34 Testing Milk and Its Products. 



Colostrum milk or milk from fresh cows will form a 

 violet colored mixture with the acid, due to the action 

 of the latter on the albumen present in such milk in 

 considerable quantities (23). 



When milk samples are preserved by means of potas- 

 ium bichromate (190), and so much of this material has 

 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, bi- 

 chromate on the precipitated casein. The difficulty is 

 still more pronounced with milk preserved with for- 

 maldehyd. 



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, according to 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 condi- 

 tion 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 sufficient for the first 

 whirling of the test bottles in the centrifuge; this will 

 brin all fat to the surface of the liquid in the bottle. 



