Whay pork taste so good and juicy?

Answer 1

Why taste?

Taste drives appetite and protects us from poisons. So, we like the taste of sugar because we have an absolute requirement for carbohydrates (sugars etc.). We get cravings for salt because we must have sodium chloride (common salt) in our diet. Bitter and sour cause aversive, avoidance reactions because most poisons are bitter (most bitter substances are bad for you - certainly in excess) and off food goes sour (acidic). Why do medicines all taste bitter? Because they are, in fact, poisons and if you take too much they will harm you. We have an absolute need for protein, and amino acids are the building blocks for proteins, so the "new" taste quality umami (pronounced: oo-marmi) which is the meaty, savoury taste drives our appetite for amino acids. This taste has been known to the Japanese for a long time - but has only recently been recognised by the West. Bacon really hits our umami receptors because it is a rich source of amino acids.

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Anatomy and Physiology of Gustation (taste)

In mammals taste buds are aggregations of 30-100 individual elongated "neuroepithelial" cells (50-60 microns in height, 30-70 microns in width), which are often embedded in specializations of surrounding epithelium, termed papillae. At the apex of the taste bud, microvillar processes protrude through a small opening, the taste pore, into the oral milieu. Just below the taste bud apex, taste cells are joined by tight junctional complexes.



Taste buds and taste papillae. Taste papillae can be seen on the tongue as little red dots, or raised bumps, particularly at the front of the tongue. These ones are actually called "fungiform" papillae, because they look like little button mushrooms. There are three other kinds of papillae, foliate, circumvallate and the non-gustatory filiform. Taste buds, on the other hand, are collections of cells on these papillae and cannot be seen by the naked eye. To illustrate the point, have a look at the diagram below. You can see that the taste buds are collections of cells situated on top of, or on the sides of, the different papillae.



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Figure 1.Papillae and taste buds

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At the base of the taste bud, afferent taste nerve axons invade the bud and ramify extensively, each fibre typically synapsing with multiple receptor cells within the taste bud .Go to top

In mammals taste buds are located throughout the oral cavity, in the pharynx, the laryngeal epiglottis and at the entrance of the eosophagus. Taste buds on the dorsal lingual epithelium are the most numerous (total number of taste buds, all classes, = 4600 per tongue) and best-studied taste end-organs. Here, taste buds are contained within four major classes of papillae.

Fungiform papillae are located on the most anterior part of the tongue and generally contain one to several taste buds per papilla. They are innervated by the chorda tympani branch of the facial (Vllth cranial) nerve. They appear as red spots on the tongue - red because they are richly supplied with blood vessels. The total number of fungiform papillae per human tongue is around 200. Papillae at the front of the tongue have more taste buds (1-18) compared to the mid-region (1-9). It has been calculated that there are 1120 fungiform taste buds per tongue.
Foliate papillae are situated on the edge of the tongue slightly anterior of the circumvallate line. They are predominantly sensitive to sour tastes. Innervated by the glossopharyngeal (lXth cranial) nerve. On average 5.4 foliate papillae per side of the tongue, 117 taste buds per foliate papillae, total = 1280 foliate taste buds per tongue.
Circumvallate papillae are sunken papillae, with a trough separating them from surrounding wall. The tase buds are in tiers within the trough of the papillae. They are situated on the circumvallate line and confer a sour/bitter sensitivity to the posterior 2/3 of the tongue. Innervated by the glossopharyngeal (lXth cranial) nerve. 3-13 circumvallate papillae per tongue with 252 taste buds per papillae, total = 2200 circumvallate taste buds per tongue
Filiform papillae (left) are mechanical and non-gustatory.
In addition there are 2500 taste buds on the epiglottis, soft palate, laryngeal and oral pharynx.

The number of taste buds declines with age.

