BACTERIOLOGICAL ANALYSES 307 



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. The bac- 

 terial content of milk powder is variable, no approximation can here 

 be offered. 



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 

 30 grams lactose 



4 grams sodium chloride 

 150 grams gelatin 

 1000 c.c- distilled water. 

 Acidity 0.1 per cent. 



Media for Yeasts and Molds 

 4 grams beef extract 

 10 grams peptone 

 12 grams agar 

 1000 grams w r hey 



