corks during processing. They are also conducting experiments to prevent or purge TCA from corks using alternative chemical treatments, steam, gas emersion and microwaving. Wineries have reduced or eliminated the use of chlorine bleach for disinfecting in the cellar. Some wineries have actually installed reverse osmosis water purification systems to completely eliminate chlorine in the water used in the cellar.

Cork taint is the biggest peril that wine buyers face since it strikes randomly, sporadically and often ferociously. It is the serial killer of wine. It is responsible for adversely affecting the sensory properties of a great deal of bottled wine each year, estimates range from 2 % to 7 % or an annual loss of approximately $10-billion worldwide. No wines are immune to it, no matter the price or pedigree.