After the latest food recall, which has been recently extended, I wondered what exactly is botulism. All I could remember from food microbiology was that it was an food borne illness caused by the botulinum toxin which is produced by the bacteria Clostridium botulinum.

Clostridium botulinum bacteria

Described on Wikipedia as:

… one of the most poisonous naturally occurring substances in the world.

Botulinum is a neurotoxin and works by interfering with nerve function leading to muscle failure. Botulism is described by the US Centers for Disease Control and Prevention (CDC) as:

A neuroparalytic illness characterized by symmetric, descending flaccid paralysis of motor and autonomic nerves, always beginning with the cranial nerves. Symptoms include double vision, blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, and muscle weakness. If untreated, illness might progress to cause descending paralysis of respiratory muscles, arms and legs.

So basically if you get botulism you may end up dieing because your lung and heart muscles fail. Lovely.

It shouldn’t be a problem especially in commercially prepared food. The situations under which Clostridium botulinum can grow are well understood. Harold McGee states:

The arch villain of the canning process is the bacterium Clostridium botulinum, which thrives in low-acid, airless conditions – oxygen is toxic to it – and produces a deadly nerve toxin. The botulism toxin is easily destroyed by boiling, but the dormant bacterial spores are very hardy and can survive prolonged boiling. Unless they are killed by the extreme condition of higher-than-boiling temperatures (which require a pressure cooker), the spores will proliferate into active bacteria when the can cools down, and the toxin will accumulate. One precautionary measure is to boil any canned produce after opening to destroy any toxin that may be there. But all suspect cans, especially those bulging from the pressure of gases produced by bacterial growth should be discarded.

The low pH (high acidity) of tomatoes and many common fruits inhibits the growth of botulism bacteria, so these foods require the least severe canning treatment, usually about 30 minutes in a bath of boiling water to heat the contents to 185-195 oF/85-90 oC. Most vegetables, however, are only slightly acid, with a pH of 5 or 6, and are much more vulnerable to bacteria and molds. They are typically heated in a pressure cooker at 240 oF/116 oC for 30 to 90 minutes.

(On Food and Cooking 2nd Edition page 298-290.)

Thus, it is a anaerobic bacteria, which prefers a low acid environment. High sugar, salt or low moisture also discourage the bacteria from reproducing. In particular, canned meat products are risky as they have a relatively high pH and a high moisture content. In fact, the FDA has strict guidelines that manufacturers should follow when canning food products. As their summary states:

The purpose of 21 CFR 108, 113, and 114 is to ensure safety from harmful bacteria or their toxins, especially the deadly Clostridium botulinum (C botulinum). This can only be accomplished by adequate processing, controls, and appropriate processing methods, such as cooking the food at the proper temperature for sufficient times, adequately acidifying the food, or controlling water activity.

And the USDA has a whole series of publications on how to carry out home canning and preservation methods. On the first page of the introduction (pdf) there is this:

Caution: To prevent the risk of botulism, low-acid and tomato foods not canned according to the recommendations in this publication or according to other USDA-endorsed recommendations should be boiled even if you detect no signs of spoilage. At altitudes below 1,000 ft, boil foods for 10 minutes. Add an additional minute of boiling time for each additional 1,000 ft elevation.

Most food processing plants are set up with HACCP which is systematic approach to the identification, evaluation, and control of food safety hazards. These plans are devised to watch for errors in the processing line, such as the internal temperature of the can not reaching 240 oF/116 oC, and having a solution to resolve the problem. These days, HACCP is a key food safety step.

It will be interesting to find out how the recent outbreak of botulism occurred. Where did the system fail and how come HACCP failed to stop the contaminated food from reaching the consumer?


2 thoughts on “Botulism

  1. My mother used to work in a canning factory (probably in the 1940’s) and she also used to do a lot of home canning in the 1960’s. Anyone who is home canning vegetables, especially things that do not have large amounts of salty brine (like olives) should cook them after opening. They need to be boiled for 15 minutes with the lid OFF (because it has to be boiled in the presence of oxygen to kill any botulism toxin). I’m not saying that every jar of home canning has botulism present, but just in case it does, it’s wise to follow this procedure with every home-canned vegetable upon opening.

  2. Mary

    Thank you for your comment.

    I should have stressed that my comments were about commercially canned food. However, given the recent outbreak perhaps we should be boiling those canned foods too.

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