Human taste sensory organs, called taste buds or gustatory calyculi, and concentrated on the upper surface of the tongue, appear to be receptive to relatively few chemical species as tastes. This contrasts markedly with the sense of olfaction, where very large numbers of different species can be differentiated.

History

Until recently, most Western sources listed only the first four of these flavors; in recent years, umami/savory has become widely, though not universally, accepted (there is even a restaurant by that name which proposes to serve only foods containing that taste).

In general, the sense of taste is often confused by smells that occur at the same time, and much of the everyday sensation of taste is at least partially derived from smell stimuli. Loss of the sense of smell (anosmia), for example when one has a cold, severely reduces one's sense of taste.

Historically, how we sense taste has been misunderstood. 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. There is no scientific foundation for these "maps", which were based on a misinterpretation of old research.

According to an ancient Chinese understanding of taste that is part of the Five Elements philosophy, there are five basic tastes, which lists spicyness instead of umami in the above list.

Saltiness

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⁺) is detected by ions channels on the tongue, leading to action potential.

Sourness

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 protons (H⁺ ions) that have dissociated from an acid.

Sweetness

Sweetness produced by the presence of sugars, some proteins and a few other substances. 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.

Bitterness

Bitterness, like sweetness, is sensed by G protein coupled receptors coupled to the G protein gustducin. Many people find bitter tastes unpleasant; many alkaloids taste bitter, and evolutionary biologists have suggested that a distaste for bitter things evolved to enable people to avoid poisoning.

The bitterest substance known is the synthetic chemical Bitrex [1] (http://www.bitrex.com/home.htm) (a trademark of UK company Macfarlan Smith) or denatonium benzoate, discovered in 1958. It is a white, odourless solid used as an aversive agent, i.e. an additive that prevents accidental ingestion of a toxic substance by humans, particularly children, and by animals.

The substance phenylthiocarbamide tastes very bitter to most people, but is virtually tasteless to others. This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics.

Umami

Savoriness or umami is the name for the taste sensation produced by the free glutamates commonly found in fermented and aged foods, and in the additive MSG, which was developed as a food additive in 1907 by Kikunae Ikeda, produces a strong umami taste.

Umami is considered basic in Japanese and Chinese cooking, but is not discussed as much in Western cuisine, where it is sometimes referred to as "savory" or "moreish."

The name comes from umami (旨味 or うまみ), the Japanese name for the taste sensation. The characters literally mean "delicious flavour."

In English, the name of the taste is sometimes spelled umame, but umami (which conforms to the romanization standards of Japanese) is much more common, as in Society for Research on Umami Taste (http://www.srut.org/index_e.html).

The same taste is referred to as Xian Wei (鮮味) in Chinese cooking.

It is believed that 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.

External links

Monell chemical senses center: http://www.monell.org/