Research note 002 — 2026-08-30 (expanded 2026-09-08)
「苦い」 Is Not a Single Taste
We make do with the single word 「苦い」 (in translation: "bitter"), but what happens in the mouth does not fit into one word. What receives bitterness is not a single sensor, but the TAS2R family, made up of multiple receptors. And for each compound, the combination of receptors it strikes is different.
Chicory's two bitterness compounds, even within the same plant, have been measured in cultured-cell experiments to mainly activate separate receptors (TAS2R43 and TAS2R46).
Because caffeine also acts through pathways other than the bitter taste receptors, one review has concluded that it cannot be regarded as bitterness's representative.
Caffeine's contribution to coffee's bitterness has been measured, across multiple sensory tests, to weaken or stay unchanged depending on whether surrounding components are present.
How far the TAS2R38 genotype governs the way vegetable bitterness is felt is still not settled uniformly — the effect appears or does not appear depending on the cooking method.
Even a single vegetable already has more than one bitterness
Chicory's two principal bitterness compounds, lactucin and lactucopicrin, are neighboring compounds within the same plant, yet the receptor that responds most sensitively to each is different — TAS2R43 for the former, TAS2R46 for the latter. Even 「チコリの苦味」 (in translation: "chicory's bitterness") is not, physiologically, a single thing.
Scope: Receptor assay in a cultured-cell system, in the cited study
Note: Receptor activation alone does not determine the intensity of the food as a whole.
Lactucin activated TAS2R43 with a threshold of 10 uM and an EC50 of 14.9 +/- 2.5 uM. Lactucopicrin was far more potent at the same receptor (threshold 0.1 uM, EC50 0.46 +/- 0.08 uM), and the paper ranks the compounds verbatim as "lactucopicrin exhibiting the highest potency, lactucin with medium potency and 11beta,13-dihydrolactucin being least potent."
Limitations: The EC50 values are read from Table 1. The body text states the potency ranking in words but does not repeat the numbers, so these figures are table-level verbatim information rather than sentence-level. Cell-based potency alone does not determine the intensity of the food as a whole, and because TAS2R43 is absent in a substantial share of people, this ranking does not apply uniformly across human populations.
Limits: Shows that TAS2R43 itself functions differently from person to person: carriers of the hT2R43 allele that encodes tryptophan at position 35 are highly sensitive to the bitterness of aloin and aristolochic acid, people without this allele cannot taste these compounds at low concentrations, and the same allele also raises sensitivity to saccharin. The authors also state that some people lack particular hT2R genes altogether.
Limitations: Lactucin was not among the tested compounds; this is not a problem specific to lactucin but concerns whether a TAS2R43-based potency ranking can be transferred to the whole population. Abstract-level extraction: allele frequency and the psychophysical n are not captured.
Limits: Cell-based potency values for TAS2R43 cannot be transferred uniformly across human populations, because the receptor gene is deleted in a substantial number of people. TAS2R43 copy number ranges from 0 to 2, and the deletion allele reaches a global frequency of 0.33 among 946 subjects across 52 populations. The paper states that more than 20% of subjects in Europe lacked this gene.
Limitations: There is no sensory test using lactucin or chicory. This limitation is inferential, linking receptor genetics to the interpretation of the in vitro EC50.
Scope: Receptor assay in a cultured-cell system, in the cited study
Note: The same paper also reports overlapping receptor activation by other chicory lactones.
Lactucopicrin activated TAS2R46 with a threshold of 0.03 uM and an EC50 of 0.35 +/- 0.06 uM, the most sensitive receptor-compound pair reported in this study; lactucin activated the same receptor with a threshold of 1 uM and an EC50 of 8.85 +/- 1.17 uM. 11beta,13-dihydrolactucin had a threshold of 30 uM, and no EC50 was determined within the concentration range tested.
Limitations: The EC50 values are based on Table 1, not on statements in the body text. Because this receptor responds broadly to sesquiterpene lactones in general, its high sensitivity to lactucopicrin is not evidence of selectivity for that compound.
