Wheat

Wheat is one of the oldest and most important cereal crops. Wheat is grown for its grain, which is ground into flour used to make breads and pastas. Wheat consists of approximately 20 species in the genus Triticum of the grass family (Poaceae). The most important wheats are: Triticum aestivum, used to make bread; T. durum, used to make pasta; and T. compactum, used to make softer cakes, crackers, cookies, and pastries.

Wheat plants have slender leaves and, in most varieties, long hollow stems. Each stem is topped by a single head or spike, which is an aggregation of 20-100 individual flower clusters called spikelets. Each flower cluster may contain up to six flowers, and each fertilized flower produces a single, edible grain.

About 13% of the mass of the ripe grain is formed by the fused layers of the fruit wall, seed coat, and aleurone. These layers, known as the bran, are nutritionally important because they contain fiber, some protein, and the vitamins thiamine, riboflavin, niacin, and vitamin A. About 84% of the grain is endosperm, a food-storage tissue that consists mainly of starch. The embryo, or germ, represents only 2.5% of the grain and contains most of the oil and protein. Because wheat provides a balance of several vitamins, starch, proteins, and oils, it is an excellent source of nutrition. Furthermore, because of its small water content, generally about 12%, wheat grains are easily transported and stored, and are resistant to microbial spoilage.

Wheat was one of the first plants to be domesticated. Along with barley (Hordeum vulgare), wheat provided the agronomic basis for development of the earliest civilizations in Mesopotamia, and has been found at archaeological sites in the region dating back to 7000 B.C. The cultivation of wheat gradually spread by trade from Mesopotamia to other parts of the world, becoming established in India by 3000 B.C., and in Europe by 2000 B.C. Wheat did not occur in the New World until brought there by the Spanish in 1520. Wheat was introduced by settlers to the United States in the early 1600s. Nowadays, wheat is grown on every arable continent, and it is the most important food for people living in temperate regions of the world (it is replaced by rice in the tropics).

By far the most important wheat is the common bread wheat (Triticum aestivum). This species evolved by a series of natural hybridizations followed by chromosome doubling (polyploidy) about 6,000 years ago in the Middle East, where its wild relatives still occur. The first hybridization is believed to have been between a primitive einkorn wheat (Triticum urartu) and an unknown species of wild goat grass (related to Triticum speltoides), each with 14 chromosomes. The resulting hybrid doubled its chromosome number and became fertile, producing emmer wheat (Triticum turgidum) with 28 chromosomes. Emmer wheat then hybridized with another wild species of goat grass (Triticum tauschii), which had 14 chromosomes, and the hybrid doubled its chromosome number to produce bread wheat with 42 chromosomes. Some of this hybridization and chromosome doubling has been duplicated experimentally in modern times, to yield an "artificial" wheat that resembles certain cultivars of bread wheat.

Winter wheats are planted in the fall and germinate before winter. The seedlings can survive cold winter temperatures—in fact, the low temperatures are needed for proper growth and development of the grain. The seedlings start growing again in the spring as soon as the frost is out of the ground, and by late spring the mature plants are ready for harvest. In contrast, spring wheats are planted in the spring and harvested in the fall.

For thousands of years, wheat was laboriously harvested using a sickle, and then threshed, or beaten, to separate the grains from the heads and flower parts (chaff). In the first half of the 1800s, the reaper was developed, which mechanized cutting and greatly reduced the amount of labor required. Nowadays, cutting the standing plants, threshing the heads, separating the grain from the chaff, cleaning the grain, and discharging it into bags, are all combined in a large, self-propelled machine called a combine.

About 25% of the world's farmland is devoted to wheat cultivation. This is more than is used for any other crop. About 566 million acres (229 million hectares) are sown to produce 478 million U.S. tons (527 million metric tons) of wheat. The world's greatest wheat producing countries are: the United States, China, Ukraine, India, Canada, Australia, Turkey, and Pakistan. The major wheat-exporting countries are the United States, Australia, Canada, and Argentina. Nearly all wheat-growing countries have breeding programs to improve the races of wheat adapted to local conditions. These programs strive to enhance qualities such as yield, disease and insect resistance, and nutrient content of the grain.

If wheat grains are eaten whole or ground whole by traditional stone grinding, all of the nutrients are retained. Modern milling methods, however, remove the germ and bran, thereby eliminating most of the proteins, oils, and nearly all vitamins. The resulting white flour stores longer and tastes good but is nutritionally impoverished. For this reason, white flour is often artificially enriched with vitamins to improve its nutritional value.

Wheat flour is primarily used to bake bread. Wheat flour is especially suited for this purpose because it contains two proteins, glutenin and gliadin (collectively known as gluten), that make a sticky, elastic dough. During baking, the dough traps bubbles of carbon dioxide produced by yeast or by chemical leaveners such as baking powder or baking soda. The trapped bubbles cause the bread to rise. (The holes you see in sliced bread were formed by trapped gas bubbles.) Other cereal grains, such as rice, barley, corn, and oats, do not contain glutenin and gliadin and therefore are not suitable for making leavened bread.

Durum wheat (Triticum durum) does not rise as well as bread wheat because of the different protein composition of its grain. Therefore, durum wheat is used for making spaghetti, macaroni, and other kinds of pasta. Other products made from wheat include bulgur, which is prepared by cooking, dehydrating, and peeling wheat. Wheat germ, which is removed in the milling process, has a large content of vitamin E and protein, and is often added to or sprinkled on other foods as a nutritional supplement. Entire wheat grains, either rolled or puffed, are often used in breakfast cereals. In addition, starch and gluten are extracted from wheat grains. The starch is used in laundering and in making a sweet syrup. The gluten is used in making monosodium glutamate (MSG), a flavor enhancer of cooked foods, especially commonly used in oriental cooking. Wheat is the most important grain fed to poultry. However, wheat is not generally fed to livestock, because it is more expensive than other suitable grains such as maize, barley, and oats.