COMPOSITION AND PROPERTIES OF CASEIN. . 33 



or alkaline solution like the other albuminoids, i.e., is pre- 

 cipitated by salts of the heavy metals, and certain complex 

 organic acids, so-called alkaloid reagents, when the acid is in 

 excess. In addition, casein is thrown down by most acid 

 aniline dye-stuffs and a few complex organic acids, but only 

 when the reaction is decidedly acid. 



Potassium alum, in suitable concentration, will throw 

 down the casein in milk, without affecting the other albu- 

 minoids present. With an excess of the salt, however, the 

 precipitate is redissolved. Other important compounds of 

 casein are those formed when milk is coagulated by rennet, 

 whereby the casein is transformed into paracasein. Like the 

 unaltered casein, this substance is readily soluble in alkalies, 

 though its calcium salt is insoluble; hence, when a soluble 

 calcium salt is present in the liquid, insoluble calcium 

 paracaseate or cheese is formed. The coagulation of milk 

 proceeds in two stages, occurring successively, the actual 

 fermentative conversion of the casein by the rennet ferment, 

 and the visible coagulation for which alone the presence of 

 lime is necessary. Paracasein is not precipitated when the 

 soluble calcium salts of the milk have been removed by 

 oxalic acid. Halliburton would confine the name para- 

 casein to the coagulated casein, calling the soluble casein 

 " caseinogen ". 



In its other properties paracasein is identical with casein, 

 but is thrown down more readily than the latter by common 

 salt, so that it can be made to coagulate, in a way, by means 

 of large quantities of salt without any addition of lime. 



No real coagulation occurs when casein is exposed to 

 high temperatures; and the solutions of its salts may be 

 boiled without suffering any alteration. In the dry state, 

 on the other hand, according to Lacqueur and Sackur, it 

 becomes partly insoluble when warmed up to 94-100 C. ; 

 whereas, according to the earlier report by Hammarsten, 



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