Context: Independently characterizes hTAS2R46 as a receptor that responds broadly not only to sesquiterpene lactones but also to clerodane- and labdane-type diterpenoids, strychnine, and denatonium; its first deorphanization was with the sesquiterpene lactone herbolide D. Because lactucopicrin belongs to the sesquiterpene lactones, the Lang 2022 result falls within an agonist class already established for this receptor.
Limitations: Lactucopicrin and lactucin themselves are not named in the abstract, nor are they shown to have been tested; this is class-level corroboration, not a direct replication. Broad tuning also cuts the other way: a receptor that responds to many sesquiterpene lactones is weaker evidence of selectivity for lactucopicrin specifically. Abstract-level extraction, with no potency figures.
This is not only a story about chicory's two compounds. In experiments using cultured cells expressing human bitter taste receptors, even for the same compound, the order of magnitude of the concentration each receptor responds to differs by receptor.
In an experiment exposing cells expressing 26 human bitter taste receptors to the mushroom-derived bitter compound oligoporin D, it moved TAS2R46 at a low concentration of 0.1 µM, while the same compound needed concentrations an order of magnitude higher to produce any response at all — 1 µM for TAS2R4, 30 µM for TAS2R1. That makes TAS2R46 about 300 times more sensitive to this compound. Even for a single compound, receptor 「感度」 (in translation: "sensitivity") is not uniform.
Scope: Layer: cell. A screen using heterologous-expression cells for 26 human bitter taste receptors, with threshold concentrations measured by calcium imaging (Table 2).
Table 2: oligoporin D (3), threshold (uM): TAS2R1=30, TAS2R4=1, TAS2R14=3, TAS2R41=30, TAS2R46=0.1. Body text: "Oligoporin D (3) was found to activate TAS2R46 already at a concentration of 100 nM."
Limitations: This is a result for a compound isolated from a single mushroom species; no sensory evaluation as a food has been carried out.
How receptors divide the work also differs from person to person. A small-scale human study looking at tendencies among habitual caffeine drinkers found that combining the mRNA abundance of TAS2R10, TAS2R43, and TAS2R46 in the taste papillae with habitual intake together explained 47% of the variation in caffeine bitterness ratings. The same study found no comparable relationship for quinine.
caffeine — perceived bitterness is associated with: Habitual caffeine intake and bitter-receptor mRNA abundance in taste papillaesupported
Scope: A small-scale human study reported in the cited paper
Note: The association does not establish the direction of causation, nor can it be generalized to every population.
n=20. gLMS bitterness ratings of 8 mM caffeine were associated with combined TAS2R10/43/46 mRNA abundance (r(18)=0.47) and with habitual intake (t(18)=2.78, p=.012), with the 2 factors together explaining 47% of the variance in ratings. TAS2R43 alone also showed r(18)=0.54, p=.014. No comparable association was seen for quinine.
Limitations: A cross-sectional study of n=20. Receptor genotypes other than TAS2R38 were not controlled for. The direction of the association (whether habitual drinkers feel it more or less strongly) is not stated explicitly in the text, and the discussion notes that prior intake-manipulation studies have reported inconsistent results.
Context: In large-scale genetic epidemiology too, people with higher genetic sensitivity to caffeine bitterness drink more coffee (+0.146 cups/day per SD) — consistent with the direction of 「常飲と感受性の関連」 (in translation: "the association between habitual drinking and sensitivity"), and interpreted as learned positive reinforcement.
Limitations: Intake is self-reported. The causal path from sensitivity to intake is not a direct measurement of a learning mechanism.
For vegetables too, how far receptor genotype explains differences in bitterness is not uniform. TAS2R38 genotype had a significant effect on the bitterness ratings of cooked turnip. However, the tendency for PAV/PAV consumers to rate bitterness higher than PAV/AVI and AVI/AVI consumers (27.6 versus 16.9 and 16.4) held only across all samples taken together and did not reach significance, and a clear significant difference appeared only when the turnip was pan-fried. The same receptor genotype can matter or not, depending on the cooking method.
Scope: UK adult consumers who rated rocket (arugula) lines (genotyped n=69) and cooked turnip (n=74). TAS2R38 PAV/PAV, PAV/AVI, and AVI/AVI diplotypes
Note: The main effect of genotype on bitterness was significant in both studies, but in neither study did pairwise comparisons within individual samples separate the genotypes.
