Growth, spores and toxins

Basic training teaches that bacteria need food, moisture, warmth and time. As a supervisor, you need to understand what happens during that time, because your controls are based on it.

Bacteria reproduce by splitting in two. Some can double every 10 to 20 minutes. If 1,000 bacteria on cooked chicken double every 20 minutes, there will be about 64,000 after two hours. The food can still look, smell and taste normal.

A group of bacteria in food follows a growth curve with four phases. In the lag phase the bacteria adapt to their surroundings, and numbers hardly change. In the log phase they multiply as fast as they can. In the stationary phase growth slows, because food and space run out and waste builds up, so numbers stay about level. In the decline phase more cells die than are produced.

The lag phase is where you want food to stay. Its length is not fixed. It is shorter in warm conditions and longer in cold ones. If food is kept chilled and used quickly, bacteria may never reach the log phase. Time also adds up. Ten minutes on a delivery bay, an hour on a prep bench and an hour on a counter all count. This is why a good system limits the total time out of temperature control, not only the time at each step.

Other factors also limit growth. Bacteria need water, so drying food, or adding salt or sugar, slows them down. Most do not grow well in very acidic food. Some need oxygen, but others grow without it, for example in the middle of a stew or in a vacuum pack. Growth is fastest at around 37°C. The danger zone is 8°C to 63°C.

Temperature affects bacteria in different ways. Chilling slows most bacteria but does not kill them. Freezing stops growth but does not kill them either, so bacteria can start again when food thaws. Proper cooking kills active bacteria, but it does not always destroy spores or toxins.

Some bacteria, including Bacillus cereus and Clostridium perfringens, can form spores. A spore is a tough, resting form of the cell. It can survive cooking. When cooked food cools slowly through the danger zone, spores can germinate into active cells, which multiply quickly. Cooking has also killed many other bacteria, so the survivors have less competition. This is why cooked rice and large meat dishes are high risk when they are cooled slowly.

Some bacteria produce toxins, which are poisons. Staphylococcus aureus and B. cereus can produce toxins in food. Some of these toxins resist heat. They survive cooking and reheating, so food can still cause illness after the bacteria themselves have been killed.

This is the key lesson for supervisors: reheating is not a rescue. Food that has been left warm for hours cannot be made safe by heating it until it is piping hot. Keep food out of the danger zone as much as you can. Cool cooked food quickly, so that spores do not have time to germinate and multiply. Never rely on a final heating step to fix an earlier failure.