The topic of the risk of developing botulism during homebrewing is a topic that circulates frequently among homebrewing circles. Recently, this topic has reared its head in the form of two sequential Experiential Brewing podcasts [1, 2]. People seem to fall into one of two camps – the “you’re going to die” camp and the “there is nothing to worry about” camp. So who is right and who is wrong?
Keep reading to find out.
This article is a part of my Fact or Fiction series on the risk from infectious organisms during home brewing.
What is Botulism?
Botulism is a form of food-borne illness caused by the bacterium Clostridium botulinum. Illness is caused by a toxin released by the bacteria. This toxin can persist in foods lacking viable bacteria, but is quickly destroyed above temperatures above 85C (185F). Botulism itself is hard to kill – the bacteria forms spores when stressed, with those spores capable of surviving boiling. This is why many canned foods must be pressure canned, as destruction of botulism spores requires temperatures above 100C/212F.
When in a suitable environment, these spores can “hatch” into viable, growing bacteria. The growth of botulism is not always obvious – some strains will cause putrefaction, producing the aroma of rotting meat. But the majority of botulism strains do not purify, meaning that there is no obvious sign of their presence. Botulism grows under a limited range of conditions: it needs a source of protein for growth, a pH above 5.0, a salt concentration below 5-10%, a sugar concentration below 30%, and an oxygen concentration below 1%. Botulism is relatively tolerant to alcohol, and is not fully suppressed until alcohol content reaches 6% ABV. The toxin is only produced by growing bacteria, and generally is not produced until 3 or more days after the bacteria begins to grow.
The botulism toxin itself is utterly terrifying. It is the most toxic substance known to human kind – literally millions of times more toxic than cyanide. The way that it functions is particularly insidious. Botulotoxin is a protease; a protein that cuts other proteins into smaller pieces. The toxin binds to neurons, which then take up the toxin. In the neuron the toxin renders the cell unable to release the neurotransmitters required to make muscles move. The end effect is flaccid paralysis – the inability of a patient to contract their muscles. Untreated, 50% of patients die due to paralysis of their respiratory muscles; they literally asphyxiate. Even with modern medical treatment, 5-10% of affected patients die, and those whose survive often experience long-term (even permanent) disability.
Hopefully the above has disabused you of any belief that botulism is harmless. From a risk-management perspective, botulism is a severe hazard – one of the most severe hazards humans know. But there is more to assessing risk than merely identifying the degree of hazard present. After all, a shark is a hazard, but is not a risk if you’re sitting in a corn field in rural Iowa.
Risk is the combination of hazard and exposure. Exposure to botulism spores is common as they are found in many foods (honey as one example), the soil, and even the air. However, we rarely encounter the vegetative (growing) bacteria, or conditions where spores can begin to grow. As such, cases of botulism are very rare – in the US there are ~15 cases of food-associated botulism per year; the remaining ~110 cases/year are in infants (who are very susceptible to the bacteria) or IV drug users who develop wound infections.
Will Botulism Grow in Wort or Must?
Of course, none of the above matters if the bacteria cannot grow in wort or must. The good news for mead makers is that botulism is unlikely to be able to grow in must, as there is not enough protein present. So even though the spores are common on honey, even a prolonged delay in the onset of fermentation should not present a risk. Ciders and wines are even more protected, as they have both a low protein content and a starting pH below 4.0. That said, the making of so-called prison wine has led to cases of botulism, largely due to the use of approaches that lead to higher pH’s and poor sanitation.
Beer wort is a very different beast. Beer wort has a high protein content and a pH permissive to the growth of botulism (>5.0). It is unclear whether hops inhibit growth of botulism, although inhibition has been observed with highly purified extracts.
The good news for conventional beer brewers is that normal brewing practices will prevent botulism from growing. The oxygen levels achieved by normal oxygenation processes are inhibitory to growth, while the acidificaiton during fermentation will suppress botulism long before it produces toxin. In other words, if you quickly chill your beer, oxygenate well, and immediately pitch a good dose of yeast, your risk is essentially zero.
There are, however, two brewing practices which may increase your risk of exposure:
- Coolship brewing. Beer chilled via a coolship has the potential for exposure to spores in the air. It may also create ideal fermentation conditions through low oxygen pickup, delayed fermentation, and prolonged time at optimal growth temperatures.
- No-chill brewing. This creates an ultra-low oxygen environment. Risk may be further increased due to delayed fermentation and prolonged time at optimal growth temperatures.
Normal brewing practices eliminate risk, but the question of whether we should stop coolship or no-chill brewing remains unanswered. The good news is that there is not one recorded case of botulism arising from either of these brewing practices, indicating that these processes can be regarded as safe. However, it is trivial to eliminate the risk, and given the extreme hazard represented by botulism, it is reasonable to take these precautions.
Coolship Brewing: Eliminating the risk during coolship brewing is trivial, and I would argue, is a natural result of best brewing practices. Pre-acidifying wort you intend to coolship by adding lactic or phosporic acid during the boil is sufficient to eliminate the risk. pH’s below 5.0 inhibit botulism, with pH’s below 4.5 reducing the risk of other pathogens. Pre-acidificaiton also has the advantage of suppressing bacteria that can produce unpleasant and toxic biogenic amines. Many home and commercial brewers have adopted the practice of pre-acidifying coolshipped worts for these reasons. This is a practice I both endorse and use myself.
No-Chill Brewing: The risk of botulism during no-chill brewing is largely overstated. Keep in mind that under ideal conditions it takes ~3 days for botulism to produce toxin. So if you are “no-chilling” overnight, or even for a day and a half, you are well within the safe period. So long as you oxygenate well and pitch a healthy dose of yeast the risk should be eliminated. However, some no-chill brewers store wort for prolonged periods of time. This represents an increased risk over conventional no-chill approaches. You can eliminate this risk by pre-acidifying the wort to a pH below 5.0, or by ceasing the practice of storing no-chill wort for more than a day.
One Final Note
I’ve thrown a lot of numbers and facts at you in this article. As much as possible I’ve linked to my sources so that you can check them out yourself. However, the bulk of the numbers regarding factors such as the time to toxin production came from a single printed source. These numbers can be found in the book:
Microorganisms in Foods 5: Characteristics of Microbial Pathogens.
By the International Commission on Microbiological Specifications for Foods. 1996, Chapman & Hall Publishers. Pages 66-110.