The major differences between solutions were the concentration of hydrogen ions and the concentration of protonated acid species. The pH was about 7.1 for the sodium acetate solution, and the pH values were 3.77, 3.59 and 3.49 for 50, 100 and 150 mg/ l acetic acid solutions, respectively. In an aqueous solution there is an equilibrium between different acetic acid species, and the extent of the equilibrium is determined by the pKa value. The ratios of different acetic acid species can be calculated by the pH value. For acetic acid, its pKa value is 4.75 at 25 C. Thus, it can be calculated that the anionic acetic acid species is more than 99% when the pH is over 6.75, and that the protonated acetic acid species accounts for more than 99% when the pH is less than 2.75. Therefore the sodium acetate solution just contained the anionic acetic acid species as its pH value was about 7.1. Therefore it can be concluded that the anionic acid species makes no contribution to sour taste, and that hydrogen ions and protonated acid species could be the chemical substances producing sour taste.