Saccharomyces cerevisiae var. diastaticus is a particularly insidious beer spoiler: a single cell is enough to cause a bottle or can of beer to explode, months after packaging. It’s come to prominence in recent months following widespread contamination in US breweries, leading to multi-million dollar recalls and lawsuits. Thankfully the threat can be managed with proper microbiological quality controls.
What is Diastaticus?
Diastaticus is a variant of Saccharomyces cerevisiae (brewer’s yeast) with the ability to modify the fermentability of beer due to an STA1 gene which causes the organism to secrete glucoamylase, an enzyme which hydrolyzes dextrins and starches into fermentable sugars. The yeast is a “hyperattenuator” as it ferments beer beyond what ordinary brewer’s yeast is capable, and is often considered to be a beer spoiler or “wild yeast”, as it causes unwanted secondary fermentations in packaged beer.
The Threat to Beer
While diastatic yeasts are sometimes used intentionally for their super-attenuating properties in the production of sours and dry saisons, they cause secondary fermentations in regular packaged products, which can lead to:
- Off-flavors & turbidity
- Alcohol level non-compliance
- Over-carbonation & gushing
- Package failure: exploding bottles & cans
- Shrapnel injuries to the consumer
It is the threat of physical harm to the consumer that most often leads to recalls. Excess CO2 generated during the secondary fermentation causes a dangerous over-pressurization leading to package failure. Bottles & cans can explode on their own or when handled by the consumer, resulting in injuries.
Damaging Secondary Fermentations
The trouble stems from the slower rate at which the dextrins & starches are fermented. Ordinary brewer’s yeast cannot utilize these complex carbohydrates, which remain in the beer once the primary fermentation completes.
Beer contaminated with even a small amount of S. diastaticus will first ferment normally in the brewery 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. Cans stored warm may become taut in as little as two weeks, while it may take many months for problems to be seen in cold stored beer.
71% of contamination events were traced to the bottling hall
Sources of Contamination
Recent press has focused on the potential for contamination stemming from yeast providers, but other sources should not be overlooked. A review of contamination events throughout Europe over a 10 year period published in the MBAA Technical Quarterly found 71% of contamination events were traced to the bottling hall. Infections can be introduced through:
- Dry hopping
- Wort ingredients, through the air
- Environmental air
- Incoming yeast
Diastaticus affects breweries of all sizes. It’s particularly common at small breweries packaging their beer for the first time. A brewpub could unknowingly have an infection for years without encountering any difficulties as their beer is freshly consumed on-site. It is only when they begin packaging their product that serious problems are seen, given the longer storage times.
Infections are generally detected by PCR detection of the STA1 gene, and PCR is the only safe means of detecting the organism as a quality control. It is possible to detect secondary fermentations by monitoring gas production with a Durham tube or by conducting post-package monitoring, but such methods do not serve as a useful quality control given the 2-6 week timeframe they entail. Microbiological methods are unable to differentiate Diastaticus from all house yeasts or provide timely results.
Real-Time PCR (also known as qPCR) is considered the gold standard for testing. It is not only the most specific, sensitive detection method available, but also produces quantitative results which are useful in tracing the source of contamination. Real-Time PCR also eliminates the cost and subjectivity of gels & cartridges associated with conventional PCR.
The Chai + PIKA Brewing Quality Solution can detect Diastaticus in beer, yeast, and equipment swab samples within 2 hours using qPCR, displaying clear objective results. Testing is performed right in the brewery using Chai’s Open qPCR instrument and PIKA Weihenstephan’s 4e Test Kit, and automated software analysis makes testing easy for everyone--no specialized education or familiarity with PCR is required. The test kits use probes for maximum specificity, and incorporate an internal inhibition control which prevents false negatives.
A single cell is enough to cause a bottle or can of beer to explode
The key challenge for testing is limit of detection: because even a single cell can cause package failure, no level of contamination is safe. Yet a contaminated yeast slurry may initially only have a miniscule fraction of wild yeasts relative to ordinary brewer’s yeast. PCR is highly sensitive, and can detect as low as 1 diastatic cell in 400,000 yeast cells, but at least one cell must make it into the PCR reaction to be detected.
Lower spoiler concentrations can be detected by pre-enriching samples with FastOrange Yeast Broth supplemented with copper sulphate 24-48 hours prior to PCR to achieve the greatest sensitivity.
The critical control points for testing are:
- Incoming yeast from suppliers
- Propagated yeast slurries prior to pitching
- Packaged bottles and cans
- The packaging environment and equipment
Direct testing of yeast early in the process reduces the costs of contamination events, while testing of the packaged product is the only way to ensure quality of the final product. If resources permit, it is also advisable to test beer prior to filtration, as it will be easier to detect wild yeast contamination in the higher yeast cell counts.
Growing in both aerobic and anaerobic environments, the yeast can live in any areas that have not been properly sanitized. Testing throughout the production process to identify the earliest stage of detection is key to locating the source of contamination.
Infections are extremely difficult to remediate, as the entire production chain must cleaned from the contamination source onwards. While it won’t be an easy process, locating the source of contamination is the first step in eradicating an infection.
Chai + PIKA Detection Products
A Real-time PCR instrument ideal for on-site production environments. Dual channel version required for PIKA kits.
Test kit containing all reagents needed to extract DNA and detect Diastaticus by qPCR.
Supplement enrichment media with copper sulphate for Diastaticus testing. Improves limit of detection to 1 cell/sample.
Yeast broth in convenient, ready-to-use enrichment bottles for direct sampling. Just add add 40 mL of sample.
Kits also available for the detection of all other beer spoilers. View the Chai + PIKA Brewing Quality Solution for more details.
MBAA Webinar "S. cerevisiae var. diastaticus—detection methods and control measures for brewing QA/QC" by Wade Begrow of Founders Brewing Co.
Incidence of Saccharomyces cerevisiae var. diastaticus in the Beverage Industry: Cases of Contamination, 2008–2017 in MBAA Technical Quarterly.