RENNET 167 



The matter was however demonstrated by Fermi and Conn. The 

 former isolated from bacterial cultures a tryptic (proteolytic) 

 ferment, and the latter isolated from milk cultures of several 

 species of bacteria a ferment having all the essential characteristics 

 of the rennet (coagulation) ferment.^ Both ferments are frequently 

 produced by the same species of bacteria, although not in equal 

 quantities. Other species only digest without previously co- 

 agulating. 



We are now in a position to consider the two chief examples 

 of these coagulation fermentations of milk. The class of organisms 

 which produce them is abundant in the dairy, though not so 

 abundant as the lactic acid organisms. The relationship between 

 the two is highly variable. It is stated by some authorities that 

 the different species favour different localities. The two examples 

 of coagulation ferments produced by bacteria may now be briefly 

 considered. 



(i.) Rennet. — Rennet is of course well known as the agent in , 

 the production of cheese. If a small quantity is added to a rela- 

 tively large quantity of milk the latter rapidly changes to a set 

 jelly, contracts, exuding the whey, and eventually becomes the firm 

 clot forming the basis of cheese. The element of the milk chiefly 

 affected js^Jhe casein, which when acted upon by rennet gives rise 

 to a body known as tyrein, the proteid constituent of the clot. 

 The whey also contains a by-product resulting from the altera- 

 tion of the casein. The clotting of rennet is not to be confused ■. 

 with the precipitation of casein by acid. The two processes are \ 

 distinct. The curd of the latter is a soluble body ; the clot of 1 

 the former is a decomposition of casein with the formation of an 

 insoluble nird, which is sweet rather than sour, and hence some- i 

 times called " sweet curdling." The chief source of rennet is the 

 mucous membrane of the stomach — where it occurs in the gastric 

 juice as rennin — and is present in largest quantity in the stomach 

 of young animals, particularly the calf It exists also in the pan- 

 creas of various animals, including man. It may be prepared in 

 an impure form by mincing the organ finely, and extracting the 

 pulpy mass with glycerine water, or solution of neutral salts.- It 

 has also been found in the stomach of birds (Roberts), and fishes 

 (Benger). 



The salts of calcium play an important part in the action of 



* Starr's Station Report, 1892, Rep. v., p. 196. 



2 The Soluble Ferments and Fermentation, J. Reynolds Green, F.R.S., 1899, 

 p. 236. 



