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.     

Ratio scales of sugar sweetness

In a series of 10 experiments, groups of Os judged the sweetness of 16 sugars. The results suggest that, for all sugars except mannose, the intensity of sweetness grows as a power function of concentration, with an exponent of about 1.3. The relative sweetness of sugars was determined using both molarity and per cent by weight. With both measures, sucrose and fructose were the sweetest sugars. The order of the remaining sugars in the sweetness hierarchy was partiy a function of the measure of concentration. The variability of the magnitude estimates of sweetness was roughly proportional to the stimulus concentration, supporting Weber’s law.     

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.     

Sweet odours and sweet tastes are conflated in memory

Certain odours and certain tastes appear to share common perceptual properties. One example is sweetness, a perceptual experience that results from stimulation of taste receptors on the tongue typically by sugars. The experiment here examined for evidence of this perceptual similarity using a novel and indirect test. Participants were exposed six times each, to three odours (strawberry, caramel, and oregano) and three tastes (sucrose, saline, and citric acid). Following a 10-min interval, participants were given a surprise frequency estimation task, in which they had to judge how often each stimulus had occurred. If sweet-smelling strawberry and caramel odours really do share this perceptual characteristic in common with sweet tasting sucrose, then frequency estimates for sucrose should be overestimated relative to non-sweet tastes. Not only was this observed, but frequency estimates for sweet tastes were also found to correlate with (1) evaluations from a later test of similarity between these sweet smells and sucrose, and (2) the degree to which these odours smelled sweet. These findings suggest a shared perceptual feature between such odours and sucrose - sweetness - under conditions where no judgment of perceptual quality was required.     

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.     

Category scales of sweetness are consistent with sweetness-matching data

Category scales were obtained for the sweetness of sucrose, fructose, and glucose, with care being taken to preclude methodological bias. These category scales were then used to predict the outcome of scale-free sweetness matching with the same three sweeteners. The matching concentrations predicted from the category scales were found to agree well with those actually obtained in several sweetness-matching studies.     

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.     

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.     

Suppression of sourness: a comparative study involving mixtures of organic acids and sugars

The degree of sourness suppression of perceptually equisour levels of citric, lactic, and malic acids by equal molar and weight amounts of sucrose, fructose, and glucose was determined in binary mixtures. Equisour acid levels were obtained by magnitude estimation. Mixture intensity ratings were collected on a categorical scale, using trained panelists. In general, equal sugar molarities and weights did not effect equivalent suppression. Instead, the perceived intensity of the sugars appeared to suppress sourness more systematically, implying that dominantly central neural mechanisms underlie suppression. This was confirmed when no significant differences were found between the suppressive abilities of sweetness-matched levels of sucrose, fructose, and an equiratio mixture of the two on citric acid sourness. The possibility of a separate receptor site/mechanism for glucose and a small peripheral component to suppression is also suggested.     

Temperature and ph effects on the relative sweetness of suprathreshold mixtures of dextrose fructose

Increasing the temperature from 5 deg to 50 deg C did not significantly alter the relative sweetness of aqueous solutions of dextrose, fructose, and their combination, as measured by the method of magnitude estimation, with a panel of 12 experienced Ss. Reducing pH from 5.8 to 2.7 caused about a 50% reduction in relative sweetness for all stimuli tested. The slopes of these lines tended to decrease slightly, whereas the Y-intercept values showed the greatest change. In all experiments the slopes of the lines were reasonably stable, averaging 1.40 vs 1.3 reported by Stevens (1961) for sucrose. The results of these experiments are discussed in relation to previously reported data on taste interactions.     

Sweetness and intensity of artificial sweeteners

In four experiments, groups of Os judged either the sweetness or the entire taste intensity of solutions of sucrose, cyclamate salts, cyclamate-saccharin mixtures, and sodium saccharin. The sensory functions obtained by magnitude estimation suggest that over the middle range of concentration the sweetness and intensity of the foregoing substances grow as power functions of concentration. As a first approximation, the exponents for sweetness and intensity are, respectively, 1.6 to 1.4 for sucrose, 1.0 to 0.8 for cyclamate salts, 0.6 to 0.85 for cyclamate-saccharin mixtures, and 0.3 to 0.6 for sodium saccharin.     

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.     

