Microbiological hazards, mainly bacteria, are the most common cause of food poisoning. They are invisible to the naked eye, so you cannot tell by looking whether food is contaminated.

This module explains how bacteria grow, which foods are most at risk, and how temperature, time and good hygiene are used to control them during manufacturing.

Bacteria are the hazard that causes most food poisoning, so this is a key module. It covers:

• Bacteria, and how they grow.
• High-risk foods.
• The temperature danger zone, and temperature control.
• Cross-contamination.
• The bacteria that commonly cause food poisoning.
• Spores and toxins.
• Viruses, moulds and yeasts.

Bacteria are tiny living organisms, far too small to see. Most are harmless, and some are useful, but harmful bacteria, called pathogens, cause food poisoning.

Pathogens cannot be seen, smelt or tasted. A product can look and smell perfect and still make people ill.

Bacteria are everywhere: in raw materials, soil, water and dust, on pests, and on people’s hands, skin and hair.

Bacteria need four things to grow: food, moisture, warmth and time. Remove any one of these, and growth slows or stops.

In the right conditions, bacteria multiply by splitting in two, roughly every 10 to 20 minutes. One bacterium can become millions in a few hours.

This is why food must spend as little time as possible at warm temperatures, during processing, packing and storage.

High-risk foods are ready-to-eat foods that support the growth of bacteria and will not be cooked again before they are eaten. Examples include:

• Cooked meat and poultry.
• Milk, cream, cheese and other dairy products.
• Cooked rice.
• Eggs and egg products.
• Shellfish and seafood.
• Prepared salads and sandwiches.

They must be kept apart from raw food, and stored chilled.

The temperature danger zone is between 5°C and 63°C. This is the range in which bacteria multiply most quickly. They grow fastest at around body temperature, 37°C.

Cold slows bacteria down, but does not kill them. Heat, at high enough temperatures for long enough, does kill them.

To keep food safe, keep cold food at or below 5°C, and hot food at or above 63°C, and move food through the danger zone as quickly as possible.

• Chill high-risk food, and keep it at or below 5°C.
• Cook food to a core temperature of 75°C, or an equivalent, such as 70°C for 2 minutes.
• Hold hot food for service or display at 63°C or above.
• Cool cooked food as quickly as possible, and put it in the fridge within two hours.
• Keep frozen food at -18°C or colder.

You will learn more about each of these controls in the modules on storage and food preparation.

Cross-contamination happens when bacteria are transferred from raw food, hands, cloths or equipment to ready-to-eat food. For example, using the same unwashed knife for raw meat, and then for cooked meat.

To prevent it, keep raw and ready-to-eat food, equipment, staff and work areas separate. Use colour-coded boards, utensils and containers to avoid mix-ups, and clean and disinfect equipment between tasks.

Bacteria that commonly cause food poisoning include:

• Salmonella, found in poultry, eggs and raw meat.
• Campylobacter, found mainly in raw poultry, and one of the most common causes of food poisoning.
• E. coli, including dangerous strains linked to undercooked beef and unwashed produce.
• Listeria, which can grow at fridge temperatures, and is linked to chilled ready-to-eat foods.
• Clostridium perfringens, linked to meat dishes left in the danger zone.

Some bacteria protect themselves by forming spores, which can survive cooking. Others produce toxins, or poisons, that ordinary cooking will not destroy.

Examples include Bacillus cereus, linked to cooked rice, Staphylococcus aureus, which often comes from people’s hands, and Clostridium botulinum.

If cooked food is cooled slowly or stored warm, spores can grow into active bacteria, and toxins can build up. This is why correct cooling and storage are so important.

Bacteria are not the only microbes that matter.

• Viruses, such as norovirus and hepatitis A, do not grow in food, but they are easily spread through poor hand hygiene, sick workers, and contaminated food or water.
• Moulds and yeasts spoil food, and some moulds produce harmful toxins.

Good personal hygiene, reporting illness, and correct storage keep all of these under control.

To control microbiological hazards:

• Wash your hands thoroughly and often.
• Keep raw and ready-to-eat food separate.
• Cook and reheat food thoroughly.
• Keep food out of the danger zone.
• Cool food quickly, and store it correctly.
• Keep surfaces and equipment clean and disinfected.
• Report illness before starting work.