RENNET. 



437 



of milk when ingested is due to this ; it is especially abundant 

 in the young while still suckling. 



It is possible that rennet is the same as pepsin; certainly 

 pepsin has a rennetic activity, and as it is purified Davis and 

 Merker show that the two activities correspond closely. 



Preparation. It is usually prepared from the fourth stomach 

 of the calf. The stomachs are dried and kept for some time ; 

 they are then cut up into small pieces and macerated in a 5 per 

 cent, salt solution, usually containing boric acid, for some days ; 

 to the solution a further 5 per cent, of salt is added, and the 

 liquid filtered ; this forms extract of rennet. By adding more 

 salt, the rennet is precipitated, and " rennet powder " is pro- 

 duced ; this consists, essentially, of the ferment, together with 

 other organic matter, and a considerable amount of salt. 



Properties. Rennet acts on casein only in neutral or acid 

 solution, and its properties are destroyed by alkalies. Like all 

 enzymes it has an optimum temperature at which it acts best ; 

 this has been found by Fleischmann to be 41 C. (105-8 R). He 

 gives the following table as showing the relative proportion of 

 milk coagulated in a given time by the same quantity of rennet 

 at different temperatures : 



TABLE CLII. 



* Given as 98 in original, but from the experimental data it appears 

 that 99 is more correct. 



At the optimum temperature, and for several degrees on either 

 side, the curd produced is very firm ; at low temperatures, 15 

 C. to 20 C., the curd is quite soft and flocculent ; and when 

 the temperature is raised to 50, the curd again becomes very 

 soft. 



By heating rennet to temperatures much above 60 C. (140 F.) 

 it loses its properties rapidly, and it also loses strength by long 

 keeping. 



