A ‘corked’ wine has a mouldy taste and smell. This is caused by chemicals leached from the cork, and is nothing to do with bits of cork floating in the wine.
Where does natural cork come from?
Natural cork is made from the bark of the Quercus suber oak tree, and Portugal is by far the largest producer. The bark is stripped from the tree, and this particular species is able to regrow its bark. Individual corks are then punched out from the bark. Various treatments are carried out along the way.
Origin of cork taint
The main culprit is a chemical called TCA, 2,4,6-trichloroanisole to be precise, although other similar compounds are also responsible. There are several possible sources of these compounds, and they involve fungi, microbes and chlorine treatments, either of the cork oaks, or the punched corks themselves.
How bad is the problem?
There was a time when the percentage of ‘corked’ bottles was estimated at between 2 and 6%. I would say that the incidence has decreased, but the problem has not been eradicated.
What are the cork producers doing about it?
There are two general approaches:-
- Change the sterilising treatment – in the past hypochlorite bleaches were used to kill off microbes in the cork. Now they use peroxides, or microwave treatments to remove unwanted life in the corks.
- Use of high tech analytical equipment, such as mass spectrometers and gas chromatographs. These can detect extremely low levels of contamination.
What are winemakers doing about it?
Apart from changing away from natural cork, the best a winemaker can do is to buy top quality corks. Samples from a batch can be tested by immersing them in wine, to see if there is any contamination.
Threshold levels of taste and smell
The threshold level is the minimum concentration at which a person can detect a smell or taste. TCA can be detected at incredibly low levels of a few parts per trillion (yes, a million-millionth). There are two interesting observations:-
- Some people are far more sensitive to cork taint than others – they have a much lower threshold level. So that one person thinks a wine is just fine, and another that it is undrinkable.
- The threshold level increases rapidly as the alcohol content increases. Which is why cork taint is very rare with bottles of spirit.
Can cork taint be completely removed by changing to screwcaps or plastic corks?
The answer is no, but the incidence will be much lower. TCA can contaminate other wooden objects in a winery, particularly oak barrels. So some contamination is always possible.