Stuck fermentation triggered by prions

Scientists have discovered the cause of “stuck fermentation” which will enable winemakers to avoid the problem in the future.

No fermentation, no wine or beer.

According to Science 2.0 a team of researchers has discovered a biochemical communication system behind the problem of “stuck fermentation” encountered by winemakers wherein yeast which should be converting sugar to alcohol and carbon dioxide prematurely shuts down, leaving the remaining sugar to be consumed by bacteria risking spoilage.

The problem has been studied extensively and, for years, biologists have been aware of an ancient biological circuit based in the yeast cell membranes that blocks yeast from using other carbon sources when the sugar glucose is present.

The circuit, known as “glucose repression”, is an efficient processor of sugar and particularly strong in the yeast species Saccharomyces cerevisiae, enabling practical fermentation processes with applications in winemaking, brewing and bread making.

Working through a prion, an abnormally shaped protein capable of self reproduction, the system enables bacteria in fermenting wine to switch yeast from sugar to other food sources without altering the yeast’s DNA.

According to Linda Bisson, a professor in the Department of Viticulture and Enology at U.C. Davis: “The discovery of this process really gives us a clue to how stuck fermentations can be avoided.”

“Our goal now is to find yeast strains that essentially ignore the signal initiated by the bacteria and do not form the prion, but instead power on through the fermentation,” she said.

The researchers found that the glucose repression circuit is sometimes interrupted when bacteria jump-start prion reproduction.

The resulting interference causes the yeast to process carbon sources other than glucose that dramatically slows down the fermentation process until it becomes “stuck”.

The study will enable better control of the fermentation process.

“This type of prion-based inheritance is useful to organisms when they need to adapt to environmental conditions but not necessarily permanently,” Bisson said.

“In this case, the heritable changes triggered by the prions enable the yeast to also change back to their initial mode of operation if environmental conditions should change again.”

Through the study the researchers were able to demonstrate that the process leading to stuck fermentation benefits both the yeast and bacteria.

As the sugar metabolism slows, conditions in the ferment become more conducive to bacterial growth and the yeast benefits by gaining the ability to metabolise additional carbon sources to glucose thus maintaining and extending their lifespan.

With a clear understanding of this process winemakers should now be better able to avoid stuck fermentations.

“Winemakers may want to alter the levels of sulfur dioxide used when pressing or crushing the grapes, in order to knock out bacteria that can trigger the processes that we now know can lead to a stuck fermentation,” Bisson said.

“They also can be careful about blending grapes from vineyards known to have certain bacterial strains or they could add yeast strains that have the ability to overpower these vineyard bacteria.”