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 centrifuged. 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 
