Sweetness is a
basic taste
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 produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
most commonly
perceived
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
when
eating
Eating (also known as consuming) is the ingestion of food, typically to provide a heterotrophic organism with energy and to allow for growth. Animals and other heterotrophs must eat in order to survive — carnivores eat other animals, herbi ...
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 inge ...
s rich in
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
s. Sweet tastes are generally regarded as
pleasurable
Pleasure refers to experience that feels good, that involves the enjoyment of something. It contrasts with pain or suffering, which are forms of feeling bad. It is closely related to value, desire and action: humans and other conscious animals ...
. 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 refined ...
, 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 element ...
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 grou ...
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 bo ...
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 solids ...
s. Some are sweet at very low concentrations, allowing their use as non-caloric
sugar substitute
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 d ...
s. Such
non-sugar sweeteners include
saccharin
Saccharin (''aka'' saccharine, Sodium sacchari) is an artificial sweetener with effectively no nutritional value. It is about 550 times as sweet as sucrose but has a bitter or metallic aftertaste, especially at high concentrations. Saccharin is u ...
and
aspartame
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 the trade names ...
. Other compounds, such as
miraculin
Miraculin is a taste modifier, a glycoprotein extracted from the fruit of ''Synsepalum dulcificum''. The berry, also known as the miracle fruit, was documented by explorer Chevalier des Marchais, who searched for many different fruits during a ...
, may alter perception of sweetness itself.
The perceived intensity of sugars and high-potency sweeteners, such as
Aspartame
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 the trade names ...
and
Neohesperidin Dihydrochalcone, are heritable, with gene effect accounting for approximately 30% of the variation.
The
chemosensory
A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecep ...
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
Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...
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 Taste detection threshold is the minimum concentration of a flavoured substance detectable by the sense of taste. Sweetness detection thresholds are usually measured relative to that of sucrose, sourness relative to dilute hydrochloric acid, salti ...
, 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
In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or .
Often only the ''useful'' or extract ...
, 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 subst ...
. 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 element ...
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 grou ...
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 bo ...
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 ma ...
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 refined ...
(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 Ketose, ketonic monosaccharide, 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 galacto ...
, 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 am ...
s are mildly sweet: alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side c ...
, glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
, and serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
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
In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive 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 cosmet ...
, the sweet component of licorice
Liquorice (British English) or licorice (American English) ( ; also ) is the common name of ''Glycyrrhiza glabra'', a flowering plant of the bean family Fabaceae, from the root of which a sweet, aromatic flavouring can be extracted.
The liq ...
root, which is about 30 times sweeter than sucrose. Another commercially important example is stevioside
Stevioside is a glycoside derived from the stevia plant, which can be used as a sweetener. Evidence of benefit is lacking for long-term effects on weight loss
Weight loss, in the context of medicine, health, or physical fitness, refers to ...
, 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 southe ...
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
Thaumatin (also known as talin) is a low-calorie sweetener and flavor modifier. The protein is often used primarily for its flavor-modifying properties and not exclusively as a sweetener.
The thaumatins were first found as a mixture of protein ...
, 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, Maurit ...
n katemfe
''Thaumatococcus daniellii'' is a plant species from Africa, known for being the natural source of thaumatin, an intensely sweet protein which is of interest in the development of sweeteners. When the fleshy part of the fruit is eaten, this molec ...
fruit. Hen egg lysozyme
Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
, an antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...
protein found in chicken egg
Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...
s, 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 chemist ...
s are sweet, including beryllium chloride
Beryllium chloride is an inorganic compound with the formula BeCl2. It is a colourless, hygroscopic solid that dissolves well in many polar solvents. Its properties are similar to those of aluminium chloride, due to beryllium's diagonal relations ...
and lead(II) acetate
Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, or Goulard's powder, is a white crystalline chemical compound with a slightly sweet taste. Like many other le ...
. The latter may have contributed to lead poisoning
Lead poisoning, also known as plumbism and saturnism, is a type of metal poisoning caused by lead in the body. The brain is the most sensitive. Symptoms may include abdominal pain, constipation, headaches, irritability, memory problems, inferti ...
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 BC ...
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 m ...
(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 component ...
