Heather Carbon

Food science graduate student Heather Carbon examines naturally occurring winegrape yeasts as part of researchers efforts to reduce sugar and improve wine flavor.

Washington State University researchers are hoping naturally-occurring yeasts on wine grapes can consume residual sugar without damaging the quality of the product.

Their research is funded by the university, Auction of Washington Wines and state wine grape growers and wineries through the Washington State Wine Commission.

Musts — the mixture of crushed grapes sitting in a tank — entering the winery have higher levels of sugar. The goal is to improve the flavor profile of the wines by letting grapes ripen for longer periods of time, said Charles Edwards, WSU professor and food scientist. The higher sugar concentrations mean the wine will naturally yield a higher level of alcohol. Too much alcohol can negatively affect wine flavor.

Wineries use different techniques to reduce that, including adding water, Edwards said. They wanted a different approach, inspired by research from Australia, involving the yeast that naturally occurs on grapes. The yeast metabolizes the sugar, but doesn't yield alcohol, unlike the fermentative yeast.

WSU is part of a project started by retired WSU plant pathology professor Dean Glawe, looking at the yeasts present on grapes, identifying more than 200 isolates. The university is screening them, Edwards said.

The yeast are microorganisms invisible to the naked eye until they grow a large enough mass, or mold, he said. 

Historically, many of the native yeast species present on grapes were thought to be the cause of a spoiled batch of wine, according to WSU. Researchers now know the yeasts can be used to enhance wine quality without spoilage issues.

The winery would likely apply the yeasts to the tanks when the must is prepared, adhering to safety guidelines and permitting.

Wineries purchase yeast cultures commercially, similar to bread yeast available in a store, Edwards said.

If the researchers are successful, Edwards said, the naturally-found yeasts could possibly be sprayed on the grapes before harvest, changing the micro-flora in the vineyard.

"There are literally hundreds of different yeasts found on an individual grape berry," he said. "It depends on the environment, the location, so many different factors. If you could change the micro-flora so that the more numerous yeasts are the desirable ones, then you've really changed the art of winemaking."

The researchers are focusing on two species that seem to have the biggest influence on the reduction of alcohol. Researchers are examining them for optimal temperature for survival and sulfite sensitivity. 

Edwards hopes to have a sense of optimal conditions and what winemakers might expect using the particular yeasts within several years.

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