METHODS OF TESTING MILK FOR CASEIN 195 



milks give satisfactory results with 30 cc. of acid. In 

 case of milk containing 3.5 to 4 per cent, of casein, 35 

 to 45 cc. of acid may be needed. The amount of acid 

 may be 3 to 5 cc. in excess of that required to effect 

 complete precipitation without affecting appreciably the 

 accuracy of the results, provided the temperature of 

 the mixture is below 80 F. In working at tempera- 

 tures under 65 F., the casein separates more slowly 

 or requires more acid to separate promptly; in such 

 cases, it is well to use for dilution water that is at a 

 temperature of 80 to 85 F. 



(4) Filtration of casein. After the casein is com- 

 pletely precipitated, pure, soft water (preferably, dis- 

 tilled) is added until the 200 cc. is reached. The 

 flask is then vigorously shaken 10 or 15 seconds, in 

 order to make the distribution of acid through the mix- 

 ture as uniform as possible. The contents of the flask 

 are then poured on a dry filter and the filtrate caught 

 in a cup. The funnels, filters and cups should all be 

 dry before being used. It is well generally to allow the 

 filtration to continue until practically all of the liquid 

 has run into the cup. Two points deserve attention in 

 this connection, (i) the rapidity of filtration and (2) 

 the appearance of the filtrate. 



(a) Rapidity of filtration. The usual time of filtra- 

 tion should not exceed 3 to 5 minutes. The rapidity 

 depends upon the temperature of precipitation and the 

 completeness of the separation of casein. In general, 

 the higher the temperature of the mixture when pre- 

 cipitated with acid, the more rapid should be the filtra- 

 tion, other conditions being uniform. In case of insuffi- 

 cient acid, the filtration is slower. 



