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.     

Perceptual integration of tertiary taste mixtures

Integration psychophysics was used to explore the taste perception of mixtures of sucrose, fructose, and citric acid. Three levels of each stimulus were varied in a 3 x 3 x 3 factorial design. Subjects rated total intensity, sweetness, and acidity of the 27 mixtures on graphic rating scales. Consistent with earlier work, the perceived total intensity of the tertiary mixtures was found to be dictated by the intensity of the (subjectively) stronger component alone (i.e., either the integrated sweetness or the acidity, whichever was the more intense). In contrast, the sweetness and acidity of the mixture were susceptible to mutual suppression: Sweetness suppressed acidity, acidity suppressed sweetness. There was, however, a difference between sucrose and fructose in their interactions with citric acid, fructose being the more susceptible to suppression. This selectivity of suppression indicates that the two sweetnesses could not have been inextricably integrated. Implications for taste coding are discussed, and the findings are reconciled in terms of two separate coding mechanisms: one for taste intensity, another for taste quality.     

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.     

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.     

Reinforcement and Inhibition of Eyelid Reflexes

Taste preferences, as measured in 48-hour, Richter-type drinking tests (test solution opposite distilled water), were determined for northern grasshopper mice (Onychomys leucogaster, ssp. breviauritus). The Ss were nine males and nine females which were individually housed within an environmental chamber. The test solutions were prepared from five sugars (fructose, glucose, lactose, maltose, sucrose), three salts (magnesium sulfate, potassium chloride, sodium chloride), and two acids (citric acid and hydrochloric acid). In randomly assigned order, each sugar and each salt solution was presented at five molar concentrations, and each acid was paired with distilled water at six levels of pH. Strong drinking preferences were shown for all concentrations of the sugars above .05?.10 M, and sugars ranked in order of preference as follows: Maltose = sucrose > glucose = fructose > lactose. Preferences were also shown for hypotonic concentrations of NaCl. The other salts and both acids, however, were indifferently preferred at low concentrations and were rejected at the higher concentrations. Taste preferences by grasshopper mice for these chemicals were similar to those exhibited by Mongolian gerbils tested with the same items. The similar patterns of preference shown by New World cricetid rodents (grasshopper mice) and Old World cricetid rodents (gerbils) suggest that conclusions concerning disparity in taste sensibilities among animal forms may be premature.     

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.     

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.     

Sweetness suppression in fructose/citric acid mixtures: A study of contextual effects

Twoexperiments investigated whether stimulus context affects ratings for mixtures of dissimilartasting substances (fructose/citric acid) to the same degree that it affects ratings for unmixed substances (fructose). In Experiment 1, replacing mixtures by equisweet unmixed fructose solutions produced virtually no response shifts. The proportion of mixtures in the stimulus set affected only slightly the degree of mixture suppression inferred from the responses. In Experiment 2, both the stimulus type (mixed or unmixed) and the stimulus distribution (positively versus negatively skewed) affected the responses. Several factors that determine the impact of contextual changes are identified: (a) the stage in stimulus processing affected—that of representation on the internal continuum or that of response selection; (b) the size and sources of variation in the affected process; and (c) the degree to which a stimulus is perceptually integrated in the context. In the present study, the sweetness of fructose/citric acid mixtures was largely, but not completely, integrated with the sweetness of unmixed fructose solutions. It is suggested that increased stimulus complexity makes mixture ratings more susceptible to contextual shifts. An analysis relating the size of the contextual shift to the degree of response variability suggests that response-selection processes are more important in determining the responses for unmixed stimuli than they are in determining the responses for mixtures.     

Ratio scales of acid sourness

Experiments were conducted to assess the relation between concentration, or pH, and the perceived sourness of 24 acids. The psychophysical functions for sourness conform to the power relation S = kCⁿ which relates sensory intensity, S, to physical concentration, C. Averaged across the 24 acids, the exponent for sourness was 0.85 for both molar and percentage concentrations, and about ?1.70 for pH concentration. The intercept, k, which is a measure of relative sourness, differed across acids. The particular measure used to designate the concentration of an acid markedly influenced its magnitude and rank order of sourness.     

