212 AGRICULTURAL BACTERIOLOGY 



milk with bacteria capable of fermenting the sugar, and 

 with others that produce propionic acid and carbon-dioxide. 

 The milk is heated to about 135 F. and the curd is removed 

 from the whey in one mass, so as not to allow it to become 

 cool, which checks the growth of Bact. Bulgaricum in the 

 curd. 



At every step in the making of this and other kinds of 

 cheese, the process is conducted in a manner that influences 

 the growth of certain groups of organisms. Of course, 

 these methods were primarily worked out entirely from the 

 standpoint of experience; but more recently their relation 

 to the action of certain bacterial groups has been more 

 definitely traced. 



In the making of cheddar cheese the salt is added to the 

 finely cut curd before it is placed in the press, but in the 

 case of Swiss cheese the salt is applied to the surface of the 

 cheese. The most marked characteristic of Swiss cheese is 

 the presence of gas-holes, ranging from the size of a cherry 

 to that of a walnut, which are scattered quite uniformly 

 through the interior of the cheese. These so called eyes 

 are formed by the fermentation of the lactates with the 

 formation of propionic acid and carbon-dioxide, the latter 

 causing the holes in the plastic curd, while the acid influ- 

 ences the flavor of the cheese. These organisms can not 

 grow in the presence of salt, and it is therefore essential 

 that an opportunity first be given for their growth. Later 

 the application of salt to the outside checks the development 

 of these gas-forming organisms as the salt g/adually pene- 

 trates the substance of the cheese. 



Mold-ripened cheese. Roquefort, a French cheese made 

 from sheep's milk, Gorgonzola, an Italian cheese, and Stil- 

 ton cheese, made in England, are illustrations of hard, firm 

 cheese that contain molds. Not only does the presence of 

 these molds confer a peculiar flavor and appearance on the 



