DAIRYING 



I 99 



Kinds of Bacteria. There are many bacteria found 

 in milk. Some work on the milk sugar, and turn it to 

 lactic acid and thus sour the milk. Others attack 

 the fat in milk and make butter rancid or strong. Still 

 others give to butter its flavor. These are called friendly 

 bacteria, and are often put into cream to give the butter 

 just the flavor desired. By cleanliness and care in cooling 

 the milk, the harmful bacteria may be held in check, so 

 that the friendly bacteria may have a chance to grow. 



Progeny of 

 a Single Germ 

 in twelve hours. 



FIG. 117. Cooling hinders growth of bacteria. (H. L. Russell, VVis. Bui. No. 62.) 



Pasteurization It has been found possible to destroy 

 all disease germs and the bacteria that are unfriendly in 

 milk by pasteurising it. In this process, the milk is 

 heated to a temperature of about 160 F., and held at that 

 point for about fifteen minutes ; then it is cooled as rapidly 

 as possible to a temperature of 50 F. This destroys the 

 germs but does not otherwise affect the milk. Many 

 machines have been invented for doing this work, but it 

 may be done in the home without the use of machinery. 



