The mineral matter of milk consists largely of potash and lime salts, and of these salts the phosphates are the most abundant. These are important, not only in the building of bone tissue, but also, as has been suggested before, in holding the casein in solution.
Water forms about 87 per cent of milk, and its chief use in this form is in holding- other materials in solution. To compare milk with other foods, we should properly think of the solid ingredients alone, since the water has no more food value than water in any other form.
Milk readily undergoes many changes, some of them harmless and some more or less harmful. The most common change is that of souring. Bacteria present in the milk act upon the sugar and change it into lactic acid. After a certain amount of this acid has been produced, the growth of the bacteria is stopped, and no further change in the sugar takes place, though undoubtedly certain other changes take place both in the fat and in the proteid.
There is no evidence that sour milk is unwholesome. The objection to it seems to be chiefly one of taste. Its use in cooking produces good results, and many prefer it for some purposes to sweet milk since it seems to produce a more tender product than does the sweet milk. On the other hand, milk may under the action of certain bacteria produce most harmful products, and poisoning from these ptomaines is not uncommon where milk has been handled in an uncleanly manner and has been poorly cared for. A more serious danger from milk is that owing to the excellent food it furnishes for almost all bacteria, it is frequently a carrier of disease. Disease germs that in water would not multiply and would probably live only for a short time, multiply abundantly in milk. It is because of the possibility of the presence of these harmful bacteria, rather than from any danger from sour milk, that we guard our milk supply carefully. Each hour that elapses between the milking of the cow and the use of the milk by the consumer, increases the number of bacteria present. One cubic centimeter of milk frequently contains from 400,000 to several million bacteria.
Efforts to guard the milk supply have been directed in two ways. The sterilization or pasteurization of all milk is often recommended; but a more satisfactory method would seem to be the insuring of cleanly conditions upon the dairy farm where the milk is produced. The next essential after cleanliness is that the milk should be cooled rapidly when first milked, since the lower temperature makes the fluid less favorable for the growth of germs.
In the household milk should be kept in perfectly clean vessels, and should be loosely, not tightly, covered, in order that there may be access of air to it, since the absence of fresh air favors the growth of certain putrefactive organisms. The entirely open vessel is only allowable in perfectly clean surroundings, not only free from dust, but with no strong flavoring substance near from which odors could be absorbed.
One form in which we often get milk is that of evaporated or condensed milk. This is simply milk from which most of the water has been removed, and which has been made sterile by heating to a high temperature. It has usually been sweetened, and the sugar acts as a preservative. While it is a convenient form for use when fresh milk is not obtainable, its large amount of sugar renders it somewhat undesirable as a common article of diet, and also makes it unfit for many cooking purposes.
There is being put upon the market now milk powder that seems to consist chiefly of the curd of the milk dried and ground. With the addition of water it forms a very fair substitute for milk.
Milk is perhaps more often adulterated than any other common article of diet. The most common form of adulteration is that of skimming or removing part of the cream. This can easily be detected, because it increases the specific gravity of the milk. To counterbalance this, water, which is slightly lighter than milk, is added in such proportion that the twice adulterated milk gives the same test as if it had not been tampered with at all.
Another adulteration that is sometimes practiced is that of adding coloring matter to the milk. This is usually done in order to conceal the blueness of the milk, when it has been watered.
Preservatives are frequently used. Of these boric acid is probably the least harmful, though some authorities contend that formaldehyde in the minute quantities in which it is used has no physiological effect. A milk that will stand in a warm place for some hours and show no tendency to sour is open to the suspicion of having been treated in some such way. Ordinary cooking soda is sometimes added to neutralize the acidity that may be present because of the age of the milk. Salicylic and benzoic acids are sometimes found, while formaldehyde is used most of all.