While pasteurization serves to destroy the most harmful of the disease bacteria as well as the lactic acid bacilli which cause souring, certain bacterial forms are capable of surviving this process, notably the bacteria which bring about putrefactive decomposition of milk. Pasteurized milk may therefore deteriorate on keeping quite as seriously as raw milk; in fact it may be more actively injurious although the change is not made evident by the development of a sour taste.

Sterilization of milk results in the destruction of all the bacteria present, and is carried out by heating to a temperature of 220° to 230° F. under pressure in an autoclave. The term is often, although incorrectly, applied to boiled milk. Boiling does not ensure complete sterilisation, unless prolonged for 30 minutes or more, but milk which has been boiled for a much shorter time may be regarded as quite safe for any ordinary purpose, provided it is protected from contamination and used within a reasonable time after treatment.

The early experiments with animals of various species fed upon heated cow's milk are summarized by Lane-Claypon, but offer little of interest, as in most cases the diet was entirely unsuitable apart from the possible deterioration on heating.

Experiments with calves fed on boiled and sterilized milk have given somewhat contradictory results. Certain American investigators71 have reported unfavorably on its use, while Dean72 recommended it. Danish experience indicates that milk from tuberculous cows can be used successfully for feeding calves.73

There is a firmly established tradition in many households that heated and especially boiled milk is highly injurious to children and more or less deleterious for adults as well, the reason usually given being that "it tends to induce constipation." Several eminent children's specialists have strenuously opposed pasteurisation of milk for babies on the ground that the heating injures its nutritive properties. On the other hand, there is much clinical evidence that boiled milk is if anything superior to raw milk." In European countries where wet nurses are rather extensively used it is quite common to have the milk of these nurses boiled in order to lessen the risk of transmission of disease, and there is no evidence that the value of the milk is impaired in any way by this procedure. Reports from children's hospitals go to show that there is comparatively little difference as far as nutritive power goes, between raw and boiled milk, and advantage being on the side of the boiled milk if anything.75 There appears to be absolutely no basis for the belief that boiled milk is more constipating or less digestible than raw milk.

While occasional cases are reported in which sick children have apparently done better on raw than on boiled milk these are counterbalanced by at least an equal number of cases in which boiled milk has proved more efficient than raw. In a recent series of experiments .with guinea pigs, boiled milk was found to be practically equivalent in nutritive value to unbeated milk.76 Park and Holt77 made a comparative study for two successive summers on groups of infants from New York tenements fed upon raw and pasteurised milk. The milk used was of good quality and modified similarly for all the children. Seventy-five and six-tenths per cent of the children in the group fed on pasteurised milk remained in good health during the entire summer, while only 33.3 per cent of those on raw milk were equally healthy. Thirteen out of the 51 children in the latter group had to be transferred to pasteurised milk during the experiment on account of severe attacks of diarrhoea.

71 Doan and Price, Maryland Agric. Exp. Sta. Bull. 77, 1901; Price, N. Y. Med. J. 79, 406, 1904.

72 Dean, Rep. Ont. Dept. Agrio. 1896, i, 06. 73 Bang, quoted by Lane-Claypon, 1. e.

A form of sterilised milk containing vitamins is prepared by Hort.77a The milk is sterilised in an autoclave, so arranged that the steam from the outer vessel passes through the milk in the inner vessel before escaping. The autoclave is maintained at 20 pounds per square inch steam pressure (about 116o C.) and passage of steam from) 3 to 5 minutes is sufficient to sterilise the milk. Taste, odor, vitamin-oontent, and emulsified condition of the fats are stated to be unaltered. Sterilised sugar may be added, to make good any loss brought about by heating. Cream is prepared from unsterilised milk and then sterilised; butter is made from sterilised milk, and condensed milk is sterilised after condensation.

74 See Lane-daypon, Oleomargarine, Ch. 9-11.

75 Finkelsteln, Therap. Monats Oct. 1907; Lene-Claypon, Local Govt. Bd. Rep. No. 63,1912.

76 Anderson, Dutcher, Ecklet, and Wilbur, Sci. 63, 446, 1921. 77 Park and Holt, Arch. Ped. 1908. 77a Hort, British Patent 166,964, 1920.

With regard to the separate constituents of milk, there is considerable evidence to show that the digestibility of caseinogen is increased by heating. Brennemann 78 was able to demonstrate that the curd formed from raw milk in the human stomach is very tough and large, while that formed from heated milk is soft and much finer in character.

McCollum and Davis79 found that while milk powder in quantities as small as two per cent of the food mixtures consisting otherwise of polished rice, purified casein, salts, and butter-fat, gave enough B for growth, milk powder which had been heated four hours in a double boiler was noticeably less efficient than the unheated product, and after heating for one hour in the autoclave it lost its growth-promoting power almost entirely. Since wheat embryo can be heated to this temperature and for this length of time without lowering its efficiency, McCollum concluded that the loss of efficiency of milk was due to the effect of heat on some factor other than B. He therefore made a series of feeding experiments in which a ration deficient only in protein and B was supplemented by various preparations from milk which had been exposed to high temperatures; whey, evaporated to small volume at or near boiling temperature and then heated in an autoclave at 15 pounds for an hour was apparently not affected by the heating and still contained sufficient B to promote good growth, as did also " protein-free milk " which had been boiled for six hours, and lactose which was heated at 15 pounds for an hour.

On the other hand, moist casein which had been heated for an hour at 15 pounds was much less satisfactory for growth than an equal quantity of unheated casein. McCollum and Davis suggest that this may be associated with the loss of sulphur by casein on heating, as demonstrated by Rettger80 and Morner.81 In view of this apparent instability of casein when exposed to heat McCollum and Davis 82 questioned the desirability of purifying casein for nutrition experiments by extraction with hot alcohol, and devised an alternative method which did not involve heating. Funk and Macallum83 could find no difference in the nutritive qualities of casein as prepared by the method of Mc-Collum and Davis or by the usual method of extraction. They confirmed the statement of McCollum and Davis as to the loss of efficiency of casein on heating in the autoclave for an hour, but found that the fault could be corrected by addition of 1 cc. of orange juice, from which they concluded that rats, in spite of their insusceptibility to scurvy, require a certain amount of the antiscorbutic vitamin in their food, and that the deterioration of the casein was due to the destruction of this factor rather than to the alteration in the character of the protein itself.

Their results are of questionable value, however, since the rations used were deficient in A.

78 Brennemann, J. Am. Med. Ass. 60, 575, 1913.

79 McCollum and Davis, J. Biol. Chem. 23, 247, 1915.

80 Rettger, Am. J. Physiol. 6, 450, 1901-2.

81 Morner, Ztschr. f. Physiol. Chem. 34, 207, 1901-2. 82 McCollum and Davis, J. Biol. Chem. 23, 231.

83 Funk and Macallum, J. Biol. Chem. 27, 51, 1917.