We’ve already seen the four conditions required for bacteria to multiply:
We’ve also looked at high risk foods. Now let’s look at the other conditions in more detail.
Bacteria need moisture to grow and this can be found in many high risk foods. Bacteria are much less likely to survive in dry food such as dried eggs or powdered milk. But any bacteria that do survive under dry conditions begin to multiply again when fluids are added to the food.
One of the reasons why salt and sugar discourage bacterial growth is because they take up moisture, thus denying it to the bacteria. Similarly, when food is frozen its moisture turns to ice and is no longer available to the bacteria.
Bacteria that cause food poisoning grow at temperatures between 5°C and 63°C (40°F-145°F). They grow most rapidly at a temperature of 37°C (98°F), which is the normal temperature of the human body.
For this reason the range of temperatures between
5°C and 63°C is known as the TEMPERATURE DANGER ZONE
Even a small number of bacteria can grow rapidly in food which is allowed to remain in the danger zone. Temperatures outside the zone are less suitable for bacteria. That’s why cooks should keep hot food hot and cold food cold.
Although bacteria thrive in warmth, most are killed off by heat. Most are killed off by a temperature of at least 70°C, providing this is reached at the centre of the food and is held for a sufficient time.
However, some bacteria and their toxins require a higher temperature for a longer period of time before they are destroyed. Pasteurisation is an example of a method used to destroy such bacteria.
In cold conditions below 5°C bacteria either do not grow or grow very slowly. At very low temperatures some will die, but many will survive and begin to grow again if returned to warm conditions.
Always remember to
STORE HIGH RISK FOODS BELOW 5°C OR ABOVE 63°C
Given moisture, warm food bacteria then need time to grow. Being careless and leaving food in the temperature danger zone allows bacteria the time they need to multiply.
Each cell multiplies by splitting into two, then two become four, four become eight and so on. This is known as binary fission.
If the temperature is suitable, the bacteria will reproduce in this way every 10 to 20 minutes. Some take even less time. This means that one cell can produce thousands of cells of bacteria in about four hours.
Things to remember about bacteria:
- Food poisoning bacteria are invisible to the naked eye.
- They usually do not cause any change in the appearance of food.
- They usually do not cause any change in the smell or taste of food.
Therefore you cannot rely on your senses to tell if food is contaminated or not.
This is Chapter Two, Part Four in a series of articles broadly similar to the course offered by the Royal Society for the Promotion of Health for its Food Hygiene Certificate.
Part Five: Common forms of food poisoning bacteria
Return to the start of Chapter Two