) 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
Chloroform, or trichloromethane, is an organic compound with chemical formula, formula Carbon, CHydrogen, HChlorine, Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to ...
, nitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5 NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale from benzene as a precursor t ...
, and ethylene glycol
Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
are sweet, but also toxic. Saccharin
Saccharin (''aka'' saccharine, Sodium sacchari) is an artificial sweetener with effectively no nutritional value. It is about 550 times as sweet as sucrose but has a bitter or metallic aftertaste, especially at high concentrations. Saccharin is u ...
, cyclamate
Cyclamate is an artificial sweetener. It is 30–50 times sweeter than sucrose (table sugar), making it the least potent of the commercially used artificial sweeteners. It is often used with other artificial sweeteners, especially saccharin; ...
, aspartame
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 the trade names ...
, acesulfame potassium, sucralose
Sucralose is an artificial sweetener and sugar substitute. The majority of ingested sucralose is not broken down by the body, so it is noncaloric. In the European Union, it is also known under the E number E955. It is produced by chlorination of ...
, alitame
Alitame is an aspartic acid-containing dipeptide sweetener. It was developed by Pfizer in the early 1980s and currently marketed in some countries under the brand name Aclame. Most dipeptides are not sweet, but the unexpected discovery of aspart ...
, 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 origina ...
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
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 ...
,[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 Gymnemic acids are a class of chemical compounds isolated from the leaves of ''Gymnema sylvestre'' (Asclepiadaceae). They are anti-sweet compounds, or sweetness inhibitors. After chewing the leaves, solutions sweetened with sugar taste like water.
...
, 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 so ...
n vine ''Gymnema sylvestre
''Gymnema sylvestre'' is a perennial woody vine native to Asia (including the Arabian Peninsula), Africa and Australia. It has been used in Ayurvedic medicine. Common names include gymnema, Australian cowplant, and Periploca of the woods, and the ...
'' and ziziphin
Ziziphin, a triterpene glycoside which exhibits taste-modifying properties, has been isolated from the leaves of '' Ziziphus jujuba'' (''Rhamnaceae'').
Among ziziphin's known homologues found in this plant, it is the most anti-sweet. However, its ...
, from the leaves of the Chinese jujube
Jujube (), sometimes jujuba, known by the scientific name ''Ziziphus jujuba'' and also called red date, Chinese date, and Chinese jujube, is a species in the genus ''Ziziphus'' in the buckthorn family Rhamnaceae.
Description
It is a smal ...
(''Ziziphus jujuba''). Gymnemic acid has been widely promoted within herbal medicine
Herbal medicine (also herbalism) is the study of pharmacognosy and the use of medicinal plants, which are a basis of traditional medicine. With worldwide research into pharmacology, some herbal medicines have been translated into modern remed ...
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 ap ...
.
On the other hand, two plant proteins, miraculin
Miraculin is a taste modifier, a glycoprotein extracted from the fruit of ''Synsepalum dulcificum''. The berry, also known as the miracle fruit, was documented by explorer Chevalier des Marchais, who searched for many different fruits during a ...
and curculin
Curculin or neoculin is a sweet protein that was discovered and isolated in 1990 from the fruit of ''Curculigo latifolia'' (Hypoxidaceae), a plant from Malaysia. Like miraculin, curculin exhibits taste-modifying activity; however, unlike mirac ...
, cause sour
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 produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
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 bud
Taste buds contain the taste receptor cells, which are also known as gustatory cells. The taste receptors are located around the small structures known as lingual papillae, papillae found on the upper surface of the tongue, soft palate, upper e ...
s on the tongue
The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
, 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
The laboratory mouse or lab mouse is a small mammal of the order Rodentia which is bred and used for scientific research or feeders for certain pets. Laboratory mice are usually of the species ''Mus musculus''. They are the most commonly used ...
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 ba ...
T1R3 prefer sweet foods to different extents. Subsequent research has shown that the T1R3 protein forms a complex with a related protein, called T1R2
Taste receptor type 1 member 2 is a protein that in humans is encoded by the ''TAS1R2'' gene.
The sweet taste receptor is predominantly formed as a dimer of T1R2 and T1R3 by which different organisms sense this taste. In songbirds, however, the T1 ...
