11 



: 



GENERAL CHARACTERS OF BACTERIA. 



of 140 if maintained for half an hour. At this temperature, 

 however, they die slowly; a temperature of 150 destroys them more 

 rapidly still, while a temperature from 170 to 180 is proportionately 

 more effective. A total destruction of bacteria, including their 

 spores, can be brought about only by a temperature above that 

 of boiling water, and this is usually accomplished, in the case of 

 liquids, in a closed chamber where the 



team can be generated under con- 

 siderable pressure. If the steam is 



llowed to collect in such a chamber 



t a pressure of 15 pounds, the tem- 

 rature, then, will be about 240. 



his temperature, kept up for half or 

 three-quarters of an hour, destroys 

 even the most resisting spores. Lab- 

 oratories usually have a small appa- 

 ratus designed for this purpose, called 

 an autoclav (Fig. 12), and this is used 

 constantly for sterilizing liquids. 



Sterilization. This is a process 

 closely related to the question of death 

 temperatures. Sterilization is some-. 

 times, to be sure, brought about by 

 adding poisonous chemicals to the 



.,. , , 



material to be sterilized; but more 

 commonly, and almost universally, 



when we are dealing with food products, sterilization is accomplished 

 by heat. If a material to be sterilized contains only active organisms, 

 it might be accomplished by subjecting it to a moderately low 

 heat, 140 to 150 F. But it is almost always a fact that anything 

 which we wish to sterilize is likely to contain spores, and, since 

 these withstand a higher temperature, no moderate heat will 

 accomplish the purpose. Even boiling is not sufficient to destroy 

 spores, so, to be sure of complete sterilization, a temperature above 

 boiling is necessary. If the object is a solid that can bear heat 

 it is simply heated at about 300 F. for an hour or so. If it is a 



FIG. 12. An autoclav used in 



sterilization of liquids under pressure 



