PRESERVATION OF MILK 89 



A sample of perfectly fresh milk should be poured into 

 three sterile flasks and incubated at 10 C., 20 C., and 37 C. 

 At frequent intervals comparative plate cultures should be 

 made from the samples by the loop method, in order to de- 

 termine the relative rapidity of growth of bacteria at the 

 various temperatures. The plates should be made at least 

 every twenty-four hours. 



Besides this approximate quantitative examination a de- 

 termination of the rate of acid development should be made 

 by removing 5 cc. of each sample each time the plates are 

 made, and determining the amount of acid therein. The max- 

 imum amount of acid formed at the various temperatures 

 should be determined, and the nature and flavor of the cur- 

 dled milk also noted. 



Exercise. Each student will procure a sample of fresh milk 

 (100 cc.), divide the same between three flasks, incubate them at 

 the temperatures indicated, and at intervals of twenty-four hours 

 prepare lactose-agar plates, using the loop method of dilution. Con- 

 tinue until no further increase in acidity is noted. 



Pasteurization of milk. A large proportion of the bac- 

 teria found in milk are vegetating cells, such forms making 

 up over 99 per cent of the bacterial content of normal milk. 

 The vegetating cells of nearly all forms of bacteria are easily 

 killed at temperatures above 60 C. In milk thoroughly pas- 

 teurized only the spores of spore-forming bacteria are left. 

 To attain this condition it is necessary to heat the milk at 

 such temperatures and for such periods as to alter its chem- 

 ical and physical nature in some degree. Some of the changes 

 are as follows : The fat globules in normal milk are, to a con- 

 siderable extent, in clusters or masses, which, on account of 

 having a greater volume for a given surface, are able to 

 overcome the resistance offered by the milk serum, and rise 



