Botulism

tarrosion

My question is naive and simple: why aren't we all dead from botulism?

- The bacteria which create the toxin are extremely hardy; spores can survive boiling water, freezing, etc.

- These bacteria are all over the place, soil, water, and so forth.

- When we cook sous vide we often create an anaerobic environment such as these bacteria require to grow.

- If I recall from Douglas Baldwin's excellent book, the center of a piece of meat may be sufficiently anaerobic for these bacteria to grow. Though interior muscle is usually sterile, bacteria can spread eg by mechanical tenderization.

- Only a tiny bit (micrograms) of neurotoxin is required to induce catastrophic effects in a human.

I believe I understand that the spores/bacteria do not produce the neurotoxin at many cooking temperatures, and further, the neurotoxin is rapidly brokend down by moderate heat (even though this doesn't kill the bacteria or spores). Rapidly chilling food after cooking is required so that the spores do not have time to germinate and produce neurotoxin.

Still it seems like one little mistake - chill too slowly after cooking sous vide, wait too long between cooking and eating, let something sit sealed in the fridge a little too long, put something too warm in the fridge which temporarily raises the temperature of a sealed bag, etc. - and major bad news.

So why aren't we all dead?

DouglasBaldwin_31850

Hi Evan,

Always a good question. My answer is rather detailed, I hope you don't mind. Let's review the steps from purchase to plate and consider the risk from Clostridium botulinum at each step:

(1) Buy your food: As you rightly point out, C. botulinum is found in soil, water, and the sludge near water. Almost all foods potentially have spores and active, vegetative cells of C. botulinum – meat, poultry, fruits, vegetables, and starches typically have 0.01 pathogens per gram while fish and shellfish have 1.0 pathogens per gram. There are seven types (A through G) of C. botulinum: types A, B, and E cause nearly all the human botulism cases, types F and G very rarely cause human botulism, and types C and D cause botulism in birds and animals. Type A and some strains of B are proteolytic, which means they can split proteins into amino acids and so produces off flavors and odors in the food. Type E and some strains of B are non-proteolytic, which means they can produce toxin without changing its flavor or odor. Type E is most often associated with fish and shellfish and can grow at refrigerator temperatures.

(2) Store your food: Types A and proteolytic B can grow at temperatures between 10 and 48 C (50 and 118 F). They grow fastest between 37 and 40 C (98.6 and 104 F); at 37 C (98.6 F), the generation (or doubling) time is about 42 minutes after an initial lag of about 5 hours. At lower and higher temperatures, the lag and generation times are longer; at 15 C (59 F), the lag is about 160 hours and the generation time is about 28 hours. The temperature growth range of types E and non-proteolytic B is 3.3 to 45 C (38 to 113 F). Not many home fridges are below 3.3 C (38 F) – many spend most their time above 5 C (41 F) and some spend most their time over 10 C (50 F) – so we expect that C. botulinum can (slowly) grow in your refrigerator. While freezing decreases the number of active, vegetative cells it has no effect on spores nor already formed toxins.

(3) “Cooking”: As you note, the vegetative cells are easy to kill – they are much easier to reduce to safe levels than the Salmonella species or Listeria monocytogenes or the pathogenic strains of Escherichia coli. But those spores are tough cookies and they pose a real risk: if even 10 spores survive and multiply to a population of 10,000 to 100,000 cells per gram of food, which is 10 to 13 generations, then they produce sufficient toxin to cause illness. Short of using a pressure cooker (or sterilizer), it's unlikely that you'll use enough heat to reduce the number of spores for storage at room temperature. But you can inhibit growth by reducing the pH level to below 4.6 or using a salt (NaCl) concentration of at least 10% (for proteolytic strains). You can also reduce the “water activity” of the food to below 0.94; a food's water activity is the ratio between its water vapor pressure and pure water's vapor pressure at a that temperature. Water activity is easy to measure with dedicated lab equipment but very hard to measure at home. You can reduce a food's water activity by drying, by adding sugar, by adding salt, by doing a bunch of different things, but it's very hard to predict how much the water activity will change. Many traditional food preservation techniques, like those used in charcuterie, combine several of these techniques so the pH doesn't have to be lower than 4.6 or the salt higher than 10%, but there is relatively little scientific research on these traditional techniques. As for destroying the toxin that was present before cooking, traditional techniques that use fairly high temperature work great: 79 C (174 F) for 20 minutes or 85 C (185 F) for 5 minutes will inactivate the toxins. But reheating to 55–60 C (130–140 F) is unlikely to sufficiently inactivate any toxins. The only other risk is if the food is heated really slowly – more than six hours until the core reaches 55 C (130 F) – so C. botulinum is able to grow significantly (3 generations) while cooking. 

(4) (a) Eating: If you cook and eat your food while it's still above 48 C (118 F), then the only risk of botulism is from food that had sufficient toxin to produce illness before cooking (and that cooking wasn't sufficient to reduce the amount of toxin). This is why people recommend boiling canned food before eating – especially home canned food – since boiling would inactivate the toxins. Since the typical concentrations in raw food are quite low, there is much more danger in not reducing the Salmonella species, L. monocytogenes, and the pathogenic strains of E. coli. (b) Cooling, storing, and reheating: If you plan to preserve the food for later, this is where the botulism risk becomes significant. The first thing is making sure the cooling is fast enough that the spores don't become active bacteria – remember the lag I mentioned earlier – so the risk of growth is lower at refrigerator temperatures. The next thing is to make sure you don't store it for too long. In my book and other places, I've recommend refrigerating for only one or two days and freezing if you plan to store it for a longer amount of time because I know that a lot of home refrigerators are warmer than 3.3 C (38 F). If you have a refrigerator thermometer (check a few places to find the warmest) then you might feel safer storing for longer. There is relatively little risk when reheating so long as you don't heat it too slowly – see Table A-2 in Appendix 4 of Health and Human Services' Fish and Fishery Products Hazards and Controls Guidance for time and temperature guidance. 

So all of this begs your question of why botulism isn't more of a problem. After all, there have not been any reported cases of botulism from food that's been cooked sous vide. I think one reason is that people cooking sous vide tend to know a lot about food safety. Another that cook-and-serve, which is how a lot of people cook sous vide, removes most the botulism risk. And those who cook-and-chill sous vide are fairly worried about botulism and use ice-water baths or blast chillers for cooling and use a thermometer to make sure their refrigerator is cold enough. We know from home canning that the risk is real, and like home canning a food-safety culture has developed around sous vide cooking. But I'd love to hear your opinions and thoughts.