Storing Wort Runs the Risk of Botulism


Clostridium botulinum — the bacteria that produces the botulinum toxin, which causes botulism. (CDC photo in the public domain)

Botulism is a rare but serious condition that can occur due to eating improperly preserved foods. One homebrewing practice that is gaining in popularity may put homebrewers at risk for botulism — using the no-chill method of wort chilling and subsequently storing (unpitched) boiled wort in sealed containers for long periods of time.

The Bacteria and its Toxin

The bacteria Clostridium botulinum is a spore-forming bacteria that mostly lives in soil. [A few other species of Clostridium can also cause botulism, but C. botulinum is the primary culprit. The name comes from the Latin word botulus, for sausage, as it was first isolated (in 1895) from a botulism outbreak traced to an improperly cured ham.] Its spores can survive boiling temperatures (212 °F/100 °C) and then begin to grow in improperly preserved foods. The bacteria produces the botulinum toxin, which poisons the food. (There are actually a series of eight similar toxins, labelled A through H, made by different strains of C. botulinum.)

The botulinum toxin, a protein, is the most deadly toxin known. As little as 100 nanograms can be lethal to an adult . (The LD50 is 1.3–2.1 ng/kg.) Symptoms of the illness usually appear 18 to 36 hours after eating the contaminated food, although this can vary. The toxin causes muscle weakness, usually starting with the facial muscles. The muscle weakness then spreads downwards. In severe cases, the victim’s ability to breath is impaired, and they must be placed on a respirator. In 5–10% of cases, botulism is fatal. The victim does not run a fever, and his or her consciousness is not affected.


Conditions That Permit Growth

Clostridium botulinum grows under anaerobic conditions, when the pH is over 4.6. Refrigerator temperatures (under 38 °F/3 °C) will impede the growth of some strains of the bacteria, but not others. (Very salty solutions will also slow its growth.) As such, when canning low acid foods (pH>4.6), the USDA recommends pressure canning. This involves heating the food, at 10 PSI, to 240 °F (116 °C) for 20 minutes. This is not sufficient to completely kill all the C. botulinum spores. However, it will greatly reduce their numbers. And, if the food is heated to 185 °F (85 °C) for 5 minutes before serving, the botulinum toxin will be denatured, and the food is generally considered safe. [Some sources give a more conservative recommendation of heating the food to 212 °F (100 °C) for 20 minutes.]

In the United States, commercial canneries are required to heat food to 250 °F (121 °C) for 3 minutes. At this temperature, all the spores should be killed in 12 seconds, but exceptions occur.

Despite these recommendations and regulations, there are over 20 cases a year of food-borne botulism in the United States. Better medical treatment means that deaths from botulism have dropped from 50% of cases (going back to the mid 20th Century) to around 5% in places with access to modern medical care. In short, botulism is rare, but serious when it does occur.

Some strains of botulism bacteria — the so called proteolytic group — produce a putrid smell in food they have contaminated. Others — of the non-proteolytic group — do not (and it’s these that also are more likely to be able to grow even when refrigerated). You cannot detect C. botulinum or its toxin in food by visual examination.

The temperature and pH recommendations for avoiding botulism given here follow the USDA’s guidelines and are echoed by every home canning publication.


Relevance to Homebrewing


Botulinum toxin (type A)

Post-boil wort generally has a pH of 5.0–5.2, in the range that C. botulinum can easily grow. It has not been exposed to temperatures around 240 °F (116 °C), and therefore the numbers of C. botulinum spores (if present) have not been reduced much. Boiling temperatures (212 °F/100 °C) will slowly kill C. botulinum spores, but you would need to boil your wort for many hours to reduce the numbers of spores (if present) to a safe level. In addition, sealed in an air-tight container, the wort is anaerobic (does not contain oxygen). The small amount of oxygen permeability that plastic shows is not sufficient to let in enough oxygen to inhibit C. botulinum. Oxygen levels over 2% are required for this.

Therefore, if you are boiling your wort, running it off to a container (such as a food-grade plastic cube) and letting it cool overnight — then storing the wort at room temperature for extended periods of time — the conditions for C. botulinum to grow are present, and you are putting yourself at risk for botulism.


