INTERACTION OF ASTRINGENCY AND TASTE CHARACTERISTICS1

The perception of astringency and basic taste in mixtures and their interaction effects were investigated by two procedures. In Experiment 1, focused and nonfocused testing procedures were compared using mixtures of low and high concentrations of alum and basic taste solutions. Both procedures yielded taste and astringency intensities that were modality‐dependent. Nonfocused testing was used in Experiment 2 to investigate the interactions of astringent phenolic (tannic acid) and nonphenolic (alum) compounds with each basic taste. Sweetness of sucrose increased with increased concentration with or without alum or tannin present. Changes in salty, bitter, and sour taste intensities were modality‐dependent. Astringency either remained unchanged or decreased with the addition of sucrose, sodium chloride, citric acid, or caffeine depending upon the taste concentration. Bitterness of tannin and alum at high concentrations was suppressed by the addition of sucrose, sodium chloride, or citric acid; sourness also decreased in the presence of sucrose or sodium chloride as well as a high level of caffeine.     

Time-course of astringent sensations

Qualitative and quantitative perceptual reactions to astringentmaterials were examined for three diverse chemical substances(alum, tannic acid and tartaric acid) at several concentrationsproducing moderate to strong levels of perceived sensation.Group discussions were held to determine language appropriateto describe the sensations arising from solutions of the threecompounds and a composite ballot of six rating scales (astringency,mouth drying, puckery feeling, mouth roughing, bitterness andsourness) was developed. For both experiments, two concentrationsof each compound were rated on the six attributes for five tosix minutes, a discrete-point time-intensity scaling procedure.All ratings showed roughly exponential decays over time. Theintensity ratings for each attribute were found to depend onboth the particular astringent substance and concentration tested.The results from experiment 2 suggested that the four tactileattributes of drying, puckery feeling, roughing, and overallastringency may not be totally interchangeable and that theremay be multiple sub-qualities in the sensory reactions groupedas astringency. It is recommended that future structure-activitystudies make use of time-intensity procedures with multiplerated attributes, using 1 g/1 alum as a reference material,since it is relatively low in perceived bitterness and sourness,but produces pronounced drying, roughing, puckery/drawing sensations.     

Responses of lipid membranes of taste sensor to astringent and pungent substances

Astringent substances and pungent substances were studied usinga multichannel taste sensor with lipid membranes. The electric-potentialpattern constructed of eight outputs from the membranes hasinformation of taste quality and intensity. Pungent substances,such as capsaicin, pipeline and allyl isothiocyanate, had noeffect on the membrane potentials of the lipid membranes. Onthe other hand, astringent substances such as tannic acid, catechin,gallic acid and chlorogenic acid changed the potentials remarkably.A principal component analysis of the patterns in electric potentialchanges caused by tbe taste substances revealed the astringencyis located between bitterness and sourness.     

Interactions of astringent substances

Two-component mixtures of astringent materials were rated forperceived intensity of astringent and taste attributes overtime. Components included alum (a complex salt), gallic acid(the monomeric component of hydrolyzable tannins), catechin(the monomeric component of condensed tannins) and citric acid.Mixtures of alum and gallic acid showed mixture suppression,in that the 50/50 mixture was less intense than either componentin astringency, drying, roughing and puckery/drawing sensations.Suppression was seen at concentration levels producing moderateto strong astringency but was absent or less pronounced at lowerconcentration levels. A similar pattern held for citric acid,although the suppressive effects were less pronounced. Catechinand gallic acid mixtures were additive. Sensory interactionsbetween astringent materials appears to depend on the substancesinvolved and their concentrations (or intensity levels).     

EVALUATION OF BITTERNESS AND ASTRINGENCY OF (+)-CATECHIN AND (-)-EPICATECHIN IN RED WINE AND IN MODEL SOLUTION

Two stereoisomeric phenolic compounds, (+)-catechin and (-)-epicatechin, were rated for perceived intensity of oral astringency and bitterness by trained judges using the scalar method. Mouth drying and mouth roughening were also assessed, since they are often associated with astringency. Amounts of 375, 750 and 1500 mg/L of each compound were tasted in red wine, and in a model system, similar in composition to a dry table wine. Preliminary tests showed that these concentrations were above the threshold level but within the range found in wine. A control sample (model solution or wine without the addition of the above phenolic compounds) was also evaluated. The results showed that the two compounds were both bitter and astringent. The high (-)-epicatechin concentration was significantly more bitter and astringent than the equal concentration of (+)-catechin in the model solution. Mouth drying and roughening ratings showed a similar increasing pattern with the ratings of astringency particularly at the higher concentrations. However, these attributes were rated differently from astringency suggesting that although they contribute to astringency, they are not subsumed by it.     

