234 AGEICULTUEAL AND INDUSTRIAL BACTBEIOLOGT 



later that they are active in bringing about the changes 

 incident to the ripening of certain kinds of cheese. 



Peoteolytic Changes in Foods 



The Ripening of Cheese. — Cheese is a product prepared 

 from milk by causing a coagulation or curdling of the 

 simple milk protein and a more or less complete removal of 

 the liquid portion or whey. Many different kinds of cheeses 

 are prepared. Probably in all more than one hundred dif- 

 ferent varieties have been described. There are several 

 factors which contribute to determine the character of a 

 cheese. They may be enumerated as follows: 



1. The kind of milk. — Cheese is prepared in various 

 countries from the milk of the cow, goat, sheep and mare. 

 These various milks differ somewhat in chemical composi- 

 tion and consequently cause variation in the composition of 

 the cheese produced. 



2. The method of causing curdling. — ^Milk may be curdled 

 either by the development or addition of acid or by the 

 addition of the enzyme rennet. The curds produced by 

 these two methods are not identical. 



3. The proportion of whey expelled. — The amount of 

 moisture left in the cheese is determined by the amount or 

 proportion of the whey squeezed out. Cheeses are some- 

 times classified as moist and dry. 



4. The size of the cheese prepared. — Small cheeses give 

 opportunity for microorganisms growing upon the surface 

 in the presence of oxygen to exert an influence on the 

 character and flavor of the cheese. This does not occur in 

 the large cheeses. 



5. Amount of salt and condiment. — Various materials 

 are used to flavor cheeses. 



6. The temperature of ripening. — The temperature at 

 which the cheese is held during the ripening process may 



