ENERGY SUPPLY OF THE CELL 



145 



in skim milk to which varying amounts of sodium lactate 

 had been added, and the fermentation stopped at a 

 different pH, but at the same concentration of undisso- 

 ciated lactic acid (see Table 30), as Van Dam had 

 shown. 



Table 30. — Effect of Added Sodium Lactate on the Lactic 



Fermentation in Skim Milk 



The conclusion is that the accumulation of a certain 

 product of fermentation will prevent the zymase of the 

 cells from further action. When this product is removed, 

 fermentation will go on until some other product limits 

 it. What will happen when this is removed also, we 

 cannot say since no means have been found as yet to 

 remove undissociated acid from the culture without 

 killing the bacteria. 



The neutralization method is used in the commercial manufacture 

 of lactic acid and of other acids, e.g., butyric acid. The buffer action 

 of casein is of great importance in the ripening process of cheese to 

 prevent too high a hydrogen ion concentration which would interfere 

 with ripening. All manipulations to remove the whey are merely 

 a means to control the ratio of casein : lactose ; this ratio determines 

 the ripening process. 



