Back in 2016, Left Hand Brewing Co. had a problem with their Milk Stout Nitro beer: customers were complaining about its fizzy mouthfeel and off-flavors.
Then their bottles started exploding.
It didn’t take long for them to connect the dip in quality to the volatile bottles. The missing link was a secondary fermentation taking place post-packaging, resulting in an off-tasting, overly-carbonated, and occasionally explosive beer.
At its best, over-fermentation is a quality control nightmare for breweries. At its worst, it’s like a Molotov cocktail, except, in this case, sugar rather than petrol is the accelerant, and yeast, not fire, causes the explosion.
The underlying issue for Left Hand Brewing Co. was yeast contamination. The culprit? A particular strain of Saccharomyces cerevisiae. In a nightmare turn of events, the fermenter had become the contaminator. This duality is known as a conditional spoilage, and it demonstrates the true complexities and nuances of the microbial world.
What made this particular strain a conditional spoilage organism?
Two words: diastatic activity.
In short, diastatic yeast can liberate sugars from the starches found in beer. This sugar is then fermented into excess carbon dioxide (CO2), off-flavors, and increased alcohol content.
Inconsistent nomenclature
In literature, it’s not uncommon to find diastatic yeast incorrectly identified as Saccharomyces diastaticus; this is problematic because it indicates that they are not members of the cerevisiae species. Likewise, they’ve been referred to as S. cerevisiae var. diastaticus. Although less erroneous than Saccharomyces diastaticus, it’s technically incorrect because it suggests there is a single type of diastatic strain, while there are many diastatic brewing strains that have a lot of differences beyond diastatic activity. Dr. Pat Gibney, a food science professor at Cornell University, compares this logic to “creating a dog breed called ‘small,’ and grouping all the different small dog breeds into it.”
Diastatic activity parallels another brewing descriptor: flocculation, which is the tendency for yeast cells to aggregate and form multicellular masses. Like flocculation, diastatic activity is merely a trait of yeast strains, rather than a species or strain variant.
Diastatic activity within the S. cerevisiae family
Interestingly, outside of the brewing industry, this trait is almost non-existent in the overall S. cerevisiae population, with the exception of a small group of isolates found in French Guiana (Krogerus et al. 2019).
This kind of locality may sound somewhat ironic at first; after all, why is this potential spoilage organism virtually only found in beer? This question returns us to the idea of conditionality. Diastatic yeasts are the fermenters of choice for specific beer styles, like Belgian-style beer, whose dry finish is the product of this activity.
The nitty-gritty details
Knowing that specific beer styles are more susceptible to diastatic yeast spoilage, it shouldn’t be surprising that beer composition plays a role. For example, beers with higher dextrin content are more vulnerable to the adverse effects of diastatic yeasts. Dextrin is a mixture of glucose chains found in starch and is an added ingredient in certain beers because of its desirable impact on the mouthfeel.
Diastatic yeasts can secrete an enzyme known as glucoamylase outside of the cell. Enzymes are proteins that help aid in chemical reactions. In this case, the glucoamylase enzyme can liberate glucose molecules from dextrins in the beer, resulting in hyper attenuation, otherwise known as excessive sugar consumption by the yeast.
As the yeast consume the free glucose molecules, the beer is fermented for a second time, resulting in an unwanted increase in alcohol and carbon dioxide.
It’s the excess carbon dioxide that is responsible for gushing beer and potentially exploding containers.
All yeasts have this glucoamylase activity, but what makes diastatic yeast unique is a gene fusion of the enzyme with a secretion signaling protein.
The secretion protein allows glucoamylase to be secreted out of the cell and into the beer. What this means is that glucoamylase is only capable of creating problems when it’s roaming free in the beer.
How do we know we have a diastatic problem?
There are a few detection methods available for identifying diastatic yeast contamination. The primary method utilizes PCR detection of the STA1 gene. However, there are limitations to this method; namely, the STA1 gene isn’t always responsible for the undesired activity, and PCR is prone to human error.
For smaller facilities where PCR may not be a viable option, selective media, like FastOrange Wild Yeast Agar and FPDM, are available. However, selective media is “not entirely conclusive nor rapid,” and the 200 µL plating volumes may not be sufficient for detection.
The Gibney Lab at Cornell University recognizes the need for better detection techniques and teamed up with a brewing yeast supply company in Chicago called Omega Yeast Labs to work on new methods to aid the brewing industry in preventing diastatic contamination. These techniques include functional and simple plating assays that assess the potential risk of diastatic contamination while taking into account genetic factors and the activity level. They’ve also formulated a selective media that improves upon the commercially available selective medias mentioned above.
In retrospect, had Left Hand Brewery Co. been aware of these techniques and implemented them into their quality control routine, this issue could have been nipped in the bud. Unfortunately, detection came far too late, which meant their response was costly.
In addition to the $2 million in losses and a nation-wide re-call, they tore apart their entire production facility, sanitizing and swapping every nook and cranny in a desperate attempt to find the contamination source. To their dismay, they came up with nothing.
The process of elimination allegedly identified their yeast supplier as the source of their problem. Unbeknownst to Left Hand Brewing Co., the strain of yeast they acquired from their distributor had diastatic activity. Feeling swindled, Left Hand Brewing Co. filed a lawsuit asking for $6 million in damages. The jury is still out on whether or not they side with the Colorado brewery, but in the time since they have switched suppliers.
Changing suppliers is a step in the right direction, but the reality is that diastatic yeast can be a product of the brewing environment. In this scenario, the quality of the supplier doesn’t matter. The environmental risk alone highlights the need for rapid detection methods that can be implemented during routine quality control testing.
For the folks at Left Hand Brewery, this ended up being a costly lesson in the complexities of microbiology. Financial losses aside, it could have been worse; after all, someone could have lost an eye to an exploding bottle.
References
- Yakobson, C. M. (2019). The Oxford Companion to Beer Definition of flocculation. Retrieved May 28, 2020, from https://beerandbrewing.com/dictionary/9yHbaDo6RA/
- Holl, J. (2017, November 20). Left Hand Sues White Labs Over Contaminated Yeast. Retrieved May 24, 2020, from https://beerandbrewing.com/left-hand-sues-white-labs-over-contaminated-yeast/
- Chai Biotechnology. (2019, August). Controlling Diastaticus in Your Brewery. Retrieved May 24, 2020, from https://www.chaibio.com/beer-spoilage/diastaticus
- Goldfarb, A. (2020, January 30). Exploding Beer Bottle Lawsuit is Settled. Retrieved May 24, 2020, from https://www.goldfarbpa.com/exploding-beer-bottle-lawsuit-is-settled/
- Burns, L. T., Sislak, C. D., Gibbon, N. L., Saylor, N. R., Seymour, M. R., Shaner, L. M., & Gibney, P. A. (2020). Improved Functional Assays and Risk Assessment for STA1+ Strains of Saccharomyces cerevisiae. Manuscript submitted for publication, Cornell University, Omega Yeast Labs, Chicago, IL.