Mould or mold is a type of tiny fungus. There are thousands of different kinds. They are often seen in wet places. Molds include all species of microscopic fungi. In contrast, microscopic fungi that grow as single cells are called yeasts.

Although some molds cause disease or food spoilage, molds are not always bad. For example, people can use them to make some kinds of cheese and antibiotics. Also, mold can be used to make penicillin.

Modern bread is sometimes wrapped in paper or plastic film, or stored in an airtight container such as a breadbox to keep it fresh longer. Bread that is kept in warm, moist environments is prone to the growth of mold. Bread kept at low temperatures, for example, in a refrigerator, will develop mold growth more slowly than bread kept at room temperature. However, unwrapped bread kept in a typical household refrigerator will turn stale quickly due to the low humidity of the air.

Calcium propionate is commonly added by commercial bakeries to retard the growth of molds.

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