Sterilized Milk

What is meant by sterilizing milk?

Heating milk for the purpose of destroying germs.

Does all cows milk contain germs?

Yes; even when handled most carefully, milk contains many germs; but when carelessly handled, and in summer, the number is enormous. While most of these are harmless or cause only the souring of milk, others are occasionally present which may produce serious diseases such as typhoid fever, diphtheria scarlet fever, cholera, tuberculosis, and many forms of diarrhoea.

Under what circumstances is it necessary to sterilize milk?

1. In warm weather when it cannot be obtained fresh; hence always in cities and towns during the summer.

2. When one cannot be certain that the cows are healthy, or that the milk has been carefully handled.

3. When the milk is to be kept for any considerable time (i.e., over twenty-four hours), especially if no ice can be had.

4. During epidemics of typhoid fever, scarlet fever, diphtheria, or any form of diarrhoeal disease.

What are the two methods of heating milk?

The first is known as sterilizing, in which the milk is heated to 212° F. for one hour or one hour and a half; the second is known as pasteurizing, in which the milk is heated to 155° or 170° F. for thirty minutes. A temperature of 155° F. continued for thirty minutes is sufficient to kill the germs of the diseases above referred to.

Will milk which has been thus treated keep indefinitely?

No; for although all the living germs may be killed, there are many undeveloped germs, or spores, which are not destroyed, and which soon grow into living germs. Milk heated to 212° F. for an hour will keep upon ice for two or three weeks; that heated to 155° F. for two or three days.

Is milk which has been sterilized always a safe food?

No; for the reason that the milk may be so old, so dirty, and so contaminated before sterilizing that it may be still unfit for food, though it contains no living germs.

Is cow's milk rendered more digestible by being heated in this way?

Sterilizing milk does not improve its digestibility but rather the contrary. Sterilized milk should be modified for infant feeding in the same way as milk which has not been heated.

Is milk in any way injured by heating to 212° F. for an hour?

There is abundant evidence that milk is rendered less digestible by such heating; also that it is more constipating, and that for some children its nutritive properties are interfered with, so that it may cause scurvy; this, however, is not seen unless it is continued as the sole food for a long period. These objections are of so much importance that this plan of heating milk is not to be recommended for general use.

When is it advantageous to heat milk to 212° F.?

For use upon long journeys, such as crossing the ocean. Milk should then be heated for one hour upon two successive days, without removing the cotton stoppers from the bottles.

Is milk in any way injured by heating to 155° F. for thirty minutes?

This point is not yet definitely settled. Such heating does not affect the taste of milk and does not render it more constipating. The unfavourable effects; if there are any, are so slight that they need not deter one from the use of pasteurized milk, even for long periods. The preference, however, should always be given to milk which is so clean and so fresh as not to require any heating.

How should milk be pasteurized?

A convenient form of apparatus is that known as Freeman's pasteurizer; another is the Walker-Gordon pasteurizer.

How should milk be cooled after pasteurizing?

Always by placing the bottles in cold water, so as to cool them rapidly; never by letting them stand at the temperature of the room, or by placing them, when warm, in an ice box.

Why is this precaution necessary?

Cooling in the air or in an ice box requires from two to four hours, and during that time a great many of the undeveloped germs may mature and greatly injure the keeping properties of the milk. In the cold water, milk can be cooled in from ten to twenty minutes if the water is frequently changed, or if ice is added to the water.