114 LABORATORY BACTERIOLOGY 



EXERCISE LIX 



PASTEURIZING AND STERILIZING MILK 



169. Milk is pasteurized, in the present acceptance pf 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 important saprophytes. It is not 

 necessanly 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. 



In the generally accepted use of the term, milk is sterilized 

 when it has been boiled. Milk, however, is a difficult substance 

 to sterilize, so that it occasionally happens that milk which has 

 been boiled for from 5 to 10 minutes still contains living organ- 

 isms (spores). 



170. Work for this Exercise. — From the fresh milk pro- 

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

 tively, of the milk. Put 25 c.c. in each of 2 large test tubes and 

 set one in the incubator and leave the other at room temperature. 

 Put 25 c.c. in each of 4 large test tubes. Sterilize two of them 

 by boiling for 30 minutes in a closed water bath, and pasteurize 

 the other two 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. It 

 should be cooled quickly by standing the tubes in cold water. 



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

 the tubes treated by each process, usiHg-^:-4eopful of milk for 

 the first plate, 3 loopfuls for the second, and ^ c.c. (measure 

 with a graduated pipette) for the third. Place one of the tubes 

 of milk treated by each process with the plate cultures, in the 