Comparison of the effects of concentration, pH and anion species on astringency and sourness of organic acids

The separate effects of concentration, pH and anion species on intensity of sourness and astringency of organic acids were evaluated. Judges rated sourness and astringency intensity of lactic, malic, tartaric and citric acid solutions at three levels of constant pH varying in normality and at three levels of constant concentration varying in pH. To assess the comparative sourness and astringency of the organic acid anions of study, binary acid solutions matched in pH and titratable acidity were also rated. As pH was decreased in equinormal solutions, both sourness and astringency increased significantly (P < 0.001). By contrast, as the normality of the equi-pH solutions was increased, only sourness demonstrated significant increases (P < 0.001) while astringency remained constant or decreased slightly. At the lowest normality tested, all solutions were more astringent than sour (P < 0.05). Although lactic acid was found to be significantly more sour than citric acid (P < 0.05), no other sourness or astringency differences among the organic acid anions were noted. This study demonstrates for the first time that astringency elicited by acids is a function of pH and not concentration or anion species, and confirms that sourness is independently influenced by concentration, pH and anion species of the acid.      

Perceptual Properties of Benzoic Acid Derivatives

Sensory properties of equimolar concentrations of benzoic acidderivatives were examined using time-intensity procedures. Significantdifferences in maximum intensities (P < 0.005) were foundfor astringency, bitterness, prickling, sourness and sweetness.Although these compounds differed only in the number and positionof the hydroxy groups, they exhibited quite different profiles.Gentisic acid had the highest sourness and bitterness maximumintensity, salicylic and gentisic acids were highest in astringency,and m-hydroxybenzoic acid was the sweetest sample. Benzoic acidhad the highest intensity of prickling feeling which lasted20 s longer than salicylic acid and 40 s longer than the othersamples which elicited lowest intensity of prickling sensation.Chem. Senses 20: 393–400, 1995.     

Turbidity as a measure of salivary protein reactions with astringent substances

Binding of tannins to proline-rich proteins has been proposed as an initial step in the development of astringent sensations. In beer and fruit juices, formation of tannin-protein complexes leads to the well-known effect of haze development or turbidity. Two experiments examined the development of turbidity in human saliva when mixed with tannins as a potential in vitro correlate of astringent sensations. In the first study, haze was measured in filtered human saliva mixed with a range of tannic acid concentrations known to produce supra-threshold psychophysical responses. The second study examined relationships among individual differences in haze development and the magnitude of astringency ratings. Mostly negative correlations were found, consistent with the notion that high levels of salivary proteins protect oral tissues from the drying effects of tannic acid.     

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 independentlyof its ability to elicit taste. First, the astringency producedby alum (10 g/l; 21.1 mM) on the non-gustatory surfaces betweenthe gum and the upper lip was measured. Subjects reported thatalum elicited sensations of astringency when the upper lip wasmoved laterally against the gum. Second, subjects dipped thetongue into two solutions—alum or a mixture solution thatapproximated the taste of alum—and attempted to determinewhich solution was more astringent. A slight tendency was foundfor subjects to identify the mixture as more astringent thanthe alum. However, the same subjects reported the alum to bemore astringent than the mixture when the same solutions wereapplied under the upper lip. These two experiments support thehypothesis that tactile stimulation is important for producingastringency, whereas taste stimulation of the anterior tongueis 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, butto varying degrees depending upon their lubricating properties.All three experiments suggest that tactile sensations causedby increased friction (decreases,in salivary lubrication) betweenoral membranes are the primary basis of astringent sensations.     

Astringent Subqualities in Acids

Astringency, astringent subqualities (drying, roughing and puckering)and sourness were compared among six acids: hydrochloric, lactic,citric, acetic, fumaric and malic acids. The attribute profilesof organic acids were similar to each other but different fromhydrochloric acid, the only inorganic acid, which was the mostastringent and the least sour. In a second experiment, two inorganicacids (hydrochloric and phosphoric) and two organic acids (citricand malic) were tested at three concentration levels. At approximatelyequal levels of overall sensory impact, the inorganic acidswere alike in astringency and sourness, receiving higher ratingsfor roughing and drying, and lower ratings for sourness thanthe organic acids. Interactions with concentration (differencesin psychophysical functions) for the subquality of drying werenoted, in addition to the differences in the astringent subqualitiesof roughing and drying seen across acids in both experiments.The higher level of astringency for inorganic acids suggeststhat the current model for tannin binding to salivary proteinsas an explanation of astringency needs to be extended to includea direct pH-dependent effect. Chem. Senses 20: 593–600,1995     

