106 EXPERIMENTAL DAIRY BACTERIOLOGY 



is disintegrated as completely as possible, when it is trans- 

 ferred to a flask of water by means of a sterile spatula, or 

 rubber-tipped rod. 



The flask is well shaken, to free the sand from cheese as 

 far as possible. In transferring with a pipette a portion of 

 this suspension to other water blanks the sample should be 

 taken immediately after shaking, before the sand has set- 

 tled. Settling may be prevented by holding the pipette in a 

 horizontal position until ready to deliver the contents. The 

 grinding material should be fine enough to avoid clogging 

 the pipette. All the instruments and material with which 

 the cheese comes in contact should be sterile ; the contami- 

 nation from the air during the grinding is so small as to 

 be negligible. 



From the various dilutions lactose-agar plates are prepared. 

 The maximum numbers of bacteria are found in cheese from 

 twenty-four to forty-eight hours after making. The number 

 decreases rapidly for some days, reaching at last a level below 

 which farther decrease is very slow. In cheese twenty-four 

 hours old over a billion bacteria per gram may be found. 



Exercise. Each student will examine quantitatively samples of 

 cheese from one to two, from ten to fifteen, and from sixty to one 

 hundred days old. 



Qualitative examination of milk for cheese making. In 

 milk imended for cheese manufacture it is desirable that acid- 

 forming organisms of the B. lactis acidi type shall be the pre- 

 dominating organisms. As is known, this organism produces 

 no gas in the fermentation of lactose, the curd produced be- 

 ing perfectly homogeneous and pleasant in flavor and odor. 

 Organisms of the B. coli communis and B. lactis aerogenes 

 types are detrimental in cheese, since they cause the cheese 

 to be filled with a greater or less number of holes produced 



