236 AGEICULTUEAL AND INDUSTEIAL BACTERIOLOGY 



cheeses produced, three only may be mentioned as illustrat- 

 ing various processes of ripening and of manufacture. 

 These are Cheddar cheese, an example of a hard cheese, and 

 Camembert and Roquefort, examples of soft cheeses. 



Ordinary Cheddar cheese is prepared from milk of good 

 quality, care being used that gas-producing bacteria of the 

 Bacterium aerogenes or Bacterium coli type are not present 

 in sufficient numbers to cause gas bubbles to develop in the 

 ripening cheese. The milk is allowed to stand until a small 

 amount of acid, usually about .2 per cent has been devel- 

 oped. The rennet is then added, whereupon the milk 

 quickly curdles. The curd is broken up by means of knives 

 and gradually shrinks until most of the whey is drained off. 

 The whey is finally removed quite completely by pressing 

 and the curd is formed into large cakes. The consistency 

 of the casein gradually changes until it becomes rubbery 

 and unites readily to form a coherent mass. The cheese is 

 then placed in the curing room until the ripening process 

 has been completed. The ripening is probably due to the 

 combined action of proteolytic enzymes originally present in 

 the milk and introduced with the rennet, and the proteo- 

 lytic and fermentative action of certain bacteria which are 

 present. Most of the lactose is soon transformed into lactic 

 acid. This acid in large part combines with the casein and 

 furnishes suitable conditions for activity of the pepsinizing 

 or proteolytic enzymes. The proteins are attacked and par- 

 tially hydrolyzed, that is, the casein becomes more and more 

 soluble in water. Flavoring substances of various types are 

 produced. It is probable that these are in part ammonia 

 and related compounds, in part volatile acids, and in part 

 the ethyl esters of various fatty acids. 



Roquefort cheese was originally made from sheep's milk 

 but milk of the cow is also used to some extent. This is a 

 characteristic soft cheese in which opportunity has been 

 afforded for the growth of a specific mold, Pencillium 



