Bread is a staple food of European, Middle Eastern and Indian (or in other words Indo-European and Semitic) cultures which is prepared by baking, steaming, or frying dough. Bread consists minimally of flour and water; salt is present in most cases; and usually a leavening agent such as yeast is used. Breads may also contain some amounts of sugar, spices, fruit (such as raisins, pumpkin or bananas), vegetables (like onion or zucchini), nuts, or seeds (such as caraway, sesame or poppy seeds). There are a wide variety of breads, with preferences differing from region to region. Fresh bread is prized for its taste and texture, and retaining its freshness is important to keep it appetizing. 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 airtight containers such as a breadbox to keep it fresh longer. Bread that is kept in warm moist environments is prone to the growth of mold. It becomes stale more quickly in the low temperature of a refrigerator, although by keeping it cool, mold is less likely to grow. The inner soft part of bread is referred to as the crumb (not to be confused with small bits called "crumbs"). The outer hard part of bread is called the crust. The latter is in common usage, however "crumb" is used mainly by professionals.