STERILIZATION ANTISEPSIS FOOD PRESERVATION 65 



other parts. Ordinarily a temperature of 150 C. for one hour, 

 170 C. for 30 minutes, or 200 C. for one minute will kill all 

 bacteria. Such exposure browns cotton of good grade only 

 slightly. One fallacy in hot-air sterilization needs to be guarded 

 against. Glassware and other apparatus must be dry before it 

 is put into the oven to sterilize. A tube containing water may 

 be left in the oven until the thermometer records a temperature 



FIG. 28. Hot-air sterilizer. 



of 200 C. in the upper corner of the sterilizer, and subsequently 

 the tube may be removed from the oven with the most of the 

 water still in it. Hot-air sterilization is employed for glassware, 

 tubes with cotton plugs, granite-ware, stone-ware, and for metals 

 not injured by heat. 



Moist heat or heat in the presence of liquid water must be used 

 whenever drying is to be avoided, especially in the sterilization of 

 culture media and various solutions. It is employed as continu- 

 ous sterilization at a single exposure and as discontinuous ster- 



5 



