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date: 12 December 2017

Cultivation and Impact of Wheat

Summary and Keywords

Wheat is the most widely grown food crop in the world and the dominant staple crop in temperate countries where it contributes between about 20% and 50% of the total energy intake. About 95% of the wheat grown is hexaploid bread wheat, with tetraploid durum wheat being grown in the hot dry Mediterranean climate and very small volumes of ancient species. About 80% of the dry weight of the mature grain is starchy endosperm. This is the major grain storage tissue, which is separated by milling to give white flour, the outer layers and germ together forming the bran. However, white flour and bran differ significantly in their compositions, with white flour being rich in starch (about 80% dry wt) and protein (about 10% dry wt) and the bran rich in fiber, minerals, vitamins, and phytochemicals.

Most of the wheat consumed by humankind is in the form of bread, noodles, pasta, and other processed foods, and the quality for processing is determined by two major characteristics: the grain texture (hardness) and the viscoelastic properties conferred to dough by the gluten proteins.

In addition to being a source of energy, wheat also contributes protein and a range of other essential and beneficial components, particularly dietary fiber. However, because most of these components are concentrated in the bran, it is important to increase the consumption of whole grain products or to improve the composition of white flour. Although there is concern among consumers about possible adverse effects of consuming wheat products on health, these are unlikely to affect more than a small proportion of the population, and wheat should form part of a healthy balanced diet for the vast majority.

Keywords: wheat, grain, protein starch, phytochemicals, vitamins, bread making, diet and health

Introduction

Wheat Production

In terms of total production wheat is the third most important cereal crop in the world, after corn and rice, with the annual production over the 5-year period from 2008 to 2012 averaging about 680 million tons (http://faostat.fao.org/site/339/default.aspx). However, it is much more widely grown than rice or corn, being the dominant crop in temperate zones and grown from Scandinavia to the south of Argentina, including highlands in the tropics (Feldman, 1995). The demand for wheat-based foods is also increasing in countries undergoing urbanization and industrialization, including countries outside its area of adaptation (such as parts of sub-Saharan Africa).

Most of the wheat grown globally is bread wheat (Triticum aestivum), but other species are grown to a lesser extent, notably about 35–40 million tons a year of durum (also called pasta) wheat (Triticum turgidum var. durum), which is adapted to the hot dry climate of the Mediterranean and similar areas. Globally, the vast majority of wheat is used for human food, and hence the quality for processing and, to an increasing extent, diet and health is an important target for improvement. In some parts of the world, such as Western Europe, substantial amounts of wheat are also used for livestock feed and for industrial purposes (notably distilling for beverages and bioethanol).

Origin and Relationships of Wheat Species

Cultivated wheat comprises diploid, tetraploid, and hexaploid species. These species have one, two, and three genomes, respectively, each genome comprising seven pairs of chromosomes. Although evidence for the consumption of cereals by humans dates back over 30,000 years (Lippi, Foggi, Aranguren, Ronchitelli, & Revedin, 2015), the cultivation of wheat probably started about 10,000 years ago, as part of the Neolithic revolution, which saw a transition from hunting and gathering to settled farming and the cultivation of crops. The earliest cultivated forms of wheat were diploid einkorn (T. monococcum var. monococcum, genome AA) and tetraploid emmer (T. turgidum var. dicoccum, genomes AABB), which probably originated from wild grasses in the southeastern part of Turkey (Dubcovsky & Dvorak, 2007). Cultivated bread wheat, which is hexaploid (genomes AABBDD), probably first appeared in the Middle East about 9,000 years ago by natural hybridization between cultivated emmer and a related wild grass (Feldman, 2001). It has since migrated across the temperate world, developing a wide range of diversity and adapting to a wide range of environments, these processes being facilitated by high genome plasticity (Dubcovsky & Dvorak, 2007).

The domestication of wheat has been associated with changes in two traits. First, in related wild species the spike shatters at maturity, resulting in dispersal of the seeds. Mutations at the Br (brittle rachis) locus have resulted in the loss of shattering, meaning that the heads can be harvested without seed loss (Nalam, Vales, Watson, Kianian, & Riera-Lizarazu, 2006). Second, wild species and ancient cultivated wheats are hulled in that the glumes adhere tightly to the grain and need to be removed before processing. Mutations at the two loci have resulted in the conversion of hulled to free-threshing forms, which include modern bread and pasta wheats (Dubcovsky & Dvorak, 2007).

In addition to modern bread and pasta wheats, small amounts of three ancient wheat species—einkorn, emmer, and spelt (T. aestivum var. spelta)—are still to be seen growing in small volumes in some countries where they are used for traditional foods. There has also been increased interest in ancient wheats in other countries in recent years as they have been suggested to be rich sources of bioactive components that may have health benefits. However, detailed comparative analyses have shown that their compositions (including gluten, dietary fiber, and phytochemical components) differ little from those of modern bread and pasta wheats (Shewry & Hey, 2015c). Nevertheless, they may differ from modern wheats in flavor components, and the use of traditional production and processing systems may also result in the food products that differ in flavor and other organoleptic properties.

The Wheat Grain

The wheat grain is a single-seeded fruit called a caryopsis. It comprises two seed tissues, the embryo (germ) and the endosperm, surrounded by several tissues derived from the maternal fruit and seed coats (the pericarp, intermediate layer, and testa) (Figure 1).

Cultivation and Impact of WheatClick to view larger

Figure 1. The component tissues of wheat grain.

(Reprinted with permission from Surget & Barron, 2005.)

