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Taste bud

Taste (or, more formally, gustation) is a form of direct chemoreception and is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons. In humans and many other vertebrate animals the sense of taste partners with the less direct sense of smell, in the brain's perception of flavor. In the West, experts traditionally identified four taste sensations: sweet, salty, sour, and bitter. In the Eastern hemisphere, piquance (the sensation provided by, among other things, chili peppers) and savoriness (also known as umami) have been traditionally identified as basic tastes as well. More recently, psychophysicists and neuroscientists have suggested other taste categories (fatty acid taste most prominently, as well as the sensation of metallic and water tastes, although the latter is commonly disregarded due to the phenomenon of taste adaptation.[citation needed]) Taste is a sensory function of the central nervous system. The receptor cells for taste in humans are found on the surface of the tongue, along the soft palate, and in the epithelium of the pharynx and epiglottis.

Contents

Overview

Psychophysicists have long suggested the existence of four taste 'primaries', referred to as the basic tastes: sweetness, bitterness, sourness and saltiness. Although first described in 1908, savoriness (also called "umami" in Japanese) has been only recently recognized as the fifth basic taste since the cloning of a specific amino acid taste receptor in 2002. The savory taste is exemplified by the non-salty sensations evoked by some free amino acids such as monosodium glutamate.[1][2][3]

Other possible categories have been suggested, such as a taste exemplified by certain fatty acids such as linoleic acid.[4][5][6] Some researchers still argue against the notion of primaries at all and instead favor a continuum of percepts,[7][8][9] similar to color vision.

All of these taste sensations arise from all regions of the oral cavity, despite the common misconception of a "taste map" of sensitivity to different tastes thought to correspond to specific areas of the tongue.[10] This myth is generally attributed to the mistranslation of a German text, and perpetuated in North American schools since the early twentieth century.[11] Very slight regional differences in sensitivity to compounds exist, though these regional differences are subtle and do not conform exactly to the mythical tongue map. Individual taste buds (which contain approximately 100 taste receptor cells), in fact, typically respond to compounds evoking each of the five basic tastes.[citation needed]

The "basic tastes" are those commonly recognized types of taste sensed by humans. Humans receive tastes through sensory organs called "taste buds" or "gustatory calyculi", concentrated on the upper surface of the tongue, but a few are also found on the roof of one's mouth, furthering the taste sensations we can receive. Scientists describe five basic tastes: bitter, salty, sour, sweet, and savory. The basic tastes are only one component that contributes to the sensation of food in the mouth—other factors include the food's smell, detected by the olfactory epithelium of the nose, its texture, detected by mechanoreceptors, and its temperature, detected by thermoreceptors. Taste and smell are subsumed under the term "flavor".

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History

In Western culture, the concept of basic tastes can be traced back at least to Aristotle, who cited "sweet" and "bitter", with "succulent", "salt", "pungent", "harsh", "puckery" and "sour" as elaborations of those two basics. The ancient Chinese Five Elements philosophy lists slightly different five basic tastes: bitter, salty, sour, sweet and spicy. Ayurveda, the ancient Indian healing science refers astringent as the sixth taste. Japanese culture also adds its own sixth taste to the basic five.[citation needed]

For many years, books on the physiology of human taste contained diagrams of the tongue showing levels of sensitivity to different tastes in different regions. In fact, taste qualities are found in all areas of the tongue, in contrast with the popular view that different tastes map to different areas of the tongue.[12][13]

Recent discoveries

The receptors for all known basic tastes have been identified. The receptors for sour and salty are ion channels while the receptors for sweet, bitter and savory belong to the class of G protein coupled receptors.[14]

In November 2005, a team of researchers experimenting on rodents claimed to have evidence for a sixth taste, for fatty substances.[15] It is speculated that humans may also have the same receptors.[16] Fat has occasionally been raised as a possible basic taste in the past (Bravo 1592, Linnaeus 1751) but later classifications abandoned fat as a separate taste (Haller 1751 and 1763). [17]

Basic tastes

For a long period, it has been commonly accepted that there are a finite number of "basic tastes" by which all foods and tastes can be grouped. Just like with primary colors, these "basic tastes" only apply to the human perception, i.e., the different sorts of tastes the human tongue can identify. Up until the 2000s, this was considered to be a group of four basic tastes. More recently, a fifth taste, savory, has been proposed by a large number of authorities associated with this field.[18]

