RELATION OF MICROORGANISMS TO CHEESE 487 



four hundred kinds of rennet-curd cheeses are made, but only twelve 

 to fifteen are of great commercial importance. With few exceptions, 

 they are made from cow's milk. From the same raw material milk, 

 rennet, and salt therefore, a wide variety of products, differing 

 in texture, taste and odor, is obtained. This fact indicates the im- 

 portance of biological factors in the changes the curd undergoes during 

 the ripening process. 



The rennet-curd cheeses may be divided into: (i) hard cheeses; 

 (2) soft cheeses; the initial difference is largely in the amount of whey 

 left in the curd during the making of the cheese. The two great groups 

 of rennet-curd cheeses gradually merge into each other in varieties 

 that by some are classed as hard cheese, by others as soft cheese. 



The rennet-cur d cheeses, as a rule, are at first tough and rubber-like 

 in . texture. The curd, which is not easily digested, is quite insoluble 

 in water and is devoid of flavor and aroma. The curd must pass 

 through a complete series of chemical and physical changes, which 

 alter its texture, solubility, and digestibility, and give to it a flavor 

 and aroma by which the different kinds of rennet-curd cheeses are 

 especially to be differentiated. 



In the hard cheeses the factors concerned in these changes act in a 

 uniform manner throughout the entire mass of the cheese, making 

 it possible to manufacture such cheeses in any desired size. In the 

 case of the soft cheeses, the ripening changes are largely due to agents 

 which grow only on the surface; the products of such agents by means 

 of diffusion gradually affect the entire mass. In order that this may 

 take place within a reasonable time, it is essential that these cheeses 



made in small sizes. Then, too, the soft texture of such cheeses 

 makes it impossible to handle them commercially in large sizes. 



CONDITIONS AFFECTING THE MAKING OF CHEESE 



QUALITY OF MILK. In the curdling of milk by rennet the solid 

 Bodies present in the milk are retained in the curd, thus the fat 

 globules are held, as are also the bacteria. The latter continue to 

 grow as they would have done in the milk except that growth 

 must take place in the form of colonies as in the solid culture media 

 of the bacteriologist. The bacteria, however, produce the same fer- 

 mentation in the curd as they would have done in the uncurdled milk. 



