Taste, is one of the five traditional senses that belongs to the gustatory system.
The five basic tastes are referred to as those of saltiness, sourness, bitterness, sweetness, and umami.
The sensation of taste is produced 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 is a form of chemoreception that takes place in the specialized taste receptors, contained in structures called taste buds in the mouth.
Taste buds are mainly on the upper surface of the tongue.
The function of taste perception is vital to help prevent harmful or rotten foods from being consumed.
The sense of taste is conveyed via three of the twelve cranial nerves.
Ageusia-complete loss of taste.
Hypogeusia-reduced sense of taste.
Dysgeusia-distortion in sense of taste.
Hypergeusia-abnormally heightened sense of taste.
It is a form of chemoreception which occurs in the specialized taste receptors in the mouth.
There are five different types of taste receptors known: salt, sweet, sour, bitter, and umami.
Each receptor has a different manner of detecting the presence of a certain compound and starting an action potential which alerts the brain.
Receptor molecules are found on the top of microvilli of the taste cells.
Serotonin is thought to act as an intermediary hormone communicating with taste cells within a taste bud, and mediating the signals being sent to the brain.
Digestive enzymes in saliva dissolve food into base chemicals that are washed over the papillae and detected as tastes by the taste buds.
Taste, along with olfaction and trigeminal nerve stimulation determines flavors of food and/or other substances.
There are 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.
The tongue is covered with thousands of papillae.
Within each tongue papilla there are hundreds of taste buds.
The filiform papillae that do not contain taste buds.
There are between 2000 and 5000 taste buds that are located on the back and front of the tongue.
Others taste buds are located on the roof, sides and back of the mouth, and in the throat.
Each taste bud has 50 to 100 taste receptor cells.
There are 5 distinct taste sensations: sweetness, sourness, saltiness, bitterness, and umami (savoriness).
Umami which translates from Japanese to delicious.
Taste buds distinguish between different tastes through detecting interactions with different molecules or ions.
The tongue’s taste receptors react to the glycosides and transduce the sweet taste sensation and the lingering bitter aftertaste by direct activation of sweet and bitter receptors.
Sweet, savory, and bitter tastes are triggered by the binding of molecules to G protein-coupled receptors on the cell membranes of taste buds.
Saltiness and sourness are appreciatee when alkali metal or hydrogen ions enter taste buds, respectively.
Basic tastes contribute only partially to the sensation and flavor of food in the mouth.
Other factors contributing to taste are : smell that is detected by the olfactory epithelium of the nose, texture that is detected through mechanoreceptors in muscle and nerves, temperature, detected by thermoreceptors, coolness and hotness through chemesthesis.
Taste senses both harmful and beneficial things, allowing the gustatory system to distinguish between safe and harmful food, and to gauge foods’ nutritional value.
Basic tastes are classified as either aversive or appetitive.
Sweetness helps to identify energy-rich foods.
Bitter foods are generally found unpleasant, while sour, salty, sweet, and umami tasting foods generally provide a pleasurable sensation.
Bitterness serves as a warning sign of poisons.
Humans cannot taste starch.
At about age 50 there is a loss tongue papillae and a general decrease in saliva production, decreasing taste perception.
Dysguesia is the distortion of tastes.
In small quantities sour and salt tastes can be pleasant in small quantities, but in larger quantities become more and more unpleasant to taste.
Sour taste can signal under-ripe fruit, rotten meat, and other spoiled foods, because of bacteria which grow in such media.
Sour taste signals acids, which can cause serious tissue damage.
The bitter taste is unpleasant, and it’s source is many nitrogenous organic molecules which include caffeine, nicotine, and strychnine, which respectively compose the stimulant in coffee, addictive agent in cigarettes, and active compound in many pesticides.
Psychological mechanisms allows humans to overcome their innate aversion to bitter taste, as caffeinated drinks are widely consumed.
Many common medicines have a bitter taste if chewed.
Sweet taste indicates the presence of carbohydrates.
Many non-carbohydrate molecules that trigger a sweet response, including many artificial sweeteners, including saccharin, sucralose, and aspartame.
The savory taste, umami, 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, antibodies, and the organic catalysts known as enzymes.
These are all critical molecules, and therefore they have the pleasurable response to their presence in the mouth.
Sweetness is produced by the presence of sugars and a few other substances and is 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 found on the taste buds.
At least two different variants of the sweetness receptors must be activated for the brain to register sweetness.
Taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1.
Average human detection threshold for sucrose is 10 millimoles per liter.
For lactose the detection threshold is 30 millimoles per liter, with a sweetness index of 0.3.
Natural sweeteners such as saccharides activate the Glucose protein coupling , which releases gustducin.
