6 A DAIRY LABORATORY GUIDE 



the milk adhering to the neck of the bottle 

 will be washed down. The vessel contain- 

 ing the acid should be kept corked in order 

 to prevent the acid taking up moisture from the air 

 and becoming too weak for use. If the acid is too 

 weak more than 17.5 cc. are used, and if too strong 

 less than 17.5 cc. are used. Good acid is colorless, 

 although it may be quite dark and at the same time 

 be useful for the Babcock test. It must, in any 

 case, be free from undissolved foreign particles. 



After adding the acid to the milk the bottles are 

 placed in a centrifugal machine and whirled. Care 

 should be taken to have the machine balanced, i. e., 

 for every bottle on one side of the machine there 

 should be a corresponding bottle on the opposite 

 side of the machine. The bottles are centrifuged 

 for five minutes and then filled with hot water up 

 to the base of the neck. Centrifuging is then con- 

 tinued for two minutes more and hot water is added 

 to bring the fat into the graduated neck. The cen- 

 trifuge is then run for one minute. 



The bottles are then ready to read, and in case 

 of whole milk one should read between the extreme 

 points of the fat column. This method of reading, 

 by comparison with the chemical method, has been 

 found to make up for the fat which remains in 

 the bottom of the bottle and cannot be removed by 

 centrifuging. The bottles should be read at a tem- 

 perature of 120 F. It is no trouble to obtain this 

 temperature if centrifuging has been done with a 

 steam machine. Where the machine is filled with 

 steam the bottles should stand for from thirty 

 seconds to one minute in a room at ordinary tem- 

 perature to allow excess of heat to pass off before 

 reading. If a hand machine is used the tester must 

 have hot water placed in it to obtain the required 



