PROTEINS 287 



of saturated ammonium sulphate. The precipitate consists main- 

 ly of caseinogen. In this respect caseinogen resembles the glob- 

 ulins. The member of this group occurring in milk also is pre- 

 cipitated with caseinogen, but its amount is extremely small. 

 Filter the mixture. Divide the filtrate from the caseinogen into 

 two portions. Heat one to boiling, acidifying slightly if neces- 

 sary. The slight precipitate is lactalbumin. Saturate the sec- 

 ond portion with ammonium sulphate. Albumin is precipitated. 



55. The usual method for the preparation of caseinogen con- 

 sists in precipitating with dilute acetic acid. This process is 

 analogous to the clotting of sour milk ; by the action of bacteria 

 milk sugar is fermented. The resulting lactic acid, when present 

 in sufficient concentration, causes the caseinogen to precipitate. 

 Dilute 25 c.c. of milk with three times its volume of water, warm 

 slightly (about to body temperature) and add 1% acetic acid 

 drop by drop, stirring and allowing a short interval to elapse 

 between the addition of successive drops. Caseinogen flocks out 

 as a heavy white precipitate. Add acid until the supernatant 

 liquid is clear. Filter and save both the filtrate (x) for use in 

 56 and the precipitate. Dissolve the precipitate in 2% sodium 

 carbonate, and reprecipitate with acetic acid. This reprecipita- 

 tion is for the purpose of freeing the caseinogen from fat, which 

 is carried down mechanically. It may be repeated several times 

 if a purer product is desired. Redissolve the caseinogen in 2% 

 sodium carbonate, filter through a wet filter to remove more of 

 the fat, and with this solution perform the biuret, Millon, and 

 xanthoproteic tests, and the test for loosely combined sulphur. 

 The presence of phosphorus in caseinogen already has been ob- 

 served. 



56. The filtrate (x) from the caseinogen prepared in 55 con- 

 tains lactalbumin and lactoglobulin, whose presence in milk was 

 demonstrated in 54. The filtrate (x) is already acid. Boil it 

 and add 2% sodium carbonate drop by drop until nearly neutral. 

 If the liquid becomes alkaline, reacidify with a few drops of 

 acetic acid. This treatment removes lactalbumin and lactoglobu- 

 lin. Filter from the precipitate and test the filtrate with Bene- 



