Potassium gymnemate and the sweet and bitter taste provoked electrically

The sweetness or bitterness of taste solutions of saccharin and quinine may be blocked, while the sweetness or bitterness evoked by electrical stimulation of the tongue by cathodal polarization is unchanged. Electrical taste stimulation apparently bypasses the most peripheral process by which taste solutions elicit sweet and bitter and thus acts directly on the receptor and/or its afferent nerve terminals.     

Both perceptual and conceptual factors influence taste-odor and taste-taste interactions

Observers are often asked to make intensity judgments for a sensory attribute of a stimulus that is embedded in a background of “irrelevant” stimulusdimensions. Under some circumstances, these background dimensions of the stimulus can influence intensity judgments for the target attribute. For example, judgments of sweetness can be influenced by the other taste or-odor qualities of a solution (Frank & Byram, 1988; Kamen et al., 1961). Experiments 1 and 2 assessed the influence of stimulus context, instructional set, and reference stimuli on cross-quality interactions in mixtures of chemosensory stimuli. Experiment 1 demonstrated that odor-induced changes in sweetness judgments were dramatically influenced when subjects rated multiple attributes of the stimulus as compared with when they judged sweetness alone. Several odorants enhanced sweetness when sweetness alone was judged, while sweetness was suppressed for these same stimuli when total-intensity ratings were broken down into ratings for the sweetness, saltiness, sourness, bitterness, and fruitiness of each solution. Experiment 2 demonstrated a similar pattern of results when bitterness was the target taste. In addition, Experiment 2 showed that the instructional effects applied to both taste-odorand taste-taste mixtures. It was concluded that the taste enhancement and suppression observed for taste-odor and taste-taste mixtures are influenced by (1) instructional sets which influence subjects’ concepts of attribute categories, and (2) the perceptual similarities among the quality dimensions of the stimulus.     

Perceptual changes in taste mixtures

Three studies were conducted to quantify perceptual changes that occur when sapid chemicals are tasted in mixture solutions. The primary effect when mixing sweetness (glucose or fructose) with salt (NaCl), sour (citric acid), or bitter (quinine sulfate) was to reduce the intensity of each taste in the mixture. The reduction was not equal for the two components, although the overall (total) taste intensity of the mixture appeared to be approximately 50% of the sum of the intensities of the unmixed components. Mixtures of sweet and salt developed an “unblended” or “clashing” taste, in which the components alternated in attempting to dominate the taste percept. Sweet mixed with either sour or bitter blended in almost all proportions. The “flavor” of sweetness in mixtures differed from that of simple sugar sweetness, suggesting that the presence of a second taste modified the qualitative aspect of sweetness. The magnitude of change in sweetness quality depended upon the sugar being rated, and upon the quality and intensity of the second, or modifying, taste.     

Quality-specific effects of aging on the human taste system

Elderly persons are known to have elevated taste thresholds, with those for bitter more affected by age, for example, than those for sweet. Do analogous quality-specific effects occur at suprathreshold levels? Young (mean age = 20.3 years, SD = 2.99) and elderly (mean age = 72.5 years, SD = 4.58) subjects made magnitude estimates of sweetness, bitterness, sourness, and saltiness for the unmixed components sucrose, caffeine, citric acid, and NaCl at three concentration levels for each. They also made magnitude estimates of the separate taste qualities in two-component mixtures of sucrose with each of the other three qualities, at various levels of the two components in each mixture. Magnitude estimates of taste intensity were interweaved with magnitude estimates of the heaviness of six weights, which subjects were to judge on the same subjective intensity scale: This is the calibration feature of the method of magnitude matching, and permits the comparison of elderly and young subjects on the absolute intensity of tastes. When unmixed components were judged, elderly subjects found the characteristic tastes of caffeine and citric acid less intense than, but those of sucrose and NaCl as intense as, younger subjects did. In judging mixtures, the elderly found bitterness, but not the other three qualities, less intense than did the young subjects.     

