Everything about Taste totally explained
Taste (or, more formally,
gustation) is a form of direct
chemoreception and is one of the traditional five
senses. It refers to the ability to detect the flavor of substances such as
food and
poisons. In humans and many other vertebrate animals the sense of taste partners with the less direct
sense of smell, in the brain's perception of
flavor. In the West, experts traditionally identified four taste sensations: sweet, salty, sour, and bitter. Eastern experts traditionally identified five tastes, including umami. They also identified five elements, five metals, five directions, etc. More recently,
psychophysicists and
neuroscientists have suggested other taste categories (
umami and
fatty acid taste most prominently, as well as the sensation of metallic and water tastes, although the latter is commonly disregarded due to the phenomenon of taste adaptation.)
Taste is a sensory function of the
central nervous system. The receptor cells for taste in humans are found on the surface of the
tongue, along the
soft palate, and in the epithelium of the
pharynx and
epiglottis.
Basic taste
Psychophysicists have long suggested the existence of four taste 'primaries', referred to as the basic tastes:
sweetness,
bitterness,
sourness, and
saltiness.
Umami, or
savoriness, is now accepted as the fifth basic taste, exemplified by the non-salty sensations evoked by some free amino acids such as
Monosodium glutamate.
Other possible categories have been suggested, such as a taste exemplified by certain
fatty acids such as linoleic acid. Some researchers still argue against the notion of primaries at all and instead favor a continuum of percepts, similar to
color vision.
All of these taste sensations arise from all regions of the oral cavity, despite the common misperception of a "taste map" of sensitivity to different tastes thought to correspond to specific areas of the tongue. This myth is generally attributed to the mis-translation of a German text, and perpetuated in North American schools since the early twentieth century . Very slight regional differences in sensitivity to compounds exist, though these regional differences are subtle and don't conform exactly to the mythical tongue map. Individual taste buds (which contain approximately 100 taste receptor cells), in fact, typically respond to compounds evoking each of the five basic tastes.
The
basic tastes are those commonly recognized types of
taste sensed by
humans. Humans receive
tastes through sensory organs called
taste buds or
gustatory calyculi, concentrated on the upper
surface of the
tongue. Scientists describe five basic tastes: bitter, salty, sour, sweet, and
umami
(described as savoury, meaty, or brothy). The basic tastes are only one component that contributes to the sensation of food in the mouth — other factors include the food's
smell, detected by the
olfactory epithelium of the nose, its
texture, detected by
mechanoreceptors, and its temperature, detected by
thermoreceptors. Taste and
smell are subsumed under the term
flavor.
History
In Western culture, the concept of basic tastes can be traced back at least to
Aristotle, who cited "
sweet" and "
bitter," with "succulent," "
salt," "
pungent," "harsh," "
astringent" and "acid" as elaborations of those two basics. The ancient Chinese
Five Elements philosophy lists slightly different five basic tastes: bitter, salty, sour, sweet, and
spicy. Japanese and Indian cultures each add their own sixth taste to the basic five.
For many years, books on the
physiology of human taste contained diagrams of the tongue showing levels of sensitivity to different tastes in different regions. In fact, taste qualities are found in all areas of the tongue, in contrast with the popular view that different tastes map to different areas of the tongue.
Recent discoveries
The
receptors for all known basic tastes have been identified. The receptors for sour and salty are
ion channels while the receptors for sweet, bitter, and umami belong to the class of
G protein coupled receptors.
In November 2005, a team of French researchers experimenting on rodents claimed to have evidence for a sixth taste, for fatty substances. It is speculated that humans may also have the same receptors. Fat has occasionally been raised as a possible basic taste in the past (Bravo 1592, Linnaeus 1751) but later classifications abandoned fat as a separate taste (Haller 1751 and 1763).
Five basic tastes
For a long period, it has been commonly accepted that there are a finite number of "basic tastes" by which all foods and tastes can be grouped. Up until the
2000s, this was considered to be a group of four basic tastes. More recently, a fifth taste, Umami, was added by a wide number of authorities in this field.
