Botrytis or botrytised are words that get used frequently, particularly with reference to sweet or dessert-style wines. So what does THAT mean?
‘Botrytis cinerea’ is a fungus. It is grey and powdery and is responsible for a number of plant diseases. As with many fungal infections, botrytis requires moist conditions to establish. If the wet or humidity continues, the damaging form, grey rot or bunch rot, can devastate a crop.
However, botrytis has a unique property which, in special circumstances, can lead to some surprising outcomes with grapes and the wine made from them.
If drier conditions follow the initial fungal infection the beneficial form, noble rot, can result. In this case, the fungus causes the loss of water through the grapes skins. This leads to raisining and the concentration of the sugars and flavour compounds held within the grapes.
Visually, the scene is not appealing. The grapes are covered in mould and really do appear rotten. But looks can be deceiving; the taste is incredible.
Because so much water has been lost and the grapes are now raisined, very little juice is yielded when the grapes are harvested and pressed. Very little juice means very little wine can be made, so consequently, wines made in this way tend to be expensive.
The juice though is so concentrated that even after primary fermentation, huge amounts of residual sugars can remain in the wine, meaning it can be very sweet indeed. But the flavours and acids are also concentrated and so balance and freshness can be retained in the wine, despite the sweetness.
Some of the most famous wines to be made in this manner are Sauternes, Tokaji Aszu and Germany’s Beerenauslese and Trokenbeerenauslese.