Taste the Bubbly

Have you ever gone to the soda fountain at your favorite fast food joint, only to be disappointed by a “flat” soda?  What is it about “fizz” that makes soda pop taste good? Why do flat beverages turn us off?  A team of researchers lead by Charles Zucker of Columbia University has succeeded in answering that question.  Their study was published last month in the journal Science.

The fizz in your soda is a result of the addition of gaseous carbon dioxide.  According to Zucker, many scientists assumed that the pleasurable sensation provided by carbonation was the result of the stimulation of mechanoreceptors (a nerve cell that responds to mechanical pressure or distortion) when the carbon dioxide bubbles popped in the mouth.  Zucker and his team, however, have shown that this is not the whole story.

Acetazolamide

Zucker and his colleagues noted some observations about carbonation that indicated that other factors might play a role in the pleasurable sensation yielded by the presence of carbon dioxide in beverages.  One important observation came from mountaineers, who take drugs such as acetazolamide to prevent altitude sickness.  Mountaineers taking this particular drug have reported that their celebratory bottle of beer or glass of champagne on top of the mountain tends to taste disappointingly flat.  This observation prompted the mountaineers to dub this side effect of the drug the “champagne blues.”

To test what mechanisms may be responsible for the “taste” of carbonation and the “champagne blues,” Zucker and his colleagues studied the electrophysiological response of taste receptor cells (TRCs) to stimulation by carbon dioxide.  To do this, they recorded the action potentials of one of the major nerves present in TRCs of the tongue after stimulation by carbon dioxide.  Interestingly, the taste system displayed action potentials in response to carbon dioxide, whether the source was a favorite carbonated beverage, carbon dioxide dissolved in buffer, or just plain gaseous carbon dioxide puffed onto the tongue! However, stimulation of the cells with pressured air did not cause the nerve cell to fire.

Next, the researchers were interested in what TRCs were responsible for the sensing of carbon dioxide.  Humans have five different populations of TRCs responsible for the different taste sensations: sweet, sour, bitter, salty, and umami (a flavor that Zucker describes in his article as “a savory sensation characterized by the taste of monosodium glutamate”).  Researchers selectively ablated each of these cell types in five different mouse lines using genetic engineering techniques.  Mice that did not have the taste receptor cell populations responsible for the sour sensation did not fire an action potential in response to stimulation by carbon dioxide.  This indicated that the TRCs responsible for sour sensations are also responsible for “tasting” carbon dioxide.

So now, the researchers knew the cell type responsible for tasting carbon dioxide, but they had not yet identified the actual molecular carbon dioxide receptor.  To do this, they looked for good candidate genes expressed in the sour-sensing cells but not in other cells. They identified a gene, the Car4 gene, that was highly specific to the sour-sensing cells (i.e., it was not present in other types of TRCs but was strongly expressed in sour-sensing cells).  Additionally, this gene was known to code for an enzyme called carbonic anhydrase 4.  This type of enzyme reversibly catalyzes the reaction in which carbon dioxide is converted into bicarbonate ions and free protons, making it an even more likely candidate for the carbon dioxide receptor.

Ribbon Model of Carbonic Anhydrase

To test if this enzyme was indeed responsible for the tasting of carbon dioxide, the researchers tested both carbonic anhydrase 4 knockout mice (genetically modified mice in which the enzyme is not expressed) and mice that had been treated with a drug that blocks the enzyme for response to carbon dioxide.  Both of these techniques resulted in very little response to carbon dioxide stimulation, but the mice were responsive to other sensations, including sour sensations.  The researchers concluded that carbonic anhydrase 4 must “function selectively as the main carbon dioxide sensor in the taste system.”

So where does the residual response to carbon dioxide come from? The team hypothesized that the other carbonic anhydrases may be responsible for the small potentials still observed.  A broadly-acting drug that inhibits multiple carbonic anhydrases removed any residual response to carbon dioxide, even at very high carbon dioxide concentrations.

Taken together, Zucker’s studies indicate that carbonic anhydrase is responsible for the taste-system detection of carbon dioxide and nerve responses.  Not surprisingly, then, the mountaineers’ drug acetazolamide happens to inhibit carbonic anhydrases. No wonder their beer and champagne tasted flat! While mechanoreceptors may be involved in the sensations provided by carbonation, it looks like we can taste carbon dioxide after all.  Zucker and colleagues have provided strong evidence for one side of a once very bubbly debate.

All images obtained from Wikimedia Commons

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~ by ktbug1987 on November 18, 2009.

2 Responses to “Taste the Bubbly”

  1. Not everyone dislikes the taste of flat pop though. Both of my brothers like to shake the beverage to get the carbonation out and then they’ll drink it. They do like regular pop too. Could their like for flat pop be a result of a sour TRC or a carbonic anhydrase 4 mutation? Or could it just be that they enjoy flat pop?

  2. I’m not really sure. Interestingly, some cultures have names for pop that mean “sour,” indicating that some people find that carbonation does yield a sour flavor–at least in some situations. It is possible that they don’t like this taste, have some mutation in cah4, or perhaps they just don’t like so many bubbles in their mouth? I personally find that most soda burns going down, but flatter soda doesn’t so much. I don’t like it totally flat, but if it’s a little less bubbly, it’s fine with me!

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