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.
Relevance to Homebrewing
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.
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.
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.]