Gustducin activates the molecule adenylate cyclase, which catalyzes the production of the molecule cAMP, or adenosine 3′, 5′-cyclic monophosphate.
Synthetic sweeteners such as saccharin activate different gLucose protein receptors and induce taste receptor cell depolarization by an alternate pathway.
Sourness taste detects acidity, and the sourness of substances is rated relative to dilute hydrochloric acid, which has a sourness index of 1.
There are a small subset of cells distributed across all taste buds in the tongue, that detect sour taste.
Protons abundant in sour substances can directly enter the sour taste cells through apically located ion channels.
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 of flavor.
Spoiled milk can develop a sour taste.
Children have a greater enjoyment of sour flavors than adults.
Sodium chloride receptors in the mouth are the simplest type of receptor.
The taste of salt is produced primarily by the presence of sodium ions.
A sodium channel in the taste cell wall allows sodium cations to enter the cell.
The saltiness of substances is rated relative to sodium chloride (NaCl), which has an index of 1.
Potassium, as potassium chloride (KCl), is the principal ingredient in salt substitutes and has a saltiness index of 0.6.
The chloride of calcium is saltier and less bitter than potassium chloride, and is commonly used in pickle brine.
People who habitually consume more sodium find that foods tasted less salty than those who consumed less sodium.
The salt taste testing utilized multiple pre-made and validated test strips that the participant held in their mouth for 3 seconds.
About 61% of men and 79% of women could detect a difference between the no-salt strip and the strip containing 0.1% sodium.
Those who could identify that the difference they tasted was a salty flavor was much smaller: between 15% and 24% for men and 24% and 32% for women.
Between 30% and 34% of men could not identify a salt concentration of 1.6% sodium while between 16% and 21% of women could not, either.
Women were better able to detect salty flavors overall, and that for men between 30 and 59 years of age, blood pressures were positively linked with both detection, tasting a difference, perception, knowing the flavor was saltiness, and salt intake.
Individuals who consumed more salty foods were also those who failed to detect difference in flavor in general and salty flavors in particular.
Those same people tended to have increasingly higher blood pressures as their minimum flavor detection and recognition levels increased.
Those who tended to consume more salt were less able to taste or identify salty flavors and had higher blood pressures than those who consumed less salt.
Bitterness is the most sensitive of the tastes.
The ability to detect bitter-tasting compounds at low thresholds is an important protective function, since a large number of natural bitter compounds are known to be toxic.
Many perceive bitterness as unpleasant, sharp, or disagreeable.
Sometimes bitterness is desirable and intentionally added via various bittering agents.
Bitter foods and beverages include: coffee, unsweetened cocoa, olives, citrus peel, dandelion greens, wild chicory, and escarole, ethanol in alcoholic beverages, additional bitter ingredients found in some alcoholic beverages including hops in beer and orange in bitters, and quinine.
See taste and smell?
The threshold for stimulation of bitter taste by quinine averages a concentration of 8 μM (8 micromolar), and taste thresholds of other bitter substances are rated relative to quinine, which is thus given a reference index of 1.
TAS2Rs, taste receptors, type 2, such as TAS2R38 coupled to the G protein gustducin are responsible for the ability to taste bitter substances.
The TAS2R family in humans is thought to comprise about 25 different taste receptors.
TAS2R receptors can recognize a wide variety of bitter-tasting compounds.
Savoriness is an appetitive taste described by its Japanese name, umami or meaty.
Savoriness can be tasted in cheese, soy sauce, many fermented and aged foods, tomatoes, grains, and beans.
Monosodium glutamate (MSG), is used as a food additive.
Monosodium glutamate is a sodium salt that produces a strong savory taste, especially combined with foods rich in nucleotides such as meats, fish, nuts, and mushrooms.
Glutamate binds to a variant of G protein coupled glutamate receptors, causing the G-protein complex to activate a secondary receptor, which ultimately leads to neurotransmitter release.
Sweetness is measured by comparing the threshold values, or level at which the presence of a dilute substance can be detected by a human taster.
Substances are usually measured relative to sucrose, which is usually given an arbitrary index of 1or 100.
Fructose is about 1.4 times sweeter than sucrose.
Glucose, is about three-quarters as sweet; and lactose, a milk sugar, is one-half as sweet.
Sourness of a substance can be rated by comparing it to very dilute hydrochloric acid.
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.
Units of dilute quinine hydrochloride can be used to measure the threshold bitterness concentration.
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 spices.
Pungency is elicited by substances such as ethanol and capsaicin causing a burning sensation by inducing a trigeminal nerve reaction together with normal taste reception.
The sensation of heat that is caused by the food’s activate nerves that express TRPV1 and TRPA1 receptors.
Some such plant-derived compounds that provide this sensation of pungency are capsaicin from chili peppers, piperine from black pepper, gingerol from ginger root and allyl isothiocyanate from horseradish.
