How to Identify Cork Taint in Wine
The waiter
reluctantly returns a bottle of opened wine to the kitchen. "The
gentleman out there says this wine is corked, and wants another
one," he says to the manager. The manager looks at the wine, and
grudgingly replies, "What's he on about, there's no bits of cork
in there! Give him another one we don't want a scene." I'm sure
this scenario is played out daily in restaurants and cafes
throughout the world. Such is the lack of understanding
surrounding cork taint or corkiness in wine. So what is cork
taint?
In practical terms, it is the
biggest peril bottled wine buyers face. It strikes sporadically,
randomly and often very ferociously. No wine, regardless of its
pedigree or price, is immune. What is worse is that it forms in the
wine after bottling, and cannot be detected until it is opened. It
is the serial killer of wine. So exactly what is it, and how
prevalent is it?
Cork taint is in
fact a set of very undesirable aroma and flavour characters that are
imparted to bottled wines following contact with their cork. Six
chemical compounds have been found to contribute to cork taint.
These are guaiacol, geosmin, 2-methylisoborneol (MIB), octen-3-ol
and octen-3-one; and the most important of them all 2,4,6
trichloroanisole. TCA as it is affectionately known is a small
and chemically simple molecule. With the exception of guaiacol,
these compounds are sensorially very potent. TCA can be detected in
dry white wine and sparkling wines at levels around two parts per
trillion (0.000000000002 grams in a litre of wine), and in red and
port wines at around five parts per trillion.
The Chemical Structure of
2,4,6 Trichloroanisole |
Such low
concentrations are difficult to conceptualise but it is
analogous to one teaspoon in a couple of thousand Olympic sized
swimming pools or one second in 32,000 years. A single gram of
pure TCA could badly taint the entire volume of wine produced in
Australia each vintage. The other less common contributors to
cork taint are not much better having sensory thresholds of
around 20 parts per trillion. So how can you tell if a wine is
cork tainted? |
For particularly badly tainted
wines it is relatively easy if you know what to look for. TCA which
is implicated in more than 80 per cent of cork tainted wines
typically has a musty, mouldy or wet hessian character. MIB
and geosmin have an earthy/muddy aroma, guaiacol is smoky
or medicinal, and octen-3-ol and octen-3-one smell distinctly of
tinned mushrooms.
While most corked wines are musty or mouldy, occasionally one of the
other characters predominates. Complex chemical mechanisms underlie
the production of TCA. The one of most importance is the conversion
of chlorophenols to chloroanisole by common microscopic fungi such
as Aspergillus sp. and Pennicilium sp., in the
presence of moisture. Chlorophenols have been used as pesticides and
as wood preservatives and as such are common environmental
pollutants. The uptake of the minutest amounts of chlorophenol by
cork tree bark during any stage of its growth, or subsequent
manufacture into cork will provide the potential for cork taint
production.
Cork bleaching with
hyperchlorite (less frequently used now, peroxide bleaching is now
favoured), also provide a ready source of chlorophenols for use by
these micro-organisms. TCA can also be formed in packing materials
and wooden shipping container floors. It can then pass either
through the air or by direct contact to previously unaffected corks.
For similar reasons TCA is a major contaminant of many other foods
and beverages. The exact incidence of cork taint in Australian wines
is hotly debated. Estimates range from one to seven per cent.
Australian Wine Research Institute records of the incidence of cork
taint seen by winemakers in thousands of bottles of wines opened as
part of their Advanced Wine Assessment Course suggest that the
figure is around five per cent. My experience in running sensory
classes for the winemaking degree at the University of Adelaide/Roseworthy
Agricultural College over the past decade would suggest a slightly
lower rate of around three per cent. Whatever the exact figure, it
is indisputable that cork taint is responsible for adversely
modifying the sensory properties of a great deal of bottled wine
each year. Arguments by even experienced tasters often arise over
whether a wine is corked. This is due to a number of reasons. The
first is that people vary greatly in their sensitivity to aromas,
taints included. A rule of thumb is that for a specific aroma
compound, the most perceptive five per cent of the population are
about 200 times more sensitive than the bottom five per cent.
Therefore when at low levels, you can be sure that not everyone will
perceive the taint. Secondly, cork taint manifests itself
differently depending on its degree. At low levels, while not being
noticed in its own right, the TCA suppresses the wine's aroma and
flavour. Under these circumstances, comparison with other bottles is
the only way in which the taint can be confidently verified.
The taint compounds themselves
also smell differently depending on their concentration. For
example, MIB is somewhat earthy at lower concentrations but when
present in large amounts has a camphorous aroma. These shifts in the
way the taint compounds smell makes them hard to pin down in some
wines. Finally to exacerbate these problems of identification,
humans quickly become adapted to the musty aroma of TCA. Continued
sniffing of a TCA affected wine can result in rapid reductions in
its perceived mustiness. In fact TCA is one of the most strongly
adaptive compounds known.
The upshot of this is if you
think a wine is corked on the first sniff, it probably is.
Subsequent sniffing is far less reliable. The question of whether a
wine is corked is also complicated by the fact that the same taints
can arise not from the cork but from wine storage in TCA-affected
oak barrels. Winemakers describe this as musty oak, and typically
associate the fault with poorly maintained old oak. However even
relatively new barrels can be affected by TCA.
The wine from a single badly
contaminated barrel when blended with hundreds of others, will
significantly affect the entire blend. Such is the potency of these
compounds. So if you open a bottle of corked wine what can you do
about it? In short, nothing. Under wine conditions TCA is a very
stable compound. After it leaches into the wine shortly after
bottling it will remain there outliving the wine itself. No amount
of subsequent breathing will clean up the wine. So what can you do?
You could take the wine back to where you purchased it. The cork is
simply part of the wine's packaging. If its failure results in the
wine not being of merchandisable quality then you have the legal
right to return it. Large wineries receive hundreds of returned
bottles each year on the basis of them being 'off'. Most of these
are subsequently found to be cork tainted. Alternatively you could
choose to purchase wines with alternative stoppers such as Stelvin
capsules or synthetics. However for a range of other reasons, cork
is still undeniably the stopper of choice for most consumers and
producers, and remains an important component of the wine packaging
mix.
To their credit wineries and
cork suppliers spend a large portion of QC budgets on identifying
tainted batches corks before they are used research into how TCA
formation is affected by the growing-making distribution process
continuing. World without TCA? It's must.
Richard Gawel
This article originally appeared in The Wine Tutor: Winestate
Magazine
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