These tissues account for about 3%, 90%, and 7–8% of the dry matter, respectively (Barron, Surget, & Rouau, 2007). The endosperm itself consists of two tissues, the starchy endosperm and a single layer of outer cells called the aleurone layer. The starch endosperm acts as a storage tissue, comprising about 80% starch and 10% protein and only small proportions of other components, notably 2–3% cell wall polysaccharides (which form dietary fiber). Whereas the starchy endosperm cells die during the later stages of grain development, the aleurone cells remain alive and during germination release enzymes that digest the starchy endosperm tissue to support the growth of the embryo and seedling. The aleurone cells also differ from the starchy endosperm cells in having thick walls, and hence about 40% dietary fiber, and being rich in protein, minerals, lipids, and phytochemicals. The embryo is also rich in protein, lipids, minerals, and phytochemicals, while the outer maternal layers comprise about 45–50% fiber.

Most food products are produced from white flour, which is derived from the central starchy endosperm cells. Milling is therefore used to separate these cells from the other grain tissues and to reduce them to fine flour. Because the wheat grain is elongated, with a pronounced ventral groove, milling is a complex process with the grain being passed between a series of cylindrical rollers, interspersed with sieves to separate the flour particles. Commercial milling therefore generates up to 30 fractions, which are called mill streams. These fractions range in composition from pure white flour to pure bran. The miller will blend them to achieve a yield of white flour corresponding to between 75% and 80% of the whole grain. The remaining fractions form the bran, which comprises the germ, the aleurone layer, and outer maternal layers.

However, because the bran tissues are rich in fiber and other components, which are beneficial for health, the extraction rate may be increased to increase the “health benefits” of the flour. High extraction flours are also used for traditional breads in many countries in Asia and North Africa, for example, for flatbreads in South Asia (chapatti, poorie, parontha) and the Middle East (tanoor in Iran, lavash and baladi in Egypt). Whole wheat flour is produced by recombining all of the milling fractions but has lower acceptability with many consumers due to the gritty texture and more astringent taste.

Grain Components

Protein

The protein content of wheat is generally 10–15% but is affected by a number of factors (Shewry, 2007). In general terms, grain protein content decreases as grain yield increases, as increased yield results mainly from greater accumulation of starch. This “yield dilution” also results in decreased contents of a range of grain components as well as protein. Because protein content is important for grain processing quality, wheat cultivars bred for bread making have higher protein contents than those bred for livestock feed or industrial use where high yield and high starch content are desirable. This difference in protein content is about 2% protein/g dry weight when bread making and feed cultivars are grown under the same conditions. Similarly, the yields of feed wheats are about 0.5 to 1.0 tons per hectare greater than those of bread making cultivars. Genetic sources of high protein have also been exploited in wheat breeding, derived from exotic lines from South America (Frondoso leading to Atlas 66) and India (Nap Hal), and from related wild wheats, notably wild emmer (Triticum dicoccum var. dicoccoides) from Israel.

There is also a strong effect of nitrogen availability on grain protein content, which may have a greater impact than genotype. Farmers in some countries (such as the United Kingdom) will therefore adjust the fertilizer application rate to maximize yield and to produce grain with the required protein content. For example, over the period 2005–2007 the Broadbalk long-term wheat experiment at Rothamsted yielded grain with a mean of 7.8% protein when 48 kgN/Ha was applied and 14.1% protein with 288 kgN/Ha.

Wheat grain proteins have been studied in great detail because of their role in determining grain processing quality (Shewry et al., 2009). They are classically divided into groups based on their extraction and solubility in a sequence of solvents, named the “Osborne fractionation” after the American scientist T. B. Osborne, who pioneered the systematic study of plant proteins at the end of the C19 and start of the C20. These groups are albumins (soluble in water), globulins (dilute saline), prolamins (alcohol–water mixtures), and glutelins (dilute acid or alkali). However, it perhaps more useful to classify them based on function, into storage proteins, structural and metabolic proteins, and defense proteins.

The major protein fraction in wheat grain is the gluten proteins, which correspond to the major type of storage protein in the starchy endosperm cells (but not in other grain tissues). Gluten proteins are generally stated to account for about 80% of the total grain protein, but the proportion varies with nitrogen availability, being greater in high-protein grain. This is because they act as a sink for nitrogen that is in excess of the normal requirements of the grain.

Gluten is a complex mixture of over 50 proteins which are classically divided into gliadins and glutelins, which are defined as prolamins and glutelins, respectively, according to the Osborne classification. However, we now know that these two fractions comprise related proteins which differ in solubility due to the fact that the gliadins are monomeric while the glutenins consist of high-molecular-weight polymers stabilized by interchain disulfide bonds. When the interchain disulfide bonds in glutenins are reduced, the individual subunits have similar solubility properties to the monomeric gliadins (in alcohol–water mixtures). Hence, both gliadins and glutenins can be defined as prolamins.

The gliadin and glutenin components can in turn be classified into several groups. Once again, both old and more recent classifications are used (summarized in Figure 2).

Cultivation and Impact of WheatClick to view larger

Figure 2. Summary of the classification of wheat gluten proteins.

(Reprinted with permission from Shewry et al., 1986.)

The α‎-gliadins, γ‎-gliadins, and ω‎-gliadins were initially defined based on their mobility on electrophoresis at low pH and the high-molecular-weight (HMW) and low-molecular weight (LMW) subunits of glutenin based on sodium dodecyl sulfate polyacrylamide electrophoresis (SDS-PAGE) after reduction of disulfide bonds. The α‎-gliadins and γ‎-gliadins are related in their properties and amino acid sequences to the LMW subunits and are together defined as sulfur-poor (S-poor) prolamins based on their high contents of the S-containing amino acid cysteine, while the ω‎-gliadins form a discrete group defined as S-poor based on the fact that most lack cysteine. The HMW subunits also form a discrete group called HMW prolamins.

