In the process of pasteurization the milk is subjected to a temperature of from 158° F. to 170° F. for half an hour or longer. It may be carried out at home in an apparatus specially constructed for the purpose. Milk thus treated will keep sweet for two or three days if stored on ice, the heat employed being considered sufficient to destroy pathogenic organisms although not their spores, and to prevent fermentative changes. Some deny that all organisms are destroyed, and regard the process as incomplete sterilization. If it is desired to secure tubercle-free milk it will be safer to sterilize or boil the milk than to trust to pasteurization. The changes in the taste and in the solids of the milk after sterilization are avoided by the use of the lower temperature employed in pasteurizing.
The objects aimed at in pasteurization are to destroy the organisms in milk, and by means of the lower temperature used to overcome the disadvantages which were found to follow the prolonged use of sterilized milk. The risks of anaemia, scurvy, etc., are said to be avoided. It seems probable that pasteurized milk, which has been properly prepared, is a perfectly safe diet for infants. The temperature should not be higher than 158° F. and the time not longer than twenty minutes, if the nutritive value of the milk is to be preserved intact. After pasteurization the milk is quite as liable to contamination and fermentative changes as fresh milk, and the same precautions must be taken to prevent fresh infection, e.g. enclosure in air-tight vessels, storage on ice, etc. The exact carrying out of the process will usually require more skill and time than are available in an ordinary household. Cases of scurvy have been reported after the prolonged use of pasteurized milk, but probably the heating had not been properly carried out. It is questionable whether anything is really gained by this process which cannot be more simply secured by boiling the milk.
By many dairy companies milk is now pasteurized in bulk and supplied to customers. So far as my own knowledge of this system goes, it has not been adopted with the view of preserving the customers' health but for the protection of the dairy company against legal liabilities from the sale of infected milk. Stale milk which has been pasteurized may contain many active poisons although few living organisms. Again, as Crozier Griffith has pointed out, while pasteurization destroys the bacteria, the spores of the subtilis group are unaffected. Unless the milk is carefully preserved on ice these spores, unchecked by the presence of living lactic acid germs, will multiply even faster than in raw milk, and may cause severe diarrhoea. In many cases the purchaser is unaware that the milk has been pasteurized and at once boils the milk on delivery. This double treatment of the milk will tend to the production of the drawbacks mentioned in connexion with sterilized milk. In the feeding of infants it is extremely important that the whole treatment which the milk has undergone before delivery should be known. The dairy pasteurization of milk may or may not be properly carried out, the milk supplied may or may not be fresh, and the result as regards the infant may or may not be successful. For these reasons the home modification of milk is to be preferred, and a guarantee should be obtained from the dairy as to the state of the milk provided.