Sum-of-squares ANOVA showed that TAS2R38 genotype had a significant effect on bitterness perception (P < 0.02). While the authors note that this suggests a significant effect on bitterness perception, they also state that the ANOVA showed no significant difference between genotypes within any particular rocket line, that the diplotype effect was less pronounced than originally hypothesized, and that there was a general tendency for non-tasters to score rocket bitterness lower than medium tasters or supertasters.
Limitations: The main effect does not survive as a pairwise difference within lines. Recruitment was around a single university, with 76.7% of participants female, limiting generalizability. Other TAS2R genotypes and fungiform papillae density were not measured, and the authors list these as factors that could contribute.
TAS2R38 genotype had a significant effect on bitterness perception (F(2,256) = 4.14, p = 0.02); across all samples, PAV/PAV consumers tended to rate bitterness intensity higher than PAV/AVI (p = 0.07) and AVI/AVI consumers (p = 0.05) (27.6 versus 16.9 and 16.4). There was no significant difference in bitterness intensity scores between PAV/AVI and AVI/AVI consumers (p = 0.99). It was only for stir-fried turnip that PAV/PAV consumers rated bitterness intensity significantly higher than PAV/AVI (p = 0.02) and AVI/AVI (p = 0.001) consumers.
Limitations: Pairwise contrasts between PAV/PAV and the other diplotypes remained at only p = 0.05 and p = 0.07 across all samples, and significant separation held for only 1 cooking method. 7 consumers with rare genotypes were excluded from the analysis. The authors themselves state that the sample is small and UK-only.
Caffeine is disqualified as the representative of bitterness
Caffeine, widely used in experiments, also acts through pathways other than the bitter taste receptors, and has been concluded to be 「苦味の普遍的なプロトタイプとしては不適」 (in translation: "unsuitable as a universal prototype of bitterness"). That no single representative can be picked out is itself evidence that bitterness is not singular.
Scope: Caffeine's experimental use as a bitter stimulus
Note: The review in question also discusses receptor-independent pathways in addition to the canonical pathway.
Caffeine acts through multiple TAS2R-independent pathways: µM affinity at adenosine receptors A1/A2A/A2B (taste cells express A2B), a rise in intracellular Ca via ryanodine receptors, ITPR3 inhibition, a rise in cGMP via PDE inhibition, and TRPA1 — activated in mice but inhibited in humans (a species difference). The conclusion: "caffeine should not be considered a prototypical bitter taste".
Limitations: A review. States explicitly that knockdown confirmation of action via endogenous T2Rs in taste cells has not been carried out. The receptors are not conserved across species, and the mechanism is species-dependent.
Caffeine (0.1–10 mM) activates mouse TRPA1 (the EC50 is stated in the text as "between 1 and 2.5 mM"), while human TRPA1 "responded to 100 uM AITC, but not to 5 mM caffeine", and suppressed the current under chase application in the presence of AITC. 8 WT mice avoided 5 mM caffeine water (P = 0.012), but 10 TRPA1-KO mice did not avoid it (P = 0.096). Of 63 capsaicin-responsive DRG neurons in WT, 20 responded to both caffeine and AITC, versus 0 of 115 in KO. The abstract closes: "caffeine does not activate human TRPA1; instead, it suppresses its activity". This is primary-level evidence that the receptor pathway for caffeine's response is not conserved across species, supporting the claim that caffeine cannot be treated as a universal bitter prototype.
Limitations: The perceptual consequence of human TRPA1 suppression (how it feels to humans) has not been tested. The authors themselves discuss human caffeine bitterness as proceeding via T2R, so this study demonstrates a species difference in a non-T2R pathway, not a replacement of the whole taste pathway. The authors state that the KO could not be used as a negative control for immunostaining.
Even in an actual beverage like coffee, caffeine's contribution to bitterness appears and disappears depending on conditions. In a raw coffee beverage made from unroasted green beans, even when it contained caffeine at more than 10 times the bitterness detection threshold of 0.67 mmol/L, no significant bitterness was detected. Caffeine being present at a sufficient concentration, and it rising up as bitterness, are not the same thing.
