METHODS OF TESTING MILK FOR CASEIN 



shade and duration of color every time. Thus one 

 should not in one titration add alkali until a deep pink 

 coloration appears, lasting for some minutes, and then 

 in another, a coloration that disappears within 5 se- 

 conds. 



(6) Calculation of results. The method of obtain- 

 ing the percentage of casein is very simple: Divide 

 by 2 the number of cubic centimeters of acid used (A) 

 and from the result subtract the number of cubic centi- 

 meters of alkali used (B) in neutralizing 100 cc. of 

 filtrate; or, expressed more briefly, divide A by 2 and 

 from the result subtract B. Expressed as a formula 

 this becomes: (A-f-2) B=Per cent, of casein in milk. 



Example: One uses 30 cc. (A) of acid in precipi- 

 tating casein and 11.95 cc. (B) of alkali in neutralizing 

 100 cc. of filtrate (one-half of filtrate from the casein 

 precipitate, corresponding to 9 grams of milk). Sub- 

 stituting 30 for A and 11.95 f r ^ m tne formula, we 

 have (30-7-2) 11.95=15 11.95=3.05 (per cent, of 

 casein in milk). 



Use of preservatives. In making a casein deter- 

 mination by this method, it is desirable when possible 

 to use milk not more than 24 hours old, which has been 

 kept in a cool place. Milk which is sour or which coag- 

 ulates on heating can not be used with satisfactory 

 results. However, by adding to fresh milk powdered 

 mercuric chloride (corrosive sublimate) in the approxi- 

 mate proportion of I part to 1,000 or 1,500 parts of 

 milk, and then keeping the mixture at a temperature of 

 50 F. or lower, one can obtain satisfactoiy results with 

 milk that had been kept 2 to 3 weeks. Milk thus treated 

 should be shaken often enough to keep the fat well 