Human taste contrast and self-reported measures of anxiety.

Successive negative taste contrast in humans was demonstrated with a common taste stimulus, i.e., cherry-flavored Kool-Aid. A total of 31 male and female college-aged participants rated a 7% sucrose solution which was cherry-flavored as less sweet when it was preceded by a 28% rather than a 7% sucrose solution which was cherry-flavored. Because drugs such as the benzodiazepines affect taste contrast in rats and act as anxiolytics in humans, the present experiment also examined whether several self-reported measures of anxiety were related to taste contrast in humans. Neither scores on Taylor's Manifest Anxiety Survey nor those on the State-Trait Anxiety Inventory were related to "sweetness" ratings or contrast effects.     

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.     

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.     

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.     

Monogeusia for fructose,glucose, sucrose,and maltose

We investigated the ability of subjects to discriminate sugars with a whole-mouth forced-choice paradigm, in which a standard solution was compared with a test solution of varied concentration. Discrimination probabilities were U-shaped functions of test concentration: for 6 subjects and pairwise combinations of fructose, glucose, and sucrose, discriminability always declined to chance over a narrow range of test concentrations. At concentrations ≦ 100 mM, maltose was indiscriminable from fructose but discriminable at higher concentrations for 4 subjects. By analogy with themonochromacy of night vision, whereby any two lights are indiscriminable when their relative intensities are suitably adjusted, we call the gustatory indiscriminability of these sugarsmonogeusia. The simplest account of monogeusia is that all information about the indiscriminable sugars is represented by a single neural signal that varies only in magnitude. The discriminability of maltose from the other sugars at higher concentrations is consistent with the hypothesis that maltose also activates a second gustatory code.     

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.     

Interactions between CO2 oral pungency and taste

Two experiments are reported in which the perceptual interactions between oral pungency, evoked by CO2, and the taste of each of four tastants--sucrose (sweet), quinine sulfate (bitter), sodium chloride (salty), and tartaric acid (sour)--were explored. In experiment 1 the effect of three concentrations of each tastant on the stimulus-response function for perceived oral pungency, in terms of both rate of change (slope) and relative position along the perceived pungency axis, was determined. In experiment 2 the effect of three concentrations of CO2 on the stimulus-response function for the perceived taste intensity of each tastant was examined. Results show that the characteristics of the mutual effects of tastant and pungent stimulus depend on the particular tastant employed. Sucrose sweetness and CO2 oral pungency have no mutual effect; sodium chloride saltiness or tartaric acid sourness and CO2 oral pungency show mutual enhancement; and quinine sulfate bitterness abates CO2 oral pungency, whereas CO2 has a double and opposite effect on quinine sulfate bitterness--at low concentrations of bitter tastant CO2 enhances bitterness, and at high concentrations of bitter tastant CO2 abates bitterness. It is suggested that the perceptual attributes of saltiness and sourness are closer, from a qualitative point of view, to oral pungency than are the attributes of bitterness and sweetness.     

Neonates' heart rate,sucking rhythm,and sucking amplitude as a function of the sweet taste

In previous studies of human newborn sucking, the effects of increasing fluid sweetness and/or volume included a decrease in sucking rate within sucking bursts and, paradoxically, an increase in heart rate. To determine whether the heart rate increase can be attributed to increased sucking amplitude for sweeter fluids, sucking and heart rates of 20 full-term infants were studied. Half sucked for three consecutive 2-min periods, first receiving small drops of water for each suck, then no fluid, then 15% sucrose. The other half experienced the reverse order. The results for sucking and heart rate were consistent with previous studies; sucking rates within bursts were slowest for sucrose and fastest for no fluid. Heart rate was higher for sucrose than for the other fluid conditions, however, only in the water-first group. The heart rate increase was significant on statistical tests which controlled for sucking amplitude as well as for several other motor variables. Sucking amplitude itself varied with fluid sweetness in the water-first group only, in which it was highest for water. There were more total sucks, longer sucking bursts, and less time between successive bursts under the sucrose condition. Multivariate statistics helped establish a set of dependent variables—sucking rate within bursts, total number of sucks, and heart rate—which most parsimoniously describes the effects of fluid sweetness. A hedonic explanation of the response of newborns to sweetness is thus reiterated.