420 MICROORGANISMS 



the foregoing that the process of canning depends on the com- 

 plete destruction of all forms of living organisms in the food. 

 The process of killing all microorganisms in a given substance 

 is known as STERILIZATION. If foods which from their mois- 

 ture content and other conditions are favorable food for bac- 

 teria are to be kept for any considerable time, they must be 

 completely sterilized and sealed up so as to be free from the 

 entrance of bacteria, for, if a single bacterium or spore is left 

 in the food in a living condition, it will soon multiply and the 

 food will be destroyed. 



Many times, however, we care to preserve food for only a 

 short time. In such cases complete sterilization is not neces- 

 sary. Furthermore, some foods, like milk, are seriously in- 

 jured by the high temperature necessary for complete steril- 

 ization. If milk is heated even to boiling, some of the proteins 

 which it contains are coagulated and the flavor of the milk is 

 greatly changed. The coagulated protein is not so easily 

 digested as it is in the fresh state and the changed flavor 

 makes it less palatable than fresh milk. Consequently, since 

 milk is usually consumed within 24 to 48 hours after it is 

 drawn from the cow, the bacterial action which might other- 

 wise cause it to sour within that time may be effectively 

 prevented by heating the milk after it is bottled to a tem- 

 perature sufficiently high to kill only the active growing bac- 

 teria which it contains. If milk is heated to 60C., or 140F., 

 for 20 minutes, all the active bacteria in it will be killed, but 

 the milk will not be changed essentially in flavor or digesti- 

 bility. Such an amount of heating is not sufficient to kill the 

 bacterial spores in the milk and these will soon begin to grow 

 and multiply and the milk will ultimately sour. The souring 

 is delayed by this process, however, and this delay usually 

 makes it possible for the milk to be delivered to the consumer 

 and consumed before the souring occurs. This process of heat- 

 ing food to a temperature sufficiently high and for long enough 

 time to kill the active bacteria but not to kill the spores, is 

 called PASTEURIZATION. You will note by the spelling of the 



