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Taste bud

Taste (also called gustation; adjectival form: gustatory) is one of the traditional five senses. Taste is that sensation produced when stimuli are taken into the mouth and react with the receptors of the taste buds. Simply put, it is a chemical reaction between stimuli (food) and receptors (taste buds).

Taste along with smell (olfaction) and trigeminal nerve stimulation (with touch for texture, also pain, and temperature) determines flavors, the sensory impressions of food or other substances.

Humans receive tastes through sensory organs called taste buds,[1] or gustatory calyculi, concentrated on the top of the tongue.[2] Taste is sensed through taste cells, which are known as taste buds. There are about 100,000 [clarification needed] 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. The sensation of taste can be categorized into five basic tastes: sweet, bitter, sour, salty, and umami. “Umami” is a Japanese word coined by the Japanese scientist Dr. Kikunae Ikeda, the discoverer of this taste as a distinct one; apparently from umai (a Japanese word meaning 'delicious' or 'savory'; not 'meaty') + mi (味 a kanji word, meaning 'taste'). Not surprisingly, it is characteristic of many Asian dishes.[3] The amino acid glutamate produces a strong umami taste.[4] The tongue is able to differentiate between the different tastes based on different molecules or ions that bind to the taste cell. Sweet, umami, and bitter taste is triggered by different molecules that bind to the G protein-coupled membrane receptors; while saltiness is from Na+ ions and sourness is from H+ ions entering the cell.[5] As taste senses both harmful and beneficial things, all basic tastes are classified as either aversive or appetitive, depending upon the effect the things they sense have on our bodies.[6] Sweetness helps to identify energy-rich foods, while bitterness serves as a warning sign of poisons.[7]

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;[8] texture,[9] detected through a variety of mechanoreceptors, muscle nerves, etc.;[10] temperature, detected by thermoreceptors; and "coolness" (such as of menthol) and "hotness" (piquance), through chemesthesis.

Introduction

As one of the senses, taste is an essential part of daily life.

History

In the West, Aristotle, who postulated c. 350 BCE[11] that the two most basic tastes were sweet and bitter,[12] was one of the first to develop a list of basic tastes.[13]

Ayurveda, an ancient Indian healing science, has its own tradition of basic tastes, comprising astringent, bitter, pungent, salty, sour, and sweet.[14][15]

Recent discoveries

The receptors for the basic tastes of bitter, sweet and umami have been identified. They are G protein-coupled receptors.[16] The cells that detect sour have been identified as a subpopulation that express the protein PKD2L1. The responses are mediated by an influx of protons into the cells but the receptor for sour is still unknown . The receptor for amiloride-sensitive attractive salty taste in mice has been shown to be a sodium channel.[17] There is some evidence for a sixth taste that senses fatty substances.[18]

Basic tastes

For a long period, it was commonly accepted[who?] that there is a finite and small number of "basic tastes" of which all seemingly complex tastes are ultimately composed. Just as with primary colors, the "basic" quality of those sensations derives chiefly from the nature of human perception, in this case the different sorts of tastes the human tongue can identify. As of the early twentieth century, physiologists and psychologists believed there were four basic tastes: bitterness, saltiness, sourness, and sweetness. At that time umami was not proposed as a fifth taste[19] but now a large number of authorities now recognize it as such.[citation needed] In Asian countries within the sphere of mainly Chinese and Indian cultural influence, piquance had traditionally been considered a fifth basic taste.[citation needed]

Bitterness

Bitterness is the most sensitive of the tastes, and many perceive it as unpleasant, sharp, or disagreeable. Common bitter foods and beverages include coffee, unsweetened cocoa, South American mate, marmalade, bitter gourd, beer, bitters, olives, citrus peel, many plants in the Brassicaceae family, dandelion greens, wild chicory, and escarole. 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[20][21] 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.[20][21][22] Plant leaves often contain toxic compounds, yet even amongst 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.[23] Amongst humans, various food processing techniques are used worldwide to detoxify otherwise inedible foods and make them palatable.[24]

The threshold for stimulation of bitter taste by quinine averages a concentration of 0.000008 M .[clarification needed][20] The taste thresholds of other bitter substances are rated relative to quinine, which is thus given a reference index of 1.[20][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.[20] 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 that is added to toxic substances to prevent accidental ingestion. This was discovered in 1958 during research on lignocaine, a local anesthetic, by MacFarlan Smith of Gorgie, Edinburgh, Scotland.

