What is the STA-1 gene in some yeast strains?

Posted by Dean Rowan on

STA-1 is a marker in yeast that indicates it has mutated or become a diastaticus variant of the normal Saccharomyces Cerevisiae. Once detected in a yeast strain, the strain is then properly identified as Saccharomyces cerevisiae variant Diastaticus

What is Diastaticus?

The presence of the STA1 gene causes the yeast to secrete glucoamylase, an enzyme which hydrolyzes dextrins and starches into fermentable sugars. The yeast is now a “hyperattenuator” as it ferments beer beyond what ordinary brewer’s yeast is capable. It's often considered to be a beer spoiler or “wild yeast”, as it causes unwanted secondary fermentations in packaged commercial beer.

The trouble stems from the slower rate at which the dextrins and starches are fermented as a result of the glucoamylase enzyme. Ordinary brewer’s yeast cannot utilize these complex carbohydrates, which remain in the beer once the primary fermentation completes, but diastaticus yeasts can, over time, ferment these sugars.

Beer contaminated with even a small amount of S. diastaticus will first ferment normally with the expected level of attenuation. After packaging, the secreted glucoamylase will continue to break down these complex carbohydrates into glucose over the subsequent months. The glucose will then be fermented by all yeasts present, producing alcohol and CO2. Since the beer is already packaged, the generated CO2 causes over-pressurization, leading both to gushing and the more dangerous effects of package failure.

The rate of the secondary fermentation depends both on the storage temperature and amount of contamination. Beer stored warm may begin to over-pressurize in as little as two weeks, while it may take many months for problems to be seen in cold stored beer.

As a result, this issue is of much greater concern to professional brewers whose product has a long shelf life, and needs to be stable for that entire time. Homebrewers, on the other hand, generally consume their beer in a shorter period of time, and tend to store it cold, which slows the secondary fermentation. Still, yeast manufacturers such as Imperial will indicated which strains are STA-1 positive, and we have reproduced those notices on our product pages (A24 Dry Hop)


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