The antimicrobial activity of organic acids is pH-dependent. The
strongest effect can be observed at low pH values, since in those
conditions organic acids are mostly in their undissociated form. The
pK a determines the ratio between dissociated and undissociated
acid forms at a given pH. At a low pH value, the undissociated form
of an acid diffuses into cells and releases protons during dissocia-
tion until equilibrium is reached. Cells of B. cereus ATCC 14579
exposed to pH 5.5 set with hydrochloric acid, lactic acid, and acetic
acid were found to be the most sensitive to acetic acid and then to lactic acid, while hydrochloric acid had the mildest effect on the cells [32]. These results may be attributed to the differences in the amounts of undissociated forms of examined acids at the applied pH (the pK a of acetic acid is 4.76; the pK a of lactic acid is 3.86). One explanation for the antimicrobial effect of organic acids is the dispersal of the proton motive force (inclusive with the effect of pH i lowering) and the accumulation of anions leading to end-product
inhibition