TESTS USED IN CHEESE-MAKING 449 



CENTRIFUGAL TEST FOR CASEIN 



The following centrifugal method has been worked 

 out at the Wisconsin station by Hart: 



Apparatus and reagents. (i) Testing-tube, with 

 neck so graduated that each division represents 0.20 

 per cent when one uses 5 cc. of milk. (2) Centrifuge, 

 of special form, run by hand, having a wheel 15 inches 

 in diameter and geared to give a speed of 2,000 revo- 

 lutions a minute. (3) Pipette for measuring 5 cc. of 

 milk. (4) Cylinder for measuring 2 cc. of chloroform. 

 (5) Dilute acetic acid containing 0.25 per cent of 

 acetic acid, prepared by diluting 10 cc. of glacial acetic 

 acid (99.5 per cent) to 100 cc. with water, and then 

 diluting 25 cc. of this solution to one liter. (6) Chloro- 

 form of the best quality. 



Method of operating test. In a testing-tube one 

 puts 2 cc. of chloroform, then on top of this 20 cc. 

 of the dilute (0.25 per cent) acetic acid. One then 

 runs in 5 cc. of milk (65 to 75 F.), after which the 

 thumb is placed over the opening of the tube and the 

 tube inverted to bring the mixture into the barrel- 

 shaped portion of the tube; then the whole is shaken 

 with a fair degree of vigor for 15 or 20 seconds, ac- 

 curately timed with watch before one. In the shaking 

 process, the chloroform takes up most of the fat and 

 the acid precipitates the casein in fine particles. The 

 sample is then ready for the centrifuge and should be 

 whirled within 30 minutes. The centrifuge is closed 

 before whirling and is brought to a speed of 2,000 

 revolutions a minute, after which it is run at this rate 

 7j^ to 8 minutes. This must be done with such pre- 

 cision that it is important to use a metronome during 

 the operation. After whirling, the testing-tubes are 



