Michael J. Gooding, Department of Agriculture, The University of Reading, Berkshire, United Kingdom
WHEAT: Chemistry and Technology, Fourth Edition
Pages 19-49
DOI: https://doi.org/10.1094/9781891127557.002
ISBN: 978-1-891127-55-7
Abstract
Wheat, maize, and rice dominate world grain production (Fig. 2.1). These grasses (family Poaceae, syn. Gramineae) are grown primarily for their grain (or caryopsis), and are thus “cereals.” The term wheat describes a number of species and subspecies in the genus Triticum, but today the most important are common, or bread, wheat (T. aestivum subsp. aestivum), which accounts for more than 90% of world wheat production, and durum wheat (T. turgidum subsp. durum), which is responsible for a further 5%. Wheats are members of the tribe Triticeae Dumort, alongside other important cereal genera, such as barley (Hordeum) and rye (Secale).
For 10,000 years, cereal cultivation has been a major factor supporting community settlement, cultural development, and population growth. The word cereal derives from Ceres, the Roman goddess associated with agriculture. The importance of cereals appears easy to explain: relative to other grain crops, yields are both high and stable; compared to root and tuber crops, cereal grain is easier to grow, transport, and store (Evans 1993). While the grain has been used for diverse foodstuffs, the exploitation of straw for roofing, livestock fodder, and bedding has also contributed to the popularity of cereal farming over the millennia (Sinclair 1998). Grain's initial status and economic importance have justified further investment in research, plant breeding, fertilizer and irrigation technology, agrochemical discovery, machinery improvement, and advisory services—all contributing to increased productivity per unit area (Fig. 2.2B), further competitive advantage, improved processing methodology, and trading efficiency (Wibberley 1989).
Wheat is the primary cereal of temperate areas, but it is also the most widely adapted, being grown from the Arctic Circle to the Equator, from sea level to 3,000 m, and in areas with between 250 and 1,800 mm of rainfall. Wheat is cultivated on more land than any other food crop (Fig. 2.2C), half of the wheat area being in developing countries. The top 10 producers include countries from Asia (China, India, Pakistan, and Turkey), North America (the United States and Canada), Europe (France and Germany), the Russian Federation, and Australia. The majority of South American wheat is grown in Argentina. Most African wheat is produced in the north (e.g., Egypt and Morocco) or in highland areas, such as in Kenya and Ethiopia, although breeding advances have extended production into the warmer tropics.
Average wheat yields are lower than those of maize and rice (Fig. 2.2B), reflecting the extensive cultivation of wheat over large areas where production is limited by water shortage. Even in much more productive areas, wheat yield can be compromised by short growing duration when it is a component of rotations that have more than one crop in a year. These systems include the rice-wheat rotations of South and East Asia and the wheat-soybean-maize systems in South America. Wheat cultivation can exploit an opportunity to maximize production and increase efficiency of land and labor use over the cropping cycle despite wheat's lower yields in such circumstances. In temperate, moist, and long-season areas where intensive production derives from responsive use of fertilizers and agrochemicals, grain production in commercial rain-fed fields can commonly exceed 10 tonnes per hectare (t/ha) and may be more limited by light interception than by water availability. Such conditions are typical in the countries where average national wheat yields exceeded 7 t/ha between 2003 and 2005 (FAOSTAT 2008), namely Ireland, the Netherlands, Belgium, the United Kingdom, Germany, New Zealand, and Denmark.
The comparatively easy storage and transportation of cereals renders them suitable for trade, and wheat is the most important grain crop in this respect (Fig. 2.2D). Total world wheat exports were valued at US$20,000 million in 2004, with more than 75% being accounted for by just five countries: the United States (>25%), Australia, Canada, France, and Argentina, where wheat exports accounted for between 0.04% of gross domestic product in the United States and 0.75% in Argentina. The greatest importers of wheat included China, Japan, Italy, Algeria, and Indonesia. Wheat therefore forms a significant component of the balance of trade of national economies, and the movement of wheat (and security of supply) is a major issue in political and economic relationships between governments. Historically, wheat supply has underpinned democracy and has supported or broken governments, the bread riots at the beginning of the French Revolution in 1789 being an oft-cited example.
