Comparative analysis of sourness intensity among inorganic and organic acids

Sourness perception is generally associated with proton concentration; however, the sourness intensity often differs among acidic substances, even at similar pH values. In this study, we examined the concentration- and pH-dependent changes in the sourness intensity of several inorganic and organic acids, including hydrochloric acid (HCl), phosphoric acid, citric acid, gluconic acid, glutamic acid, lactic acid, and, malic acid, using human sensory evaluation. For all the acids tested, the sourness intensity increased with increasing concentration, although the magnitude of the response differed among the acid species. HCl, phosphoric acid, and malic acid showed comparable concentration–response profiles, whereas gluconic acid, lactic acid, and glutamic acid exhibited weaker sourness. Analyses based on the measured pH values showed that acids with similar pH values did not necessarily produce similar sourness intensities, indicating that pH alone does not fully determine sour taste perception. At a fixed pH of 2.5, gluconic acid and malic acid were perceived to be significantly sourer than HCl. These results demonstrate that sourness intensity depends on the acid species rather than the proton concentration alone, even under identical pH conditions, highlighting the pH-independent aspects of sourness perception. Such acid-specific effects may be relevant for understanding sour taste mechanisms and optimizing acid selection in food formulations.