Assessment of the taste interaction between two qualitatively similar-tasting substances: a comparison between comparison rules

The taste interaction between sucrose and fructose was assessed by using three different comparison procedures: the summated response comparison, the factorial plot comparison, and the equimolar comparison rule. The perceived sweetness intensities were obtained on a ratio scale by using a functional measurement approach in combination with a two-stimulus procedure. The conclusions obtained from each of the three comparison rules were identical. The taste interaction between sucrose and fructose could be explained to a large extent, but not completely, by the apparent taste "interactions" within sucrose and fructose as single substances. It is argued that the apparent taste interaction within a large number of single sugars and between two of these sugars in a mixture is somewhat synergistic at low sweetness levels, additive at intermediate sweetness levels, and suppressive at high sweetness levels.     

Gustatory responses during periods of controlled and ad lib cigarette smoking

Three psychophysical methods, differential sensitivity, perceived intensity, and degree of liking, were used to measure gustatory responses among nonsmokers and among smokers who were placed on 2-week schedules of increased and decreased cigarette consumption. Varying the smoking levels had little influence on the smokers’ sensitivity to the tastes of sucrose and sodium chloride. Throughout the experiments, nonsmokers were slightly more sensitive than smokers. Lower intensity ratings were ascribed to both compounds by smokers than by nonsmokers. Degree of liking of the test solutions was unrelated to smoking. A subsequent experiment showed that 45 nonsmokers were slightly, but not significantly, more sensitive than were 45 smokers to the tastes of sucrose, sodium chloride, citric acid, and quinine hydrochloride, with no differences in their sensitivity to the odor of 2-butanone and to the viscosity imparted by carrageenan gum. There was a slight inverse relationship between sensitivity and the number of cigarettes smoked per day. Smokers liked coffee significantly better and drank more cups per day than did nonsmokers. Few changes in body weight were observed, despite the smokers’ subjective assessment that their appetites and food intake were inversely related to the number of cigarettes smoked.     

Sensory scales of taste intensity

The subjective intensity of taste was scaled by the method of magnitude estimation in which Os assigned numbers to designate the apparent strength ofstimulus concentrations. Substances used were sucrose, dextrose, maltose, fructose, saccharin, Sucaryl, sodium chloride, and quinine sulfate. For aqueous solutions of each substance, taste intensity was found to increase as a power function of concentration by weight. Some approximate exponents were: sucrose, 1.3; sodium chloride, 1.4; quinine sulfate. 1.0. The magnitude scale for sucrose was compared with the category scale obtained by a commonly used rating procedure. The category scale turned out to be highly nonlinear.     

Effects of taste quality and intensity on oral perception of viscosity

Oral assessments of viscosity were obtained with the method of magnitude estimation. Subjects judged the viscosity of a series of aqueous solutions thickened to one of six viscosity levels (1–2025 centistokes) with a food-grade gum, sodium carboxymethylcellulose. The solutions contained one of several concentrations of caffeine, citric acid, sodium chloride, or sucrose. The presence of taste substances significantly altered the perception of solution viscosity for only the thickest solutions. Increasing concentrations of citric acid and sodium chloride produced progressive decreases in perceived viscosity, and increasing sucrose concentrations produced small increases in perceived viscosity. Caffeine did not affect judgments of solution viscosity. The most likely explanation for these findings is that subjects detected differences in the Newtonian behavior of the thickened solutions, differences that were produced by the addition of taste substances.     

Human taste response as a function of locus of stimulation on the tongue and soft palate

Taste recognition thresholds and psychophysical intensity functions were determined for NaCl, sucrose, QHC1, urea, and citric acid for four loci on the tongue and on the soft palate. The results showed greater differences between loci than previously reported. Contrary to older data, the threshold for bitter was found to be lower for the fungiform papillae at the front of the tongue and for the soft palate than for the vallate papillae. For all compounds, the slopes of the intensity functions varied with the locus of stimulation. The functions for most compounds were steepest at the vallate and foliate loci.     

