GENERAL CHEMISTRY OF MILK 95 



INFLUENCE OF CALCIUM CHLORID ON RENNET ACTION 



Per cent. CaCh. Time required for coagu- 



lation in minutes. 



No coagulation. 



0.01 No coagulation. 



0.02 610 



0.05 46 



0.1 4 



0.5 1 



I.0 3 



6.0 8 



10.0 20 



Milk which has been kept in metallic vessels does not coagulate 

 with rennet as readily as normal milk, since some of the metal is 

 dissolved. The effect of metal is very pronounced, and rusty 

 cans have a retarding effect, since some iron combines with the 

 lactic acid in milk. Platinum and tin have the least retarding 

 effect (Olsen). 



4. Temperature is of great importance in rennet coagulation, 

 and influences the speed of reaction and the character of the 

 coagulum. The lower the temperature, the longer the time re- 

 quired for coagulation; the longer the milk with rennin remains 

 cold, the less rennin is necessary to coagulate milk when it is 

 finally warmed. At 60° C. there is no rennet action, but if rennet 

 is added to milk in the cold it coagulates even when heated to 

 100° C. At 60° C, therefore, the transformation of casein into 

 paracasein is inhibited, but not the action of calcium salt. The 

 following table (Fleischmann) shows the influence of temperature 

 on rennet action : 



INFLUENCE OF TEMPERATURE ON RENNET ACTION 



Time required for Relation to optimum 



Temperature. coagulation in minutes. temperature. 



20° C 32.7 18 



25° C 14 44 



30° C 8.47 71 



40° C 6.15 98 



41° C 6.06 100 



42° C 6.12 98 



50° C 12 50 



The table shows that the optimum temperature is 41° C. 



At 15.5° C. the curd is flocculent, spongy, and soft; at 25° to 

 45° C. it is fine and soUd; at 50° C. it is soft and gelatinous. 



Heating milk precipitates calcium salts; the greater the heat, 

 the larger is the amount of calcium salt precipitated. Rennet 

 action is consequently retarded in proportion to the degree of 

 heat applied. According to Rupp, the time required for coagula- 

 tion in milk heated up to 65° C. is slightly less than that in raw 

 milk. At 70° C. there is slight retardation, and at 75° C. the time 

 is almost doubled. Van Slyke states that rennin normally fails to 

 coagulate unless some soluble calcium salt or some acid is added. 

 The coagulum of heated milk is flocculent and not as firm as that 

 of raw milk. 



