MILK. 697 



in lime-water is not precipitated by phosphoric acid, but an opaque fluid is 

 obtained containing casein and calcium phosphate in suspension. The solu- 

 tion is precipitated by alum, zinc sulphate, cupric sulphate, etc. Caseinogen 

 solutions are not coagulated by heat, but, like milk, are covered with a scum. 



Experiment 89. (Preparation of casein.} To a mixture of 400 c.c. of milk 

 and 1 liter of water add gradually enough (but not more) of acetic acid to pre- 

 cipitate the casein, which also carries down the fat. Decant the liquid, then 

 filter, first through muslin, then through paper, reserving liquid for Experi- 

 ment 93. Wash the coagulum well with water, press it as dry as possible, 

 then grind it with 100 c.c. of alcohol, and allow to stand for an hour. Filter, 

 dry the coagulum between filter-paper, place it with 200 c.c. of ether into a 

 stoppered bottle and let stand for a day. Collect the precipitate on a filter and 

 add filtrate to the alcoholic filtrate from above; the mixture will be used for 

 Experiment 92. Rub the casein in a mortar until the ether is evaporated. 

 To purify it from fat, mix with water and add drop by drop a 1 per cent, 

 solution of sodium hydroxide until the greater part of the casein is dis- 

 solved. The mixture, which should not be alkaline after thoroughly stirring 

 it, is then filtered, when some fat and suspended matter is left behind, while a 

 fairly clear filtrate is obtained. Acidify faintly with acetic acid, wash the 

 precipitated casein with water, alcohol, and ether, and dry between paper. 



Tests for casein. 



1. Apply the xanthoproteic, Millon's, and biuret reactions. 



2. Dissolve casein in a 1 per cent, solution of sodium carbonate; 

 neutralize with acetic acid, when casein is precipitated. 



3. Mix some casein in a mortar with freshly precipitated cal- 

 cium carbonate and water. Casein dissolves, while carbon dioxide 

 is liberated. 



4. Dissolve casein in lime-water and add dilute phosphoric acid 

 until the solution is neutral. No precipitate is formed, but the 

 liquid becomes turbid. 



5. Heat a mixture of casein with sodium carbonate and potassium 

 nitrate in a crucible (or dry test-tube) until all organic matter has 

 been destroyed. Dissolve mass in water, acidify with nitric acid, 

 filter, and add ammonium molybdate. A yellow precipitate is formed, 

 showing the presence of phosphorus in the casein. 



Experiment 90. (Separation of the proteins.} Saturate 20 c.c. of fresh milk with 

 powdered sodium chloride: a precipitate consisting of caseinogen and fat is 

 formed. Filter, wash the precipitate with saturated solution of sodium 

 chloride, rub the moist precipitate with 20 c.c. of water, allow to stand for 

 twenty-four hours, and filter. The solution contains caseinogen in the 

 same condition in which it is found in milk. To a portion of the solution 

 add acetic acid : casein is precipitated. To another portion add a solution of 

 rennin and some calcium chloride, heat to 40 C. (104 F.) for a short time, 

 when a precipitate of paracasein is formed. 



Saturate the filtrate from caseinogen and fat with magnesium sulphate : 