Bitterness
The bitter taste is perceived by many to be unpleasant, sharp, or disagreeable. Common bitter foods and beverages include
coffee, unsweetened
chocolate,
bitter melon,
beer, uncured
olives,
citrus peel, many plants in the
Brassicaceae family,
dandelion greens and
escarole.
Quinine is also known for its bitter taste and is found in
tonic water.
The most bitter substance known is the synthetic chemical
denatonium. It is used as an
aversive agent that's added to toxic substances to prevent accidental ingestion. This was discovered 1958 during research on
Lignocaine, a local anesthetic, by
Macfarlan Smith of
Edinburgh,
Scotland.
Research has shown that TAS2Rs (taste receptors, type 2) such as
TAS2R38 coupled to the
G protein gustducin are responsible for the human ability to taste bitter substances. They are identified not only by their ability to taste for certain "bitter" ligands, but also by the morphology of the receptor itself (surface bound, monomeric). Researchers use two synthetic substances,
phenylthiocarbamide (PTC) and
6-n-propylthiouracil (PROP) to study the
genetics of bitter perception. These two substances taste bitter to some people, but are virtually tasteless to others. Among the tasters, some are so-called "
supertasters" to whom PTC and PROP are extremely bitter. This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics. In addition, it's of interest to those who study
evolution since PTC-tasting is associated with the ability to taste numerous natural bitter compounds, a large number of which are known to be toxic.
Saltiness
Saltiness is a taste produced primarily by the presence of
sodium ions. Other ions of the
alkali metals group also taste salty. However the further from sodium the less salty is the sensation. The size of
lithium and
potassium ions most closely resemble those of sodium and thus the saltiness is most similar. In contrast
rubidium and
cesium ions are far larger so their salty taste differs accordingly. Potassium, as
potassium chloride - KCl, is the principle ingredient in
salt substitutes.
Other
monovalent cations, for example
ammonium, NH
4+, and
divalent cations of the
alkali earth metal group of the
periodic table, for example calcium, Ca
2+, ions generally elicit a bitter rather than a salty taste even though they too can pass directly through ion channels in the tongue, generating an
action potential.
Sourness
Sourness is the taste that detects
acidity. The mechanism for detecting sour taste is similar to that which detects salt taste. Hydrogen
ion channels detect the concentration of
hydronium ions (H
3O
+ ions) that are formed from acids and water.
Hydrogen ions are capable of permeating the amiloride-sensitive channels, but this isn't the only mechanism involved in detecting the quality of sourness. Other channels have also been proposed in the literature. Hydrogen ions also inhibit the potassium channel, which normally functions 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 in this specific manner. In addition, it has also been suggested that weak acids, such as CO
2 which is converted into the
bicarbonate ion HCO
3– by the
enzyme carbonic anhydrase, to mediate weak acid transport.
Sweetness
Sweetness is produced by the presence of
sugars, some proteins 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" need to be activated for the brain to register sweetness. The compounds which the brain senses as sweet are thus compounds that can bind with varying bond strength to two different sweetness receptors. These receptors are T1R2+3 (heterodimer) and T1R3 (homodimer), which are shown to be accountable for all sweet sensing in humans and animals. The average human detection threshold for
sucrose is 10 millimoles per litre. For
lactose it's 30 millimoles per litre, and
5-Nitro-2-propoxyaniline 0.002 millimoles per litre.
Umami
Umami is the name for the taste sensation produced by compounds such as
glutamate, and are commonly found in
fermented and aged foods. In English, it's sometimes described as "meaty" or "savoury". The word is from the
Japanese word 旨味, うまみ whose characters literally mean "delicious flavour." Umami is now the commonly used term by taste scientists. The same taste is referred to as
xiānwèi (鮮味 or 鲜味) in
Chinese cooking. Umami is considered a fundamental taste in Chinese and Japanese cooking, but isn't discussed as much in
Western cuisine.
