262 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



placed in iced-water they may be kept for twelve hours 

 without an increase in bacteria. 



All apparatus used for collecting, measuring, dilut- 

 ing, and plating the samples must be sterilized by heat- 

 ing at a temperature of at least 175 C. for one hour. 



There are two methods of counting bacteria in milk: 

 The plate method, in which the number of bacteria per 

 c.c. of milk is determined by counting the colonies de- 

 veloping on agar plates to which a measured quantity 

 of milk has been added; and the direct microscopic 

 method, in which a small measured quantity of milk is 

 spread over a definite area on a glass slide, dried, fixed 

 and stained, and the bacteria counted under the micro- 

 scope. The plate method is the oldest and best under- 

 stood and is recommended for general purposes. The 

 microscopic method is useful when rapid results are de- 

 sired, as when samples are examined for the purpose of 

 classifying or grading milk. 



PLATE METHOD 



Medium. The medium used is standard beef extract 

 agar, which should be prepared according to the follow- 

 ing directions: 



" To 1000 c.c. of water add 5 grammes of peptone 

 and 3 grammes of beef extract. The peptone shall be 

 the best available, and the beef extract shall be Liebig's 

 where this is obtainable. Weigh the containing dish and 

 its contents. 



" Dissolve the peptone and beef extract by boiling 

 and replace the water lost through evaporation. 



" While still hot filter through filter paper. 



" Add 12 grammes of oven-dried agar, or 15 grammes 

 of market agar, and dissolve by boiling, or in an autoclav. 



