208 AGRICULTURAL BACTERIOLOGY 



variations in the methods of manufacture, and in the con- 

 ditions under which the ripening takes place, permit the 

 growth of different classes of bacteria in the different types 

 of cheese. 



The amount of whey allowed to remain in the cheese will 

 cause a difference not only in moisture, but also in sugar, 

 and hence in the acidity of the product, since all of the 

 sugar will be fermented by the acid-forming bacteria of the 

 milk, which in large part are retained in the curd. The 

 type of cheese, such as the Camembert and Brie, in which 

 a large amount of whey is left is called soft cheese; those 

 from which the whey is more completely expressed, as 

 represented by the Cheddar and Swiss types, are called 

 hard cheese. The latter are usually made in considerably 

 larger sizes, and in the case of the American or cheddar 

 cheese may be made of any desired size. In the case of the 

 soft cheese, the ripening is, in part, due to the action of 

 organisms that grow on the surface of the cheese, and act 

 on the curd by the secretion of soluble enzymes that diffuse 

 into the cheese. If the cheese is too large, the outer layers 

 will be ripe before the inner portions, and the softness 

 makes the handling of large cheese impossible. 



Cheddar cheese. In the making of cheddar cheese, the 

 most common variety made in America, it is essential that 

 the milk contain great numbers of lactic bacteria, since it 

 is necessary that a relatively large amount of acid be 

 formed during the making of the cheese. If the milk is too 

 sweet, or, in other words, too low in bacteria, the maker 

 adds a starter prepared from pure cultures of lactic bac- 

 teria, just as the butter-maker does for the control of the 

 cream-ripening process. The action of the rennet that is 

 used to curdle the casein is also facilitated by the acid 

 reaction of the milk. Within a few minutes from the time 

 the rennet is added the milk curdles, and as soon as it is 



