Killer toxins hold solution to explosive alcohol problem

Bottles of craft beer can explode when diastatic yeasts – a variant of brewer’s yeast (Saccharomyces cerevisiae) – makes its way into the wrong product by accident, causing a secondary fermentation that can spoil fresh beer by augmenting the alcohol content, changing the flavour and creating an excess of carbon dioxide.

However, proteins called killer toxins – produced naturally by many strains of S. cerevisiae –suppress diastatic strains and may help curb the issue. A group of brewers and microbiologists, led by the American Society of Microbiology, explored how these killer toxins could help brewers remedy potentially contaminated beers.

Explosive recalls

The past year has seen a number of recalls caused by alcoholic beverages exploding in the bottle, with former Top Gear host Jeremy Clarkson among those forced to pull products from store shelves​​ to curb the risk to consumers.

“If you’ve got a diastatic contamination, most of the time you just throw away the beer, and that’s expensive,”​ said microbiologist and senior author Paul Rowley, Ph.D., at the University of Idaho. “What we show in the paper is that we can add the killer yeast at the point of contamination. It’s a remediation procedure to prevent the diastatic strains from taking off.”​

Large brewers avoid the problem by pasteurising beer, but this is an expensive process that is prohibitive to craft brewers. Some brewers also worry that pasteurisation can alter the taste of their beers, according to Rhinegeist brewery microbiologist and the study’s co-author Nicholas Ketchum

Effective toxins

Ketchum and his team subjected 34 diastatic strains of yeast to Saccharomyces strains producing eight known killer toxins. The most effective toxin, K1, prevented the growth of more than 91% of diastatic strains tested.

He admitted there were still more unknowns more than knowns regarding the process – the efficacy of the toxins, for example, seemed to depend on the amount of total yeast in the mix and not just diastatic strains. Rowley is also currently investigating how widespread the problem is among small brewers.

“Yeasts are a lot more complicated than we might think,”​ Rowley concluded.