Although the gluten proteins are the major storage proteins in wheat, smaller amounts of other storage proteins related to the globulin storage proteins of dicotyledonous plants also occur. These are “7S type storage globulins” in the aleurone cells and embryo and “triticins” (related to 11S globulins) in the starchy endosperms cells.

The “structural and metabolic proteins” include proteins associated with cell structures, such as cell walls and membranes, and enzymes. Finally, a range of small sulfur-rich proteins, many of which are related to prolamins, display inhibitory activity to exogenous hydrolytic enzymes or other biological activities which may contribute to broad spectrum resistance to pests and pathogens. In fact, it has been estimated that two thirds of the albumins present in wheat flour are inhibitors of α‎-amylase. Most of these proteins belong to a family of inhibitors of trypsin and α‎-amylase, which are related to the gluten proteins. They were initially defined as “CM proteins” on account of their solubility in chloroform:methanol mixtures and occur in monomeric, dimeric, and tetrameric forms, with the dimeric wheat inhibitors being particularly active against human salivary and pancreatic α‎-amylases (See Shewry et al., 2009, for a detailed review of wheat proteins.)

Carbohydrates

The mature wheat grain consists of 85% (w/w) carbohydrate, of which 80% is starch, about 7% low molecular mass mono-, di-, and oligosaccharides including fructans (fructo-oligosaccharides) and about 12% cell wall polysaccharides (Stone & Morell, 2009).

The most abundant oligosaccharides in wheat grain are fructans, which comprise an average of 5–7 fructose units. The fructan content of whole wheat grain varies between about 1% and 2% dry weight, with the concentration in bran (up to about 4%) being approximately twice that in white flour (about 1.5%) (Shewry & Hey, 2015b). Wheat also contains the disaccharides sucrose (comprising glucose and fructose units; 0.54–1.55% of dry weight) and maltose (two glucose units; 0.05–0.18% of dry weight) and the trisaccharide raffinose (galactose, glucose, and fructose units; 0.19–0.68% of dry weight) (reviewed by Stone & Morell, 2009).

Starch is essentially present only in the starchy endosperm cells, where it is deposited in granules in amyloplasts. Wheat starch granules occur in two size ranges. Large A granules have diameters between about 15 and 30 µm and are initiated early in development, while small B granules have diameters below about 10 µm and are initiated later in development within tubular protrusions from the same amyloplasts that contain A granules (reviewed by Bechtel, Abecassis, Shewry, & Evers, 2009). The A granules are much less numerous than B granules, accounting for only 3–4% of the total number in the mature grain but for about 50–75% of the total weight of starch.

Starch is a mixture of two glucose polymers: amylose, which comprises single unbranched (1→4) α‎-linked chains of up to several thousand glucose units, and amylopectin, which is highly branched (with (1→6) α‎-linkages as well as (1→4) α‎-linkages) and may comprise over 100,000 glucose unit residues. As in most species, amylose and amylopectin occur in a ratio of 1:3 in wheat starch. The proportion of amylose is of particular interest as it affects the formation of “resistant starch” that escapes digestion in the small intestine (reducing the glycemic load) and forms part of the dietary fiber fraction that is fermented by microorganisms in the colon (with additional health benefits, discussed below).

Mutations affecting the amylose:amylopectin ratio are well characterized in diploid cereals (corn, barley, rice), but the polyploid nature of bread wheat means that mutations in all three genomes are required to observe a clear phenotype. Low-amylose (waxy) cereals have essentially 100% amylopectin and have unusual properties (notably high viscosity and water retention) that are exploited in the food industry, for example, for the production of refrigerated and frozen foods. The waxy phenotype results from mutations in granule-bound starch synthase (GBSS), the single enzyme that catalyzes amylose synthesis. The full waxy phenotype has not been reported to occur naturally in polyploid bread wheat, but partial waxy lines with mutations in one or two of the GBBS genes are quite common and have reduced proportions of amylose (down to about 20%). Partial null lines can be readily crossed to obtain complete waxy types of bread and durum wheat with very low amylose contents (about 0–2%).

The high-amylose phenotype can result from the reduced activities of several enzymes involved in starch synthesis and amylose contents of up to about 40% have been achieved by combining natural mutations (reviewed by Lafiandra, Riccardi, & Shewry, 2014)., However, high-amylose starches differ in their processing properties from conventional starches, particularly in swelling and viscosity, which may limit the ability to produce acceptable food products from high-amylose wheats.

The different tissues of the wheat grain vary widely in their contents and compositions of cell wall polysaccharides. In the outer maternal layers of the grain, the cell walls account for 45–50% of the dry weight and have similar compositions to the cell walls of the vegetative tissues of the plant: about 90% polysaccharide (60% xylans, 30% cellulose) and 12% lignin (a complex polymer of aromatic alcohols). The cell walls of the starchy endosperm and aleurone cells lack lignin and have similar compositions, being rich in arabinoxylan (AX) and (1→3,1→4)-β‎-D-glucan (β‎-glucan), with smaller amounts of cellulose ((1→4)-β‎-D-glucan) and glucomannan. However, whereas cell wall components account for about 40% of the dry weight of the aleurone cells, they account for only 2–3% in the starchy endosperm cells. The contents of AX and β‎-glucan present in white flour (starchy endosperm) vary by about twofold.

Minor Grain Components: Phytochemicals and B Vitamins

Wheat contains a wide range of minor components, some of which have established or proposed health benefits. However, all are concentrated in the bran (aleurone, outer layers, and embryo) with only low concentrations in white flour. Analyses of wholemeal fractions are compared in Table 1.

