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members of the group of simple proteins, which experiment shows are invaluable for growth. After the clotting of milk-the coagulation of the caseinogen-these valuable simple proteins are found in the whey. Both caseinogen and the lactalbumen and lactglobulin rank high as food proteins of first-class biological value.
66. The fat, which is of importance both as an actual source of energy and of building material, is also the vehicle for vitamins A and D. The fat is present in the form of an extraordinarily fine emulsion, which probably makes it very susceptible to the attack of fat-splitting enzymes. Probably, too, the high content of unsaturated fatty acids makes it of particular value to the organism. As regards the content of vitamins, it has been shown that it is variable, being at its maximum in summer when the cows are at pasture.
67. The carbohydrate exists in the form of lactose, a sugar which is not very susceptible to the attack of yeasts. Possibly one of its main assets is the fact that, unlike dextrose and cane sugar, it is comparatively free from sweetness, so that in this way milk does not become nauseating or even distasteful. Although lactose is not readily fermentable by yeasts, it is very open to attack by certain micro-organisms, such as occur, for example, in the souring of milk.
68. The salt content of human milk is about three times less than that of cows' milk. The content of lime in the ash of milk is about the same in both milks, whereas cows' milk is richer in phosphorus. These various salts of milk yield a basic ash which, on consumption, may play an important rôle in the maintenance of the acid base balance in the tissue fluids.
69. When one speaks of milk as a food in general terms it is commonly assumed that the reference is to cows' milk. It must not, however, be overlooked that, good as a food though cows' milk may be, it is not the ideal nutriment for the human infant. It must be emphasised that the milk secreted by the female of any species is peculiarly fitted for the maintenance, and perhaps even more particularly the rate of growth, of the young of the same species. This special and unique correlation of the quality of the many different milks with the physiological factor of growth is most clearly brought out, emphasising at the same time the really fundamental importance of milk as a food, in the following table :-
Animal,
Human Horse
Cow Goat
Sheep
Pig Cat Dog Rabbit
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TABLE 4.
Composition of milk of various animals.
100 parts of milk contain.
Period in days required to
double birth
weight.
Protein.
Salts. Calcium.
Phosphoric
acid.
1.6
0.2
0.03
0.05
2.0
0.4
0-12
0.13
3.5
0.7
0.16
0.20
3.7
0-8
0.20
0.28
4.9
0-8
0.25
0.29
5-2
0.8
0.25
0-31
9.5
7-0
1.0
7.4
1-3
0-45
0.51
10.4
2.5
0.89
1.00
885241600
70. This table renders it very evident that there is a marvellous adaptation of the lacteal secretion to the needs of the developing organism, and, secondly, that the ideal food for the human infant is human milk, and, indeed, probably the milk of its own mother.
71. Although it is clear that the mother's milk is unquestionably the best for the infant, cows' milk in one form or another is the most commonly employed substitute when for any reason the mother's supply is deficient or absent. Provided it is pure, it is or can be rendered a good substitute.
In
(d) The effect of pasteurisation on the constituents of milk. 72. In what follows, the term pasteurisation applies almost exclusively to treatment by the holding process in which the milk is heated to a temperature of 145°-150° F. for half an hour. appraising evidence concerning the nutritional value of milk so pasteurised, it is necessary to remember that any faults which may be attributed to it are exaggerated when the processing is carelessly conducted. Observations made without due inquiry as to details in the treatment actually received by the milk employed may therefore tend to obscure the issue.
73. Of the chief organic constituents of milk, the fat and sugar suffer no appreciable change during the treatment, and though the fate of the proteins calls for some discussion, there is adequate evidence to show that their essential physiological value is unaffected. It may be said, indeed, without hesitation that there is no evidence to show that the process has any effect upon the value of milk as a source of energy to the body or as a supply of formative material for the living tissues in general. On the other hand, there is evidence for the occurrence of certain significant changes of
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