COMPOSITION OF MILK

Milk as a Food

Milk contains all the elements necessary in a balanced diet for the adult and furnishes an almost perfect food for the suckling. Milk is lacking in iron and roughage and is therefore not suitable for the sole diet of adults. This is compensated for in the infant by a large amount of iron which is contained in the spleen and furnishes this necessary element during the suckling period. There is no other single food that will so well promote the growth and development in young children. Milk is easily digested, is palatable, and forms one of our best and most important articles of food. It is asserted that the consumption of milk in the United States will average about 0.6 of a pint per capita per day. In many countries there is practically no milk used. In the United States about 16% of the dietary consists of milk and its products.

Milk is an animal secretion produced by the mammary gland and is exceedingly complex in its composition. It consists chiefly of water containing various solids in solution. Cow’s milk consists of 87% water and 13% solids. The solids consist of fats in emulsion, milk sugar, albumin, casein and mineral matter. Milk also contains such gases as oxygen, nitrogen and carbon dioxide. It contains enzymes, phosphatids and vitamins, also antibodies and other substances.

Fresh normal milk is an opaque fluid of white or yellowish-white color and has a sweetish taste and rather pleasant odor. In reaction milk is amphoteric, that is, it is acid to litmus and alkaline to turmeric. The specific gravity of cow’s milk is from 1.027 to 1.035. Under the microscope it is found to contain fat globules and cells, also bacteria and other objects. The oxygen and nitrogen that are found in milk are thought to be carried into it mechanically from the air during the process of milking. Lecithin, cholestrin, citric acid, lactosin, orotic acid and ammonia are found in milk in small quantities.

Proteins

Casein, lactalbumin, and lactoglobulin are the three proteins found in milk and are usually constant in a given species.

Casein is found nowhere in nature except in the secretion of the mammary glands. It is highly specialized and, as a nucleoalbumin, contains a certain amount of phosphorus. Lactalbumin is a protein found in quantities varying from 0.2% to 0.8%. It is similar to the serum albumin in blood and coagulates at 70° C. There is a mere trace of lactoglobulin in milk. It is insoluble in water.

Fat

Milk contains fat in emulsion; that is, the fat is suspended in the serum of milk in the form of emulsion. The fat is lighter than the milk serum and therefore rises to the top in the form of cream, or the fat may be separated from the milk by centrifugal force. Cream does not consist in fat globules alone but contains the same constituents as the milk, only it is very much richer in fat. By agitating the cream, as in a churn, or by means of shaking, the fat globules will coalesce and form into lumps of butter. It is stated by some authors that cream contains a larger number of bacteria than skim milk. The fore milk, or that which is first milked from the udder, has a small amount of fat; the last that is taken, which is known as the strippings, may contain as much as 9% or 10% of fat. An increase in the temperature of the milk will retard the rising of the fat and if the temperature is kept above 65° C. for any length of time, cream will not form on top. Therefore, it follows that a moderately low temperature will increase the rapidity with which fat rises to the top of the milk.

Parkes says, “Milk should not have less than 12.5% of total solids of which 3.5% is fat and 0.7% is salts; ... the percentage of cream by volume not less than 10%.”

Infants placed on a diet of milk that is too rich in fats will thrive for a while, but in time will lose weight and show other symptoms, indicating the inability of Innate to digest the food. The stools become hard and dry, of a pale color and composed largely of fat soaps. This shows that Innate is unable to furnish sufficient alkaline bases through the body to saponify the excessive amount of fat in the intestines. This results in a condition resembling acidosis.

Vitamins

Milk is found to contain all three vitamins, A, B, and C, known as the fat-soluble A, water-soluble B, and water-soluble C. Cream is rich in vitamin A. This vitamin increases growth and promotes nutrition and is therefore valuable in the diet of the young. Vitamin A and B are not destroyed by heat, unless it be a very great degree for a long period of time; they also resist drying and age. For this latter reason milk may be used in cooking and still retain the virtues of these two vitamins, as well as do evaporated milk and dried milk. Vitamin C is impaired and may be completely destroyed by high temperature and deteriorates with age and with drying. There is no article of food (diet) that is superior to milk in dietetic and nutritional value, and especially is this true of butter fat. The standard by which milk is tested is the amount of butter fat it contains.

Lactose or Milk Sugar

Milk contains what is known as milk sugar or lactose. It is white crystalline sugar, has a slightly sweet taste and is soluble in water. When acted upon by microörganisms it is changed into glucose, and the glucose is then reduced to lactic acid, the lactic acid being the cause of the milk souring. Milk sugar is found nowhere else in Nature.