Botrytis Cinera, a fungal infection can be bad news to a viticulturist, but interestingly, infected grapes can sometimes be desirable for their use in dessert wines. In these cases, it is referred to as “Noble Rot.” The difference between spoilage and nobility here really just depends on the weather conditions late in the growing season.
Botrytis loves moisture and therefore, if there’s too much rainfall or soupy, foggy weather while there are grapes on the vine, botrytis can set in. It is a fungus that necrotizes (eats) flesh, but requires a wound to the berry in order to get in. Therefore, it is more likely to really take hold later in the season, once the berries have sugar in them. Birds and insects come into the vineyard, try to snack on the berries, and then if there’s lots of moisture, the botrytis gets in through the little bite marks.
If things then stay wet in the vineyard, there’s going to be a likely problem with the crop as a white hairy layer will cover the clusters of grapes and the fungus will start to eat the berries' tissue. There ARE different ways in which viticulturists will manage and try to prevent botrytis. The specifics depend on whether the vineyard is sustainable (most are heading this way,) organic, or biodynamic.
However, if after the initial infection, the weather dries out, and it is very close to or exactly at the time of harvest, then this can be a perfect recipe for Noble Rot. In this case, the botrytis will suck the moisture out of the berries, thereby condensing the sugars, acids, and other flavor components in the fruit. Some winemakers will ask the viticulturist to let the infected berries hang on the vine for several days so that they dry out even further. Basically, they’re then making wine from semi-raisined grapes.
It makes for very sweet wines with a specific “noble rot” flavor (I think it tastes a teeny bit like subtle acetone- but in a good way,) which is prized as a delicacy.
Some wines made this way (if you’re curious to try them) are Sauternes from France (probably the best known,) Tokay from Hungary, Spatlese from Germany, and Ausbruch from Austria.
Did you know?
On top of yeast (a fungus’) initial alcoholic fermentation, many wines- especially reds- undergo a second fermentation known as malolactic fermentation in which certain bacteria convert malic acid into lactic acid. This lactic acid has a smoother feel and can take the bite off of a too sour wine.
Fungus, bacteria- yum!