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 



10.0 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 has 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 



