Noble Rot Infects Some Of The World’s Greatest Dessert Wines
Bunch rot is the bane of the winegrower’s existence. However, one fungus, Botrytis cinerea or noble rot, is required to make some of the world’s most delicious and expensive dessert wines. Botrytis bunch rot is responsible for annual crop losses of 1-10% in California. However, proper climatic conditions transform this lowly bunch rot into “noble rot”. Noble rot dehydrates and shrivels the healthy grapes, providing the raw materials for the decadently sweet Sauternes of France and Germany's exotic Trockenbeerenauslese, Berenauslese and Rieslings, as well as others.Merlot and Cabernet Sauvignon are thick-skinned, loose-clustered varieties and are therefore less prone to rot.
The fungi are a fascinating group of living organisms. 100,000 species have already been identified and twice that number may exist.
One fungus, Penicillium roqueforti, was first found in caves near the French village of Roquefort and, as the story goes, cheese left in a cave a few weeks was later found to have acquired a tart, pungently fragrant character from the fungus infection. Today, only cheeses from around these caves can use the name Roquefort. Similarly, the fungus Penicillium camemberti gives Camembert cheese its unique flavor.
What’s this got to do with wine?
There’s a group of over 50 different parasitic fungi that grow on grapes, producing what is called bunch rot. Growers spray to avoid bunch rot, and moldy grapes account for considerable crop loss each year. Bunch rot organisms emit unpleasant odors. Sour rot, a bacterial/bunch rot complex, produces acetic acid imparting a vinegary smell. Aspergillis growth gives off a “smelly sock” odor, while Penicillium smells just plain rotten, making these grapes unsuitable for winemaking. These three common bunch rots are secondary pathogens, growing due to damage where grapes split naturally, or from growing too closely together in a cluster.
The Botrytis spores are present in the vineyards throughout the year, remaining dormant until proper weather conditions of cool temperatures and high humidity prevail. Not only must specific weather conditions prevail, but they must do so at the right time of the season. The rain must arrive when the grapes are at full maturity of 22-25% sugar. Below 20%, there is too much acidity in the grape. The morning fog from a nearby body of water can settle on the grape surface initiating an “infection”. The fungus pokes minute holes in the berries’ skin allowing water to evaporate, causing shriveling. Hot weather (above 90 degrees F.) will dry up the infection and cause raisining. However, continued wet weather or high humidity following the initial onset of Botrytis results in a variety of fungal infections without the dehydration of the fruit. After this cycle of morning sun and afternoon fog continues for 3-10 days, grapes dry and shrivel into what resemble fuzzy raisins. Clusters or even single berries infected with noble rot are harvested individually, hence the term “special select late harvest”.
Favorable temperature and humidity make further noble rot occur and successive harvests are made. The selection process is perhaps the most important step because when the conditions are right for Botrytis they are also right for undesirable bunch rots. The infection must be clean Botrytis because other bunch rots have a mustiness or moldiness like stinky cheese. Botrytis (noble rot) flavors are best when they are in harmony with ripe fruit flavors.
Noble rot produces the “Nectar of the Gods”
An attribute of these late harvest wines is their dark color.Botrytis mold (noble rot) produces an enzyme that causes browning, giving the wines their yellow color, which intensifies to a golden hue as they age. Dessert wines made from botrytized grapes have considerably greater aging potential than table wines. Like Madeira and Port, the high sugar concentration preserves the wine against microbial growth as it ages in the bottle or after it is opened. From the seductive aromas alone the taster can derive pleasure from simply smelling the rich apricot and honeysuckle perfume of a late harvest wine. Intense sweetness followed by the scintillating jolt of acidity can make the wine drinker’s mouth enjoy a unique pleasure. Why else have these wines been given the accolade “nectar of the Gods”?
California's San FranciscoBay provides both the cool temperatures and high humidity necessary for Botrytis. While California is dry in terms of rainfall, certain areas in Napa, Sonoma and Mendocino counties get fog and cloud cover until due to the marine influence. This fog, thick enough to be mist, rests on the grapes stimulating Botrytis growth and the “noblest of rots”.