540 



MILK 



are removed after the thermometer shows the desired tempera- 

 ture of 145° to 150° F., and the water is then cooled to the same 

 temperature by addition of cold water. The bottles in the 

 basket are placed into the pail again and held for twenty to thirty 



minutes. Then cold water is run 

 into the pail until the bottles 

 have cooled. 



Frost has described a test for 

 determining whether milk has actu- 

 ally been heated to 60° C. The 

 basis of the test depends on stain- 

 ing leukocytes broken down by 

 heating and susceptible to staining. 

 The test is made as follows: The 

 stain is prepared by adding about 

 7 grams of Griibler's dry methyl- 

 ene-blue to 100 c.c. distilled water. 

 After shaking frequently for several 

 hours the solution is filtered. Saf- 

 ranin and Coget's thionin can 

 also be used, but methylene-blue 

 gives the best results. One- part 

 of this stain is added slowly to 5 

 parts of milk so as to prevent co- 

 agulation. After standing for fif- 

 teen to thirty minutes the mix- 

 ture is centrifugahzed and the 

 sediment smeared on a slide. 

 After drying, it is ready for ex- 

 amination. A Stewart-Slack tube 

 answers well for centrifugalizing. 

 In raw milk the entire field is 

 stained a light blue, the intensity 

 of the stain depending upon the 

 thickness of the film. Clear areas 

 consisting of fat globules and 

 leukocytes appear in the blue background. The fat globules are 

 smaller than the leukocytes. Under high power the leukocAi;es 

 are practically colorless. Mononuclear cells are well stained and 

 are not considered, the test depending on the lack of stain of 

 polymorphonuclear cells. In heated milk the background is not 

 as deeply stained as in raw milk. Leukocytes are always more 

 deeply stained than the background, and can be seen under low 

 powers as dark blue areas in a lighter blue field. Under the oil- 

 immersion lens leukocytes are less irregular in outhne than those 



SIDE 



Fig. 221. — Wire basket holding 

 bottles for pasteurization of milk. 

 (Rogers in Circular 197, Bureau of 

 Animal Industry, U. S. Dept. of 

 Agriculture.) 



