i66 FERMENTATION IN MILK 



4. The Coag-ulative Fermentations of Milk 



We now come to a series of fermentations about which there 

 is considerable obscurity, for of all the fermentations the exact 

 process has been least worked out in the case of these. The 

 prominent feature of their action, however, as far as it concerns 

 milk, is that it results in the formation ofasemi-g^elatinous clot 

 or jelLy, which soon after its formation undergoes a species of 

 contraction and shrinkage similar to the coagulation of blood. 

 Hence for convenience we have assumed the name of thej^coagu- 

 lative fermentations " of milk, meaning by that the clotting of the 

 t^ casein as distinguished from its curdling by acid. The process 

 might perhaps equally well be termed the non-acid fermentations 

 of milk. 



We shall obtain a clearer view of these fermentations by con- 

 sidering for a moment two facts, one having relation to alimentary 

 digestion and the other to bacterial products. When milk enters 

 the stomach it becomes coagulated, owing to the effect upon it of 

 the rennin contained in the gastric juice.^ The acid action is 

 prevented in all probability by being neutralised by the alkaline 

 salts contained in the milk. In this way, the rennin, and not the 

 acid, obtains primary action on the consumed milk, and its action 

 , is that of a coagulation fermentation. If the acid predominated we 

 should get a curdling as is obtained in lactic acid fermentation, 

 when the casein, being dissociated from the lime salts and not 

 soluble, would be precipitated as a flocculent curd. Here then is 

 a simple example of the fermentations we are about to consider 

 taking place in the stomach naturally. 



The second fact to be borne in mind, is that certain bacteria 

 produce chemical ferments allied to, or identical with, the enzymes 

 {see p. 1 52). The class of bacteria in general which liquefy gelatine 

 have been found to possess prgteejytic^jcha racters like ^Jrypsin 

 (Lehmann, Hueppe), and many of them ^so produce a ferment 

 L_which coagulates milk without rendering it acid. Here then are 

 two functions of bacteria analagous to the double fermentation 

 capacity of the gastric juice, the one coagulating like rennin, the 

 other digesting like pepsin and trypsin. 



For a number of years these general principles had been held 

 as probably the explanation of certain facts known to physiologists. 



1 Gastric juice is composed in the main as follows : water, 99 per cent. ; 

 hydrochloric acid, 0-3 ; organic ferments (pepsin and rennin), 0-3 ; and salts (in- 

 cluding calcium chloride), 0-3 per cent. 



