Taste Chemistry

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It appears that some psychological process allows humans to overcome their innate aversion to bitter taste, as caffeinated drinks are widely consumed and enjoyed around the world. Many common medicines have a bitter taste if chewed; the gustatory system apparently interprets these compounds as poisons. In this manner, the unpleasant reaction to the bitter taste is a last-line warning system before the compound is ingested and can do damage. Sweet taste signals the presence of carbohydrates in solution. Since carbohydrates have a very high calorie count saccharides have many bonds, therefore much energy , they are desirable to the human body, which evolved to seek out the highest calorie intake foods.

They are used as direct energy sugars and storage of energy glycogen. However, there are many non-carbohydrate molecules that trigger a sweet response, leading to the development of many artificial sweeteners, including saccharin , sucralose , and aspartame. It is still unclear how these substances activate the sweet receptors and what adaptational significance this has had. The savory taste known in Japanese as "umami" was identified by Japanese chemist Kikunae Ikeda of Tokyo Imperial University , which signals the presence of the amino acid L-glutamate , triggers a pleasurable response and thus encourages the intake of peptides and proteins.

The amino acids in proteins are used in the body to build muscles and organs, transport molecules hemoglobin , antibodies , and the organic catalysts known as enzymes. These are all critical molecules, and as such it is important to have a steady supply of amino acids, hence the pleasurable response to their presence in the mouth. In Asian countries within the sphere of mainly Chinese and Indian cultural influence, pungency piquancy or hotness had traditionally been considered a sixth basic taste.

Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars and a few other substances. Sweetness is often connected to aldehydes and ketones , which contain a carbonyl group. Sweetness is detected by a variety of G protein coupled receptors coupled to the G protein gustducin found on the taste buds. At least two different variants of the "sweetness receptors" must be activated for the brain to register sweetness. Compounds the brain senses as sweet are thus compounds that can bind with varying bond strength to two different sweetness receptors.

Membrane Receptor Taste Stimuli

For lactose it is 30 millimoles per liter, with a sweetness index of 0. The gustducin then activates the molecule adenylate cyclase , which catalyzes the production of the molecule cAMP , or adenosine 3', 5'-cyclic monophosphate. This molecule closes potassium ion channels, leading to depolarization and neurotransmitter release. Synthetic sweeteners such as saccharin activate different GPCRs and induce taste receptor cell depolarization by an alternate pathway. Sourness is the taste that detects acidity. The sourness of substances is rated relative to dilute hydrochloric acid , which has a sourness index of 1.

By comparison, tartaric acid has a sourness index of 0. Sour taste is detected by a small subset of cells that are distributed across all taste buds in the tongue. Sour taste cells can be identified by expression of the protein PKD2L1 , [25] although this gene is not required for sour responses.

There is evidence that the protons that are abundant in sour substances can directly enter the sour taste cells through apically located ion channels. It has also been proposed that weak acids such as acetic acid, which are not fully dissociated at physiological pH values, can penetrate taste cells and thereby elicit an electrical response. According to this mechanism, intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell.

By a combination of direct intake of hydrogen ions which itself depolarizes the cell and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter. The most common food group that contains naturally sour foods is fruit , such as lemon , grape , orange , tamarind , and sometimes melon. Wine also usually has a sour tinge to its flavor, and if not kept correctly, milk can spoil and develop a sour taste. Many of these candies contain citric acid or malic acid.

The simplest receptor found in the mouth is the sodium chloride salt receptor. Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of the alkali metals group also taste salty, but the further from sodium, the less salty the sensation is. A sodium channel in the taste cell wall allows sodium cations to enter the cell. This on its own depolarizes the cell, and opens voltage-dependent calcium channels , flooding the cell with positive calcium ions and leading to neurotransmitter release. This sodium channel is known as an epithelial sodium channel ENaC and is composed of three subunits.

An ENaC can be blocked by the drug amiloride in many mammals, especially rats. The sensitivity of the salt taste to amiloride in humans, however, is much less pronounced, leading to conjecture that there may be additional receptor proteins besides ENaC to be discovered. The size of lithium and potassium ions most closely resemble those of sodium, and thus the saltiness is most similar. In contrast, rubidium and caesium ions are far larger, so their salty taste differs accordingly.

Other monovalent cations , e. But the chloride of calcium is saltier and less bitter than potassium chloride, and is commonly used in pickle brine instead of KCl.

