How Noble Rot Produces Sweet Wine
The Role of Botrytis Cinerea in Winemaking and Viticulture
Jun 5, 2009
They say it was a brave man who first ate an oyster. However it was an even braver man, most likely a Hungarian, who first decided to harvest, vinify and then drink a wine made from the rotten, dry and shrivelled remains of grapes. Even braver, when you consider his country was on the brink of being invaded by the Otteman Empire. Since then, the role played by the necrotrophic fungus Botrytis cinerea has become synonymous with either ruining vintages or by producing some of the most sublime white wines found on the planet.
When is Noble Rot a Good Thing?
The key factors governing whether this fugal pathogen acts to destroy or dignify, depend on the mesoclimate in which the grapes are grown. For 90% of the the worlds growing regions, once established Botrytis cinerea will fervently invade the whole vineyard. It causes bunches to disintegrate and liquefy on the vine turning the grapes into a grey mess of sporolating rot, often refered to as 'Bunch rot'. For the fortunate few, invasion, spread and destruction can be kept in check by special climatic factors that can give rise to the form of beneficial Botrytis, commonly referred to as Nobel Rot.
For a grape, being infected with the fungal spores of Botrytis cinerea is a serious matter. The spores are carried by the wind and spread rapidly throughout the vineyard and if they find an entry point, the grape's had it. Damaged tissue offers perfect infection points, whereby the spore grows a network of entangled mycelia that releases pectilytic enzymes that dissolve the grape cell walls. This causes the grape tissue around the infection point to soften and disintegrate. Whilst the rot quickly spreads throughout the whole bunch, elongated branch like structures, conidiophores, are set up to release more and more spores into the vineyards, hastening the spread of infection.
So, how are some of the world most brilliant wines produced from such a devastating fungus? The secrets in the mesoclimate. Firstly there needs to be a water source near by. This is so that towards the end of summer and throughout autumn the cooling night air over still warm water will help set up dense cool morning mists. These morning mists produce the humid conditions Botrytis requires to allow the spread the infection. Secondly, so as to limit the rampant destructive growth and promote dehydration within the rotten bunches, the mists need to be burnt off and followed by warm dry afternoons. The loss of moisture is critical as it determines the quality of Botrytised fruit left on the vine. The drying concentrates the juice which is the crucial feature in the development of the sensory properties of this style of wine.
The Noble Wines of Europe
All the wines that use Botrytis cinerea come from white grape varieties, thus they avoid the problems associated with anthocyanin oxidation. Most of the varieties used, tend to mature late in the season and have relatively thick skins that are essential to hold the soft rotten bunches together.
France produces some great examples of Botrytised wines. These Range from the Chenin Blanc based wines of the Coteaux du Layon and the Quarts de Chaume in the Loire Valley to the majestic Sauternes and Montbazillac of Bordeaux's château winemakers. The styles vary according to what grape variety is used, but a common feature is the moderate levels of alcohol which commonly exceed 11%.
German wines can often be misleading giving very little indication as to the sweetness level or whether or not the grapes that went into making the wine have any Noble rot included. The Prädikat system means that fruit going into the Beerenauslese (BA) and Trockenbeerenauslese (TBA) wines do not necessarily have to contain any botrytised fruit, but they usually do. The ranking system ensures the desired sweetness and guarantees the grapes are derived from either individual bunches (BA) or individual berries (TBA). Often made from Riesling, Gewürztraminer, Pinot Gris and Muscat that in good years create phenomenal wines that balance high natural acidity against intensely concentrated sweetness. From the best producers, the very small amount made each year can exchange hands for vast sums of money and will develop in the cellar for decades.
Hungary, thought to be the home of this style of sweet wine, owes a lot of its character to the native Furmint and Harslevelu grape varieties. Here, the grapes are harvested in an advanced state of rot, often shrivelled and oozing juice from the dusty bunches. They are then placed in tubs, called puttonyos, which hold between 25kg to 30kg of grapes and trodden or crushed to release the must. The must, which is so dense and sweet it forms a paste is then mixed with a lighter, non-Botrytised base wine, in 130-litre wooden vats. The number of puttonyos added determines the sweetness of the final product and is often mentioned on the bottles label. Once mixed, it undergoes a slow and cool fermentation which can take several months to complete. A practise unique to these wine is to leave the bung slightly off the barrel following fermentation to allow a degree of oxidation to occur. This amount of oxidation often becomes the hallmark of the producers unique style.