Potential Objections

I imagine that many homebrewers, upon hearing this, will have some objections. For example, some will say, “But I’ve done this before and never had any problems.” That is true, but irrelevant. Botulism is rare. In the US, with over 300 million people, there only around 20 cases of food-borne botulism in adults per year. You wouldn’t expect it to happen frequently enough that there would be multiple cases of this traced to homebrewing yet. Even though no-chill brewing has been popular for awhile in Australia, how many brewers have saved their wort for an extended amount of time?

Also, producing wort that is not contaminated by common wort-spoiling or beer-spoiling bacteria or wild yeasts is not proof that C. botulinum spores are destroyed. Common wort-spoilers and beer-spoilers are killed by boiling temperatures; C. botulinum is not.

Other brewers may say, “But, pathogens can’t grow in beer.” This is true — as far as we can tell — but C. botulinum isn’t a pathogen. It’s not the bacteria that kills you, its the toxin secreted by the bacteria. (There are mycotoxins from fungi in the genus Fusarium that can also poison beer. This fungus can infect barley and maltsters test for it so it doesn’t show up in malt.)

Also, most importantly, wort isn’t beer. Although pathogens can’t survive in beer, they certainly can survive in sugary solutions. Infant botulism, for example, is most commonly caused by infants eating raw honey. (And furthermore, botulism is fairly common in prisons when inmates try to make their own homemade prison hooch, such as pruno, from sugary solutions.)

Finally, a brewer might ask, “If boiling doesn’t kill C. botulinum spores, why isn’t botulism more common in homebrew?” When beer is fermented, the pH drops — usually to 4.0–4.4 — below the threshold that inhibits growth of the bacteria. If C. botulinum spores are present, they can’t grow and produce enough toxin in the time between the wort being cooled and fermentation finishing to cause a problem.



Packaging boiled wort in a sealed container certainly falls outside of commercial regulations for safe food packaging in the US and the guidelines for home canning.

No-chill brewing, in which hot wort is sealed in food-grade containers to cool overnight, is likely safe if that wort is pitched with brewers yeast in a reasonable amount of time. What is reasonable is hard to say, however, because we would need to know how long it takes C. botulinum to grow to dangerous levels in wort. Storing the wort for a couple days almost certainly shouldn’t be a problem — but beyond that, it’s hard to say. C. botulinum grows slowly, but it only needs to produce a tiny amount of toxin to be a threat.

If the wort can be refrigerated, the risk of botulism drops, but is not eliminated. Likewise, if the pH of the wort could be adjusted downward — by adding food-grade acid — to 4.5 or lower, the threat would eliminated. Of course, this would be impractical and may lead to overly tart beer. Also, wort that has been stored could be reheated to 185 °F (85 °C) for 5 minutes, as a precautionary measure, although this requires the wort to be chilled again, defeating the purpose of the no-chill technique.


Risk Assessment

The risk of botulism from storing wort for an extended period of time is low. However, the risk is real, and the consequences are severe, perhaps fatal. The good news is that it is easy to avoid the problem. I would advise no-chill brewers to pitch their worts as soon as is feasible, and there should be no problem. It takes awhile for the bacteria to grow, so overnight chilling and a few days of sitting around should be fine. Beyond that, it is hard to say. The risk will always be small, but that needs to be weighed against the large, negative consequences.

Some strains of botulism-causing bacteria create putrid smells or give of gas. If your wort smells bad, the container swells, or it foams when you open it, be prudent and don’t use that wort.

Perhaps the worst case scenario is a batch of homebrew poisoning multiple people. If a homebrewer ferments wort that has been sitting around for months, he or she should take care to ensure it is safe before taking it to a party, homebrew club meeting, or sending it to a homebrew contest. It is one thing for a homebrewer to be willing to take a risk; it is entirely another thing to put others at risk. If homebrewers are aware of the risk, hopefully they will avoid the problem by pitching promptly. No one should ever be sickened by botulism from stored wort when it can so easily be prevented.

