[Qo MILK 



Estimation of Casein. 



Casein (or caseinogen) is present in milk, not in ordinary 

 solution but probably in a state of colloidal suspension, since it can 

 be completely removed from milk by filtering the latter through 

 unglazed porcelain. It is combined with calcium salts, and these 

 calcium casein compounds are precipitated by the action of 

 rennet on milk. A somewhat similar action takes place 

 on saturating milk with various metallic salts, such as MgS04, 

 NaCl, etc. Coagulation of milk with acids, however, results in the 

 throwing down of casein free from lime salts. 



Casein possesses acidic properties, in that it dissolves in dilute 

 solutions of alkalies, and replaces carbon dioxide from metallic 

 carbonates. 



To 10 grams of milk are added, with constant 

 stirring, about lO c.c. of a saturated solution of MgSO^ 

 in water, and solid MgSO^ is then added until saturation 

 of the whole volume of liquid is attained. 



The precipitated casein is filtered from the whey, and 

 is washed several times with saturated MgSO^ solution. 

 The solid is then washed into a Kjeldahl flask, and the 

 percentage of nitrogen estimated as usual, by heating 

 with concentrated sulphuric acid, and distilling after 

 rendering alkaline. 



The proportion of nitrogen in casein being 15-65 per 



cent., the factor 6-38 f = — ^ j is employed for convert- 

 ing the percentage of nitrogen into percentage of casein. 



Analysis of Sour Milks. 



When milk is kept for some time, a certain quantity of lactic 

 acid is produced by fermentation of the milk sugar, and the milk 

 becomes acid or " sour." The lactic acid causes a coagulation of 

 the casein, and the ordinary methods of analysis cannot be 

 conveniently applied directly to the coagulated sample. 



A good method of dealing with milk in this condition is that 

 proposed by WeibuU. 



