76 Agricultural Bacteriology. 



-acts only in an acid medium. The reaction of the con- 

 tents of the normal stomach is acid. Certain kinds of 

 .stomach trouble, and indigestion are due to a lack of acid 

 in the stomach, the food remains unacted upon by the 

 .stomach juices, while putrefactive changes occur which 

 are marked by belching of gas and offensive breath. 



The immense number of lactic acid bacteria that are 

 present in the milk are concentrated in the curd just as 

 are the fat globules of the milk. In the warm moist curd 

 they grow rapidly and change the sugar of the whey to 

 lactic acid. This acid enables the pepsin of the rennet 

 extract to act, changing the tough, rubbery, insoluble 

 curd to a soft and partially soluble form. Without the 

 lactic acid bacteria to produce this change the cheese does 

 not ripen. They are absolutely essential to the ripening 

 of cheese. Owing to the favorable growth conditions, 

 during the making and in the first stages of the ripening 

 of the cheese, an immense number of lactic acid bacteria 

 are found therein. This number rapidly decreases with- 

 in a few days as the sugar is soon completely fermented, 

 and conditions are no longer favorable for growth. 



Green cheese has none of the flavor which a ripe cheese 

 must have in order to make it valuable as a commercial 

 product. With increasing age the flavor becomes more 

 and more marked, until it may become so strong as to be 

 undesirable. The cause of the typical flavor of cheddar 

 or American cheese, as it is often called, is unknown. It 

 has been found that the cheese must be made in a certain 

 way and ripened under certain conditions, and unless 

 this is done the product will not have the flavor of ched- 

 dar cheese. For example it does not seem possible to 

 make a cheese from pasteurized milk, or to omit the salt- 

 ing of the curd. Such things strongly indicate the work 

 of bacteria in the formation of the flavoring substances. 



