186 



MILK 



"3. The precipitation of the casein by dilute acetic acid. 



"4. The agitation of the precipitate with chloroform to remove 

 the fat. 



"5. The application of a definite centrifugal force in order 

 to mass the casein into the pellet. 



"6. Reading the percentage of casein." 



The test is carried out in the following manner: A suitable 

 rack is provided to hold the tubes and 2 c.c. of pure chloroform 

 poured into each tube from a buret. On top of the chloroform are 

 poured 20 c.c. of a 0.25 per cent, acetic acid solution. For con- 

 venience a 10 per cent, solution of 99.5 per cent, acetic acid is kept 

 in stock. Previous to making the test 25 c.c. of this stock solu- 

 tion are diluted to 1 liter with water. The temperature of the 



Fig. 67. — A convenient rack for supporting the tubes. (Bull. 156, November, 

 1907, Univ. of Wis. Agri. Exp. Sta.) 



acetic acid should be 21° C. (70° F.), but a variation of 5 degrees 

 on either side is not material. The next step is to measure into a 

 pipet exactly 5 c.c. of the milk sample. After this is placed in the 

 tube the mouth of the tube is closed with the thumb and the chlo- 

 roform brought down into the barrel by rotation. The tube is 

 then shaken vigorously for fifteen to twenty seconds. The chlo- 

 roform is broken up and comes in contact with the fat, which is 

 dissolved. After shaking, the tubes should not be allowed to stand 

 for more than fifteen to thirty minutes. If a large series of tests 

 is to be made, the chloroform and acetic acid should be placed in 

 all tubes before the milk sample is added. The tubes are then 

 placed in a centrifuge and whirled for seven and a half to eight 

 minutes, counting from the time when a speed of 2000 revolutions 



