The proteins of milk, which form the curd or larger part of the solids, according to Van Slyke[29] are in the form of casein and albumen. There are 3.6 parts casein to 1 part soluble proteins, but these figures vary somewhat at times. Casein is insoluble in pure water, but dissolves readily in water to which an alkali or calcium carbonate is added. The soluble protein in the form of lactalbumen is one of the constituents of whey. This substance contains more sulphur than does casein, but no phosphorus.
Whey is the opalescent fluid which remains when the casein is precipitated, and is composed of water 93.8%, total ash 0.44% (König).
Mineral salts, 0.7% of milk, are made up of calcium, potassium, sodium, magnesium, iron, sulphur, phosphorus, and chlorine. Milk is so rich in calcium that it requires only 400 c.c. (or about 2½ cups) to furnish 1 gram of calcium. This is the amount believed to be necessary for the welfare of man each day and this must be derived from food.[30]
Water.—The fluid part of milk is composed chiefly of water, constituting 87% of whole milk.
Milk as a food for infants will be discussed in another chapter.
As has already been said, no food has so far been discovered which could be effectually substituted for milk. There is no food, however, which requires more attention in its selection and care. It is very susceptible to both odors and flavors, absorbing them both readily, as will be found if milk be placed in the same compartment with foods of strong odor and flavor, without being properly covered and protected. This is particularly noticeable with cucumbers, melons, etc.
Milk also furnishes a splendid medium for bacterial growth, and if left exposed to the air, put into unclean receptacles, or kept in a warm place, will immediately become more or less contaminated, after which it is unwise to use it. Sterilization and pasteurization will in a measure overcome the bacterial contamination, but milk purchased from a dairy which is not clean or milked under unsanitary conditions will remain dirty, hence unfit for human consumption. When the morning’s milk supply is brought to the house it should be in clean, well-stoppered bottles, but before placing it in the ice-box the tops of the bottles should be carefully wiped off with a wet cloth to remove any superficial dust which may be adhering to them. Every time a portion of the milk is removed thereafter the tops should be again cleansed before the milk is poured out. This is a wise precaution, and often prevents contamination from the hands, etc.
The amount of water in milk prevents its being an adequate food for adults except in certain pathological conditions. However, it furnishes a supplementary food unequaled by any other beverage known. There are fortunately only a few individuals who are unable to drink milk. There are many who fancy they cannot do so, but if the nurse has the ingenuity to utilize some of the various methods whereby milk is made more digestible, it will generally be found that the patient can take it without trouble. In cases of personal dislike, if the milk is flavored or colored or made up into soup, cocoa, chocolate, junket, custards, blanc-mange, etc., it will usually prove acceptable.
Application of Heat.—A word as to the changes which are brought about as the result of heat as applied to milk. These changes are demonstrated in the two methods commonly used in the preparation of milk known as “pasteurization” and “sterilization.” Pasteurization is rather an indefinite term to use, unless the time and the temperature to which the milk is subjected are given. According to Morse and Talbot “the term sterilization should never be applied to the processes used in the preparation of milk for the feeding of infants, because the milk is never rendered bacteriologically sterile by them.”[31]
As a rule the flavor and odor of milk are not changed by heat until the temperature reaches nearly to the boiling point. A scum then forms on boiling milk, composed of casein 50.86%, fatty matter 45.42%, ash 4.72% (Rosenau). Prolonged boiling changes the color of milk from a creamy white to a yellowish brown which deepens with boiling. This is due to the caramelization of the milk sugar. Cream will not rise (or its rise will be very slow) on milk which has been subjected to a temperature of 150° F. for thirty minutes or more because the fat droplets are broken down so that they cannot hold together at that temperature and become more completely distributed throughout the fluid.[32]