Spatial and temporal suppressive behavior of bitter-sweet mixtures

Sensory sweetness and sourness interactive response of sucrose-citric acid mixture based on synergy and antagonism

The clarity of taste sensation interaction is a key basis for promoting the food sensory science research and its application to the beverage and food additive industries. This study explored the synergy and antagonism effect of sucrose-citric acid mixture and established an optimized method to determine the human sweetness and sourness interactive response. Sucrose-citric acid mixtures were evaluated by the “close type” question. According to the sensory difference strength curves and Weber–Fechner law, citric acid increased the sucrose’s absolute threshold (0.424–0.624%) and weber fraction (20.5–33.0%). Meanwhile, sucrose increased citric acid’s absolute threshold (0.0057–0.0082%) and decreased its weber fraction (17.96–9.53%). By fitting absolute threshold and weber fraction variation equations, the sweet–sour taste sensory strength variation models (SSTVM) were derived, which could be used to explain the synergy and antagonism effect of sweet–sour taste. According to the SSTVM, the interactive response to sweet–sour taste could be quantitatively calculated. The high coincidence between SSTVM and human evaluation (1.02% of relative error) indicated that it could be applied in the food industry, health management, and intelligent sensory science.

Perceived intensity and persistence of heterogeneous taste mixtures

Perceived intensity and persistence of single taste stimuli, binary and ternary heterogeneous taste mixtures in a sweet acidulated system were assessed. The effects of a sweetener, aspartame (APM) on sourness, and of citric acid (CA) buffered with different levels of its sodium salt (NaCit) on sweetness were examined. A sensory panel scored intensity and duration of single and mixed stimuli formulated with APM (0, 1, 2 or 4 mM), CA (50 mM) and NaCit (0, 10, 20 or 40 mM). The analysis of frequency of qualities assigned by subjects depicts a great similarity in the intensity as well as in the persistence attributes. All stimuli were perceptually measurable into the four taste qualities, but sour and sweet were the most frequent responses obtained when tasting solu tions. Perceived sourness dropped by adding NaCit or APM to the acid solution. Sour persistence showed roughly equivalent results. The intensity and duration of sweet quality rose with APM concentration. A drop in the pH of the solutions diminished sweet persistence but not intensity scores. These data suggested the operation of similar mechanisms for processing intensity and persistence in the present conditions.

Receptor agonism and antagonism of dietary bitter compounds

Food contains complex blends of structurally diverse bitter compounds that trigger bitterness through activation of one or more of the ∼25 human TAS2 bitter taste receptors. It remains unsolved, however, whether the perceived bitterness of binary bitter-compound mixtures can be considered an additive function of all bitter-inducing chemicals in the mouth, suggesting that little mutual interaction takes place among bitter substances or if mixture suppression and synergism occurs. Here we report on two natural sesquiterpene lactones from edible plants, which stimulate distinct sets of hTAS2Rs in transfected cells. Both chemicals also robustly inhibit different but overlapping subsets of agonist-activated hTAS2Rs. These findings demonstrate that mixtures of bitter compounds, because they normally occur in human foodstuff, likely elicit bitter perception in a complex and not in a merely additive manner. An unexpected implication of this discovery is that, during evolution, the naturally occurring bitter taste receptor antagonists have shaped some of the pharmacological properties of the receptors, such as overlapping recognition profiles and breadth of tuning.

The taste transduction channel TRPM5 is a locus for bitter-sweet taste interactions

Ordinary gustatory experiences, which are usually evoked by taste mixtures, are determined by multiple interactions between different taste stimuli. The most studied model for these gustatory interactions is the suppression of the responses to sweeteners by the prototype bitter compound quinine. Here we report that TRPM5, a cation channel involved in sweet taste transduction, is inhibited by quinine (EC(50)=50 microM at -50 mV) owing to a decrease in the maximal whole-cell TRPM5 conductance and an acceleration of channel closure. Notably, quinine inhibits the gustatory responses of sweet-sensitive gustatory nerves in wild-type (EC(50)= approximately 1.6 mM) but not in Trpm5 knockout mice. Quinine induces a dose- and time-dependent inhibition of TRPM5-dependent responses of single sweet-sensitive fibers to sucrose, according to the restricted diffusion of the drug into the taste tissue. Quinidine, the stereoisomer of quinine, has similar effects on TRPM5 currents and on sweet-induced gustatory responses. In contrast, the chemically unrelated bitter compound denatonium benzoate has an approximately 100-fold weaker effect on TRPM5 currents and, accordingly, at 10 mM it does not alter gustatory responses to sucrose. The inhibition of TRPM5 by bitter compounds constitutes the molecular basis of a novel mechanism of taste interactions, whereby the bitter tastant inhibits directly the sweet transduction pathway.

The science and complexity of bitter taste

Food choices and eating habits are largely influenced by how foods taste. Without being the dominant taste sensation, bitter taste contributes to the complexity and enjoyment of beverages and foods. Compounds that are perceived as bitter do not share a similar chemical structure. In addition to peptides and salts, bitter compounds in foods may include plant-derived phenols and polyphenols, flavonoids, catechins, and caffeine. Recent studies have shown that humans possess a multitude of bitter taste receptors and that the transduction of bitter taste may differ between one compound and another. Studies of mixture interactions suggest further that bitter compounds suppress or enhance sweet and sour tastes and interact with volatile flavor molecules. Caffeine, a natural ingredient of tea, coffee, and chocolate, has a unique flavor profile. Used as a flavoring agent, it enhances the sensory appeal of beverages. Research developments on the genetics and perception of bitter taste add to our understanding of the role of bitterness in relation to food preference.