MICROBIAL ECOLOGY DURING VINIFICATION

(NWW) - Wine Spoilage Yeasts

Kloeckera apiculata is the most common occurring yeast species on mature grapes. Other yeasts that may be isolated from grapes and must include species of Brettanomyces,Dekkera (the sporulating counterpart of Brettanomyces), Candida, Pichia, Hansenula andTorulopsis (Lafon Lafourcade, 1983). Diseased and damaged grapes have significantly higher populations of spoilage yeasts that can affect the outcome of the fermentation. These yeasts can metabolize the sugar in the grapes (which contain 160-240 g/L) mainly in the form of glucose and fructose. These yeasts can result in strong off-flavors and may be tolerant to sulfur dioxide.

Kloeckera

Kloeckera apiculata has been reported to grow in the presence of up to 150 mg/L of SO2. It is also cold tolerant (10-15°C) and is able to survive until the end of fermentation. This can result in thick scum formation, ethyl acetate and amyl acetate (banana skin smell) formation. Its presence depletes nutrients needed by Saccharomyces cerevisiae for a successful fermentation. Kloeckera apiculata and its counterpart Hanseniaspora are fermentative yeasts and can grow in abundance early in native fermentations. They can represent the dominant species in unsulfited juice and must.

Using microscopy, if more than 5-10 non-Saccharomyces yeast are observed in a field under 40x magnification prefermentation, there will most likely be problems with the fermentation.Kloeckera has a distinct morphology when viewed using a microscope. It is described as lemon-shaped due to repeated budding at both poles. It will grow in 1-2 days in culture whileBrettanomyces requires 3-7 days to grow.

Brettanomyces

The most common form of yeast spoilage is due to Brettanomyces bruxellensis. This yeast produces volatile phenols and acetic acid. Examples of flaws include aromas described as “medicinal” in white wines and “leather” or “horse sweat” in red wines. Other aromas descriptors include barnyard, wet dog, tar, tobacco, creosote, plastic and band aid.

Brettanomyces can infect red wine 6-10 months after barreling and can spoil bottled wines as well. It can also be transmitted by fruit flies. It also grows on the disaccharide cellobiose, a by-product of toasting in barrel production. Control of this yeast is difficult due to its tolerance to sulfur dioxide.