254 THE BOOK OF CHEESE 



The large number of bacteria in the cheese is very 

 striking. The number as given in the accompanying table 

 is not that actually in the cheese, as it is very difficult to 

 obtain the sample in suitable condition for plating. 1 



The principal action of the lactic acid-forming bacteria 

 in the cheese ripening is the changing of the milk-sugar 

 or lactose into lactic acid and the formation of small 

 amounts of other substances, such as acetic, succinic 

 and formic acids, alcohol, aldehydes and esters and some 

 gases, carbon dioxide and hydrogen. While the amount 

 of these substances other than lactic acid is small, it is 

 thought that the effect of these on the cheese may be im- 

 portant. Heinemann shows 2 that lactic acid exists in 

 two optical modifications, the levorotatory and dextroro- 

 tary acids. In cheese they are usually found in the inac- 

 tive or racemic form, the levorotatory and dextrorotary 

 acids being present in equal amounts. What impor- 

 tance the question of optical activity of the lactic acid 

 may assume is not definitely known. Just as some groups 

 of bacteria have a specific effect on the lactose, producing 

 only one modification of lactic acid, so bacteria attacking 

 lactic acid may exercise a selective action and use only 

 one or the other optically active modification. In other 

 words, the early flora of cheese-ripening bacteria may 

 determine the later flora by the production of a form of 

 lactic acid attacked by one group of bacteria and not by 

 another, and the effect on the flavor will differ accordingly. 

 The amount of lactic acid in the cheese increases for a 

 time, then decreases. 



1 Harding, H. A., The r61e of the lactic acid bacteria in the 

 manufacture and in the early stages of ripening of Cheddar 

 cheese, N. Y. (Geneva) Exp. Sta. Bui. 237, 1903. 



2 Heinemann, P. G., The kinds of lactic acid produced by 

 lactic acid bacteria, Jour. Biol. Chem., Vol. 2, pages 603-608. 