Scope: Layer: food. A raw coffee beverage prepared at 54 g/L unroasted coffee powder from green beans (Colombian origin, ground under liquid nitrogen). 18 panelists, 0-5 point scale.
"low intensities for sweet (0.5), sour (1.25), and astringency (1.0), no significant bitter taste was detectable, although the concentration of the caffeine was more than factor 10 above its bitter detection threshold (0.67 mmol/L)"
Limitations: Limited to 1 lot of green beans from a single origin. The bitterness value of the raw coffee beverage itself without caffeine added is not shown in the figure (only the value after addition).
In roasted coffee, conversely, surrounding components weaken caffeine's bitterness. In an experiment combining coffee's high-molecular-weight fraction with caffeine, sensory bitterness intensity dropped from 5.0 for caffeine alone to 2.5 (±0.3) under the combined condition.
Scope: Layer: food (reconstitution of a high-molecular-weight fraction isolated from coffee). A 5-point scale, 0 (not detectable) to 5 (strongly detected), 18 panelists, t-test (α=0.05).
Relative to the caffeine (7.5 mmol/L) reference = 5.0 (ref), caffeine+5-CQA+HMW>10kDa (at the original extraction concentration) = 2.5 (±0.3), p≤0.05.
Limitations: Limited to the HMW fraction derived from a single commercial coffee product (Dallmayr Capsa Espresso Decaffeinato).
Meanwhile, when caffeine and 5-caffeoylquinic acid were mixed in equimolar amounts, nuclear magnetic resonance analysis confirmed complex formation, yet sensory bitterness intensity barely changed — 4.8 (±0.2) against a reference of 5.0. Forming a chemical bond and bitterness changing are, again, different things.
Scope: Layer: solution. 18 panelists, 0-5 point scale, t-test (α=0.05).
"No significant difference in bitter taste perception could be observed between the reference caffeine solution (intensity 5.0) and the equimolar caffeine-chlorogenic acid complex 4.8 (±0.2)."
Limitations: A result from a single molar ratio (equimolar).
Bitterness is not even a fixed value. Sugar and salt (though a considerable concentration is needed) suppress quinine's bitterness, and simply changing the order in which you drink changes the rise and strength of the bitterness in the next sip. For orange juice, it has also been shown that no single marker compound can represent its bitterness.
quinine — perceived bitterness is suppressed by: Relatively high concentrations of sucrose, aspartame, and sodium chloridesupported
Scope: 0.1 mM quinine hydrochloride, in the human study cited
Note: The concentration required and the matrix are essential to interpretation.
Verbatim quotation from the abstract: An almost 80% inhibition of bitterness (calculated as concentration %) of a 0.1 mM quinine hydrochloride solution required 800 mM of sucrose, 8 mM of aspartame, and 300 mM NaCl. Sucrose, aspartame, and NaCl were all effective, but only at relatively high concentrations, and aspartame was about 100 times more efficient than sucrose on a molar basis. Phosphatidic acid (1.0 w/v%) and tannic acid (0.05 w/v%) reached comparable inhibition (81.7% and 61.0%) at far lower concentrations. In binding experiments, quinine did not bind to NaCl, sucrose, or aspartame, with the unbound fraction for these compounds close to 100%, which the authors attribute to these compounds being hydrophilic.
Limitations: An aqueous-solution test in which a 5 ml sample was held in the mouth for 15 s with a 20 min interval between samples; there is no food matrix. The panel comprised 11 volunteers, a small size for magnitude estimation. Because bitterness was scored on a 5-point scale anchored to quinine standard solutions of 0.01-1.00 mM, the reported percentages are inhibition rates converted to concentration, not a direct drop in scale value.
An independent human study on salt-bitter binary mixtures, using quinine hydrochloride as the bitter stimulus and NaCl, LiCl, KCl, L-arginine:L-aspartic acid, sodium acetate, and sodium gluconate as salts at 0, 0.1, 0.3, and 0.5 M, reports that perceived bitterness was suppressed by salt in most cases, though the degree of suppression varied. For urea, 3 sodium salts and a lithium salt all suppressed bitterness while KCl did not, identifying the active ingredient as the sodium or lithium ion, with an effect independent of perceived saltiness.
