This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Osborne in his classical work on the wheat flour proteins reported only a small fraction soluble in water. This fraction contained leucosin and albumin, plus proteose or non-protein nitrogen. A small fraction was extracted by dilute salt solution and this globulin fraction constituted about 0.6 per cent of the wheat kernel. Gliadin, a prolamin, constituting about 40 per cent of the total protein, was soluble in alcohol. The remaining protein, called glutenin, was found to be insoluble except in dilute acids or alkalies.
But later work has shown that the terms "globulin" and "albumin" have little meaning in connection with flour proteins and that neither "gliadin" nor "glutenin" may be an individual chemical entity. Blish in "The Wheat Flour Proteins" says, "It seems that as we have become increasingly familiar with the behavior and properties of flour proteins, we have found ourselves progressively more and more hesitant in the matter of making dogmatic assertions and published statements relative to their nature, their identities, and their individualities."
Effect of salts on protein extracted. Gortner, Hoffman, and Sinclair found that N/1 solutions extract the following average percentages of the total wheat flour proteins: KF 13, KC1 23, KBr 37, and KI 64. They showed that the amount of protein extracted varied not only with the nature of the salt used but also with its concentration.
Gortner, Hoffman, and Sinclair have found that for increasing peptizing effect on wheat proteins the following anions arrange themselves in a lyotropic series: F < SO4 < CI < tartrate < Br < I. The cations have a less distinct effect on solubility of the flour proteins but give the following order of peptization: Na < K < Li < Sr < Mg < Ca.
Mangels and Bailey in studying the gelatinizing action of reagents on starch noted a Hoffmeister series as follows: CI < B < I < CNS < salicylate <OH.
Rich has shown that the amount of protein extracted or peptized by N/2 MgSO4, 10 per cent NaCl, and N/4 NaCl solutions varies directly with increasing ash content of the flour. In general, the maximum peptization occurred with N/4 solutions. A further increase in salt concentration causes dehydration and eventually salting out of the protein micelle.
Is gluten a single protein? McCalla and Rose say that the most soluble 10 to 15 per cent of the gluten protein is probably distinct, but the remainder is a single protein complex which can be divided into a great many progressively different fractions. They suggest the terms "glutenin" and "gliadin" should be discarded.
Rich states that it is "apparent that the protein of wheat is a single individual and that knowledge of the nature of differences in protein 'quality' between 'strong' and 'weak' flours can best be solved by studying the protein as a whole instead of splitting it into arbitrary fractions and studying each fraction separately." He believes that if the dispersing agent or the method of isolation is changed, even slightly, there is no established limit as to the number, quantities, and character of the protein fraction that can be isolated.
Is gluten composed of distinct groups or individual proteins? The greatest difficulty in fractional precipitation of the flour proteins can be traced to the agents used to disperse them, for no solvent has been found from which all the protein can be recovered without loss or irreversible change in original properties. Blish says that "irreversible and destructive hydrolytic changes occur almost at once, and proceed at a relatively high rate when gluten is dispersed in weak alkali." On the other hand, in dilute acetic acid, an appreciable amount of hydrolysis is detectable after a few hours standing, but at ice temperature hydrolytic changes are slow. Addition of an equal volume of alcohol to dilute acetic acid protects against hydrolytic change, but may split off other components. Urea is used for peptizing proteins, but because it gelatinizes starch must be used on washed gluten.
Salicylate is one of the anions in the lyotropic or Hoffmeister series causing greatest peptization of flour proteins. Sodim salicylate appears to peptize the flour proteins with no observable hydrolysis or denaturation.
Blish states that it is probable that only a few components or component groups constitute the flour proteins and that they may be precipitated as mixtures containing progressively varying proportions of these few components. "We can merely summarize our present viewpoint, which is as follows: There is a considerable portion of flour protein that does not enter into the formation of gluten, but it is not yet possible to distinguish a sharp and definite boundary-line between the gluten and non-gluten portions. With reference to gluten, itself, we recognize a definite fraction consisting of approximately 10-12 per cent that is extremely resistant to dispersion. We recognize the 'gliadin' fraction, which accounts for 50 per cent or more of the gluten, and which may or may not be a mixture of components having very similar properties. The remaining portion we regard as doubtful, but until proven otherwise we prefer to look upon it as a mixture of a 'glutenin' and a lipoid-protein complex." Work with flour proteins "is peculiarly a problem in which any new discovery is likely to invalidate much that has gone before and to necessitate an entire re-examination of the whole situation."
Lecithio-protein. Flour may contain a lecithio-protein somewhat similar to that found in egg yolk. Blish says that alcohol added to acetic acid solutions in fractioning flour proteins may cause irreversible splitting off of an important gluten component, a "lecithio-protein" or some similar lipoid-protein complex. Blish adds that he suspects the "lecithio-protein" is a very important gluten constituent and that its importance and amount in flours have been underestimated. "At present we feel that a thorough study and understanding of the nature and properties of the lipoid-protein complex will contribute greatly to the solution of several flour problems that are of the greatest fundamental interest and importance."
Gliadin. Gliadin, either as a multiple protein, or as a component of the gluten complex, is soluble in 60 to 70 per cent alcohol. In water it swells to a sticky mass, being least soluble at its isoelectric point about pH 6.5. With increasing acidity it becomes more soluble, reaching a maximum with a pH range of 2.0 to 3.0. With hydrogen-ion concentration greater than pH 2 the solubility decreases gradually. With increasing alkalinity, the solubility of gliadin increases more rapidly than on the acid side, up to pH 13.1, the most alkaline of the solutions used by Tague.
Glutenin. It is the glutenin, classified as a glutelin, that gives the desirable baking qualities to wheat flour, qualities not possessed by any other cereal. Glutenin is insoluble in water, but is increasingly soluble in dilute acids and alkalies. Sharp and Gortner state that the maximum solubility occurs on the acid side at pH 3.0 and on the alkaline side at pH 11. The isoelectric range, as determined by Sharp and Gortner, is from pH 6.0 to 8.0, and by Bungenberg de Jong as pH 5.6. Glutenin swells in water. If prepared with sodium salicylate it is said to resemble gluten in its coherence and tenacity.