Thought to have been growing since Paleolithic times and cultivated for at least 6,000 years, wheat is the world's largest cereal-grass crop. Its status as a staple is second only to rice. One reason for its popularity is that — unlike other cereals — wheat contains a relatively high amount of gluten, the protein that provides the elasticity necessary for excellent breadmaking. Though there are over 30,000 varieties of wheat, the three major types are hard wheat, soft wheat and durum wheat. Hard wheat is high in protein (10 to 14 percent) and yields a flour rich in gluten, making it particularly suitable for yeast breads. The low-protein (6 to 10 percent) soft wheat yields a flour lower in gluten and therefore better suited for tender baked goods such as biscuits and cakes. Durum wheat, although high in gluten, is not good for baking. Instead, it's most often ground into semolina, the basis for excellent pasta. In the United States, wheat is also classified according to the time of year it is sown — namely, spring wheat and winter wheat (which is actually sown in the fall). The unprocessed wheat kernel, commonly known as a wheat berry, is made up of three major portions — bran, germ and endosperm. Wheat bran, the rough outer covering, has very little nutritional value but plenty of fiber. During milling, the bran is removed from the kernel. It's sold separately and used to add flavor and fiber to baked goods. Wheat germ, essentially the embryo of the berry, is a concentrated source of vitamins, minerals and protein. It has a nutty flavor and is very oily, which causes it to turn rancid quickly. Wheat germ is sold in both toasted and natural forms and is used to add nutrition to a variety of foods. Wheat germ oil, an extraction of the germ, is strongly flavored and expensive. The wheat endosperm, which makes up the majority of the kernel, is full of starch, protein, niacin and iron. It's the primary source of many wheat flours. In addition to flour, wheat is available in several other forms including wheat berries, cracked wheat and bulghur wheat. Wheat berries are whole, unprocessed kernels, whereas cracked wheat is the whole berry broken into coarse, medium and fine fragments. Both are sold in health-food stores and may be cooked as cereal, or in pilafs, breads or other dishes. See also kamut.