I20 LABORATORY BACTERIOLOGY 



EXERCISE LIV 



PASTEURIZING AND STERILIZING MILK 



173. 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 important saprophytes. It is not 

 necessarily sterile, although it sometimes is. The temperature 

 should be from 60° to 68° C. and the time for heating 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). 



In this exercise students may work in small groups. 



174. Work for this exercise. From the fresh milk provided, 

 make 2 agar plates, using i and 2 loopfuls, respectively, of 

 the milk. Put 25 cc. in each of 6 large test tubes and set one 

 in the incubator and leave one at the room temperature. 

 Boil two of them for 30 minutes in a closed water bath, and 

 pasteurize the remaining 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 min- 

 utes. 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, using i loopful of milk for 

 the first plate, 3 loopfuls for the second, and .25 cc. (measure 

 with a graduated pipette) for the third. Place in the incubator 



