THE DETECTION OF PASTEURIZED MILK 159 
by centrifugal force, smeared on slides, dried, and fixed with 
formalin. The staining of this dried, fixed film is done 
with a one-half saturated aqueous solution of methylene blue. 
The nuclei of the cells in the raw milk stain well (Fig. 21). 
They appear as if drawn out and there is frequently no indi- 
cation of cell body, the area around the nuclei fusing into the 
background. If a cell body is observed it has a reticulated ap- 
pearance. In pasteurized milk, on the other hand, the nuclei of 
these cells are smaller, more compact, stain deeper, and the cells 
which are considerably smaller, are outlined by a clear, circular 
zone around the nucleus. The difference is noticeable when com- 
paring raw milk with milk heated to 60° for 30 minutes and is 
very marked if the milk has been heated to 63° for 30 minutes. 
Figures 23 to 26 represent preparations made from raw 
milk (23), pasteurized at 58° C. (24), 60° C. (25), and 63° C. 
(26) and stained with Wright’s blood stain. The milk sedi- 
ment was spread on slides, dried, and then stained in the 
usual way. 
Derails oF PROCESS 
Stain of background.—The body of the smear, probably 
the casein or milk serum, takes the stain in a varying degree. 
In raw milk the background is usually found to be dark and 
continuous. By this is meant that the entire smear is stained 
deeper than the nuclei of the leucocytes which are imbedded 
in it. Usually these are not stained at all. Whether it is 
actually dark or light depends very evidently upon the thick- 
ness of the film and this unfortunately cannot be controlled. 
In well pasteurized samples the stain of the background is 
found to be light and variable. By this is meant that the 
stain in the background is lighter than the nuclei of the con- 
tained leucocytes. It is variable in that it is usually darker 
about the leucocytes, forming a more or less distinct ‘‘dark 
halo’’ about them. 
Stain in cells—In raw milk the leucocytes are unstained 
and in the typical preparations appear as clear areas in a 
dark blue field. They are easily differentiated from the fat glob- 
ules by their larger size and more or less irregular outline. 
