16 EXPERIMENTAL DAIRY BACTERIOLOGY 



more complete flocculation of the proteid so as to permit 

 ready filtration. 



Agar is not injured by prolonged heating, as is gelatin. The 

 finished medium should he clear when liquid, and slightly 

 opaque when solid. It should be of a firm jelly-like consist- 

 ency, not soft, mushy, or easily broken. 



Exercise. Each student will prepare 1 liter of agar. 



It frequently happens in the preparation of media that the 

 material becomes seeded with resistant spore-bearing organ- 

 isms. These do not come from the materials used in the 

 media, but frequently from unclean utensils. All utensils 

 media cookers, funnels, pipettes, etc. should therefore 

 be washed with hot 1 water and drained as soon as possible, in 

 order to free them from the adhering media. If allowed to 

 stand, germ growth takes place in this material, and the 

 utensils become so abundantly seeded as to make sterili- 

 zation of the media prepared in such vessels difficult or 

 impossible. 



Meat infusion should be used whenever possible for media, 

 but beef extract can be substituted, 3 grams of Liebig's Beef 

 Extract being used for each liter of medium. Egg albumen 

 must then be used as a clearing agent, to remove the finely 

 dividend material. The white of one egg for each liter of me- 

 dium should be added to the distilled water before adding 

 the other ingredients. The albumen should be well beaten 

 and thoroughly mixed with the water. Then proceed as when 

 meat infusion is used. 



Preparation of milk. The milk to be used as a culture 

 medium should be as fresh as possible. It should not have 

 an acidity above 0.18 per cent calculated as lactic acid. The 

 normal acidity of perfectly fresh milk ranges from 0.12 to 



