THE DETECTION OF PASTEURIZED MILK 161 
used. When a temperature of 63° C. (145° F.) is maintained, 
the average diameter of the cells is only slightly greater than 
half the diameter of the cells in raw milk. It is evident from 
Figure 14 that the shrinking is less at 60° C. (140° F.) than 
it is at 63° C. The shrinking continues with the increase 
of heat. See Figures 15 to 18. 
The table below gives the comparative size of the milk cells 
in raw and pasteurized samples. All of the figures given are 
the average of at least twenty-five cells. 
SIZE OF MILK CELLS 
Raw Pasteurized 
11.2 5.6 
9.8 6.0 
10.4 7.5 
10.9 7.0 
12.9 6.4 
11.9 7.2 
11.8 6.8 
11.0 6.2 
13.0 5.7 
12.0 6.0 
Grand average 11.4 6.4 
Temperature at which change takes place.—The staining 
reaction typical of properly pasteurized milk occurs only 
when milk hag been subjected to a particular temperature 
for a definite length of time. With the time constant, say 
thirty minutes, no change takes place in the leucocytes until 
the temperature nears 60° C. It is true that the nuclei begin 
to absorb the dye at temperatures from 56 to 60° C. At first 
the nuclei stain only lightly and there is an almost invariable 
tendency for them to be greenish in color even at 60° C. It 
is usually possible to differentiate milk pasteurized at 60° C. 
from that pasteurized at 63° C. and above, by the way the 
nuclei take up the stain. It is only when a temperature of 
63° C. is reached that the nuclei become deeply stained. Be- 
yond this temperature there is no change in the character of 
the stain although the nuclear fragments may be more com- 
pact and discrete and the size of the cell reduced. The definite 
11 
