Chemical Composition

Since the price of eggs is determined entirely by considerations other than chemical composition, and eggs are never produced primarily for industrial uses in which the components are separated from each other, there has been no economic reason for the study of the causes and extent of variations in composition, and our information on these points is very meager as compared, for example, with the corresponding data for milk. Differences in composition seem usually due to different proportions of white and yolk. According to Langworthy the proportion of yolk (and therefore of fat) is greatest in the eggs of those breeds which are best adapted to fattening. Other things being alike the edible portion of white-shelled and dark-shelled eggs shows essentially the same composition and nutritive value.

The average composition of eggs of different kinds, as given by Langworthy, is shown in Table 12, the fuel values being recalculated by the use of the now accepted factors.

The figures given for hens' eggs in this table are the average of 60 American analyses compiled by Atwater and Bryant,' in which the protein varied from 11.6 to 16.0 per cent and the fat from 8.6 to 15.1 per cent. The estimated averages of European writers fall well within these limits, but are apt to be somewhat higher in fat than the American average as given above. Thus the estimate of König, which is widely quoted, allows 12.55 per cent protein and 12.11 per cent fat.

Speaking in round numbers, we may say that the edible portion of the egg contains 72 to 75 per cent of water, about 1 per cent of ash, 12 to 14 per cent protein, 10 to 12 per cent fat; or about three fourths water, one eighth protein, and one eighth fat. Of the edible portion the yolk constitutes (by weight) a little over one third and the white a little under two thirds; and these are of very different composition. The white is about

1 United States Department of Agriculture, Office of Experiment Stations, Bulletin 28.

seven eighths water and one eighth protein (chiefly albumin) with a small amount of ash, consisting mainly of common salt with smaller amounts of potassium salts. The yolk is about one half water, one third fat, and one sixth protein with more ash than the white, including relatively large amounts of phosphorus, calcium, and iron in organic combination. Thus the yolk is a much more concentrated food material than the white, containing in a given weight about seven times as much energy, as well as larger amounts of protein and of the chief ash constituents. The vitamins of the egg are also contained chiefly in the yolk.

The nature of the nutrients in eggs is of almost as much interest and importance as their amount. The fact that when an egg is kept at a proper temperature for about three weeks without the addition of anything from without, it produces a chick so well developed as to begin at once to walk and to eat the same food as the adult, suggests that the egg must contain substances which are very efficient as sources both of the energy and the materials for growth and development.

The fat of egg is practically all in the yolk, and like milk fat it exists in a finely emulsified condition, so that it is capable of digestion in the stomach, as well as in the intestine. Volhard has reported an experiment in which 78 per cent of the fat of egg yolk was digested in the stomach. A large proportion of the egg fat, probably at least one fourth, consists of phosphorized fats (lecithins together with closely related substances, such as kephalins). Egg lecithin is usually taken as typical of the "phosphorized fats," "phospholipines," or "phosphatids." It has the chemical structure of a fat in which one of the fatty acid radicles is replaced by a radicle of phosphoric acid in combination with a nitrogenous organic base (choline). The typical lecithin molecule thus contains one atom each of phosphorus and nitrogen; that described by Hoppe-Seyler had the composition C44H90NPO9.

Recent investigations, especially those of McCollum and his associates at the Wisconsin Agricultural Experiment Station, appear to demonstrate that the nature of the fatty acids in both the ordinary fat and the phosphorized fat of the egg is influenced by the food of the hen. This is consistent with earlier observations relating to the influence of the food upon butter fat and upon the fat of the adipose tissues.

Dissolved in the fat of the egg yolk is a yellow coloring matter to which the name "lutein " has been given. Recently Palmer has shown this to be the same substance as the yellow plant pigment called carotin.

The proteins of egg are of much interest, and those of the yolk and of the white are quite different in their properties. The fact that egg white contains so little of any other substances than proteins and water makes it easy to observe the behavior of the proteins. Egg white is therefore largely used as a material with which to demonstrate the properties of proteins - particularly of the albumins, since ovalbumin is the chief protein of the egg white. According to Osborne and Campbell1 egg white also contains small quantities of three other proteins called conalbumin, ovomucin, and ovomucoid. The chemical constitution of these minor proteins has not been studied. Ovalbumin has been purified in quantity by Osborne, Jones, and Leavenworth, and studied with reference to the amino acids yielded on hydrolysis; the results, together with those for ovovitellin, are shown below.

Ovovitellin is the chief protein of the egg yolk. It is believed to exist largely in chemical combination with lecithin. When freed from lecithin, it has nearly the composition of casein, as shown by the following analyses, due chiefly to the work of Osborne.

1 See references at the end of the chapter.

Those who desire a fuller account of the chemistry of the egg proteins should consult the original papers of Osborne and Campbell. (See references at end of chapter.)

Casein and ovovitellin are regarded as the two typical phosphoproteins.

The percentages of the various amino acids obtained on hydrolysis of ovalbumin and ovovitellin, by Osborne and his associates, were as given below (Table 13).

A comparison with the corresponding data given in Chapter III (Table 5) shows that ovovitellin resembles casein in the amino acid radicles which it contains as well as in its elementary composition.

TABLE 13. PERCENTAGES OF AMINO ACIDS FROM EGG PROTEINS