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.
[Illustration: FIG. 1]
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.