These hot or spicy sensations provided by such foods and spices plays an important role in a diverse range of cuisines.
This pungency sensation is called chemesthesis.
Chemesthesis is not a taste because the sensation does not arise from taste buds.
In chemesthesis, a different set of nerve fibers carry the sensation to the brain.
Foods like chili peppers activate nerve fibers directly, manifesting as sensation interpreted as hot results from the stimulation of somatosensory fibers on the tongue.
Some substances activate cold trigeminal receptors, even when not at low temperatures,sensation can be tasted in peppermint, spearmint, menthol, ethanol, and camphor.
This coolness is only a perceived phenomenon.
Unripe fruits, tea, red wine, rhubarb, some and unripe persimmons and bananas contain tannins or calcium oxalate that cause an astringent or puckering sensation of the mouth.
Food and drink, certain medicines or amalgam dental fillings can cause a metallic taste, considered an off flavor.
It may be caused by galvanic reactions in the mouth, as that caused by dental work, as dissimilar metals used may produce a measurable current.
Some artificial sweeteners are perceived to have a metallic taste, which is detected by the TRPV1 receptors.
Blood may have a metallic taste.
A metallic taste in the mouth classified under the symptom dysgeusia refers to distortions of the sense of taste, and can be caused by various kinds of medication, including and occupational hazards, such as working with pesticides.
The taste of chalk has been identified as the calcium component of that substance.
CD36 taste receptor binds to fat molecules, specifically, long-chain fatty acids), and it has been localized to taste bud cells.
CD 36 taste receptors are located in the circumvallate and foliate papillae of the tongue.
There is clear association between CD36 receptor quantity and the ability to taste fat.
Other possible fat taste receptors, G protein-coupled receptors GPR120 and GPR40, and monovalent cation channel TRPM5.
The main form of fat that is ingested is triglycerides, which are composed of three fatty acids bound together.
In this state of bound fatty acids, triglycerides are able to give fatty foods unique textures that are often described as creaminess.
The fatty acids that make up triglycerides are hydrolysed into fatty acids via lipases, and the taste is commonly related to other, more negative, tastes such as bitter and sour due to how unpleasant the taste is for humans.
Low concentrations of fatty acids can create an overall better flavor in a food, however, a high concentration of fatty acids in certain foods is generally considered inedible.
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
Therefore, there are few regularly consumed foods rich in fat taste, due to the negative flavor that is evoked in large quantities.
Flavored foods to which fat taste makes a small contribution include olive oil and fresh butter, along with various kinds of vegetable and nut oils.
Temperature can be an essential element of the taste experience, as part of the culture.
In a given culture traditionally served hot food is often considered distasteful if cold, and vice versa.
Similarly, beverages are served hot, cold or room temperature depending on cultural norms.
Structures that allow for taste to their relevant endpoints in the human brain:
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 of the soft palate, where the greater petrosal, lesser palatine and zygomatic nerves synapse.
The soft palate taste signals are carried through the greater petrosal nerves to the facial nerve.
Spicy foods cause nasal drip because the lesser palatine nerve sends signals to the nasal cavity.
Similarly, the zygomatic nerve sends signals to the lacrimal nerve that activate the lacrimal gland; which is the reason that spicy foods can cause tears.
The lesser palatine and the zygomatic are maxillary nerves from the trigeminal nerve.
Afferents of the vagus nerve carry taste from the epiglottal region of the tongue.
The lingual nerve, branch of the trigeminal nerve, is interconnected with chorda tympani in that it provides all other sensory information from the ⅔ of the tongue.
The reticular formation is signaled to release serotonin during and after a meal to suppress appetite,
Salivary nuclei are signaled to decrease saliva secretion after eating.
The hypothalamus and thalamus connections regulate hunger and the digestive system by hormonal mechanisms.
The Edinger-Westphal nucleus reacts to taste stimuli by dilating and constricting the pupils.
The frontal operculum is speculated to be the memory/association hub for taste.
A supertaster is a person whose sense of taste is significantly more sensitive than average, and is likely due to an increased number of fungiform papillae.
Aftertastes arise after food has been swallowed, and can differ from the food it follows.
Medicines may have a lingering aftertaste, as they can contain certain artificial flavor compounds.
An acquired taste refers to an appreciation for a food or beverage that is unlikely to be enjoyed by a person who has not had substantial exposure to it.
Patients with Addison’s disease, pituitary deficiency or cystic fibrosis sometimes have a hyper-sensitivity to the five primary tastes.
G protein-coupled receptors exist for the basic tastes of bitter, sweet and savory.
The cells that detect sourness have been identified as a subpopulation that express the protein PKD2L1.