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. 



DETAILS 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. 



