Tongue’s sour-sensing cells taste carbonation

Protein splits carbon dioxide to give fizz its flavor

The light, sparkly fizz of champagne owes its taste to the tongue’s sense of sour. New studies in mice reveal how the tongue tastes carbonation, solving an old puzzle of why some mountain climbers get the “champagne blues.”

FIZZ-TASTIC Studies in mice have shown the importance of an enzyme on certain taste cells in tasting carbonation. Here, sour-sensing cells and the enzyme light up a mouse tongue. J. Chandrashekar et al./Science AAAS 2009

Tasting fizz begins with a special protein that’s tethered to sour-sensing taste cells on the tongue, researchers report in the Oct. 16 Science. This protein, the enzyme carbonic anhydrase 4, splits carbon dioxide into bicarbonate ions and free protons, which stimulate the sour-sensing cells.

Scientists have long thought that the taste of carbonated beverages emerged from the physical bursting of bubbles on the tongue, says study author Charles Zuker, a neuroscientist now at Columbia University who did the work while at the University of California, San Diego. But bubbly drinks still taste distinctly carbonated when they are imbibed in a pressure chamber where bubbles don’t burst.

To understand how carbonation fits into the sensory repertoire, Zuker, Nick Ryba of the National Institute of Dental and Craniofacial Research in Bethesda, Md., and their colleagues measured nerve activity in mouse taste cells. When the rodents were given carbon dioxide in the form of club soda or gaseous CO2, their taste cells responded robustly to CO2.

The researchers then genetically engineered mice that were missing one of the five kinds of taste cell—sweet, salty, umami, bitter and sour. Taste-sensing nerves fired in response to carbon dioxide except in the “sourless” mice, pinpointing the role of the sour-sensing taste cells.

Combing through the genes that are turned on in sour-sensing cells, one “rose to the top,” says Zuker. It was the gene encoding carbonic anhydrase 4, one enzyme in an important class that helps maintain appropriate levels of CO2 and acidity in the body.

The researchers also dosed normal mice with a chemical that inhibits the enzyme. The chemical quashed the nerve-firing response to carbonation but didn’t eliminate it entirely. Some taste may have lingered because carbon dioxide’s protons also stimulate the trigeminal nerve, which doesn’t directly respond to taste but registers minor sensory irritants such as pepper or mint, says Alexander Bachmanov of the Monell Chemical Senses Center in Philadelphia.

Bachmanov calls the new work “elegant.” Taste “is a very challenging system to study,” he says. “Everything is very small but very complex.”

In the bigger picture, tasting carbonation may have allowed animals to sense CO2 produced in foods that had fermented or gone bad, akin to how bitter-sensing taste cells warn of potential toxicity, says Zuker.

Humans have turned this flavor into a pleasure—bottles of bubbly often kick off a celebration. But mountain climbers often pop a bottle at the peak and find that it tastes horrible, says Zuker. The new research suggests an explanation for these “champagne blues.” A drug that many climbers take to prevent altitude sickness inhibits the carbonic anhydrase enzyme. When taste cells don’t get their proton hit, champagne falls flat. 

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