Many food-poisoning bacteria have to multiply to high numbers in food before they are likely to cause illness. Bacteria need food, moisture, warmth and time in order to grow.

Food

Pathogens like to multiply in nutritious foods like meat, chicken, fish (particularly shellfish), cooked rice and pasta, milk products and eggs. They also like anything which contains these foods such as meat pies, sandwiches, gravy, salads, etc.

Pathogens will grow in both raw and cooked foods. Raw meat and poultry contain pathogens, but thorough cooking will kill the pathogens and make the food safe to eat. Eating food which has not been cooked or heat-treated may lead to food poisoning.

The following foods have often been involved in food poisoning:

  • unpasteurised (untreated) dairy products such as milk and cheese;
  • foods containing raw eggs such as mayonnaise and mousse;*
  • uncooked shellfish such as oysters and mussels.

Bacteria are also found in cooked food if it has not been cooked properly or if it gets contaminated after cooking. Sometimes bacteria can produce spores when foods are cooked can become active bacteria again if the cooked food is kept in warm temperatures (within the growth zone). Cooked foods must therefore be protected from contamination and kept at the right temperature (either hot or cold).

Moisture

Most foods contain enough moisture (water) for the bacteria to grow. Bacteria will not grow in dry foods such as milk powder and soup mixes. But once water is added they can grow again.

Warmth / Temperature

Some bacteria grow at low temperatures (usually below 20°°C) and some at high temperatures (above 45°C).

But most like warm temperatures between 5°C and 63°C, commonly referred to as the growth or 'danger' zone. The optimum temperature for growth is about 37°C (human body temperature), when pathogens multiply most quickly. They will stop growing altogether above 63°C, but to destroy bacteria temperatures must rise further. A temperature of 70°C for 2 minutes is recommended to kill pathogens during the normal cooking process.

Time

In ideal conditions (i.e. in moist foods at 37°C) bacteria will grow and multiply by dividing into two every 20 minutes. After 6 hours, in these conditions, one bacterial cell could become 131,072 bacteria.

Other Factors which affect Growth

  • Acidity or alkalinity of foods ( most bacteria like neutral conditions (pH value of 7).
  • Oxygen ( pathogens vary in their oxygen requirements.
  • Other bacteria in the same food compete for the same nutrients, so many will die.

* Note: Since raw eggs may sometimes contain food-poisoning bacteria, the Government advises that people who will catch an infection easily (such as babies, toddlers, older people, those who are already ill and pregnant women) should only eat eggs that are thoroughly cooked until both the yolk and white are solid. They suggest that for other people there is little risk from eating cooked eggs.