THE USEFUL MICROBES 
45 
milk, and turn it into lactic acid. The lactic 
acid curdles the milk, and so makes it sour. 
Milk pasteurized at the usual heat sours in the 
ordinary way when allowed to stand for some 
time, though if a heat of about 180 degrees is 
used, even the lactic acid microbes are killed. 
The milk then spoils through the action of 
certain other microbes which have survived. 
Milk that has soured through the action of the 
lactic-acid microbes is not unhealthy to drink, 
however. If it were not for the unpleasant 
taste, it would be just as healthy as fresh milk. 
Sometimes other changes take place in milk, 
rather than the usual souring. The milk 
becomes blue, or red, or yellow in color, or 
bitter in taste, or slimy to the touch. Such 
changes are sometimes called diseases of milk. 
They also are caused by microbes that find 
their way into it. Nearly all of these unpleas¬ 
ant changes can be prevented, however, by 
thoroughly purifying the containers used for 
the milk, and by observing sanitation in the 
barns where the cows are kept. 
How does 
mil\ change? 
