662 BACTERIOLOGICAL ANALYSES 



Dilutions for Numerical Counts. For dilutions sterile, glass 

 stoppered 250 cc. flasks are conveniently used. In the case of milk 

 or cream use 2 cc of sample and 198 cc. of sterile water. The milk 

 or cream is most readily measured and transferred to the dilution 

 flask by the use of sterile, straight stem, bulbless pipettes. This 

 constitutes the first dilution. 



In the case of butter, weigh two grams of the sample into a 

 tared flask. Mix with enough sterile water at a temperature of 98 

 to 100 F. to make up 100 cubic centimeters. This constitutes the 

 first dilution. If yeast and mold counts only are made of butter, 

 dilutions may be dispensed with entirely. In this case the butter 

 sample is melted at low heat (not exceeding 100 F), and 1 cc. of 

 this warm melted butter is transferred direct to the petri dish to 

 which previously was added 1 cc. of tartaric acid and the plating is 

 finished by pouring over the mixture of melted butter and tartaric 

 acid 10 cc. of nutrient agar. 



Further dilutions of milk, cream and butter are made in a similar 

 manner as the first dilution, using in the place of the original sample, 

 portions of the diluted sample. Dilutions should be sufficient to 

 limit the number of colonies on the plates to about 50 to 100 colonies 

 per plate. Whole milk as it arrives at the factory averages a total 

 count of from about 100,000 to 1,000,000 bacteria per cc. Cream at 

 gathered cream creameries contains from about 500,000 to 

 500,000,000 bacteria per cc. before pasteurization and from about 

 1,000 to 300,000 bacteria per cc. after pasteurization. Butter made 

 from raw gathered cream usually contains from about 1,000,000 to 

 2,000,000 bacteria per cc., and butter made from properly pasteur- 

 ized gathered cream contains from about 2,000 to 1,000,000 bacteria 

 per cc. In properly pasteurized cream butter the count of yeast and 

 molds is usually below 10 and often it is 0. When above 10 colonies 

 of yeast or mold or both, either the pasteurization was imperfect, 

 or the butter was recontaminated after the pasteurization of the 

 cream, or the count includes colonies which, though they may thrive 

 in the special medium used for yeast and molds are other than yeast 

 and molds. 



