With the exception of those soft drinks that contain phosphoric acid, in most other acidic foods acidity is due to the presence of weak organic acids. These do not dissociate completely into protons and conjugate base in solution but establish equilibrium:

The partial dissociation of weak acids, such as acetic acid, plays an important role in their ability to inhibit microbial growth. Although addition of strong acids has a more profound effect on pH but at the same pH, they are less inhibitory than weak lipophilic acids. This is because microbial inhibition by weak acids is directly related to the concentration of undissociated acid. These undissociated lipophilic acid molecules can pass freely through the membrane, in doing so they pass from an external environment of low pH where the equilibrium favours the undissociated molecule to the high pH of the cytoplasm. At this higher pH, the equilibrium shifts in favour of the dissociated molecule, so the acid ionizes producing protons. These protons tend to acidify the cytoplasm.