Milk is universally used as an article of food by civilized and by many uncivilized peoples. It is one of the cheapest, most easily digested, and perfect foods that we use. Mother's milk is the natural aliment for infants. Composition

Milk contains more than 85 per cent, of water and variable proportions of protein, fat, carbohydrate, and mineral salts. It contains two proteins. Fibrinogen, or casein, is the most important because it constitutes six-sevenths of all the protein in cow's milk. Lactalbumin, the second protein of milk, is similar to serum albumin of blood. The fat of milk consists chiefly of stearin, palmitin, and olein. The proportion of fat varies much in different kinds of milk. The carbohydrate of of milk is lactose, or milk-sugar. It occurs in an almost unvarying amount in each variety of milk. The other constituents are subject to variation. The mineral salts in milk are numerous, but slight in quantity. Their relative percentages in the ash of human milk are as follows:

The following table gives the comparative composition of milk of different kinds:

Comparative Composition Of Various Kinds Op Milk

Human milk and cow's milk are not very dissimilar in the amount of energy that they might generate, but their food value, as estimated by the ratio of their protein to the calories that they might produce, is very different. Human milk is poor in protein and rich in carbohydrate.

The average composition of human milk is:

It is slightly alkaline, and has a specific gravity of from 1028 to 1034. The following analyses, made by Harrington for Rotch, illustrate the variability of human milk:

The fats and proteins may vary very much more than these analyses would lead one to think. The following analyses show how greatly a mother's milk may vary from time to time:

Cow's milk varies almost or quite as much as human milk. The most constant ingredients in it are milk-sugar and mineral matter. The following is an average analysis of cow's milk:

It is slightly acid in reaction and has a specific gravity of 1029 to 1033. The milk of a cow may vary in composition at different times. It is usually richer in cream for a time after her calf is born than later. Food, excitement, and illness will also cause it to vary. So long as the animal is upon a uniform diet and is healthy, her milk will not vary noticeably in composition or amount. Different breeds give milk of varying degrees of richness:

Variations In Milk

Good cow's milk should contain from 3 to 5 per cent, of fat.

Variation

The proportions of the constituents of milk vary from the filling of the cow's udder after calving to the time when it ceases to be secreted. After the first week it is usually the richest and remains about the same for months, providing the animal's diet is uniform. Ultimately the milk becomes less abundant, but the percentage of solids increases.

The flavor of milk is frequently modified by diet; certain plants may make it disagreeable.

Milk is more nearly uniform when it is obtained habitually from a herd of cows; that procured from one animal varies much from time to time.

Adulteration

As it is purchased from dealers, milk is at times modified in composition or adulterated. The addition of water is the commonest mode of adulteration. If the water is pure, its addition is harmless, except as it lessens the nutritive value of a given quantity of the milk. Coloring-matter is added to make milk look richer. Milk is sometimes slightly thickened by the addition of starch. Preservatives, such as salicylic and boric acids and formaldehyd, are mixed or dissolved in it. They prevent its souring rapidly. Although these agents are used in very small percentages, and, if taken occasionally, would probably be harmless, they must be regarded as deleterious when administered habitually, as they may be in milk.