Milk is coagulated or curdled by many fruit and vegetable acids, as the housewife well knows, using milk in pies containing certain acid fruits, such as lemons, or in soup containing tomatoes. The hydrochloric acid of the stomach at once causes a similar coagulation, though the curds are tougher and more leathery. The milk forms into curds immediately upon entering the stomach. This is the natural process of milk digestion and is the chief reason why it should be drunk slowly, otherwise the curds will form in too large sizes, thus pressing upon the entrance to the stomach and causing distress. The tough, large curds formed by the hydrochloric acid, are difficult for invalids or for very delicate stomachs to digest.
If an alkali, such as limewater, is added, to neutralize the acids of the stomach, the curds do not form, or are re-dissolved, and digestion is aided. One sixth limewater to five-sixths milk is the proper proportion.
Milk Tests. In testing the value of milk, or the value of a cow, butter makers and farmers gauge it by the amount of butter fat in the milk, while the cheese maker tests the milk for the proportion of protein (casein). The amount of butter fat depends upon the feed and water, and upon the breed. The milk from Jersey and Guernsey cows yields about five per cent butter fat. If the total nutrient elements fall below twelve per cent, it is safe to assume that the milk has been watered.
In cheese and butter there is no sugar; it remains in the buttermilk and the whey, both of which the farmer takes home from the factories to fatten his hogs.
Preserving Milk. Many forms of bacteria thrive in milk and it is needless to say that the utmost cleanliness should be observed on the part of the dairyman in the care and cleanliness of his cows, in the cleanliness of the milk receptacles, and in the place in which the milk is allowed to stand over night. Care and cleanliness in the home is quite as important.
If milk could be kept free from bacteria, it would keep sweet almost indefinitely. At the Paris Exposition, milk from several American dairies was kept sweet for two weeks, without any preservative, except cleanliness and a temperature of about forty degrees. The United States Bureau of Animal Industry states that milk may be kept sweet for seven weeks without the use of chemicals.
The best method for the housewife to follow is to keep the milk clean, cool, and away from other foods.
Pasteurized Milk. In pasteurizing milk the aim is to destroy as many of the bacteria as possible without causing any chemical changes or without changing the flavor. One can pasteurize milk at home by placing it in an air tight bottle, immersing the bottle to the neck in hot water, heating the water to one hundred and forty-nine degrees F for a half hour and then quickly cooling the milk to fifty degrees, by immersing the bottle in cold water. The rapid cooling lessens the cooked taste. Many of the best dairies pasteurize the milk in this way before it is marketed.
Sterilized Milk. Milk is sterilized to destroy all bacteria, by boiling it. It must sometimes be boiled one, two or three successive days. Sterilized milk remains sweet longer than pasteurized milk, but more chemical changes are produced and the flavor is changed.
Formerly borax, boric acid, salicylic acid, formalin and salt petre were used to keep the milk sweet, but this adulteration is now forbidden by the pure food laws.