Yeasts represent a major cause of wine defects and spoilage. Moreover, since yeasts are an expected part of the natural flora of grapes and must, their growth before, during, and after the wine fermentation is difficult to control. For example, Kloeckera apiculata, one of the yeasts involved in the early stages of a natural fermentation, can produce high enough levels of various esters (mainly ethyl acetate and methylbutyl acetate) to cause an ester taint, which has a vinegar-like aroma. Once vigorous growth of S. cerevisiae begins, other yeasts are generally unable to compete and grow. However, if S. cerevisiae does not become well-established (i.e., during natural fermentation), other yeasts, including Zygosaccharomyces bailii, can grow and produce acetic and succinic acids. Growth of this organism is especially a problem in sweet wines, due to its ability to tolerate high osmotic pressure and high ethanol concentrations.
Growth of yeasts during aging of wine, either in barrels or bottles, is a particularly serious spoilage problem. The main culprits are species of Brettanomyces/Dekkera, and Bret-tanomyces bruxellensis in particular (note that Brettanomyces/Dekkera exists in one of two forms, where Brettanomyces is the asexual, nonsporulating form and Dekkera is the sexual, sporulating form of the same yeast). These yeasts are common contaminants of wineries and the oak barrels used for aging. Growth of these organisms may lead to volatile phenol-containing compounds that give the wine a disagreeable "mousy" aftertaste (although mousy taints may also be caused by bacteria).
Brettanomyces may produce a variety of other taints, variously described as "barnyard," "horse sweat," "band-aid," and "wet dog." A common marker or signature chemical for "Brett" spoilage is 4-ethyl phenol, whose presence in suspect wines can be routinely monitored. There are also various film yeasts, including species of Pichia, Candida, and Hansenula, that grow on the surface of wine during barrel aging, much like the flor yeast whose growth in sherry making is desirable. Under ordinary cir cumstances, however, growth of film yeast can lead to oxidation of ethanol and formation of acids, esters, acetaldehyde, and other undesirable end products. Most of these spoilage yeast can be controlled or managed by SO2 addition, maintenance of proper anaerobic conditions (topping off of barrels), barrel management, and good sanitation practices, or in extreme cases, by sterile filtration.
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