320 MANUAL OF MILK PRODUCTS, 



reason for this is that too long a contact with chloroform may 

 vitiate the results. No harm, however, results if the tubes 

 after shaking and before centrifuging are allowed to stand 15 to 

 30 minutes, but longer standing than this can introduce an 

 error. The tubes are next placed in the centrifuge with the 

 revolving wheel 15 inches in diameter w^hen the tubes stand ex- 

 tended. The machine should be run closed. After the speed 

 of 2000 revolutions a minute has been attained, the test is run 

 seven and one-half to eight minutes. A slight variation from 

 the above speed will not introduce a serious mistake. A range 

 of 50 revolutions on either side is allowable. The time should 

 be regulated by a watch. 



As soon as the tubes have been properly whirled they should 

 be removed and placed in the rack, which supports them in an 

 upright position. If properly worked, the casein will rest as a 

 sharply defined white mass above the chloroform now holding 

 the fat in solution, while above the casein will be a water-clear 

 solution of the other milk solids. The tubes should be allowed 

 to stand at least ten minutes before reading. This is impera- 

 tive. After suddenly relinquishing the centrifugal force from 

 the casein pellets, a proper time is necessary for them to regain 

 a constant volume. This requires at least ten minutes. After 

 the first ten minutes practically no change in volume results, 

 even on standing 24 hours at room temperature. 

 Measuring the casein. 



Hold the tube in a perpendicular position with the scale on 

 a level with the eye. Observe the divisions which mark the 

 highest and the lowest limits of casein. The difference between 

 them gives directly the per cent of casein in the milk. Read- 

 ings can easily be taken to half divisions or one-tenth of a per 

 cent, and with care and practice even to five-hundredths of a 

 per cent. The lines of division between the casein and chloro- 

 form on the bottom and the dilute acetic acid solution on the 

 top are usually straight lines, and no doubt need arise concern- 



