84 DAIRY LABORATORY GUIDE 



possible to prevent darkening of surface. Cut cylin- 

 ders into two equal parts by a diagonal cut. Place 

 in cool running water for twelve to eighteen hours. 

 This usually renders the potato neutral. Sterilize 

 in steam sterilizer for thirty minutes on three con- 

 secutive days. Place in sterile test tubes, slanting 

 surface upward, and plug with sterile cotton. Steri- 

 lize once after plugging. 



Milk Media 



Heat fresh milk for fifteen minutes in' steam steri- 

 lizer at 200 to 212 F. Place in ice-box over night. 

 Titrate, and if less than .2 per cent acid place in 

 tubes and sterilize for twenty minutes on four con- 

 secutive days in a steam sterilizer. If more than 

 .2 per cent acid reduce to .15 per cent by the addition 

 of one- tenth normal solution of sodium hydroxide. 

 Place in sterile test tubes with sterile cotton plugs. 

 Make cultures of the above media from milk, cheese, 

 ice cream, and butter from various sources. Note 

 behavior and color of culture on the various kinds 

 of media. Make plate cultures of agar and gelatine 

 by warming them to melting point and adding a 

 drop of milk. Shake thoroughly and pour into sterile 

 petra dish and allow to cool. Place in incubator for 

 twelve to twenty-four hours at 100 F. in case of the 

 agar plates and 80 F. in the case of gelatine plates. 

 Examine every few hours or as often as convenient. 



Make medium more condensed for quantitative 

 analysis of liquids. 



After washing with cleaning mixture prepared in 



