BACTERIOLOGICAL ANALYSES 375 



rnilk and milk powder, use tared flasks holding about 150 cubic 

 centimeters, weigh into them two grams of the product and add 

 enough sterile water at a temperature of 98 degrees F. to make 

 up 100 cubic centimeters. Use a sterile spoon or spatula to 

 transfer the product to this flask. A wide-mouth flask is 

 preferable. 



The above represents the first dilution. The flask should be 

 carefully shaken until a homogeneous mixture is obtained and 

 the soluble portions have been completely dissolved. 



From this dilution further dilutions are made in sterile 

 water in glass-stoppered flasks, according to requirements. The 

 dilutions should be sufficient to limit the number of colonies on 

 the plates to about 50 to 100 per plate. Whole milk, as it arrives 

 at the factory, usually shows from 100,000 to 1,000.000 bacteria 

 per c.c. Evaporated milk should be practically sterile unless the 

 can shows signs of fermentation in which case the number of 

 bacteria present will depend on the age of the sample can ; 

 dilutions as high as 1 : 1,000,000 are recommended in such cases. 

 Plain condensed bulk milk when fresh contains from about 

 1,000 to 100,000 bacteria per c.c., when several days old and in 

 the absence of refrigeration, its germ content is often much 

 greater. Sweetened condensed milk averages from about 500 to 

 500,000 bacteria per c.c. 



Plating. -For plating the following media are recommended : 



Media for Total Counts and also for acidifiers 

 4 grams beef extract 

 10 grams peptone 

 30 grams lactose 

 4 grams sodium chloride 

 12 grams thread agar 

 1000 c.c. distilled water. 

 Acidity 0.1 per cent. 



For acidifiers add 1 c.c. of sterile litmus solution to each 

 plate before pouring the agar. 



Media for Liquefiers 



4 grams beef extract 

 10 grams peptone 



