MILK may be defined as the normal secre tion of the mammary glands. The milk of all mammals is similar in qualitative composition, consisting essentially bf water, fat, proteins, milk-sugar and salts or ash. Colostrum, the fluid secreted for a short time immediately after giving birth to the young, is composed of sim ilar substances, but differs considerably from normal milk in its quantitative composition and physiological properties. The average percen tage composition of some of the more import ant milks is approximately as follows: Human milk varies to such an extent that any attempt to state its average composition is liable to be misleading. The percentage of any constituent, and especially that of proteins, may differ widely from that given above while the milk is still entirely normal. Such differences are found not only in the milk of different women, but in that of the same woman at dif ferent periods of lactation.
Cows' milk differs from human milk in con taining less sugar and considerably more proteins and ash. The proteins are also of a somewhat different character, the casein being more easily coagulated and forming a denser curd. Goats' and ewes' milks' being rich in fats and proteins are well adapted to the manufac ture of cheese and are largely used for this pur pose in some parts of Europe. Asses' and mares' milks have been recommended as prefer able to cows' milk for infant feeding, since they show some resemblance to human milk in the amount and nature of the proteins which they contain. In this country, however, the milk of the cow is the only one of commercial importance. Unless otherwise explained all of the statements which follow will be understood to refer to cows' milk, but many of them are true of the milk of other mammals as well.
Cows' The constitution of cow's milk has been concisely stated by Richmond: "It is essentially an aqueous solution of milk sugar, albumin and certain salts, holding in suspension globules of fat and in a state of semi-solution, casein together with mineral matter. Small quantities of other substances are also found.' As regards its physical prop erties, milk is an opaque, white or yellowish fluid, somewhat heavier and more viscous than water, having a faint characteristic odor .and a
slightly sweetish taste. The yellowish color is due to the fat and the opacity and viscosity in part to the fat and in part to the casein and lime salts present. The specific gravity is usually between 1.029 and 1.034 at 15.5 C. (60° F.). Normal fresh milk shows toward litmus an amphoteric reaction, and reacts acid with phenol-phthaline. This property is at tributed to the presence of phosphates and of carbonic add. The gases contained in cows' milk, carbonic acid with small amounts of oxygen and nitrogen, are for the most part evaporated in the usual processes of handling the milk and therefore need not be further con sidered here.
Proportions of Water and The proportion of water in cows' milk varies con siderably, depending upon breed, individuality, period of lactation, etc. While the average amount is about 87 per cent, the mixed milk of a herd may easily show as much as or as little as 85 per cent of water corresponding re spectively to 12 or 15 per cent of solids. The milk of a single healthy cow in normal condi tion may sometimes contain as little as 10 or as much as 18 per cent of solids, while in extreme cases even these latter limits may be passed. The writer has found 19.88 per cent of solids in the milk of a perfectly healthy cow and 27.40 per cent in that of a cow having fever. Over 16 per cent of solids in the mixed milk of a herd or over 18 per cent in that of an in dividual is, however, comparatively rare.
While much depends upon the indi vidual cow, it is well known that some breeds tend to yield richer milk than others. The fol lowing figures, obtained by averaging the rec ords of tests made at the New York and New Jersey agricultural experiment stations, serve to illustrate the variation in richness of milk yielded by different breeds: servation being continued in each case for 10 months: In the last few days of lactation, when the yield becomes very small, the proportion of solids often rises to a marked degree, some times reaching 20 per cent or over.