Gustatory cross adaptation between sweet-tasting compounds

Taste profiles were obtained for 16 compounds after adaptation to sucrose, saccharin, and water. Sucrose adaptation reduced the sweetness of all sweet compounds. Saccharin adaptation, when analyzed over all compounds, also reduced sweetness, but the effect was less than that of sucrose. It is concluded that there may be a single receptor mechanism for the sweet quality. Adaptation to sucrose also increased the saltiness, sourness, and bitterness of the other compounds slightly. This increase should be attributed to the taste : induced in water by adaptation to sucrose rather than a potentiation of the other compounds per se.     

Effects of solution viscosity on perceived saltiness and sweetness

Subjects used magnitude estimation to judge the perceived saltiness or sweetness of a series of aqueous solutions containing five suprathreshold concentrations of NaCl or sucrose and thickened with sodium carboxymethylcellulose (CMC). In the first experiment, CMC-H (high viscosity form) was used to thicken a series of sucrose and NaCl solutions to six viscosity levels (1–2,025 centistokes). At the highest viscosity levels, significant decreases occurred in the perceived taste intensity of only the lower concentrations of sucrose and NaCl. A second experiment determined that variations in the quantity of solution sampled from cups did not systematically influence judgments of saltiness when the starting volume was 10 ml. In the third experiment, aqueous solutions containing sucrose or NaCl were thickened with the low (L), medium (M), or high (H) viscosity form of CMC (1–1,296 centistokes). CMC-L-thickened solutions produced little or no suppression of perceived taste intensity, whereas viscous CMC-H solutions produced significant reductions in perceived saltiness and sweetness.     

Perception of sweetness and bitterness in different vehicles

In the present study, we investigated taste-taste, taste-vehicle, and simultaneous taste-vehicle-taste mixtures. Subjects made estimates of the sweetness and bitterness of 27 stimuli. Sucrose (292, 585, and 1170 mM), caffeine (13, 26, and 52 mM), and binary mixtures of low (292-13 mM), middle (585-26 mM), and high (1170-52 mM) levels of both components were dispersed in water, carboxymethylcellulose (CMC) 1% w/v, and gelatin 6% w/v. The sweetness and bitterness of the sucrose-vehicle-caffeine combinations were significantly weaker than the respective sucrose-vehicle and caffeine-vehicle combinations. The emerged mutual suppressive effects were asymmetrical and persisted when both tastants were presented in CMC and gelatin. Moreover, the increase in vehicle consistency and the simultaneous addition of another taste reduced the perceived intensity of a taste either presented alone or dissolved in water. For both sweetness and bitterness, the total taste suppression observed was always significant.     

Effects of context on sweet and bitter tastes: unrelated to sensitivity to PROP (6-n-propylthiouracil).

In a double-shifting context paradigm, subjects gave magnitude estimates of the perceived intensity of qualitatively dissimilar taste substances (saccharin and quinine, sucrose and quinine) or qualitatively similar ones (saccharin and sucrose), with each pair of substances taking on different contextual sets of concentrations in different sessions. The dissimilar pairs produced substantial differential effects of context (e.g., a particular concentration of saccharin or sucrose was judged more intense than a particular quinine in one contextual setting, less intense in another), but the similar pair did not. This result accords with the hypothesis that differential context effects depend on qualitative similarity. Contrary to expectations, however, the magnitude of the context effect did not differ in tasters and nontasters of the bitter substance 6-n-propylthiouracil (PROP), groups previously shown to differ in sensitivity to bitterness in saccharin. Similarity judgments suggest that saccharin and sucrose were qualitatively alike for all subjects, regardless of sensitivity to PROP.     

