LABORATORY WORK IN DAIRY BACTERIOLOGY 305 



b. Determine the number of liquefying colonies. 



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

 of the Bad. lactis acidi type? 



d. Calculate the percentage of acid bacteria liquefiers and 

 miscellaneous bacteria in the milk. By reference to page 181 

 draw a conclusion as to which samples were probably badly 

 contaminated. 



*No. 58. Bacteria in Fresh Cream. Dilute one c.c. of freshly 

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

 separator, 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. 



*No. 59. Bacteria in Ripened Cream. Repeat experiment No. 

 57, 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. 



No. 60. Analysis of Cream at Successive Intervals. Obtain 

 some sweet separator cream, and set it to ripen in the ordinary 

 way. Make a series of litmus gelatin plates from the cream, 

 when first separated, and others at 6 hours later, 12 hours later 

 and at the time of churning. The dilution of cream in the suc- 

 cessive experiments should be increased from about 1,000 to 

 1,000,000. Incubate at 70, and carefully determine the number 

 of bacteria, their types and percentages. Note particularly the 

 increase in the percentage of lactic acid bacteria at the successive 

 periods of the ripening. 



No. 61. 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 1,00,000. 



No. 62. Analysis of Butter. Weigh out upon accurate chem- 

 ical scales 5 grams of freshly made butter. Place this in a sterile 