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Bitterness is the most sensitive of the tastes, and many perceive it as unpleasant, sharp, or disagreeable, but it is sometimes desirable and intentionally added via various bittering agents. Common bitter foods and beverages include coffee , unsweetened cocoa , South American mate , bitter gourd , olives , citrus peel , many plants in the family Brassicaceae , dandelion greens, wild chicory , and escarole. The ethanol in alcoholic beverages tastes bitter, [30] as do the additional bitter ingredients found in some alcoholic beverages including hops in beer and orange in bitters.

Quinine is also known for its bitter taste and is found in tonic water. Bitterness is of interest to those who study evolution , as well as various health researchers [23] [31] since a large number of natural bitter compounds are known to be toxic. The ability to detect bitter-tasting, toxic compounds at low thresholds is considered to provide an important protective function.

This has allowed several loss of function mutations that has led to a reduced sensory capacity towards bitterness in humans when compared to other species. This was discovered in during research on lignocaine , a local anesthetic, by MacFarlan Smith of Gorgie , Edinburgh , Scotland. These two substances taste bitter to some people, but are virtually tasteless to others. The variation in sensitivity is determined by two common alleles at the TAS2R38 locus. Gustducin is made of three subunits. When it is activated by the GPCR, its subunits break apart and activate phosphodiesterase , a nearby enzyme, which in turn converts a precursor within the cell into a secondary messenger, which closes potassium ion channels.

This leads to a build-up of potassium ions in the cell, depolarization, and neurotransmitter release. It is also possible for some bitter tastants to interact directly with the G protein, because of a structural similarity to the relevant GPCR. Savory, or savoriness is an appetitive taste [12] and is occasionally described by its Japanese name, umami [41] [42] or meaty. This taste is also present in tomatoes, grains, and beans. Ajinomoto was later identified as the chemical monosodium glutamate MSG , and increasingly used independently as a food additive, [6] [51] it is a sodium salt that produces a strong savory taste, especially combined with foods rich in nucleotides such as meats, fish, nuts, and mushrooms.

Some savory taste buds respond specifically to glutamate in the same way that "sweet" ones respond to sugar. Glutamate binds to a variant of G protein coupled glutamate receptors. This causes the G-protein complex to activate a secondary receptor, which ultimately leads to neurotransmitter release. The intermediate steps are not known. Measuring the degree to which a substance presents one basic taste can be achieved in a subjective way by comparing its taste to a reference substance.

Sweetness is subjectively measured by comparing the threshold values, or level at which the presence of a dilute substance can be detected by a human taster, of different sweet substances. The sourness of a substance can be rated by comparing it to very dilute hydrochloric acid HCl. Relative saltiness can be rated by comparison to a dilute salt solution. Quinine , a bitter medicinal found in tonic water , can be used to subjectively rate the bitterness of a substance.

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In the human body a stimulus refers to a form of energy which elicits a physiological or psychological action or response. Sensory receptors are the structures in the body which change the stimulus from one form of energy to another. Sensory receptors are modified ends of sensory neurons ; modified to deal with specific types of stimulus, thus there are many different types of sensory receptors in the body.

The neuron is the primary component of the nervous system, which transmits messages from sensory receptors all over the body. Taste is a form of chemoreception which occurs in the specialised taste receptors in the mouth. To date, there are five different types of taste receptors known: Each receptor has a different manner of sensory transduction: It is a matter of debate whether each taste cell is tuned to one specific tastant or to several; Smith and Margolskee claim that "gustatory neurons typically respond to more than one kind of stimulus, [a]lthough each neuron responds most strongly to one tastant".

This enables the body to make "keep or spit out" decisions when there is more than one tastant present. Receptor molecules are found on the top of microvilli of the taste cells. Sweetness is produced by the presence of sugars , some proteins, and a few other substances. Sourness is acidity , [66] [67] and, like salt, it is a taste sensed using ion channels.

The protons that are released then block potassium channels, which depolarise the cell and cause calcium influx. In addition, the taste receptor PKD2L1 has been found to be involved in tasting sour. The amino acid glutamic acid is responsible for savoriness, [71] [72] but some nucleotides inosinic acid [47] [73] and guanylic acid [71] can act as complements, enhancing the taste.

