6. As milk is usually taken into the body in liquid form, the common
tendency is to regard it as a beverage, rather than as an important
source of nourishing food material. However, a knowledge of its
composition, as well as the fact that milk becomes a solid food in the
stomach and must then be dissolved in the process of digestion, will
serve to show that milk contains solids. That it possesses all the
elements required to sustain life and promote health is proved by the
fact that a child may live for months on milk alone and during this time
increase in weight.
7. The solids contained in milk are proteins, fat, carbohydrate in the
form of sugar, and mineral salts, besides which, of course, water occurs
in large quantities. The sugar and fat of milk serve as fuel; the
mineral salts are chiefly valuable for the growth of bones and teeth and
for their effect on the liquids of the body; and the proteins, like the
fat and sugar, serve as fuel, but they also make and repair the muscular
tissues of the body.
In considering the food substances of milk, it will be well to note also
that they vary according to the breed, feeding, and individual
characteristics of the cow. Jerseys and Guernseys give milk rich in fat
and total solids, and while Holstein cows give a greater quantity of
milk, such milk has a smaller proportion of fat and total solids. As a
rule, though, the composition of milk may be considered as approximately
3.3 per cent. protein, 4 per cent. fat, 5 per cent. carbohydrate, and
.7 per cent. mineral matter, making a total of 13 per cent. This
indicates the quantity of actual food material in milk, the remainder,
or 87 per cent., being water.
8. PROTEIN IN MILK.--Because of the double usefulness of protein--to
serve as fuel and to make and repair muscular tissue--this element is
regarded as an important ingredient of milk. The protein in milk is
called _casein_. The opaque whiteness of milk is largely due to the
presence of this substance. As long as milk remains sweet, the lime
salts it contains hold this casein in solution; but when it sours, the
salts are made soluble and the casein thickens, or coagulates. In
addition to casein, milk contains a small amount of protein in the form
of _albumin_. This substance, upon being heated, coagulates and causes
the formation of the skin that is always found on the top of milk that
has been heated. The skin thus formed contains everything that is found
in milk, because, as it forms, casein is dried with it and sugar and
fat, too, are caught and held there. It is the protein of milk and its
characteristic coagulation that are made use of in the making of cheese.
In cooking, the protein of milk is probably more affected than any of
the other substances, but the degree to which the digestion of milk is
thus affected is not definitely known, this being a much
disputed question.
9. FAT IN MILK.--The other substance in milk that serves as fuel, or to
produce energy, is fat. It occurs in the form of tiny particles, each
surrounded by a thin covering and suspended in the liquid. Such a
mixture, which is called an _emulsion_, is the most easily digested form
in which fat is found. The fat in milk varies more than the other food
substances, it being sometimes as low as 2 per cent, and again as high
as 6 per cent. However, the average of these two, or 4 per cent., is the
usual amount found in most milk.
As has been mentioned, the fat globules of milk rise to the top because
fat is lighter than water, so that when milk has been undisturbed for
some time the top, which is known as _cream_, will be found to contain
most of the fat. Because of the fat it contains, the cream is yellower
in color than the milk underneath. If the cream is beaten, or churned,
these fat particles will adhere in a mass, advantage of this fact being
taken in the making of butter.
10. CARBOHYDRATE IN MILK.--The carbohydrate contained in milk is in the
form of sugar called _lactose_. It is unlike other sugars in that it is
not very sweet and does not disagree with most persons nor upset their
digestion. For this reason, it is often given to children, invalids, and
persons who have digestive disturbances. However, it is like other
carbohydrates in that in solution it ferments. The result of the
fermentation in this case is the production of _lactic acid_, which
makes the milk sour. With the fat, lactose makes up the bulk of the
energy-producing material of milk, and while this is important it is
only secondary when compared to the tissue-building power of the protein
and minerals. Besides being an important part of milk itself, lactose is
a valuable by-product in the manufacture of cheese. After being taken
from _whey_, which is the clear, straw-colored liquid that remains when
the curd, or coagulated portion, is completely removed from the milk,
the lactose is refined and sold in the form of a powder that is used for
various kinds of infant and invalid feeding.
11. MINERAL MATTER IN MILK.--Considerable quantities of mineral salts,
which are chiefly _lime_, _potash_, and _phosphates_, are found in milk.
As has already been pointed out, these are important in the building of
bone and hard tissue in the body, but in addition they help to keep the
fluids of the body in the right condition. Because of the work they do,
these mineral salts are necessary in the building of the bodies of
growing children, and are useful for repair and the regulation of the
body processes in adults. In cheese, butter, and cream, which are the
products of milk, less of the mineral salts are found in proportion to
the quantity than in whole milk, skim milk, and whey.
12. WATER IN MILK.--The percentage of water in milk is much greater than
that of all the other food substances combined, there being more than
six times as much. While this quantity seems very large, it is an
advantage, for milk provides nourishment to persons when they can take
neither solid nor more condensed food. On the other hand, the water is a
disadvantage, for it is responsible for the rapid spoiling of milk. This
fact is clearly shown in the case of condensed milk, where the water is
partly or completely evaporated, for milk of this kind keeps much longer
without spoiling than either whole or skim milk.