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Sweetness is a basic taste most commonly perceived when eating foods rich in sugars. Sweet tastes are generally regarded as pleasurable. In addition to sugars like sucrose, many other chemical compounds are sweet, including aldehydes, ketones, and sugar alcohols. Some are sweet at very low concentrations, allowing their use as non-caloric sugar substitutes. Such non-sugar sweeteners include saccharin, aspartame, sucralose and stevia. Other compounds, such as miraculin, may alter perception of sweetness itself.
The perceived intensity of sugars and high-potency sweeteners, such as aspartame and neohesperidin dihydrochalcone, are heritable, with gene effect accounting for approximately 30% of the variation. [1]
The chemosensory basis for detecting sweetness, which varies between both individuals and species, has only begun to be understood since the late 20th century. One theoretical model of sweetness is the multipoint attachment theory, which involves multiple binding sites between a sweetness receptor and a sweet substance.
Studies indicate that responsiveness to sugars and sweetness has very ancient evolutionary beginnings, being manifest as chemotaxis even in motile bacteria such as E. coli. [2] Newborn human infants also demonstrate preferences for high sugar concentrations and prefer solutions that are sweeter than lactose, the sugar found in breast milk. [3] [4] Sweetness appears to have the highest taste recognition threshold, being detectable at around 1 part in 200 of sucrose in solution. By comparison, bitterness appears to have the lowest detection threshold, at about 1 part in 2 million for quinine in solution. [5] In the natural settings that human primate ancestors evolved in, sweetness intensity should indicate energy density, while bitterness tends to indicate toxicity. [6] [7] [8] The high sweetness detection threshold and low bitterness detection threshold would have predisposed our primate ancestors to seek out sweet-tasting (and energy-dense) foods and avoid bitter-tasting foods. Even amongst leaf-eating primates, there is a tendency to prefer immature leaves, which tend to be higher in protein and lower in fibre and poisons than mature leaves. [9] The "sweet tooth" thus has an ancient heritage, and while food processing has changed consumption patterns, [10] [11] human physiology remains largely unchanged. [12]
A great diversity of chemical compounds, such as aldehydes and ketones, 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. Sucrose in solution has a sweetness perception rating of 1, and other substances are rated relative to this. [13] For example, another sugar, fructose, is somewhat sweeter, being rated at 1.7 times the sweetness of sucrose. [13] 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.
The sweetness of 5% solution of glycine in water compares to a solution of 5.6% glucose or 2.6% fructose. [14]
A number of plant species produce glycosides that are sweet at concentrations much lower than common sugars. 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.
Name | Type of compound | Sweetness |
---|---|---|
Lactose | Disaccharide | 0.16 |
Maltose | Disaccharide | 0.33 – 0.45 |
Trehalose (α,α-trehalose) | Disaccharide | max. 0,45 [21] |
Isomaltulose | Disaccharide | 0.40 - 0.50 [22] |
Sorbitol | Polyalcohol | 0.6 |
Galactose | Monosaccharide | 0.65 |
Glucose | Monosaccharide | 0.74 – 0.8 |
Glycine | Amino acid | 0.6 – 0.86 |
Sucrose | Disaccharide | 1.00 (reference) |
Xylitol | sugar alcohol | 1,02 [23] |
Fructose | Monosaccharide | 1.17 – 1.75 |
Sodium cyclamate | Sulfonate | 26 |
Steviol glycoside | Glycoside | 40 – 300 |
Aspartame | Dipeptide methyl ester | 180 – 250 |
Acesulfame potassium | Oxathiazinone dioxide | 200 |
Sodium saccharin | Sulfonyl | 300 – 675 |
Sucralose | Modified disaccharide | 600 |
Thaumatin | Protein | 2000 |
Neotame | Aspartame analog | 8000 |
Sucrooctate | Guanidine | 162,000 (estimated) |
Bernardame | Guanidine | 188,000 (estimated) |
Sucrononic acid | Guanidine | 200,000 (estimated) |
Carrelame | Guanidine | 200,000 (estimated) |
Lugduname | Guanidine | 230,000 (estimated) |
Some variation in values is not uncommon between various studies. Such variations may arise from a range of methodological variables, from sampling to analysis and interpretation. Indeed, the taste index of 1, assigned to reference substances such as sucrose (for sweetness), hydrochloric acid (for sourness), quinine (for bitterness), and sodium chloride (for saltiness), is itself arbitrary for practical purposes. [18] Some values, such as those for maltose and glucose, vary little. Others, such as aspartame and sodium saccharin, have much larger variation.