Bitterness

Bitterness is the most sensitive of the tastes, and is perceived by many to be unpleasant, sharp, or disagreeable. Common bitter foods and beverages include coffee, unsweetened cocoa, South American mate, marmalade, bitter melon, beer, bitters, olives, citrus peel, many plants in the Brassicaceae family, dandelion greens and escarole. Quinine is also known for its bitter taste and is found in tonic water. The threshold for stimulation of bitter taste by quinine averages 0.000008 M.[19] The taste thresholds of other bitter substances are rated relative to quinine, which is given an index of 1.[19][20] 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.[19] The most bitter substance known is the synthetic chemical denatonium, which has an index of 1,000.[20] 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 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.[21] 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).[22] 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.[23] This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics.

In addition, it is of interest to those who study evolution, as well as various health researchers[19][24] since PTC-tasting is associated with the ability to taste numerous natural bitter compounds, a large number of which are known to be toxic. The ability to detect bitter-tasting, toxic compounds at low thresholds is considered to provide an important protective function.[25][19][24] 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.[26] Amongst humans, various food processing techniques are used worldwide to detoxify otherwise inedible foods and make them palatable.[27] 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. [28]

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.[19][20] Potassium, as potassium chloride - KCl, is the principal ingredient in salt substitutes, and has a saltiness index of 0.6.[19][20]

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

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.[19][20] The mechanism for detecting sour taste is similar to that which detects salt taste. Hydrogen ion channels detect the concentration of hydronium ions that are formed from acids and water.

Hydrogen ions are capable of permeating the amiloride-sensitive channels, but this is not the only mechanism involved in detecting the quality of sourness. Other channels have also been proposed in the literature. Hydrogen ions also inhibit the potassium channel, which normally functions 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 in this specific manner. In addition, it has also been suggested that weak acids, such as CO2 which is converted into the bicarbonate ion by the enzyme carbonic anhydrase, to mediate weak acid transport. The most common food group that contains naturally sour foods is the fruit, with examples such as the lemon, grape, orange, and sometimes the melon. Wine also usually has a sour tinge to its flavor. If not kept correctly, milk can spoil and contain a sour taste.

Sweetness

Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars, some proteins 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" need to be activated for the brain to register sweetness. The compounds which 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 are shown to be accountable for all sweet sensing in humans and animals.[29] Taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1.[19][20] The average human detection threshold for sucrose is 10 millimoles per litre. For lactose it is 30 millimoles per litre, with a sweetness index of 0.3[19], and 5-Nitro-2-propoxyaniline 0.002 millimoles per litre.

Savoriness

Savoriness is the name for the taste sensation produced by amino acids such as glutamate. The compounds that generate savoriness are commonly found in fermented and aged foods. It is also described as "meatiness", "relish", or having a "rich" taste. Savoriness is considered a fundamental taste in Chinese, Japanese and Korean cooking, but is not discussed as much in Western cuisine.

Humans have taste receptors specifically for the detection of the amino acids, e.g., glutamic acid. Amino acids are the building blocks of proteins and are found in meats, cheese, fish, and other protein-heavy foods. Examples of food containing glutamate (and thus strong in savoriness) are beef, lamb, parmesan, and roquefort cheese as well as soy sauce and fish sauce. The glutamate taste sensation is most intense in combination with sodium ions, as found in table salt. Sauces with savory and salty tastes are very popular for cooking, such as Worcestershire sauce for Western cuisines and soy sauce and fish sauce for Oriental (East Asian) cuisines.

The additive monosodium glutamate (MSG), which was developed as a food additive in 1907 by Kikunae Ikeda, produces a strong savory taste. Savoriness is also provided by the nucleotides 5’-inosine monophosphate (IMP) and 5’-guanosine monophosphate (GMP). These are naturally present in many protein-rich foods. IMP is present in high concentrations in many foods, including dried skipjack tuna flakes and kombu used to make "dashi", a Japanese broth. GMP is present in high concentration in dried shiitake mushrooms, used in much of the cuisine of Asia. There is a synergistic effect between MSG, IMP, and GMP which together in certain ratios produce a strong savory taste.

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.[30][31]

Further sensations

The tongue can also feel other sensations, not generally classified as tastes or included in the five human tastes. These are largely detected by the somatosensory system.