Limitations: The abstract does not isolate the magnitude of suppression specific to quinine, and the detailed ion analysis was carried out for urea rather than quinine. Panel size is not given in the abstract. The concentrations here (0.1-0.5 M NaCl) lie in a similarly high range to the 300 mM NaCl of Nakamura 2002, consistent with this claim that suppression requires relatively high concentrations.
Limits: Reports that binary taste-mixture interactions are not a fixed property of human taste but vary with PROP taster status. Using the sucrose/quinine hydrochloride and NaCl/quinine hydrochloride mixtures among 4 pairs, supertasters rated all qualities and overall mixture intensity higher than medium and non-tasters, and in the sweet-bitter mixture, non-tasters did not show the suppression of sweetness intensity at the highest QHCl concentration that was evident in super- and medium tasters.
Limitations: The group difference stated in the abstract concerns the suppression of sweetness by quinine, not the suppression of quinine's bitterness by sucrose or NaCl. The abstract has no explicit statement about group differences in bitterness suppression. This therefore limits the generality of a single suppression value but does not directly contradict it. Group size is not given in the abstract.
consumption sequence can change The maximum intensity, rise rate, and time-intensity area of the following bitter stimulusestablished
Scope: 6 compounds tested in paired order
Note: Magnitude and susceptibility were compound-specific. Reproduced across 2 systems, aqueous solutions and actual beverages. The direction of the order effect is compound-dependent.
All 6 compounds showed carryover or sensitization, and the magnitude and susceptibility were compound-specific.
Limitations: Panel size is not stated in the abstract (full text needed, unchanged). However, the matching criteria for stimulus concentrations, the number of pairs presented in 1 session, and the rinsing procedure are given in the abstract and are supplemented by the related record.
Reproduces, with an independent design, the phenomenon that consumption order changes the perceived bitterness of a following stimulus, and adds that the effect is asymmetric. Verbatim: "the bitterness of chicory was rated lower after roasted coffee consumption. Evaluation of this stimulus pair in opposite sequence revealed no change in the bitterness of roasted coffee after chicory consumption." The authors relate this to roasted coffee's broader pattern of receptor activation overlapping with, and desensitizing, the receptors that chicory compounds act on.
Limitations: Because this uses actual food rather than the single-compound solutions of Cubero-Castillo 2001, the comparison is at the level of phenomenon, not compound. The direction here is suppression of the second stimulus, whereas the earlier study reported both carryover and sensitization depending on the compound. The effect size and statistics for the order comparison are not captured in the verbatim.
A panel of 20-27 screened judges determined the thresholds of limonin and naringin in water, sucrose solution, citric acid solution, and a citrus-juice model system. The threshold in distilled water was 1 mg/l for limonin and 20 mg/l for naringin, and sucrose, citric acid, and their combination raised both thresholds several-fold. In a model with 1.2% citric acid plus 5% sucrose, 2.5% dextrose, and 2.5% fructose, limonin's maximum threshold of 11 mg/kg occurred at pH 3.8, and the maximum recognition threshold for limonin in natural orange juice was 6.5 mg/kg, also at pH 3.8. No pH optimum was seen for naringin.
Limitations: Supports only the acid and sugar side of the interplay. Polymethoxyflavones and limonoid glycosides were not examined. The abstract was reconstructed from OpenAlex (single pathway). 1973 methodology.
Context: Verbatim: 'There are two types of bitterness, namely immediate bitterness and delayed bitterness, in citrus fruits, imparted by two different types of compounds.'; 'The immediate bitterness is largely conferred by naringin and neohesperidin.'; 'the delayed bitterness is mainly produced by limonin of limonoids.'; 'The bitter-tasting naringin is the major contributor to the bitter taste in grapefruit and neohesperidin in sour orange.' In this way, an independent review likewise attributes the bitterness of citrus juice not to a single marker compound but to multiple compound classes (flavanone neohesperidosides and limonoids).