Table 1. Variation in the Contents of Phytochemicals (Phenolics and Terpenoids) and B Vitamins in Whole Grain Wheat

Component

Number of lines analyzed

Range of content (µg/g dry weight)

Phenolics

Total phenolic acids

150

326–1171

Free phenolic acids

150

3–30

Conjugated phenolic acids

150

76–297

Bound phenolic acids

150

208–878

Bound ferulic acid

150

162–721

Alkylresorcinols

150

241–677

Lignans

73

3.4–22.70

Terpenoids

Total tocols

150

27.6–79.7

α‎-Tocopherol (vitamin E)

150

9.1–19.9

Total sterols (including stanols)

150

670–959

β‎-Carotene

8

0.11–0.24

Lutein

8

0.93–1.3

Zeaxanthin

8

0.23–0.44

Methyl donors

Betaine

155

747–2940

Choline

155

76–280

B vitamins

Thiamine (B1)

75

6.90–11.7

Riboflavin (B2)

75

0.48–1.07

Niacin (B3)

26

0.69–1.18

Pyridoxine (B6)

75

1.44–3.16

Folates (B9)

150

0.32–0.77

Source: Shewry and Hey (2015b).

Phenolic compounds contain at least one aromatic ring bearing at least one hydroxyl group and are the most abundant and complex group of secondary products present in cereal grains. The major phenolic compounds are phenolic acids, which are derived either from cinnamic acid or benzoic acid. They occur in three forms: as free compounds, as soluble conjugates with LMW compounds (such as sugars and sterols), and bound to cell wall polysaccharides (particularly arabinoxylan). Other phenolic compounds with possible health benefits are alkylresorcinols, which are phenolic lipids comprising a 1,3-dihydroxylated benzene ring with an alkane chain at position 5, and lignans, which are polyphenols derived from the amino acid phenylalanine.

The second major group of phytochemicals in wheat are terpenoids, which are derived from 5-carbon isoprene units. The major groups of terpenoids in wheat grain are sterols, which are steroid alcohols; tocols, which comprise a chromanol ring with a C16 phytol side chain; and carotenoids, which are derived from long polyene chains of 35–40 carbons The major wheat sterols are 4-desmethyl sterols with small amounts of 4α‎-monomethyl sterols and 4,4-dimethyl sterols. Significant amounts of saturated forms of sterols, called stanols, are also present, and a substantial proportion of both sterols and stanols are modified to form esters and glycosides. Tocols also occur in saturated forms (tocopherols) and unsaturated forms (tocotrienols), with both tocopherols and tocotrienols occurring as four types which differ in the positions of methyl groups on the chromanol ring. Although the name “vitamin E” is commonly applied to all tocols, they differ in biological activity, with α‎-tocopherol being the most active form.

Wheat carotenoids are classified into oxygen-containing xanthophylls (which include lutein and zeaxanthin) and unoxygenated carotenes (which include α‎-carotene and β‎-carotene). Some carotenoids, including carotenes, are converted to vitamin A (retinol) in mammals and hence called provitamin A. The levels of carotenoids in cereal grain are generally low, but the content of lutein is higher in durum wheat as it is largely responsible for the yellow color favored by consumers.

Wheat is a very rich source of betaine (glycine betaine (N,N,N-trimethylglycine)) with lower amounts of its precursor compound choline, with both being concentrated in the bran (which may contain over 1% dry weight betaine). These two compounds are classified with folate (vitamin B9) as “methyl donors” (as discussed below).

Wheat is an important source of B vitamins: thiamine (B1), riboflavin (B2), niacin (B3), pyridoxine (B6), biotin (B7), and folate (B9). Niacin (B3) is of particular concern as only a proportion of the total present in cereals (about 20% of that in wheat grain) is bioavailable.

It is also clear from Table 2 that the contents of phytochemicals and B vitamins vary widely between wheat lines, although the extent of this variation differs between groups. For example, free phenolic acids vary by 10-fold, but total sterols only by about 1.5-fold. The existence of such wide variation will pose a challenge for food processors and retailers if products with enhanced levels of these components are developed in the future.

Table 2. Summary of Proposed and Established Health Benefits of Components Present in Wheat Grain

Component

Proposed health benefit (for cereals or other foods)

Supported by approved health claim?

Dietary fiber

Reduces post-prandial glycaeic response (and risk of type 2 diabetes)

yes

Reduces intestinal transit time

Increases fecal bulk

Reduces cholesterol and risk of coronary heart disease

Reduces risk of colorectal cancer

no

Reduces risk of breast cancer

Reduces risk of stroke

Prebiotic effects

Stimulates immune responses

Resistant starch

Reduces post-prandial glycemic response

Other benefits as part of dietary fiber above

yes

Fructans

Prebiotic effects

no

Promote calcium (and iron?) absorption

Betaine

Normal homocysteine metabolism (reduced risk of coronary heart disease)

yes (not for cereals)

Choline

Normal homocysteine metabolism (reduced risk of coronary heart disease)

no

Phenolic acids

Improve vascular function

no

Antitumor properties

“Antioxidant”

Alkylresorcinols

Antimicrobial

no

“Antioxidant”

Lignans

Phytoestrogen

no

Antitumor

Antimicrobial

Sterols, stanols, and derivatives

Reduce serum cholesterol and risk of coronary heart disease

yes

Anticancer effects

no

Tocols

Vitamin E activity

yes

Prevention of neurodegeneration

no

Induction of immune responses

Anticancer

Cholesterol lowering

“Antioxidant”

Note: A number of components have been described as having “antioxidant” properties, but the in vivo significance of this broad but readily measured activity is debated.

(*) Approved by EFSA (EU) or FDA (USA).

Source: Shewry and Hey (2015a).