Even some inorganic compounds are sweet, including beryllium chloride and lead(II) 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. [24]
Hundreds of synthetic organic compounds are known to be sweet, but only a few of these are legally permitted[ where? ] as food additives. For example, chloroform, nitrobenzene, and ethylene glycol are sweet, but also toxic. Saccharin, cyclamate, aspartame, acesulfame potassium, sucralose, alitame, and neotame are commonly used.[ citation needed ]
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, [25] 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). [26] Gymnemic acid has been widely promoted within herbal medicine as a treatment for sugar cravings and diabetes.
On the other hand, two plant proteins, miraculin [27] and curculin, [28] 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.
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. [29]
Human studies have shown that sweet taste receptors are not only found in the tongue, but also in the lining of the gastrointestinal tract as well as the nasal epithelium, pancreatic islet cells, sperm and testes. [30] It is proposed that the presence of sweet taste receptors in the GI tract controls the feeling of hunger and satiety.
Another research has shown that the threshold of sweet taste perception is in direct correlation with the time of day. This is believed to be the consequence of oscillating leptin levels in blood that may impact the overall sweetness of food. Scientists hypothesize that this is an evolutionary relict of diurnal animals like humans. [31]
Sweetness perception may differ between species significantly. For example, even amongst the primates sweetness is quite variable. New World monkeys do not find aspartame sweet, while Old World monkeys and apes (including most humans) all do. [32] Felids like domestic cats cannot perceive sweetness at all. [33] The ability to taste sweetness often atrophies genetically in species of carnivores who do not eat sweet foods like fruits, including bottlenose dolphins, sea lions, spotted hyenas and fossas.
To depolarize the cell, and ultimately generate a response, the body uses different cells in the taste bud that each express a receptor for the perception of sweet, sour, salty, bitter or umami. Downstream of the taste receptor, the taste cells for sweet, bitter and umami share the same intracellular signalling pathway. [34] Incoming sweet molecules bind to their receptors, which causes a conformational change in the molecule. This change activates the G-protein, gustducin, which in turn activates phospholipase C to generate inositol trisphosphate (IP3), this subsequently opens the IP3-receptor and induces calcium release from the endoplasmic reticulum. This increase in intracellular calcium activates the TRPM5 channel and induces cellular depolarization. [35] [36] The ATP release channel CALHM1 gets activated by the depolarization and releases ATP neurotransmitter which activates the afferent neurons innervating the taste bud. [37] [38]
The color of food can affect sweetness perception. Adding more red color to a drink increases its perceived sweetness. In a study darker colored solutions were rated 2–10% higher than lighter ones despite having 1% less sucrose concentration. [39] The effect of color is believed to be due to cognitive expectations. [40] Some odors smell sweet and memory confuses whether sweetness was tasted or smelled. [41]
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 hypothesized that to evoke a certain taste, a molecule must contain some structural motif (called a sapophore) that produces 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 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 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.
B-X theory was 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 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 (MPA) 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. [42] 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.
Aspartame is an artificial non-saccharide sweetener 200 times sweeter than sucrose and is commonly used as a sugar substitute in foods and beverages. It is a methyl ester of the aspartic acid/phenylalanine dipeptide with brand names NutraSweet, Equal, and Canderel. Aspartame was approved by the US Food and Drug Administration (FDA) in 1974, and then again in 1981, after approval was revoked in 1980.
A sugar substitute is a food additive that provides a sweetness like that of sugar while containing significantly less food energy than sugar-based sweeteners, making it a zero-calorie or low-calorie sweetener. Artificial sweeteners may be derived through manufacturing of plant extracts or processed by chemical synthesis. Sugar substitute products are commercially available in various forms, such as small pills, powders, and packets.
Umami, or savoriness, is one of the five basic tastes. It is characteristic of broths and cooked meats.
Neohesperidin dihydrochalcone, sometimes abbreviated to neohesperidin DC or simply NHDC, is an artificial sweetener derived from citrus.
Saccharin, also called saccharine, benzosulfimide, or E954, or used in saccharin sodium or saccharin calcium forms, is a non-nutritive artificial sweetener. Saccharin is a sultam that is about 500 times sweeter than sucrose, but has a bitter or metallic aftertaste, especially at high concentrations. It is used to sweeten products, such as drinks, candies, baked goods, tobacco products, excipients, and for masking the bitter taste of some medicines. It appears as white crystals and is odorless.
