There are various methods of sterilizing milk. We have already referred to the sterilizing action of boiling the milk, but by the term " sterilization " is meant the exposure of milk to a temperature of 212° F. or higher for a period varying from half an hour to an hour and a half. By some it is boiled and by others it is treated by steam. It may be carried out at home when a special sterilizer is used, such as Soxhlet's, Arnold's, Cathcart's, etc. The milk may be sterilized in bulk on delivery and afterwards modified by the addition of cream, sugar, etc., or, the necessary dilution and additions having been made, the prepared milk for twenty-four hours is put into bottles, each containing enough for one meal, and all the bottles are put in the sterilizer and heated; the bottles are then put aside on ice and are opened as required. They are usually made so that a rubber teat can be fixed around the opening, and the child fed direct from the bottle.
The advantage claimed is that all organisms, pathogenic as well as non-pathogenic, are destroyed in a much more thorough manner than by boiling the milk for one or two minutes. The object aimed at is to prevent the occurrence of disease by specific contagion in the milk or by fermentative changes in the milk. By some it is claimed that the milk is rendered more digestible, by others this is denied. Hutchison states that sterilization alters the taste of the milk, destroys the fine emulsion of the fat, coagulates the lactalbumin, and renders the casein less easy of digestion. In all probability the digestibility of the milk is not altered to any marked extent. Holt states that the constipating effects of sterilized milk are soon noticed.
The advantage of destroying the organisms in cows' milk has already been emphasized, and sterilizing will effect this object more fully than simple boiling, although certain spores may still resist the effects of the high temperature. Milk which has been sterilized will also keep from souring longer than milk not so treated, but it will none the less become stale. Milk which has been swarming with organisms will not be rendered pure by sterilizing, and the various toxins produced in milk which has already been kept too long will not be destroyed by prolonged heating. There is also a biological aspect to the question. Experience has shown that sterilized milk is apt to lead to anaemia, rickets, and more especially scurvy. Out of 379 cases of infantile scurvy collected in America, the previous diet in 107 cases was sterilized milk. This means that some vital property in fresh milk has been destroyed. Milk which has been sterilized has not only been boiled, it has been boiled to death. So generally has this been recognized that the process is now considerably shortened, and many consider that from ten to twenty minutes is a sufficient length of time. As a complete diet for infants sterilized milk is not to be recommended, and its use might with advantage be reserved for special conditions of ill-health which only the physician can determine. It will be found useful as a temporary resort when milk has to be transported a long distance, or when the suspicion of infection in the milk (typhoid, tuberculosis) renders special precautions advisable. It has no advantage over boiled milk in the prevention or cure of infantile diarrhoea.
Sterilized milk is also supplied by the dairy companies. The same objections which have been urged above apply here, and there are others. The process of sterilization requires skilled supervision, and one can never depend on dairy milk having been prepared according to exact scientific methods. There are but too many opportunities of scamping the work. Again, while one can usually tell as to the freshness of untouched milk, it is impossible to form an estimate as to how many days sterilized milk has been kept and how many times it has been re-sterilized. Sterilization forms an excellent method of preserving for future sale the surplus supply of fresh milk at the end of the day. For these reasons if sterilization is employed it is very much better to carry out the process at home.