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The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor).^[1] Taste is the perception stimulated when a substance in the mouth reacts chemically with taste receptor cells located on taste buds in the oral cavity, mostly on the tongue. Taste, along with the sense of smell and trigeminal nerve stimulation (registering texture, pain, and temperature), determines flavors of food and other substances. Humans have taste receptors on taste buds and other areas, including the upper surface of the tongue and the epiglottis.^[2]^[3] The gustatory cortex is responsible for the perception of taste.

The tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye.^[2] Within each papilla are hundreds of taste buds.^[1]^[4] The exceptions to this is the filiform papillae that do not contain taste buds. There are between 2000 and 5000^[5] taste buds that are located on the back and front of the tongue. Others are located on the roof, sides and back of the mouth, and in the throat. Each taste bud contains 50 to 100 taste receptor cells.

Taste receptors in the mouth sense the five basic tastes: sweetness, sourness, saltiness, bitterness, and savoriness (also known as savory or umami).^[1]^[2]^[6]^[7] Scientific experiments have demonstrated that these five tastes exist and are distinct from one another. Taste buds are able to tell different tastes apart when they interact with different molecules or ions. Sweetness, savoriness, and bitter tastes are triggered by the binding of molecules to G protein-coupled receptors on the cell membranes of taste buds. Saltiness and sourness are perceived when alkali metals or hydrogen ions meet taste buds, respectively.^[8]^[9]

The basic tastes contribute only partially to the sensation and flavor of food in the mouth—other factors include smell,^[1] detected by the olfactory epithelium of the nose;^[10] texture,^[11] detected through a variety of mechanoreceptors, muscle nerves, etc.;^[12] temperature, detected by temperature receptors; and "coolness" (such as of menthol) and "hotness" (pungency), by chemesthesis.

As the gustatory system senses both harmful and beneficial things, all basic tastes bring either caution or craving depending upon the effect the things they sense have on the body.^[13] Sweetness helps to identify energy-rich foods, while bitterness warns people of poisons.^[14]

Among humans, taste perception begins to fade during ageing, tongue papillae are lost, and saliva production slowly decreases.^[15] Humans can also have distortion of tastes (dysgeusia). Not all mammals share the same tastes: some rodents can taste starch (which humans cannot), cats cannot taste sweetness, and several other carnivores, including hyenas, dolphins, and sea lions, have lost the ability to sense up to four of their ancestral five basic tastes.^[16]

Basic tastes

The gustatory system allows animals to distinguish between safe and harmful food and to gauge different foods' nutritional value. Digestive enzymes in saliva begin to dissolve food into base chemicals that are washed over the papillae and detected as tastes by the taste buds. The tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye. Within each papilla are hundreds of taste buds.^[4] The exception to this are the filiform papillae, which do not contain taste buds. There are between 2,000 and 5,000^[5] taste buds that are located on the back and front of the tongue. Others are located on the roof, sides and back of the mouth, and in the throat. Each taste bud contains 50 to 100 taste-receptor cells.

The five specific tastes received by taste receptors are saltiness, sweetness, bitterness, sourness, and savoriness (often known by its Japanese name umami, which translates to 'deliciousness').

As of the early 20th century, Western physiologists and psychologists believed that there were four basic tastes: sweetness, sourness, saltiness, and bitterness. The concept of a "savory" taste was not present in Western science at that time, but was postulated in Japanese research.^[17] By the end of the 20th century, the concept of umami was becoming familiar to Western society.

One study found that salt and sour taste mechanisms both detect, in different ways, the presence of sodium chloride (salt) in the mouth. Acids are also detected and perceived as sour.^[18] The detection of salt is important to many organisms, but especially mammals, as it serves a critical role in ion and water homeostasis in the body. It is specifically needed in the mammalian kidney as an osmotically active compound that facilitates passive re-uptake of water into the blood.^{[citation needed]} Because of this, salt elicits a pleasant taste in most humans.

Sour and salt tastes can be pleasant in small quantities, but in larger quantities become more and more unpleasant to taste. For sour taste, this presumably is because the sour taste can signal under-ripe fruit, rotten meat, and other spoiled foods, which can be dangerous to the body because of bacteria that grow in such media. Additionally, sour taste signals acids, which can cause serious tissue damage.

