EXAMINATION OF MILK. 17 



small quantity of calcium chloride, coagulation takes place 

 at once. 1 



(6) Influence of Acids and Alkalies on the Coagulation of 

 Milk by Rennin. Into each of three test-tubes A, B, and C 

 put 10 cc. of milk. To B add 10 drops of dilute hydrochloric 

 acid (1 cc. of hydrochloric acid, sp. gr. 1.183, to 150 cc. of 

 water). No separation of casein should take place. To C 

 add 1 to 2 drops of a concentrated solution of sodium carbon- 

 ate. To A add neither acid nor alkali. To each of the three 

 tubes add one-half a cubic centimeter or 10 drops of the 

 rennin solution and note the order in which the coagulation 

 takes place. The milk in B coagulates first, then that in A. 

 The milk in C does not coagulate at all or coagulates ex- 

 tremely slowly. Acids aid the rennin coagulation; alkalies 

 interfere with it or prevent it entirely. The coagulation of 

 milk in the stomach results from the simultaneous action of 

 the hydrochloric acid and the rennin ferment. The experi- 

 ment may also be performed in another way. Dilute the 

 rennin solution so that one-half of a cubic centimeter or 10 

 drops of it will just bring about the coagulation of 100 cc. 

 of milk in ten minutes or will no longer coagulate it. Now 

 add to 10 cc. of milk 10 drops of the dilute hydrochloric acid 

 (0.25 per cent.) and then 10 drops of the diluted rennin solu- 

 tion. Coagulation results before the expiration of ten min- 

 utes. 



V. LACTIC ACID FERMENTATION. 2 



To a solution of 50 g. of cane-sugar in 500 cc. of water 

 add 20 g. of precipitated chalk and about 30 cc. of sour milk, 

 and let the mixture stand, in an open flask or one loosely 



1 Arthus and Pages, Arch, de Physiologic, 1891, pp. 331 and 540. 



2 For a better method of preparing lactic acid see Die Zersetzung stick- 

 stofff reier organischen Substanzen durch Bacterien von Dr. O. Emmerling, 

 p. 32. O. 



