16 CONSTITUENTS OF MILK 



duce clottable solutions on dispersion by acidification after 

 solution in alkali. Clots produced by the action of rennin 

 cannot be redispersed, a fact that suggests some alteration in 

 structure. Schryver found that calcium caseinogenate solu- 

 tions on warming, and sodium caseinogenate solutions after 

 treatment with carbon dioxide in the cold, would produce clots 

 with rennin and suggested that these observations point to the 

 formation of caseinogen by the action of heat- in the former, and 

 carbon dioxide in the latter, and that clot formation is produced 

 by the action of rennin on the free caseinogen or metacasein- 

 ogen (see p. 7). 



Some observers have stated that a change in reaction occurs 

 during the action of rennin but Hewarden 24 found that hydrogen 

 ions were not necessary for the coagulation of milk or solutions 

 of caseinogen containing calcium. The author has found that 

 the curd produced from milk by rennin usually has an acidity 

 equivalent to 8.3 to 8.8 c.cms. of normal acid per litre of milk, 

 an amount which is identical with the acidity of the caseinogen 

 in the solution from which it is produced. 



Caseinogen is also clotted by the action of trypsin and other 

 enzymes, but in the case of trypsin there is definite evidence of 

 proteoclastic cleavage with the formation of soluble com- 

 pounds containing nitrogen and phosphorous. 



Heating milk to 70 C. and upwards, retards the velocity 

 of the rennin reaction by partial destruction of the enzyme and 

 precipitation of the calcium salts: refrigeration also prevents 

 the formation of the characteristic curd but this property is 

 regained on heating to 37 C. (Morgenrath). 



The optimum reaction temperature for rennin is about 40 C. 

 and at temperatures exceeding this it is gradually weakened and 

 finally destroyed: the destruction by heat follows the law of a 

 monomolecular reaction. The velocity of the rennin reaction 

 follows the usual laws until 40 C. is reached when the observed 

 values become smaller than the calculated values owing to 

 partial weakening of the enzyme by heat. Some of the results 

 obtained by Field on this subject are given in Table IV. 



