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Effect of repeated presentation on sweetness intensity of binary and ternary mixtures of sweeteners.
The purpose of the present study was to determine the effect of repeated presentation of the same sweet stimulus on sweetness intensity ratings. The sweet stimuli tested in this study were binary and ternary blends of 14 sweeteners that varied widely in chemical structure. A trained panel evaluated the sweetness intensity over four sips of a given mixture presented at 30 s intervals. The individual components in the binary sweetener combinations were intensity-anchored with 5% sucrose, while the individual sweeteners in the ternary mixtures were intensity-anchored with 3% sucrose (according to formulae developed previously). Each self-mixture was also evaluated (e.g. acesulfame-K-acesulfame-K). The main finding of this study was that mixtures consisting of two or three different sweeteners exhibited less reduction in sweetness intensity over four repeated sips than a single sweetener at an equivalent sweetness level. Furthermore, ternary combinations tended to be slightly more effective than binary combinations at lessening the effect of repeated exposure to a given sweet stimulus. These findings suggest that the decline in sweetness intensity experienced over repeated exposure to a sweet stimulus could be reduced by the blending of sweeteners.
Gurmarin suppression of licking responses to sweetener-quinine mixtures in C57BL mice.
Gurmarin (Gur) is a peptide that selectively suppresses responses of the chorda tympani nerve to sweet substances in rats and mice. In the present study, we examined the effect of Gur on behavioral responses to sweet substances in C57BL mice. To accomplish this, we developed a new short-term lick test and measured numbers of licks for 10 s for sweet substances mixed with quinine hydrochloride (QHCl) in water-deprived mice. Numbers of licks for sucrose mixed with 1 or 3 mM QHCl increased with increasing concentration of sucrose from 0.01 to 1.0 M. Oral infusion with 30 micro g/ml Gur produced significant decreases in responses to concentration series for sucrose mixed with 3 mM QHCl, whereas no such effect by Gur was observed in responses to QHCl alone or QHCl-mixed HCl, NaCl or monosodium glutamate. The Gur suppression of QHCl-mixed sucrose responses, which otherwise lasted for 2-3 h, rapidly returned to approximately 80% of control levels after oral infusion with beta-cyclodextrin. These results are comparable to neural data previously found in chorda tympani responses, and thereby provide further evidence for Gur as a sweet response inhibitor in C57BL mice. In the other aspect, our newly developed short-term test can also provide a tool for measurements of taste-guided behavioral responses to sweeteners.
Synergistic responses of the chorda tympani to mixtures of umami and sweet substances in rats.
It has been known that umami substances such as monosodium L-glutamate (MSG) and 5'-inosine monophosphate (IMP) elicit a unique taste called 'umami' in humans. One of the characteristics of the umami taste is synergism: the synergistic enhancement of the magnitude of response produced by the addition of 5'-ribonucleotides to MSG. In addition to this well-documented synergism, we report here for the first time on another type of synergism between a glutamate receptor agonist, L-AP4, and sweet substances, by analyzing the chorda tympani responses in rats. The results are as follows: (i) when L-AP4 was mixed with one of the sweet substances, such as sucrose, glucose, fructose and maltose, large synergistic responses were observed. (ii) These synergistic responses, except to L-AP4 + sucrose, were not suppressed by sweet taste suppressants, gurmarin and pronase E. (iii) These synergistic responses were not suppressed by either metabotropic or ionotropic glutamate receptor antagonists. (iv) Fibers that responded well to the binary mixtures of L-AP4 and sweet substances also responded well to NaCl and HCl, but very weakly to sucrose. These findings are different from the characteristics of synergism between glutamate and IMP. The multiple transduction mechanisms for the umami taste in rat taste cells are discussed.
A psychophysical investigation of binary bitter-compound interactions.
