112 Dairy Bacteriology. 



are not those that usually occur, but are characterized by 

 the curdling of the milk from the action of rennet enzyms. 

 5. Action of rennet. Heating milk causes the soluble lime 

 salts to be precipitated, and as the curdling of milk by ren- 

 net (in cheese-making) is dependent upon the presence of 

 these salts, their absence in heated milks greatly retards 

 the action of rennet. This renders it difficult to utilize 

 heated milks in cheese-making unless the soluble lime salts 

 are restored, which can be done by adding solutions of cal- 

 cium chlorid. 



Sterilization. As ordinarily used in dairying, steriliza- 

 tion means the application of heat at temperatures approxi- 

 mating, if not exceeding, 212 F. It does not necessarily 

 imply that milk so treated is sterile, i. e., germ-free; for, 

 on account of the resistance of spores, it is practically im- 

 possible to destroy entirely all these hardy forms. If milk 

 is heated at temperatures above the boiling point, as is 

 done where steam pressure is utilized, it can be ren- 

 dered practically germ-free. Such methods are employed 

 where it is designed to keep milk sweet for a long period 

 of time. The treatment of milk by sterilization has not 

 met with any general favor in this country, although it has 

 been more widely introduced abroad. In most cases the 

 process is carried out after the milk is bottled; and consid- 

 erable ingenuity has been exercised in the construction of 

 devices which will permit of the closure of the bottles after 

 the sterilizing process has been completed. Milks heated 

 to so high a temperature have a more or less pronounced 

 boiled or cooked taste, a condition that does not meet with 

 general favor in this country. The apparatus suitable for 

 this purpose must, of necessity, be so constructed as to with- 

 stand steam pressure, and consequently is considerably 