Humans have evolved taste receptors specifically for the detection of the
amino acids, for example
glutamic acid. Amino acids are commonly found in meats, cheese, fish and other protein-heavy foods. Examples of food containing glutamate (and thus strong in the umami taste) are beef, lamb,
parmesan and
roquefort cheese as well as
soy sauce and
fish sauce. The glutamate taste sensation is most intense in combination with sodium ions, as found in table salt. Sauces with umami and salty tastes are very popular for cooking, such as
worcestershire sauce for Western cuisines and
soy sauce and fish sauce for
Asian cuisines.
The additive
monosodium glutamate (MSG), which was developed as a food additive in 1907 by
Kikunae Ikeda, produces a strong umami taste. Umami is also provided by the
nucleotides
5’-inosine monophosphate (IMP) and
5’-guanosine monophosphate (GMP). These are naturally present in many protein-rich foods. IMP is present in high concentrations in many foods, including dried
skipjack tuna flakes used to make
dashi, a
Japanese broth. GMP is present in high concentration in dried
shiitake mushrooms, used in much of the
cuisine of Asia. There is a synergistic effect between MSG, IMP and GMP which together in certain ratios produce a strong umami taste.
Some Umami 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.
More sensations
The tongue can also feel other sensations, not generally called tastes
per se or included in the five human tastes. These are largely detected by the
somatosensory system.
Astringency
Some foods, such as unripe fruits, contain
tannins or
calcium oxalate that cause an astringent or rough sensation of the mucous membrane of the mouth or the teeth. Examples include
tea,
rhubarb,
grapes and unripe
persimmons and
bananas.
Less exact terms for the astringent sensation include: "rubbery", "hard", "styptic", "dry", "rough", "harsh" (especially for wine) and "tart" (normally referring to sourness) . The Chinese have a term for this: 澀 (
sè), the Koreans have 떫다 (
tteolda), the Japanese call it 渋い (
shibui), while Thai have ฝาด (fard), the Malay use
kelat, Filipinos use
pakla, and in Russian there's
вяжущий (vyazhuschiy) or
тёрпкий (tjorpky).
In the Indian tradition, one of the 6 tastes (sweet, sour, salty, bitter, hot/pungent and astringent)
(External Link
) is astringency (Kasaaya in Sanskrit). This is more or less in line with the Japanese approach to umami.
Coolness
Some substances activate cold
trigeminal receptors. One can sense a cool sensation (also known as "fresh" or "minty") from, for example,
spearmint,
menthol, ethanol or
camphor, which is caused by the food activating the
TRP-M8 ion channel on
nerve cells that also signal cold. Unlike the actual change in temperature described for sugar substitutes, coolness is only a perceived phenomenon.
Fat
Recent research has revealed a potential taste receptor called the
CD36 receptor to be reacting to
fat, more specifically,
fatty acids. This receptor was found in
mice, but probably exists among other
mammals as well. In experiments, mice with a genetic defect that blocked this receptor didn't show the same urge to consume fatty acids as normal mice, and failed to prepare
gastric juices in their
digestive tracts to digest fat. This discovery may lead to a better understanding of the biochemical reasons behind this behaviour, although more research is still necessary to confirm the relationship between CD36 and the perception of fat.
Kokumi
Some Japanese researchers refer to a flavor called kokumi which has been described variously as continuity, "mouthfulness", mouthfeel and thickness.
Metallic taste
Most people know this taste (for example Cu2+, FeSO4, or blood in mouth), but it isn't only taste but olfactory receptors worked in this case (Guth and Grosch, 1990). Metallic taste is commonly known, however biologists are reluctant to categorize it with the other taste sensations. One of the primary reasons is that it isn't one commonly associated with consumption of food. Proponents of the theory contest that the sensation is readily detectable and distinguishable to test subjects. Therefore, metallic should be added as one of the basic types of sensations in the chemical receptor senses.
Spiciness or hotness
Substances such as ethanol and capsaicin cause a burning sensation by inducing a trigeminal nerve reaction together with normal taste reception. The sensation of heat is caused by the food activating a nerve cell called TRP-V1, which is also activated by hot temperatures. The piquant sensation, usually referred to as being "hot" or "spicy", is a notable feature of Mexican, Hungarian, Indian, Szechuan, Korean, Indonesian, central Vietnamese, and Thai cuisines.
