STERILIZATION 35 



6. Sterilization. Many of the most serious diseases 

 of man and domestic animals are caused and spread by 

 bacteria ; these minute organisms are also responsible 

 for the souring of milk and the putrefaction of meat, 

 eggs, and other food materials. It is therefore im- 

 portant to study the means by which their activities can 

 be checked or their vitality destroyed altogether. 



Milk, beer, flesh, and all substances from which all 

 living germs have been removed or destroyed, are said 

 to be sterile, and will keep indefinitely if, after steriliza- 

 tion, further access of bacteria is prevented. 



Various methods of sterilization are in daily use, the 

 chief of which are based upon the destructive action of 

 heat, or upon the poisonous nature of certain chemical 

 compounds. 



By maintaining a low temperature, food-stuffs may be 

 kept for an indefinite time without change. In a frozen 

 state meat, milk, and fruit of all kinds are sent long 

 distances without damage, and it is well known that 

 fruit, jam, milk, butter, bread, and meat keep best in 

 cool, dry situations. In such cases it must be borne in 

 mind that the low temperature merely checks the 

 development of the bacteria present, and does not 

 destroy them ; a sterile condition cannot be produced 

 in this way, and when the temperature is raised the 

 organisms present begin their growth and work of 

 destruction with unabated vigour. 



The keeping qualities of foods of all kinds are greatly 

 increased by roasting, baking, boiling, and steaming, for 

 most bacteria are killed by these processes. 



The most certain and complete sterility is obtained by 

 the application of heat. A temperature of 150 C, main- 

 tained for an hour, is sufficient to destroy all forms of 



