Harmful bacteria that cause food poisoning will multiply in particular conditions. Bacteria require certain conditions in order to grow and reproduce. Harmful bacteria require food, they need a supply of nutrients and usually find this in moist, protein-rich foods. They need moisture, without moisture, the growth of bacteria slows down and may stop. Dry foods do not support the growth of bacteria. They need warmth as food kept at the wrong temperature will allow harmful bacteria to multiply. Bacteria also require time to multiply. Food poisoning bacteria, under optimum conditions, can quickly multiply to dangerous levels.
Bacteria need moisture in order to stay alive. This is why they cannot multiply in dried foods such as powdered milk, or uncooked pasta. However, as soon as liquid is added to dried food, the reconstituted products are likely to provide conditions, which are ideal for bacterial growth.
Food poisoning bacteria will multiply within a certain range of temperatures. This range of temperatures is called the danger zone and high-risk foods should spend as short a time as possible at these temperatures. When the temperature falls below 5°C, such as in a refrigerator, the multiplication of bacteria will slow down or stop. At 18°C bacteria will multiply, but this is not the ideal temperature for multiplication. The ideal temperature is higher than this, but this is still in the danger zone. 37°C is the temperature at which harmful bacteria will multiply at the fastest rate. This is the average temperature of the human body. Cooking food at 70°C or above for sufficient time will kill most harmful bacteria.
Harmful bacteria multiply by splitting in two. This process is called binary fission. Most bacteria will multiply every 10 to 20 minutes. This means that after 10 to 20 minutes, one single bacterium will become two. After another 10 to 20 minutes, these two bacteria will become four. After a further 10 to 20 minutes, the four bacteria will have multiplied to become eight, and so on. It will take only a few hours for one bacterium to multiply to over 10 million bacteria.
Some foods are called ‘high risk foods’ because it is easy for harmful bacteria to multiply in them, even after they have been cooked. Harmful bacteria that cause food poisoning can grow and reproduce on a range of foods. They are most likely to multiply on food that is both moist and high in protein i.e. poultry, eggs, meat products, fish, milk and milk products, dairy products, cooked rice and pasta and meat.
Try to answer these questions yourself:
1. What four things do bacteria need in order to grow and reproduce?
2. Harmful bacteria multiply by splitting into two. What is this process called?