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.