Wheat Proteins, Known and Unknown

Dale K. Mecham, George H. Brother.

A search for new uses for wheat and for means of extending its present uses goes on steadily. Always to be borne in mind is that wheat is usually a more expensive source of protein, starch, or other constituents than crops with which it must compete. When wheat is used for food, its extra cost is justified by its unique value in making bread and other baked goods. Bread is made from wheat flour because the proteins in wheat enable leavened wheat-flour Boughs to retain gas bubbles in a manner that provides the desired porous structure of breadstuffs. Flours made from other seed crops lack this characteristic.

The properties, and therefore the possible uses, of wheat proteins depend on the methods of their separation from the other components of wheat and, more basically, on their chemical and physical behavior. Accordingly, we shall discuss the separation and identification of the proteins before we consider their possibilities for utilization. Also to be stressed is that about six times as much starch as protein can be recovered from wheat. Protein can be produced for industrial purposes, therefore, only when suitable markets are also available for the starch.

The total protein content of wheats ranges from 6 to 22 percent, depending on soil, weather conditions, and variety. The commercial classification of wheats as hard or soft in kernel texture reflects somewhat the protein content. A protein content of 12 to 12.5 percent (on a moisture-free basis) can be considered as intermediate. Most soft wheats contain less protein. Most hard wheats contain more.

Several different proteins are present in wheat kernels, but the principal and most characteristic ones are those that make up wheat gluten. The gluten proteins may be separated as a coherent, somewhat rubbery, but extensible mass by gently kneading a flour-water dough in a stream of water; starch and water-soluble constituents are washed away and the crude gluten remains.

Because they are largely responsible for the usefulness of wheat as a bread-making cereal, the gluten proteins have always been of interest in connection with the use of wheat in foods. As far as the industrial nonfood utilization of wheat proteins is concerned, they are of primary interest also because they make up the major portion of the total wheat proteins and can be separated by relatively simple mechanical procedures.

THE SEPARATION of the gluten proteins from wheat ordinarily begins in the flour-milling process. The bran and germ, which contain no gluten proteins, are separated mechanically from the endosperm, which contains the proteins. Then the particles of endosperm are ground to flour of the fineness desired. About 70 percent of the wheat proteins are recovered in flour of various grades. Of the flour proteins, approximately 80 percent are gluten proteins. (The figures vary somewhat with wheats of differing hardness, weight, and total protein content.)

In commercial practice, the gluten proteins are usually separated from other flour components by mechanical washing processes of different types. In the oldest method, a flour-water dough is made and then kneaded in an excess of water. As starch is removed from the dough, the starch-water mixture is replaced with fresh water until all the readily removable starch is washed out and the gluten remains as the characteristic wet mass. Starch is recovered by methods similar to those used for the industrial production of other vegetable starches.

Recently modifications and variations in the basic process have come into use. In one, a very soft dough, formed by adding more than the usual proportion of water, is subjected to a tumbling action in the wash water. In another, developed by Department scientists, a soft dough or batter is mixed with additional water to form a suspension in which the gluten proteins form small particles, large enough to be screened away from the starch but small enough to permit rapid and,complete separation of starch from the gluten. These modifications provide more rapid and more nearly continuous processes.

Other methods of separating the proteins have been developed. One a wet-milling process similar to that used in the production of cornstarch is applied to the entire wheat kernel. The proteins so recovered are greatly changed in properties and have been used only as animal feed.

In another process, wheat flour is treated with dilute alkali solutions, in which the proteins dissolve, the undissolved starch is removed, and the proteins are recovered by neutralizing the alkaline protein solution. The protein obtained is suitable for the production of monosodium glutamate, although its physical properties have been altered by exposure to alkali.

The wet crude gluten obtained by the mechanical washing processes contains water to the extent of about two-thirds of its weight. Its drying is an important engineering and economic problem. If heat is used, the physical properties are altered so that the gluten no longer forms the typical rubbery mass with water, and the product is known as devitalized gluten. If heating of the gluten is avoided, it will again form a rubbery mass with water; the product is gum gluten.

The processes by which gluten can be dried without heating are costly. Vacuum drying can be used. When the vacuum is applied, however, the gluten mass puffs up to many times its original size so that the capacity of drying equipment is reduced. Or, the gluten can be extruded in a thin sheet or in ribbons or subdivided by other means and dried in an air current; here again the capacity of the necessary equipment is low. The difficulties in the drying process are indicated by the relative market prices of devitalized and gum glutens, which were approximately 16 and 30 cents a pound, respectively, in 1950.

