All proteins contain nitrogen, carbon, hydrogen, oxygen, sulphur; some contain also phosphorus and iron. However, there are many forms of protein, all possessing widely varied chemical and physical properties. By such experiments as those that have been performed and others much more complicated and extended, scientists have learned enough about these substances to classify them as follows :

I. Simple Proteins:

A. Albumen, found in egg, blood, milk, etc. Properties: (1) Coagulates with heat

(2) Soluble in water

(3) Soluble in weak salt solution

(4) Insoluble in strong salt solution

B. Globulin, found in egg, muscle, blood, milk, etc. Properties: (1) Coagulates with heat

(2) Insoluble in water

(3) Soluble in weak salt solution

(4) Insoluble in strong salt solution

C. Alcohol-Soluble Proteins

(1) Glutenin and gliadin, found in wheat, which, when mixed with water, unite to form gluten

(2) Legumen of peas and beans

D. Albuminoids or proteins found in skeleton and framework.

Principal form is collagen, which is found in the white fibers of connective tissue and which, when boiled with water, forms gelatin Properties: (1) Dissolve in hot water (2) Form a jelly when cold

E. Extractives

In the animal body proteins are constantly breaking down into simpler substances, some of which contain nitrogen. At the time of killing, some of these nitrogenous sub-stances are in the tissues; they can be extracted with water and for this reason the name "Extractives " has been given them. As a human food they serve as mild stimulants and also as flavoring materials. It is not thought, however, that they can be used as building stones. If an animal is fed on a diet in which extractives are the only nitrogenous substances, it dies of what is called nitrogen starvation.

The extractives are not coagulated by heat; they will, there-fore, remain in solution when meat juice is heated. Not so, however, the albumen and globulin of meat juices; they are coagulated at a certain temperature and rise to the surface and form a scum. A beef broth, therefore, in which the proteins have been coagulated and removed by skimming or straining is wholly lacking in tissue-forming substances; it is mildly stimulating, however, i.e. it spurs on the various processes of the body and may be the means of supplying water in a very acceptable form.

II. Conjugated Proteins. - Proteins which contain some other molecule united to the protein molecule

A. Phospho-proteins. - Are compounds of protein with a phosphorus-containing substance. They are especially adapted to furnish the material for tissue growth. They are present in the casein of milk and vitellin of egg yolk.

B. For other forms of protein see "Chemistry of Food and

Nutrition," H. C. Sherman.

Eggs

Embryo or germ -from which chicken develops

Twisted white cords upon which the yolk revolves -

Air space skin lining the shell

Shell-

Section of a Hen's Egg

Composition of Edible Portion of Eggs - (H. C. Sherman) Water, 73.7%; Fat, 10.5%; Protein, 14.8%; Ash, 1.0%.

Composition of Yolk of Egg Water, 50.9%; Fat, 31.75%; Protein, 16.2%; Ash, 1.09%.

Composition of White of Egg - (Hutchinson) Water, 85.7%; Fat, 0.25%; Protein, 12.6%; Ash, 0.59%.

Eggs contain all the elements necessary for the growth of the body, as from the egg the young chick is formed. They are lacking, however, in carbohydrates, so it is better to eat them with bread, rice, potato, or other starchy foods. The yolk of the egg forms a valuable source of iron and phos-phorus for the body.

The edible portion of newly laid eggs almost fills the shell, but as the shells are porous the water evaporates, and air, containing decomposing bacteria, takes its place. As it is the presence of these bacteria which causes the egg to spoil, the principle of egg preservation is to prevent the evaporation of water by filling the pores of the shell. This is accomplished by placing the eggs in a solution of water glass. A fresh egg, containing more water than a stale one, will sink in water.