ON MILK AND MILK-TESTING *jy 



How may an acetic acid solution of proper strength 

 be made? 



Add to lo c. c. of pure glacial acetic acid 90 c. c. 

 of water. Take 25 c. c. of this solution and make 

 it up to 1,000 c. c. by the addition of water. The 

 solution then contains 0.25 per cent of acetic acid. 



How is the Hart casein test carried out? 



Add 2 c. c. of the chloroform, 20 c. c. of the dilute 

 acid, and 5 c. c. of the milk, in the order named, to 

 the test tube. The temperature of the milk and 

 acid solution must be within 5° of 70° F. A lower 

 temperature tends to give a higher reading, and a 

 higher temperature has the reverse eflfect. The 

 thumb is placed over the opening and the tube 

 inverted several times and shaken with some vigor 

 for not more than 20 nor less than 15 seconds. The 

 agitation must be just sufficient to thoroughly mix 

 the contents and yet not form an emulsion. The 

 tubes may be whirled in the centrifuge at once or 

 may stand 20 to 25 minutes before whirling, if 

 necessary. The speed of a revolving disk 15 inches 

 in diameter should be approximately 2,000 revolu- 

 tions per minute, and should continue seven and 

 one-half to eight minutes. If the test has been 

 properly made there will be found in the bottom of 

 the tubes on taking them from the centrifuge a 

 layer of the chloroform fat solution and immedi- 

 ately over it the layer of casein. Allow the tubes 

 to stand 10 minutes after removing from the ma- 

 chines to allow the casein to come to a constant 

 volume. Then read the test. 



