As mentioned above, eggs have been used from time immemorial as a source of nourishment. While eggs of many different species of birds have been at different times used for food, the egg of the domestic fowl is now used almost exclusively by the peoples of Europe and America, and of these the egg of the hen is used in vastly the greatest numbers. The hen's egg is practically the only egg found in the metropolitan markets, the eggs of the duck, goose, turkey, and guinea fowl being used principally in the rural districts where they are produced. While eggs contain a considerable amount of fat, approximating ten per cent, this does not make a sufficient amount of fat, even in the half dozen eggs that might be taken in one day's rations, so that we would be justified in looking upon the egg as an appreciable source of fat. We eat the egg because of the protein. It is a most wholesome source of protein, and therefore is classified here as a nitrogenous food. The fat of the egg is contained in the yolk, which is about one third fat. Egg fat, as extracted with ether, is a yellow oil composed largely of olein with a slight admixture of palmitin and stearin. Egg protein consists of a mixture of several albumens and globulins. The principal albumen of the white is called ovalbumen, the principal protein of the yolk is vitellin. The egg does not possess a clearly marked flavor. For this reason it is customary in the cooking of eggs and milk in custards, neither of which constituent possesses a marked flavor, to add some flavoring material. It often happens, however, that the food used by the hen possesses a strong flavor, and this may, in turn, influence the egg, giving it a flavor which may be disagreeable. If eggs are not absolutely fresh they are likely to get a strong, disagreeable flavor. It is important to be able to test an egg to determine its freshness. Several methods have been devised. If one clasps an egg in the two hands looking through the egg at a candlelight, the hands shutting out all the light that does not come through the egg, a perfectly fresh egg seems translucent and unclouded. In an egg which has been incubated, or has been for a few days under a setting hen, a dark spot is visible, which increases in size according to the length of time the incubation has continued. An egg in which decay has begun is dark-colored. Another method of determining the freshness of eggs is to float them in a brine made in the proportion of two ounces of salt to a pint of water. In such a brine a fresh egg will sink at once to the bottom, while eggs one, two, and three days old will rise nearer to the surface. After the third day the surface of the shell will appear above the liquid, and with increasing age a progressively larger and larger portion of the shell will be exposed above the level of the liquid. Eggs are, as a rule, easily digestible, even when hard boiled, if they are masticated until they are reduced to a smooth, creamy consistency. Eggs are used very largely in combination foods where they are mixed with milk, flour, starch, etc., in the making of custards, puddings, breads, and cakes. Such combinations will be discussed later. In the separate cooking of eggs we may describe the simplest first. The boiling of an egg may be accomplished in three different ways: If an egg is dropped into a pint of actually boiling water, and if the water is kept actively boiling for three minutes, the egg at the end of that time will be in the condition known as "soft-boiled." The soft-boiled egg when properly prepared should present the yolk in a creamy condition, and the white soft and gelatinous, but cooked through. If the boiling is continued for a longer period the white and yolk become progressively more and more dense and hard until after fifteen or twenty minutes of boiling the yolk finally becomes mealy in consistency and the white hard and leathery. However, even in this condition the yolk may be very easily masticated, and the white, if masticated sufficiently long to reduce it to a fine state of division, is not difficult to digest. In soft boiling eggs there should be about a pint of water to each egg, otherwise if several eggs are put into, say, a quart of water, the temperature will be so appreciably lowered by the introduction of the eggs that the active boiling will cease for a minute or two. At the end of the three-minute period, if the eggs are taken out, it will be found that they are barely warm through, the white being not yet coagulated.

Another method of boiling eggs is to put the eggs into actively boiling water, the proportion of one egg to a pint of water, and stand them aside away from the stove for a period of six minutes. At the end of that time they will have been cooked through, and will represent the condition similar to the soft-boiled egg mentioned above. This method has the advantage of cooking the yolk much more thoroughly than can be done in the three-minute process.

A third method used by some is to put the eggs into cold water, proportioned as above, a pint to an egg, and let the water come to a boil. As soon as the water begins to boil lift the eggs out of the water and throw a dash of cold water over them. This last method produces results very similar to the second method described. Some prefer one of these methods and some another, it really doesn't matter from a dietetic standpoint. Let one use the method which he likes best, feeling assured that that is the proper method. The only thing that does matter is this: that whenever the white of the egg is coagulated by the heat, it must be divided by. mastication into the minutest possible divisions. When so divided it is quickly digested and changed in the stomach to peptone, absorbed as such, changed in the epithelium to serum globulin and serum albumin, and carried to the tissues to be used as are the serum albumin and serum globulin derived from the digestion of meat proteins.

The importance of eggs as a source of iron has been mentioned above, and a special egg preparation, egg lemonade, has been mentioned. It is probable that no more easily digested food can be prepared than egg lemonade. While it is prepared in a special way when it is prescribed for the purpose of increasing the red coloring matter of the blood, if it is to be used for purposes of general nutrition the whole egg, yolk and white, is made into a glass of egg lemonade, through the addition of the juice of half a lemon, sugar to taste, water sufficient to fill the glass and iced. As one is preparing such a lemonade, he will notice that after the lemon juice has been added to the egg and beaten, the egg presently loses its ropiness and becomes limpid like milk, though yellow in color. This change in the consistency of the egg from viscid to limpid marks a change in the chemical composition of the egg. After it becomes limpid it is no longer unmodified albumin, globulin, and vitellin, but has, under the influence of the citric acid of the lemon, been changed to acid albuminate. This acid albuminate represents a mid stage of digestion of proteins. Of the digestive process it remains simply to change the acid albuminate to proteoses and peptone. This is quickly accomplished by the pepsin of the gastric juice. Invalids who can with difficulty retain solid foods, and digest them, find in egg lemonade a food not only most pleasing to the taste, but one which is readily retained and easily digested and assimilated.

Other preparations of raw egg may be valuable to serve as a variety in cases where it is necessary to give a considerable number of eggs for days or weeks in succession. While raw egg may be taken directly from the shell, and swallowed quickly without chewing, to many people this is a difficult process which may even lead to nausea. Among the vehicles that may be used to carry raw eggs, there may be enumerated, besides the lemon juice mentioned above, orange juice, grape juice, milk, coffee, cream, and cocoa. When used with any of these vehicles the eggs should, of course, be beaten, the flavor of the vehicle usually veiling the flavor of the egg, which to many people is not particularly pleasing. Eggnog was originally prepared from milk and eggs flavored with whisky or brandy and sweetened with sugar; however, the recipe now most used in hospitals omits the liquor and substitutes any strong flavor.