RELATION OF MICROORGANISMS TO CHEESE. 347 



mercial 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 importance 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 and they meet in types which are 

 classified with difficulty. 



The rennet-curd cheeses of the cheddar group are at first tough and 

 rubber-like in texture, due to their curd, which is not easily digested, 

 is soluble in water only in small part, and, further, is devoid of flavor 

 and aroma. The curd must pass through a complex series of chemical 

 and physical changes, altering its texture, solubility and digestibility, and 

 giving to it 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 controlled by 'agents existing only on the surface, the products 

 of such agents by means of diffusion gradually affecting the entire mass. 

 In order that this may take place within a reasonable time, it is essential 

 that these cheeses be 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 fermentation in the curd as they would have done in 

 the uncurdled milk. 



The butter maker can control, through pasteurization and the use of 

 pure lactic cultures, the fermentation of the cream. The pasteurization 

 may be so efficient as to destroy all non-spore-forming bacteria since the 



