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Sweetness is one of the five basic tastes, and is almost universally regarded as a pleasurable experience. In the English language, things that are pleasant in a more general sense are often called sweet, in phrases such as "sweet rest," "sweet revenge," or "home sweet home." In many other languages, both ancient and modern, the adjective meaning sweet can also be used to describe things that are in any way very good: the author of the 104th Psalm of the Bible wrote, "My meditation of him shall be sweet: I will be glad in the LORD."
Examples of sweet substances
Main article: Sweetener
A great diversity of chemical compounds are sweet. Among common biological substances, all of the simple carbohydrates are sweet to at least some degree. Sucrose (table sugar) is the prototypical example of a sweet substance, although another sugar, fructose, is somewhat sweeter. Some of the amino acids are mildly sweet; alanine, glycine, and serine are the sweetest; some other amino acids are perceived as both sweet and bitter.
A number of plant species produce glycosides that are many times sweeter than sugar. The most well-known example is glycyrrhizin, the sweet component of licorice root, which is about 30 times sweeter than sucrose. Another commercially important example is stevioside, from the South American shrub Stevia rebaudiana. It is roughly 250 times sweeter than sucrose. Another class of potent natural sweeteners are the sweet proteins such as thaumatin, found in the West African katemfe fruit. Hen egg lysozyme, an antibiotic protein found in chicken eggs, is also sweet.
Even some inorganic compounds are sweet, including beryllium chloride and lead acetate. The latter may have contributed to lead poisoning among the ancient Roman aristocracy: the Roman delicacy sapa was prepared by boiling soured wine (containing acetic acid) in lead pots.
Hundreds of synthetic organic compounds are known to be sweet. The number of these that are legally permitted as food additives is, however, much smaller. For example, chloroform and nitrobenzene are both highly sweet, but also toxic. As of 2005, seven artificial sweeteners are in widespread use: saccharin, cyclamate, aspartame, acesulfame potassium, sucralose, alitame, and neotame.
A few substances alter the way sweet taste is perceived. One class of these inhibits the perception of sweet tastes, whether from sugars or from highly potent sweeteners. Commercially, the most important of these is lactisole, a compound produced by Domino Sugar. It is used in some jellies and other fruit preserves to bring out their fruit flavors by suppressing their otherwise strong sweetness.
Two natural products have been documented to have similar sweetness-inhibiting properties: gymnemic acid, extracted from the leaves of the Indian vine Gymnema sylvestre, and ziziphin, from the leaves of the Chinese jujube (Ziziphus jujuba). Gymnemic acid has been widely promoted within herbal medicine as a treatment for sugar cravings and diabetes mellitus.
On the other hand, two plant proteins, miraculin and curculin, cause sour foods to taste sweet. Once the tongue has been exposed to either of these proteins, sourness is perceived as sweetness for up to an hour afterwards. While curculin has some innate sweet taste of its own, miraculin is by itself quite tasteless.
The sweetness receptor
Despite the wide variety of chemical substances known to be sweet, and knowledge that the ability to perceive sweet taste must reside in taste buds on the tongue, the biomolecular mechanism of sweet taste was sufficiently elusive that as recently as the 1990s, there was some doubt whether any single "sweetness receptor" actually exists.
The breakthrough for the present understanding of sweetness occurred in 2001, when experiments with laboratory mice showed that mice possessing different versions of the gene T1R3 prefer sweet foods to different extents. Subsequent research has shown that the T1R3 protein forms a complex with a related protein, called T1R2, to form a G-protein coupled receptor that is the sweetness receptor in mammals.
Historical theories of sweetness
The development of organic chemistry in the 19th century introduced many new chemical compounds and the means to determine their molecular structures. Early organic chemists tasted many of their products, either intentionally (as a means of characterization) or accidentally (due to poor laboratory hygiene). One of the first attempts to draw systematic correlations between molecules' structures and their tastes was made by a German chemist, Georg Cohn, in 1914. He advanced the hypothesis that in order to evoke a certain taste, a molecule must contain some structural motif (called a sapophore) that produced that taste. With regard to sweetness, he noted that molecules containing multiple hydroxyl groups and those containing chlorine atoms are often sweet, and that among a series of structurally similar compounds, those with smaller molecular weights were often sweeter than the larger compounds.
In 1919, Oertly and Myers proposed a more elaborate theory based on a then-current theory of color in synthetic dyes. They hypothesized that in order to be sweet, a compound must contain one each of two classes of structural motif, a glucophore and an auxogluc. Based on those compounds known to be sweet at the time, they proposed a list of six candidate glucophores and nine auxoglucs.
From these beginnings in the early 20th century, the theory of sweetness enjoyed little further academic attention until 1963, when Robert Shallenberger and Terry Acree proposed the AH-B theory of sweetness. Simply put, they proposed that in order to be sweet, a compound must contain a hydrogen bond donor (AH) and a Lewis base (B) separated by about 0.3 nanometres. According to this theory, the AH-B unit of a sweetener binds with a corresponding AH-B unit on the biological sweetness receptor to produce the sensation of sweetness.
A later refinement of this theory was the AH-B-X theory proposed by Lemont Kier in 1972. While previous researchers had noted that among some groups of compounds, there seemed to be a correlation between hydrophobicity and sweetness, this theory formalized these observations by proposing that in order to be sweet, a compound must have a third binding site (labeled X) that could interact with a hydrophobic site on the sweetness receptor via London dispersion forces. Later researchers have statistically analyzed the distances between the presumed AH, B, and X sites in several families of sweet substances to estimate the distances between these interaction sites on the sweetness receptor.
The most elaborate theory of sweetness to date is the multipoint attachment theory proposed by Jean-Marie Tinti and Claude Nofre in 1991. This theory involves a total of eight interaction sites between a sweetener and the sweetness receptor, although not all sweeteners interact with all eight sites. This model has successfully directed efforts aimed at finding highly potent sweeteners, including the most potent family of sweeteners known to date, the guanidine sweeteners. The most potent of these, lugduname, is about 225,000 times sweeter than sucrose.
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