What does 
pasteurizing 
do to mil\? 
42 WORLD OF INVISIBLE LIFE 
This method is used, by the companies that 
sell milk, in almost every city in the United 
States. It has brought about a great lessening 
in the amount of sickness caused by impure 
milk. While it does not cause any great 
changes in the milk, it does stop to some extent 
the formation of cream, and it weakens one of 
the healthful things called vitamins in it. But' 
it makes the milk so much safer that we over¬ 
look these small drawbacks. There is no doubt 
but that it has saved countless lives, especially 
those of babies, who live almost entirely on 
milk. 
When milk is allowed to stand in a warm 
place, it always becomes sour. For a long time, 
nobody knew just why this happened. Now 
we know that it is the work of a group of 
microbes that are always present in milk and 
which multiply rapidly except when the milk 
is kept cold. They are known as the lactic-acid 
microbes. As they increase in numbers, they 
act on the milk-sugar, which is one of the 
substances making up the drops of fat in the 
