Noble rot

Botrytis cinerea punctures the grape skin. Water evaporates through the holes while sugars, acids and flavour compounds stay behind and concentrate.

It also creates aromas the grape never had — honey, saffron, ginger, dried apricot — which is why botrytised wine tastes distinct from wine merely made from very ripe grapes.

It requires damp, misty mornings followed by dry, sunny afternoons. Without the drying phase the same fungus becomes destructive grey rot.

Because affected berries develop unevenly, picking is done in repeated passes over weeks, selecting only what is ready. Yields are minute, which is why these wines are expensive.

Botrytis cinerea is a fungus, and the same organism produces two completely different outcomes depending on the weather.

In damp conditions that stay damp, it becomes grey rot: it destroys the fruit, produces mouldy off-flavours and ruins the crop. In conditions where humid mornings alternate with dry, sunny afternoons, it becomes noble rot.

Noble rot punctures the grape skin with fine filaments, allowing water to evaporate while sugar, acid and flavour compounds concentrate in what remains. It also metabolises some of the grape's acid and produces its own compounds — the honey, saffron, ginger and dried-apricot character that no other method gives.

The alternating cycle is why the great botrytis regions are all beside water that generates morning mist: the Ciron and Garonne at Sauternes, the Layon in Anjou, the Bodrog and Tisza at Tokaj, and the shallow Neusiedlersee in Austria.

Because rot spreads unevenly, picking is done in repeated passes over several weeks, selecting only affected berries. Yields are minute, and that — rather than oak or ageing — is the principal reason these wines are expensive.