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Topic hub — 2026-09-08

Bitter taste receptors

The mechanism behind tasting bitterness cannot be explained by a single receptor alone. Human bitter taste receptors are often discussed together as a gene family called TAS2R, but in practice they divide into narrow receptors that respond only to particular substances and receptors that respond broadly to multiple, structurally different substances, and even for the same compound, sensitivity varies greatly from receptor to receptor. Much of the evidence for this division of labor comes from experiments that express a receptor in cultured cells and measure its activation, which is a different layer from sensory experiments that directly measure the bitterness intensity or threshold a person experiences in the mouth. The TAS2R38 genotype, often cited as the representative example of individual variation, is linked to the bitterness perception of propylthiouracil and phenylthiocarbamide, but how much it explains varies greatly by substance, and genotype alone does not determine the whole of whether or how strongly something tastes bitter. The fact that a receptor was activated in the laboratory does not by itself mean bitterness in the mouth, either. Here, following only claims that have been confirmed, we lay out separately: the division of labor among receptors, the difference between the layers being measured, individual variation centered on TAS2R38, the gap between activation and perception, and the suppression of bitterness via receptors.

What this page establishes

• That the range of substances each receptor responds to differs from receptor to receptor has been measured in experiments that examined multiple receptors side by side.
• That TAS2R38 genotype explains part of the individual variation in how PROP and PTC taste bitter has been measured in one study, but the proportion it explains varies greatly by substance.
• A receptor responding in a cell or in a computation, and a person perceiving bitterness in the mouth, have each been confirmed in experiments at different layers, and studies directly connecting the two are still few.
• Why multiple TAS2R38 variants remain in the population is still at the hypothesis stage and has not yet been confirmed.

Receptors divide the labor — broad responders and narrow ones

Human bitter taste receptors are collectively called the TAS2R family, and they mediate part of oral bitterness perception. Early functional-identification experiments using heterologous expression systems showed that specific T2Rs function as bitter taste receptors.

How narrow each receptor's response range can be has been confirmed in an experiment that screened mouse receptors. Among the receptors examined, theobromine activated only Tas2r121, at a threshold concentration of 0.03 mmol/L, and every other receptor's column was blank.

Within the same screen, there are also compounds that, conversely, move multiple receptors at the same concentration. Phenanthroline activated Tas2r105, Tas2r121, Tas2r122, Tas2r135, and Tas2r144 as common cognate receptors, all at a threshold concentration of 1.0 mM.

In human receptors too, combinations of receptors corresponding to particular groups of substances have been shown. Hop-derived bitter acids have been reported to bind three receptors, TAS2R1, TAS2R14, and TAS2R40 (within the scope of the abstract).

A bitter peptide's receptor response is narrower still, tending to select a particular receptor. The bitter dipeptides Gly-Phe and Gly-Leu (25 mM) activated hTAS2R1 most strongly among several receptors, and the tasteless dipeptide Gly-Gly did not activate any of the receptors up to 100 mM.

Once peptides form a mixture, the number of receptors that respond widens. Cells expressing hTAS2R1, hTAS2R4, hTAS2R14, and hTAS2R16 responded to the bitter fraction (2.5 mg/mL, a bitterness equivalent to 0.125 mM quinine) of a trypsin digest of casein.

Even for the same compound, the order of magnitude of the concentration required can differ by receptor. Oligoporin D, from a mushroom, activates TAS2R46 at a threshold concentration of 0.1 µM, while the same compound requires 30 µM to move TAS2R1 — TAS2R46 being about 300 times more sensitive.

Measuring in cells vs. measuring by sensory panel

There are also examples that have tracked a receptor's role in both a cell assay and a sensory evaluation. Individual variation in the bitter aftertaste of saccharin and acesulfame K is linked to multiple TAS2R31 missense mutations, and in vitro receptor assays have also confirmed that each independently changes the receptor response (within the scope of the abstract).

On the other hand, there are also examples where only the cell-based threshold concentration has been measured, and a sensory-side follow-up is not included within the scope of this claim. Oligoporins A, B, D, and E all activated TAS2R1 at a threshold concentration of 30 µM, but oligoporin F did not activate the same receptor.

There is also an example that directly linked a physiological response in the mouth to subjective ratings. The depolarization amplitude during propylthiouracil stimulation, measured by electrogustometry on the tongue, correlated significantly with the bitterness intensity subjects reported, and also correlated with fungiform papillae density.

TAS2R38 and individual variation — PROP, PTC, and Brassica

TAS2R38 genotype has been reported to have a significant effect on the perceived bitterness of Brassica vegetables. In UK consumer panels that rated rocket lines and cooked turnip, a main effect of genotype on bitterness perception was confirmed among the PAV/PAV, PAV/AVI, and AVI/AVI diplotypes.

