FERMENTATION 197 



curd and whey. The curd is casein together 

 with the fat globules carried down as it precipi- 

 tates. The whey is a dilute saline solution of 

 milk-albumin, milk sugar, etc. 



Test the chemical reaction. The mixture is 

 still neutral. Milk may also be curdled by acid, 

 either added artificially or produced in the milk 

 itself by lactic acid fermentation of milk sugar. 

 The absence of an acid reaction in the above exper- 

 iment excludes precipitation through acid fermen- 

 tation of milk sugar. Casein prepared free from 

 milk sugar is also precipitated by rennin. Finally, 

 rennin, extracted by the method given above, does 

 not act upon milk sugar, but rapidly precipitates 

 casein. 



Analogy suggests that the specific action of 

 the rennin may be the splitting of casein and that 

 the precipitation may be a secondary process. 

 The following experiments determine this matter. 



Experiments of Arthus and Pages. 1 Prepare 

 two solutions, A and B. 



A. 



Milk 100 c.c. 



Neutral oxalate of 



potassium 1 % 5 c.c. 

 Rennin 1 to 250 4 c.c. 



B. 



Milk 100 c.c. 



Neutral oxdate of 



potassium 1 % 5 c.c. 

 Water 4 c.c. 



Arthus and Pages : Archives de physiologic, 1890, p. 334. 



