438 BUTTEK, CHEESE, ETC. 



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

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

 addition of water to milk causes it to be coagulated more slowly 

 by rennet and the curd is less firm. By heating milk, the action 

 of rennet is delayed, owing to the removal of some of the soluble 

 calcium compounds. By the addition of soluble lime salts, the 

 milk will be curdled by rennet in the usual manner. 



Alkalies destroy the power of rennet to curdle milk ; borax 

 acts an an alkali, boric acid being inert to rennet, and other 

 alkaline salts as citrate prevent the curdling. 



Testing of Rennet. It is important to know what the 

 strength of rennet preparations are i.e., the amount of milk 

 that will be curdled by 1 part in a definite time at a definite 

 temperature. This may be estimated as follows : 5 c.c. of a 

 rennet extract or 0'5 gramme of a rennet powder are made up to 

 100 c.c. with distilled water. After thorough mixing, 1 c.c. is 

 measured out by means of a pipette and added to 100 c.c. of 

 separated milk of acidity 20, which has been brought to a tem- 

 perature of 35 C. ; the milk and rennet solution are well stirred 

 immediately and the exact time at which the rennet was added 

 noted. The milk should be contained in a beaker, which is 

 placed in a water-bath kept at 35 C., and gently stirred with 

 a thermometer till it is found, by the path becoming visible, 

 that the milk has coagulated ; the exact time which has elapsed 

 from the addition of the rennet till the coagulation sets in is 

 noted. 



The strength of the rennet i.e., quantity of milk that will be 

 coagulated by 1 part in forty minutes is calculated by the 

 following formula : 



Let x = quantity of milk coagulated. 



p = proportion between milk and rennet taken. 

 t = the time. 



Then x = ^- P . 



The value of p is 2,000 when 5 c.c. was diluted to 100 c.c. and 

 1 c.c. taken, and 20,000 when 0-5 gramme was taken. 



If the time taken is less than five minutes, or more than ten 

 minutes, it is advisable to make another determination, using a 

 small or larger proportion of rennet to milk. 



The Ripening of Cheese. The work of Freudenreich, Lloyd, 

 Duclaux, Bochicchio, and Babcock and Eussell has shed much 

 light on the nature of ripening. 



Rdle of Micro-organisms. When the curd is precipitated 

 by rennet it carries down with it the bulk of the micro-organisms 

 present in the milk. The first of these to develop are the lactic 



