49. Glutenins

These form a large part of nitrogen compounds of the cereal grains and possibly of other seeds. They are insoluble in water, alcohol, and neutral salt solutions, but readily dissolve in very dilute acids and alkalies. The glutenin of the wheat, found in the tenacious substance that is left after washing the starch out of wheat flour, is the best-known protein of this class and is an important constituent of wheat flour, existing there to over 40 per cent of the total protein.

50. Alcohol-soluble proteins.1 -; Alcohol-soluble proteins have been found in all the cereal grains so far examined. The principal ones to be mentioned are gliadin from wheat, zein from corn, and hordein from barley. Gliadin is practically as abundant in the wheat kernel as is the glutenin with which it is associated, the two together constituting over 80 per cent of the total proteins of that cereal. The proportion of gliadin in wheat flour has much to do with its quality for bread-making purposes. It appears that the best bread flour contains about twice as much gliadin as glutenin.

51. Albuminoids

This term, according to the classification in common use in the United States, has been understood as including various proteins such as the albumins, and globulins. The latter classification recommended confines the term to the proteins found chiefly in the animal body in such parts as the cartilages, bones, feathers, hair, hoofs, horns, and nails. These proteins are also obtained from the threads of silkworms and from sponges. The albuminoids have group names, such as collagen in cartilage and bone, keratins in feathers, hair, hoofs, horns, nails, and similar exterior tissues, fibroin in the threads of silkworm, and spongin in the framework of sponges.

1 Osburn proposes the name prolamine. Science, Vol. XXVI, pp. 417-427.

Gelatin, so well known to the housewife, is derived from collagen. It is a matter of common observation that when meat containing tendons (cartilage) is submitted to the action of boiling water, there is obtained in the extract a gelatinous substance which becomes evident when the extract is cooled. This gelatin is insoluble in cold water, but dissolves in hot. As a dry commercial article, it is a tenacious substance, which, when prepared in thin layers, is transparent When collagen and gelatin are acted upon by tannic acid, as, for instance, when the skin of an animal is treated with an extract from hemlock or oak bark, the result is a substance which does not putrefy and which gives to the tanned hide the properties of leather. Gelatin is much used in various food preparations.

It is characteristic of the keratins such as hair and horn that they contain a relatively large proportion of sulfur, the analysis of horn and hair showing as high as 5 per cent, the average amount in horn being 3.3 per cent. The keratin bodies serve to give rigidity and wearing qualities to certajn exterior animal tissues.

52. Histones, Protamines

The proteins in these two groups do not occur as such in nature, and are only obtained by separating them from some combination. The two groups are alike in being basic in character and in being found in the spermatozoa of fishes. Histones have also been obtained from the blood corpuscles of a goose and from the white blood corpuscles of thymus glands.