However, the view that people with the genotype that tastes bitterness more strongly reject Brassica vegetables was not directly supported by the same consumer studies. It has been reported that the genotype's effect on perceived bitterness does not carry through to liking or acceptance.

The group who perceive PROP as strongly bitter is called "supertasters," and while there is a view that treats this as an amplification of oral sensation in general, there is also criticism that rejects this generalization. It has been pointed out that TAS2R38 alleles alone cannot explain supertasting.

There is also a hypothesis-stage explanation for why TAS2R38 diversity is maintained. A balancing-selection hypothesis has been proposed in which the non-taster AVI allele encodes a functional receptor that responds to bitter toxins other than PTC, so that heterozygotes perceive a broader repertoire of bitter toxins.

How much TAS2R38 explains varies in size depending on the substance in question. The bitterness threshold response to goitrin correlated significantly with both the PROP and PTC threshold responses, but the variance explained by TAS2R38 genetic variation was only r²=0.16 for goitrin, smaller than PROP's 0.50 or PTC's 0.57 (within the scope of the abstract).

For PROP's own bitterness intensity too, TAS2R38 alone cannot explain all of the variance. In a study of 243 participants, TAS2R38 diplotype explained 29.3% of the variance in PROP bitterness ratings at the highest concentration, and, independently of this, TAS2R4 diplotype additionally explained a further 7.7%. Between the group homozygous for the low-function haplotypes at both and the group homozygous for the high-function haplotypes at both, the mean rating differed by about 3.1-fold.

Differences in sensitivity due to genotype can also narrow depending on additives. When L-arginine was added in graded amounts to a PROP solution, bitterness intensity increased significantly only in subjects homozygous for AVI/AVI, and some non-sensitive individuals came to taste PROP as bitter for the first time only once the lowest concentration, 0.8 mM, was added.

Receptor activation is not bitterness

Bitterness perception is not determined by the presence or absence of a receptor alone. People who lack the TAS2R43 gene make up about 18–30% of the population, but even they perceive caffeine's bitterness.

The fact alone that a receptor has been activated in a test tube does not prove the bitterness of a particular dish. Larb khom's bitterness is described, in an introduction to the regional dish, as coming from cattle or buffalo bile. Bile acids contained in bile have been shown, in receptor assays using heterologous expression cells, to activate human bitter taste receptors, and human TAS2R1 responded to cholic acid and taurocholic acid at 100 µM, and to lithocholic acid at a low concentration of 0.3 µM. However, this is not a sensory evaluation that used larb khom itself as the sample.

There are also cases where binding to a receptor is predicted only by computation rather than by experiment, a stage even earlier than confirming activation. Tangeretin and nobiletin were predicted, in a molecular docking simulation against the three-dimensional structure of T2R46, to interact with a binding-pocket residue.

Suppressing bitterness via receptors — inhibitors, salt, aroma compounds

Receptor activity can also be moved by salt. The review states that at low sodium concentrations of 10 mM (0.058% sodium chloride) or below, sodium stimulates the saltiness receptor while at the same time suppressing the activity of the bitter taste receptor TAS2R16.

There are also examples where an aroma compound directly hinders receptor activation. Guaiacol and α-terpineol, derived from tea withered with pinewood smoke, inhibited TAS2R14 activation by pre-withering tea extract by 37% and 21% respectively (within the scope of the abstract).

Suppression by an aroma compound has also been reported for caffeine's receptor activation. TAS2R43 is one of the receptors sensitive to caffeine, and the aroma compound (R)-citronellal completely blocked this activation (within the scope of the abstract).

How strongly the same aroma compound acts differs by receptor. TAS2R46 is likewise activated by caffeine, but suppression by (R)-citronellal was not as strong as for TAS2R43 (within the scope of the abstract).

Frequently asked

Is a dislike of bitterness genetic?

That genes are involved in individual differences in bitterness sensitivity has been measured, but the explanation is not exhausted by a single gene. In a study of 243 people looking at the substance PROP, it has been reported that TAS2R38 type accounted for 29.3% of the variation in bitterness ratings, while another gene's type independently added a further 7.7%. Consumer studies of Brassica vegetables have also reported that TAS2R38 type has a significant effect on perceived bitterness, but it has not been consistently confirmed that people with the more bitter-tasting type go so far as to reject the vegetables. A pooled analysis examining the association between bitterness sensitivity and vegetable/legume liking and intake has also reported that the association between the two was small and not significant. How confident the answer is depends on which substance is meant, and on whether the question concerns the stage of tasting or the stage of rejection.

