85 



Examine the tubes inoculated during the preceding ex- 

 ercise. 



Allow the inoculated tubes to remain in the incubator for 

 several days and note whether or not any of them develop 

 after the first 24 hours. If they do examine them micro- 

 scopically to determine if the culture is pure. Observe in 

 these cultures the difference between immediate destruction 

 and the retarding of the growth of the bacteria. 



EXERCISE 



PASTEURIZING AND STERILIZING MILK. 



150. Explanatory note. Milk is pasteurized, in the 

 present acceptance of the term, when all of the pathogenic 

 bacteria which it may happen to contain (with the exception 

 of the spores of Anthrax) are destroyed with the more im- 

 portant Saprophytes. It is not necessarily sterile although 

 it sometimes is. The temperature and time for heating is 

 from 60-68 C. for 20 minutes. 



In this exercise it is the purpose to study the effect of this 

 process on the bacteria of milk and to compare its effect with 

 that of sterilization. 



151. General directions. From the fresh milk pro- 

 vided, make 3 agar plates, using i, 2 and 3 loopfuls respect- 

 ively of the milk. Put 20 c.c. in a large tube and set it in 

 the incubator. Put 25 c.c. in each of 4 large test tubes. 

 Sterilize 2 of them by boiling for 30 min. in a closed water 

 bath, and pasteurize the other 2 by heating them in the 

 water bath for 30 minutes at 65 C. It requires about 10 

 minutes for the milk in the tubes to reach the temperature of 

 the water, leaving the milk exposed to the temperature of 

 the water for 20 minutes and cool. 



After the tubes are cooled make 3 agar plates from one of 

 the tubes treated by each process, using i loopful of milk for 



