Fresh on last weeks cheese-centric post, we’re once again diving into a cheese making topic – the risk of botulism from cheese and how to limit that risk. I’ve written about botulism before – I did a deep dive 2020 into the topic as it relates to beer, and back in 2018 I wrote a post on general fermented food safety that touches on the topic. Cheese (and other fermented milk products) represents another area of fermented foods where botulism can be an issue.
What is Botulism
Botulism is a disease brought on by the consumption of botulinum toxin – a highly toxic protein secreted by the bacteria Clostridium botulinum. This toxin is the most poisonous substance known to human kind, and kills in a horrific manner. The toxin enters the nerves which control the contraction of our muscles, and once in the nerve, blocks the release of the neurotransmitter acetylcholine. Acetylcholine is needed to make our muscles contract, and as such the toxin prevents us from contracting our muscles, leading to flaccid paralysis. Without treatment, about half of patients will die as they loose the ability to inhale and thus die of slow asphyxiation. With treatment it is lethal in ~10% of cases globally, and 3-5% of cases in industrialized nations. Because botulism damages the nerves that control muscle contractions, the majority of patients who survive the initial disease suffer physical disabilities that last for decades. Long-term psychological issues are also common in survivors. Botulism infection is considered a catastrophic disease – meaning that if you get it you’ll end up in the hospital, even in the hospital you may die, and even if you survive you’ll suffer long-term consequences.
If that wasn’t nefarious enough, Clostridium botulinum itself is a real bastard. While the bacteria itself is somewhat of a lightweight – exposure to oxygen will kill it, as will exposure to modest heat (conventional pasteurization temperatures will finish it off), the bacterium can form spores which are incredibly hard to kill, with some surviving up to temperatures of 121C/250F! Clostridium botulinum grow well at room temperature, and in the absence of oxygen. Which is where the biggest threat comes from – spores can survive conventional canning approaches, and if the food is not acidic enough (below a pH of 4.5) or salty enough (>10% salt), the spores can germinate. The resulting bacteria can now grow, contaminating the canned food with the lethal toxin. But botulism can grow anywhere that provides a food source (a bit of protein will do), an absence of oxygen, and a pH and salinity that aren’t too oppressive. The interior of a cheese – especially a high-moisture cheese – being a prime environment.
Botulism is nightmare fuel!
…or is it?
While quite dangerous, Clostridium botulinum is not encountered very often. It is a normal part of soil microbiota, but they are a somewhat rare part of the soil microbiome and may even be absent from some soils. The exact determinants of which soils will harbour Clostridium botulinum are not completely understood, but factors such as soil pH and bacteria in the soil which complete for nutrients appear to play a role. In addition, botulism will not grow on the above-ground part of a plant (it cannot survive in oxygen), meaning that while plants can be contaminated with spores carried by dust or insects, the amount of spores present on leafy greens, spices, and fruit, tends to be low. Foods grown in-ground (e.g. potatoes, onions, garlic) are at a higher risk of carrying spores as they are in continual contact with soil.
Many foods are naturally resistant to botulism – bacteria will not survive, and spores will not germinate (“hatch” and form living bacteria) in acidic foods (pH below 4.5), in salt concentrations above 10%, or in alcohol concentrations above 2.5% ABV.
If you’ve read my previous posts on this topic, nothing I wrote about above is new. But in the case of cheese I need to address one additional point of the biology of Clostridium botulinum. There are two distinct lineages (essentially sub-species) of the bacteria that different in two major ways:
- Mesophilic strains: These bacterial strains only grow at temperatures above 12C/54F, with an optimum around 37C/98F. These strains of botulism are the ones which most often kill people as they produce highly durable spores which can only be killed at temperatures of 121C/250F. These are the ones which are an issue with canned foods, and are why the FDA (and most other food safety organizations) recommend pressure canning for most forms of food preservation.