Fattiness

Recent research has revealed a potential taste receptor called the CD36 receptor to be reacting to fat, more specifically, fatty acids.[32] This receptor was found in mice, but probably exists among other mammals as well. In experiments, mice with a genetic defect that blocked this receptor didn't show the same urge to consume fatty acids as normal mice, and failed to prepare gastric juices in their digestive tracts to digest fat. This discovery may lead to a better understanding of the biochemical reasons behind this behaviour, although more research is still necessary to confirm the relationship between CD36 and the perception of fat.

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 may exist in humans as well is unknown.[33][34]

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 or the teeth. 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".[35]

In the Indian tradition, one of the 6 tastes [36] is astringency (Kasaaya in Sanskrit, the other five being sweet, sour, salty, bitter and hot/pungent).

In wine terms, "dry" just means opposite of "sweet". If a wine gives you a cotton-like feeling in the mouth, then it means that there are a lot of tannins in it, not that it is necessarily dry. There are dry wines that do not give the rough feeling on the cheeks.

Metallicness

Most people know this taste (e.g. Cu2+, FeSO4, or blood in mouth), however it is not only taste, but also olfactory receptors at work in this case (Guth and Grosch, 1990). Metallic taste is commonly known, however biologists are reluctant to categorize it with the other taste sensations. One of the primary reasons is that it is not one commonly associated with consumption of food. Proponents of the theory contest that the sensation is readily detectable and distinguishable to test subjects and that therefore, "metallic" should be added as one of the basic types of sensations in the chemical receptor senses.

Prickliness or hotness

Substances such as ethanol and capsaicin cause a burning sensation by inducing a trigeminal nerve reaction together with normal taste reception. The sensation of heat is caused by the food activating nerves that express TRPV1 and TRPA1 receptors. Two main plant derived compounds providing 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 also other spices like ginger and horseradish plays an important role in a diverse range of cuisines across the world, such as Ethiopian, Peruvian, Hungarian, Korean, Indonesian, Lao, Malaysian, Mexican, South Asian, Southwest Chinese (including Sichuan cuisine), and Thai cuisines.

If tissue in the oral cavity has been damaged or sensitised, ethanol may be experienced as pain rather than simply heat. Those who have had radiotherapy for oral cancer thus find it painful to drink alcohol.[citation needed]

This particular sensation is not considered a taste in the technical sense, because it is carried to the brain by a different set of nerves. Although taste nerves are also activated when consuming foods like chili peppers, the sensation commonly interpreted as "hot" results from the stimulation of somatosensory (pain/temperature) fibers on the tongue. Many parts of the body with exposed membranes but without taste sensors (such as the nasal cavity, under the fingernails, or a wound) produce a similar sensation of heat when exposed to hotness agents.

Coolness

Some substances activate cold trigeminal receptors. One can sense a cool sensation (also known as "fresh" or "minty") from, e.g., spearmint, menthol, ethanol or camphor, which is caused by the food activating the TRPM8 ion channel on nerve cells that also signal cold. Unlike the actual change in temperature described for sugar substitutes, coolness is only a perceived phenomenon.

Numbness

Both Chinese and Batak Toba cooking include the idea of 麻 , or mati rasa the sensation of tingling numbness caused by spices such as Sichuan pepper. The cuisine of Sichuan province in China and of North Sumatra province in Indonesia, often combines this with chili pepper to produce a 麻辣 málà, "numbing-and-hot", or "mati rasa" flavor.[37]

Heartiness (Kokumi)

Some Japanese researchers refer to the kokumi in foods laden with alcohol- and thiol-groups in their amino acid extracts which has been described variously as continuity, mouthfulness, mouthfeel, and thickness.

Temperature

Temperature is an essential element of human taste experience. Food and drink that—within a given culture—is considered to be properly served hot is often considered distasteful if cold, and vice versa.