Sweet taste signals the presence of carbohydrates in solution.^{[citation needed]} Since carbohydrates have a very high calorie count (saccharides have many bonds, therefore much energy),^{[citation needed]} they are desirable to the human body, which evolved to seek out the highest-calorie-intake foods.^{[citation needed]} They are used as direct energy (sugars) and storage of energy (glycogen). Many non-carbohydrate molecules trigger a sweet response, leading to the development of many artificial sweeteners, including saccharin, sucralose, and aspartame. It is still unclear how these substances activate the sweet receptors and what adaptative significance this has had.

The savory taste (known in Japanese as umami), identified by Japanese chemist Kikunae Ikeda, signals the presence of the amino acid L-glutamate. The amino acids in proteins are used in the body to build muscles and organs, and to transport molecules (hemoglobin), antibodies, and the organic catalysts known as enzymes. These are all critical molecules, and it is important to have a steady supply of amino acids; consequently, savory tastes trigger a pleasurable response, encouraging the intake of peptides and proteins.

Pungency (piquancy or hotness) had traditionally been considered a sixth basic taste.^[19] In 2015, researchers suggested a new basic taste of fatty acids called "fat taste",^[20] although "oleogustus" and "pinguis" have both been proposed as alternate terms.^[21]^[22]

Sweetness

Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars and substances that mimic sugar. Sweetness may be connected to aldehydes and ketones, which contain a carbonyl group. Sweetness is detected by a variety of G protein coupled receptors (GPCR) coupled to the G protein gustducin found on the taste buds. At least two different variants of the "sweetness receptors" must be activated for the brain to register sweetness. Compounds the brain senses as sweet are compounds that can bind with varying bond strength to two different sweetness receptors. These receptors are T1R2+3 (heterodimer) and T1R3 (homodimer), which account for all sweet sensing in humans and animals.^[23]

Taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1.^[24]^[25] The average human detection threshold for sucrose is 10 millimoles per liter. For lactose it is 30 millimoles per liter, with a sweetness index of 0.3,^[24] and 5-nitro-2-propoxyaniline 0.002 millimoles per liter. "Natural" sweeteners such as saccharides activate the GPCR, which releases gustducin. The gustducin then activates the molecule adenylate cyclase, which catalyzes the production of the molecule cAMP, or adenosine 3', 5'-cyclic monophosphate. This molecule closes potassium ion channels, leading to depolarization and neurotransmitter release. Synthetic sweeteners such as saccharin activate different GPCRs and induce taste receptor cell depolarization by an alternate pathway.

Sourness

Sourness is the taste that detects acidity. The sourness of substances is rated relative to dilute hydrochloric acid, which has a sourness index of 1. By comparison, tartaric acid has a sourness index of 0.7, citric acid an index of 0.46, and carbonic acid an index of 0.06.^[24]^[25]

Sour taste is detected by a small subset of cells that are distributed across all taste buds called Type III taste receptor cells. H+ ions (protons) that are abundant in sour substances can directly enter the Type III taste cells through a proton channel.^[26] This channel was identified in 2018 as otopetrin 1 (OTOP1).^[27] The transfer of positive charge into the cell can itself trigger an electrical response. Some weak acids such as acetic acid, can also penetrate taste cells; intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell. By a combination of direct intake of hydrogen ions through OTOP1 ion channels (which itself depolarizes the cell) and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter.^[28]

The most common foods with natural sourness are fruits, such as lemon, lime, grape, orange, tamarind, and bitter melon. Fermented foods, such as wine, vinegar or yogurt, may have sour taste. Children show a greater enjoyment of sour flavors than adults,^[29] and sour candy containing citric acid or malic acid is common.

