The word 'botrytis' is one that you'll find from time to time on wine labels. It sounds like some kind of disease and in a way that's exactly what it is. It's the name of a fungus; a small, yeast-like variety that makes its living by infecting the skins of grapes under certain climatic conditions. It makes the grapes shrivel by making the skins prone to evaporation, leaving them shrunken and covered with a furry mould. Perhaps it was desperation that led someone somewhere to look at their botrytis grapes and go ahead and make a wine anyway, despite their appearance. The strange thing is that you end up with a great wine.

The French, who were probably the first to discover this, call the fungal infection 'la porriture noble' or the noble rot, a description that includes the word 'noble' because of its beneficial effects. What the evaporation caused by botrytis does is concentrate the grape sugars. This means that when you press the grapes there's very little liquid, but what there is, is intensely flavoured and high in sugar. But what it doesn't mean, and this needs to be said, is that you obtain simply a sweet white wine. What you obtain is a wine that is sweet, but with a sweetness balanced by acidity, intensely perfumed, rich in tastes and flavours and in some cases, such as Chateau d'Yquem from the Sauternes, you get one of the greatest wines that are made in the world.

Perhaps the best known wines that are made in this way are the French ones; those from Sauternes and Barsac in the Bordeaux. The Semillon grape, grown traditionally in these areas, is more subject to botrytis than others, which may explain why the style evolved here. But the climactic conditions that allowed these areas to experiment with botrytis grapes can be found elsewhere in the world, as can the Semillon grape, so increasingly these wines can be found from other countries. For the grower it's a high-risk business; not every year has the right conditions and bad weather is a constant threat to the late harvesting of the grapes. For the consumer it's one of the wine world's bargains - the cost of making it is rarely reflected in the final price.

There's no doubt that the fashion for dessert wines has waned, probably because dessert wines and unsubtle sweet German table wines have got confused. That's a shame, because a the sweetness produced by botrytis and the sweetness produced by less expensive means are as different as chalk and cheese.

Wine of the Week

Beaumont Goutte d'Or 1997

This is a South African variant, whose name translates to 'a drop of gold'. It has all the richness and intensity of flavour and like all wines of this style, is best savoured in small quantities. Although wines like this are nearly always produced as an accompaniment to desserts, when chilled they are a good match for fatty foods, for example foie gras, with which it makes a wonderful combination.

Available from Berry Bros. & Rudd, €16.44 (37.5 cl)