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.
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Relevance to Homebrewing
Botulinum_toxin_3BTA
Botulinum toxin (type A)
Post-boil wort generally has a pH of 5.05.2, 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 Ive 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 wouldnt 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 cant grow in beer. This is true as far as we can tell but C. botulinum isnt a pathogen. Its 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 doesnt show up in malt.)
Also, most importantly, wort isnt beer. Although pathogens cant 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 doesnt kill C. botulinum spores, why isnt botulism more common in homebrew? When beer is fermented, the pH drops usually to 4.04.4 below the threshold that inhibits growth of the bacteria. If C. botulinum spores are present, they cant grow and produce enough toxin in the time between the wort being cooled and fermentation finishing to cause a problem.