Limitations: The flavanone contributors named belong to bitter species (pomelo, grapefruit, bitter/sour orange) and are largely absent from sweet orange. Because polymethoxyflavones are not singled out as bitterness contributors, this review does not support the PMF portion of the interplay finding above.
Context: As a sentence absent from the PubMed abstract, states that the effect of sugars on the perceived bitterness of limonoids and PMFs in orange juice was "quantitatively elucidated." This pairs with the existing finding of sub-threshold bitterness enhancement by PMFs, suggesting an as-yet-unrecorded direction — bitterness suppression or modulation by sugars. Verbatim: "For the first time, sub-threshold concentrations of PMFs were shown to enhance the perceived bitterness of limonoids. Moreover, the influence of sugars on the perceived bitterness of limonoids and PMFs in orange juice relevant concentration ranges was quantitatively elucidated."
Limitations: Because the full text has not been obtained, whether the sugar effect runs toward suppression or enhancement, the quantitative values, and the sugars involved (sucrose, fructose, glucose, etc.) are unknown. This is only a lead found from a difference in how the abstract was paraphrased; turning it into a new claim would need the full text checked.
Show dataingredient-orange-juice —has_bitterness_generated_by→ "The interplay of limonin, limonoid glycosides, polymethoxyflavones, organic acids, and sugars"
The suppressing substance itself can also flip direction with concentration. Raising tannic acid to 0.05% suppresses quinine's bitterness to about 61% of the control, but raising the concentration further to 0.15% weakens the suppression instead, and the bitterness score climbs back above the control, to 1.5. The study attributes this reversal to the astringency that tannic acid itself carries. Suppression and enhancement swap places purely through a difference in the concentration of the same substance.
quinine can show higher bitterness scores under: Tannic acid suppresses quinine bitterness to about 61% of the control up to 0.05 (w/v)%, but at 0.15 (w/v)% this reverses and the bitterness score exceeds the control (the direction of the effect flips with dose).preliminary
Scope: The specific concentrations used in the cited human study.
Note: This is presented as an example of interaction between descriptors, not as proof that astringency is a form of bitterness.
The relationship is non-monotonic. Raising tannic acid from 0.005 to 0.05 (w/v)% lowered the bitterness score of 0.1 mM quinine to about 61% of the control, but at 0.15 (w/v)% no further inhibition occurred — suppression weakened, and the resulting bitterness score of 1.5 was significantly higher than the control level. The unbound quinine fraction remained at about 72%, and unlike phosphatidic acid, tannic acid does not compete strongly at the receptor site (an adsorption effect of about 27.5% plus masking of the remaining receptor site of nearly 33%, totaling 61.0%), so the authors attribute this reversal not to a change in binding but to the astringency of the high-concentration tannic acid solution itself.
Limitations: The authors themselves state explicitly that the explanation by astringency is unproven, and the astringency intensity data are described as not shown. Because bitterness and astringency were rated on the same bitterness scale, the increase at 0.15 (w/v)% may partly reflect confusion between descriptors rather than a true increase in bitterness. Aqueous solution only; panel of 11.
Show datacompound-quinine —can_show_higher_bitterness_scores_with→ "Tannic acid suppresses quinine bitterness to about 61% of the control up to 0.05 (w/v)%, but at 0.15 (w/v)% this reverses and the bitterness score exceeds the control (the direction of the effect flips with dose)."
Even the way bitterness fades differs by compound. Cyclization products that form from hop-derived iso-alpha acids during beer aging show a harsh, lingering bitterness, with reported threshold concentrations ranging from 5 to 70 µmol/L (as reported within the scope of the abstract).
Scope: trans-specific conversion products formed during beer aging. Quantified in stored commercial beer
Note: Explains the change in the quality (not only intensity) of beer bitterness with storage. The threshold range given in the abstract is for the compound set, not per-compound values.
Tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, and epitetracyclocohumol were identified as previously unknown trans-specific iso-α-acid conversion products. The abstract states that these compounds show a harsh, lingering bitterness with threshold concentrations ranging from 5 to 70 µmol/L, and their formation during beer aging was confirmed by HPLC-MS/MS.
