Under the Microscope: Dealing with Diastaticus in the Brewhouse

Determining whether a yeast is a contaminant is like deciding whether a plant in your garden is a weed—it all depends on whether you want it there. The insatiability of diastatic strains can be a danger if uninvited. Properly managed, it can be an asset.

Don Tse Dec 16, 2021 - 10 min read

Under the Microscope: Dealing with Diastaticus in the Brewhouse Primary Image

Photo: Rattiya Thongdumhyu/Shutterstock

A few years ago, diastatic yeast got an eruption of attention that occasionally spilled over into mainstream news; breweries were forced to recall beers and explain why to local drinkers. There was even a high-profile civil lawsuit, with Colorado’s Left Hand Brewing—which had to destroy $2 million in beer—accusing White Labs of selling “contaminated and defective” yeast in 2016. Diastatic yeasts have gained such a negative reputation that many breweries simply play it safe and ban them from the premises.

Of course, there is nothing new about diastatic yeast—they’ve been around and making beer for centuries. However, the proliferation of smaller breweries making a broader variety of beer styles—and often employing a wider range of yeasts—increases the likelihood of the occasional cross-contamination.

What Is Diastaticus?

Saccharomyces cerevisiae var. diastaticus are strains of ale yeast with a special ability: They secrete an enzyme called glucoamylase, which breaks down otherwise unfermentable dextrins into simple sugars. The yeast can then ferment these simple sugars, leading to a beer that is drier, stronger, and more highly attenuated.

If this happens after packaging—when the diastatic yeast are uninvited and unexpected—this additional fermentation can lead to over-carbonation and changes in flavor. Besides the sensory impact, the higher alcohol can push the beer out of regulatory spec. Over-carbonated kegs may produce excess pressure and foam, leading on-premise accounts to return them. In the worst-case scenario, cans or bottles can explode.


What determines the presence of the diastaticus variant is a gene sequence known as STA1. If a yeast strain is STA1-positive—often denoted as STA1+—then it has the potential to be a diastatic yeast. However, diastaticus is more complicated than the mere presence or absence of STA1. A better understanding of diastatic yeast can help you as brewers add it to your toolkits with confidence.

When Diastaticus Is Your Friend

When used properly, diastatic yeast can create wonderfully (intentionally) dry beer—Belgian-style saison is the best-known application, but some other Belgian ale strains also can be diastatic.

Many diastatic yeast strains also produce characteristic spicy phenolics—these are denoted as POF+ (phenolic off-flavor positive). However, strains that are STA1+ are not necessarily POF+ (phenolic), or they may produce phenolics to different degrees. For example, you could employ a diastatic yeast to ferment a tart, dry beer that lacks that spicy character. Incidentally, you can also brew a saison with the classic phenolic traits yet without diastaticus—Lallemand’s new Farmhouse strain is a hybrid developed to do just that.

Either way, if you want full attenuation and a predictably dry beer, diastaticus can be your friend.

The keys to working with diastatic yeast include airtight sanitation, careful testing, and knowing how the strain behaves.


“We’ve never switched away from using it,” says Chris Kinast, head brewer at Perennial on Lockwood, a brewpub run by Perennial Artisan Ales in St. Louis. Diastatic yeast, he says, are “necessary for saison, especially ‘clean’ saison fermented without Brettanomyces or Lactobacillus. We have learned how to wrangle it.”

That wrangling includes Perennial’s chamomile-laced flagship Saison de Lis. Through trial and error and meticulous data collection, Kinast and the brewing team know that they only need to achieve a lower level of carbonation in the tank via spunding and CO2. Then, after packaging, they know that the diastatic yeast will further carbonate the beer to the appropriate level. They use a warm room to condition the beer before it leaves the brewery, so it’s ready before it goes to market. The process is similar to what many Belgian breweries do when bottle-conditioning.

However, not all diastatic yeast behave the same; some are quicker to ferment, while others are slower. Also, as with any yeast, behavior can change depending on the beer, the temperature, and other factors. “We learned on the fly and gathered data,” Kinast says.

If you’re working with diastatic yeast for the first time, talk to your yeast supplier and other brewers using the same strain—but always gather your own data. Your brewing processes could cause different results, so develop SOPs specific to your brewery when working with each diastatic strain.

