212 CHEESE 



Rennet. 



Rennet, which is largely employed for the manufacture of cheese 

 owing to its action in precipitating casein (paracasein), owes its 

 activity in this respect to an enzyme, or unorganised ferment, named 

 chyjfzosin, which occurs in the stomachs of young animals living 

 on milk. Rennet is generally prepared from the fourth stomach of 

 the calf by extracting the dried stomach with dilute salt solution. 

 Rennet is largely employed directly in the form of this solution, or 

 may be purchased as more concentrated and purer solutions, or in 

 the solid form as pellets or powder, which has been precipitated 

 from the saline solution by addition of further quantities of salt. 



The rapidity of precipitation of casein (or more correctly para- 

 casein, see p. 207) by rennet depends upon several factors, such as 

 the " strength " of the rennet, the temperature, and the degree of 

 acidity (p. 182) of the milk. The more acid the milk, the more 

 rapid the action, while the optimum temperature is about 40° C. 

 A temperature as high as this, however, is never employed in 

 practice, as it would be almost impossible to retain the fat in the 

 curd. A temperature of 29° C. (84° F.) is most commonly used. 



208. Determination of Rennet Strength. 



Rennet strength may be defined as the volume in 

 c.c. of milk which will be coagulated in forty minutes, 

 at a temperature of 35°C., by i c.c. of rennet solution 

 or I gram of rennet powder. 



Five c.c. of rennet solution, or 5 grams of rennet 

 powder, are made up to 100 c.c. with distilled water, 

 and I c.c. of this solution is withdrawn by means 

 of a pipette, and forcibly blown into 100 c.c. of 

 separated milk warmed to 35° C, and contained in a 

 beaker, which is immersed in a larger beaker containing 

 water at the same temperature. The mixture is well 

 stirred by means of a thermometer, and the exact time 

 (in seconds) noted between the addition of the rennet 

 and the commencement of the curdling. The strength 

 of the rennet is then calculated by simple proportion, 

 as shown by the following example. 



