LABORATORY WORK 309 



After diluting the milk make six litmus gelatin plates from 

 •each sample. It is best to have three of these plates diluted twice 

 as highly as the other three. After cooling, place at a temperature 

 of 70 . After two days, examine and determine whether there are 

 any rapidly liquefying colonies that are likely to destroy the plates; 

 if so, make a study of the plates at once. If possible, however, keep 

 the plates for four days before studying them. 



Study of plates. — After the plates have grown (2 to 4 days), 

 study as follows : 



a. Determine total number of bacteria per c.c. of milk. Com- 

 pare with the total number found on the agar plates. 



b. Determine the number of liquefying colonies. 



c. Determine the number of acid bacteria per c.c. Are they 

 ■of the Bact. lactis acidi type ? 



d. Calculate the percentage of acid bacteria — liquefiers and 

 '-miscellaneous bacteria in the milk. Draw a conclusion as to which 

 samples were probably badly contaminated. 



35. Bacteria in Fresh Cream. — Dilute one c.c. of freshly 

 separated cream with sterile water. If this is separated by a sep- 

 arator, the dilution should be about 1,000. If the cream has been 

 separated by the gravity method, the dilution should be higher, 

 since the cream is older, and should be as high as 100,000. After 

 diluting, make a series of four plates in litmus gelatin, incubate at 

 70 and study as described in No. 56. 



36. Bacteria in Ripened Cream. — Repeat experiment No. 

 .35, using, however, some ripened cream that is just ready for 

 churning. In this case the dilution must be much higher, and 

 probably never less than 1,000,000. After incubating, determine 

 the variety and numbers of bacteria present. 



37. Analysis of Butter Milk. — Make an analysis of the 

 bacteria in butter milk in litmus gelatin. The dilution in this case 

 should not be less than 100,000. 



38. Analysis of Butter. — Weigh out upon accurate chem- 



