WSU News Archive

By Brian Clark, College of Agricultural, Human and Natural Resource Sciences

PULLMAN, Wash. - Every time you take a breath, you inhale fungi and fungal spores. In every cubic meter of air there are thousands of spores released by hundreds of species of fungi, according to a study conducted by scientists at the Max Planck Institute in Germany. 

No wonder, then, that winemakers are concerned about controlling fungi.

 

Wine is the byproduct of the interaction of a fungus with grapes. But not just any fungus; winemakers use special strains of yeast to get the fermentations they want. And they go to great lengths to avoid contamination by microorganisms that can ruin a fermentation otherwise destined to yield a great wine.

 

 

At Washington State University, wine microbiologist Charles Edwards and his team of graduate students are focused on controlling fermentations so that winemakers get the best product possible with the least expense and hassle.

 

"Spoilage is a threat to wine quality everywhere," said Edwards. "We are working to understand the biology and ecology of the various fungi that can affect fermentation.

 

"We work closely with the Washington wine industry to provide research that addresses the challenges they face, in the present as well as long term," he said.

 

 

Brettanomyces yeast cells (large, smooth ovals) are trapped in the long chains of chitosan molecules. (Electron scanning microscope image at 15,000x magnification)

One of the issues winemakers want to get a grip on is Brettanomyces contamination. Brettanomyces is a genus of yeast collectively - if not exactly affectionately - referred to as "Brett." The word comes from the Greek for "British fungus" because it was discovered in English ales.

 

While some strains of Brett can, in small numbers, produce desirable sensory results in fermentations, those same strains, in larger populations, can produce unpleasant results. Barnyard and rancid odors are the sensory extremes, but even less obvious effects are unpleasant to many consumers.

 

"At present, the main ways to control Brett include the use of sulfur dioxide (SO2), low temperatures, filtering, good sanitation in the winery and a chemical sold under the name Velcorin," said doctoral student Biljana Petrova.

 

SO2 limits growth of some strains of Brett, but others are much less sensitive to it. The same is true of using temperature to control the yeast. Alternatively, filtering removes microorganisms, but such filters are expensive.

 

And some strains of Brett can survive for long periods in extremely low population densities. If even a few organisms survive SO2, low temperatures or filtering, a Brett outbreak still may occur.

 

 

Bijlana Petrova is a graduate student from Macedonia working on her Ph.D. in Charles Edwards' lab.

"I'm working on control of Brett using chitosan," said Petrova. Chitosan is a naturally occurring chemical derived from the chitinous shells of crustaceans as well as other sources.

 

"It's a very interesting molecule,” Petrova said. "It has certain properties that, we theorize, enable it to adhere to the outer membrane of a Brett cell and somehow disrupt the yeast's metabolism.

 

"It also causes Brett cells to agglutinate - to stick together and sink to the bottom of the fermentation tank,” she said. "Then the winemaker is able to rack off the liquid and leave the wine behind."

 

 

Petrova said chitosan already is approved for use by winemakers in Europe to reduce heavy metal content and the hazing those metals can cause in wine.

 

"The company that sells it there wants to register it for use in the U.S. They need scientific data to demonstrate its safety and efficacy in order to get it approved here," she said.

 

Chitosan might just pack a double punch against spoilage organisms.

 

"The bacterium Acetobacter is another spoilage organism,” Petrova said. "It converts the alcohol in wine into acid and is used to deliberately turn wine into vinegar. But it can also destroy wine. It would be great if winemakers had one safe and relatively inexpensive compound to treat two problems."

 

Petrova's research is supported in part by a Fulbright scholarship. In her native Macedonia, she conducted research on yeast ecology during wine fermentation.

 

"Even without adding Saccharomyces" - the genus of yeast preferred by brewers and winemakers - "the wild yeasts will finish the fermentation in about the same amount of time, five to 10 days," she said. But the results are wildly unpredictable, she said, and often undesirable.

 

"Macedonia is a small country with an ancient winemaking tradition but only a small scientific community," she said. "So I was thrilled to be able to come to Washington to study with Dr. Edwards. Wine microbiology is an exciting area of research and, with so many new wine regions emerging, this work is applicable all over the world."

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