Wild Yeast Isolation

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Wild yeast isolation, sometimes known as "yeast wrangling" or more formally "bioprospecting", 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 from Sui Generis blog and Jeff Mello from Bootleg Biology for providing the information in this article.

Isolating Pure Cultures


  • Sui Generis Blog 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.
  • Sui Generis Blog guide to more types of agar plates, and video tutorial.


  • Bootleg Biology defines three methods of catching a wild yeast.
  • Sui Generis Blog video tutorial for catching wild yeast from fruit or from the air.
  • David Thornton of SouthYeast Labs describes his favorite method of catching wild yeast/bacteria: "Most of our cultures are taken from over ripened fruit macerated in its own juice and left to ferment 24 hours at the location in a small erlynmeyer with a perforated cap. Always grows something, and almost always get an alcohol fermenting strain, and of the alcohol fermenting strains id say 10% are worth brewing with. I like this method because I can stir my loop and do a streak on regular UBA aerobically, then pull favorable looking colonies for yeast and lacto and do a quadrant streak on selective media for isolation." [1]



  • Sui Generis Blog video guide on how to visually identify usable yeasts versus potential pathogens and molds on agar plates.

Growing and Testing


Slants are a good option for longer term storage (3 months to potentially 2+ years). For instructions on how to make slants at home capable of storing any microbe for potentially 2+ years, see Bryan's video on Sui Generis Brewing (requires a pressure cooker).

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 [2].

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 with 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 yeast cell count should be high enough at this point to out-compete bacteria, and lower the pH of the wort within a few days. The pH of the 1 gallon batch therefore does not need to be lowered, although the brewer may choose to do so anyway. For safety reasons, test to make sure that the beer fully attenuated and has a pH of 4.5 or less. Allow for one month after fermenting the 1 gallon batch of beer before sampling. 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. David Thornton from SouthYeast Labs estimates that only about 10% of yeast that can fully attenuate produces favorable results, so failures are to be expected (keep trying!) [1]. Fermenting a few batches of beer at different temperatures is a useful method for identifying the ideal fermentation temperature range for the yeast.


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 ~99.9% safe to drink. However, several studies can give us some guidance on this, as well as Bryan from Sui Generis Blog.

  • The addition of hops in the starter wort will inhibit or kill some gram positive bacteria pathogens (L. monocytogenes and S. aureus) [3][4].
  • 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 (survival is another matter) [5][6].
  • Two studies have shown that E. coli can survive in moderate strength beer for at least 28 days (more than 30 days in one study) if the beer is stored cold (39-41°F or 4-5°C) [3][7]. 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 4.6, and alcohol should be present for at least one month before tasting. The smell of feces or vomit indicates that bad bacteria may be present; if so dump it out [8].
  • 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 [3]. However, some of these food poisoning bacteria can survive the low pH of the stomach (~2.0) long enough to cause illness, so relying on a low pH alone is not adequate. Bryan of Sui Generis Blog recommends a pH of 4.5 or less, 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 [4].
  • The risk of botulism toxin is extremely low, but not 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 [9][10]. However, proteins in the wort may allow growth at a lower pH than 4.6 (studies have not been done on wort, only on meats) [4]. The presence of more than 5% oxygen in the wort will also reduce the risk [11].
  • To our knowledge, there have been no documented cases of food poisoning from beer [5]. 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 of Sui Generis Blog on MTF [4]. For more information on the potential for pathogens surviving in beer, see this Sui Generis Blog article..
  • In the words of Bryan from Sui Generis Blog: "If you have an immunodeficiency or are immunosuppressed: Obviously, don't take medical advice off the internet. Talk to your medical doctor 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." [5]
  • See also this thread on MTF.
  • See also this MTF thread on the mechanisms of botulism poisoning, and how it applies to different brewing processes.
  • Further explanation of what makes microbes "pathogenic" by Bryan from Sui Generis Blog and a related MTF discussion.

See Also

Additional Articles on MTF Wiki

External Resources





  1. 1.0 1.1 Conversation with David Thornton on MTF. 09/06/2015.
  2. Collecting Wild Yeast. Manoaction. Homebrewtalk. Oct 30, 2012.
  3. 3.0 3.1 3.2 Growth and survival of foodborne pathogens in beer. Menz G., Aldred P, Vriesekoop F . Oct 2011.
  4. 4.0 4.1 4.2 4.3 Conversation with Bryan of Sui Generis Blog on MTF regarding when it is safe to taste wild beer. 7/6/2015.
  5. 5.0 5.1 5.2 Fact or Fiction? Can Pathogens Survive in Beer? Sui Generis Blog.
  6. The growth and survival of food-borne pathogens in sweet and fermenting brewers' wort. Menz G1, Vriesekoop F, Zarei M, Zhu B, Aldred P. May 2010.
  7. Survival of foodborne pathogenic bacteria (Bacillus cereus, Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, and Listeria monocytogenes) and Bacillus cereus spores in fermented alcoholic beverages (beer and refined rice wine). Kim SA1, Kim NH, Lee SH, Hwang IG, Rhee MS. March 2014.
  8. Conversation with Bryan of Sui Generis Blog on MTF regarding when it is safe to taste wild beer. 7/5/2015.
  9. Toxin Production by Clostridium Botulinum in Media at pH Lower Than 4.6. Nobumasa, Tanaka. Journal of Food Protection®, Number 3, February 1982, pp. 214-284, pp. 234-237(4)
  10. Preventing Foodborne Illness: Clostridium botulinum. University of Florida IFAS Extension. Retrieved 7/5/2015.
  11. Growth and toxin production by Clostridium botulinum in steamed rice aseptically packed under modified atmosphere. Kasai Y, Kimura B, Kawasaki S, Fukaya T, Sakuma K, Fujii T. May 2005.