Sensory irritation and taste produced by NaCl and citric acid: effects of capsaicin desensitization

Abstract

Three experiments were conducted to measure the sensory irritation produced by two prototypical gustatory stimuli: citric acid and NaCl. The stimuli were applied to the tip of the tongue on filter paper disks. The first experiment revealed that solutions of NaCl and citric acid that produced approximately equal taste sensations also produced similar amounts of irritation; that the psychophysical functions for irritation were approximately twice as steep as the functions for taste; and that irritation grew over time for NaCl but not for citric acid. When viewed as a percentage of the taste sensation at 25 s, NaCl irritation averaged 23% at the lowest concentration and 70% at the highest concentration; citric acid irritation averaged 44% at the lowest concentration and 98% at the highest concentration. The second experiment investigated whether the irritation produced by these two stimuli was mediated via capsaicin-sensitive (CS) fibers. The experiment included a pre-test, an irritation treatment with either capsaicin (a desensitizing agent) or zingerone (a non-desensitizing agent), a 15 min rest period and a post-test. Reductions in irritation and taste occurred following treatment with both capsaicin and zingerone. A third experiment demonstrated that the majority of the effect of zingerone on taste and irritation was due to a perceptual context effect. After the context effect was taken into account, capsaicin desensitization remained significant for both salt taste and salt irritation at the highest concentration. A similar pattern of results for citric acid suggests that both citric acid and NaCl produce irritation in part via CS fibers. The results are discussed in terms of the ability of subjects to discriminate the gustatory and chemesthetic components of oral sensations and the role of salt and acid irritation in flavor perception.