Bread is a staple food of Europe, the Middle East and India that is prepared by baking, steaming or frying dough. The primary ingredients are flour and water. Salt, fat and a leavening agent such as yeast are usually used too. There are many types of breads which may contain other ingredients: milk, egg, sugar, spice, fruit (such as raisins), vegetables (such as onion), nuts (such as walnuts) or seeds (such as poppy seeds). Fresh bread is prized for its taste and texture, and retaining its freshness is important to keep it appetising. Bread that has stiffened or dried past its prime is said to be stale. Modern bread is often 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 mould. It becomes stale more quickly in the low temperature of a refrigerator, although by keeping it cool, mould is less likely to grow. The inner, soft part of bread is referred to as the crumb, which is not to be confused with small bits of bread that often fall off, called crumbs. The latter term is in common use, while crumb is an esoteric word used mainly by culinary professionals. The outer hard portion of bread is referred to as the crust.