Wild Yeast Isolation

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Wild yeast isolation, sometimes known as yeast wrangling, refers to the process of catching wild yeast, and isolating it with agar plates as a pure culture. This article will also contain information on growing up a captured culture in wort rather than isolating yeast cultures on agar plates.

Special thanks goes out to Bryan Heit from Sui Generis blog and Jeff Mello from Bootleg Biology for providing the information in this article.

Isolating Pure Cultures

Preparation

  • Bryan Heit's video guides on setting up a clean workspace, building and using an alcohol lamp, and aseptic techniques.
  • Bootleg Biology's guide on creating agar plates using wort, agar powder, and yeast nutrient.
  • Bryan Heit's guide to more types of agar plates, and video tutorial.

Catching

  • Bootleg Biology defines three methods of catching a wild yeast.
  • Bryan Heit's video tutorial for catching wild yeast from fruit or from the air.

Isolating

  • Bootleg Biology's guide to streaking agar plates to isolate the captured yeast cultures.
  • Bryan Heit's video guide on how to streak agar plates.
  • Bryan Heit's video guide on streaking plates and further isolating yeast.

Identifying

  • Bryan Heit's video guide on how to visually identify usable yeasts versus potential pathogens and molds on agar plates.

Growing and Testing

Growing and Testing Without Plating

While using agar plates to isolate yeast colonies is the most effective way to culture wild yeast, it is not the only way. Wild yeast should first be caught using DME wort as outlined in Bootleg Biology's Method 1 or Method 2. Hopping the wort will help decrease bacteria if that is desired. The wort's pH should be lowered to 4.5 or lower with lactic acid to avoid bacteria as much as possible. Molds may still grow in the yeast starter even with the lower pH, and should be removed by scraping them off of the surface of the wort. Signs of a small krausen within 1-3 days is a good sign that viable wild yeast has been collected. After another few days, the yeast will start dropping to the bottom of the collection vessel [1].

Once the wort has fermented out (allowing 2 weeks total is a good rule of thumb), decant the beer and pitch the collected yeast into 500ml of starter wort, again lowering the pH of the starter wort to 4.5 or lower by using lactic acid. After the starter wort has been fermented, the yeast should have enough of a population to ferment out a 1 gallon batch of wort. Keep the recipe of the wort simple, and in the 1.050 gravity range. The population of the yeast should be high enough to out-compete bacteria and lower the pH of the wort within a few days, so the pH of the 1 gallon batch should not need to be lowered. After fermenting out the 1 gallon batch of beer, first smell the beer. If it smells like feces or vomit, do not sample it and throw it away. If the beer smells ok, feel free to sample the beer to see if the wild yeast produced a good tasting beer. Fermenting a few batches of beer at different temperatures is a useful method for identifying the ideal fermentation temperature range for the yeast. Wild yeasts can be highly or moderately estery, or could have undesirable flavors [1].

Safety

An often asked question when it comes to tasting beer fermented from a wild culture that wasn't plated and isolated is, "When is it safe to drink?" To our knowledge, no studies have shown when a wild caught beer is scientifically 100% safe to drink. However, several studies can give us some guidance on this.

  • The addition of hops in the starter wort will inhibit or kill some gram positive bacteria pathogens (L. monocytogenes and S. aureus) [2][3].
  • A low pH is not the only requirement for making beer safe. A combination of alcohol, low pH, and hops will ensure that pathogens cannot grow [4][5].
  • Two studies have shown that E. coli can survive in moderate strength beer for at least 28 days (more than 30 says in one study) if the beer is stored cold (39-41°F or 4-5°C) [2][6]. Therefore, the wild culture should be stored at room temperature for a least a month before tasting. Additionally, the pH of the beer should be less than 4.5, and alcohol should be present for at least one month before tasting. The smell of feces or vomit indicates that these bacteria may be present; dump it out if so. [7].
  • In unfermented wort, E. coli O157:H7 and Salmonella Typhimurium were shown to grow at a pH of 4.3, but stopped growing at a pH of 4.0 [2]. However, some of these food poisoning bacteria can survive the low pH of the stomach (~2.0), so relying on a low pH alone is not adequate. Bryan Heit recommends a starting pH of 4.5, the use of well sanitized equipment, and monitoring the starter for the rapid onset of fermentation without putrid smells. Some oxidative yeasts that are the first "barriers" to bacterial growth may be inhibited at 4.0 pH [3].
  • The risk of botulism toxin is extremely low, but not non-zero. Generally, C. botulism is not able to grow and produce toxin in low protein (non-meats) substances at a pH lower than 4.6 [8][9]. However, proteins in the wort may allow growth even at a lower pH (studies have not been done on wort, only on meats) [3]. The presence of more than 5% oxygen in the wort will also reduce the risk [10].
  • To our knowledge, there have been no documented cases of food poisoning from beer [4]. However: "It is well established that at least a dozen human pathogens can survive in fully fermented beer; moreover, sour-brewing (and home brewing in general) offers an additional opportunity for pathogens to gain a foothold or produce toxins which persist into the final product. Reality is that the source of most food poisonings is never established. A few industries (milk, meat, canning, etc) are under close scrutiny, and as such we have firm numbers for rates of food-borne illnesses from those industries. Neither commercial nor homebrewing is subject to that monitoring, so any cases of disease arising from contaminated products from either source would likely go unidentified." - Bryan Heit on MTF [3]. For more information on the potential for pathogens surviving in beer, see Bryan Heit's Blog article..
  • In the words of Bryan Heit from Sui Generis Blog: "If you have an immunodeficiency or are immunosuppressed: Obviously, don't take medical advice off the internet. Talk to your MD to see if you are at risk of infection - especially if you're condition/treatment increases your risk of fungal infections. If so, or if you are concerned, stick to beers that are fermented using commercial strains of Saccharomyces yeast." [4]

See Also

References

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