68 Report of the Department of Bacteriology of the 



The effect of acid formation upon the action of the rennet may 

 be directly observed by adding measured amounts of acid to 

 milk and noting the decrease in the time required to curdle 

 portions of the same with rennet. 



In the following experiment 12^ litres (25 pounds) of milk was 

 used. The time required for the action of the rennet according 

 to the Monrad rennet test was determined at the beginning of the 

 experiment and after the addition of successive portions of acid, 

 100 cc. (3^ oz.) of the milk being used in each rennet test. Be- 

 fore each test, after the first, 2.5 cc. (1-12 oz.) of chemically pure 

 lactic acid diluted with 100 cc. of water was added to the milk. 



Table I. — Effect of Aciu Upon the Curdling Action of the Rennet. 



From this it is clearly seen that the addition of acid quickens 

 the rate of curdling by rennet since the addition of 12.5 cc. of acid 

 (about 0.1 per ct.) shortened the time of curdling from 3 minutes 

 45 seconds to 43 seconds. In other words the addition of o.i per 

 ct. of acid to fairly sweet milk changed it as much as is allowable 

 for milk intended for cheese -making. 



This should help to emphasize the importance of delivering 

 milk intended for cheese-making before the bacteria in it form 

 any considerable quantities of acid. 



It is also interesting to observe how much more noticeable is 

 the effect from the. first addition of acick Here the time of cur- 

 dling was shortened i minute 43 seconds, while the addition of an 

 equal amount of acid at the close of the experiment shortened the 

 time of curdling but 4 seconds. This is ([uite in accord with 

 factory experience where the addition of sour milk has an imme- 

 diate effect upon the rate of rennet action although the acid ad ded 

 amounts to only a very insignificant proportion. 



