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Sour or bitter to the taste
Common clues:
Caustic; Sour; Bitter; Biting; Harsh; Sharp-tongued; Tart; Having a bite; Like a sourball; Astringent; Acid; Like lemon juice; Like vinegar; Lip-puckering
Crossword puzzle frequency: 2 times a year
Maggie vs. Grapefruit

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 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.

The bitter taste is perceived by many to be unpleasant, sharp, or disagreeable. Evolutionary biologists have suggested that a distaste for bitter substances may have evolved as a defense mechanism against accidental poisoning. 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, discovered in 1958. It is used as an aversive agent that is added to toxic substances to prevent accidental ingestion.

Research has shown that TAS2Rs (taste receptors, type 2) such as TAS2R16 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.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Taste"