Farm Dairying 



251 



356. Changes in Milk. Bacteria are the active agents 

 of change in milk. The souring of milk is due to the 

 formation of acid by bac- 

 the acid 

 sufficient 



in 



teria. When 



accumulates 



quantity, it combines with 



the protein to form the 



clabber. If bacteria are 



kept out of the milk, it will 



keep sweet indefinitely. 



The flavors developed in 



milk and butter are due 



to the presence of certain 



kinds of bacteria. Some give the butter undesirable 



flavor, and some greatly improve the flavor. The flavor 



of butter, however, may be controlled by destroying 



all the bacteria in the milk or cream by Pasteurization. 



(1 367.) After the milk or cream has been freed from 



the desirable, as well as undesirable germs, by the 



process mentioned, it is then cooled and desirable ones 



Fig 161. Microscopic appearance of 

 ordinary milk showing fat globules 

 and bacteria in the milk. The 

 cluster of bacteria on left side are 

 lactic acid -forming germs. After 

 Russell, University of Wisconsin. 



Progeny of 



a Single Germ 

 In twelve hours. 



Fig. 162. Cooling hinders growth of bacteria. After Russell 



