A\TUSEPTI(\'< 161 



This interpretation seems probable as the dissociation 

 constant of propionic acid is 1.3 X 10\ The fractions 

 of undissociated acid are 



at pH 5 0.43 



at pH 6 0.072 



at pH 7 0.0076 



at pH 8 0.0007. 



In neutral or alkaline media, the fraction of undissoci- 

 ated acid is so small that even a high concentration of 

 proprionate would give only a very small amount of acid. 

 But when the s.olution becomes acid, the undissociated 

 portion increases rapidly from less than 1% at neutral- 

 ity to 43% of the total propionate at pH 5. 



A similar effect by the undissociated fraction of an 

 acid has been observed by Rogers and Whittier as early 

 as 1928. They found that the lactic fermentation by 

 Streptococcus lactis ceases, regardless of pH, when the 

 concentration of undissociated lactic acid becomes more 

 than 0.017 molar. 



The strong action of the undissociated acids as com- 

 pared with that of their ions may be explained by the 

 ability of the undissociated molecules to pass through 

 the membrane and through protoplasm "with great rap- 

 idity, according to the simple laws of diffusion" while 

 ''the permeability of cells to ions is a decidedly complex 

 phenomenon, more or less limited in its extent, and in- 

 volving theoretical equilibria which may be approached 

 very slowly, and perhaps never be attained" (Jacobs, 

 1940). 



Benzoic acid and sulfur dioxide cause an increased lag 

 period as well as a decreased multiplication rate. Figure 

 31 shows the growth of yeast at pH 4.25, with increasing 

 amounts of sodium benzoate. The curves with NaoSOs 

 are similar. 



The length of the lag period and the rate of multipli- 

 cation depend on pH as well as on the concentration of the 