The two main plants providing this sensation are chili peppers (those fruits of the Capsicum plant that contain capsaicin) and black pepper.
If tissue in the oral cavity has been damaged or sensitised, ethanol may be experienced as pain rather than simply heat. Those who have had radiotherapy for oral cancer thus find it painful to drink alcohol.
In many cases, this particular sensation isn't considered a taste, so much as a painful reaction to certain marginally damaging chemicals on the taste receptors and the skin of the tongue. While the taste nerves are activated when consuming foods like chili peppers, the reaction commonly interpreted as "hot" is derived from the tongue's pain nerves firing.
Numbness
Chinese cooking includes the idea of 麻 má, the sensation of tingling numbness caused by spices such as Sichuan pepper. The cuisine of Sichuan province often combines this with chili pepper to produce a 麻辣 málà, "numbing-and-hot", flavor.
Temperature
Temperature is an essential element of human taste experience. Food and drink that — within a given culture — is considered to be properly served hot is often considered distasteful if cold, and vice versa.
Some sugar substitutes have strong heats of solution, as is the case of sorbitol, erythritol, xylitol, mannitol, lactitol and maltitol. When they're dry and are allowed to dissolve in saliva, heat effects can be recognized. The cooling effect upon eating may be desirable, as in a mint candy made with crystalline sorbitol, or undesirable if it's not typical for that product, like in a cookie. Crystalline phases tend to have a positive heat of solution and thus a cooling effect. The heats of solution of the amorphous phases of the same substances are negative and cause a warm impression in the mouth.
Supertasters
A supertaster is a person whose sense of taste is significantly more acute than average. Women are more likely to be supertasters, as are Asians, Africans and South Americans. Among individuals of European descent, it's estimated that about 25% of the population are supertasters. The cause of this heightened response is currently unknown, although it's thought to be, at least in part, due to an increased number of fungiform papillae. The evolutionary advantage to supertasting is unclear. In some environments, heightened taste response, particularly to bitterness, would represent an important advantage in avoiding potentially toxic plant alkaloids. However, in other environments, increased response to bitter may have limited the range of palatable foods. In our modern, energy-rich environment, supertasting may be cardioprotective, due to decreased liking and intake of fat, but may increase cancer risk via decreased vegetable intake. It may be a cause of picky eating, but picky eaters are not necessarily supertasters, and vice versa.
Aftertaste
Aftertaste is the persistence of a sensation of flavor after the stimulating substance has passed out of contact with the sensory end organs for taste. Some aftertastes may be pleasant, others unpleasant.
Alcoholic beverages such as wine, beer and whiskey are noted for having particularly strong aftertastes. Foods with notable aftertastes include spicy foods, such as Mexican food (for example chili pepper), or Indian food (such as curry).
Medicines and tablets may also have a lingering aftertaste.
Acquired taste
An acquired taste is an appreciation for a food or beverage that's unlikely to be enjoyed, in part or in full, by a person who hasn't had substantial exposure to it, usually because of some unfamiliar aspect of the food or beverage, including a strong or strange odor, taste, or appearance. The process of “acquiring” a taste involves consuming a food or beverage in the hope of learning to enjoy it. In most cases, this introductory period is considered worthwhile, as many of the world's delicacies are considered to be acquired tastes. A connoisseur is one who is held to have an expert judgment of taste.
Factors affecting taste perception
Many factors affect taste perception, including:
- Aging
- Color/vision impairments
- Hormonal influences
- Genetic variations - See Phenylthiocarbamide
- Oral temperature
- Drugs and chemicals
- CNS Tumors (esp. Temporal lobe lesions) and other neurological causes
- Plugged noses
- Zinc deficiency
It is also important to consider that flavor is the overall, total sensation induced during mastication (for example taste, touch, pain and smell). Smell (olfactory stimulation) plays a major role in flavor perception.
Disorders of taste
ageusia (complete loss)
Further Information
Get more info on 'Taste'.
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