Solids Other Than Fat

These are as follows:

Letheby gives the percentage of lactose as 5.2, which is too high for the average.

Lactose

Lactose is an important ingredient of milk. When chemically pure it is hard, transparent, white, and crystalline. It is now obtainable in much purer commercial form than formerly, when in addition to its diuretic effect, if given therapeutically, it often excited fermentation and even glycosuria. It has but a faintly sweetish taste, and in its properties is allied to both sucrose and starch. Its chief function in human milk is to supply energy for heat production for the infant whose muscles are not yet active in developing this force, and it constitutes about one half of the total solids of milk, exclusive of fat; but the quantity present in cow's milk varies greatly, and, as a general rule, it bears an inverse ratio to the amount of fat and casein present. It is less liable than cane sugar to ferment in the stomach; and alone it is not susceptible to alcoholic fermentation, but in the presence of fermenting nitrogenous material it is converted into lactic acid, making the milk sour. It is promptly absorbed from the alimentary canal, not remaining over an hour in the stomach.

Casein And Albumin

The principal albuminoid of milk is casein, but there are a half dozen proteids which have been described, showing different behaviour on coagulation, in polariscopic rotation, etc. There is an albumin present called lactalbumin, which is coagulable by heat and forms the tenacious "scum" which floats on top of boiled milk. This albumin plays an important role by surrounding the minute oil globules of the milk and preventing them from agglutination - in other words, it helps maintain the fat in permanent fine emulsion. It is contained in solution in whey. Traces of peptones have also been found in milk.

Casein itself is non-coagulable by heat, even by boiling, but it is coagulated in firm tough clots by acids, such as the hydrochloric acid of the gastric juice, and by many organic acids which occur as products of malfermentation in the stomach. Lactic acid is the common agent in forming the coagulae. Casein is also coagulated, but less firmly, by the special milk-curdling ferment "rennin," which is actively present in the infant stomach, and in the third stomach of the calf. The casein clot formed by rennin, unlike that of lactic acid, is not redissolved by neutralisation with alkalies. Casein is present in milk chiefly in an alkaline form as potassium caseinate, and in conjunction with calcium phosphate.