THE USEFUL MICROBES 
41 
in the past it often caused epidemics of various 
diseases. Some way had to be found to kill 
any germs in it before it was used for drinking. 
It was especially necessary to do this in cities, 
where the milk had to be brought a long 
distance from dairies whose sanitary conditions 
could not be known to those drinking the milk. 
At first, boiling the milk was tried, for 
boiling kills all the microbes. But boiling also 
causes certain changes in the milk that make 
it less easy to digest, besides lessening its food 
value. It also changes the taste. 
For these reasons, another way of treating 
the milk is now followed, which kills all the 
harmful microbes. This method is called 
pasteurization. It is named after Louis Pasteur, 
a famous French scientist who discovered a 
great many facts about microbes. It consists 
of heating the milk hot enough to kill the 
harmful microbes, but not hot enough to cause 
any marked changes in the milk itself. The 
milk is heated to 142 to 145 degrees for thirty 
minutes. Then the milk is cooled rapidly. 
How can they 
be \illed? 
