Psychophysical evidence that oral astringency is a tactile sensation

Aluminum potassium sulphate [AIK(SO4)2 12H2O—‘alum’] was tested for its ability to elicit oral astringency independently of its ability to elicit taste. First, the astringency produced by alum (10 g/l; 21.1 mM) on the non-gustatory surfaces between the gum and the upper lip was measured. Subjects reported that alum elicited sensations of astringency when the upper lip was moved laterally against the gum. Second, subjects dipped the tongue into two solutions—alum or a mixture solution that approximated the taste of alum—and attempted to determine which solution was more astringent. A slight tendency was found for subjects to identify the mixture as more astringent than the alum. However, the same subjects reported the alum to be more astringent than the mixture when the same solutions were applied under the upper lip. These two experiments support the hypothesis that tactile stimulation is important for producing astringency, whereas taste stimulation of the anterior tongue is not. Third, after the application of alum, lubricating rinses (water, sucrose, Salivart®, corn oil, and the subjects' own saliva) were compared for their ability to decrease astringency. The lubricants reliably decreased original astringency, but to varying degrees depending upon their lubricating properties. All three experiments suggest that tactile sensations caused by increased friction (decreases,in salivary lubrication) between oral membranes are the primary basis of astringent sensations.