Mousey. Wet Dog. Burnt plastic. Barnyard. When wine fans hear these terms, they invariably think of a flawed wine. But what about cedar, tamarind, lavender and smoked meat? Would you associate those aromatics with something as sinister sounding as a “spoilage yeast”?
According to new findings by the enology department at the University of California at Davis, all those aromatics have one thing in common: they can come from yeasts in the genus brettanomyces. Brett, as it is often referred to, has become synonymous with unwanted characteristics in wine, implicated as the downfall of many a bottle. The unpleasant aromas and flavors result from certain compounds associated with brett, especially 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG). Once these two chemicals are produced during fermentation, they combine with other elements in the wine to produce off tastes and odors.
But according to the UC Davis team, brett yeasts can contribute aromas and flavors wine lovers enjoy, too. Linda Bisson, professor of yeast microbiology and functional genetics at U.C. Davis, and her colleague Lucy Joseph released a brett aroma wheel in January. The wheel is the result of a study the two conducted on a collection of 83 strains of brettanomyces, the genus that covers a number of yeasts, from the common B. bruxellensis, an integral and welcome component of any Belgian beer, to the more exotic UCD615, which prefers white wine. They identified 17 strains of brett yeast as positive and five as negative.
The aroma wheel breaks down aromatics into categories such as putrid, spicy, fruity and animal, and then into more specific identifiers like marshy, black pepper, tamarind and wet dog. Bisson put the wheel to the test by buying 36 bottles sold at K&L Wine Merchants in California whose online tasting notes used descriptors that appear on the aroma wheel but which were not described as flawed or "bretty." She then tested the wines for signs of brett. “All were positive for some Brett or lactics. Only 19 could be shown to have viable Brett; the other 17 had viable Lactobacillus/Pediococcus. Brett is commonly found together with the lactics so it makes it challenging to determine which organism made the character.”
So does this mean winemakers can wholeheartedly employ brettanomyces in their fermentation regimen? “I would say for most people the positives are not outweighing the risks," Bisson told Wine Spectator. "Brett is hypermutable and hypervariable. We think this variability in off-character expression is also due to masking effects in some wines that we and others have seen, but why and how the off-characters get masked is not known."
Ask a room full of winemakers about brett, and some will say it must be avoided at all costs, while others say it can add positive elements to a wine in low amounts. "There are winegrowing regions and styles where moderate brettanomyces contributions seem to be important to the traditional character of those wines and they are not over-the-top spoiled," said Bisson. "The bottom line is brett is not a yeast to be trusted. The negative characters can be overpowering.”
Brettanomyces is hard to control. It can lurk in vineyards, or it can find its way into barrels or other storage containers and thereby contaminate an entire winery.
Should winemakers shun the yeast altogether? “I think that ultimately we will understand enough about this yeast to be able to use it with confidence and control its activity in the winery," said Bisson. "I liken it to the malolactic fermentation, which was considered a spoilage fermentation 60 years ago. Now the malolactic signature compounds are important to specific styles of wine.”
The new aroma wheel for wines partially fermented by Brettanomyces yeast (Click to expand view.)