Threat: Diastatic Yeast and Cross-Contamination

What is diastatic yeast?

Diastatic yeast has high attenuation ability. And that’s not because it is more efficient at metabolizing more kinds of wort sugars than other yeast (kind of the opposite, interestingly). Diastatic yeast achieves high attenuation by sourcing fermentable sugars (mostly glucose) from starch using its own amylase. It doesn’t have to rely on the diastatic power that malt enzymes like alpha- and beta-amylase provide in wort. That means it can sit down to dinner in places a brewer wouldn’t want it to, like in finished beer.

Production of the glucoamylase enzyme — encoded for by yeast’s STA1 gene — makes diastatic yeast strains unique among beer brewing strains. Glucoamylase is a ​“diastase,” which in Latin just means something along the lines of ​“enzymes that are able to break down starches” (it’s where the word ​“diastatic” comes from). Strains that have the STA1 gene are referred to as ​“STA positive,” STA+, or STA1+, etc. You’ll see this noted in yeast strain descriptions as an important heads up that these strains have special diastatic ability.

A common misconception

A common misconception is that diastatic yeast is more resilient to cleaning protocols than other yeast. The truth is it’s eradicated by the same methods that get rid of non-diastatic yeast. A cross-contamination with diastatic yeast is just more noticeable because of the consequences — it is more likely to spoil your beer than another cross-contaminating ale strain, due to hyper attenuation and/or phenolic off-flavors (most diastatic strains are phenolic (but not all).

The threat

Though brewers use diastatic yeast strains intentionally for high attenuation and/or unique flavor and aroma characteristics, like in saison, for example, if a brewer is not expecting and accounting for diastatic activity, as would be the case with an accidental cross-contamination, the extra fermentation this yeast is capable of causing can be a dangerous surprise; it’s not only a potential embarrassment, or expensive product loss, it’s also a legitimate safety hazard because of the increased pressure in packaging. It’s critical that a brewery protects itself against its (unintentional) presence.

You may be more vulnerable that you think, especially if you’re not:

• Using multiple, validated detection protocols. A combination of both PCR-based detection and microbial plating is helpful in balancing possible false negatives and positives. 
• Optimizing your growth media for reliable results. Standard LCSM recipes have some surprising gaps which overlook commonly used diastatic brewing strains.
• Aware of new tests that detect diastatic activity directly (not via copper resistance) There’s more available to do than what is typical best-practice.
• Considering to what degree a contaminating strain is diastatic. Did you know there’s variation in diastatic strain intensity? Knowing more about diastatic yeast overall can go a long way for improving your protection.