Glutamic acid binds to a variant of the G protein-coupled receptor, producing a savory taste. The tongue can also feel other sensations not generally included in the basic tastes. These are largely detected by the somatosensory system. In humans, the sense of taste is conveyed via three of the twelve cranial nerves.


The AH,B glycophore and general taste chemistry - ScienceDirect

The facial nerve VII carries taste sensations from the anterior two thirds of the tongue , the glossopharyngeal nerve IX carries taste sensations from the posterior one third of the tongue while a branch of the vagus nerve X carries some taste sensations from the back of the oral cavity. The trigeminal nerve cranial nerve V provides information concerning the general texture of food as well as the taste-related sensations of peppery or hot from spices. Substances such as ethanol and capsaicin cause a burning sensation by inducing a trigeminal nerve reaction together with normal taste reception.

Some such plant-derived compounds that provide this sensation are capsaicin from chili peppers , piperine from black pepper , gingerol from ginger root and allyl isothiocyanate from horseradish. The piquant "hot" or "spicy" sensation provided by such foods and spices plays an important role in a diverse range of cuisines across the world—especially in equatorial and sub-tropical climates, such as Ethiopian , Peruvian , Hungarian , Indian , Korean , Indonesian , Lao , Malaysian , Mexican , New Mexican , Singaporean , Southwest Chinese including Szechuan cuisine , Vietnamese , and Thai cuisines.

This particular sensation, called chemesthesis , is not a taste in the technical sense, because the sensation does not arise from taste buds, and a different set of nerve fibers carry it to the brain. Many parts of the body with exposed membranes but no taste sensors such as the nasal cavity, under the fingernails, surface of the eye or a wound produce a similar sensation of heat when exposed to hotness agents. Asian countries within the sphere of, mainly, Chinese , Indian , and Japanese cultural influence, often wrote of pungency as a fifth or sixth taste.

Some substances activate cold trigeminal receptors even when not at low temperatures. This "fresh" or "minty" sensation can be tasted in peppermint , spearmint , menthol , ethanol, and camphor. Caused by activation of the same mechanism that signals cold, TRPM8 ion channels on nerve cells , unlike the actual change in temperature described for sugar substitutes, this coolness is only a perceived phenomenon. Some foods, such as unripe fruits, contain tannins or calcium oxalate that cause an astringent or puckering sensation of the mucous membrane of the mouth.

Examples include tea , red wine , rhubarb , some fruits of the genus Syzygium , and unripe persimmons and bananas. Less exact terms for the astringent sensation are "dry", "rough", "harsh" especially for wine , "tart" normally referring to sourness , "rubbery", "hard" or "styptic".

Taste Receptors

When referring to wine, dry is the opposite of sweet, and does not refer to astringency. Wines that contain tannins and so cause an astringent sensation are not necessarily classified as "dry", and "dry" wines are not necessarily astringent. In the Indian Ayurvedic tradition , one of the six tastes is astringency kasaaya. A metallic taste may be caused by food and drink, certain medicines or amalgam dental fillings. It is generally considered an off flavor when present in food and drink.

The Chemistry of Taste

A metallic taste may be caused by galvanic reactions in the mouth. In the case where it is caused by dental work, the dissimilar metals used may produce a measurable current. The distinctive taste of chalk has been identified as the calcium component of that substance. The CaSR receptor is commonly found in the gastrointestinal tract , kidneys , and brain.

Whether closely related genes in mice and humans means the phenomenon exists in humans as well is unknown. Recent research reveals a potential taste receptor called the CD36 receptor. Other possible fat taste receptors have been identified. G protein-coupled receptors GPR and GPR40 have been linked to fat taste, because their absence resulted in reduced preference to two types of fatty acid linoleic acid and oleic acid , as well as decreased neuronal response to oral fatty acids.

Monovalent cation channel TRPM5 has been implicated in fat taste as well, [96] but it is thought to be involved primarily in downstream processing of the taste rather than primary reception, as it is with other tastes such as bitter, sweet, and savory. Proposed alternate names to fat taste include oleogustus [97] and pinguis, [21] although these terms are not widely accepted. The main form of fat that is commonly ingested is triglycerides , which are composed of three fatty acids bound together.

