358 CHEESE 



Sometimes small pieces of the dried and salted 

 stomachs are added to milk in order to coagulate it but 

 most cheesemakers now use a brine extract of rennet 

 containing the active principle rennin. Home-made 

 extracts are prepared by soaking the " veils " or stomachs 

 in .3 to .6 per cent, solution of salt for three or four 

 weeks, after which the liquid is filtered through charcoal. 

 The extract should be dark-coloured but clear, turbidity 

 indicating objectionable decomposition. 



Commercial extracts in much stronger brine (16 per 

 cent, solutions) are available as well as rennet " powders " 

 and " tabloids " : these are more reliable, being more 

 regular in strength than home-prepared material, and 

 less likely to contain bacteria and other impurities which 

 give rise to tainted or faulty cheese. 



Rennet is a very active substance, exceedingly small 

 amounts being able to coagulate large quantities of milk. 

 One part of ordinary extract, which is itself a very weak 

 solution of the active enzyme, will very frequently curdle 

 as much as 5000 to 6000 times its volume of milk ; it 

 has been calculated that one part of pure rennin will coagu- 

 late 30 million parts of milk. 



The extracts are very sensitive to the action of light, 

 heat, and certain chemical agents. They should be kept 

 cool and in the dark, not in glass bottles, for the alkali 

 of the glass dissolves out and damages the enzyme. 



Rennin is most active in neutral or in very dilute acid 

 or alkaline solutions ; strong alkalis or strong acids soon 

 destroy it. 



Its coagulating power increases from about 10 C, 

 below which it ceases to act, up to the optimum, about 

 39 to 42 C. ; above this temperature it decreases until 

 at about 70 C. its strength rapidly diminishes even in 



