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Cork Taint

“Cork taint” is a term used to describe wine faults associated with the presence of certain organohalogen compounds which lend wine an off-putting mouldy smell and taste. These are probably the most infamous of wine faults, and also the most controversial. The smell of cork taint is often described as mouldy, musty, reminiscent of a damp basement, and generally unpleasant. When a bottle of wine is “corked” (i.e., it is affected by cork taint), its natural aromas are reduced significantly. A severely corked bottle of wine can be undrinkable. Most commonly at fault is the 2,4,6-trichloroanisole molecule, better known as TCA, followed by 2,4,6-tribromoanisole (TBA), 2,3,4,6-tetrachloroanisole (TeCA), and pentachloroanisole (PCA), though various other molecules have also been associated with the problem.

TCA is typically transferred to a wine from the cork stopper (hence the term “corked” wine), but it may also come from other sources, and be introduced either via the cork or before bottling. Several factors can contribute to the presence of TCA, amongst them contaminated wooden barrels, storage and transport conditions, and cleaning products in the winery.

TCA is primarily produced as a result of the interaction between microbes and chlorinated phenolic compounds (chlorophenols) present in natural cork. More specifically, microbes convert chlorophenols into chlorinated anisole derivates (chloroanisoles), which are then present in the cork and dissolve into the wine. Chloroanisoles can nevertheless also arise in the absence of microbes.

The chlorophenols which precede TCA are industrial pollutants found in many pesticides and wood preservatives. They can enter the wine-making process at several stages. They can be taken up by cork trees, they can be a product of the chlorine bleaching process used to sterilise or bleach wood and paper, and they can migrate from other objects such as shipping pallets that have been treated with chlorophenols. The microbes that produce TCA can be mould-forming fungi that live in small pores in the bark of cork trees, airborne fungi, or bacteria like Aspergillus spores, Penicillium spores, Actinomycetes, Botrytis cinerea, Rhizobium spores, and Streptomyces.

Interestingly, TCA is not directly perceived by the human olfactory apparatus. Studies have shown that the TCA molecule actually suppresses olfactory signal transduction, thereby distorting an individual’s perception of smell and leading them to “smell” cork taint.2 So counterintuitively, it is odour suppression that causes us to perceive an unpleasant aroma in corked wine.

It is estimated that 99% of the population can detect TCA when it reaches a concentration of 200-300 parts per million3, but individual detection thresholds can be significantly lower than this, and there is a huge variation in individual sensitivity to TCA. Some people can detect even faintly corked wine, whilst others do not smell it when the concentration of TCA is orders of magnitude higher. This means that there is a significant degree of ambiguity surrounding the problem of cork taint, both in terms of its entry into the wine-making process as well as its perception when a bottle of wine is opened.

From a commercial point of view, cork taint not only leads to unhappy customers but also increases transactional costs within the wine industry due to the corrective action required upon detecting a corked bottle (e.g., removing the tainted bottle from the supply chain, returning the bottle to the supplier or producer, etc.). The annual cost of cork taint to the wine industry worldwide has been estimated to be at an excess of $10 billion.4

In addition, many wineries suffer from “systemic TCA”, which occurs when cork taint infiltrates a winery by means other than cork and affects the entire production. Significantly, this means that even wines sealed with screw-caps can be corked if the winery of origin suffered from systemic TCA. It also means that cork taint can enter the wine-making process from the moment a cork is still bark on a tree to the moment a wine is bottled and sealed (which can occur even after shipping, as with wine shipped in bulk), and it can occur even when cork is entirely absent from the process. This all makes cork taint very difficult to eradicate completely.

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References

  1. Wine Science, J. Goode, Octopus Publishing Group, 2005.
  2. 2,4,6-Trichloroanisole is a potent suppressor of olfactory signal transduction, H. Takeuchi, H. Kato and T. Kurahashi, Proceedings of the National Academy of Sciences, 2013.
  3. Wine Science: Principles and Applications, R. S. Jackson, Elsevier, 2014.
  4. Estimating a consumer rejection threshold for cork taint in white wine, J. Prescott, L. Norris, M. Kunst and S. Kim, Food Quality and Preference Vol. 16, 2005.