Ignore the storage guidelines for bottled vege products at your peril.
ANSWER: Refrigeration helps prevent the production in bottled vegetables of a toxin that is 100,000 times more lethal than Sarin, the so-called weapon of mass destruction. How can a harmless-seeming jar of tomatoes or olives in oil potentially contain such a powerful poison?
The answer is through the presence of Clostridium botulinum, a bacterium that can produce the neurotoxin botulinum that causes the potentially fatal illness botulism.
Botulinum inhibits a neurotransmitter in the nervous system, leading to paralysis. It starts in the muscles of the face and spreads towards the limbs. In severe cases, botulism paralyses the muscles required for breathing, resulting in respiratory failure and death.
Given its severity, any suspected case of botulism is treated as a medical emergency and food manufacturers are rigorous in their efforts to prevent Clostridium botulinum from poisoning their products.
In the 1970s and 1980s, before food-safety scientists fully understood how and in what conditions botulinum was produced, there were several outbreaks of botulism. In the US in 1985, 37 people got the illness after eating a garlic-in-oil preparation made in a restaurant.
Home-bottled garlic in oil was linked to other cases of botulism in California, Florida and Denmark. And between 1994 and 1998, more than 100 cases of the illness in Italy were traced to home-prepared vegetables stored in oil or water.
In 1998, a case of botulism type A (from the most lethal botulinum toxin) was linked to home-prepared mushrooms covered in oil, and two further cases implicating oil-covered mushrooms occurred in the UK.
The involvement of mushrooms is not unexpected, because Clostridium botulinum is commonly found in soil and water, which means produce that comes in close contact with the ground is at risk. But that doesn’t mean the fresh mushrooms you buy at the greengrocer are hazardous, because botulinum is produced only under certain environmental conditions.
Clostridium botulinum also likes a less acidic environment. So, food manufacturers use acidity regulators such as citric acid, or add vinegar, to keep the pH below 4.6.
In the case of tomatoes, their pH is normally just below 4.6 and decreases further once they’re dried and the natural-acid components are concentrated. So, in their sun-dried state, they have a reduced susceptibility to botulinum production.
However, a low pH doesn’t stop other microbial contamination, such as from yeast. Once a jar is opened and oxygen is present, yeast can grow, although that doesn’t pose a health danger. And as any baker of bread will tell you, yeast doesn’t like cool temperatures such as those in a refrigerator.
Clostridium botulinum is also averse to low temperatures. The bacterium thrives in temperate conditions, which is why we’re instructed to store opened jars of vegetables in the fridge. The lower the temperature, the less likely it is that botulinum will produce its harmful toxin.
But even at 8°C, Clostridium botulinum can produce toxins within 10 days or fewer. For that reason, the label on bottled-vegetable products often states that they must be eaten within a certain time of opening.
This article was first published in the September 21, 2019 issue of the New Zealand Listener.