24 Milk and Its Products. 



in which the casein exists. Formerly it was sup- 

 posed that the casein is in solution. This idea 

 was brought about by the fact that it is impossible 

 to filter the casein from the milk, even though it 

 is passed through several thicknesses of fine paper. 

 Later, though, it was found that when milk is 

 passed through a fine porcelain filter a certain part 

 is removed, and it was then supposed that there 

 were two forms of casein, one of which was in 

 solution and the other in suspension in very fine 

 particles of a colloidal or gummy character. Still 

 later investigations have shown that in all proba- 

 bility a large part of the casein is in this extremely 

 fine colloidal state. When milk is subjected to 

 the action of weak organic or mineral acids, to 

 rennet or to certain vegetable substances, the casein 

 is precipitated in a flocculent mass. Casein is not 

 acted upon by heat. The albumin of the milk is 

 in all respects similar to blood albumin. It is 

 rendered insoluble by a heat of about 180° F., but 

 it is not acted upon by weak acids or rennet, and 

 in this way it is chiefly distinguished from the 

 casein. The fibrin of milk, if present, is in ex- 

 tremely minute quantities. It is supposed to be 

 the same as blood fibrin, and coagulates upon ex- 

 posure to the air, but is never present in sufficient 

 quantity to form a clot, as in the case of blood. 

 Its coagulation is hindered by a reduction of tem- 

 perature, and it has been supposed that when it does 

 coagulate it forms a sort of network of threads 

 through the mass of milk. 



