Sense of Taste: Taste

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Taste (or, more formally, gustation; adjectival form: "gustatory") 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.

Basic tastes

For a long period, it was commonly accepted 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. Until the 2000s, the number of "basic" tastes was considered to be four. 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, wild chicory, escarole and lemons. 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. Additionally, the taste receptor PKD2L1 has been found to be involved in tasting sourness.^[29]
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.^{[clarification needed]} 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. Sour candy is especially popular in North America^[30] including Cry Babies, Warheads, Lemon drops, Shock tarts and Sour Skittles and Starburst. Many of these candies contain citric acid.

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.^[31] 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, Thai and Korean cooking, but is not discussed as much in Western cuisine, at least prior to the introduction of the umami concept in the West.
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.^[32]^[33]

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.^[34] 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.^[35]^[36]

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".^[37]
In the Indian tradition, one of the 6 tastes ^[38] is astringency (Kasaaya in Sanskrit, the other five being sweet, sour, salty, bitter and hot/pungent).
In wine terms, "dry" is the opposite of "sweet" and does not refer to astringency. Wines that contain tannins and that cause astringent sensations in the mouth are not necessarily classified as "dry", and "dry" wines are not necessarily astringent.

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.^[39] 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 argue that the sensation is readily detectable and distinguishable to test subjects.

Prickliness or hotness

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 má, 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.^[40]

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.

Sense of Taste

Monday, July 19, 2010

Taste