162 UNIVERSITY OF WISCONSIN STUDIES 



change in staining reaction between the leucocytes of raw 

 and pasteurized milk takes place at the temperature of 63 C. 



When milk is subjected to this temperature for varying 

 lengths of time it is found that the definite staining reaction 

 takes place when the temperature is maintained for about 

 twenty minutes. When the time of exposure is eighteen min- 

 utes or less, some of the leucocytes do not take the stain in 

 the characteristic way. So it may be stated that the character- 

 istic staining reaction occurs when the milk is subjected to 

 a temperature of 60 to 63 C. for upwards of twenty minutes. 



Effect of stain on bacteria. The heat which fixes the 

 nuclei of the leucocytes for staining curiously renders the 

 bacteria less likely to take the stain. It is only in raw milks 

 then that the bacteria stain well. In well pasteurized milks 

 they stain poorly. Frequently the bacteria take the stain with 

 a varying degree of intensity, but whether this is due to in- 

 herent differences in the bacteria or to the effect of the heat 

 on them is undetermined. The reaction of the bacteria to 

 the stain has up to now shown illogical variations, but in 

 general the following conclusions seem warranted. The 

 bacteria in raw milk are always stained, usually well and 

 sometimes very beautifully. In the sediment from properly 

 pasteurized milk examined within a few hours after pasteuri- 

 zation the bacteria are usually invisible. Old pasteurized 

 milk may show the nuclei of the cells well stained and the 

 bacteria also. In this case we assume that the bacteria which 

 stain have been introduced or grown subsequent to the pro- 

 cess of pasteurization. 



THE MICROSCOPICAL PICTURE OF A PROPERLY PASTEURIZED MILK 



When a milk has been stained by the Wet Process described 

 in this paper and the sediment mounted without the addition 

 of water is examined under the microscope, the following 

 picture will be obtained if it has been pasteurized at a tem- 

 perature of 63 C. (145 F.) or above, and held at that 

 temperature for twenty minutes or more. The nuclei of prac- 

 tically all of the leucocytes are in compact masses. They 

 are well stained. The background is lighter than the nuclei 