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Cells in taste papillae
Supporting cells - contain microvilli, appear to secrete substances into lumen of taste bud.
Sensory receptor cell - has peg-like extensions projecting into lumen. These contain the sites of sensory transduction.
Basal cells - these differentiate into new receptor cells. They are derived from surrounding epithelium. The cells are continuously renewed every 10 days or so.
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Figure 2. A taste receptor cell



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Taste transduction
There are five basic tastes: salt, sour, sweet, bitter and umami.

Salt taste
Salt is sodium chloride (Na+ Cl-). Na+ ions enter the receptor cells via Na-channels. These are amiloride-sensitive Na+ channel (as distinguished from TTX-sensitive Na+ channels of nerve and muscle). The entry of Na+ causes a depolarization, Ca2+ enters through voltage-sensitive Ca2+ channels, transmitter release occurs and results in increased firing in the primary afferent nerve.
Sour taste
Sour taste is acid and acid is protons (H+). H+ ions block K+ channels. K+ channels are responsible for maintaining the cell membrane potential at a hyperpolarized level (close to the K+ equilibrium potential of around -85mV). Block of these channels causes a depolarization, Ca2+ entry, transmitter release and increased firing in the primary afferent nerve. Go to top
Sweet taste
There are receptors in the apical membrane that bind glucose (sucrose - a combination of glucose and fructose - and other carbohydrates). Binding to the receptor activates adenylyl cyclase, thereby elevating cAMP. This causes a PKA-mediated phosphorylation of K+ channels, inhibiting them. Depolarization occurs, Ca2+ enters the cell through depolarization-activated Ca2+ channels, transmitter is released increasing firing in the primary afferent nerve.
Bitter taste
Bitter substances cause the second messenger (IP3) mediated release of Ca2+ from internal stores (external Ca2+ is not required). The elevated Ca2+ causes transmitter release and this increases the firing of the primary afferent nerve.
Umami taste
Umami is the taste of certain amino acids (e.g. glutamate, aspartate and related compounds). It was first identified by Kikunae Ikeda at the Imperial University of Tokyo in 1909. Recently it has been shown1,2 that the metabotropic glutamate receptor (mGluR4) mediates umami taste. Binding to the receptor activates a G-protein and this may elevate intracellular Ca2+. (Go to top)
Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of Soy sauce), may stimulate the umami receptors. But, in addition, there are ionotropic glutamate receptors (linked to ion channels), i.e. the NMDA-receptor, on the tongue. When activated by these umami compounds or soy sauce, non-selective cation channels open, thereby depolarizing the cell. Calcium enters, causing transmitter release and increased firing in the primary afferent nerve
1Chaudhari et al, (1996) The taste of monosodium glutamate: membrane receptors in taste buds. J. Neurosci. 16, 3817-3826.
2Kurihara & Kashiwayanagi (1998) Introductory remarks on umami taste. Annals NY Acad Sci 855, 393-397.

Monosodium glutamate
Monosodium glutamate is the main ingredient of Soy sauce. This is added to foods to enhance their flavour. It probably works by activating NMDA receptors which are found in taste cells. NMDA receptors are integral receptor-ion channel complexes and when they open they allow an influx of Na+ and Ca2+ ions. This will depolarise the taste receptor cell and act as an excitatory influence. Then, far less of a particular taste will be required to cause the further depolarisation necessary to bring about transmitter release.
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Receptors
Sweet and bitter taste receptors have recently been cloned. A summary of the different types of receptor responsible for each of the 5 taste modalities is given below.





Salt receptor

ENaC (Epithelial Sodium (Na) channel)
ubiquitously expressed
only functional in anterior tongue

Bitter receptor family - T2Rs

50-80 members
expressed in small subset of all taste papillae
expressed in cells that also express a-gustducin
70% of gustducin cells in circumvallate & foliate papillae express T2Rs
Sweet and umami receptors
Heteromeric receptors made up of a combination of different subunits, coded for by a small gene family - T1R

