THE NITROGENOUS MATTER IN MILK. 15 



excess of the others, and forming about 80 per cent of the total nitro- 

 genous compounds. Casein contains nuclein, a substance which is 

 not found in albumin, and which is characteristic of the cell nucleus. 

 It is rich in phosphorus, and strongly resists the action of pepsin 

 solutions. While it has the properties of an acid it is also able to 

 form saline compounds with bases, and is insoluble in water. On 

 the other hand, its compound with lime (calcium oxide) in which 

 form it is present in milk is soluble in water, or, more correctly 

 speaking, forms with water a bulky colloidal substance, which, when 

 milk is filtered through porous clay cells, does not pass into the 

 nitrate, and is not absorbed when milk is passed through porous 

 clay plates (Lehmann plates). The other albuminoids present in 

 milk are in true solution, i.e. when milk is filtered through porous clay 

 cells they pass into the filtrate. In order to distinguish the casein 

 present in milk, which is in combination with lime, from pure casein, 

 it is called the caseous matter of milk. A very small portion of this 

 caseous matter, at most from *5 to 1 per cent, is removed from the 

 milk in the separators by centrifugal force, and forms the chief 

 constituent of the separator residue. When milk spontaneously 

 becomes sour, or is coagulated by the addition of acids, the lime 

 which it contains is separated from the caseous matter, and the 

 insoluble casein coagulates in the form of a clot. Under the action 

 of rennet, casein is converted into paracasein and curd protein. The 

 former, provided there is a sufficiency of lime salts present to effect 

 precipitation, is precipitated, and the latter remains in solution. In 

 both cases the clots thus formed enclose mechanically the particles 

 of fat present in the milk. When milk is coagulated by rennet, or 

 by the addition of substances which act as dehydrating agents, as, 

 for example, neutral salts or alcohol, the precipitate thrown down 

 contains not merely the fat, but also the calcium phosphate in sus- 

 pension in the milk. If, on the other hand, milk is coagulated by 

 acids, or is allowed to become spontaneously sour, the greater portion 

 of the suspended mineral salts is left in solution, and the coagulated 

 casein contains only minute quantities of calcium phosphate. 



The extent to which the caseous matter is precipitated varies in 

 the case of milk derived from different sources. Even in the same 

 sample of milk the caseous matter is not coagulated to the same 

 extent, even although the conditions under which coagulation takes 

 place are similar. As a rule, the coagulation obtained is greatest 

 immediately after milking, and diminishes with the lapse of time. 



