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

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