Limitations: Abstract-level extraction. The 5-70 µmol/L range is given for the compound set as a whole, with no per-compound assignment. The solvent or matrix, number of panelists, and threshold method for the threshold test are not stated in the abstract.
Quantitative LC-MS/MS profiling of 56 hop-derived sensometabolites found that proton-catalyzed cyclization of trans-iso-α-acids is the quantitatively dominant reaction, generating tricyclic and tetracyclic compounds (tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, epitetracyclocohumol) with a persistent, harsh bitterness, and that these accumulate in beer as storage time and temperature increase. The authors propose controlling the beer's initial pH and keeping storage temperature low to delay the onset of the harsh bitter aftertaste.
Limitations: Abstract-level extraction. The number of beers, storage temperature, duration, and pH values are not stated in the abstract. The recommendations on pH and temperature are the authors' inference from the chemistry, not a reported sensory storage trial.
Context: An independent group quantified the degradation of trans-iso-α-acids during storage of commercial lager beer by HPLC, derived kinetic parameters (activation energy, rate constant), and concluded that the iso-α-acid loss 'can be used in the beer industry to predict the alteration of the bitterness of beer during storage.' This independently confirms the precursor chemistry (storage-driven trans-iso-α-acid loss) behind the harsh-bitterness conversion products.
Limitations: Does not identify tricyclic or tetracyclic conversion products or conduct a sensory test. The sensory attribution of the harsh, lingering bitterness to those compounds still relies solely on the Munich (Intelmann/Hofmann) group.
Even among the transformation products generated by heating hop beta-acid under wort-boiling conditions, how the aftertaste lingered differed. Recognition thresholds split across a range of 7.9 to 90.3 µmol/L: cohulupone showed a short-lasting, iso-alpha-acid-like bitterness at a low threshold of 7.9 µmol/L, while hydroxytricyclocolupone gave a coarse bitterness that lingered long on the back of the tongue and throat at 14.7 µmol/L (as reported within the scope of the abstract). Inside the single phrase 「後を引く苦味」 (in translation: "lingering bitterness") sits both the kind that ends quickly and the kind that lingers long.
Scope: Beta-acid transformation products from heat treatment of colupulone under wort-boiling conditions. Subsequently quantified in commercial beer
Note: Shows that not only alpha-acid derivatives but also hop beta-acids contribute measurable bitterness, and that even at similar threshold magnitudes, the temporal quality differs among homologs. Panel size and threshold method are not stated in the abstract.
Heat treatment of the hop beta-acid colupulone under wort-boiling conditions gave cohulupone, hulupinic acid, nortricyclocolupone, and multiple tricyclocolupone epimer pairs as the main bitter products. Depending on structure, the recognition threshold ranged from 7.9 to 90.3 µmol/L. The lowest were cohulupone at 7.9 µmol/L (described as a short-lasting, iso-alpha-acid-like bitterness) and hydroxytricyclocolupone at 14.7 µmol/L (described as a long-lingering, prickly bitterness perceived at the back of the tongue and throat). Concentrations differed markedly between beer types.
Limitations: Abstract-level extraction. Panel size, threshold method, and tasting matrix are not stated in the abstract, and no DoT (dose-over-threshold) calculation is reported, so the abstract does not establish that these compounds actually exceed threshold in the beers analyzed.
The system of language itself does not confine bitterness to a single box. A document for wine sensory-panel training lists sweet, sour, salty, umami, and bitter as the five basic tastes, with astringency listed alongside them in a separate category.
Scope: OIV's ‘Sensory Analysis of Wine’ document (March 2016), Part I b), the section on tests for judging sensory acuity. Covers the sensory panel training program for grapes and grape-derived beverages (moûts, vins et autres boissons d'origine vitivinicole).
Note: The same section lists the five basic tastes to be checked in stimulus-detection training as ‘sweet, sour, salty, umami, bitter,’ with astringency (astringence) appended alongside it in a separate category, as "ainsi que" (in addition). The document’s own opening Avertissement states that it has not gone through the member-state resolution review procedure and "cannot be treated as an OIV resolution," being a subcommittee consensus document instead — so this claim is recorded as a record of institutional positioning, not a claim of established scientific consensus.
