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Milk, the well-known white fluid, the secretion of the mammary gland, obtained from the cow, though the milk of goats and asses has been used.  It is one of our most important foods, as it contains all the elements needed for the body and is easily digested.  When examined by a microscope, it is seen to consist of a large number of round substances or globules, which are transparent and very small and float in a colorless fluid. These globules are made of fat, with a thin coating somewhat like the white of an egg, called casein.  When the milk stands, the larger globules rise to the top and make the cream, which is the fattest or richest part of the milk.  When the cream is shaken, as in a churn, these globules break and the fat runs together, making lumps, which are called butter.  The casein which surrounds the globules of fat, and is also dissolved in the liquid, if it becomes sour, either naturally by exposure to the air or by the addition of rennet or an acid of any kind, collects into masses called curd.  This change is brought about partly by minute forms of animal life, called microbes, which get into the milk from the air.  This is one reason why it is so necessary to purify by heat, usually hot water, all the articles used about milk.  Sterilization and pasteurizing of milk are effective in removing or neutralizing the intrusion of bacteria.  The clots or curds, made by the addition of rennet to the milk, are pressed into blocks and make cheese, which is a very rich food, containing all the fat of the milk, as do cream and butter, and the casein, also, which is an albuminous substance.  Condensed milk is prepared by sweetening the milk and evaporating it, until it loses about half or three fourths of its bulk.  It is poured into tins while hot, and sealed.  When used, it is diluted with several times the quantity of water.  The adulteration of milk by adding water, starch or chalk is frequent in large cities, and has called for boards of inspectors and produced instruments, known as lactometers, for detecting it.  The most common fraud, however, in the sale of milk is removing the cream.  Supplying a large city with cream and milk creates a great business and employs many men: those who milk the cows at all hours; the railroad employees who run the great milk-trains; the large dealers who distribute it to the wagons; and the drivers of the milk-carts whose noisy clatter disturbs the early-morning nap, but whose faithful labors in heat and cold furnish our milk and cream for breakfast.  A quart of milk at eight cents is as nourishing as a pound of beefsteak at 18 cents; while a pound of American cheese, costing 20 cents, authorities affirm, contains almost as much nourishment as two pounds of the best beefsteak.