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Sweetness is a basic taste most commonly perceived when eating
food Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is ...
s rich in sugars. Sweet tastes are generally regarded as pleasurable. In addition to sugars like
sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refi ...
, many other
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...
s are sweet, including
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...
s,
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...
s, and
sugar alcohol Sugar alcohols (also called polyhydric alcohols, polyalcohols, alditols or glycitols) are organic compounds, typically derived from sugars, containing one hydroxyl group (–OH) attached to each carbon atom. They are white, water-soluble solid ...
s. Some are sweet at very low concentrations, allowing their use as non-caloric sugar substitutes. Such non-sugar sweeteners include saccharin and aspartame. 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. 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 Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
bacteria such as ''E. coli''. Newborn human infants also demonstrate preferences for high sugar concentrations and prefer solutions that are sweeter than
lactose Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from ' (gen. '), the Latin word for milk, plus the suffix ...
, the sugar found in breast milk. 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. In the natural settings that human primate ancestors evolved in, sweetness intensity should indicate energy density, while bitterness tends to indicate
toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subs ...
. 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. The 'sweet tooth' thus has an ancient heritage, and while food processing has changed consumption patterns, human physiology remains largely unchanged.


Examples of sweet substances

A great diversity of
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...
s, such as
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...
s and
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...
s, are sweet. Among common biological substances, all of the simple
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ...
s are sweet to at least some degree.
Sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refi ...
(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. For example, another sugar,
fructose Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...
, is somewhat sweeter, being rated at 1.7 times the sweetness of sucrose. Some of the
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s are mildly sweet: alanine, glycine, and serine are the sweetest. Some other amino acids are perceived as both sweet and bitter. The sweetness of 20% solution of glycine in water compares to a solution of 10% glucose or 5% fructose. A number of plant species produce glycosides that are sweet at concentrations much lower than common sugars. The most well-known example is
glycyrrhizin Glycyrrhizin (or glycyrrhizic acid or glycyrrhizinic acid) is the chief sweet-tasting constituent of ''Glycyrrhiza glabra'' (liquorice) root. Structurally, it is a saponin used as an emulsifier and gel-forming agent in foodstuffs and cosmetics. ...
, the sweet component of licorice root, which is about 30 times sweeter than sucrose. Another commercially important example is stevioside, from the
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...
n 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 Africa West Africa or Western Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mau ...
n katemfe fruit. Hen egg lysozyme, an
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
protein found in chicken eggs, is also sweet. 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. 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 compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
s are sweet, including beryllium chloride and lead(II) acetate. The latter may have contributed to lead poisoning among the
ancient Roman In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 B ...
aristocracy: the Roman delicacy ''sapa'' was prepared by boiling soured
wine Wine is an alcoholic drink typically made from fermented grapes. Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different varieties of grapes and strains of yeasts are ...
(containing
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
) in lead pots. Hundreds of synthetic organic compounds are known to be sweet, but only a few of these are legally permitted as food additives. For example, chloroform, nitrobenzene, and ethylene glycol are sweet, but also toxic. Saccharin, cyclamate, aspartame, acesulfame potassium, sucralose, alitame, and
neotame Neotame, also known by the trade name Newtame, is a non-caloric artificial sweetener and aspartame analog by NutraSweet. By mass, it is 8000 times sweeter than sucrose. It has no notable off-flavors when compared to sucrose. It enhances orig ...
are commonly used.


Sweetness modifiers

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,Kinghorn, A.D. and Compadre, C.M. Alternative Sweeteners: Third Edition, Revised and Expanded, Marcel Dekker ed., New York, 2001. a compound produced by
Domino Sugar Domino Foods, Inc. (also known as DFI and formerly known as W. & F.C. Havemeyer Company, Havemeyer, Townsend & Co. Refinery, and Domino Sugar) is a privately held sugar marketing and sales company based in Yonkers, New York, United States, that ...
. 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
India India, officially the Republic of India ( Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the ...
n 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 Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level (hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...
. 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 tongue is a muscular organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive process, and is the primary organ of taste. The tongue's upper surface (dorsum) is covered by taste bu ...
, 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 In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
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 G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
that is the sweetness receptor in mammals. 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. 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. Sweetness perception may differ between species significantly. For example, even amongst the
primate Primates are a diverse order (biology), order of mammals. They are divided into the Strepsirrhini, strepsirrhines, which include the lemurs, galagos, and lorisids, and the Haplorhini, haplorhines, which include the Tarsiiformes, tarsiers and ...
s sweetness is quite variable. New World monkeys do not find aspartame sweet, while Old World monkeys and apes (including most humans) all do. Felids like domestic cats cannot perceive sweetness at all. The ability to taste sweetness often atrophies genetically in species of carnivores who do not eat sweet foods like fruits, including
bottlenose dolphin Bottlenose dolphins are aquatic mammals in the genus ''Tursiops.'' They are common, cosmopolitan members of the family Delphinidae, the family of oceanic dolphins. Molecular studies show the genus definitively contains two species: the co ...
s, sea lions, spotted hyenas and
fossa Fossa may refer to: Animals * Fossa (animal), the common name of a carnivoran mammal of genus ''Cryptoprocta'' endemic to Madagascar * ''Fossa'', the Latin genus name of the Malagasy civet, a related but smaller mammal endemic to Madagascar Pla ...
s.


Sweet receptor pathway

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. 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. The
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals, men's professional tennis governing body * American Technical Publishers, employee-owned publishing company * ', a Danish pension * Armenia Tree Project, non ...
release channel CALHM1 gets activated by the depolarization and releases ATP neurotransmitter which activates the afferent
neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...
s innervating the taste bud.


Cognition

The
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
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. The effect of color is believed to be due to cognitive expectations. Some odors smell sweet and memory confuses whether sweetness was tasted or smelled.


Historical theories

The development of
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
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 Hygiene is a series of practices performed to preserve health. According to the World Health Organization (WHO), "Hygiene refers to conditions and practices that help to maintain health and prevent the spread of diseases." Personal hygiene refer ...
). 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 Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...
atoms are often sweet, and that among a series of structurally similar compounds, those with smaller
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
s 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 Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
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 In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...
donor A donor in general is a person, organization or government which donates something voluntarily. The term is usually used to represent a form of pure altruism, but is sometimes used when the payment for a service is recognized by all parties as rep ...
(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 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.


MPA theory

The most elaborate theory of sweetness to date is the multipoint attachment theory (MPA) proposed by
Jean-Marie Tinti Jean-Marie is both a given name and a surname. Notable people with the name include: * Jean-Marie Abgrall (born 1950), a French psychiatrist, criminologist, specialist in forensic medicine, cult expert, and graduate in criminal law * Jean-Marie C ...
and
Claude Nofre Claude may refer to: __NOTOC__ People and fictional characters * Claude (given name), a list of people and fictional characters * Claude (surname), a list of people * Claude Lorrain (c. 1600–1682), French landscape painter, draughtsman and etcher ...
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.


References


Cited


General

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Further reading

* * {{Authority control Gustation Culinary terminology