The characteristics of gluten gain in importance because it is the protein of the flour as a whole, regardless of whether it is composed of one, few, or many components, that gives the baking quality to flour. When water is added to flour, the proteins gradually absorb about 200 per cent of their weight in water.

When flour is made into a stiff dough and kneaded for a short time, the starch can be washed out, leaving a small part of the original dough which is known as gluten. Gluten is rubbery, tenacious, and elastic. It is usually a light gray or slightly yellowish color. A large part of the gluten is composed of the proteins gliadin and glutenin. In addition to the protein, the gluten as washed from the dough contains some starch which is entangled in the gluten, some lipoids, mineral salts, and water. The amount of all these constituents of gluten varies with the manipulation in washing, the kind of water used, and the character of the flour itself. The physico-chemical properties of gluten have been extensively studied and applications made to baking bread.

Since gliadin and glutenin have about the same isoelectric point, ranging around pH 6.5 to 7.0, Bailey states, "it might be anticipated that maximum coherence and extensibility of gluten and dough should be encountered in about the same range."

Heat coagulation of gluten. Gluten seems to have no definite coagulation temperature. Alsberg and Griffing have reported that heating the gluten decreases its power to swell in acid solutions, the swelling power decreasing during heating from 50° to 80°C. Except at the temperatures 60° to 65°C. no information was obtained that indicated a definite coagulation temperature. They state that denaturation takes place over the whole range from 50° to 80°C. The swelling power was not impaired but probably increased at temperatures from 30° to 50°C.

Gluten quality. Different flours vary in their baking qualities. Millers and bakers speak of those having poor baking qualities as "weak" flours, and those having good baking qualities as "strong" flours. The definition of good baking quality commonly accepted by millers and bakers is the one suggested by an English worker, Humphries: "Strong flour is one capable of making large well-piled loaves." This definition excludes flours "that produce large loaves, but do not rise satisfactorily." Thus strong flours and good baking quality are synonymous to a certain extent. A weak flour does not have good baking qualities, but there is no distinct line between weak and strong flours. A flour may also be too strong for good baking qualities. Such a flour is called gluten "bound."

Gortner and Sharp have reported that weakness of flour may be divided into three classes as follows:

1. Weakness due to an adequate quantity of gluten but of inferior quality.

2. Weakness due to an inadequate quantity of gluten of good quality.

3. Weakness due to factors influencing yeast activity, diastatic and proteolytic enzymes, hydrogen-ion concentration, etc.

Even though flour may have an adequate quantity of gluten for good baking qualities, if the quality is inferior it does not produce a good loaf of bread.

Gluten quality and growth of wheat. In the flour the protein may be thought of as a partially dried hydrogel. When proteins are dried to contain little or no water, less than 5 per cent, they may lose the power of swelling in water. If they contain 10 to 15 per cent of moisture they may retain the power of swelling in water. Flour contains 10 to 13 per cent of moisture, so that the swelling property is not destroyed. However, the moisture content of flours varies somewhat; but of more importance to the quality of the protein is the rate of dehydration of the protein when it is deposited in the wheat kernel. It is well recognized in colloidal chemistry that the past history of a colloid influences its future reactions. The conditions during the growth of the wheat influence the later reactions of the protein. In the sap of the wheat the protein is in the form of a sol; later as it is deposited in the wheat it is found in the form of a gel. With ripening and further dehydration of the protein, dried hydrogel is formed. The speed at which this takes place, depending upon climatic and moisture conditions, affects the quality of the protein. The mineral salts and the lipoid content of the flour also influence the quality of the gluten, which has been discussed under ash.

Gluten quality and colloidal state. Gortner and Doherty suggest that the size of the gluten particle is a factor in determining its properties. They emphasize that strong flours give glutens with nearly "perfect colloidal" gels with highly "pronounced physico-chemical properties," whereas weak flours give glutens whose properties are less colloidal; or strong flours contain gluten in a less dispersed state, and weak ones contain gluten more highly dispersed.