Wheat, like other cereals, is multifunctional, providing a concentrated source of carbohydrate (mostly starch), with useful amounts of protein, fat, minerals, vitamins, and fiber. Wheat provides between a fifth and a quarter of the energy and protein in the worldwide human diet. Two thirds of the crop is used directly in products for human consumption, and wheat is the main source of calories for 1.5 thousand million people (Reynolds et al 1999). Varieties of wheat are preeminent among cereals in the ability of doughs made from their flours to trap carbon dioxide liberated from fermentation. This allows leavened foods to be baked, bread in diverse forms being by far the most important such food. Yeast spores can be found naturally on the surface of cereal grains, so fermentation occurs readily in wheat dough left to rest. It is likely, therefore, that leavening is of prehistoric origin. Certainly wheat doughs have been baked to produce primitive flat or dense breads from at least the Neolithic era (10,500–7,500 b.p.). Bread continues to be the “staff of life” and is arguably the world's oldest convenience food. Many types of bread have been developed and have become associated with their region of origin. Low-density pan and hearth breads are familiar worldwide but particularly in the West. In China and surrounding countries, rolls of fermented dough are steamed (rather than baked) to produce a bread with a dense crumb and thin white skin. Flat breads include chapatis (India and Pakistan), naan (N. India), roti or rotta (India), injera (Ethiopia), and in Arabian countries, khoubs or pitta. In Central and South America, products that are more commonly made from maize, such as tortillas and tacos, can also be partly or wholly made from wheat. Biscuits (cookies) are a common outlet for wheat varieties not suited to leavened bread production. In many Asian countries, different types of noodle are made by cutting sheeted dough into thin strips that are then boiled and dried, ready for subsequent cooking. Pastes produced from the semolina of durum wheat are extruded to form different kinds of pasta. Alternatively, semolina grains can be steamed to produce couscous, or durum wheat can be harvested prematurely and the grains parched to produce frekah (Middle East, North Africa) or Gruenkern (Germany). Other processes lead to a plethora of breakfast cereals. Some wheat is simply prepared by soaking for use in porridge, broth, or pudding. Bulgur and “wurld” wheat can be prepared by removing part or all of the bran, respectively, from whole grain.
Of the remaining third of the wheat harvest, i.e., that which is not used directly for human consumption, much is included in livestock rations. While less important than maize as an animal feed, wheat has a different geographical range than maize, can equal maize in energy value, often betters maize with respect to protein concentration, and is particularly important for non-ruminants.
Wheat starch, like starch from other cereals, can be fermented for use in alcoholic beverages. Wheat that has a low nitrogen content, and thus is less suited to bread-making, can give a high spirit yield for grain whisky production or be used in the barley-brewing industry to aid head retention in pasteurized beers. In areas of the world where local supply exceeds demand for wheat-based foods and livestock feed, fermentation of low-nitrogen-content grain is increasingly used to produce bioethanol and hence reduce reliance on nonrenewable sources of energy. Experimentally, wheat grain has produced up to 46.5 L of ethanol per 100 kg of grain (Rosenberger 2005), while commercial plants have approached 40 L, representing significant positive energy balances when full life-cycle analyses have been performed (Malça and Freire 2006). Industrial exploitation also includes using the starch as a stiffening or surface coating agent in the manufacture of paper and board; as an adhesive in the manufacture of corrugated boxes (Jones 1987); as a loading agent in the production of resins and plastics; as a fermentation substrate in the production of antibiotics, vitamins, and hormones; as a gelling agent or emulsifier in paint; as well as in a host of other more-minor uses.