Effects of context on sweet and bitter tastes: Unrelated to sensitivity to PROP (6-npropylthiouracil)

In a double-shifting context paradigm, subjects gave magnitude estimates of the perceived intensity of qualitatively dissimilar taste substances (saccharin and quinine, sucrose and quinine) or qualitatively similar ones (saccharin and sucrose), with each pair of substances taking on different contextual sets of concentrations in different sessions. The dissimilar pairs produced substantial differential effects of context (e.g., a particular concentration of saccharin or sucrose was judged more intense than a particular quinine in one contextual setting, less intense in another), but the similar pair did not. This result accords with the hypothesis that differential context effects depend on qualitative similarity. Contrary to expectations, however, the magnitude of the context effect did not differ in tasters and nontasters of the bitter substance 6-n-propylthiouracil (PROP), groups previously shown to differ in sensitivity to bitterness in saccharin. Similarity judgments suggest that saccharin and sucrose were qualitatively alike for ail subjects, regardless of sensitivity to PROP.     

Gustatory cross adaptation: Sourness and bitterness

In five experiments, Sa were presented with a variety of sour and bitter compounds after the tongue was rinsed with distilled H₂0, QHCl, urea, or citric acid. All the acids tested were significantly less sour following adaptation to citric acid than after adaptation to distilled H₂O. The taste of these acids was not affected by rinsing the tongue with QHCl or urea. QHCl adaptation markedly reduced the bitterness of some compounds, while having little effect on others, including urea and citric acid. Both urea and citric acid had smaller but reliable effects on the bitterness of QHCI. These apparently incompatible results do not seem to be the result of a simple verbal confusion between sourness and bitterness. Some compounds were not affected by any of the adapting conditions. The coding mechanisms for the sourness of acids appears to be relatively simple, while that for bitterness is more complex.     

The effectiveness of different sweeteners in suppressing citric acid sourness

The exact mechanism that causes taste suppression in a perceptually heterogeneous mixture, and the locus of that mechanism, are as yet unknown. The present study was designed to explore the idea that mixture suppression is a perceptual phenomenon and not the result of physical, chemical, or receptor-substance interactions. An investigation was carried out as to whether perceptually similar taste stimuli give rise to the same sensory interactions when mixed with a substance of a different taste quality. In the first study, five different sweeteners (sucrose, fructose, aspartame, saccharin, and sorbitol) were matched in perceived sweetness intensity, in order to obtain five perceptually similar stimuli. Every equisweet sweetener concentration was mixed with each of four citric acid concentrations. In a second study, the sourness-suppressing effects of two sweeteners, sucrose and aspartame, were compared at four different concentration levels. Sourness scale values of unmixed citric acid, the unmixed sweeteners, and the citric acid/sweetener mixtures were assessed with a functional measurement approach in combination with a two-stimulus procedure. The equisweet sweeteners were equally effective in suppressing the perceived sourness intensity of citric acid over the concentration range used. The side tastes of the sweeteners, if present, did not have a substantial effect on the degree of sourness suppression.     

Detection and apparent taste intensity of salt—acid mixtures in two media

Gustatory responses to mixtures of sodium chloride and citric acid were measured in two media of dispersion, distilled water and green bean puree, by fifteen experienced Ss. The two psychophysical methods employed, detection thresholds and apparent taste intensity, showed good agreement. Salt sharply depressed apparent sourness in both media, as well as interfering with detection of sourness. Citric acid influenced apparent saltiness in a complex manner, dependent upon concentration, media, method, and tile individual S. In water, lower concentrations of acid generally enhanced and higher concentrations depressed saltiness. Within both media, half the Ss indicated an enhancement of saltiness with increasing acidity and half indicated the reverse. Higher acid levels interfered with detection of saltiness. This biphasic response to binary taste stimuli are discussed and compared with pertinent findings from previous literature.     