The aim of this study was to determine if taste interactions occur when bitter stimuli are mixed. Eight bitter stimuli were employed: denatonium benzoate (DB), quinine-HCl (QHCl), sucrose octaacetate (SOA), urea, L-tryptophan (L-trp), L-phenylalanine (L-phe), ranitidine-HCl, and Tetralone. The first experiment constructed individual psychophysical curves for each subject (n = 19) for each compound to account for individual differences in sensitivities when presenting bitter compounds in experiment 2. Correlation analysis revealed two groupings of bitter compounds at low intensity (1, L-trp, L-phe, and ranitidine; 2, SOA and QHCl), but the correlations within each group decreased as the perceived intensity increased. In experiment 2, intensity ratings and two-alternative forced-choice discrimination tasks showed that bitter compounds generally combine additively in mixture and do not show interactions with a few specific exceptions. The methods employed detected synergy among sweeteners, but could not detect synergy among these eight bitter compounds. In general, the perceived bitterness of these binary bitter-compound mixtures was an additive function of the total bitter-inducing stimuli in the mouth.
Understanding creaminess perception of dairy products using free-choice profiling and genetic responsivity to 6-n-propylthiouracil.
Flavor and texture contribute to the perception of creaminess in dairy products, but the nature of this interaction is not well understood. Previous studies on the genetic ability to perceive the bitter compound 6-n-propylthiouracil (PROP) reveal the existence of individual differences in creaminess perception. The objective of the present study was to use PROP-classified subjects to gain insight into this individual variation to better understand the cues for creaminess in dairy products, and to ascertain the contributions of flavor and texture to the integrated perception of creaminess. Ten nontasters and 10 supertasters of PROP participated in the study. Subjects evaluated nine commercial dairy products using Free-Choice Profiling (FCP), a type of descriptive analysis that allows subjects to rate products on individual lists of descriptors. Generalized Procrustes Analysis was used to develop separate consensus spaces for nontasters and supertasters. The models for both groups accounted for approximately 54% of the variance in the data and were resolved in two dimensions (a dairy flavor/texture axis and a sweet-sour continuum). The products were arranged in a similar pattern along the dimensions in both models. However, nontasters used a limited number of simple terms (sour, sweet, milky and mouthcoating) to describe the products, whereas supertasters used a more complex vocabulary (rich, buttery, creamy, light, grainy, gritty and sandy). The model for nontasters gave equal weight to the sweet-sour and dairy flavor/texture dimensions (28 and 26% variance, respectively); whereas, the model for supertasters relied more heavily on the dairy flavor/texture dimension (34% variance), and less so on the sweet-sour dimension (20% variance). These data suggest that the overall impression of creaminess was similar for nontasters and supertasters, but the cues the two groups used to judge creaminess differed.
Estimation of the individual firing frequencies of two neurons recorded with a single electrode.
When monitoring neurons with a single extracellular electrode, it is common to record action potentials from different neurons. A recurring problem with such recordings is to identify which neuron is active. Sorting spikes into separate classes is possible if each neuron discharge spikes differing by their shapes and sizes. However, this approach is not applicable when the spikes are indistinguishable. In this paper, we develop a method for estimating the respective firing frequencies of two neurons, producing indistinguishable spikes. It is based on the fact that, when a neuron fires a spike, there is an interval of time during which the probability of generating a second spike is very low. If a spike occurs during this 'silent period', it is likely to be generated from another neuron and the number of occurrences of such 'doublets' can be used to estimate the respective frequencies of two spike trains. We demonstrate here that a simple relation holds between the frequency of doublets d, the respective frequencies of the two neurons A and B, fA and fB, and a chosen value Delta shorter than the silent period, d=2fAfBDelta. This relation holds for a wide class of firing processes. We used this method to analyze responses from Drosophila taste sensilla. We first checked if the method was consistent with results obtained with stimuli that elicit responses of two taste neurons firing distinguishable spikes. We then applied this method to the study of a pair of taste neurons involved in the coding for salt taste in Drosophila melanogaster.
The influence of sodium salts on binary mixtures of bitter-tasting compounds.
