From the above experiments, it can be speculated that the
fundamental substance producing a sour taste is the hydrogen
ion, including free hydrogen ions and undissociated hydrogen ions
(in protonated acid species), and that the intensity of sour taste is
determined by the total concentration of free and undissociated
hydrogen ions. The anionic acid species (without hydrogen ions)
does not produce a sour taste but can intensify or weaken the
intensity of the sour taste, or can show other attributes, such as
a bitter taste. The effect of anionic acid species on the intensity
of sour taste is determined by its structure.