Acesulfame potassium, also known as acesulfame K or Ace K, is a synthetic calorie-free sugar substitute often marketed under the trade names Sunett and Sweet One. In the European Union, it is known under the E number E950. It was discovered accidentally in 1967 by German chemist Karl Clauss at Hoechst AG. In chemical structure, acesulfame potassium is the potassium salt of 6-methyl-1,2,3-oxathiazine-4(3H)-one 2,2-dioxide. It is a white crystalline powder with molecular formula C
4H
4KNO
4S and a molecular weight of 201.24 g/mol.
Diet or light beverages are generally sugar-free, artificially sweetened beverages with few or no calories. They are marketed for diabetics and other people who want to reduce their sugar and/or caloric intake.
Neotame, also known by the brand name Newtame, is a non-caloric artificial sweetener and aspartame analog by NutraSweet. By mass, it is 7,000 to 13,000 times sweeter than sucrose. It has no notable off-flavors when compared to sucrose. It enhances original food flavors. It can be used alone, but is often mixed with other sweeteners to increase their individual sweetness and decrease their off-flavors. It is chemically somewhat more stable than aspartame. Its use can be cost effective in comparison to other sweeteners as smaller amounts of neotame are needed.
Monellin, a sweet protein, was discovered in 1969 in the fruit of the West African shrub known as serendipity berry ; it was first reported as a carbohydrate. The protein was named in 1972 after the Monell Chemical Senses Center in Philadelphia, U.S.A., where it was isolated and characterized.
Lactisole is the sodium salt and commonly supplied form of 2-(4-methoxyphenoxy)propionic acid, a natural carboxylic acid found in roasted coffee beans. Like gymnemic acid, it has the property of masking sweet flavors and is used for this purpose in the food industry.
Aftertaste is the taste intensity of a food or beverage that is perceived immediately after that food or beverage is removed from the mouth. The aftertastes of different foods and beverages can vary by intensity and over time, but the unifying feature of aftertaste is that it is perceived after a food or beverage is either swallowed or spat out. The neurobiological mechanisms of taste signal transduction from the taste receptors in the mouth to the brain have not yet been fully understood. However, the primary taste processing area located in the insula has been observed to be involved in aftertaste perception.
A taste receptor or tastant is a type of cellular receptor that facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity and other locations. Molecules which give a sensation of taste are considered "sapid".
Taste receptor 2 member 38 is a protein that in humans is encoded by the TAS2R38 gene. TAS2R38 is a bitter taste receptor; varying genotypes of TAS2R38 influence the ability to taste both 6-n-propylthiouracil (PROP) and phenylthiocarbamide (PTC). Though it has often been proposed that varying taste receptor genotypes could influence tasting ability, TAS2R38 is one of the few taste receptors shown to have this function.
Gustducin is a G protein associated with taste and the gustatory system, found in some taste receptor cells. Research on the discovery and isolation of gustducin is recent. It is known to play a large role in the transduction of bitter, sweet and umami stimuli. Its pathways are many and diverse.
Pentadin, a sweet-tasting protein, was discovered and isolated in 1989, in the fruit of oubli, a climbing shrub growing in some tropical countries of Africa. Sweet tasting proteins are often used in the treatment of diabetes, obesity, and other metabolic disorders that one can experience. These proteins are isolated from the pulp of various fruits, typically found in rain forests and are also used as low calorie sweeteners that can enhance and modify existing foods.
Taste receptor type 2 member 4 is a protein that in humans is encoded by the TAS2R4 gene.
Taste receptor type 2 member 9 is a protein that in humans is encoded by the TAS2R9 gene.
T1R2 - Taste receptor type 1 member 2 is a protein that in humans is encoded by the TAS1R2 gene.
Taste receptor type 1 member 3 is a protein that in humans is encoded by the TAS1R3 gene. The TAS1R3 gene encodes the human homolog of mouse Sac taste receptor, a major determinant of differences between sweet-sensitive and -insensitive mouse strains in their responsiveness to sucrose, saccharin, and other sweeteners.
The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception stimulated when a substance in the mouth reacts chemically with taste receptor cells located on taste buds in the oral cavity, mostly on the tongue. Taste, along with the sense of smell and trigeminal nerve stimulation, determines flavors of food and other substances. Humans have taste receptors on taste buds and other areas, including the upper surface of the tongue and the epiglottis. The gustatory cortex is responsible for the perception of taste.