148 MANUAL OF MILK PRODUCTS 



Microscopic test for heated (pasteurized) milk. (Frost and 

 Ravenel.) 



About 15 c.c. of milk are centrifuged for five minutes, or 

 long enough to throw down the leucocytes. The cream layer 

 is then completely removed with absorbent cotton and the 

 milk drawn off with a pipette, or a fine-pointed tube attached 

 to a Chapman air pump. Only about 2 mm. of milk are left 

 above the sediment which is in the bottom of the sedimentation 

 tube. 



The stain, which is an aqueous solution of safranin 0, soluble 

 in water, is then added very slowly from an opsonizing pipette. 

 The important thing is to mix stain and milk so slowly that 

 clotting does not take place. The stain is added until a deep 

 opaque rose color is obtained. After standing three minutes, 

 by means of the opsonizing pipette, which has been washed 

 out in hot water, the stained sediment is then transferred to 

 slides. A small drop is placed at the end of each of several 

 slides and spread by means of a glass spreader, as in Wright's 

 method for opsonic index determinations. 



In an unheated milk the polymorphonuclear leucocytes have 

 their protoplasm slightly tinged or are unstained. 



In heated milk the polymorphonuclear leucocytes have their 

 nuclei stained. In milk heated to 63° C. or above, practically 

 all of the leucocytes have their nuclei definitely stained. When 

 milk is heated at a lower temperature, the nuclei are not all 

 stained above 60° C. The larger number, however, are stained. 



