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equilibrium among those mineral constituents which are necessary for the normal growth of bone, and it is sure that certain vitamins, present in characteristically small amounts, but serving functions essential to normal nutrition, are destroyed to an extent which varies from case to case and depends to no small degree on the precise conditions of treatment. The practical significance of alterations such as these calls for discussion.
74. A few words, however, may be first devoted to the question of the proteins. Some 5 per cent. of the lactalbumin may be coagulated during the holding process, but this represents a very small proportion of the total protein present, and is in any case not necessarily lost from the milk as consumed. While the essential nutritional value of the proteins, when digested, is unimpaired, the question whether they remain equally digestible after pasteurisation has been much debated and has led to many experimental inquiries. Experiments in vitro have suggested that the proteins of raw milk are somewhat more open to the influence of the digestive ferments than they are after pasteurisation, but the results described have varied with the conditions of experiment, and in no case have the conditions been such as occur in vivo. Normal digestion of the proteins does not begin while they are in the colloid form displayed in milk. When the fluid, raw or pasteurised, enters the stomach the casein is coagulated and rendered insoluble, and on this solid form the digestive ferments act with special readiness. In the adult, such coagulation is due to the acidity of the gastric juice, in the infant, to the action of the ferment rennin. The case of the infant is the more important, and the effect of pasteurisation on coagulation by rennin has been frequently investigated (Lane-Claypon, 1916). Laboratory experiments have shown that it may be somewhat slowed, but the circumstance is of little significance. The stages of the normal gastric digestion of milk proteins are not easy to reproduce outside the body. For a final decision as to whether pasteurisation has any significant effect on their digestibility, reliance must be placed rather on experiments made with living animals, or, better, with infants. In such experiments the percentage nitrogen loss through the bowel, due to incomplete digestion, is compared when milk, raw or pasteurised respectively, is fed. A survey of the data published during a number of years leaves the complete conviction that any effect of pasteurisation on the digestibility of the milk proteins may be dismissed as wholly unimportant for practical issues.
75. The same cannot be said of the effect on the availability of those important mineral constituents, calcium and phosphorus, on which the growth of the skeleton makes a continuous call. It has long been known that when milk is heated a proportion of the lime and phosphoric acid it contains are precipitated together in an insoluble form. Once again, however, direct experiment must decide whether this change is such as to affect appreciably the nutritional value of the milk, or, incidentally, whether any other