igS BACTERIA AND OTHER MICROORGANISMS IN CHEESE. 



they are necessary to the ripening is proved by the fact that cheeses 

 do not ripen normally when they are ripened in chloroform vapor, 

 which prevents bacteria growth, but allows the enzyme action to 

 continue as usual. Although some of the digestive changes go on as 

 usual in these cheeses, the ripening does not become complete, 

 and the cheeses never develop either the same final chemical char- 

 acter or the flavors of cheeses in which the bacteria have had an 

 opportunity for growth. 



Chemical Action of Bacteria. When cheese-ripening was 

 first studied, it was believed to be primarily due to the action of 

 bacteria. We have already seen that certain kinds of bacteria have 

 the power of changing casein to peptone the liquefying type and 

 this change in the cheese-ripening was at first supposed to be due to 

 the growth of these peptonizing bacteria. But later it became evi- 

 dent that the liquefying bacteria are not common in cheeses, espe- 

 cially in the better grades. If present at the beginning they rapidly 

 decrease in numbers, until they almost or entirely disappear, a fact 

 which forced the conclusion that they cannot contribute materially 

 to the ripening of cheeses. More recently, it has been claimed that 

 certain "acid liquefiers" i.e., peptonizing bacteria that at the same 

 time produce acid are intimately connected with the ripening. 

 But there does not yet appear to be much evidence for this. 



These facts led to a suggestion that the ripening is due really to 

 the lactic acid bacteria. These do not liquefy gelatin and do not 

 ordinarily have any power of changing casein to peptone. They 

 produce lactic acid which curdles the milk, after which they ap- 

 parently cease to act upon it at all. Hence, it would not seem that 

 they could digest cheese. But if the acid which they produce is 

 neutralized by the presence of some alkaline, like carbonate of soda, 

 the bacteria continue to grow, and eventually produce the peptoniza- 

 tion of the casein. Moreover, the grade of the cheeses is very 

 closely dependent upon the growth of lactic bacteria, and cheese 

 from which lactic acid bacteria are excluded by aseptic milking will 

 not ripen normally, while they would do so if the acid germs were 

 present. All of these facts together led to the conclusion that it is 

 this peptonizing power of the lactic acid bacteria, under certain con- 



