364 



OTHER MILK PRODUCTS. 



Rennet. — This substance is an enzyme produced in tlie stomachs 

 of mammals ; it occurs in the human stomach, and the curdling 

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

 in the young while still suckling. 



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° F.). 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 : — 



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

 keeping. 



The action of rennet is affected by the acidity of the milk ; 

 the larger the amount of acid, the more rapid the action ; the 



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

 that 99 is more correct. 