[Wikepedia has articles on botulism (the condition), C. botulinum, and the botulinum toxin that agree well with information from the CDC and other reputable, academic sources. Thanks to Graham Cox (homebrewer with a degree in food science), Bob Stempski (no-chill brewing enthusiast), and James Spencer (you know who he is) for their discussions on this topic.]


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  1. I create starter wort by boiling Dried Malt Extract for about 15 minutes, and then pressure treating in a sealing mason jar in a pressure cooker for 20 minutes. When it cools, it forms an airtight seal for long term storage. Do you see any botulism risks with this method?

    • Chris Colby says:

      Pressure cooking for 20 minutes (at 10 PSI or above) falls within the USDA guidelines for canning low acid foods.

  2. Can adding campden tablets with the hot wort into the container have any impact on the bacteria?

    I also leave some air/headspace in my cubes (it is impractical for me to completely purge it) would say 2L of headspace add back enough O2 to inhibit the bacteria?

    Will hop oils from cube hopping have any impact?

    • Chris Colby says:

      These are great questions. I’d actually like to use them for a Q and A post later. But here are the short answers:

      Campden tablets won’t work at wort (or beer) pH. They work in wine because the pH (in the low to mid 3s) causes the potassium metabisulfite in the tablets to release sulfur dioxide (SO2) gas. At post-boil wort pH (5.0–5.2),or beer pH (4.0–4.4), the SO2 will remain mostly bound to the metabisulfite.

      Leaving more headspace would likely lead to an _Acetobacter_ (vinegar-producing aerobic bacteria) bloom on the surface of your wort (and probably would do little to inhibit _C. botulinum).

      Hops have some anti-bacterial properties vs. Gram positive bacteria, and _C. botulinum_ is Gram positive. So, hops might help somewhat, but I sure wouldn’t rely on them. (You still need to use sanitary practice when making a double IPA; hops alone won’t kill every last bacterial cell in your wort.)

  3. Nicely done. How great to note some of the problems of low-acid anaerobic storage. Canning has its own rules and lore. There are a few more things that might be useful for brewers who want to make canned starter wort to think about.

    One is that when canning at altitude in a pressure canner you have to adjust time or pressure for each 1000 feet. Many people live a thousand or so feet above sea level and don’t have to adjust regular cooking or brewing times, so it may be a surprise to learn that.

    Lots of those USDA publications include warnings that home-canned products that may not conform to all the suggested practices be “boiled in an open pan for 15 minutes before tasting or eating.” Bet not many brewers reboil their canned wort for 15 minutes, then cool for a starter. Good idea to do it, though, when you think about it.

    Hmm. I wonder if somebody from the USDA or the CDC would be willing to suggest times and pressure for home canning wort?

  4. This is why I process my canned starter wort at 15psi for 15 minutes to reach that magical 250F. I don’t want to take any chances. Not sure why someone would use the reduced heat at 10psi vs 15…

  5. What about starters or harvested yeast slurries? is there a potential for culturing botulism along with your yeast if it sits in the fridge for a while before brew day?

    • Chris Colby says:

      The pH of yeast starters or yeast slurries would be below 4.6, so you don’t have to worry.

  6. Can the same principles in this article be applied to E. coli, or is that another article? 🙂

    • SpringDog says:

      Sorry, I have to protest as a public health veterinarian (and amateur brewer) that the science in this article is misguided. Your statement “Common wort-spoilers and beer-spoilers are killed by boiling temperatures; C. botulinum is not” is correct for the SPORE but incorrect for the bacterium, C. botulinum. The bacterium, which produces the spore in anaerobic conditions, is easily destroyed by heating. This means you would have to NOT boil the wort and the the bacterium would have to present in the wort during your non-chill for the risk you are discussing to exist.

      While the spore can pre-form in the environment, the real risk is when people don’t properly heat preserves. The bacterium, C. botulinum, is still present, and in the anaerobic conditions in the jar (or a fermenting container in your example) the bacterium produces spores which are deadly and resistant to heating. If you are doing a 60 minute boil, you’re acceptably covered (just compare it to milk pasteurization, which although admittedly only reduces the microbiological load to an acceptable level, involves heating milk to 72 degrees Celsius for 15 seconds).