Minerals: Iron, Zinc and Selenium

Deficiencies in mineral micronutrients, notably iron and zinc, affect more than 2 billion people worldwide. Although micronutrient deficiency is particularly acute in countries with poor dietary intakes, iron deficiency anemia also affects a significant proportion of women in the United Kingdom and other countries with adequate diets. Cereals are significant sources of minerals in the diet, providing about 40% of the total intake of iron in U.K. adults (with about 15% coming from bread).

However, there are two major concerns relating to the use of wheat as a source of minerals. First, the concentrations of some mineral micronutrients (including iron and zinc) are lower in the grain of modern wheat cultivars than in older types of wheat. Evidence for this comes from two sources: comparative studies of new and old varieties grown together in replicate field trials and analysis of archived samples from historic trials. This trend is illustrated in Figure 3, which compares the contents of iron and zinc in grain of 26 cultivars grown on six sites.

Cultivation and Impact of WheatClick to view larger

Figure 3. Trends in the concentrations of iron (Fe) and zinc (Zn) in wheat cultivars in relation to release date. Data are means of six environments (trial sites and/or seasons). The curves are fitted with a quadratic equation.

(Reprinted with permission from Zhao et al., 2009).

The decreases in the concentrations of iron and zinc coincide with the introduction of modern semi-dwarf cultivars, but it is not known whether these result solely from yield dilution (as discussed above for protein content) or also from effects of dwarfing genes on mineral uptake and partitioning between the grain and other parts of the plant.

The second concern relates to the location and form of the minerals in the grain. Both iron and zinc are concentrated in the aleurone and embryo, with very low concentrations in the white flour. Furthermore, most of the minerals in these tissues are present as complexes with phytate (myo-inositolphosphate, 1,2,3,4,5,6-hexa-kisphosphate), which are largely insoluble with low bioavailability to humans and livestock. Hence, increasing total grain concentrations of iron and zinc may not be sufficient to ameliorate dietary deficiencies.

Selenium is an essential micronutrient for mammals but is also toxic when present in excess. Cereals are major dietary sources of selenium in many parts of the world, but the content of selenium in the grain varies widely, from about 10 µg/kg to over 2,000 µg/kg. The concentration of selenium in wheat is largely determined by the availability in the soil, and wheat produced in Western Europe may containing only about a tenth of the selenium that is present in wheat grown in North America (where the soils are more rich in selenium). Because the import of wheat from North America into Western Europe has declined over the last 25 years, the intake of selenium in the diets of these countries has also decreased. Attempts to identify and exploit genetic variation in the ability of the wheat plant to take up selenium from low-selenium soils have failed, and the most attractive option is to apply selenium to the crops in fertilizer, a practice called biofortification. Unlike iron and zinc, selenium is not concentrated in the aleurone or bound to phytate, and bioavailability is not a problem. However, sulfur competes with selenium for the same mechanisms of uptake and transport, and the increasing practice of applying sulfur fertilizer to wheat being grown for bread making is likely to have adverse effects on the selenium status.

The Role of Grain Proteins in Determining Grain Processing Properties

Wheat Gluten Proteins and Bread Making Quality

The ability to make dough and bake bread from wheat but not from related cereals, such as barley and rye, is determined by the structures and properties of the gluten proteins. Although the gluten proteins are deposited in discrete “protein bodies” in the developing starchy endosperm cells, these fuse during the later stages of grain maturation and desiccation to form a continuous protein matrix in the cells. When the grain is milled and the flour mixed with water to form dough, the gluten proteins are brought together to form a continuous viscoelastic network. When yeast is added, the carbon dioxide released is trapped as bubbles in the gluten protein network, resulting in expansion of the dough and the formation of bread with a light porous crumb structure when the protein is denatured by baking.

A fairly high protein content is required for bread making, the precise concentration depending on the process. For example, The Chorleywood bread process, which is widely used for factory bread production in the United Kingdom and some English-speaking countries, has a high requirement for protein, a minimum of 13% (on a 14% moisture basis). However, an adequate content of protein is not sufficient for good bread making performance, as the quality of the proteins is also crucial. In particular, the gluten should have an appropriate balance of extensibility and elasticity, with highly elastic (strong) doughs being required for bread making and highly extensible doughs for biscuits (cookies).

The HMW subunits are particularly important in determining dough strength due to their ability to form HMW polymers (reviewed by Shewry et al., 2003). Variation in dough strength between genotypes is associated with allelic variation in the numbers of the HMW subunit genes that are expressed and the properties of the encoded proteins. This has led to the identification of “quality-associated” HMW subunit alleles, which are routinely selected in wheat breeding programs.

Puroindolines and Grain Hardness

Many non-experts are aware that bread wheats are “hard and strong,” and it is often assumed that these two characteristics are related. This is not the case, but they are co-selected in breeding programs because they are the two major traits that determine bread making performance. Grain texture (hardness) is important because it determines how the grain behaves during milling and the suitability of the flour for different end uses. In the starchy endosperm cells of hard wheat, the starch granules are tightly bound to the gluten protein matrix, meaning that more energy is requiring for milling. Because of this adhesion milling also results in starch damage in hard wheat, which allows water to be absorbed during bread making.

Bread wheats show wide variation in hardness, with about 60% and 80% of the variation being controlled by allelic variation at the Hardness (Ha) locus on chromosome 5D. This locus has been characterized in detail showing the presence of two genes encoding related proteins called puroindolines A and B (Pin A and Pin B). Pin A and B are small sulfur-rich proteins related to wheat gluten proteins. However, they are characterized by the presence of a short sequence containing 3 or 5 residues of the amino acid tryptophan. The puroindolines are considered to contribute directly to grain hardness by binding to the surface of the starch granules to prevent the adhesion to the gluten matrix. Thus, the “wild-type” phenotype is soft rather than hard. The mechanism is not known but it is probably that the tryptophan-rich domain plays a role.

