ON MILK AND MILK-TESTING 53 



small cylinder and transferred to the test bottle, 

 allowing it to run down the side of the neck. Any- 

 adhering milk will be washed down if the bottle is 

 rotated during the process. The acid should run 

 under the milk until all is added. Then mix it 

 quickly and thoroughly by holding the bottle in a 

 slanting position and whirling the body in a circle 

 while the top remains stationary. 



When should the test bottles be whirled in the 

 centrifuge? 



Immediately upon mixing with the acid and 

 while still hot. The test would not be spoiled if 

 the bottles stood for a time before whirling, but it 

 would be necessary to heat them to the proper tem- 

 perature before beginning the operation. 



How long and how often should the bottles be 

 whirled? 



The first whirling should last five minutes. 

 Water is then added until the fat rises to the base 

 of the neck. Then whirl for two minutes to wash 

 the fat free from any undissolved substance. Hot 

 water is again added until the bottom of the fat 

 column is some distance above the zero mark. Then 

 the final whirling is given for one minute. 



How should the water be added? 



Hot, with some form of pointed tube or pipette 

 and allowed to fall directly on the fat to wash out 

 undissolved particles. In the Mitchell-Walker test 

 the water enters the bottles from a revolving water 

 centrifuge while the machine is running. 



