Taste - Wikipedia, the free encyclopedia
Basic tastes
For a long period, it was commonly accepted
[who?]
that there is a finite and small number of "basic tastes" of which all
seemingly complex tastes are ultimately composed. Just as with
primary colors,
the "basic" quality of those sensations derives chiefly from the nature
of human perception, in this case the different sorts of tastes the
human
tongue
can identify. As of the early twentieth century, physiologists and
psychologists believed there were four basic tastes: sweetness,
sourness, saltiness, bitterness. At that time
umami was not proposed as a fifth taste
[20] but now a large number of authorities recognize it as the fifth taste.
[citation needed] 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.
[citation needed]
Sweetness
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. These receptors are
T1R2+3 (heterodimer) and T1R3 (homodimer), which account for all sweet
sensing in humans and animals.
[21] Taste detection thresholds for sweet substances are rated relative to
sucrose, which has an index of 1.
[22][23] The average human detection threshold for sucrose is 10 millimoles per liter. For
lactose it is 30 millimoles per liter, with a sweetness index of 0.3,
[22] and
5-Nitro-2-propoxyaniline 0.002 millimoles per liter.
Sourness
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 0.7,
citric acid an index of 0.46, and
carbonic acid an index of 0.06.
[22][23]
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,
[24]
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. This transfer of positive charge
into the cell can itself trigger an electrical response. 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 mechanism by which
animals detect sour is still not completely understood.
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. Children in the US show a greater enjoyment of sour flavors than adults,
[25] and
sour candy is popular in North America
[26] including
Cry Babies,
Warheads,
Lemon drops,
Shock Tarts and sour versions of
Skittles and
Starburst. Many of these candies contain citric acid.
Saltiness
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. 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.
[citation needed] The saltiness of substances is rated relative to
sodium chloride (NaCl), which has an index of 1.
[22][23] Potassium, as
potassium chloride - KCl, is the principal ingredient in
salt substitutes, and has a saltiness index of 0.6.
[22][23]
Other
monovalent cations, e.g.
ammonium, NH
4+, and
divalent cations of the
alkali earth metal group of the
periodic table, e.g. 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.
Bitterness
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,
beer (due to
hops),
bitters,
olives,
citrus peel, many plants in the
Brassicaceae family,
dandelion greens, wild
chicory, and
escarole.
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
[22][27]
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.
[22][27][28] Plant leaves often contain toxic compounds, yet even amongst
leaf-eating
primates, there is a tendency to prefer immature leaves, which tend to
be higher in protein and lower in fiber and poisons than mature leaves.
[29] Amongst humans, various
food processing techniques are used worldwide to detoxify otherwise inedible foods and make them palatable.
[30]
The threshold for stimulation of bitter taste by quinine averages a concentration of 8E-6
M (8 micromoles).
[22] The taste thresholds of other bitter substances are rated relative to quinine, which is thus given a reference index of 1.
[22][23] For example,
Brucine
has an index of 11, is thus perceived as intensely more bitter than
quinine, and is detected at a much lower solution threshold.
[22] The most bitter substance known is the synthetic chemical
denatonium, which has an index of 1,000.
[23] It is used as an
aversive agent (a
bitterant) that is added to toxic substances to prevent accidental ingestion. This was discovered in 1958 during research on
lignocaine, a local anesthetic, by
MacFarlan Smith of
Gorgie,
Edinburgh,
Scotland.
Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as
TAS2R38 coupled to the
G protein gustducin are responsible for the human ability to taste bitter substances.
[31] 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).
[32]
The TAS2R family in humans is thought to comprise about 25 different
taste receptors, some of which can recognize a wide variety of
bitter-tasting compounds.
[33]
Over 550 bitter-tasting compounds have been identified, of which about
100 have been assigned to one or more specific receptors.
[34]
Recently it is speculated that the selective constraints on the TAS2R
family have been weakened due to the relatively high rate of mutation
and pseudogenization.
[35] 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. The variation in sensitivity
is determined by two common alleles at the TAS2R38 locus.
[36] This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics.
Umami
Umami is an
appetitive taste
[10] and is described as a savory
[37][38] or
meaty[38][39] taste. It can be tasted in
cheese[40] and
soy sauce,
[41] and while also found in many other fermented and aged foods, this taste is also present in tomatoes, grains, and beans.
[40] Monosodium glutamate (MSG), developed as a food additive in 1908 by
Kikunae Ikeda,
[42] produces a strong umami taste.
[41] See
TAS1R1 and
TAS1R3 pages for a further explanation of the amino-acid taste receptor. A
loanword from
Japanese meaning "good flavor" or "good taste",
[43] umami (旨味?) is considered fundamental to many
Eastern cuisines
[44] and was first described in 1908,
[45] although it was only recently recognized in
the West as a basic taste.
[41][46]
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.
[47][48]
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