18 SCIENCE AND PRACTICE OF DAIRYING. 



different, and that in milk there is only one albuminoid, viz., casein. 

 According to him, the changes which the milk undergoes, as above 

 described, are to be accounted for by the fact that casein in solution, 

 and when precipitated, acts differently. Lactoprotein and albumin arc, 

 as Duclaux assumes, nothing else than casein in conditions more or less 

 soluble in water. 



Among the most recent investigations on the nature of the nitrogenous 

 substance of milk, undoubtedly the most valuable work is that by Soldner, 

 entitled, The Salts of Milk and their Relations to the Conditions of Casein. 



Soldner opposes to Hammarsten's vaguely expressed theory that casein 

 and calcium phosphate are present in the milk in solution, the exact and 

 well authenticated theory that the caseous substance of the milk must be 

 regarded as consisting of a neutral calcium compound of casein, and that 

 the action of the rennet does not depend on the presence of calcium phos- 

 phate, but chiefly on the presence of a soluble lime salt. Further on, in 

 the Chapter on the Preparation of Casein, we will have an opportunity of 

 again referring to Soldner's work. 



Within the limits of to 100 C., the amount of acid or neutral salts 

 which is necessary to effect the precipitation of casein, decreases with an 

 increase of temperature ; while within the limits of and 42 C., the 

 length of time which elapses before the spontaneous coagulation of the 

 milk takes place also decreases with the increase of temperature. Normal 

 sodium carbonate, caustic alkalies, normal sodium phosphate, and other salts, 

 which effect the precipitation of solutions of calcium phosphate, although 

 they are themselves solvents of casein, yet in the process of coagulation 

 cause its precipitation. This is effected by the fat and casein becoming 

 mechanically entangled with the precipitated tricalcium phosphate, and 

 carried down with it. The addition to milk of a small quantity of a 

 caustic alkali, or of a carbonate of the alkalies, diminishes its opacity. 



Solutions of caseous matter, on standing at temperatures of over 50 C., 

 become covered with a skin, and when heated in close air-tight vessels to 

 130 to 140 C. become coagulated, and exhibit greater laevo-rotatory pro- 

 perties than solutions of albumin; and are precipitated by dilute acids, by 

 most of the salts of the heavy metals, by alcohol, and by rennet, provided 

 the dissolved calcium salts necessary for this purpose are present. The 

 heat equivalent of casein, according to Stohmann's investigations, amounts 

 to 5715 calories per gram of substance. Schiibler gives the specific gravity 

 of fresh casein as I'lOO, and of boiled casein as 1-259. According to the 

 investigations of the author, the pure nitrogenous matter of milk at 15 C. 

 has a specific gravity of 1'486. 



Of equal interest, both from a theoretical and practical point of view, 

 is the relationship which exists between the nitrogenous constituents of 



