This week I take a look at the yeast variant diastaticus more formally known as saccharomyces cerevisiae var. diastaticus. This is a form of wild yeast that can lead to the unwanted extended fermentation of sugars in a finished beer.
The Diastaticus Problem
As mentioned above diastaticus is a genetic variant of the normal Saccharomyces cerevisiae (common brewing) yeast. It is commonly found in the wild and around many breweries, so it is fairly easy to get cross contamination in a typical brewing environment, and is also sometimes present even in certain brewing strains.
The main problem with diastaticus variants is that many of them will ferment residual carbohydrates that a normal cerevisae strain would not ferment, and also can do it over an extended period. This can unfortunately lead to fermentation after bottling or kegging creating issues with stability, over-attenuation, changes in flavor, over-carbonation and even bottles bursting.
A yeast is classified as a diastaticus variant if it carries a “signature” gene called STA1. So all diastaticus variants carry the STA1 gene, though not all strains will express the gene resulting in the extended fermentation issue above. Still it is known that if a yeast strain has the STA1 marker gene, it is capable of developing the stability and over-attenuation problem given the right environment. This article was derived from a wyeast article found here if you want to read more about the details on diastaticus and STA1.
The Root of the Problem
Diastaticus is common in many wild yeasts, so you can have contamination at the yeast supplier or brewery. The wyeast article linked above notes that a study (Meier-Dӧrnberg, Jakob, Michel & Hutzler, 2017) in Europe found that only a single contamination of STA1 occurred at a yeast supplier, and recently yeast labs have been taking extra steps to test for an prevent diastaticus contamination.
In 92% of the cases, the breweries themselves were the cause of contamination with the bottling/filling area making up the vast majority (71%). Most of the remaining issues (29%) came from the brewhouse, fermentation cellar and storage cellar.
Diastaticus Solutions
Yeast labs, now being widely aware of the issue have implemented additional steps to test for and eliminate diastaticus contamination in their strains. In addition, the major yeast labs have started publishing STA1 as part of their yeast data – typically listed simply as “STA1” positive or negative. A yeast that is STA1 negative does not have the gene, and cannot express itself in the unwanted fermentation of complex carbohydrates.
However as diastaticus is common in wild yeast strains, and over 90% of the cross contamination takes place in the brewery, both home and commercial brewers need to be aware of the issue and take additional steps to ensure proper cleaning and sanitation. Bottling and filling areas are of a particular concern and small items like fillers, hoses, kegs and bottles need to be meticulously cleaned and sanitized. However the fact is that contamination can occur anywhere on the cold side of brewing to include chilling, fermentation, aging, and storage. Pasteurization is often used for commercial beer to kill off any yeast in the finished product.
Proper care and pitching if yeast is also important. While yeast providers are taking extra precautions now, you can easily cross contaminate when making a starter or pitching your yeast. Those reusing yeast from one batch to another must be particularly careful as it is easy to get diastaticus in your sample when harvesting or propagating yeast.
I hope you enjoyed this week’s article on diastaticus. Thanks for joining me on the BeerSmith Home Brewing Blog. Be sure to sign up for my newsletter or my podcast (also on itunes…and youtube…and streaming radio station) for more great tips on homebrewing.
