Three astringents — grape seed extract (GSE), epigallocatechin-gallate (EGCG) from green tea, and aluminum sulfate (Alum) — were evaluated by volunteers for perceived astringency intensity over the course of 80 oral exposures (sips). The astringency intensity curves grew exponentially over exposures, regardless of the compound or concentration ( Figure 1 A and Figure S1 in the Supplemental Information ). Each astringent solution, however, elicited distinct maxima (a + y0) depending on the concentration level. Weakly concentrated solutions never reached the astringency maxima of more concentrated solutions of the same compound. In addition, the rate (b) at which maximal intensity was reached over trials varied with both the chemical structure and concentration ( Figure 1 B). Here, Alum and EGCG solutions reach their astringency maxima faster at high concentrations than low, whereas GSE solutions reach their maximum at the same rate regardless of the concentration.

(A) Perceived astringency ratings over 80 oral exposures of 1.5 g/L (open circles) and 0.5 g/L (solid circles) of grape seed extract solutions. Values represent mean ± SEM. 21 subjects were tested. Least squares fitted curves follow the equation: f = y0 + a ∗ (1– exp(–b ∗ x)). Asterisk: p < 0.05. (B) Parameters (b) [Rate, left panel] and (a + y0) [Maximum, right panel] calculated from the fitting curves of three different astringents (grape seed extract (circles), epigallocatechin-gallate (squares) and aluminum sulfate (triangles)) tested at 3 concentrations: 0.5, 0.75, and 1.5 g/L. (C) Perceived fattiness (left panel) and astringency (right panel) rated after drinking: five sips of tea (orange circles) or water (dark brown circles) interspersed between ingestion of 5 g of dry meat, or five sips of tea only (green triangles). Values represent mean ± SEM. 21 subjects were tested and each test was performed in triplicate. Asterisk: p < 0.05, tea rinsing versus water rinsing versus tea only. (D) Perceived fattiness (left panel) and astringency (right panel) rated immediately after eating 5 g of dry meat, followed by: nothing (2.5 min of rest time, green triangles), 5 sips of tea (orange circles), or 5 sips of water (dark brown circles). Values represent mean ± SEM (n = 21). Each test was performed in triplicate. Asterisk: p < 0.05, tea rinsing versus water rinsing versus dry meat only.

After establishing that weak astringents could elicit strong astringency with repeated sampling, we asked subjects to rate fattiness and astringency, after ingesting pieces of fatty food (dried meat) alternating with multiple sips of one of two rinsing solutions (tea or water). Astringent rinses affected oral sensations. In particular, the perceived fattiness was less pronounced after drinking tea than after drinking water ( Figure 1 C left panel). Thus, astringent consumption during meals provided a greater reduction of oral fattiness compared to water rinses. We also observed a more significant growth of astringency sensation with multiple sips without eating a fatty food, indicating that fat reduced the build-up of astringency ( Figure 1 C right panel). Similarly, there was a greater increase of fatty sensation from repeated fatty food consumption without any rinsing ( Figure 1 D left panel). These observations support the hypothesis that these sensations oppose each other perceptually and lie at different ends of an oral rheological/tribological sensory spectrum.

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The oral perceptions of fattiness and astringency largely depend on tactile sensations []. Frictional rubbing of the oral epithelia is very important for food tactile perception. For example, foods are commonly assessed mechanically ex vivo using a tribometer, which measures the traction force between two bodies in rubbing motion []. The basic rheology of oral lubricants, such as saliva and fats from foods, has previously been studied [], but very little is known about the cumulative rheological effects of astringent stimuli over multiple oral exposures, such as occurs when sipping red wine during a meal. Our new results show three fundamental properties of multiple oral exposures to weak astringents: the asymptotic limits to growth in perceived astringency, the role of stimulus strength in determining these limits, and the role of chemical structure to astringency growth rate. Thus, the nature of the astringent stimulus greatly impacts the evolution of the perceived astringency over multiple exposures. Furthermore, we support the hypothesis that oral fatty sensations and astringency represent opposite ends of an oral spectrum extending from ‘slippery’ at one end to ‘dry’ and ‘rough’ at the other. This provides an explanation of how these sensations interplay over the course of a meal and maintain the balance of a moderate position along a tribological scale of oral sensations. Work is still required to determine how different wines, teas and acidic foods, such as pickles and sorbets, vary in their efficacy of ameliorating oral fatty sensations during meals, and whether differences among individuals in the degree of cleansing effect by astringents is linked to their respective differences in oral tactile sensitivities. Astringent foods are desired with meals as they appear to provide a pleasant sensation of ‘cleanness’ in the mouth, removing after-tastes and fatty mouth coating sensations, as referred to in their international conceptualization as “palate cleansers”.