EFFECTIVENESS OF RINSES IN ALLEVIATING BITTERNESS AND ASTRINGENCY RESIDUALS IN MODEL SOLUTIONS1,2

A two‐part study determined the effectiveness of gum‐based rinses with or without oil for alleviating residuals of a bitter (0.8 g/L caffeine solution) and an astringent (1g/L alum solution) stimuli in serial responses using a sip and spit method. In Experiment 1, rinsing with deionized water was compared to rinsing with 0.3% xanthan gum in water alone or with 5% corn oil and 0.55% carboxymethyl cellulose (CMC) in water alone or with 5% or 10% corn oil. The 0.3% xanthan gum and 5% corn oil mixture resulted in the highest stimulus intensity difference before and after rinsing, whereas 0.55% CMC had a value of nearly zero for residual effect. Results of a two‐alternative forced choice test used in Experiment 2 indicated that both were equally effective for reducing bitterness residuals, but 0.55% CMC solution resulted in the lowest (p=0.007) residual effect for astringency. Therefore, the 0.55% CMC rinse was considered an effective interstimulus rinse to use for assessing both bitterness and astringency in model solutions.     

Sensory integration in citric acid/sucrose mixtures

The scale values of perceived sweetness, sourness and totaltaste intensity of unmixed sucrose, unmixed citric acid andseveral citric acid/sucrose mixtures were assessed, using afunctional measurement approach in combination with a two-stimulusprocedure. The data showed that the scale values obtained werelinear with perceived taste intensity. It was demonstrated thatcitric acid suppresses the sweetness of sucrose and that, inversely,sucrose suppresses the sourness of citric acid. However, thissuppressive effect was not symmetrical in the range of concentrationsused. While the degree of sweetness suppression depended onlyon the citric acid level, the degree of sourness suppressiondepended on the sucrose as well as on the citric acid concentration.With regard to the perceived total taste intensity of citricacid/sucrose mixtures, it was shown that the sum of sweetnessand sourness approximately equals the total taste intensity.The implications of the present findings for the analytic—syntheticcontroversy and for taste interaction theories are discussed.     

Responses of Primate Taste Cortex Neurons to the Astringent Tastant Tannic Acid

In order to advance knowledge of the neural control of feeding,we investigated the cortical representation of the taste oftannic acid, which produces the taste of astringency. It isa dietary component of biological importance particularly toarboreal primates. Recordings were made from 74 taste responsiveneurons in the orbitofrontal cortex. Single neurons were foundthat were tuned to respond to 0.001 M tannic acid, and representeda subpopulation of neurons that was distinct from neurons responsiveto the tastes of glucose (sweet), NaCl (salty), HCI (sour),quinine (bitter) and monosodium glutamate (umami). In addition,across the population of 74 neurons, tannic acid was as wellrepresented as the tastes of NaCI, HCI quinine or monosodiumglutamate. Multidimensional scaling analysis of the neuronalresponses to the tastants indicates that tannic acid lies outsidethe boundaries of the four conventional taste qualities (sweet,sour, bitter and salty). Taken together these data indicatethat the astringent taste of tannic acid should be consideredas a distinct taste quality, which receives a separate representationfrom sweet, salt, bitter and sour in the primate cortical tasteareas. Chem. Senses 21: 135–145, 1996.     

The addition of CO2 to traditional taste solutions alters taste quality

Previous studies of the effect of carbonation on taste perception have suggested that it may be negligible, manifesting primarily in increases in the perceived intensity of weak salt and sour stimuli. Assuming CO2 solutions in the mouth stimulate only trigeminal nerve endings, this result is not altogether surprising; however, there are neurophysiological data indicating that CO2 stimulates gustatory as well as trigeminal fibers. In that case, carbonation might alter the quality profile of a stimulus without producing substantial changes in overall taste intensity--much as occurs when qualitatively different taste stimuli are mixed. To address this possibility, subjects were asked to rate the total taste intensity of moderate concentrations of stimuli representing each of the basic tastes and their binary combinations, with an without added carbonation. They then subdivided total taste intensity into the proportions of sweetness, saltiness, sourness, bitterness and 'other taste qualities' they perceived. The addition of carbonation produced only small increases in ratings of total taste intensity. However, rather dramatic alterations in the quality profiles of stimuli were observed, particularly for sweet and salty tastes. The nature of the interaction is consistent with a direct effect of carbonation/CO2 on the gustatory system, although the possibility that at least some of the observed effects reflect trigeminal-gustatory interactions cannot be ruled out.      

