MILK. 219 



milk is acid in reaction, through the inception of lactic acid fer- 

 mentation, or from any other cause, no film will form when heat 

 i applied, but instead a true coagulation will occur. When milk 

 is boiled certain changes occur in its odor and taste. These changes, 

 according to Rettger, are due to a partial decomposition of the milk 

 proteins and are accompanied by the liberation of a volatile sul- 

 phide, probably hydrogen sulphide. 



The milk-curdling enzymes of the gastric and the pancreatic 

 juice have the power of splitting the caseinogen of the milk, 

 through a process of hydrolysis, into soluble casein and a peptone- 



FIG. 74- 





NORMAL MILK AND COLOSTRUM. 

 a, Normal milk ; b, Colostrum. 



like body. This soluble casein then forms a combination with the 

 calcium of the milk and an insoluble curd of calcium casein or 

 casein results. The clear fluid surrounding the curd is known 

 as whey. 



The most pronounced difference between human milk and cow's 

 milk is in the protein content, although there are also differences 

 in the fats and likewise striking biological differences difficult to 

 define chemically. It has been shown that the caseinogen of human 

 milk differs from the caseinogen of cow's milk in being more diffi- 

 cult to precipitate by acid or coagulate by gastric rennin. The 

 casein curd also forms in a much looser and more flocculent manner 

 than that from cow's milk and is for this reason much more easily 

 digested than the latter. Interesting data relative to the composi- 

 tion of milk from various sources, may be gathered from the fol- 



