The sour taste of acids. The hydrogen ion and the undissociated acid as sour agents

In the first of two experiments, the sourness of aqueous solutionsof HCl and of several carboxylic acids was measured by meansof a fitter-paper method. Perceived sourness of the carboxylicacids was positively correlated with the dissociation constantK_al. In contrast to HCl, the H⁺ concentration in carboxylicacids appeared not to be the only factor in eliciting sourness.Decreasing pH by buffering of the carboxylic acids with theirsodium salts did not lower perceived sourness. The rank-orderof the acids, according to the amount of NaOH needed for titration,depends on the pH to which it is titrated. The rank-order ofpH 4.4 reasonably fits the sourness rank-order. In the secondexperiment, adaptation and cross-adaptation of HCl, tartaric-lactic-,and acetic acid were measured. Neither self- nor cross-adaptationwas observed in the case of HCl, whereas with the carboxylicacids self- and mutual cross-adaptation did occur. The presentresults suggest that HCl-sourness and the sourness of carboxylicacids are elicited by different receptor processes.