254 MILK HYGIENE 



Sterilization: Dry sterilization. Petri dishes, pi- 

 pettes, empty test tubes, etc., are usually sterilized by 

 dry heat. 



A common gas-stove oven of large size may be used. 



Empty test tubes are sterilized when the heat causes 

 the cotton plugs to turn slightly brown ; petri dishes and 

 pipettes should be exposed to the full heat for at least 

 one-half hour, and are best sterilized in dust-proof 

 copper boxes in which they may be kept sterile for a 

 long time. 



Such a petri dish box may be made 4J x 4J x 10 

 inches with a hinged cover on one side closing over the 

 edges and a ring handle on top. 



A convenient pipette box is one 11 x 2 x 2 inches, 

 square in section rather than round to prevent rolling, 

 with a ring handle on the end cover. 



These boxes must be made without solder, on ac- 

 count of the heat to which they are to be exposed in dry 

 sterilization. 



Autoclave sterilization. Media, dilution water, and 

 the rubber stoppers used in the centrifugal apparatus 

 should be sterilized in the autoclave. 



They should be kept under fifteen pounds steam 

 pressure, which gives a temperature of about 250 F. 

 for at least half an hour. 



