200 CLEAN MILK 



of milk to be used that will conform to the tube and scale adopted, 

 allowing percentage of casein to be read directly. 3. The precip- 

 itation 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 a 

 pellet. 6. Reading the per cent, of casein. 



Special tubes (Fig. 60) made by E. H. Sargent & Co., Chi- 

 cago, are used. The tubes are 5^4 inches long and graduated so 

 that each division on the scale = 0.2 per cent, of casein when 5 c. c. 

 of milk are used in the test. A centrifuge is used with a revolving 

 wheel 15 inches in diameter and giving 2,000 revolutions per 

 minute. A 20th Century Babcock fat tester, made by the Creamery- 

 Package Co., Chicago, may be employed. It is necessary that the 

 diameter of the machine and the number of revolutions performed 

 shall be exactly as directed. The pockets for the tubes are made 

 especially for the casein tube and the bottom of the pocket is 

 cushioned from the graduated part of the tube by inserting the 

 ends of the graduated tubes into holes bored out for them in corks 

 (see Fig 60). 20 c. c. of glacial acetic acid in 0.25 per cent, 

 solution is used for each tube. The test : Place tubes in a rack, 

 graduated-ends down. Drop 2 c. c. of C.P. chloroform into each 

 tube; then on top of the chloroform, drop 20 c. c. of the acetic acid 

 solution at approximately 70 F. Then add 5 c. c. of milk by 

 pipette at temperature ranging from 65 "-75" F. Shake mixture 

 with thumb over aperture of tube for 15 to 20 seconds by the 

 watch. Place tubes in the centrifuge and rotate. When machine is 

 revolving 2,000 revolutions per minute keep this speed for y l / 2 to 8 

 minutes, and remove the tubes to a rack with graduated ends down. 



To secure correct number of revolutions, buy a metronome at a 

 music store. Then count how many revolutions of the wheel occur 

 for each turn of the crank. Thus 1 turn of crank = 36 revolutions, 

 then 55 turns per minute = 2,000 revolutions per minute. The 

 metronome is set to audibly beating this time, i. e., 55 beats per 