Comparisons of collections of hard and soft genotypes have shown that whereas the former express genes encoding “wild-type” Pin A and B proteins, the latter have either a deletion of the Pin a gene or mutations in the Pin b gene (Morris, 2002). Although there may be subtle differences in the precise effects of these different alleles on texture, this variation has not been exploited in breeding or grain trading and lines are classified into only two types. The simple genetics also mean that texture is readily manipulated during breeding. The equivalent regions of chromosomes 5A and 5B of both hexaploid bread and tetraploid wheats have independent deletions of the genes encoding both puroindolines, and durum wheat is therefore ultra-hard and forms coarse semolina on milling rather than fine flour (farina).

A proportion of the variation in hardness within bread wheat is still unexplained and is probably related to effects of other proteins (including minor Pin variants, which are also expressed) and the structure and mechanical properties of the cell walls. Environment and nutrition may also affect grain hardness, with grain produced under hot conditions with high nitrogen having a vitreous appearance as opposed to the usual floury appearance.

Contribution of Wheat to Diet and Health

The role of wheat in providing essential nutrients and other beneficial components to diets has been referred to above. The benefits of some components, such as protein, vitamins, and minerals, are so well established that discussion is not required here. However, a number of other components present in wheat grain have been proposed to contribute to diet and health, as summarized in Table 2. These components are becoming of increasing interest as the relationship between diet and health becomes clearer, in particular in relation to reducing the risk of chronic diet-related conditions such as obesity, type 2 diabetes, and cardiovascular disease (CVD), which are associated with the adoption of a western diet and lifestyle.

Protein Quality for Human Nutrition

Wheat contributes significantly to the total daily intake of protein in many countries. Even in the United Kingdom, which has a varied diet, bread contributes about 10% of the adult daily intake of protein and the contribution is much greater in developing countries in which wheat contributes up to 70% of the total energy intake.

Protein nutritional quality depends on the proportions of “essential amino acids” that cannot be synthesized by animals and hence must be provided in the diet. If only one essential amino acid is limiting, the others will be broken down and excreted. Although only 10 amino acids are strictly essential (lysine, isoleucine, leucine, phenylalanine, tyrosine, threonine, tryptophan, valine, histidine, and methionine), cysteine is often also included as it can only be synthesized from methionine and the proportions of cysteine and methionine are often combined. The requirements for essential amino acids are lower for adult humans, where amino acids are required only for maintenance, than for human children, where they are also required for growth, and for rapidly growing livestock.

Wheat grain has low levels of several essential amino acids, particularly of lysine with a mean of about 2.2 g/100 g protein in white flour and 2.9 g/100 g protein in wholemeal, compared with a recommendation of 4.5g/100 g protein in adults (reviewed by Shewry & Hey, 2015b). The lower contents of lysine and other essential amino acids in white flour compared with whole grain results from the higher proportion of gluten proteins that have unusual amino acid compositions with high contents of glutamine and proline and low contents of lysine and other essential amino acids. Because the proportion of gluten proteins increases with total grain protein content, the deficiencies are exacerbated in high-protein grain. In practical terms, the deficiencies in essential amino acids in wheat grain are usually compensated for in mixed diets by their higher contents in other protein sources such as legumes (which are rich in lysine) and meat. However, the specific deficiencies are taken in account when developing formulations for livestock feed and may be a problem where wheat forms a high proportion of the human diet, particularly for children.

Dietary Fiber

Dietary fiber is essential for human health, with the vast majority of the dietary intake coming from plant sources. Cereals are important sources of fiber, accounting for over 40% of the daily intake in U.K. adults, with bread alone accounting for about 20%. Whole wheat grain contains 11.5–15.5% dietary fiber, the major components being cell wall polysaccharides with smaller proportions of lignin, fructans, and resistant starch (see Shewry & Hey, 2015b).

The health benefits of fiber from wheat and other cereal grain have been studied in detail, particularly in relation to the consumption of whole grain products. Much of our current knowledge has been reviewed by the U.K. Scientific Advisory Committee on Nutrition (SACN, 2015). This report concludes that there is good evidence for benefits of cereal fiber relating to cardio-metabolic health and colorectal health. Benefits for cardio-metabolic health include reducing blood pressure and levels of total cholesterol, low-density lipoprotein (LDL) cholesterol, and triacylglycerol in the blood and reducing the risk of cardiovascular disease, coronary events, and type 2 diabetes. Recognized benefits relating to colorectal health include increased fecal weight, reduced intestinal transit time, and reduced risk of colorectal cancer. Other studies did not meet the criteria for inclusion in the SACN (2015) review but nevertheless provide persuasive evidence for wider beneficial effects of cereal fiber. These include reduced risk of breast, small intestinal, and pancreatic cancers.

The mechanisms of action of fiber are not fully understood, and comparisons of studies are not always easy due to variation in the fractions studied. In particular, soluble β‎-glucan fractions from oats and barley have been widely studied, in addition to wholegrains and total, soluble, and insoluble fiber fractions, which in some cases include fiber components from other food sources. Nevertheless, it is clear that several mechanisms may contribute to the health benefits of fiber, including increasing fecal bulk, decreased transit time and reduced exposure to carcinogens due to binding to fiber as well as faster transit and dilution effects with greater bulk, changes in the rate of food digestion and the absorption of glucose in the small intestine, and fermentation in the colon to produce short-chain fatty acids (SCFAs), including butyrate, which has a beneficial effect on colonocytes as well as systemic effects that affect appetite and cholesterol synthesis.

