Woman's Institute Library of Cookery. Volume 2: Milk, Butter and Cheese; Eggs; Vegetables

by Woman's Institute of Domestic Arts and Sciences

NUTRITIVE VALUE OF EGGS

5. Like milk, eggs are often spoken of as a perfect food. Still, as has been pointed out, they are not a perfect food for man, but they are of especial nutritive value and should be used freely in the diet just as long as their cost neither limits nor prohibits their use. An idea of how they compare with other nutritious foods can be obtained from Fig. 1, which shows that eight eggs are equal in food value to 1 quart of milk or 1 pound and 5 ounces of beefsteak. A better understanding of their food value, however, can be gained from a study of their composition.

6. Since an egg is an undeveloped chick that requires only the addition of warmth to develop it into a living, moving creature made of muscles, bones, and blood, it is evident that this food contains considerable tissue-building and energy-producing material. The exact proportion of this material, as well as the other substances found in eggs, is given in the food chart shown in Essentials of Cookery, Part 1. The chart relating to the composition of eggs points out that the edible portion of the whole egg consists of 73.7 per cent. of water, 14.8 per cent. of protein, 10.5 per cent. of fat, and about 1 per cent. of ash, or mineral matter. The protein, which is chiefly in the form of albumen, and the fat are the most digestible of these elements, while the mineral constituents are as valuable for the growing child as for the chick. When the total weight of an egg is taken into consideration, the shell constitutes about 11 per cent., the yolk 32 per cent., and the white 57 per cent. The composition of the yolk and the white differs somewhat, the yolk having the greater food value, a fact that is also clearly indicated in the chart. The white contains a larger proportion of water than the yolk, but the yolk contains the most of the fat and more protein and mineral matter, or ash, than the white. In addition, the chart shows that the number of calories to the pound of whole egg is 700, of egg yolk is 1,608, and of egg white is 265.

7. PROTEIN IN EGGS.--The nature of the food substances in eggs is of nearly as great importance as their amount, for they not only determine the value of this food in the body, but influence its cooking. That protein is present in both the yolk and the white is apparent from the fact that they coagulate when heat is applied. Because eggs are high in protein, containing 14.8 per cent. of this substance, they may be regarded as equivalent to a meat dish, and it is only when they are extremely high in price that they cannot be frequently substituted for meat to advantage. They are often used to take the place of milk, too, for eggs and milk are more alike in nutritive value than any other two protein foods; but, of the two, milk yields the cheaper form of protein. Like meat and milk, eggs are rich in all those food materials which enter into the construction of bone, muscle, and blood.

8. FAT IN EGGS.--A study of the food chart previously mentioned will show that eggs contain proportionately almost as much fat as protein and that nearly all this fat is found in the yolk. Since fat produces more heat or energy, weight for weight, than any other food substance, and since eggs contain neither starch nor sugar, it is evident that the fat of this food is the main source of the energy-producing material. Fat in eggs occurs in the form of an emulsion, or tiny particles, and, like the fat of milk, is very readily digested. It is for this reason that both of these foods are particularly well adapted to the diet of both children and adults. The presence of quantities of protein and fat and the absence of carbohydrate in eggs indicate that the proper thing to combine with this food, in order to have a well-balanced meal when eggs are eaten, is carbohydrate in some form.

9. MINERALS IN EGGS.--Eggs are especially valuable for the mineral salts they contain, chief among which are lime, phosphorus, sulphur, iron, potassium, and sodium. For this reason, the addition of eggs to any kind of diet supplies a large amount of the minerals that are needed for bone, blood, and tissue building. A favorable point concerning the minerals found in eggs is that they are not affected to any extent by cooking. Therefore, in the preparation of any dish, if eggs are added to other foods, that dish will contain an additional amount of mineral salts, plus the nutritive value of the eggs.

10. DIGESTIBILITY OF EGGS.--In connection with the discussion of the food substances of which eggs are composed, it will be well to note how these affect the digestibility of this food. But just what is meant by this characteristic with reference to eggs must first be understood. In some foods, digestibility may mean the length of time required for them to digest; in others, the completeness of the digestion; and in still others, the ease and comfort with which the process of digestion proceeds. In the case of eggs, digestibility refers to the quantity of this food that is absorbed, that is, actually dissolved and permitted to enter the blood stream. The nutritive value of eggs is not so high as would naturally be supposed, for, although the protein, fat, and mineral salts of an egg make up about one-fourth of its contents, one egg equals in nutritive value only 1/2 cupful of milk, a small potato, or a medium-sized apple. However, when the proportion of the nutritive material that the body retains from this food, or its digestibility, is considered, eggs rank extremely high, it having been determined by experiments that 97 per cent. of the protein and 95 per cent. of the fat are assimilated. A point worthy of note in this connection, though, is that eggs contain no cellulose, such as that found in grains, vegetables, and fruits. Therefore, in order to add the much-needed bulk to the diet, foods that do contain cellulose should be served with eggs.

11. Whether or not the cooking of eggs has any effect on their digestibility is a matter that has also been investigated. The results of the experiments made indicate that cooking makes some difference with the rate of digestion, but very little with its thoroughness. So far as the rapidity of digestion is concerned, there is very little difference between raw eggs and slightly cooked eggs; but hard-cooked eggs, although they may be digested as completely as soft-cooked ones, require longer time for the accomplishment of the process. This is due to the fact that the whites of hard-cooked eggs are so firm in texture that, unless they are finely chopped or thoroughly masticated, the digestive juices are not able to act on them quickly. As a result, portions of them may escape digestion or remain in the digestive tract for some time and decompose. For this reason, hard-cooked eggs are usually excluded from the diet of children and invalids, and even healthy adults should be careful to masticate them thoroughly.