112 PHYSIOLOGY OF ALIMENTATION. 



a solid mass which contracts after a time and squeezes out 

 a yellowish fluid the whey. The clot consists of casein 

 in the meshes of which there is found the fat of the milk. 

 The whey contains lactalbumin and lact globulin, two pro- 

 tein bodies which have nothing to do with the curdling of 

 the milk, together with the sugar of the milk and most 

 of its salts. 



In the coagulation of milk during ordinary digestion three 

 substances are involved caseinogen, caseinase, and certain 

 salts. It was first shown by HAMMARSTEN that calcium 

 salts play an important role in the curdling of milk. As 

 will be shown immediately, however, calcium represents 

 only one of a large number of salts which can influence 

 the coagulation of milk. Various theories have been pro- 

 posed from time to time to explain fehe interaction of the 

 three substances. These have in the main been of a chem- 

 ical nature, and have assumed the production of true chem- 

 ical combinations between caseinogen and calcium, etc. 

 The recent work of CONRADI and of LOEVENHART speak 

 against this idea and indicate that in the curdling of milk we 

 are dealing with a physical process in which the colloid casein- 

 ogen which exists in ordinary milk in a liquid or sol state is 

 converted into the solid or gel state which we term casein. 

 Caseinogen and casein (casein and paracasein of HAMMAR- 

 STEN) are therefore the same chemically, but different physi- 

 cally in that the former consists of small particles suspended 

 in the liquid portion of the milk, while the latter represents 

 these same particles clumped. The passage from the sol to 

 the gel state is influenced by the ferment caseinase and cer- 

 tain salts. 



Casein can be precipitated by all methods used for this 

 purpose much more easily than can caseinogen. But if 

 the proper concentration is employed the latter is brought 

 down also. It is concluded from this that casein exists in 

 a coarser state of aggregation than caseinogen. Nor can 

 the differences which have been said to exist between case- 





