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The science of taste

By JOE O'CONNELL, cbbqa Past President
First posted August 24, 2001
Modified by BILL WIGHT, cbbqa member
Updated December 10, 2002

Taste in humans is the brain's detection of a stimuli, which the brain interprets as the basic tastes:  salty, sour, sweet, bitter and, possibly, umami.  (The latter, fifth taste is still under investigation and appears to relate the the glutamate found, for example, in MSG.)

Each of these five "basic tastes" are registered via a series of chemical reactions in the taste cells of the taste buds.  Five separate biochemical pathways underlie each of these five basic tastes.  Each taste cell (usually called "taste buds") detect all five basic tastes, so that individual taste cells are not programmed, or "tuned," to respond to only one kind of taste stimulus.

Salty

As described in Scientific American, salts, such as sodium chloride (NaCl), trigger taste cells when sodium ions (Na+) enter through ion channels on microvilli at the cell's apical, or top, surface.  (Sodium ions can also enter via channels on the cell's basolateral, or side, surface.)

The accumulation of sodium ions causes an electrochemical change called depolarization that results in calcium ions (Ca++) entering the cell.  The calcium, in turn, prompts the cell to release chemical signals called neurotransmitters from packets known as vesicles.  Nerve cells, or neurons, receive the message and convey a signal to the brain.  Taste cells repolarize, or "reset," themselves in part by opening potassium ion channels so that potassium ions (K+) can exit.  Id.

Here is an illustration,  curtsey of Scientific American Magazine

Sour

What the human tongue perceives as "sour" is actually the perception of "acidic".  As described by Scientific American, acids taste sour because they generate hydrogen ions (H+) in solution.  Those ions act on a taste cell in three ways:  by directly entering the cell;  by blocking potassium ion (K+) channels on the microvilli;  and by binding to and opening channels on the microvilli that allow other positive ions to enter the cell.  The resulting accumulation of positive charges depolarizes the cell and leads to neurotransmitter release.  Id.

Sweet

Scientific American explains that sweet stimuli, such as sugar or artificial sweeteners, do not enter taste cells but trigger changes within the cells.  They bind to receptors on a taste cell's surface that are coupled to molecules named G-proteins.  This prompts the subunits (a, b and g) of the G-proteins to split into a and bg, which activate a nearby enzyme.  The enzyme then converts a precursor within the cell into so-called second messengers that close potassium channels indirectly.  Id.

Bitter

The taste that we perceive as bitter, such as quinine and coffee, results from stimuli that, as Scientific American explains, also act through G-protein-coupled receptors and second messengers.  In this case, however, the second messengers cause the release of calcium ions from the endoplasmic reticulum.  The resulting buildup of calcium in the cell leads to depolarization and neurotransmitter release.  Id.

Umami -- the fifth taste

Scientists have recently identified a fifth taste, called Umami.  Amino acids such as glutamate, which stimulates the umami taste, are known to bind to G-protein-coupled receptors and to activate second messengers.  Scientific American admits, however, that the intermediate steps between the second messengers and the release of packets of neurotransmitters are unknown.  Id.


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