34 



Testing Milk and Its Products. 



been added that the milk has a dark yellow or reddish 

 color, the mixture of milk and acid will turn greenish 

 black, and a complete solution is rendered extremely 

 difficult on account of the toughening effect of the bichro- 

 mate on the precipitated casein. An indistinct separa- 

 tion of the fat is also sometimes obtained in such samples, 

 but this difficulty can generally be overcome by using a 

 little less than the regular quantity of acid. 



39. Whirling bottles. After the milk and the acid 

 have been completely mixed, the test bottle is at once 

 placed in the centrifugal machine or tester and whirled 

 for four or five minutes at a speed of 600 to 1200 revo- 

 lutions per minute, the proper speed being determined 

 by the diameter of the tester (66). It is not absolutely 

 necessary to whirl the test bottles in the centrifuge as 

 soon as the milk and the acid are mixed, although this 

 method of procedure is much to be preferred ; they may 

 be left in this condition for any reasonable length of time 

 (24 hours, if necessary) without the 



test being spoiled. If left until the 

 mixture becomes cold, the bottles 

 should, however, be placed in warm 

 water (of about 160 F.) for about 15 

 minutes before whirling. 



Four minutes at full speed is suffi- 

 cient for the first whirling of the test 

 bottles in the centrifuge; this will 

 bring the fat to the surface of the 

 liquid in the bottle. 



40. Adding water. Hot water is now added by means 

 of a pipette or some special device (Fig. 10), until the 

 bottles are filled to near the scale on the neck (80). The 



FIG. 10. Oil stove for 

 heating water. 