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.[26] 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).[27] 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.[28] Over 550 bitter-tasting compounds have been identified, of which about 100 have been assigned to one or more specific receptors.[29] Recently it is speculated that the selective constraints on the TAS2R family have been weakened due to the relatively high rate of mutation and pseudogenization.[30] 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.[31] This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics.

Saltiness

Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of the alkali metals group also taste salty, but the further from sodium the less salty the sensation is. The size of lithium and potassium ions most closely resemble those of sodium and thus the saltiness is most similar. In contrast rubidium and cesium 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.[20][25] Potassium, as potassium chloride - KCl, is the principal ingredient in salt substitutes, and has a saltiness index of 0.6.[20][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.

Sourness

"Sour" redirects here. For other uses, see Sour (disambiguation).
Look up sour in Wiktionary, the free dictionary.

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.[20][25]

Sour taste is detected by a small subset of cells that are distributed across all taste buds in the tongue. Sour taste cells can be identified by expression of the protein PKD2L1,[32] although surprisingly this gene is not required for sour responses. There is evidence that the protons that are abundant in sour substances can directly enter the sour taste cells. This transfer of positive charge into the cell can itself trigger an electrical response. It has also been proposed that weak acids such as acetic acid, which are not fully dissociated at physiological pH values, can penetrate taste cells and thereby elicit an electrical response. According to this mechanism, intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell. By a combination of direct intake of hydrogen ions (which itself depolarizes the cell) and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter. The mechanism by which animals detect sour is still not completely understood.

The most common food group that contains naturally sour foods is fruit, such as lemon, grape, orange, tamarind and sometimes melon. Wine also usually has a sour tinge to its flavor, and if not kept correctly, milk can spoil and develop a sour taste. Sour candy is popular in North America[33] including Cry Babies, Warheads, Lemon drops, Shock tarts and Sour Skittles and Starburst. Many of these candies contain citric acid.

Sweetness

Main article: Sweetness
See also: Miraculin and Curculin

Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars and a few other substances. Sweetness is often connected to aldehydes and ketones, which contain a carbonyl group. Sweetness is detected by a variety of G protein coupled receptors 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 thus 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.[34] Taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1.[20][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,[20] and 5-Nitro-2-propoxyaniline 0.002 millimoles per liter.

Umami

Main article: Umami

Umami is an appetitive taste[6] and is described as a savory[35][36] or meaty[36][37] taste. It can be tasted in cheese[38] and soy sauce,[39] and while also found in many other fermented and aged foods, this taste is also present in tomatoes, grains, and beans.[38] Monosodium glutamate (MSG), developed as a food additive in 1908 by Kikunae Ikeda,[4] produces a strong umami taste.[39] See TAS1R1 and TAS1R3 pages for a further explanation of the amino-acid taste receptor. A loanword from Japanese meaning "good flavor" or "good taste",[40] Umami (旨味) is considered fundamental to many Eastern cuisines[41] and was first described in 1908,[42] although it was only recently recognized in the West as a basic taste.[39][43]

Some umami 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.[44][45]

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. Quinine, a bitter medicinal found in tonic water, can be used to subjectively rate the bitterness of a substance.[46] 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.[46] More formal chemical analysis, while possible, is difficult.[46]

Relative saltiness can be rated by comparison to a dilute salt solution.[47]

The sourness of a substance can be rated by comparing it to very dilute hydrochloric acid (HCl).[48]

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.[49] Substances are usually measured relative to sucrose,[50] which is usually given an arbitrary index of 1[51][52] or 100.[53] Fructose is about 1.4 times sweeter than sucrose; 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][49]

Functional structure

Main article: Gustatory system
Bitterness

Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as TAS2R38 are responsible for the human ability to taste bitter substances.[54] They are identified not only by their ability to taste certain bitter ligands, but also by the morphology of the receptor itself (surface bound, monomeric).[55]

Saltiness

Saltiness is a taste produced best by the presence of cations (such as Na+
, K+
 or Li+
)[56] and, like sour, it is tasted using ion channels.[56]

Other ions of the alkali metals group also taste salty, but the less sodium-like the ion is, the less salty the sensation.[citation needed] As the size of lithium and potassium ions is close to that of sodium, they taste similar to salt.[citation needed] In contrast, the larger rubidium and cesium ions do not taste as salty.[citation needed]