When Diastaticus Is Not Your Friend

When invited and properly managed, a diastatic strain can be your ally; when it’s not wanted or expected, it can be your nemesis. Especially if your brewery uses different yeast strains, cross-contamination should be front of mind when working with any that are diastatic.


Another area of concern: mutation. The behavior of diastatic yeast can change with re-pitching. While a STA- yeast will not mutate into an STA+ strain—if that happens it’s cross-contamination, not mutation—any STA+ strain can change the degree to which it is diastatic, either positively or negatively. That can push attenuation in either direction. (At Perennial, Kinast says he’s seen a formerly diastatic yeast lose its diastatic power and fail to properly carbonate a beer.)

Robust quality assurance and persistent data collection are the best ways to manage STA+ yeast.

Controlling Diastaticus

Confusingly, not all STA1+ yeast strains are diastatic. The key is the presence of a promoter gene sequence; if a strain’s DNA lacks the promoter gene, it will not be diastatic. Some labs are even using CRISPR gene-editing technology to remove the promoter gene from STA1+ strains, thus eliminating their diastatic properties while preserving other potentially desirable traits—such as phenolic production.

Thus, it’s important to know exactly which strains you’re getting from your supplier. Knowledge about diastaticus has increased dramatically over the past few years, and so has testing for it. If you have doubts, test your yeast.

Re-pitching yeast inevitably comes with some risk of cross-contamination, which can be minimized to the smallest possible degree. Some breweries dedicate separate sets of hoses, clamps, and tanks—or, in extreme cases, entirely separate brewhouses. Careful sanitation is obviously critical.


When Perennial goes into a tank, they remove all ports and use an extra dose of caustic on everything. Whatever can be autoclaved, is. This is followed by a heavier, longer dose of peracetic acid. All this sanitation is preventative: Nothing can reduce the risk of cross-contamination to zero, so testing is equally important.

Testing for Diastaticus

The simplest form of testing is plate testing. There are several plate media that can test for diastaticus. Using starch or copper, these media allow diastatic yeast to grow where other yeast cannot. While plate testing is simple and economic, the results can take several weeks, and false positives are possible.

The next level of testing is polymerase chain reaction (PCR) testing. PCR tests use primers to test for the presence of a specific sequence of DNA. If the DNA sequence is present, the primer will grow the sequence exponentially to the point where it can be detected. Because the primer is specific to the DNA sequence, the risk of false positives is greatly reduced.

Next, qPCR testing is an enhancement of PCR—the “q” stands for “quantitative.” A qPCR test uses florescent dye to measure not only the presence of a DNA sequence but also the amount of that DNA present. Newer tests can detect as little as a single cell of STA1+ yeast in one milliliter of wort or beer.

For example, Cleveland-based Midwest Microbio sells qPCR testing equipment designed specifically for brewers. “This technology has been out for a while,” says Aras Klimas, Midwest Microbio’s founder and chief technology officer. “But the solutions weren’t as refined and were cost-prohibitive. We created a solution that is optimized for the [brewing] industry.”

Klimas says that because many brewing processes can inhibit traditional qPCR testing, Midwest Microbio has developed multiple solutions depending on the beer being brewed. Hazy IPAs and fruit beers can throw off traditional qPCR tests, but Midwest Microbio’s system factors these issues into its procedures.

The Midwest Microbio tester is a six-inch cube. The consumables for a test—such as media and pipettes—can cost as little as a few dollars, and the results are ready in as little as 90 minutes. qPCR can be used not only to test for the presence of STA1+ and the promoter gene but also for how much of that DNA is present in the fluid. This means that breweries can ensure they are pitching the correct yeasts and that they haven’t mutated. If there is cross-contamination during fermentation, the brewery can detect it and save money by not dry-hopping or packaging the off-spec beer. Breweries also can use these qPCR tests at multiple points to help identify the source of an unwanted STA1+ contamination.

Despite the understandable trepidation among some brewers about diastatic yeast, it’s possible to manage them. They remain essential to the classic versions of certain styles. With investments in sound procedures and testing, diastatic yeast can be useful tools in variety-driven breweries of any size.

Don Tse is an internationally recognized beer writer and beer judge, working from his home base in the middle of North America’s barley belt.