September 12, 2024

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by American Society for Microbiology

When diastatic strains of Saccharomyces cerevisiae, or brewer’s yeast, land in the wrong place, they can become a craft beer brewer’s nightmare. Diastatic yeasts are variant strains of yeasts that secrete glucoamylase, an enzyme that can break down dextrins into simple sugars, which can spoil fresh beer by augmenting the alcohol content, changing the flavor and in worst-case scenarios, making bottles explode.

One potential solution to yeast overgrowth in beer production may be found in a recent study published in Applied and Environmental Microbiology. Researchers, including brewers and microbiologists, discovered that certain proteins called killer toxins, naturally produced by many varieties of S. cerevisiae, can suppress problematic diastatic strains, offering a way to manage the issue effectively.

“Typically, if there’s a diastatic contamination, the entire batch of beer has to be discarded, which is costly,” explained microbiologist and study lead Paul Rowley, Ph.D., from the University of Idaho. “Our study demonstrates that introducing killer yeast at the point of contamination acts as a remediation strategy, preventing the diastatic strains from proliferating.”

Diastatic strains of S. cerevisiae are critical in crafting Belgian-style saison beers known for their higher alcohol content. Challenges arise, according to Rowley, when these strains inadvertently become part of the brewing process for pale ales and other beer types, triggering unwanted secondary fermentation. Despite stringent monitoring practices at breweries, these strains can occasionally go undetected.

Rowley notes that these diastatic variants are difficult to distinguish visually on an agar plate. “They look identical to non-diastatic types,” he explained. The significant difference lies in a genetic change in the diastatic yeasts which enables them to break down residual starches more effectively. “In appearance, yeast remains yeast,” he added.

Large breweries sidestep contamination issues by pasteurizing beer, although it’s costly and some small brewers claim it alters the flavor. Nicholas Ketchum, a microbiologist at Rhinegeist brewery in Cincinnati, Ohio and co-author of a recent study, oversees monitoring diastatic contamination there.

The initiative started when Ketchum was teaching applied microbiology and brewing at a community college in Cincinnati. While preparing a lecture about wild yeasts and killer toxins, he conceived the idea that these proteins could be a cost-effective solution to diastatic contamination. He conducted initial tests, shared early results at the World Brewing Congress in 2020, and later discussed the findings on a beer brewer’s podcast, which intrigued Rowley.

In Rowley’s laboratory, researchers including Victor Zhong and Ximena Garcia tested 34 diastatic yeast strains against Saccharomyces strains that produce eight different killer toxins. The most potent, K1 toxin, halted growth in over 91% of the diastatic strains tested.

Ketchum mentioned the next objectives involve understanding the mechanism better and devising ways to make it applicable for craft breweries. He noted that there’s a lot still unknown about how the process works, highlighting that toxin effectiveness may vary based on overall yeast concentrations, not solely on diastatic types. Additionally, Rowley is exploring the prevalence of this issue among smaller breweries.

The potential relationship between the amount of yeast and the effect of toxins is also under examination by Rowley’s team.

Rowley continues to investigate killer toxins, which haven’t been well studied. “Yeasts are a lot more complicated than we might think,” he said.

More information:

Applied and Environmental Microbiology (2024). doi.org/10.1128/aem.01072-24

Journal information:

Applied and Environmental Microbiology

Provided by

American Society for Microbiology

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