Some sugar substitutes have strong heats of solution, as is the case of sorbitol, erythritol, xylitol, mannitol, lactitol, and maltitol. When they are dry and are allowed to dissolve in saliva, heat effects can be recognized. The cooling effect upon eating may be desirable, as in a mint candy made with crystalline sorbitol, or undesirable if it's not typical for that product, like in a cookie. Crystalline phases tend to have a positive heat of solution and thus a cooling effect. The heats of solution of the amorphous phases of the same substances are negative and cause a warm impression in the mouth.[38]

Supertasters

A supertaster is a person whose sense of taste is significantly sharper than average. Women are more likely to be supertasters, as are Asians, Africans, and South Americans. Among individuals of European descent, it is estimated that about 25% of the population are supertasters. The cause of this heightened response is currently unknown, although it is thought to be, at least in part, due to an increased number of fungiform papillae.[39] The evolutionary advantage to supertasting is unclear. In some environments, heightened taste response, particularly to bitterness, would represent an important advantage in avoiding potentially toxic plant alkaloids. However, in other environments, increased response to bitter may have limited the range of palatable foods. In a modern, energy-rich environment, supertasting may be cardioprotective, due to decreased liking and intake of fat, but may increase cancer risk via decreased vegetable intake.[citation needed] It may be a cause of picky eating, but picky eaters are not necessarily supertasters, and vice versa.

Aftertaste

Aftertaste is the persistence of a sensation of flavor after the stimulating substance has passed out of contact with the sensory end organs for taste. Some aftertastes may be pleasant, others unpleasant.

Alcoholic beverages such as wine, beer and whiskey are noted for having particularly strong aftertastes. Foods with notable aftertastes include spicy foods, such as Mexican food (e.g., chili pepper), or Indian food (such as curry).

Medicines and tablets may also have a lingering aftertaste, as can certain artificial flavor compounds, such as aspartame (artificial sweetener).

Acquired taste

An acquired taste is an appreciation for a food or beverage that is unlikely to be enjoyed by a person who has not had substantial exposure to it, usually because of some unfamiliar aspect of the food or beverage, including a strong or strange odor, taste, or appearance. The process of "acquiring" a taste involves consuming a food or beverage in the hope of learning to enjoy it. Many of the world's delicacies are considered to be acquired tastes. A connoisseur is one who is held to have an expert judgement of taste.

Taste combinations — appetitive plus aversive

Salty, sweet and savory are "appetitive," and bitter and sour are "aversive." Appetitive tastes drive us toward essential nutrients. Aversive tastes alert us to potentially harmful substances. Mixing appetitive with aversive sends conflicting messages to the brain. Confusion is the result, and rejection tends to be the first reaction, as the negative signal can be useful, lifesaving information. Adults nevertheless acquire tastes for some foods that send mixed signals. Coffee with cream or sugar might be an example of this. Olives, strong cheese, sweet and sour Chinese cuisine might be additional examples. Other possible combinations are just about out of the question for most people. Few would enjoy the taste of pickles with cocoa for example.[40]

Factors affecting taste perception

The perception of a mixture of ingredients does not simply equal the sum of the components. Several of the basic tastes compete with each other, so that adding one can reduce the perceived intensity of another. Lemonade, for example, is made by combining lemon juice (sour), sugar (sweet), and water. Without the sugar, the lemon juice—water mixture tastes very sour. The more sugar is added, the less sour the result tastes. Another example is tonic water, made by combining quinine (extremely bitter), sugar (sweet), and water. The bitterness causes many people to not perceive tonic water as sweet, even though it contains as much sugar as an ordinary soft drink.

Many factors affect taste perception, including:

  • Aging
  • Color/vision impairments
  • Hormonal influences
  • Genetic variations; see Phenylthiocarbamide
  • Oral temperature
  • Drugs and chemicals
  • Natural Substances (such as Miracle fruit, which temporarily makes sour foods taste sweeter)
  • CNS Tumors (esp. Temporal lobe lesions) and other neurological causes[41]
  • Plugged noses
  • Zinc deficiency

It is also important to consider that flavor is the overall, total sensation induced during mastication (e.g. taste, touch, pain and smell). Smell (olfactory stimulation) plays a major role in flavor perception.

In some cases, what you see can affect what you taste. For example, if you eat a potato while looking at an apple, you may have the sensation you are eating an apple.

Innervation

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

Disorders of taste

Taste modulators

Compounds so called taste modulators that enhance the sweet and salty flavors of foods could combat obesity and heart disease. Researchers have discovered tiny compounds that make foods taste sweeter, saltier and more savory than they really are, which could reduce the sugar, salt and monosodium glutamate typically added. Several of these taste enhancers are being tested in commercial foods. Whether people will consume fewer calories if their foods become tastier remains to be seen; people might eat lots of sweet foods for reasons that have nothing to do with taste.[42]

See also

References

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