The off-flavour transferred from cork to wine comes from the compound 2,4,6-trichloranisole (TCA for short). No one really knows if it comes from the use of pesticides in the cork oak forests of Portugal, from the processing of cork, or from storage and transport. The fact is, when it's there TCA renders wine anything from dull to undrinkable.
The problem of corked wine is a big headache for the wine industry because it takes just one small imperfect cork to undermine all the energy, natural and man-made, that goes into producing a bottle of wine.
Precisely because it's so damaging, the incidence of corked wines is hotly disputed. The cork industry puts it at between 0.5 and 2 per cent. At a seminar on cork in London last month, wine industry representatives put it higher. Waitrose suggested 3 per cent, Australia's Southcorp (which sells 30 million bottles a year in the UK) said 4 per cent, while an Adelaide survey takes 5 per cent as a rule of thumb.
Whatever the figure, any taint is unacceptable. "If customers are disappointed with a wine, whether or not they recognise cork-taint, they won't buy it again. And that unnecessarily damages the reputation of the wine producer, as well as the region and the country," says Helen Robinson, a wine buyer for Tesco.
The concern in the wine industry at the damage done by TCA has led to a groundswell of indignation against the complacency of the cork industry, and a search for alternatives. Increasingly wines, particularly the everyday kind, have got not corks but stoppers made from synthetic materials. New generation synthetic stoppers such as Supremecorq and Integra, are claimed to be TCA-free, incapable of leaving a taint, and recyclable. Plastic "corks" are catching on fast.
But as they are a relatively young product, the synthetics attract questions. Tony Smith, managing director of ASA Ltd in Australia, has gradually moved away from cork towards Integra, a stopper that he's spent A$3m (pounds 2m) developing. He believes synthetic "corks" are fine for everyday wines meant to be drunk within two years (an estimated 96 per cent of wines). But he has yet to be convinced that they are right for a fine wine, which needs laying down for more than two years.
The challenge of the "polymer pretenders" has so frightened the cork industry that it has begun to fight back. Its Quercus Report, aimed at determining the cause of taint and eliminating it, was inconclusive. But the French producer Sabate is advocating Altec, a new "non-agglomerated cork which offers near protection against cork risk and leakage". Amorim, the giant Portuguese cork producer, claims to have found a fault-free, ozone-washing process. The producer has even enlisted the heavyweight support of Len Evans, the "soul" of Australian wine, who has, for a fee, been extolling cork's virtues.
The most emotive claim against plastic corks is that they are a threat to the wildlife of Portugal's forests. And the cork industry is pointing to the findings of the Leatherhead Food Research Association, which has cast doubt over synthetics, asking whether some plastic stoppers may produce an off-taste. According to the cork producers it is natural cork, anyway, that is preferred by wine-makers and drinkers.
Unfortunately, it seems the best wine stopper, the Stelvin closure, or screwcap, is still light-years away from public acceptance. The screwcap is widely acknowledged by the wine industry to be the best closure for wine. In 1997, Tesco championed it, selling half a million screwcapped bottles in a promotion that drew attention to the pitfalls of cork. And in Australia, all of Yalumba's reference samples are now kept under screwcap.
According to Brian Walsh, senior wine-maker at Yalumba, "Stelvin closures are superior to anything else ... they're a technical success but a commercial failure". Southcorp, Australia's biggest wine company, agrees that "the screwcap gives the best seal for wine". The problem, simply, is the screwcap's downmarket image.
Meanwhile, though, we can all contribute to standards. If the wine you open smells a bit like the inside of a mouldy walnut shell - take it back.
Wines of the Week
1998 Frascati Colli di Catone, pounds 3.99, Asda (left) This aromatic, unoaked dry Roman white made from the Malvasia del Lazio and Malvasia di Candia grapes has the typical Frascati nuttiness with good intensity of fruit and more character than you'd normally expect at this price.
1998 Finca el Retiro Tempranillo, Mendoza, pounds 5.49, Waitrose (right) Taking the Spanish Tempranillo grape, Italian winemaker Alberto Antonini has made this spicy Argentinian red with sumptuous black-fruit flavours and a shading of oak.