In order to study potential mixture interactions among bitter compounds, selected sodium salts were added to five compounds presented either alone or as binary bitter-compound mixtures. Each compound was tested at a concentration that elicited 'weak' perceived bitterness. The bitter compounds were mixed at these concentrations to form a subset of possible binary mixtures. For comparison, the concentration of each solitary compound was doubled to measure bitterness inhibition at the higher intensity level elicited by the mixtures. The following sodium salts were tested for bitterness inhibition: 100 mM sodium chloride (salty), 100 mM sodium gluconate (salty), 100 and 20 mM monosodium glutamate (umami), and 50 mM adenosine monophosphate disodium salt (umami). Sucrose (sweet) was also employed as a bitterness suppressor. The sodium salts differentially suppressed the bitterness of compounds and their binary combinations. Although most bitter compounds were suppressed, the bitterness of tetralone was not suppressed, nor was the bitterness of the binary mixtures that contained it. In general, the percent suppression of binary mixtures of compounds was predicted by the average percent suppression of its two components. Within the constraints of the present study, the bitterness of mixtures was suppressed by sodium salts and sucrose independently, with few bitter interactions. This is consistent with observations that the bitter taste system integrates the bitterness of multi-compound solutions linearly.
Taste disturbance and its recovery after middle ear surgery.
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Gender-specific olfactory sensitization: hormonal and cognitive influences.
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Flavor processing: perceptual and cognitive factors in multi-modal integration.
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Complex taste--taste of D-amino acids.
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Qualitative differences of divalent salts: multidimensional scaling and cluster analysis.
Sensations from salts of iron, calcium, magnesium, and zinc with different anions were studied using a sorting task and multidimensional scaling (MDS). Ten divalent salts were adjusted in concentrations such that the mean intensity ratings were approximately equal. Stimuli were sorted on the basis of similarity to minimize any semantic influence and were examined with and without nasal occlusion to eliminate retronasal cues. Compounds representing the four primary tastes and astringency were also sorted. Similarity estimates were derived from sorting and were submitted to MDS. Divalent salts fell outside the area of the space defined by the four primary tastes. The nose-open condition showed that some of the divalent salts have unique metallic sensations along with astringency. The groupings obtained were corroborated using single-linkage cluster analysis. An iron group was most distinctive in metallic sensations; calcium and magnesium salts were primarily bitter; and zinc salts were characterized by astringency. When nasal cues were not available, the sensations from the divalent salts were mainly explained by bitterness and astringency. Results were consistent with a previous evaluation of divalent salts using descriptive analysis.
Differential covariation in taste responsiveness to bitter stimuli in rats.
Variation exists in the sensitivity of individual rodents and humans to different bitter tastants. An absence of uniform correlation in responsiveness to different bitter substances across individuals within a species suggests heterogeneity in the mechanisms underlying stimulus processing within this taste modality. Here, we examined taste responsiveness of individual rats to three bitter compounds (quinine hydrochloride, denatonium benzoate, and cycloheximide) in short-term lick tests to determine the magnitude of covariation among responses to these stimuli and infer commonalities in their receptor and neural mechanisms. Rats were tested with a given pair of bitter stimuli during three sessions comprising randomized trial blocks of six concentrations of each stimulus + deionized water. Psychophysical functions were generated for individual rats for respective stimulus pairs, and concentrations of each stimulus that produced equivalent lick suppression relative to water were correlated across animals. Behavioral taste responsiveness to quinine hydrochloride strongly covaried with responsiveness to denatonium benzoate (r = +0.82). Lick responsiveness to quinine was less robustly correlated with that to cycloheximide (r = +0.44), and denatonium and cycloheximide responses failed to correlate. These results imply substantial overlap in the bitter taste coding mechanisms for quinine and denatonium but some degree of independence in the mechanisms responsible for gustatory processing of cycloheximide. More generally, these data reinforce the notion that bitter taste processing is not a homogeneous event.
Linoleic and oleic acids alter the licking responses to sweet, salt, sour, and bitter tastants in rats.