Saltiness

Saltiness taste seems to have two components: a low-salt signal and a high-salt signal. The low-salt signal causes a sensation of deliciousness, while the high-salt signal typically causes the sensation of "too salty".^[30]

The low-salt signal is understood to be caused by the epithelial sodium channel (ENaC), which is composed of three subunits. ENaC in the taste cells allow sodium cations to enter the cell. This on its own depolarizes the cell, and opens voltage-dependent calcium channels, flooding the cell with positive calcium ions and leading to neurotransmitter release. ENaC can be blocked by the drug amiloride in many mammals, especially rats. The sensitivity of the low-salt taste to amiloride in humans is much less pronounced, leading to conjecture that there may be additional low-salt receptors besides ENaC to be discovered.^[30]

A number of similar cations also trigger the low salt signal. The size of lithium and potassium ions most closely resemble those of sodium, and thus the saltiness is most similar. In contrast, rubidium and caesium ions are far larger, so their salty taste differs accordingly.^{[citation needed]} The saltiness of substances is rated relative to sodium chloride (NaCl), which has an index of 1.^[24]^[25] Potassium, as potassium chloride (KCl), is the principal ingredient in salt substitutes and has a saltiness index of 0.6.^[24]^[25]

Other monovalent cations, e.g. ammonium (NH₄⁺), and divalent cations of the alkali earth metal group of the periodic table, e.g. calcium (Ca²⁺), ions generally elicit a bitter rather than a salty taste even though they, too, can pass directly through ion channels in the tongue, generating an action potential. But the chloride of calcium is saltier and less bitter than potassium chloride, and is commonly used in pickle brine instead of KCl.^{[citation needed]}

The high-salt signal is still very poorly understood as of 2023. Even in rodents, this signal is not blocked by amiloride. Sour and bitter cells trigger on high chloride levels, but the specific receptor is still being identified.^[30]

Bitterness

Bitterness is one of the most sensitive of the tastes, and many perceive it as unpleasant, sharp, or disagreeable, but it is sometimes desirable and intentionally added via various bittering agents. Common bitter foods and beverages include coffee, unsweetened cocoa, South American mate, coca tea, bitter gourd, uncured olives, citrus peel, some varieties of cheese, many plants in the family Brassicaceae, dandelion greens, horehound, wild chicory, and escarole. The ethanol in alcoholic beverages tastes bitter,^[31] as do the additional bitter ingredients found in some alcoholic beverages including hops in beer and gentian in bitters. Quinine is also known for its bitter taste and is found in tonic water.

Bitterness is of interest to those who study evolution, as well as various health researchers^[24]^[32] since a large number of natural bitter compounds are known to be toxic. The ability to detect bitter-tasting, toxic compounds at low thresholds is considered to provide an important protective function.^[24]^[32]^[33] Plant leaves often contain toxic compounds, and among leaf-eating primates there is a tendency to prefer immature leaves, which tend to be higher in protein and lower in fiber and poisons than mature leaves.^[34] Amongst humans, various food processing techniques are used worldwide to detoxify otherwise inedible foods and make them palatable.^[35] Furthermore, the use of fire, changes in diet, and avoidance of toxins has led to neutral evolution in human bitter sensitivity. This has allowed several loss of function mutations that has led to a reduced sensory capacity towards bitterness in humans when compared to other species.^[36]

The threshold for stimulation of bitter taste by quinine averages a concentration of 8 μM (8 micromolar).^[24] The taste thresholds of other bitter substances are rated relative to quinine, which is thus given a reference index of 1.^[24]^[25] For example, brucine has an index of 11, is thus perceived as intensely more bitter than quinine, and is detected at a much lower solution threshold.^[24] The most bitter natural substance is amarogentin, a compound present in the roots of the plant Gentiana lutea, and the most bitter substance known is the synthetic chemical denatonium, which has an index of 1,000.^[25] It is used as an aversive agent (a bitterant) that is added to toxic substances to prevent accidental ingestion. It was discovered accidentally in 1958 during research on a local anesthetic, by MacFarlan Smith of Gorgie, Edinburgh, Scotland.^[37]

Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as TAS2R38 coupled to the G protein gustducin are responsible for the human ability to taste bitter substances.^[38] They are identified not only by their ability to taste for certain "bitter" ligands, but also by the morphology of the receptor itself (surface bound, monomeric).^[18] The TAS2R family in humans is thought to comprise about 25 different taste receptors, some of which can recognize a wide variety of bitter-tasting compounds.^[39] Over 670 bitter-tasting compounds have been identified, on a bitter database, of which over 200 have been assigned to one or more specific receptors.^[40] It is speculated that the selective constraints on the TAS2R family have been weakened due to the relatively high rate of mutation and pseudogenization.^[41] Researchers use two synthetic substances, phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP) to study the genetics of bitter perception. These two substances taste bitter to some people, but are virtually tasteless to others. Among the tasters, some are so-called "supertasters" to whom PTC and PROP are extremely bitter. The variation in sensitivity is determined by two common alleles at the TAS2R38 locus.^[42] This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics.