      The real risk of no-chill brewing is your wort (which you can consider a giant nutrient bacterial broth at that point) is exposed to spoilage bacteria for a longer period of time, and doesn’t have the quick addition of yeast to wipe out the competition. C. botulinum isn’t aerobic…it won’t be joining the party from exposure to air. You definitely need to be on your hygiene game to pull this method off to avoid contamination with all those wonderful bugs that make those sour beers so tasty, but risk from botulinum toxin is negligible.

      • Chris Colby says:

        Sorry, but as a PhD in biology, I have to say the science in this article is solid. It’s straight out of the USDA’s canning recommendations and the FDA’s requirements for commercial food packaging. I ran the article by a friend who recently earned a degree in food science (and is also a homebrewer) and I’ve had a couple other friends of mine with PhDs in biology review it.
        _C. botulinum_ forms spores. It’s just plain wrong to state that the spore is something different from the active bacterium — it’s just a stage in the bacterium’s life cycle. And, it’s a stage meant for dispersal. While boiling will kill the active form of the bacteria, it won’t kill the spores and that’s the concern.
        Also, your information on how milk is pasteurized is way out of date. It hasn’t been done that way since the 1970s.

        • I have no specific biology training, but if I understand SpringDog’s point, and assuming that the transfer from the kettle to the cube doesn’t introduce anything new, the C. botulinum spore would have to be present in the wort from the start, since the bacterium would be killed by the boiling process.

          If that is correct, then the real question seems to be “how much C. botulinum spore would typically be present in the wort at the start of the boil?”

          And a follow up question re. beer vs. wort… is it accurate to say that any toxins secreted by the C. botulinum would persist from the wort to the beer, even after the fermentation goes through its cycle?

          • Chris Colby says:

            Spores of _C. botulinum_ are common. Many fresh foods, and especially root vegetables, are coated with the spores. These don’t present a problem in raw or cooked food because the spores pass through adults before they can do any harm. [They can harm infants, however, which is why doctors say to never feed honey (which frequently contains spores) to infants.] They become a risk when food is heated, then stored anaerobically (over pH 4.6). It would be interesting to find out the typical level of spores in malt and hops.

    • Chris Colby says:

      _E. coli_ is killed in boiling liquids. _C. botulinum_ isn’t because it forms spores.

  7. Nice article. But is this simply a hypothesis or is there any evidence or research on the subject?
    links to peer reviewed articles would be nice.



    • Chris Colby says:

      This is not just a hypothesis. This is the standard recommendation for people who can food at home and the legal requirement for companies that package foods (or sell their foods in the US). There are links to the USDA canning guidelines in the article, and the Wikipedia articles also have links for more background information. There aren’t any peer reviewed articles specifically on botulism in wort, but people have been canning all sorts of food for a long time and so this article reflects the generally accepted general guidelines for packaging food safely.

  8. It’s a pity there’s only 150 years of research and evidence on this topic, Chris, and much of it by that Pasteur character, who was probably French and a commie.

    • Chris Colby says:

      Yeah, if there were 151 years of research, and the results agreed with what I want to believe, I’d be more inclined to take the words of the “experts.”

  9. For anyone interested, Chris provided some follow up information with James Spencer on BasicBrewing Radio. Chris White of White Labs also talked about Colby’s article. The episode is listed under “May 1, 2014 – Yeast vs IBUs” and is in the later half of the show:

    Thanks, Chris, for writing this up and having the guts to post it!

  10. David G of Napier, NZ says:

    I cann’t bring any science to the discussion, being numerically challenged, but I use the No Chill method regularly to give me some flexibility in the process by saving me from strife late in the day when my wife needs assistance and/or the kitchen for domestic duties. After the boil I put my pot out of the way with lid on and wrap with some old towels secured with bungy cord and forget until sometime next day or so. It takes about 18 hours untouched to get to pitching temperature. If necessary you can hurry it up by cooling it. Then put in fermenter as usual. The water savings are not a factor where I live. I don’t know why you would want to store the wort for much longer.
    Thanks for yet another interesting article and discussion

  11. Emperor Pho says:

    Much vitriol on the Aussie no chill forums about this article you have proposed Chris Colby. Unfortunately, despite anecdotes to the contrary, and much to my disappointment, you are indeed correct. It would be nice to live in a world where deadly anaerobic bacteria were killed by boiling but unfortunately they are not. No amount of, “I haven’t died yet and I’ve done it for years!” stories make it possible.

