"Great burgundy smells of shit." Thus, famously, wrote Anthony Hanson, Master of Wine, in the first edition of the Faber classic on the wines of Burgundy more than 20 years ago. Being neither sensationalist nor coprophiliac, Mr Hanson deleted the offending phrase from the second edition, but didn't change his opinion. He meant what he smelt, as did other critics who used to find hamster cages in the great Hermitage of the Rhône, and those who appreciated the sweaty-saddle character of Hunter Valley Shiraz.

Since then, technological progress in winemaking has taught us that the niffs we once so admired are nothing other than manifestations of a damaging yeast in wine called Brettanomyces, or brett, as it is nicknamed in the trade. So if you've ever had a horsey wine, outside a Countryside Alliance party, you may have praised its stable-like smell, but what you should know is that it almost certainly suffered from brett. In fact, according to the Bordeaux oenologist Pascal Chatonnet, brett affects a good two-thirds of red wines. At low levels, it's acceptable, and may, according to some winemakers, although not Chatonnet, even add complexity. But when it crosses the threshold, as it does in three out of 10 red wines, it's a problem.

Chatonnet came to London recently to deliver a masterclass in malodorous wines to the wine press. Many of us, me included, have been guilty at some stage or another of giving red wines Old-MacDonald-and-his-farm-like compliments when what we were smelling and tasting was the not-so-great smell of brett. It's increasingly common in red wines, Chatonnet expounded, because "the demand for super-ripe, low-acid wines from grapes in warm climates - such as syrah and mourvèdre - increases the likelihood of contamination." How do you spot it? Brett comes with a distinct wet horsey whiff, an animal-like taste in the wine itself and a bitter aftertaste.

But that's not all that can go wrong with wine, whether red or not. The cork taint problem, TCA, ruins at least one in 20 wines and affects many more adversely. As if that's not enough, Chatonnet lobbed a hand grenade: it's also possible to find similar mouldy smells in wines not affected by cork taint. How come? Apparently because a wine can be contaminated by other materials in a winery, such as oak barrels and wooden roofs. This environmental taint, known as TBA, is indistinguishable from TCA, says Chatonnet, so it's possible to find a "corky" taint in a wine with a screwcap. Yet more fetid features identified by the fragrant Chatonnet include volatile acidity that turns a wine vinegary and sorbic acid that gives off geranium odours.

Did I mention that M. Chatonnet had come to London under the auspices of Amorim, the cork manufacturer? Should we suspect him of being an undercover agent for a cork industry determined to reverse the tide of bad publicity? I don't think so. He didn't try to defend the problems of cork taint, which he fully admitted. What he suggested, though, was that the finger of suspicion that normally points to cork as the source of a wine's problem may not always be pointing in the right direction. So far, so foul. Yet however scientific the analysis, one person's gamey will be far too farmyardy for another. Most of us are pretty tolerant. Which is perhaps just as well if the Bordelais boffin is right.