The beneficial effects of colonic fermentation of fiber led Gibson and Roberfroid (1995) to develop the concept of stimulating the growth of beneficial colonic bacteria by providing non-digestible carbohydrates in the diet (prebiotics) The most widely studied prebiotic (which is usually used as a standard) is inulin, a β‎-(2,1)-linked fructan from tubers of Jerusalem artichoke (Helianthus tuberosus), but wheat non-starch polysaccharides, fructans, and resistant starch also have prebiotic effects.

Although colonic fermentation of carbohydrates is beneficial for health in most individuals, this may not be the case for those suffering from bowel conditions. In particular, fructans and raffinose are the major components in wheat of a group of small fermentable carbohydrates which have been termed FODMAPs (fermentable oligo-, di-, and monosaccharides and polyols). It has been suggested that a low-FODMAP diet improves the management of irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) (Crohn’s disease and ulcerative colitis), by reducing fermentation in the colon (Muir & Gibson, 2013). Hence, wheat lines that are low in FODMAPs could be of interest for developing low-FODMAP food products.

Phytochemicals

Although health benefits are often claimed for phytochemicals, there is little direct evidence for benefits of long-term human consumption in humans, as opposed to short-term interventions or in vitro studies. Consequently, the benefits of most phytochemicals have not been accepted for health claims by the FDA or EFSA, exceptions being tocols (vitamin E), plant sterols, and stanol esters, which have accepted benefits in reducing blood cholesterol and therefore the risk of cardiovascular disease.

There has been a particularly active debate about the benefits of phenolics, as they exhibit strong antioxidant activity and the total phenolic content of wheat is strongly correlated with total antioxidant activity. While the relevance of antioxidant properties for human health continues to be debated, there is increasing evidence that phenolic compounds, including ferulic acid, which is the major phenolic acid in wheat, improve vascular function in short-term interventions in humans and in animal models (Del Rio et al., 2013; Suzuki et al., 2007).

Betaine and choline are termed “methyl donors” because they can donate methyl groups in methylation reactions, substituting for folates, and whole wheat grain is a particularly important source of dietary betaine (Craig, 2004). Of particular importance is the remethylation of plasma homocysteine in the serum. Homocysteine is produced by demethylation of methionine and is a major risk factor in CVD. It can be removed either by remethylation to methionine, using folate, betaine, or choline as a methyl donor; by metabolism to give cysteine; or by conversion to S-adenosylhomocysteine.

Adverse Effects of Wheat on Human Health

Over the last few decades, consumers in many countries have become increasingly aware of the relationship between their diet and health, with attention focused increasingly on the perceived adverse effects of wheat. This has led to increasing demand for wheat-free or gluten-free products, which may be depleted in the beneficial components (such as fiber, minerals, and B vitamins) usually provided by wheat products.

One concern that is readily refuted is that wheat products contribute disproportionally to increases in obesity and type 2 diabetes. There is no scientific basis for this suggestion (Brouns, van Buel, & Shewry, 2013) and it is counterintuitive that the prevalence of these conditions is higher in developed countries in which wheat contributes a low proportion of the diet than in developing countries where wheat may contribute up to 70% of calories. The second concern is that wheat consumption is associated with an increased prevalence of adverse reactions, including intolerances (notably celiac disease), allergies, and other conditions.

Allergy and intolerance are well-defined conditions that result from reactions of the immune system to foreign components. Allergies are associated with the production of a specific class of antibody called IgE and reactions to ingested wheat products include skin, respiratory, and gastrointestinal symptoms. A range of proteins has been implicated, notably α‎-amylase inhibitors and gluten (gliadin and glutenin) proteins (reviewed by Tatham & Shewry, 2008). Despite popular perception, a detailed meta-analysis of allergies to plant foods showed that allergy to wheat is rather rare, with a prevalence below 0.5% in children (Zuidmeer et al., 2008). Wheat allergy contracted in infancy is also frequently outgrown by teenage years. Reports for the prevalence of sensitization of adults to wheat, determined by skin prick test or serum immunoglobulin E, range from 0 to 3.6%, but these values have not been confirmed by food challenge (Zuidmeer et al., 2008).

Celiac disease (CD) is an autoimmune condition that affects the small intestine, resulting in malabsorption, weight loss, fatigue, abdominal pain, vomiting, and diarrhea. Patients therefore suffer from malnutrition including iron anemia and folate deficiency. However, some individuals may present only mild or no symptoms. The role of wheat gluten proteins in triggering CD was established over 50 years ago, and recent work has identified some 30 short amino acid sequences in gliadin and glutenin proteins that are able to trigger a response in susceptible individuals (Sollid, Qiao, Anderson, Gianfrani, & Konig, 2012).

There has been wide debate about the extent to which celiac disease may, or may not, be increasing in prevalence, and this is difficult to assess because of increased awareness of the condition (particularly in adults) and improved methods of diagnosis. Although reports of prevalence vary between countries, there is a broad consensus that the average prevalence in countries with populations of European descent is about 1%. Increases in prevalence have also been reported in some countries, including Scandinavia and the United States (Lohi et al., 2007; Ludvigsson et al., 2013; Myléus et al., 2009).

Celiac disease may be associated with neurological conditions, notably peripheral neuropathy and gluten ataxia, in which the cerebellum is damaged, while dermatitis herpetiformis is a form of CD, which presents as a chronic skin disease. However, the prevalences of both of these conditions is very low with no evidence of increases.

In recent years an increasing number of individuals have reported symptoms related to wheat consumption which are not allergic or autoimmune responses. This has led to the definition of a new condition called “non-celiac gluten sensitivity” (NCGS) (Sapone et al., 2012). A wide range of symptoms has been reported, ranging from gastrointestinal symptoms to tiredness, headache, dermatitis, pains in muscles and joints, depression, anxiety, and anemia. However, in most cases the reported responses are to wheat, rather than to gluten (Biesiekierski et al., 2013), meaning that the role of gluten has not been established. It is therefore possible that other components, such as FODMAPs, contribute, and it may be more accurate to describe the syndrome as “non-celiac wheat sensitivity” (NCWS).