Effects of gymnemic acid concentration and time since exposure on intensity of simple tastes: A test of the biphasic model for the action of gymnemic acid

At several intervals following exposure to gymnemic acid, subjectsjudged the sweetness, bitterness, saltiness, and sourness ofsimple taste stimuli. The experiment was expressly designedto test Kennedy and Halpern's (1980) biphasic model for theaction of gymnemic acid. The model predicts selective suppressionof sweet taste immediately following exposure to gymnemic acidbut nonselective disruption of tastes with the passage of time.The data show dramatic reductions in sweet taste which recoverwith time but no reductions in bitterness, saltiness, or sournessat any time following exposure to any of a wide range of gymnemicacid concentrations.     

Green tea polyphenol epigallocatechin gallate activates TRPA1 in an intestinal enteroendocrine cell line, STC-1

A characteristic astringent taste is elicited by polyphenols. Among the polyphenols, catechins and their polymers are the most abundant polyphenols in wine and tea. A typical green tea polyphenol is epigallocatechin gallate (EGCG). Currently, the mechanism underlying the sensation of astringent taste is not well understood. We observed by calcium imaging that the mouse intestinal endocrine cell line STC-1 responds to the astringent compound, EGCG. Among major catechins of green tea, EGCG was most effective at eliciting a response in this cell line. This cellular response was not observed in HEK293T or 3T3 cells. Further analyses demonstrated that the 67-kDa laminin receptor, a known EGCG receptor, is not directly involved. The Ca(2+) response to EGCG in STC-1 cells was decreased by inhibitors of the transient receptor potential A1 (TRPA1) channel. HEK293T cells transfected with the mouse TRPA1 (mTRPA1) cDNA showed a Ca(2+) response upon application of EGCG, and their response properties were similar to those observed in STC-1 cells. These results indicate that an astringent compound, EGCG, activates the mTRPA1 in intestinal STC-1 cells. TRPA1 might play an important role in the astringency taste on the tongue.     

TASTE INTENSITIES OF OIL-IN-WATER EMULSIONS WITH VARYING FAT CONTENT

The objective of this study was to determine the effect fat has on the intensity of sweet, salty, sour, bitter and umami tastes in oil-in-water emulsions. The first experiment used two levels of fat (9% and 17% in oil-in-water emulsions) and two intensities of each taste (high and low). We compared the taste intensities of these emulsions to the intensities of oil-free samples with equal total volume, and to oil free samples of the same aqueous taste compound concentrations. Because of potential confusion between taste intensity and viscosity, we repeated the experiment, having panelists rate both thickness and taste intensity. Diluting with oil, compared to diluting with water, decreased bitterness, but increased the intensity of salty, sweet, sour and umami tastes. When compared to samples with equal aqueous taste compound concentrations, fat suppressed bitterness, but had no effect on the other tastes.     

Sensory evaluation of acids by free-choice profiling

The technique of free-choice profiling was applied in orderto characterize the sensory properties of some common organicand inorganic acids. Analysis of panelists' scores by generalizedProcrustes Analysis (GPA) provided information on the relationshipsamong samples and assessors for both the consensus and individualconfigurations. Results indicated that on a weight basis (w/vor v/v), acids differed in their flavor and taste dynamics.Acids were described differently by individual panelists. The GPA resulted in three important principal axes (PA). Thefirst PA had astringency/mouthfeel as the most important factor,while bitterness and sourness were the most important for thesecond and the third PAs, respectively. At 0.08% (w/v or v/v),the inorganic acids, hydrochloric and phosphoric, were moreastringent than sour. The bitterness of succinic (S) was intenseas was the sourness of fumaric, malic and the combinations offumaric:malic (FM), citric:malic (CM) and citric:fumaric (CF).The sensory characteristics of adipic and quinic were perceivedto be very weak at this concentration. The relationship betweenastringency and pH was more evident than was the relationshipbetween pH and sourness.     

The influence of acid on astringency of alum and phenolic compounds

Astringency of aqueous solutions of phenolic compounds (grape seed tannins, tannic acid, catechin and gallic acid) increased upon addition of citric acid, whereas the astringency of alum was reduced. Astringency of alum was decreased equivalently by addition of equi-sour levels of lactic acid, citric acid or hydrochloric acid. The difference between alum and the phenolic compounds is speculated to result from chemical modifications affecting binding of the astringents with oral proteins rather than cognitive differences. Chelation of the aluminum ion in alum by acids reduces its availability for interacting with salivary proteins or epithelial proteins. In contrast, the increased astringency produced upon acidification of phenolic compounds is speculated to result from the pH driven increase in the affinity of the phenols for binding with proteins. These results suggest that alum cannot be used interchangeably with phenolic astringents in psychophysical studies.