A COMPARISON OF ACID AND RENNET CURDS 185 



H. Urea fermentation, by urea bacteria. 



Urea + water + hydrolytic endo-enzyme = ammonium carbonate. 

 (producing intramolecular change) 



H 2 N 



\ 



C=0 



EXERCISE 11. A COMPARISON OF ACID AND RENNET 



CURDS 



Apparatus. Three 200 c.c. flasks containing 100 c.c. each 

 of sterile skim milk; 200 c.c. fresh skim milk; small funnel; 

 eighteen large test tubes; absorbent cotton; 10% lactic acid; 

 5% phenol. 



Cultures. B. prodigiosus; Bad. lactis addi; B. megater- 

 ium. 



Method. 1. Inoculate flasks containing 100 c.c. sterile 

 milk with B. prodigiosus, Bad. ladis addi and B. megaterium. 



2. Place about 30 c.c. fresh skim milk in a 200 c.c. flask. 



3. Add 10% lactic acid drop by drop, shaking constantly. 



4. When the first finely divided curd appears, titrate. 

 What degree and percent of acid were necessary to curdle the 

 milk? 



5. Titrate fresh skim milk. 



6. Prepare 150 c.c. of 0.5% phenol milk (by adding 

 15 c.c. of 5% phenol to 135 c.c. of milk and sterilize). 

 Mix well and titrate again. Is the acidity of the milk 

 increased perceptibly by the addition of phenol? 



7. As soon as curd appears in inoculated flasks, titrate. 

 Determine the degree and percent of acidity present. 



8. Allow the cultures to develop several days until 

 decided proteolysis is evident. Then titrate again and 