The pathogenesis of NCGS/NCWS is not understood, but it may include a range of conditions, including the stimulation of the innate immune system. The lack of understanding of the components involved and mechanisms poses a challenge for diagnosis and for estimates of prevalence. Until recently the latter have been based on “self-reporting” rather than blind challenges, with estimates ranging up to 10% or more (reviewed by Shewry & Hey, 2016). The use of strict diagnostic criteria and blind challenges will undoubtedly result in much lower estimates, perhaps as low as those reported for allergy and celiac disease.

Hence, although the prevalence of adverse reactions to wheat is still not established, it is certainly much lower than the numbers of consumers in Europe and North America who have decided to reduce, or eliminate, their consumption of wheat due to perceived health problems. In fact, the reduced consumption of wheat is of concern to health professionals because gluten-free foods may be depleted in the beneficial components that conventional wheat products contribute to our diet: fiber, vitamins, minerals, and phytochemicals.

Challenges for Wheat Production and Utilization

Increasing Production

Despite the decreasing consumption of wheat products in some Western countries, the global consumption of wheat continues to increase. This is due partially to population growth but also to other factors. Although much of the increased consumption is in countries where wheat is traditionally grown and consumed, the increased requirements may be beyond their current production capacity. For example, Egypt produced an average of over 8 million tons of what a year over the period 2007–2011 but still imported over 9 million tons a year (data from FAO Stat). Increasing wheat yields therefore remains a priority in many countries, particularly where yields are currently limited by abiotic (heat and drought) and biotic (pests and pathogens) stresses.

The consumption of wheat is also increasing in countries where it is not traditionally consumed in large amounts This is associated with social and economic changes, with urbanization and industrialization being associated with the adoption of a more Western-style diet. This is happening in India and China, where wheat imports have decreased due to vast increases in production, but also in sub-Saharan Africa where the climate is not appropriate for intensive wheat production. For example, Nigeria imported an average of over 3.5 million tons over the period 2007–2011, compared to production of 117,000 tons. The comparable values for 1961–1965 were 17,600 tons production and 35,000 tons imported (data from FAO Stat). We do not know whether the range of adaptation of wheat can be widened to increase the production in such countries: if not, they will increasingly rely on imports.

Reducing the Nitrogen Requirement for Production and Processing

Although the global yield of wheat in 2013 was 3.26 tons per hectare, the yield in Western Europe was 7.49 tons per hectare (FAO Stat). Although many factors affect yield, one of the most important is the availability of resources, notably water and nitrogen fertilizer. Nitrogen fertilization is of particular concern for several reasons: the manufacture of fertilizer has a high energy requirement, it is expensive and a major input cost for farmers, and it can have an adverse environmental footprint.

However, the requirement for nitrogen fertilization cannot be avoided, as 1% grain protein present represents 1.75 kg of nitrogen per ton of grain. Hence, a yield of 10 tons per hectare containing 10% protein will in theory require a minimum of 175 kg/Ha of nitrogen. In fact, the calculation is more complex, as some nitrogen may be available from the soil (from mineralization and deposition from the atmosphere) while a variable fraction of the available nitrogen is recovered as grain protein. For example, it can be calculated that in the United Kingdom the grain protein represents about 86% of the nitrogen applied as fertilizer, but only about 66% of the total available nitrogen, including that present in the soil (personal communication of Malcolm Hawkesford [Rothamsted Research] based on data in Barraclough et al., 2010), while lower efficiencies may be achieved in other countries where application is less precise. This means that there is an opportunity to reduce the nitrogen requirement for wheat production by improving the intrinsic ability of the plant to take up nitrogen from the soil and incorporate it into grain proteins.

A second opportunity for reducing the nitrogen requirement is by reducing the protein content required for making bread and other foods. The nitrogen requirement for bread making varies, with the process being particularly high (about 13%) for the Chorleywood process (see above). This may require the farmer to apply nitrogen above the optimum for grain yield, increasing the cost of production. Developing new types of wheat with good processing properties at low grain protein content would therefore have a range of benefits (economic, strategic, and environmental).

Improving the Health Benefits of White Flour

The vast majority of wheat products consumed globally are made from white flour, including breads, other baked products, and pasta and noodles. Although some success has been achieved in increasing the proportion of whole wheat products consumed in some countries (notably Western Europe), many consumers remain resistant due the stronger flavor and coarse texture. Furthermore, white wheat products are also favored in countries undergoing urbanization and industrialization, where they may replace traditional foods made from less highly refined cereals. It therefore important to increase the contents of bioactive components in the starchy endosperm rather than in the whole grain in order to deliver benefits in the high-volume refined products which are in greatest demand by consumers.

Acknowledgments

I am grateful to Dr. Malcolm Hawkesford (Rothamsted Research) for discussions. Rothamsted Research receives strategic funding from the Biotechnological and Biological Sciences Research Council (BBSRC) of the United Kingdom.

Suggested Readings

Carver, B. F. (Ed.). (2009). Wheat science and trade. Ames, IA: Wiley Blackwell.Find this resource:

Gooding, M. J., & Davies, W. P. (1997). Wheat production and utilisation: Systems, quality and the environment. Wallingford, U.K.: CAB International.Find this resource:

Khan, K., & Shewry, P. R. (Eds.). (2009). Wheat: Chemistry and technology (4th ed.). St. Paul, MN: AACC.Find this resource:

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