, 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 protein family, evolution ...
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
Leptin (from Ancient Greek, Greek λεπτός ''leptos'', "thin" or "light" or "small") is a hormone predominantly made by adipose cells and enterocytes in the small intestine that helps to regulate Energy homeostasis, energy balance by inhib ...
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 of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s sweetness is quite variable. New World monkey
New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ( ...
s do not find aspartame
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 the trade names ...
sweet, while Old World monkey
Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboons ...
s and ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s (including most humans) all do. Felids
Felidae () is the family of mammals in the order Carnivora colloquially referred to as cats, and constitutes a clade. A member of this family is also called a felid (). The term "cat" refers both to felids in general and specifically to the dom ...
like domestic cats
The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of t ...
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 common ...
s, sea lion
Sea lions are pinnipeds characterized by external ear flaps, long foreflippers, the ability to walk on all fours, short and thick hair, and a big chest and belly. Together with the fur seals, they make up the family Otariidae, eared seals. ...
s, spotted hyena
The spotted hyena (''Crocuta crocuta''), also known as the laughing hyena, is a hyena species, currently classed as the sole extant member of the genus ''Crocuta'', native to sub-Saharan Africa. It is listed as being of least concern by the IUC ...
s and fossas.
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
Umami ( from ja, 旨味 ), or savoriness, is one of the five basic tastes. It has been described as savory and is characteristic of broths and cooked meats.
People taste umami through taste receptors that typically respond to glutamates and ...
. Downstream of the taste receptor
A taste receptor or tastant is a type of cellular receptor which 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 locat ...
, 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
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role ...
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
Transient receptor potential cation channel subfamily M member 5 (TRPM5), also known as long transient receptor potential channel 5 is a protein that in humans is encoded by the ''TRPM5'' gene.
Function
TRPM5 is a calcium-activated non-selecti ...
channel and induces cellular depolarization
In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
. The ATP release channel CALHM1
Calcium homeostasis modulator 1 (CALHM1) is a pore-forming subunit of a voltage-gated ion channel and a voltage-gated ATP channel that in humans is encoded by the ''CALHM1'' gene.
Function
Central nervous system
CALHM1 was identified by a ti ...
gets activated by the depolarization and releases ATP neurotransmitter which activates the afferent neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
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 associ ...
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 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.Clayden, J.; ...
in the 19th century introduced many new chemical compounds and the means to determine their molecular structure
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determ ...
s. 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
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
groups and those containing chlorine
Chlorine is a chemical element with the Symbol (chemistry), 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 betwee ...
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 associ ...
in synthetic dye
A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...
s. 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
The name Robert is an ancient Germanic given name, from Proto-Germanic "fame" and "bright" (''Hrōþiberhtaz''). Compare Old Dutch ''Robrecht'' and Old High German ''Hrodebert'' (a compound of '' Hruod'' ( non, Hróðr) "fame, glory, honou ...
and Terry Acree
Terry is a unisex given name, derived from French Thierry and Theodoric. It can also be used as a diminutive nickname for the names Teresa or Theresa (feminine) or Terence or Terrier (masculine).
People
Male
* Terry Albritton (1955–2005), A ...
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 a ...
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
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
(B) separated by about 0.3 nanometre
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
s. 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
Lemont Kier (born September 13, 1930) is an American chemist and researcher in the field of drug design and medicinal chemistry. He is the recipient of the American Association of Pharmaceutical Scientists 2008 Research Achievement Award in Drug De ...
in 1972. While previous researchers had noted that among some groups of compounds, there seemed to be a correlation between hydrophobicity
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
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 force
London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, fluctuating induced dipole bonds or loosely as van der Waals forces) are a type of intermolecular force acting between at ...
s. 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 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
Guanidine is the compound with the formula HNC(NH2)2. It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine predominantly in patients experie ...
sweeteners. The most potent of these, lugduname
Lugduname (from lat. ''Lugdunum'' for ''Lyon'') is one of the most potent sweetening agents known. Lugduname has been estimated to be between 220,000 and 300,000 times as sweet as sucrose (table sugar), with estimates varying between studies. It w ...
, is about 225,000 times sweeter than sucrose.
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Culinary terminology