Gustducin is made of three subunits. When it is activated by the GPCR, its subunits break apart and activate phosphodiesterase, a nearby enzyme, which in turn converts a precursor within the cell into a secondary messenger, which closes potassium ion channels.^{[citation needed]} Also, this secondary messenger can stimulate the endoplasmic reticulum to release Ca2+ which contributes to depolarization. This leads to a build-up of potassium ions in the cell, depolarization, and neurotransmitter release. It is also possible for some bitter tastants to interact directly with the G protein, because of a structural similarity to the relevant GPCR.

Umami

Umami, or savoriness, is an appetitive taste.^[13]^[17] It can be tasted in soy sauce, meat, dashi and consomme. A loanword from Japanese meaning "good flavor" or "good taste",^[43] umami (旨味) is considered fundamental to many East Asian cuisines,^[44] such as Japanese cuisine.^[45] It dates back to the use of fermented fish sauce: garum in ancient Rome^[46] and ge-thcup or koe-cheup in ancient China.^[47]

Umami was first studied in 1907 by Ikeda isolating dashi taste, which he identified as the chemical monosodium glutamate (MSG).^[17]^[48] MSG is a sodium salt that produces a strong savory taste, especially combined with foods rich in nucleotides such as meats, fish, nuts, and mushrooms.^[49]

Some savory taste buds respond specifically to glutamate in the same way that "sweet" ones respond to sugar. Glutamate binds to a variant of G protein coupled glutamate receptors.^[50]^[51] L-glutamate may bond to a type of GPCR known as a metabotropic glutamate receptor (mGluR4) which causes the G-protein complex to activate the sensation of umami.^[51]

Measuring relative tastes

Measuring the degree to which a substance presents one basic taste can be achieved in a subjective way by comparing its taste to a reference substance.

Sweetness is subjectively measured by comparing the threshold values, or level at which the presence of a dilute substance can be detected by a human taster, of different sweet substances.^[52] Substances are usually measured relative to sucrose,^[53] which is usually given an arbitrary index of 1^[54]^[55] or 100.^[56] Rebaudioside A is 100 times sweeter than sucrose; fructose is about 1.4 times sweeter; glucose, a sugar found in honey and vegetables, is about three-quarters as sweet; and lactose, a milk sugar, is one-half as sweet.^[b]^[52]

The sourness of a substance can be rated by comparing it to very dilute hydrochloric acid (HCl).^[57]

Relative saltiness can be rated by comparison to a dilute salt solution.^[58]

Quinine, a bitter medicinal found in tonic water, can be used to subjectively rate the bitterness of a substance.^[59] Units of dilute quinine hydrochloride (1 g in 2000 mL of water) can be used to measure the threshold bitterness concentration, the level at which the presence of a dilute bitter substance can be detected by a human taster, of other compounds.^[59] More formal chemical analysis, while possible, is difficult.^[59]

There may not be an absolute measure for pungency, though there are tests for measuring the subjective presence of a given pungent substance in food, such as the Scoville scale for capsaicine in peppers or the Pyruvate scale for pyruvates in garlics and onions.

Functional structure

Taste is a form of chemoreception which occurs in the specialised taste receptors in the mouth. To date, there are five different types of taste these receptors can detect which are recognized: salt, sweet, sour, bitter, and umami. Each type of receptor has a different manner of sensory transduction: that is, of detecting the presence of a certain compound and starting an action potential which alerts the brain. It is a matter of debate whether each taste cell is tuned to one specific tastant or to several; Smith and Margolskee claim that "gustatory neurons typically respond to more than one kind of stimulus, [a]lthough each neuron responds most strongly to one tastant". Researchers believe that the brain interprets complex tastes by examining patterns from a large set of neuron responses. This enables the body to make "keep or spit out" decisions when there is more than one tastant present. "No single neuron type alone is capable of discriminating among stimuli or different qualities, because a given cell can respond the same way to disparate stimuli."^[60] As well, serotonin is thought to act as an intermediary hormone which communicates with taste cells within a taste bud, mediating the signals being sent to the brain. Receptor molecules are found on the top of microvilli of the taste cells.