Response to bitter substances in primates: Roles of diet tendency and weaning age

In primates, the perception of bitter taste may be an essential mechanism for avoiding the ingestion of bitter, and often toxic, substances. Bitterness sensitivity varies across the different primate species and, for bitter thioure substances (e.g., phenylthiocarbamide—PTC), within species. Primates respond to bitterness by displaying aversive affective reactions, and whether these serve for conspecifics as information on the taste of food is still unclear. The aim of this study was to investigate the response to two bitter substances (quinine and PTC) in 11 primate species (N = 29 individuals) hosted at the Ménagerie du Jardin des Plantes, in relation to their main diet tendency and weaning age. We tested primate reactions to bitterness with a two-bottle method. The study individuals showed a strong aversion to bitter taste, minimizing consumption of both bitter solutions. All of the individuals were PTC-taster phenotypes, but the degrees of sensitivity to PTC varied among the individuals. Across-species comparisons revealed that whereas the degree of frugivory of the species had a negative effect on the consumption of bitter solutions by the individuals, a later weaning age seems to be a better predictor for the occurrence of aversive affective reactions. Although the low sample size does not allow for excluding interindividual variability, our results support the hypothesis that affective reactions to bitterness may be trustworthy information for conspecifics during the learning process. Thus, the evolution of the appropriate perceiver systems to convert affective displays into true affective signals could be a shared trait among human and nonhuman primates.     

An application of the τ scale of taste: Interaction among the four qualities of taste

A series of 10 standard solutions, spaced equidistant on the τ scale, were prepared for each of the basic tastes, sweet, bitter, sour, and salty. In addition, four other concentrations of each basic taste, plus 96 compound solutions of two different solutes, were prepared as test stimuli. The Ss, four specialists in taste testing and four laymen, were asked to specify the taste of each test stimulus by matching it with the standard series. In this way, both the precision of the matching procedure and the enhancement or masking of one taste by another were measured in terms of τ, which is a nearly logarithmic function of concentration. The average standard deviations of the matchings among the Ss were in the order of 0.3 to 0.5 units on the τ scale, which corresponds to a value between 1 and 2 decilogs of concentration. Among the interactions observed was the mutual masking of sucrose and quinine sulfate.     

Covariation in individuals’ sensitivities to bitter compounds: Evidence supporting multiple receptor/transduction mechanisms

People vary widely in their sensitivities to bitter compounds, but the intercorrelation of these sensitivities is unknown. Our goal was to investigate correlations as a function of individual sensitivities to several bitter compounds representative of different chemical classes and, from these correlations, infer the number and variety of potential bitterness transduction systems for these compounds. Twenty-six subjects rated and ranked quinine HCl, caffeine, (?)-epicatechin, tetralone, L-phenylalanine, L-tryptophan, magnesium sulfate, urea, sucrose octaacetate (SOA), denatonium benzoate, andn-propylthiouracil (PROP) for bitterness. By examining individual differences, ratings and rankings could be grouped into two general clusters—urea/phenylalanine/tryptophan/epicatechin, and quinine/caffeine/SOA/denatonium benzoate/tetralone/magnesium sulfate—none of which contained PROP. When subjects were grouped into the extremes of sensitivity to PROP, a significant difference was found in the bitterness ratings, but not in the rankings. Therefore, there are also subjects who possess diminished absolute sensitivity to bitter stimuli but do not differ from other subjects in their relative sensitivities to these compounds.     

There's more to taste in a coloured bowl

The flavour and pleasantness of food and drinks are affected by their colour, their texture or crunch, and even by the shape and weight of the plate or glass. But, can the colour of the bowl also affect the taste of the food it contains? To answer this question we served popcorn in four different coloured bowls, and participants rated sweetness, saltiness, and overall liking. The sweet popcorn, in addition to being sweet, was perceived as saltier when eaten out of a coloured (as compared to a white) bowl, and vice versa for the salty popcorn. These results demonstrate that colour in bowl design can be used to elicit perceptions of sweetness and saltiness in real foods.     

