WASHINGTON (AP) -- Like the fizz of a soft drink? Thank your tongue's sour-sensing taste buds.

Nope, it's not the popping bubbles that gives the "taste" of carbonation. Try a swallow inside a pressure chamber -- where the bubbles don't burst -- and the sensation's the same.

So says a report in Friday's edition of the journal Science, where researchers tackled a bubbly puzzle: How do we taste the carbon dioxide that gives carbonated drinks their fizz?

After all, the human tongue is supposed to sense just five flavors: bitter, sweet, salty, sour and umami, sometimes called savory.

It turns out that the taste buds that let us sense sourness have an enzyme on their surface that interacts with carbon dioxide, said researchers from the University of California, San Diego, and the National Institutes of Health.

They made the discovery in mice, whose sense of taste resembles that of humans. They gave the animals sips of club soda or a little buzz of carbon dioxide gas and recorded how the tongue signaled the sensation to the brain. Both soda and the gas produced similar sensations. But when they tested mice bred to have no sour taste buds, the brain never got its sensory alert. Further probing uncovered the enzyme responsible.

Why isn't the tingle merely sour? Carbon dioxide also stimulates somatosensory cells in the mouth, cells that give touch sensations, so presumably it's that unique combination.

But given that carbonating water was first tried not quite 250 years ago, why would mammals have evolved a way to sense carbon dioxide? The scientists couldn't say, but speculated it might have been a protective mechanism, to avoid fermenting foods that give off the gas.

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