T1Rs (3 genes distantly related to mGluRs)
T1R1 - expressed in fungiform papillae
T1R2 - expressed in circumvallate & foliate
- both may couple with transducin, not gustducin
T1R3 - expressed in 30% of all taste buds
T1R1+3 = amino acid receptor (umami)
T1R2+3 = sweet receptor

Umami is possibly mediated by both mGluR4 and T1R1+3 receptors


Sour receptors
Sour is the taste of acid, i.e. protons (H+). Three possible receptor mechanisms:

H+ blocks K+ channels
H+ ions go through ENaC channels
H+ ions go through a proton channel
References for taste receptors
(1) Mammalian Sweet taste receptors. Nelson, G. et al (2001) Cell, 106, 381-390
(2) An amino-acid taste receptor. Nelson, G. et al (2002) Nature 416 (14 March), 199-204
(3) A plethora of taste receptors. Kinnamon, S.C. (2000) Neuron, 25, 507-510.


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Artificial Sweetners
Saccharin - Discovered in 1879 when a Johns Hopkins worker inadverently licked his fingers. Saccharin is only sweet to humans. Bees/butterflies which normally crave the sweetness of nectar, do not treat it as a desirable substance.

Cyclamate - Discovered by accident. A graduate student at the University of Illinois in 1937 was smoking a cigarette that came into contact with some.

Aspartame - James Schlatter licked fingers in preparing to pick up a peice of weighing paper. It is a combination of two naturally occurring amino acids (aspartic and phenylalanine). Alitame, similar to aspartame in that it combines two amino acids (alanin and aspartic acid) into a dipeptide, is about 2,000-times sweeter than sugar.

Have a look at the structure of sweetners

Sucralose - A chloride-containing carbohydrate product some 600-times sweeter than sugar. Discovered when a foreign student (Shashikant Phadnis) working in Prof Leslie Hough's lab at King's College, London, misunderstood a request for "testing" as "tasting".

Some plant proteins, e.g. Monellin and Thaumatin, taste 10,000 times as sweet as sucrose (a disaccharide made up of a glucose and a fructose molecule). Salts of lead and beryllium also taste sweet.

Certain artifical sweetners (e.g. saccharin) lead to the generation of IP3 and a rise in intracellular Ca2+ due to release from internal stores.

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Modifying taste
Taste exhibits almost complete adaptation to a stimulus - perception of a substance fades to almost nothing in seconds. Taste can be suppressed by local anaesthetics applied to the tongue. Amiloride, a blocker of epithelial Na channels, reduces salt taste in humans and adenosine monophosphate (AMP) may block the bitterness of several bitter tasting agents. Naturally occuring compounds include, gymnemic acid (a product of the Indian tree/shrub Gymnema sylvestre) decreases the sweet perception by competitive inhibition of the sweet receptor. Artichokes have the opposite effect, enhancing sweet taste (the active compounds in this case are chlorogenic acid and cynarin) by suppression of sour and bitter taste receptors. Miracle fruit turns sour tastes sweet. The active ingredient, "miraculin", binds to a site near the sweet receptor. When sour substances then are tasted, a conformational change in the taste cell membrane occurs in such a way as to bring the miraculin molecule into contact with the sweet receptor, activating it.

