This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Bungenberg de Jong states that gliadin is a protein that swells easily in water, is readily peptized to a colloidal solution by dilute alkali or acid and in water forms a sticky mass that can easily be stretched into threads. Glutenin in water is a flaky mass without much coherence which swells in, but is difficult to peptize in dilute acid. It peptizes easily in alkali.
Bungenberg de Jong suggests that the plastic and elastic properties of gluten are partially due to the two proteins' adhering to each other because of opposite charges throughout a definite pH range. If this is true, he says that at the pH at which the charge on each protein is the same either a maximum or minimum in physical properties should occur, i.e., turbidity, coherence, farinogram curves (which is a measure of consistency of the dough). The point at which this maximum occurs might be altered by the presence of other proteins, such as albumin and globulin with isoelectric points much lower than that of glutenin, hence would be negatively charged at the pH range at which the gliadin-gluten complex occurs. He also discusses the possibility of salts altering the maximum point. Jong represents this maximum or minimum range schematically. See Fig. 44.
Gliadin, below its isoelectric point pH 6.4, is positively charged, this charge increasing as the pH is lowered. However, about pH 5.6, the isoelectric point of glutenin, the glutenin is negatively charged. In Bungen-berg de Jong's experiments maximum turbidity occurred at pH 6.1.
Gluten quality and proteolytic enzymes. Balls and Hale ascribe part of the quality of gluten to the action of the proteolytic enzymes. They state that the proteolytic enzymes change the colloidal character of wheat proteins, the effects of which are shown in the gluten. "Proteinases usually produce first a coagulation of the protein; later the coagulated material is broken down and perhaps ultimately dissolved. If this rule holds for flour, in the first phase of proteinase action the gluten would probably become more tenacious; in the second phase it would be broken down to a thinner, more nearly liquid material." They add that a small amount of proteinase may be beneficial, a larger amount harmful. "Usually there is too much, rather than too little."
Fig. 44. - Bungenberg de Jong's representation of effect of charge on gliadin and glutenin on gluten properties. The charge of components is represented by positive and negative signs. J. Soc. Chem. Ind. 52: 391T, 1933.
The baking qualities of gluten are determined largely by its hydration capacity, its power of cohesion, and its elasticity. The hydration capacity results in a larger or smaller volume when mixed with water. With a larger volume the particles of the gluten form a greater surface for contact with each other, thus increasing the sponge-like character of the dough. Also with increased swelling the gluten becomes more tender and less tenacious; with less swelling the volume is smaller but the tenacity may be greater. The swelling can be so great that there is a limit to the amount of stretching the gluten will stand before breaking. The cohesive-ness of the gluten gives less chance for the gluten particles to be pulled apart; the elasticity permits stretching under pressure or when pulled.
Hydration capacity. By hydration capacity is meant the swelling of the gluten in water, whatever the mechanism by which it is brought about, whether by absorption or other means. Gortner and Doherty have reported that a strong gluten has a faster rate of hydration, or imbibition as they express it, and also a higher maximum hydration capacity than a weak gluten.
Distribution of water in dough. Alsberg in "Starch and Flour Quality" states that in bread dough about 50 per cent of the water is bound moisture or water of hydration incapable of serving as a solvent for other substances. The starch holds approximately half of this bound water, the gluten the remainder. Starch at room temperature can absorb about 30 per cent of its weight of water, whereas gluten may absorb 200 per cent of its weight. But starch constitutes so great a percentage of the flour that as a result the quantity of water it binds is nearly as great as that bound by gluten. The remaining 50 per cent of the water in bread dough, which may serve as a solvent for other substances and form steam, cannot be separated readily from the dough by mechanical means. It is held in the interstices of the dough by surface and mechanical forces.
The proportion of water added. The proportion of water added also affects the character of the dough and baked product. If too little water is added the maximum cohesiveness and elasticity of the gluten is not attained.
Water-binding capacity of flour, dough, and bread. Kuhlmann and Golossowa found that the water-binding capacity of the flours they tested was in the following order: Soya, rye, corn maize, durum wheat, soft wheat, and potato. No complete parallelism existed between the protein content of the flours and their water-binding capacity. Rye starch had a water-binding capacity of 78 to 80 per cent, whereas that of wheat starch was 40 to 42 per cent. A direct relationship was found between the water-absorbing capacity of the flour and its water-binding capacity.
The water-binding capacity of bread doughs was about the same as flour, but a sharp increase occurred in bound water during baking so that a dough with a water-binding capacity of 60 per cent changed to about 85 per cent in hot bread.
Kuhlmann and Golossowa found that the factors which increased the water-binding capacity of the dough decreased moisture loss in baking and in drying out or aging of the bread after baking. Both the method of making the dough and materials used influenced the water-binding capacity. Doughs made by a straight-dough method bound more water than those made by the sponge method. When the flour for the sponge method was scalded, the water-binding capacity was markedly increased over dough or bread made by the sponge method without scalding and slightly higher than dough made by the straight-dough method. The addition of maltose and buttermilk increased the water-binding capacity of the doughs and bread.