THE DETECTION OF PASTEURIZED MILK 155 



usually allowed this to stand in the 37 C. incubator for sev- 

 eral hours (over night) and then filtered it through paper. 

 Such a stain when added to milk, part for part, should pro- 

 duce a " cadet blue". Some powdered dyes may need to be 

 used in greater concentration, others, especially the medicinal 

 methylene blue, in smaller quantities. 



This solution seems to be quite permanent and most of my 

 evidence indicates that it keeps indefinitely. 



Mixing stain and milk. The addition of the dye to the 

 milk can be done in any convenient way. At first I put 

 the milk in a flask or beaker and added concentrated stain 

 slowly while the milk was kept moving by a rotary motion 

 of the vessel. When a weaker solution as recommended above 

 is used, the method of combining them is quite immaterial 

 except that it is never safe to drop the milk into the dye. It 

 is better to add the dye to the milk. The mixing should be 

 thoroughly done. 



Time of action of stain. The action of the stain requires 

 several minutes and it has been my practice to mix the stain 

 with the milk and then allow it to act for ten or fifteen minutes 

 before it is centrifuge*!. A longer action up to one hour is not 

 injurious, but if the stain is allowed to stand too long in con- 

 tact with the milk the cells of the raw milk may take on the 

 stain, but even under these conditions a careful study of such 

 cells would enable one to recognize the difference between 

 them and the cells of properly pasteurized milk. 



Centrifugation. Any form of centrifuge tube can be 

 used for collecting the sediment. The most convenient are 

 those which have a rubber stopper at the lower end because 

 of the ease with which the sediment can be removed. At first 

 I used the Stewart-Slack tubes, but lately I have made a 

 larger tube of the same style. In this way I always get 

 an abundance of cells for examination, and avoid the use 

 of a special head on the centrifuge. The tubes are made from 

 three-eighth inch glass tubing and are four inches long. One 

 end is plugged with a rubber stopper (No. 00), the other is 

 left open. These tubes can be put into the ordinary head of 

 any centrifuge. The centrifuge should be run long enough 