The dried crude gluten produced by the usual washing processes contains 75 to 80 percent protein. The non-protein material consists mainly of 5 to 15 percent carbohydrates ( chiefly residual starch) and 5 to 15 percent fats of different types. More vigorous and prolonged washing will remove much of the starch; gluten containing 85 to 90 percent protein can be obtained in that way. Glutens separated from a wide variety of wheats fall within these ranges of gross composition; when they are wet they form the typical gluten mass.

BECAUSE GLUTEN gives bread its structural framework, it might be expected that the higher the gluten content of a flour, the greater would be its bread-making value. All glutens do not have this ability to the same degree, however. In fact, the variations are so great that, of two flours containing equal amounts of gluten, one could make excellent bread by a variety of procedures, while the other might not produce satisfactory bread by any process. Equally significant differences in behavior can be found in flours in the lower range of protein content used for making cake and cookies.

Those variations present the milling and baking industries with a continuing problem because the quality of their products must be kept uniform. A baker selects flours of different characteristics for use in different products and he tries to obtain the flours best suited to his baking processes. Some bread flours require long mixing and fermenting to produce good bread. Others are easily overmixed or over-fermented, but they will yield good bread if they are handled properly. In some shops precise temperature control may be difficult, or adherence to a rigid dough-handling schedule may be impossible; a flour with tolerance to such factors then is required. On the other hand, in a highly mechanized bakery a rigid production schedule may be necessary. Then successive lots of flour must be uniform in characteristics. The miller must buy wheats of widely varied properties and blend and mill them to produce flours of different types but of reasonably uniform qualities within a type in order to satisfy the bakers who buy his flour.

THESE PROBLEMS have stimulated studies of the properties of gluten by both chemical and physical methods in the hope that the fundamental causes of the variations and the essential characteristics of good-quality gluten for different uses would be found. Such knowledge would be of practical value to the milling and baking industries. The suitability of lots of wheat for specific uses could be predicted more accurately and easily, and the proper processing conditions in the bakery for lots of flour specified.

Actually, such information could have a broader usefulness. It could insure that, whatever its variety, wheat would be processed into the type of Product for which it was best suited. Some varieties are grown because they Yield well or are disease-resistant but are of inferior quality for milling and baking. New methods of using such wheats might be developed if the basic reasons for their lower quality were known. Finally, physical and chemical properties essential for specific uses could be determined and set up as standards for new wheat varieties developed by plant breeders.

Although much useful information has been accumulated, the basic problem is unsolved; the fundamental differences in the proteins of flours of varied characteristics are still unknown.

INVESTIGATIONS in this field may be said to begin with the work of T. B. Osborne and his associates at the Connecticut Agricultural Experiment Station. A report of their studies was published in 1907.

Osborne concluded that the protein of gluten has two distinct components, gliadin and glutenin, which are present in nearly equal amounts. He also noted the presence of residual starch and fatty materials in gluten, and he studied the nongluten proteins of wheat flour and of -the wheat kernel.

The portion of gluten that was soluble in 60 to 70 percent ethyl alcohol Osborne called gliadin. The remaining portion, glutenin, could not be dissolved in water or water-alcohol mixtures, but was soluble in dilute acids and alkalies. Besides the differences in solubility, differences in chemical composition between gliadin and glutenin were noted. The physical properties of wet gliadin and glutenin prepared by Osborne's methods are markedly different from those of wet gluten. Gliadin forms a soft and sticky mass of gluelike consistency; glutenin swells in water to form tough, inextensible particles which do not cohere well.

Until about 1925, Osborne's conclusions on the components of gluten were not seriously questioned, although attempts to predict and interpret the behavior of flours of varied baking characteristics on the basis of the properties of gliadin and glutenin were not satisfactory. For example, it had been proposed (apparently from a consideration of the physical properties of typical preparations) that the ratio of glutenin to gliadin might determine the characteristics of any specific gluten. When attempts were made to determine this ratio for various glutens, it was found that slight variations in experimental procedures produced large changes in the distribution of protein between the gliadin and glutenin fractions. Such results were not in accord with those expected, if only two distinct and individual proteins were present. Furthermore, it was demonstrated that glutenin prepared by Osborne's method had undergone irreversible changes as a result of its exposure to alkali.