- Psychrophillic strains: These bacterial strains will grow at temperatures as low as 1.5C/35F, meaning not even refridgeration can prevent them from growing. However, the spores of these strains are much less durable than those of the mesophilic strains, with 10 minutes of heating at 90C/195F sufficient to eliminate these spores.
Botulism in Cheese
This brings me to the risk of botulism in cheese. The good news is that cheese-associated botulism is very rare – globally, under 20 cases a decade are linked to cheese (note: link is to a PDF). That is not to say that there is no risk, but the risk is minimal if you make and store cheeses with due care and attention. In fact, if you raise your own milk-producing animals, they are hundreds of times more likely to kill you than is cheese-associated botulism (or botulism in general).
The risk from cheese comes from two places – contamination of milk, and contamination from additives. Contaminated milk is the major cause of cheese-associated botulism cases globally, but is easy to avoid – simply follow the safe milk collection practices discussed in my last post. The major risk here is dust and dirt carrying spores into milk – it will not come from the animals themselves.
The second risk is from additives such as spices, fruits, and vegetables. These can be contaminated with botulism spores, which can then be introduced into the cheese. Foods which grow below-ground (e.g. garlic an onions) are far more likely to have spores than are things which grow above ground. That doesn’t mean that a green herb has no risk of botulism, or that an onion is guaranteed to have it – you’re just far more likely to find it on the latter. An herb could be contaminated by dust, insects carrying spores, or even a farm worker dropping a harvested plant. Likewise, many onions are free of spores – botulism, while it lives in soil, is a pretty rare bacterium.
Reducing the Risk in Cheese
Luckily, reducing the risk of botulism from cheese is fairly straight forward.
Keep your milk clean, or buy from a trusted source: Any commercial (grocery store) milk should be safe, as will be any milk collected at home (or by a farmer) using a closed-circuit milking machine (e.g. one which doesn’t expose milk to the air during collection). Hand milking is a higher risk, but even then, the risk is low so long as you take care to prevent soil and hair from falling into the milk.
Clean your additives: Whole herbs/leaves and fruit should be cleaned in water to remove any surface spores (botulism won’t grow in/on the leaves, and will only be deposited on the surface). Whole spices can also be cleaned in this manner.
Cook your additives: If compatible with your recipe, blanching or cooking your additives can greatly reduce risk as it will eliminate the psychrophilic strains. Steaming, blanching, roasting, toasting, and frying are all cooking methods that will kill these strains.
Watch your temperatures: The spores of mesophilic strains will not germinate at conventional cheese-cave temperatures (10-13C). Meaning, if you’ve eliminated the psychrophilic strains, keeping your cheese ages at or below cheese cave temperatures will ensure that none of the mesophilic strains grow.
Only use additives in low-moisture, higher-salt cheeses: While the pH of cheese is rarely low enough to suppress spore germination (<4.5) and the salt level is never high enough to prevent germination (>10%), the combined effect of acidity and salinity can often suppress germination at pH’s and salt concentrations that on their own are not suppressive. This is especially true if this acidity and salinity is paired with low water activity (e.g. a hard/dry style cheese). Such a cheese, held at an appropriate temperature, is unlikely to support the germination of botulism spores or the growth of the bacteria. This PDF contains a risk assessment of different cheese styles for botulism based on these criteria.
Be careful with root vegetables: While onions and garlic are wonderful additions to cheese, they are also among the most risky as they grow in the soil and therefore have a higher likelihood of carrying bacterial spores. The best practice is to cook the root vegetable to eliminate any psychrophilic strains. Further protection can be found by always keeping the cheese at cheese cave (10-13C) or refrigerator (4C) temperatures after the root vegetable is added. Ideally, cheeses should be consumed quickly once the root vegetable is added. You can also choose styles of cheese which are less hospitable to botulism, as discussed in the previous point, to reduce risk.
Relax, Don’t Worry, Have Some Cheese
The risk of botulism is a complex topic that can be affected by many different factors. But the main message I’d like to impart is when it comes to cheese, the risk is low. Moreover, you can add an additional margin of safety by following some simple guidelines when making and aging cheese.