    • Chris Colby says:

      I suspected as much. I’ve never quite understood people who get angry at the facts or a person who presents them. Hopefully, this information will save some homebrewers from a making a fatal mistake. And for those who choose to take the risk, hopefully they don’t serve their beers to others.

  12. My question might sound dumb however I’ll ask it anyways. I am completely unfamiliar how ”Brewing kits”- the ones you buy and simply pitch the yeast, are preserved since I have only done one onced. I do not plan on using the no chill method ever but do those kits can possibly represent any c. botulism danger ?!

    • Chris Colby says:

      That’s not a dumb question at all. If you buy a wort kit in the US, Canada, or the EU, their food packaging requirements would require that the wort be produced in such a way that botulism was excluded.
      (And if you buy a beer kit based on liquid malt extract, botulism can’t survive in it.)

      • Thanks for the answer !

        Another question, by the time the wort is chilled down to temp, by the time you pitch yeast and by the time fermentation really starts, is there any risks of botulism ?!

        By the way great article.

        • Chris Colby says:

          No, there is no risk of botulism when beer is produced in the normal way. It takes at least 3 days for botulism to grow to the point where it can be a problem.

  13. Allan M Tiso says:

    What about freezing the wort after cooling or quicker yet cooling by freezing?

  14. What about kettle souring with lacto from grains? Could be more a practice more dangerous than a classical sponteanous fermentation because of the lack of oxygen in the acidification phase? We work in anaerobe at 105f, and even if we pre-acidify the wort at <4,5 ph it doesn't seems the costridium must surely die just for the ph drop (instead it would surely die with oxygen), and once the toxin is produced nothing can eliminate it in our process, not the boil, not the alcohol…maybe i'm paranoid but i feel that i'd be more confident in tasting a "classical" spontaneous fermentation (but obviously not before the ph has dropped and once the alcohol is present) than a kettle sour beer made with grain.

    One partially reassuring fact is that the costridium is typically correlated with meat, vegetables and not with cereals, but the for me it's not enough.

    Typical spontaneous fermentation trusts on oxygen to outcompete clostridium while kettle souring do the same with the acidification but i'm wrong if i say (for what i've read) that, for clostridium, oxygen is a much more strong enemy than pH?

  15. I know this article is about storing wort, but are you aware of any information regarding low ABV beers?

    I’m curious if there’s any research supporting the level of alcohol needed to make beer safe, either directly or as a proxy for changes to the pH during fermentation. I ask because I brewed a very low alcohol beer (a little 2-row and a bunch of carapils) that finished at 1.57% ABV (1.021 to 1.009), but now I’m worrying if there’s bad stuff growing in the small keg of it. (Well, I’m sure there is, but is it within a safe level for my immune system and to prevent toxin formation?) Would such a small fermentation still reduce the pH sufficiently? Is that ABV enough to keep it safe?

    It has been 3 days and I could easily go through the stuff (I probably take in more ethanol from gargling with mouthwash!), which is probably what I’ll do since it sounds like at serving temperature and with a little bit of ethanol it should be fine for at least a few days. I just don’t want to run into problems.


  16. Interesting article, what I get out of it is if using No Chill, pitch yeast as soon as possible after cooling, don’t let it sit around for a month. I plan to pitch yeast as soon as cooled sufficiently, ie 24 hours. Also seems the same risk would occur if the fermentation failed to start within a couple days of pitching?

  17. David Pritchard says:

    Would the DME manufacturing process not remove all botulism spores, or do we expect there to still be spores present in modern brand name DME’s?
    Also! If a batch of beer was made using a yeast starter where the starter wort had not been done in a pressure canner as suggested here and left for five days before pitching yeast, would any botulism bacteria present (if any) be present in every bottle of finished product? I have drunken three bottles of a brew that was made in this way (before being enlightened to the risk) without any issues, can I now safely say the rest of the brew is fine???

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