The effect of oral drying and astringent liquids on the perception of mouth wetness

A feeling of mouth dryness occurs from actual drying of the oral surfaces or from sampling astringent substances such as polyphenols (e.g., tannins in brewed tea and wine), which bind proline-rich proteins in saliva to reduce its lubricity. Here we investigated the interactions between physical drying and the effect of polyphenols on the subjective state of oral hydration. Twelve subjects rated the perceived wetness/dryness of their mouth using a labeled magnitude scale, after the mouth was dried with air for 35 s, or the subjects waited for an equal period of time during which the mouth was not dried. Subsequently, 1.5 mL volumes of an astringent solution (5 g L− 1 tannic acid in distilled water), distilled drinking water, or a sweet solution (40 g L− 1 sugar in mango tea with no tannins) were introduced into the mouth. After swishing and swallowing, the subject rated the wetness of the mouth for 4.3 min. The liquids were found to differ in their ability to wet the mouth (p < 0.0001). The least wet sensations were reported for the astringent solution, on average; however, the differences among liquids were not equally pronounced at all times during the observation period (p < 0.02). When the mouth was normally hydrated (i.e., had not been dried), the wetting effectiveness of the three liquids, based on the ratings, differed most greatly immediately after they had been received and swallowed. In contrast, when the mouth was dried, the liquids did not differ at this time. That the astringent solution did not have less wetting effectiveness in the dried mouth was attributed to the absence of precipitable salivary proteins. The findings suggest that the refreshment value of astringent drinks, based on their perceived wetting effectiveness, may vary with the state of oral hydration.