Scientists describe seven basic tastes: bitter, salty, sour, astringent, sweet, pungent (eg chili), and umami. There are however five basic tastes that the tongue is sensitive to: salt, sweet, bitter, sour, and umami, the taste of MSG. Umami is a Japanese word meaning "savory" or "meaty" and thus applies to the sensation of savoriness -- specifically, to the detection of glutamates, which are especially common in meats, cheese and other protein-heavy foods. The action of umami receptors explains why foods treated with monosodium glutamate often taste fuller or just better.
Umami, which has been quietly enjoyed by Eastern civilizations for years, was recently brought to the forefront of western thought by the discovery by the University of Miami of the actual receptors responsible for the sense of umami, a modified form of mGluR4, in which the end of the molecule is missing. The researchers named it 'taste-mGluR4'. The discovery of the receptor is interesting especially since the receptor for bitter has not yet been identified.
Main five basic tastes
Saltiness is a taste produced by the presence of sodium chloride (and to a lesser degree other salts). The ions of salt, especially sodium (Na+), can pass directly through ion channels in the tongue, leading to an action potential.
Sourness is the taste that detects acids. 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 have dissociated from an acid. Hydrogen ions are capable of permeating the amiloride-sensitive sodium channels, but this is not the only mechanism involved in detecting the quality of sourness. Hydrogen ions also inhibit the potassium channel, which normally functions to hyperpolarize the cell. Thus, 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.
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 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 several different sweetness receptors. The differences between the different sweetness receptors is mainly in the binding site of the G protein coupled receptors. The average human detection threshold for sucrose is 10 millimoles per litre. For lactose it is 30 millimoles per liter, and 5-Nitro-2-propoxyaniline 0.002 millimoles per litre.
Bitterness is the taste which detects bases. Bitterness, like sweetness, is sensed by G protein coupled receptors coupled to the G protein gustducin. Many people find bitter tastes to be unpleasant; many alkaloids taste bitter, and evolutionary biologists have suggested that a distaste for bitter things evolved because it enabled people to avoid accidental poisoning. The bitterest substance known is the synthetic chemical denatonium, marketed as the trademarked Bitrex , discovered in 1958. Denatonium benzoate is a white, odourless solid used as an aversive agent, and can be an additive that prevents accidental ingestion of a toxic substance by humans, particularly children, and by animals. It is commonly used on denaturizing ethanol. The synthetic substance phenylthiocarbamide (PTC) tastes very bitter to most people, but is virtually tasteless to others; furthermore, among the tasters, some are so-called "supertasters" to whom PTC is 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. Quinine, the anti-malarial prophylactic, is also known for its bitter taste and is found in tonic water. Bitter taste receptors are known specifically as T2R's (taste receptors, type 2). 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).
Savouriness is the name for the taste sensation produced by the free glutamates commonly found in fermented and aged foods. In English, it is sometimes described as "meaty" or "savoury". In the Japanese, the term umami is used for this taste sensation, whose characters literally mean "delicious flavour." Umami is now the commonly used term by taste scientists. The same taste is referred to as xianwèi in Chinese cooking. Savoury is considered a fundamental taste in Japanese and Chinese cooking, but is not discussed as much in Western cuisine.
Examples of food containing these free glutamates (and thus strong in the savoury taste) are parmesan and roquefort cheese as well as soy sauce and fish sauce. It is also found in significant amounts in various unfermented foods such as walnuts, grapes, broccoli, tomatoes, and mushrooms, and to a lesser degree in meat. The glutamate taste sensation is most intense in combination with sodium. This is one reason why tomatoes exhibit a stronger taste after adding salt. Sauces with savoury and salty tastes are very popular for cooking, such as tomato sauces and ketchup for Western cuisines and soy sauce and fish sauce for East Asian and Southeast Asian cuisines. Since not every glutamate produces a savoury-like taste sensation, there is continuing investigation into the exact mechanism of how the savoury taste sensation is produced.
The additive monosodium glutamate (MSG), which was developed as a food additive in 1907 by Kikunae Ikeda, produces a strong savoury taste. Savoury is also provided by the nucleotides disodium 5’-inosine monophosphate (IMP) and disodium 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 taste. A subset of savoury taste buds responds specifically to glutamate in the same way that sweet ones respond to sugar. Glutamate binds to a variant of G protein coupled glutamate receptors.
A Sixth Taste?
In November 2005, it was reported that a team of French researchers experimenting on rodents claimed to have evidence for a sixth taste, for fatty substances.Investigator Philippe Besnard and his team believe the CD36 receptors that they found on rodents, were important for evolutionary reasons - to ensure animals ate a high energy diet when foods were scarce. It is speculated that humans may also have the same receptors. Fat has occasionally been raised as a possible basic taste since at least the 1800s. Read more...
Role of Temperature as an 'False Heat' or False Coolness'
False Coolness -- Some substances activate cold trigeminal receptors. One can sense a cool sensation (also known as "cold", "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 signal cold. The reactions behind this sense are therefore analogous to those behind the hot sense. Unlike the actual change in temperature described for sugar substitutes, coolness is only a perceived phenomena.
Spiciness or (false) heat --Substances such as ethanol and capsaicin cause a burning sensation by inducing a trigeminal nerve reaction together with normal taste reception. The heat is caused by the food activating a nerve cell ion channel called TRP-V1, which is also activated by hot temperatures. The sensation, usually referred to as "hot" or "spicy", is a notable feature of Mexican, Indian, Tex-Mex, Szechuan, Korean, and Thai cuisine. The two main plants providing this sensation are chili peppers (those fruits of the Capsicum plant that contain capsaicin) and black pepper.
Some foods, such as tea or unripe fruits, contain tannins that constrict organic tissue. The best example of this is unripe persimmons, whose juice causes a very unpleasant astringent sensation on any part of the mouth it touches. Less exact terms for the astringent sensation include: "rubbery", "hard", "styptic", "dry", "rough", "harsh" (especially for wine) and "tart" (normally referring to sourness).
More information on the Molecules of Taste
Lindemann, Bernd (2000). "A taste for umami". Nature Neuroscience
Huang A. L., et al. Nature, 442. 934 - 938 (2006)
Ishimaru Y., et al. PNAS, 103. 12569 - 12574 (2006)
Science of Chocolate