EXAMINATION OF MILK. 15 



III. PRECIPITATION WITH MAGNESIUM SULPHATE. 



I 



r ~i 



Precipitate: casein (sodium) Filtrate: milk-sugar, albumin, 



and fat. salts. 



Casein may also be precipitated from milk, presumably 

 in combination with alkali, in a form soluble in water or 

 dilute salt solutions. The precipitation of the casein in this 

 form results from the saturation of the milk with different 

 salts, e.g., magnesium sulphate. Saturate 100 cc. of milk 

 with magnesium sulphate (about 50 g. are required) by shak- 

 ing it with the finely powdered salt in a flask. When the 

 salt is entirely or almost entirely dissolved, filter through a 

 filter moistened with a saturated solution of magnesium sul- 

 phate, and wash several times with magnesium sulphate 

 solution. The precipitate contains casein and fat besides a 

 globulin-like body (lactoglobulin) present in milk in very 

 small quantity. In the filtrate we may prove the presence 

 of albumin by heating to boiling. The well-washed and still 

 moist precipitate of casein and fat is ground in a mortar with 

 100 cc. of water, when the casein goes into solution. The 

 mixture is allowed to stand till next day, when the fat will 

 have come to the top ; it is then clarified by filtering several 

 times. We thus obtain a light-bluish opalescent solution, 

 scarcely ever entirely clear, which, however, may be used to 

 determine the amount of rotation. The solution is Isevo- 

 rotatory. On the addition of acetic acid the casein is precipi- 

 tated. 



IV. ACTION OF RENNIN ON MILK. 



(a) Coagulation of Milk. One-tenth (0.1) gram of the 

 commercial rennet powder is dissolved in 100 cc. of water. 

 The solution is usually somewhat cloudy, but it may be used 