Covariation in individuals' sensitivities to bitter compounds: evidence supporting multiple receptor/transduction mechanisms

People vary widely in their sensitivities to bitter compounds, but the intercorrelation of these sensitivities is unknown. Our goal was to investigate correlations as a function of individual sensitivities to several bitter compounds representative of different chemical classes and, from these correlations, infer the number and variety of potential bitterness transduction systems for these compounds. Twenty-six subjects rated and ranked quinine HCl, caffeine, (-)-epicatechin, tetralone, L-phenylalanine, L-tryptophan, magnesium sulfate, urea, sucrose octaacetate (SOA), denatonium benzoate, and n-propylthiouracil (PROP) for bitterness. By examining individual differences, ratings and rankings could be grouped into two general clusters--urea/phenylalanine/tryptophan/epicatechin, and quinine/caffeine/SOA/denatonium benzoate/tetralone/magnesium sulfate-none of which contained PROP. When subjects were grouped into the extremes of sensitivity to PROP, a significant difference was found in the bitterness ratings, but not in the rankings. Therefore, there are also subjects who possess diminished absolute sensitivity to bitter stimuli but do not differ from other subjects in their relative sensitivities to these compounds.     

Odorous and pungent attributes of mixed and unmixed odorants

In order to explore functional properties of the olfactory and common chemical senses as well as their relation to the total nasal sensation experienced, various concentrations of two pungent odorants were presented alone and in the presence of different backgrounds of the other irritant. Stimuli comprised formaldehyde (at 1.0, 3.5, 6.9, and 16.7 ppm), ammonia (at 210, 776, 1,172, and 1,716 ppm), and their 16 possible binary mixtures. Subjects were asked to estimate the total nasal perceived intensity, and then to assess the olfactory (odor) and common chemical (pungency) attributes of the evoked sensations. The results showed that stimulus-response functions for pungency are steeper than those for odor. Furthermore, odor was always hypoadditive in mixtures (i.e., mixtures were perceived as less intense than the sum of their components), whereas pungency was, mainly, additive, and even suggested hyperadditivity. Total perceived intensity of the stimuli, alone and in mixtures, followed the stimulus-response patterns for pungency, which, therefore, emerged as the dominating attribute used by subjects in scaling the explored range of concentrations. The relationship between total nasal perceived intensity of the mixtures and that of their components reflected hypoaddition, resembling the outcome for the odor attribute.     

Evidence for neural inhibition in bittersweet taste mixtures.

Three lines of evidence from psychophysical experiments implied that mutual suppression of bitter and sweet tastes is due to neural inhibition rather than chemical interactions in solution or competition of molecules for common receptor sites. Removal of sweetness from bittersweet mixtures caused the bitterness to increase. This was accomplished by adaptation to sucrose or by treatment with Gymnema sylvestre, neither of which affect the concentration of sucrose on the tongue. Such increases in the bitterness of mixtures, independent of the concentration of the sweet masking substance, are difficult to reconcile with suppression by means of chemical interactions. Similar dependence of suppression on perceived intensity (and independence from concentration) was observed with mixtures of phyenylthiocarbamide and sucrose. Tasters of phenylthiocarbamide showed stronger suppression of sweetness than nontasters. This result was also inconsistent with molecular interactions causing suppression, which would have resulted in the same degree of suppression for the two groups. Instead, these findings support neural explanations of mixture suppression, such as antidromic inhibition or occlusion.     

Hunger alters the expression of acquired hedonic but not sensory qualities of food-paired odors in humans

To test whether expression of hedonic and sensory odor qualities acquired by association with sweet and bitter tastes depend on hunger state, hungry volunteers experienced odors paired with sucrose, quinine, or water and then were tested under different hunger states manipulated with energy preloads. Acquired liking for sucrose-paired odors was evident following a low-energy or control preload but not a high-energy preload; however, odor sweetness increased in all preload conditions. Acquired dislike and increased bitterness of quinine-paired odors were independent of preloading. These data demonstrate hunger-dependent expression of acquired liking for flavors through flavor-flavor associations in humans and demonstrate independence between acquired hedonic and sensory qualities of odors.