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Regional localisation of taste on the tongue (Taste maps)
There has been some controversy as to whether the familiar taste maps of the human tongue, which appear in every textbook, are correct. Taste sensation can be localised on the tongue but does the tongue have regions that are more sensitive to one taste modality than another? Fungiform papillae are concentrated on the anterior tip of the tongue and anterior lateral margins in humans and it has been demonstrated that NaCl threshold was inversely related to the number of fungiform papillae (more papillae = more sensitivity, lower threshold). In a study of human fungiform papillae it was found that taste buds can respond to NaCl only or to both NaCl and sucrose. The responses to NaCl and sucrose occurred in different cells within the taste bud. Thus, one can infer that fungiform papillae are salt-sensitive but this does not mean they are insensitive to other tastes. Bitter receptors are not uniformly distributed over the tongue. In rats the bitter receptors are expressed in a subset of taste cells in all papillae but they are more concentrated in foliate and circumvallate papillae situated at the sides and the back of the tongue. Furthermore, alpha-gustducin, which is the G-protein coupled to the T2R bitter receptors (see below), is expressed more in circumvallate than fungiform papillae in the rat. One rather more empirical approach to resolving this question is to stimulate the different areas of the tongue directly. Thermal stimulation of the anterior sides of the tongue in humans (fungiform papillae and the chorda tympani nerve) evokes sweet and salt/sour taste. While thermal stimulation of the rear of the tongue (foliate/circumvallate papillae and glossopharyngeal nerve) causes a different relationship between temperature and taste to the anterior stimulation. One can conclude that the classical "taste map" is an over simplification. Sensitivity to all tastes is distributed across the whole tongue and indeed to other regions of the mouth where there are taste buds (epiglottis, soft palate), but some areas are indeed more responsive to certain tastes than others.

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Relay to the brain


Figure 3. Innervation of the tongue

Taste receptor cells do not have an axon. Information is relayed onto terminals of sensory fibres by transmitter. These fibres arise from the ganglion cells of the cranial nerves Vll (facial - a branch called the chorda tympani)) and lX (glossopharyngeal) (see Figure 2).The first recordings from sensory fibres showed an optimal response to one stimuli, but a smaller response to other taste stimuli.

Taste is determined by the pattern of active (firing) fibres, i.e. by "across-fibre pattern" rather than "labelled-line".

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Central pathways
Primary gustatory fibres synapse centrally in the medulla (in a thin line of cells called the nucleus of the solitary tract). From there the information is relayed (1) to the somatosensory cortex for the conscious perception of taste and (2) to the hypothalamus, amygdala and insula, giving the so-called "affective" component of taste. This is responsible for the behavioural response, e.g. aversion, gastric secretion, feeding behaviour.

Supertasters

It has been found that some people have more than the normal number of taste papillae (and taste buds). They are distinguished by their increased density of fungiform papillae and their exterme sensitivity to the chemical n-propylthiouracil (PROP). Supertasters - 25% of the population (and more women than men) - tend not to like green vegetables and fatty foods.

% of population *density of taste papillae cm-2
supertasters 25 165
normal tasters 50 127
non-tasters 25 117

* at the tip of the tongue (from Yackinous & Guinard, Appetite (2000) 38, 201-209).



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Strange taste facts

Taste is mainly smell. Hold your nose, close your eyes, and try to tell the difference between coffee or tea, red or white wine, brandy or whisky. In fact, with blocked nose (clothes peg or similar) you can't tell the difference between grated apple and grated onion - try it! Of course, this is because what we often call taste is in fact flavour. Flavour is a combination of taste, smell, texture (touch sensation) and other physical features (eg. temperature).



Smelly fruit

The durian fruit smells horrible. Some people cannot bear to eat it because it smells so foul. But it is called the "King of Fruits" and tastes delicious. It is very large (can be the size of a football) and comes from South East Asia.





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Links to other sites:


Notes on smell
Chemoreception, a vaste resource of everything to do with taste and smell, wierd and mainstream.

Return to Tim Jacob homepage


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Bibliography:
Signals and Perception edited by David Roberts (2002). Open University - Palgrave Macmillan.
The Senses by Barlow by H.B and Mollon, J.D. (1982). Cambridge University Press.
Handbook of olfaction and gustation by Doty, R.L. (1995). Marcel Dekker.
The Chemical Senses by Moncrieff, J.W. (1967), pp 108-112.
Neurophysiology by Carpenter, R.H.S. (1995), 3rd edition.

Answer 2

Because. Pork here isn't really pork. It's someone's good sense in taking Jonathan Swift up on his "Modest Proposal." It's about time someone used that good idea!

Answer 3

your wasting peoples time and im sorry i wasted mine responding to your stupid question

Answer 4

because you are an idiot

Answer 5

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Answer 6

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