THE CONSTITUENTS OF MILK 17 



lime water and diluted alkalies. The separation depends 

 only and solely upon the breaking down of the casein 

 calcium compound. By rennet (chymosin, etc.) casein 

 is precipitated as flakes (human milk and donkey milk), 

 or as a firm gelatinous mass {e.g. cow's milk). The dif- 

 ferent modes of precipitation do not appear to come 

 from the differences in the casein, but are due to the dif- 

 ferent salt content of the different kinds of milk. Under 

 the influence of chymosin, casein is transformed into 

 paracasein, which in its compound with lime is insoluble 

 in water and, therefore, may be precipitated in this way. 



The action of the rennet ferments is quite different 

 from the precipitation of casein by acids. "With the pre- 

 cipitation of paracasein, an albumose-like proteid re- 

 mains in solution (whey proteid), so the action of the 

 rennet ferment is to split the casein. By the growth 

 of bacteria in milk, the casein is often thrown out of 

 solution, either as a result of the formation of acids 

 or by the chymosin-like effect of ferments produced by 

 bacteria. Sometimes the separation occurs from the 

 joint action of the ferments and the acids that are 

 formed. In pepsin digestion, casein is dissolved with the 

 formation of albumoses (caseoses) and paranuclein, 

 which is rich in phosphorus. In the organism, para- 

 nuclein is dissolved by the pancreatic .ii^ice, is absorbed, 

 and is excreted with the urine as phosphoric acid. 



It has often been said that casein, as it is found in 

 the milk of different animals, is not the same and, as 

 evidence of this, reference has, incorrectly, been made 

 to the different ways in which casein separates from 

 milks acted upon by rennet, A more important indica- 

 tion, although the observation requires confirmation, is 

 in the claim that in the pepsin digestion of human milk 

 no paranuclein is formed. According to Hammarsten's 

 thorough work, casein appears to show no chemical dif- 



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