The Challenge: Saccharomyces cerevisiae var. diastaticus

Is your brewery immune to wild yeast?

The wild yeast Saccharomyces cerevisiae var. diastaticus – often its name is shortened to S. diastaticus – is not a new threat to conventional brewers. Every brewer should be aware that in the worst case S. diastaticus can ruin the whole production. After seeing some severe contaminations with S. diastaticus in the last months, especially in the U.S. (ref. 12), it is more commonly realized that wild yeasts generally are bearing an immense damage potential. No brewery is immune to contaminations of wild yeasts, no matter if microbrewery or big production, no matter if Germany or the U.S. S. diastaticus contaminations are existing worldwide and even spreading today, especially when brewing Sours side by side with conventional beers.

The Problem

Wild yeasts usually grow together with brewer’s yeasts. Their names are Brettanomyces, Pichia, Candida, Hansenula, Saccharomyces cerevisiae var. diastaticus… Especially the super attenuating members of the wild yeasts mostly come in low numbers, so they stay undetected during the production process. Only after brewer’s yeast has done its job – fermenting most of the available sugars – the time for wild yeasts has come: Some yeasts as S. diastaticus further break down the more complex carbohydrates like starches and dextrins by extracellular glucoamylase enzymes, starting a secondary fermentation. When super attenuating yeasts contaminate beer, they can take long time until they start growing, often weeks or even months after bottling. However, once a secondary fermentation is starting in finished beer, the results can be not only off-flavors, haze and often a raise of the alcohol content exceeding governmental regulations, but even more dangerously a considerable pressure formation in the packages due to over carbonation. Concerning these risks, the detection of wild yeasts in beer will usually always lead to a complete recall of all contaminated beer from the market.

Saccharomyces cerevisiae var diastaticus

The strain description of Saccharomyces cerevisiae var. diastaticus gives the following information: This yeast is facultative anaerobic, can ferment typical sugars similar to Saccharomyces cerevisiae plus starches and dextrins. Its morphology is comparable to the bottom fermenting brewer’s yeast Saccharomyces pastorianus, cells are oval to egg-shaped, mostly single or in pairs. S. diastaticus suspended in water can survive 10 min at 59.5°C, but all cells die when exposed to 60.5°C for 10 min. This is about 8°C higher than culture yeast or many other wild yeasts can survive at similar conditions. Fermenting wort with O.G. 1073 S. cerevisiae var. diastaticus can lead to a limit of attenuation of 1004, which is equivalent to 1°p (ref. 1).

The Solutions

When you face a contamination with S. diastaticus, conventional analysis by microscopy will probably not detect it during the production process, as this wild yeast looks just as your brewer’s yeast. To test for S. cerevisiae var. diastaticus, we are recommending a combination of the following methods covering enrichment, sensory and PCR:

 

  1. Enrichment: There are several selective enrichment media for the detection of wild yeasts in general available. Their principle is that growth of culture yeasts is more or less suppressed, while growth of wild yeasts should be enhanced. The choice of the enrichment medium always influences the growth rates both of the wild yeast and the brewer’s yeast. Commonly used media are e.g. lysine-medium, crystal-violet or MYGP-CuSO4 (Analytica Microbiologica, European Brewery Convention (EBC) methods: 6, 8, 9). Furthermore there are LWYM, Clen medium or Nystatin medium (American Society of Brewing Chemists (ASBC): ref. 13). From our own in-house experiences, we recommend working with CuSO4 based media as – besides PCR analysis – we got the best results for Non-Saccharomyces wild yeast detection with MYGP + CuSO4 (ref. 5). Other publications (ref. 4) figured out that with CuSO4 medium you can detect 80% of the tested wild yeast strains, which is a higher number than with the compared enrichment media. A combination of different media for wild yeast detection can be reasonable to increase the chance of finding the beer spoiling wild yeasts. Get more information how to use CuSO4 based media here.
  2. Continuous sensory testing of your brew is always essential, and we recommend documenting all results. If you notice any deviations in your beer, which can be off-flavors, a lighter taste, or higher carbonization, always take into consideration that this might be an indicator for a beginning microbiological contamination. Sensorial changes due to a wild yeast contamination often are detectable only after some weeks or months of storage, as many wild yeast contaminations may grow slowly.
  3. Additionally, monitor residual extract, pressure, and ABV of your beers, as a contamination with wild yeasts may significantly change these values. The residual extract may decrease, while pressure and ABV might increase due to activity of wild yeasts. Again, be aware that this effect can take a long time, from weeks to months.
  4. The most reliable and fastest analysis today to get knowledge about the identity of a wild yeast is PCR testing, especially when testing for S. cerevisiae var. diastaticus, as there is no selective medium available for its detection . The sensitivity and specificity of PCR analysis is unreached by any other method – you can detect 1 spoiler cell within 400,000 brewer’s yeast cells. If you cannot do PCR in your lab today, let us know and we will recommend you a service lab, or we can assist you in getting started with PCR testing.

