3 82 



FUNDAMENTALS OF AGRICULTURE. 



Sterilization and Pasteurization. Since the souring 

 of milk depends on the growth of the bacteria found 

 therein it follows that if these are destroyed, e. g., by 



heating the milk to the 

 boiling point, it will keep 

 for a long period, per- 

 haps indefinitely, pro- 

 vided all the bacteria 

 have been destroyed in 

 the heating process. The 

 milk has then been ster- 

 ilized. As the process 

 of sterilization changes 

 the properties of the milk 

 and may decrease some- 

 what its value as a food 

 for infants or invalids, 

 the heating is now not 

 generally carried so far, 

 but only to the point of 

 pasteurization. By "pas- 

 teurization " is meant the application of sufficient heat 

 to destroy all bacteria in the milk, but not their spores. 

 This is accomplished by heating the milk for twenty 

 minutes at 140 F., or for shorter periods at higher 

 temperatures. If the milk is cooled promptly after 

 being heated, it will not have a cooked taste, and will 

 have similar properties as fresh, so-called raw milk. 

 Owing to its freedom from bacteria pasteurized milk 

 will remain sweet for several days if kept at a low 

 temperature. 



Preservation of the milk by heat and subsequent 

 cooling is the only legitimate method of improving the 

 keeping qualities of milk. The laws of most states in 

 the Union prohibit the use of chemicals for preserving 

 milk. The reason for this is that chemicals decrease 

 the digestibility of the milk, and may seriously affect 

 the health of infants and sick persons who depend 

 largely on milk for their nourishment. 



THE BLACK SPACE SHOWS NUMBER 

 OF BACTERIA IN NORMAL FRESH 

 MILK; THE WHITE SQUARE THE 

 NUMBER AFTER PASTEURIZA- 

 TION. 