If a receptor activates, is it tasted as bitter?

Experiments that express a receptor in cultured cells and measure its response, and experiments in which a person perceives the strength of bitterness in the mouth, are looking at different layers. First, there are examples where perception does not necessarily disappear even without the receptor. Part of the population lacks the TAS2R43 gene itself, but there are reports that people who lack this receptor still perceive caffeine's bitterness unchanged. Second, there is also a range in the level of evidence that a receptor has responded. There are examples where a component was shown, by computational prediction alone, to bind to the structure of a bitter taste receptor, and this is only a stage earlier than confirmation of activation by experiment. In another example, it has been confirmed as far as showing that bile acids actually activate a human bitter taste receptor expressed in cells, but there is not yet a sensory evaluation that measured the bitterness of tasting the dish that uses that bile itself.

Does having a TAS2R38 type make broccoli too bitter to eat?

In consumer panels that rated rocket lines and cooked turnip, TAS2R38 genotype has been reported to have a significant effect on perceived bitterness. In one study, consumers of a particular type tended to rate bitterness intensity higher than consumers of other types. However, the view that people with the type that tastes bitterness more strongly reject the vegetable is not consistently supported within the same body of studies. The authors of one study state that the main reason for acceptance or rejection seems to relate more to the leaves' hotness than to bitterness. A pooled analysis examining multiple studies of the association between bitterness sensitivity and vegetable/legume intake also found the association between the two to be small and not significant. Whether a genotype makes something taste bitter, and whether people actually dislike it and avoid eating it, need to be measured again as separate questions.

Do people who taste PROP as strongly bitter also taste other flavors more strongly?

People who rate PROP as strongly bitter are called supertasters, and there is a study presenting the view that they perceive a variety of bitter and sweet substances more strongly, and also perceive stimulation from alcohol and capsaicin as a stronger burning sensation. This study also reports that the frequency is higher in women. However, there is also disagreement with this generalization. A review points out that the heightened response of people who report a secondary bitter taste from irritants is independent of PROP bitterness, and it is also stated that the TAS2R38 gene alone cannot explain the construct of supertasting. The view that someone has strong oral sensation in general, and the view that someone has a strong response to one particular substance, need to be carefully distinguished.

What is not known

• Why multiple TAS2R38 types are maintained in the population has a hypothesis, but the authors themselves state that the maintenance mechanism itself is unknown, and it has not yet been confirmed.
• Whether the suppression of receptor activation by aroma compounds actually leads to a weakening of bitterness felt in the mouth has not yet been measured outside cell experiments.
• The report that aroma compounds from pinewood-smoke withering suppress receptor activation has not had its full text made available, and could only be confirmed as far as the abstract.
• The statement that low concentrations of sodium suppress bitter taste receptor activity comes from a review's citation, and confirmation has not extended as far as the full text of that primary study.

Read in the encyclopedia

The encyclopedia entries below are in Japanese.

• human TAS2R receptor family — the gene family that bundles together human bitter taste receptors. The range of substances each member responds to differs
• TAS2R38 — a receptor with the common PAV and AVI haplotypes. Responds to propylthiouracil and phenylthiocarbamide
• TAS2R1 — a receptor strongly activated by bitter dipeptides and tripeptides
• TAS2R14 — one of the receptors with a broad range of agonists
• TAS2R31 — a receptor belonging to the TAS2R30-46 cluster, involved in the response to saccharin and others
• TAS2R43 — one of the receptors sensitive to caffeine
• TAS2R46 — a receptor with compounds, such as oligoporin D, that it responds to at low concentrations
• 6-n-propylthiouracil (PROP) — a compound used as the standard probe for taster classification
• phenylthiocarbamide (PTC) — a probe compound for taster classification alongside PROP
• supertaster (concept) — a term for the group of tasters who rate PROP as strongly bitter. The generalization is disputed

Go deeper in the research notes

• Twenty-Five Bitter Taste Receptors: What Do They Divide Between Them? — follows the range of agonists for each receptor, and the history of its nomenclature and identification
• A Dislike of Broccoli Might Be Genetic — follows the relationship between TAS2R38 genotype and the perceived bitterness of Brassica vegetables
• Does any seasoning remove bitterness? — Sugar and salt — reads, separately across human sensory studies, cooking, receptor assays, and the electronic tongue, how bitterness changes when another taste is added
• Cheese's Bitterness Is Written and Erased by Microbes — follows the process by which bitter peptides arise and disappear, and their response at the receptor

Source ledger

The primary sources and limitations behind the claims listed here can each be checked from the source link inside its card. The full list is gathered in the Source ledger.