Other monovalent cations, e.g., ammonium, NH+
4, and divalent cations of the alkali earth metal group of the periodic table, e.g., calcium, Ca2+
, ions, in general, elicit a bitter rather than a salty taste even though they, too, can pass directly through ion channels in the tongue.[citation needed]

Sourness

Sourness is acidity,[57][58] and, like salt, it is a taste sensed using ion channels.[56] Hydrogen ion channels detect the concentration of hydronium ions that are formed from acids and water.[citation needed] In addition, the taste receptor PKD2L1 has been found to be involved in tasting sour.[59]

Sweetness

Sweetness is produced by the presence of sugars, some proteins, and a few other substances.[citation needed] It is often connected to aldehydes and ketones, which contain a carbonyl group.[citation needed] Sweetness is detected by a variety of G protein-coupled receptors coupled to a G protein that acts as an intermediary in the communication between taste bud and brain, gustducin.[60] These receptors are T1R2+3 (heterodimer) and T1R3 (homodimer), which account for sweet sensing in humans and other animals.[61]

Umami-ness

The amino acid glutamic acid is responsible for umami,[62][63] but some nucleotides (inosinic acid[41][64] and guanylic acid[62]) can act as complements, enhancing the taste.[41][64]

Glutamic acid binds to a variant of the G protein-coupled receptor, producing an umami taste.[65][66]

Further sensations

The tongue can also feel other sensations not generally included in the basic tastes. These are largely detected by the somatosensory system.

Calcium

In 2008, geneticists discovered a CaSR calcium receptor on the tongues of mice. The CaSR receptor is commonly found in the gastrointestinal tract, kidneys, and brain. Along with the "sweet" T1R3 receptor, the CaSR receptor can detect calcium as a taste. Whether closely related genes in mice and humans means the phenomenon exists in humans as well is unknown.[67][68]

Coolness

Some substances activate cold trigeminal receptors even when not at low temperatures. This "fresh" or "minty" sensation can be tasted in spearmint, menthol, ethanol, and camphor. Caused by activation of the same mechanism that signals cold, TRPM8 ion channels on nerve cells, unlike the actual change in temperature described for sugar substitutes, this coolness is only a perceived phenomenon.

Dryness

Some foods, such as unripe fruits, contain tannins or calcium oxalate that cause an astringent or rough sensation of the mucous membrane of the mouth. Examples include tea, red wine, rhubarb, and unripe persimmons and bananas.

Less exact terms for the astringent sensation are "dry", "rough", "harsh" (especially for wine), "tart" (normally referring to sourness), "rubbery", "hard" or "styptic".[69]

When referring to wine, dry is the opposite of sweet, and does not refer to astringency. Wines that contain tannins and so cause an astringent sensation are not necessarily classified as "dry," and "dry" wines are not necessarily astringent.

In the Indian Ayurvedic tradition, one of the six tastes is astringency (kasaaya).[70]

Fattiness

Recent research reveals a potential taste receptor called the CD36 receptor that reacts to fat (fatty acids, more specifically).[71] This receptor was found in mice.

Heartiness (kokumi)

Some Japanese researchers refer to the kokumi of foods laden with alcohol and thiol-groups in their amino acid extracts, and this sensation has also been described as mouthfeel.

Numbness

Both Chinese and Batak Toba cooking include the idea of 麻 (má or mati rasa), a tingling numbness caused by spices such as Sichuan pepper. The cuisines of Sichuan province in China and of the Indonesia province North Sumatra often combine this with chili pepper to produce a 麻辣 málà, "numbing-and-hot", or "mati rasa" flavor.[72] These sensations although not taste fall into a category of Chemesthesis.

Spiciness
Main articles: Piquance and Scoville scale

Substances such as ethanol and capsaicin cause a burning sensation called Chemesthesis, piquance, spiciness, hotness, or prickliness by inducing a trigeminal nerve reaction together with normal taste reception. The sensation of heat is caused by the food's activating nerves that express TRPV1 and TRPA1 receptors. Two main plant-derived compounds that provide this sensation are capsaicin from chili peppers and piperine from black pepper. The piquant ("hot" or "spicy") sensation provided by chili peppers, black pepper, and other spices like ginger and horseradish plays an important role in a diverse range of cuisines across the world—especially in equatorial and sub-tropical climates, such as Ethiopian, Peruvian, Hungarian, Indian, Korean, Indonesian, Lao, Malaysian, Mexican, Southwest Chinese (including Szechuan cuisine), Vietnamese, and Thai cuisines.

