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 8 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. 
