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 fiocculent 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. 
