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Food Focus August 2008

Bulk cooked food can breed bulk bacteria

Although not a notifiable disease in New Zealand, in 2007 Clostridium perfringens was the second highest agent linked with outbreaks of foodborne illness, surpassed only by Campylobacter

Clostridium perfringens is a normal component of decaying vegetation and the bacteria is commonly found in the gut of animals. It usually makes its way into our food on meat (contaminated during slaughter) and foods of animal origin, as well as through soil and dust on vegetation.

Sometimes called the ‘food service germ’, most cases of illness have arisen from food cooked in bulk that has been cooled too slowly or left for long periods at room temperature. In these situations where large quantities of food have been served, many people will be affected if an outbreak does occur. (See the article ‘Turkey goes belly up’.)

In 2007, 13 outbreaks were attributed to C. perfringens (11 of these stemmed from mass catering) and 87 people fell ill. Outbreaks in New Zealand have been reported from minestrone soup; spinach boiled with fried bean curd; gravy; rolled or stuffed meat and poultry; roast pork and lamb; cold cooked chicken; and wontons. Implicated foods include meat (lamb, beef, pork); poultry (turkey and chicken); fresh produce; and rice/noodles/pasta.

Cells multiply very quickly

C.perfringens has two forms: spores and vegetative cells. The spores are very resilient and can survive cooking. When the spores germinate they produce vegetative cells. Warm temperatures and the absence of air allow spores of C. perfringens to germinate, releasing vegetative cells. If we eat food containing these cells they produce more spores in our intestines and release the toxin that makes us ill.

The cells can double in number every seven minutes at optimal growth temperatures (43–47°C) and in acidic, anaerobic conditions (such as inside intestines). Unfortunately for us, these conditions can occur in thick stews, gravy or pies, where cooking drives off air (or pastry excludes air), and the food cools too slowly through the temperature danger zone (60–4°C).

Controlling the breeding ground

Because C. perfringens is adapted to life inside intestines, it is able to turn a pie, stew or gravy into a nasty surprise – vegetarian curries or soups are not exempt either. Slow cooling and poor reheating of food are the main culprits as they allow C. perfringens to grow. By far the best technique for avoiding contamination by C. perfringens cells is to follow the 4Cs (clean, cook, cover, chill).

C.perfringens spores are highly resistant to most control factors – even surviving being boiled for at least an hour! Any toxin produced in food as spores develop can be deactivated if food is held at 60°C for five minutes. In some foods, if numbers of bacteria are large enough to release sufficient toxin into food to cause illness, the food is usually spoiled and therefore unlikely to be eaten.

Vegetative cells are rapidly killed with cooking and freezing, and slowly decline under refrigeration. They can also be controlled by changing the pH (the optimal pH range is 6–7, which is the range of many cooked meat and poultry products). Reducing the water content of the food by adding salt, or using combinations of salt with preservatives, such as sodium nitrite, are also effective control measures.

Visible symptoms

Illness is usually apparent within 10–12 hours of eating contaminated food but can occur anywhere between 6 and 24 hours. Symptoms include profuse watery diarrhoea with severe abdominal pain – vomiting and nausea are rare – and recovery is usually rapid (within 24 hours). Hospital treatment is generally not required. People with low immunity may suffer more severe effects but the disease is rarely fatal.

Other members of the Clostridium genus also cause well-known diseases from the release of toxins. C. tetani causes tetanus or lock-jaw and people can contract the disease through wound contamination. Food contaminated with C. botulinum causes botulism, but the disease is rare in New Zealand (no cases reported since 1985). Exposure to the bacteria is usually from home-canned or home-bottled products that have not been heated enough during processing.

More information about foodborne illnesses and Clostridium perfringens is available here.