Taste

Overview
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 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. Eastern experts traditionally identified a fifth, called umami. More recently, psychophysicists and neuroscientists have suggested other taste categories (umami and 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.)

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 taste
Psychophysicists have long suggested the existence of four taste 'primaries', referred to as the basic tastes: sweetness, bitterness, sourness, and saltiness. Umami is now accepted as the fifth basic taste, exemplified by the non-salty sensations evoked by some free amino acids such as monosodium glutamate.

Other possible categories have been suggested, such as a taste exemplified by certain fatty acids such as linoleic acid. Some researchers still argue against the notion of primaries at all and instead favor a continuum of percepts , 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. This myth is generally attributed to the mis-translation of a German text, and perpetuated in North American schools since the early twentieth century. 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.

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 ones mouth furthering the taste sensations we can receive. Scientists describe five basic tastes: bitter, salty, sour, sweet, and umami (described as savory, meaty, or brothy). 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.

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. Japanese and Indian cultures each add their own sixth taste to the basic five.

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.

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 umami belong to the class of G protein coupled receptors.

In November 2005, a team of French researchers experimenting on rodents claimed to have evidence for a sixth taste, for fatty substances. It is speculated that humans may also have the same receptors. 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). Meanwhile, scientists from the Monell Chemical Senses Center continue to research the five basic tastes, especially Umami.

Five 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, ie. the different sorts of tastes our tongue can identify. Up until the 2000s, this was considered to be a group of four basic tastes. More recently, a fifth taste, Umami, was added by a wide number of authorities in this field.

Bitterness
The bitter taste is perceived by many to be unpleasant, sharp, or disagreeable. Common bitter foods and beverages include coffee, unsweetened chocolate, bitter melon, beer, uncured 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 most bitter substance known is the synthetic chemical denatonium. 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. 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). 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. 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 since PTC-tasting is associated with the ability to taste numerous natural bitter compounds, a large number of which are known to be toxic.

Saltiness
Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of the alkali metals group also taste salty. However the further from sodium the less salty is the sensation. 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. Potassium, as potassium chloride - KCl, is the principle ingredient in salt substitutes.

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 mechanism for detecting sour taste is similar to that which detects salt taste. Hydrogen ion channels detect the concentration of hydronium ions (H3O+ 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 HCO3– by the enzyme carbonic anhydrase, to mediate weak acid transport.

Sweetness
Sweetness 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. The average human detection threshold for sucrose is 10 millimoles per litre. For lactose it is 30 millimoles per litre, and 5-Nitro-2-propoxyaniline 0.002 millimoles per litre.

Umami
Umami (旨味, うまみ) is the name for the taste sensation produced by compounds such as glutamate, and are commonly found in fermented and aged foods. In English, it is also described as "meatiness", "relish", or "savoriness". The Japanese word comes from umai (旨い) for yummy, keen, or nice. Umami is now the commonly used term by taste scientists. The same taste is referred to as xiānwèi (鮮味 or 鲜味) in Chinese cooking. Umami is considered a fundamental taste in Chinese and Japanese 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 commonly found in meats, cheese, fish, and other protein-heavy foods. Examples of food containing glutamate (and thus strong in umami) 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 umami and salty tastes are very popular for cooking, such as worcestershire sauce for Western cuisines and soy sauce and fish sauce for Asian cuisines.

The additive monosodium glutamate (MSG), which was developed as a food additive in 1907 by Kikunae Ikeda, produces a strong umami. Umami 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 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 umami.

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.

Fattiness?
Recent research has revealed a potential taste receptor called the CD36 receptor to be reacting to fat, more specifically, fatty acids. 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. '''

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

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, rhubarb, grapes 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",. The Chinese have a term for this: 澀 (sè), the Koreans have 떫다 (tteolda), the Japanese call it 渋い (shibui), while Thai have ฝาด (fard), the Malay use kelat, Filipinos use pakla, and in Russian there is вяжущий (vyazhuschiy) or тёрпкий (tjorpky).

In the Indian tradition, one of the 6 tastes (sweet, sour, salty, bitter, hot/pungent and dry) is astringency (Kasaaya in Sanskrit). This is more or less in line with the Japanese approach to umami.

Metallicness
Most people know this taste (e.g. Cu2+, FeSO4, or blood in mouth), but it is not only taste but olfactory receptors worked 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. 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 a nerve cell called TRP-V1, which is also activated by hot temperatures. The piquant sensation, usually referred to as being "hot" or "spicy", is a notable feature of Mexican, Hungarian, Indian, Szechuan, Korean, Indonesian, central Vietnamese, and Thai cuisines.

The two main plants providing this sensation are chili peppers (those fruits of the Capsicum plant that contain capsaicin) and black pepper.

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.

In many cases, this particular sensation is not considered a taste, so much as a painful reaction to certain marginally damaging chemicals on the taste receptors and the skin of the tongue. While the taste nerves are activated when consuming foods like chili peppers, the reaction commonly interpreted as "hot" is derived from the tongue's pain nerves firing.

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 TRP-M8 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.

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.

Supertasters
A supertaster is a person whose sense of taste is significantly more sharp 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. 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. 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.

Acquired taste
An acquired taste is an appreciation for a food or beverage that is unlikely to be enjoyed, in part or in full, 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. In most cases, this introductory period is considered worthwhile, as many of the world's delicacies are considered to be acquired tastes. A connoisseur is one who is held to have an expert judgment of taste.

Factors affecting taste perception
Many factors affect taste perception, including: 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.
 * Aging
 * Color/vision impairments
 * Hormonal influences
 * Genetic variations; see Phenylthiocarbamide
 * Oral temperature
 * Drugs and chemicals
 * CNS Tumors (esp. Temporal lobe lesions) and other neurological causes
 * Plugged noses
 * Zinc deficiency

Disorders of taste

 * ageusia (complete loss)