Fungus can be a friend or foe. Without fungus there would be no athlete’s foot or mildewed carpets, no green furry bread or mystery food in the back of the fridge. But there would also be no penicillin or blue cheese, no mushrooms...or wine! Just as in the real world, the wine world has good fungi, bad fungi, and just plain ugly fungi. Here are a few ways the fungus among us shapes wine for better or for worse.
Brewer’s yeast, or Saccharomyces cerevisiae, is the primary agent responsible for transforming grape juice into wine. Though S. cerevisiae is the most common yeast in winemaking (and baking and brewing), it is rarely the only yeast species involved in a (spontaneous) fermentation. Grapes brought in from harvest are usually teeming with a variety of "wild yeast" from the Kloeckera and Candida genera, as well as Brettanomyces (which in large amounts might be categorized under "The Bad" below as a wine fault). According to the staunchly traditional Rioja winemaker Lopez de Heredia, “Yeasts are vital to the quality of the wines since they cause fermentation; these micro-organisms [sic] are typically found in the soil, and are spread over the grape skins by insects and the wind. The main microflora in La Rioja are Scharomyces [sic], Kloekera apiculata [sic] and Tomlaspora rosei [sic].”
Botrytis cinerea a.k.a Noble Rot is a necrotrophic fungus--that’s right, a parasite that kills its host and feeds on its dead matter--that causes grapes to shrivel and decay. It typically occurs in mature grapes in moist growing conditions. If conditions stay wet the fungus produces “grey rot” which destroys the grapes. However, if the infected grapes are exposed to drier conditions they produce what is known as “noble rot.” Noble rot intensifies sweetness and adds flavor. Wines produced with these grapes are known as botrytized wines. Famous botrytized wines include Bordeaux’s Sauternes, Hungary’s Tokaji Aszu, and Germany’s Spätlese.
Cork Taint is a wine defect that occurs when airborne fungi come into contact with chlorophenol compounds producing 2,4,6-trichloroanisole (TCA) or 2,4,6-tribromoanisole (TBA). Generally, the cork of a wine is responsible for this defect. It is easily identified by its characteristic smell, which evokes wet cardboard or a moldy basement. Though harmless to consume, it affects the quality of the wine, destroying the aromas and flavors that make wine enjoyable, regardless of the price or quality of the wine. If a wine is corked send it back! Restaurants and retailers will (or at least should) happily give you a new bottle free of charge.
Powdery Mildew is a fungus that lives on the green tissue of the plant, including the leaves and young berries. It can result in crop loss or poor quality wine. In organic farming practices, viticulturists often apply a light sulfur spray, which eliminates the fungus.
Black Mold is a typically innocuous but unsightly fungus found in cellars. Cellars should have about 70% humidity and with humidity comes mold. It does not affect the quality of the wine (unless it gets into bottles or barrels) but it might destroy your labels if you’re not careful!