This particular sensation, called Chemesthesis, is not a taste in the technical sense, because the sensation does not arise from taste buds and a different set of nerve fibers carry it to the brain. Foods like chili peppers activate nerve fibers directly; the sensation interpreted as "hot" results from the stimulation of somatosensory (pain/temperature) fibers on the tongue. Many parts of the body with exposed membranes but no taste sensors (such as the nasal cavity, under the fingernails, surface of the eye ([cornea]) or a wound) produce a similar sensation of heat when exposed to hotness agents. Asian countries within the sphere of, mainly, Chinese, Indian, and Japanese cultural influence, traditionally consider piquance a sixth basic taste.

Temperature

Temperature can be an essential element of the taste experience. Food and drink that—in a given culture—is traditionally served hot is often considered distasteful if cold, and vice versa. For example, alcoholic beverages, with a few exceptions, are usually thought best when served cold, but soups—again, with exceptions—are usually only eaten hot. A cultural example is soda. In North America it is almost always preferred cold, regardless of season. In South America lukewarm soda is almost exclusively consumed in winter.[citation needed]

Other concepts

Supertasters

Main article: Supertaster

A supertaster is a person whose sense of taste is significantly more sensitive than average. The cause of this heightened response is likely, at least in part, due to an increased number of fungiform papillae.[73] Study have shown that supertasters require less fat and sugar in their food to get the same satisfying effects. However, contrary to what one might think, these people actually tend to consume more salt than the average person. This is due to their heightened sense of the taste of bitterness, and the presence of salt drowns out the taste of bitterness. (This also explains why supertasters prefer salted cheddar cheese over non-salted.)[74]

Aftertaste

Main article: Aftertaste

Aftertastes arise after food has been swallowed. An aftertaste can differ from the food it follows. Medicines and tablets may also have a lingering aftertaste, as can certain artificial flavor compounds, such as aspartame (artificial sweetener).

Acquired taste

Main article: Acquired taste

Acquired taste is an appreciation for a food or beverage that one is likely to initially dislike. Many of the world's delicacies are considered acquired tastes.

Innervation

Taste is brought to the brainstem by 3 different cranial nerves:

Disorders of taste

See also

Notes

Footnotes

a. ^ It has been known for some time that these categories may not be comprehensive. In Guyton's 1976 edition of Textbook of Medical Physiology, he wrote:

On the basis of physiologic studies, there are generally believed to be at least four primary sensations of taste: sour, salty, sweet, and bitter. Yet we know that a person can perceive literally hundreds of different tastes. These are all supposed to be combinations of the four primary sensations...However, there might be other less conspicuous classes or subclasses of primary sensations",[75]

b. ^ Some variation in values is not uncommon between various studies. Such variations may arise from a range of methodological variables, from sampling to analysis and interpretation. In fact there is a "plethora of methods"[76] Indeed, the taste index of 1, assigned to reference substances such as sucrose (for sweetness), hydrochloric acid (for sourness), quinine (for bitterness), and sodium chloride (for saltiness), is itself arbitrary for practical purposes.[48]

Some values, such as those for maltose and glucose, vary little. Others, such as aspartame and sodium saccharin, have much larger variation. Regardless of variation, the perceived intensity of substances relative to each reference substance remains consistent for taste ranking purposes. The indices table for McLaughlin & Margolskee (1994) for example,[20][77] is essentially the same as that of Svrivastava & Rastogi (2003),[78] Guyton & Hall (2006),[48] and Joesten et al. (2007).[51] The rankings are all the same, with any differences, where they exist, being in the values assigned from the studies from which they derive.

As for the assignment of 1 or 100 to the index substances, this makes no difference to the rankings themselves, only to whether the values are displayed as whole numbers or decimal points. Glucose remains about three-quarters as sweet as sucrose whether displayed as 75 or 0.75.

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Reference #30 (Wooding et al.) is helpful, but it is incorrect. The discovery that variants in the TAS2R38 gene underlie the ability to taste PTC and PROP was reported a year earlier in: Kim, U.-K., Jorgenson, E., Coon, H., Leppert, M., Risch, N., and D. Drayna. Positional Cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide. Science 299:1221-1225 (2003). I was the senior and communicating author on both of these papers.

Dennis Drayna, PhD NIDCD/National Institutes of Health

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