PASTEURIZING AND STERILIZING MILK 105 
EXERCISE XLIV 
PASTEURIZING AND STERILIZING MILK 
159. 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. 
160. Work for this exercise. From the fresh milk provided, 
make 2 agar plates, using 1 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 1 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 