The free fatty acids (FFAs), linoleic and oleic acids, commonly found in dietary fats can be detected by rats on the basis of gustatory cues following conditioned taste aversion pairings. FFAs depolarize the membrane potential of isolated rat taste receptor cells by inhibiting delayed rectifying potassium channels. This study examined the licking response of rats to sweet, salt, sour, and bitter taste solutions when 88 muM linoleic acid, 88 muM oleic acid, or an 88 muM linoleic-oleic acid mixture was added to the solutions. The presence of linoleic, oleic, and the linoleic-oleic acid mixture in sweet solutions produced increases in the licking responses, whereas adding linoleic, oleic, and the linoleic-oleic acid mixture to salt, sour, or bitter taste solutions produced decreases in licking responses when compared with the licking responses to the solutions in the absence of the FFAs. We conclude that FFAs may act in the oral cavity to depolarize taste receptor cells and therefore to increase the perceived intensity of concomitant tastants, thus contributing to the enhanced palatability associated with foods containing high dietary fat.
Subthreshold olfactory stimulation can enhance sweetness.
The impact of olfactory perception on sweetness was explored in a model solution using odorants at subthreshold concentrations. First, the impact of 6 odorants, previously described in the literature as congruent with sweetness, was investigated at suprathreshold level in a sucrose solution. Ethyl butyrate and maltol were selected as they had the highest and the lowest sweetness-enhancing properties, respectively. Second, the impact on sweetness of the 2 odorants was investigated at subthreshold concentrations. A system delivering a continuous liquid flow at the same sucrose level, but with varying odorant concentrations, was used. At a subthreshold level, ethyl butyrate but not maltol significantly enhanced the sweetness of the sucrose solution. This study highlights that olfactory perception induced by odorants at a subthreshold level can significantly modulate taste perception. Finally, contrary to results observed with ethyl butyrate at suprathreshold levels, at subthreshold levels, the intensity of sweetness enhancement was not proportional to ethyl butyrate concentration.
An analysis of 5'-inosine and 5'-guanosine monophosphate taste in rats.
Inosine monophosphate (IMP) and guanosine monophosphate (GMP) elicit an umami taste in humans and synergistically increase the intensity of the umami taste of monosodium glutamate (MSG). Conditioned taste aversion (CTA) studies in rodents indicate that these nucleotides and MSG elicit quite similar tastes, but recent physiological evidence suggests that these nucleotides and MSG may not activate the same population of taste receptors and therefore may not elicit identical taste qualities. This study reports the findings of several behavioral experiments with rats that compared the taste properties of IMP and GMP with each other and with those of MSG. Well-trained rats were able to detect both nucleotides at nanomolar concentrations, but they did not respond to either nucleotide in two-bottle preference tests or brief-access CTA tests at concentrations less than 0.5 mM. Discrimination experiments found that the tastes of these nucleotides could not be discriminated from each other, but both could be discriminated from MSG, even when the taste of Na(+) was controlled. Overall, these experiments indicate the taste properties of the two 5'-ribonucleotides are quite similar to each other, and even though they may elicit an umami sensation, these sensations are not identical to the taste of MSG.
Riboflavin-binding protein exhibits selective sweet suppression toward protein sweeteners.
Riboflavin-binding protein (RBP) is well known as a riboflavin carrier protein in chicken egg and serum. A novel function of RBP was found as a sweet-suppressing protein. RBP, purified from hen egg white, suppressed the sweetness of protein sweeteners such as thaumatin, monellin, and lysozyme, whereas it did not suppress the sweetness of low molecular weight sweeteners such as sucrose, glycine, D-phenylalanine, saccharin, cyclamate, aspartame, and stevioside. Therefore, the sweet-suppressing activity of RBP was apparently selective to protein sweeteners. The sweet suppression by RBP was independent of binding of riboflavin with its molecule. Yolk RBP, with minor structural differences compared with egg white RBP, also elicited a weaker sweet suppression. However, other commercially available proteins including ovalbumin, ovomucoid, beta-lactogloblin, myoglobin, and albumin did not substantially alter the sweetness of protein sweeteners. Because a prerinse with RBP reduced the subsequent sweetness of protein sweeteners, whereas the enzymatic activity of lysozyme and the elution profile of lysozyme on gel permeation chromatography were not affected by RBP, it is suggested that the sweet suppression is caused by an interaction of RBP with a sweet taste receptor rather than with the protein sweeteners themselves. The selectivity in the sweet suppression by RBP is consistent with the existence of multiple interaction sites within a single sweet taste receptor.