304 MANUAL OF MILK PRODUCTS 



Rennet tests 



Many cheese-makers prefer some form of rennet test such 

 as Monrad's or Marschall's to the alkaline solution acid test. 

 The development of acidity in milk is due to the formation 

 of lactic acid by the bacteria, and as the acidity increases, the 

 number of bacteria increases also with great rapidity ; in fact, 

 the number of bacteria becomes quite large before the acidity 

 increases enough to be measured by the alkaline tests. Hast- 

 ings and Evans have found that the "rennet test is a much 

 more delicate index of the bacteriological condition of the milk 

 during the early phases of the acid fermentation than is the 

 titration method. In other words, the curdling time by rennet 

 under otherwise constant conditions is profoundly influenced 

 by the most minute quantities of acid." Their conclusion is 

 that since the rennet test is a much more sensitive measure of 

 the bacteria and development of acid in the milk, it is the better 

 test for the cheese-maker. 



The Monrad rennet test is used by cheese-makers for deter- 

 mining the ripeness of milk. Five c.c. of rennet extract is 

 measured into a 50 c.c. flask by means of a pipette ; the pipette 

 is rinsed with water, and the flask filled to the mark with 

 water; 160 c.c. of milk is now measured into the tin basin 

 from the cylinder and slowly heated to exactly 86 F. ; 5 c.c. 

 of the dilute rennet solution is then quickly added to the warm 

 milk and the time required for coagulation noted. 1 Milk 

 sufficiently ripe for cheddar cheese-making will coagulate in 

 30 to 60 seconds, according to the strength of the rennet ex- 

 tract used. 



The Marschall rennet test is used for the same purpose as 

 the Monrad test. The directions for this test are as follows : 

 Fill the small glass with pure water to the mark, pour into it 

 1 c.c. of rennet extract and rinse the pipette in the same water. 



1 Decker, Cheese Making, Revised ed., 1909, p. 39. 



