Peter R. Shewry, Rothamsted Research, Harpenden, Hertfordshire, United Kingdom; Renato D'Ovidio and Domenico Lafiandra, Universita degli Studi della Tuscia, Viterbo, Italy; John A. Jenkins and E. N. Clare Mills, Institute for Food Research, Norwich, United Kingdom; Ferenc Békés, CSIRO Plant Industry, Canberra, ACT, Australia
WHEAT: Chemistry and Technology, Fourth Edition
Pages 223-298
DOI: https://doi.org/10.1094/9781891127557.008
ISBN: 978-1-891127-55-7
Abstract
The word protein means primary substance, according to Mulder and Berzelius, who proposed the name in 1838 (Tracey 1967). However, the study of wheat proteins has a longer history, starting with the famous description of gluten published by Jacopo Beccari, professor of chemistry at the University of Bologna, in 1745 (Beccari 1745). In his article “De frumento” (“concerning corn or grain”), he described the separation of wheat into two fractions, “amylaceum” (starchlike), which was soluble in water and had properties similar to those of sugars, and “glutinosum,” which was insoluble and sticky and resembled substances of animal origin (i.e., proteins). Readers are referred to Bailey (1941), who provides an excellent commentary and “modern” translation of this fascinating work. Further studies were reported by Parmentier (1773), who showed that gluten was largely soluble in vinegar (i.e., acetic acid) and partially soluble in spirits of wine, and by Einhof (1805, 1806), who showed that part of gluten was soluble in alcohol and that similar fractions were present in barley and rye, thus establishing the major property that was eventually used to define prolamins as a group. Taddei (1819) reported further studies of these properties, separating gluten into fractions that were soluble (gliadins) or insoluble (zymon, later called glutenin) in alcohol, while O'Brian (1895a, b) reported studies of gluten and other wheat seed proteins.
This early work, and similar studies of maize (Gorham 1821; Bizio 1822a, b) and oats (Kreusler 1869), laid the basis for the work of Thomas Burr Osborne (1859–1929), who can be regarded as one of the founding fathers of protein chemistry. Working at the Connecticut Agricultural Experiment Station, Osborne published some 250 papers, including studies of seed proteins from 32 species, over the period 1886–1928. His studies of wheat are recorded in research papers (Osborne and Vorhees 1893, Osborne 1907) and in his monograph The Vegetable Proteins (1924).
Osborne adapted the system of protein classification proposed by the “Recommendations of the Committee on Protein Nomenclature” (1908), which divided proteins into three major types: simple, conjugated, and derived. Osborne concluded that the proteins in plant tissues were “simple” and comprised four major types, which have since become known as “Osborne fractions”: albumins, globulins, prolamins, and glutelins. The gluten proteins of wheat classically fall into two of these groups, with the alcohol-soluble gliadins being defined as prolamins and the alcohol-insoluble glutenins as glutelins. The water-soluble albumins and salt-soluble globulins of wheat grain are a mixture of structural, metabolic, and storage proteins, while at least some of the insoluble structural proteins may be extracted with dilute acid or alkali in the glutelin fraction. Readers should note that enzymes and enzyme inhibitors, with the exception of the chloroform/methanol-soluble proteins, are covered in Chapter 11.
The literature on wheat grain proteins is massive, and much of our knowledge has resulted from small “incremental” advances. However, it is also possible to identify significant technical advances that can be seen as landmarks. These include the introduction of electrophoresis on starch gel (Jones et al 1959, Woychik et al 1961) and polyacrylamide (Lee 1962) matrices, sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) (Bietz and Wall 1972), two-dimensional electrophoresis (2-DE) of genetic stocks (Wrigley and Shepherd 1973), the use of chaotropic agents for protein separation (Meredith and Wren 1966), and the introduction of high-performance liquid chromatography (HPLC) (Bietz 1983).
A more conceptual advance that has had a crucial impact on our view of gluten proteins is the recognition that solubility or insolubility in aqueous alcohols depends largely on whether the protein subunits are assembled into polymers stabilized by interchain disulfide bonds, although the initial studies that led to this concept were made on other cereals. As early as 1939, Bishop noted that the extraction of hordein from barley grain was increased in the presence of bisulfite, while Foster et al (1950) reported that the extractability of zein from maize was increased in the presence of a reducing agent. Detailed studies of barley (Lontie et al 1953, Lontie and Voets 1959) and maize (Sodek and Wilson 1971, Landry et al 1972) established that the fractions extracted by aqueous alcohol followed by aqueous alcohol containing a reducing agent were related in composition and properties, and this was subsequently also established for wheat by Bietz and Wall (1973). This was a crucial step toward the acknowledgment that gliadin and glutenin are composed of related subunits, with their solubility being determined by their patterns of disulfide bond formation: gliadin subunits either contain no cysteine residues or form only intrachain disulfide bonds, while glutenin subunits form inter- and intrachain bonds.
Although much of the work described here was conducted because of the importance of wheat proteins in determining processing properties, the contribution of wheat proteins to human nutrition should not be overlooked. On a worldwide scale, about 70% of the protein available for human consumption is derived from plant sources, with cereals contributing more than 70% of this.
Wheat, as one of the “three major cereals” (with rice and maize), contributes almost a third of the world's cereal protein production (FAO 1970). However, the nutritional quality of wheat grain is limited by the low content of lysine, and it is therefore necessary to mix cereal grains with other sources of protein (such as animal protein or legume seeds) to provide a balance of essential amino acids. Improvement of nutritional quality has therefore also been a stimulus for some work on wheat protein and is discussed in more detail in Chapter 7 of this volume.