Taste-aroma interactions in a ternary system: a model of fruitiness perception in sucrose/acid solutions

Cross-modal interactions between aroma, sweetness, and acidity were studied. A series of samples was presented to trained panelists who assessed strawberry flavor intensity using magnitude estimation with a reference modulus. The delivery of aroma stimuli from the different solutions was measured by monitoring exhaled breath using atmospheric pressure chemical ionization-mass spectrometry to determine whether there were any physicochemical effects on volatile release; no significant differences were noted. Three-dimensional predictive models were built to describe perceived strawberry flavor intensity as a function of concentrations of sucrose, acid, and volatiles. Analysis of the data identified two groups of panelists with different responses: For Group 1, increasing sucrose and/or acid levels also increased the perceived flavor intensity. For Group 2, changing sucrose concentrations had little effect, but increasing acid and/or volatile levels did. The results show different effects of organic and inorganic acids on perception, as well as clear interactions between the modalities of taste (sugar and acid) and aroma. The clustering of panelists' responses suggests that this phenomenon may depend on prior associations between the fruity flavor and the tastants.     

Ingestive responses of human newborns to salty, sour, and bitter stimuli.

Human newborns (1-4 days old) were offered two fluids differing in taste for 3 min each. The volumes ingested were measured. Infants offered water and bitter or sour solutions did not ingest them differentially, which corroborated earlier observations with weaker solutions. A sucrose solution was used to raise baseline ingestion above that of water. Infants offered the sucrose solution with and without urea, citric acid, or sodium chloride consumed less of it when citric acid was added. They were indiffferent to the addition of urea or sodium chloride. The failure to observe intakes lower than that of water suggests that newborns maximally inhibit their ingestion of water. The effects of sex, age, birth weight, and individual consistency on intake were assessed.     

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.     

Memory improvement by glucose, fructose, and two glucose analogs: a possible effect on peripheral glucose transport

The memory-improving action of post-training, noncontingent injections of glucose was investigated in a series of experiments which examined the effects of several substances that interact with glucose metabolism on the retention of a conditioned emotional response and on blood glucose levels in male hooded rats. Although post-training glucose injections of 1, 2, and 3 g/kg all produced similar increases in blood glucose, only 2 g/kg improved retention, suggesting that attainment of a particular blood glucose level is not critical for memory improvement. Post-training injections of a range of insulin doses (0.25-4 IU/kg) failed to affect retention. Post-training injection of fructose (the same doses as were used for glucose) had no effect on blood glucose levels and, as with glucose, only the 2 g/kg dose improved retention. This finding suggests that blood glucose levels are not critical for the memory-improving effect, that glucose and fructose may act on the same substrate and, because fructose does not act directly on the brain, it raises the possibility that both substances act peripherally. Post-training injections of 2-deoxyglucose and 3-O-methylglucose both improved retention. The fact that these mostly nonmetabolized glucose analogs were effective suggests that the memory-improving action of glucose may depend on the activation of a membrane glucose transport mechanism. The implications of the possible action of glucose on peripheral transport mechanisms for understanding the effect of reinforcers on memory are discussed.     

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.     

Cross-enhancement of the sour taste on single human taste papillae

The subjective intensity of one taste quality can be increased by prior exposure of the tongue to a different taste quality stimulus. This phenomenon, called cross-enhancement, may be the result of interactions among the physiological mechanisms that code taste quality. Another possible explanation is that the water solvent of the second stimulus acquires a taste after exposure of the tongue to the first stimulus. This water taste could add to the taste of the solute in the second stimulus and result in an increase of its subjective intensity. A third possibility is that taste receptors on the tongue may be sensitized by exposure to a taste stimulus. Using a small number of highly trained subjects, we have demonstrated that sucrose can enhance the intensity of an acid taste on the single papilla. Neither water taste nor sweet taste system activation played any role in the mediation of this enhancement. Through a series of experimentally derived inferential steps, we conclude that this phenomenon depends on the removal of protons from the acid receptors. In addition, we have demonstrated in the single papilla, that suppression of the acid taste when in mixture with sucrose can occur without sweet system activity. We conclude that sugars, through their capacity to bind protons, act to reduce the availability of protons to the acid receptors.