Once you detect growth of wild yeast, you need to collect more information as it helps you not only to find and eliminate the contamination source, but – even more important – prevent the following brews from spoilage!

4e Detection Kit for Detection and Identification of Beer Spoilers

To sum it up:

Wild yeast detection generally can take a long time as these species might grow slowly, but never underestimate their hazard potential!

Include wild yeast testing into your routine quality control!

Take this challenge, especially when also producing sour beers!

References / More information:

  1. Andrews, J., and Gilliland, R.B. (1952) Super-attenuation of beer: A study of three organisms capable of causing abnormal attenuations. Journal of the Institute of Brewing 58:189-196
  2. Lin, Y. (1981) Formulation and testing of cupric sulphate medium for wild yeast detection. Journal of the Institute of Brewing 87: 151-154
  3. Taylor, G.T. and Marsh, A.S. (1984) MYGP + Copper, a medium that detects both Saccharomyces and non-Saccharomyces wild yeast in the presence of culture yeast. Journal of the Institute of Brewing 90: 134-145
  4. Van der Aa Kühle, A. (1998) Detection and identification of wild yeasts in lager breweries. International Journal of Food Microbiology 43(3): 205-13
  5. Vogeser, G. (2014) Specific detection of bacteria and yeasts in downstream process control of beer and related products. 75th ASBC Annual Meeting, Chicago, IL
  6. Analytica EBC, (2011): 4.2.5. Saccharomyces Wild Yeasts, URL: http://www.analytica-ebc.com/index.php?mod=contents&scat=22 (Date 17/11/22)
  7. Analytica EBC, (2011): 4.2.5.1 Cu-differentiation, URL: http://www.analytica-ebc.com/index.php?mod=contents&method=226 (Date 17/11/22)
  8. Analytica EBC, (2011): 4.2.6 Non-Saccharomyces Yeasts, URL: http://www.analytica-ebc.com/index.php?mod=contents&method=228 (Date 17/11/22)
  9. Analytica EBC, (2011): 4.2.7 Dekkera (formerly Brettanomyces), URL: http://www.analytica-ebc.com/index.php?mod=contents&method=229 (Date 17/11/22)
  10. Analytica EBC (2011): 5.1.3.5 YM (or MYGP) + CuSO4, URL: http://www.analytica-ebc.com/index.php?mod=contents&method=315 (Date 17/11/22)
  11. Torres, L. (2017): When Yeast Attack: The Story of Bell’s and Saccharomyces cerevisiae var. diastaticus. CBC 2017: 14th Craft Brewer’s Conference, Washington D.C.
  12. Brewbound, Kendall, J. (2017), URL: https://www.brewbound.com/news/left-hand-files-lawsuit-white-labs-contaminated-yeast (Date 17/11/28)
  13. American Society of Brewing Chemists ASBC, (2009): Microbiological Control, URL: http://methods.asbcnet.org/methods/MicrobiologicalControl-5.pdf (Date 17/11/29)

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Spoilt product or positive enrichments are further discriminated – fast and specific by PCR analyses the same day as the samples are received. We further identify unknown bacteria, yeasts or molds from product or the production area within 3 to 5 days by DNA sequencing.

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FastOrange B

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