3.4. The chemistry of sour taste
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.
According to the speculation, all the phenomena mentioned
above can be reasonably illustrated. For example, a sodium acetate
solution without adjusted pH had no sour taste, as there were no
undissociated hydrogen ions (pH 7.1) and the concentration of free
hydrogen ions was less than its sour taste threshold. In terms of the
three sodium acetate solutions at pH 4.0, although the concentration
of free hydrogen ions was below the sour taste threshold of
hydrogen ions, they all had an obvious sour taste, as there existed
protonated acetic acid species which contained undissociated
hydrogen ions. The total concentrations of all hydrogen ion species
in 50, 100, and 150 mg/l sodium acetate solutions were 0.65, 1.3
and 2.0 mM, respectively which are higher than the sour taste
threshold of hydrogen ions.
In order to ascertain the effect of the anionic acid species on the
intensity of sour taste, the sour taste thresholds of nine kinds of