Lists the five basic tastes to be confirmed in stimulus-detection training as sweet, sour, salty, umami, and bitter, and appends astringency alongside them, in addition (in a separate category).
Limitations: The document itself states that it has not gone through the procedure for an OIV resolution and is a consensus document.
In another test, too, samples of sweet, sour, salty, and bitter were presented on separate strips from samples of astringency from tannin and irritation from capsaicin. The need to deliberately separate out adjacent sensations, just to keep the range that the word bitterness refers to intact, is itself a sign that bitterness is not a sensation that stands complete on its own.
Scope: A task in which sweet, sour, salty, and bitter samples were identified separately from astringent and pungent samples, presented on different test strips
Note: This is a result showing that separate answer columns could be used to measure them; it is not proof that each sensation is received by only one kind of sensor.
Sweet, sour, salty, and bitter samples were placed on separate paper strips from samples of tannin-induced astringency and capsaicin-induced pungent stimulation, and separate answer columns were also provided.
Limitations: This was a test of identifying the type of sensation; it did not measure intensity, duration, or which sensor alone was involved.
Hop bitter acids, tea catechins, cacao's theobromine and diketopiperazines, citrus naringin and limonin, bitter gourd's momordicine, chicory's lactucin, olive's oleuropein, and quinine — the chemical identity behind 「苦い」 (in translation: "bitter") is a different substance for every ingredient, and how it is felt changes with receptor, concentration, order, and coexisting components. This is why NIGAMI LAB does not assign a fixed bitterness score to an ingredient, and instead stores it as a conditional relationship.
An honest note
Findings about receptors are mainly measurements from cultured-cell systems, and perception in the mouth is not simply determined by receptor response. Results measured with human sensory panels, such as the interaction between caffeine and coffee, are written up separately from these.
The studies included here include some still at the abstract-checked stage (see each card's review-status indicator).
Because there have been revisions to the nomenclature, this note does not fix an exact count of TAS2R receptors.
A more detailed breakdown of how the work divides among receptors is treated separately in Research note 040; the time course, adaptation, and carryover of bitterness in Research note 019; and the origin of words for bitterness and a map across languages in Research note 025.
Frequently asked
Is caffeine the main cause of coffee's bitterness?
Not simply. In a raw coffee beverage made from unroasted green beans, it has been reported that even when it contained caffeine at more than 10 times the bitterness detection threshold, no significant bitterness was detected. In roasted coffee, one experiment found that combining it with coffee's high-molecular-weight fraction dropped caffeine-alone sensory bitterness intensity to roughly half. Meanwhile, when caffeine and 5-caffeoylquinic acid were mixed in equimolar amounts, bitterness barely changed even though chemical complex formation was confirmed. So caffeine's concentration alone cannot explain coffee's bitterness.
Are there individual differences in how bitterness is felt?
Yes. A small-scale study of habitual caffeine drinkers reports that combining the mRNA abundance of several TAS2R receptors in the taste papillae with habitual intake explained 47% of the variation in caffeine bitterness ratings. The same study found no comparable relationship for quinine. There are also reports that people lacking the gene for the TAS2R43 receptor exist at a certain frequency across world populations, with more than 20% of subjects in Europe lacking this gene. So individual differences exist separately for each compound, and are not all explained by variation in a single receptor.
What is not known
How far the cell-level responses of TAS2R43 and TAS2R46 correspond to the strength of bitterness actually felt when eating a food has not yet been confirmed.
Whether sugars affect the bitterness of limonoids in orange juice in the direction of suppression or enhancement is not settled, because the full text has not been checked.
Whether the phenomenon in which bitterness suppression reverses with tannic acid concentration comes from confusion with astringency or from a genuine change in bitterness is stated as unproven even by the authors themselves.
For the association between caffeine sensitivity and daily intake, the direction is not yet clear — whether more-sensitive people drink more, or drinking more changes sensitivity.
The sensory attribution of the lingering-bitterness compounds that form during beer aging has not been traced beyond the reports of a limited number of research groups.