Botulinum, from black olives in water to homemade preserves: which foods is it found in?

" You can't die from botulism in 2025 , and there can't be so many poisonings, with cases first in Sardinia and now in Calabria. There is too much ignorance and superficiality in food preservation and preparation and in the fight against infections. We need to be more careful, and it seems to me that the topic is not on the prevention agenda." This was stated to Adnkronos Salute by Matteo Bassetti , director of Infectious Diseases at the San Martino Polyclinic Hospital in Genoa, speaking about the suspected botulism poisoning in Cosenza that has reportedly caused the death of a 52-year-old man and left several people hospitalized in the Cosenza hospital.

In Italy, "the home-produced foods most responsible for botulism cases are black olives in water, canned mushrooms in oil, canned turnip greens, and canned meat and fish (especially tuna)." This is the finding from the Italian National Institute of Health on its website page dedicated to botulism, a neuroparalytic disease caused by clostridial toxins (Clostridium botulinum, the most well-known of the clostridia that produce botulinum toxin).

Symptoms of botulism

"Foodborne botulism can affect people of all ages and is not transmittable from person to person," experts point out. "Symptoms typically appear a few hours to over a week after consuming contaminated food (6 hours, 15 days). However, in cases of foodborne botulism occurring in Italy, symptoms typically appear within 24-72 hours after consuming the contaminated food. Obviously, the earlier the symptoms appear, the more severe the illness will be." First in Sardinia, now in Calabria, several cases of possible foodborne botulism have been reported in Italy in recent weeks.

Symptoms can range from mild, self-resolving forms to very severe, potentially fatal forms. The most common symptoms are: blurred and double vision; dilated pupils; difficulty keeping the eyelids open; slurred speech; difficulty swallowing; dry mouth and throat; and constipation. "In the most severe forms, respiratory failure occurs, which can be fatal due to blockage of nerve conduction to the muscles responsible for respiration. The symptoms, as they explain, are characteristic of botulism: it has a symmetrical progression (affecting both the right and left hemispheres of the body) and a descending progression (from the head, neck, chest, and even paralysis of the limbs), and manifests as flaccid paralysis."

Since the spores of botulinum toxin-producing clostridia are ubiquitous, they can also contaminate foods and raw materials. However, not all foods are at risk of botulism. A food product—the ISS warns—is at risk of botulism if it contains the conditions for germination and vegetative development of the spores of botulinum toxin-producing clostridia. Botulinum toxins, in fact, are produced during the multiplication of the microorganism.

Non-risky foods

Since one of the essential conditions for spores to develop is the absence of oxygen, fresh foods (salad, bread, pasta) are not at risk of botulism. However, canned and semi-canned foods that are not acidic or have not undergone acidification/fermentation treatments are at risk. Industrially produced canned foods are generally not dangerous because the production technologies are well standardized and allow for the control of the development and toxigenicity of clostridia that produce botulinum toxin. Home-made canned foods are more dangerous because they are often made following recipes passed down over time and revised to reduce the amount of protective agents (primarily vinegar and salt), without any scientific basis.

Homemade preserves

To make safe vegetable preserves at home, "you need to cook the vegetables in a solution containing equal parts water and vinegar, jar the vegetables, add oil if necessary, and pasteurize after sealing the jar. Pasteurization is a heat treatment that can be done at home by completely immersing the filled, hermetically sealed jar in water, which is then brought to a boil for a time proportional to the size of the jar. For 350-400 g jars, 15-20 minutes of treatment may be sufficient," advise the ISS experts.

Jams and marmalades should instead be made using equal parts fruit and sugar. If the sugar content is reduced, lemon juice should be added in sufficient quantities to make the fruit acidic enough (pH <4.6) to inhibit the growth of clostridia that produce botulinum toxin. To be safe, olives and other pickled products should be prepared using 100 g of salt per liter of water, the experts continue.

"Homemade canned meat or fish should be avoided. These products cannot be acidified or treated with sufficient amounts of salt or sugar to inhibit the growth and toxigenicity of botulinum toxin-producing clostridia. Their safety depends on the destruction of any spores present, using a thermal process called sterilization. This process is carried out with superheated steam at a temperature of 121 degrees Celsius and a pressure of 2 atmospheres and cannot be performed with the tools normally found in home kitchens," concludes the ISS.

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