Food rots when microscopic organisms, such as fungi and bacteria, start to live and grow on it. These organisms are found everywhere. They are in the air, on shelves and tabletops, and on your skin. When food is left exposed, or is touched by fingers, it is easy for micro-organisms to settle on it, feed, and build up their numbers. You can keep food from spoiling quickly by storing it in a sealed container. Food is also protected by freezing it or drying it or by adding chemicals to it that prevent the micro-organisms from growing. Although some types of micro-organisms spoil our food, we use other micro-organisms to make food. For example, if you look at the list of ingredients on a container of yogurt or sour cream, you will see that they contain "bacterial culture." The bacteria produce the yogurt and sour cream from milk. Other types of micro-organisms make cheese from milk.

Yeast, which is a type of fungus, is used by bakers to make traditional bread. The yeast is added to the dough mixture, where it feeds on the sugar and produces tiny bubbles of carbon dioxide gas. This gas is what makes the bread rise and forms the tiny holes that you see in a slice of bread. Other foods that micro-organisms help produce are wine, pickles, and chocolate.

(from the files of the former CBC4Kids)