3.2. The contribution of the protonated organic acid species to sour taste In order to verify whether the protonated acid species makes a contribution to sour taste, different concentrations of sodium acetate solutions were prepared and adjusted to pH 4.0 using hydrochloric acid. The intensity of sour taste of these solutions was then scored (Fig. 2). Fig. 2 shows that the intensity of sour taste for these sodium acetate solutions increased with an increase in concentration of protonated acetic acid species. In the acetate solutions, the concentrations of hydrogen ions were identical, but the concentrations of protonated acetic acid species were different. Thus the results suggested that the protonated acetic acid species cannot only produce a sour taste, but the higher its concentration, the stronger is the sour taste.