In this state, triglycerides are able to give fatty foods unique textures that are often described as creaminess. But this texture is not an actual taste. It is only during ingestion that the fatty acids that make up triglycerides are hydrolysed into fatty acids via lipases. The taste is commonly related to other, more negative, tastes such as bitter and sour due to how unpleasant the taste is for humans. Richard Mattes, a co-author of the study, explained that low concentrations of these fatty acids can create an overall better flavor in a food, much like how small uses of bitterness can make certain foods more rounded.

However, a high concentration of fatty acids in certain foods is generally considered inedible. Volunteers were able to separate the taste of fatty acids into their own category, with some overlap with savory samples, which the researchers hypothesized was due to poor familiarity with both.

The researchers note that the usual "creaminess and viscosity we associate with fatty foods is largely due to triglycerides", unrelated to the taste; while the actual taste of fatty acids is not pleasant. Mattes described the taste as "more of a warning system" that a certain food should not be eaten. There are few regularly consumed foods rich in fat taste, due to the negative flavor that is evoked in large quantities.

Foods whose flavor to which fat taste makes a small contribution include olive oil and fresh butter, along with various kinds of vegetable and nut oils. Some Japanese researchers refer to the kokumi of foods. This sensation has also been described as mouthfulness, []: Temperature can be an essential element of the taste experience.

Food and drink that—in a given culture—is traditionally served hot is often considered distasteful if cold, and vice versa. For example, alcoholic beverages, with a few exceptions, are usually thought best when served at room temperature or chilled to varying degrees, but soups—again, with exceptions—are usually only eaten hot. A cultural example are soft drinks. In North America it is almost always preferred cold, regardless of season. A study suggested that humans can taste starch specifically, a glucose oligomer independently of other tastes such as sweetness.

However, no specific chemical receptor has yet been found for this taste. The glossopharyngeal nerve innervates a third of the tongue including the circumvallate papillae. The facial nerve innervates the other two thirds of the tongue and the cheek via the chorda tympani. The pterygopalatine ganglia are ganglia one on each side of the soft palate. The greater petrosal , lesser palatine and zygomatic nerves all synapse here.

The greater petrosal, carries soft palate taste signals to the facial nerve. The lesser palatine sends signals to the nasal cavity ; which is why spicy foods cause nasal drip. The zygomatic sends signals to the lacrimal nerve that activate the lacrimal gland ; which is the reason that spicy foods can cause tears. Both the lesser palatine and the zygomatic are maxillary nerves from the trigeminal nerve. The special visceral afferents of the vagus nerve carry taste from the epiglottal region of the tongue.

NST receives input from the amygdala regulates oculomotor nuclei output , bed nuclei of stria terminalis, hypothalamus, and prefrontal cortex. NST is the topographical map that processes gustatory and sensory temp, texture, etc. Saltiness is sensed by taste receptor cells that respond primarily to sodium chloride. The proteins in the cell membranes involved in transforming the presence of salt into nervous signals are epithelial sodium channels that allow the sodium ions to enter the cells, initiate the release of chemical neurotransmitters , and stimulate adjacent gustatory afferent axons nerve cells that carry taste information to the brain.

Sourness hydrochloric acid, citric acid, or acetic acid is likewise sensed by taste receptor cells in ways that directly affect ion channels. Protons either enter via the epithelial sodium channels or block epithelial potassium channels to initiate the cellular response. The bitterness of quinine and calcium is also sensed by blocking potassium channels in taste receptor cell membranes. In contrast, other tastes including sweetness as in sucrose, fructose, and artificial sweetener are sensed by actually binding to specific membrane receptor proteins in taste receptor cells.

The chemicals sensed as sweet bind to selective sites on a membrane receptor in a "lock-and-key" fashion implying that only chemicals of a particular shape can fit in the binding site and initiate the response. Once the sweet chemical is bound, the membrane receptor initiates a series of chemical reactions inside the cell, leading eventually to a change in the flow of ions across the membrane and the release of neurotransmitter.

Likewise, the bitterness of some chemicals is sensed by binding to other membrane receptors and then initiating a response. The taste of some amino acids is initiated by binding to a specific site on chemical-gated ion channels channels that open when a chemical is bound where the amino acid the chemical acts as the key.

The umami taste of monosodium glutamate is sensed by binding to another type of membrane receptor similar to the synaptic glutamate receptors of the brain that allows ions to cross cell membranes. Taste and smell are well-known chemical senses; however, the specific genes and proteins involved in some tastes have not yet been fully identified. Also available from http: