How bacteria grow

Bacteria are tiny living things. You cannot see, smell or taste them, so food that looks fine can still be dangerous. Most bacteria are harmless. The harmful ones are what food safety rules are designed to control.

Harmful bacteria reach a kitchen in many ways. They arrive on raw meat, poultry, eggs and unwashed vegetables. They come in on people, especially on hands and in noses. Pests, waste, dirty cloths and dirty equipment carry them too.

Bacteria need four things to multiply: food, moisture, warmth and time. Take one away and you slow them down or stop them. Moist, protein-rich food suits them well. Dried food is lower risk, until water is added to it. Salt, sugar and acid also make food harder for bacteria to grow on, which is why jam and pickles keep well. Always follow the label, because many of these foods need a fridge once opened.

In good conditions, some bacteria can double every 10 to 20 minutes. If they double every 20 minutes, one bacterium becomes more than 1,000 in under three and a half hours. This is why a short time out of the fridge matters.

The danger zone is 8°C to 63°C. Bacteria multiply in this range, and growth is fastest around 37°C, which is body temperature. A warm kitchen is a good place for them to grow.

Cold slows most bacteria down, but it does not kill them. Freezing stops them growing, but it does not kill them either. When food warms up, they can start again. Thorough cooking kills most bacteria. This is why so much of this course is about temperature.

High-risk foods are ready-to-eat foods that bacteria grow on easily. Examples are cooked meat and poultry, cooked rice, dairy products, egg dishes, gravies and sandwich fillings. Keep them out of the danger zone as much as you can.

Time matters as much as temperature. Take only what you need from the fridge. Put food back as soon as you have finished with it. Do not leave food in the danger zone while you do another job.

Imagine a tray of cooked chicken left on a warm bench while you deal with a delivery. It has food, moisture and warmth, and every minute gives it more time. That is everything bacteria need. Putting the tray in the fridge first takes away the warmth.