Researchers say the refreshing bite you get from an ice-cold beer or soda comes from a chemical reaction that's going on inside your mouth — a reaction that turns the beverage's carbon dioxide bubbles into irritating carbonic acid.
That's right: It's not the bubbles. It's the acid.
"Carbonation bite is an acidic chemical sensation rather than a purely physical, tactile one," Bruce Bryant, a sensory biologist at the Monell Chemical Senses Center in Philadelphia, said in a news release Wednesday. Bryant is one of the authors of a study on the biology behind the "bite," published in the open-access PLOS ONE.
Bryant and his colleagues found that the bubbles do enhance the overall sensation of carbonation — but by stimulating the sense of touch rather than taste.
Feeling the pressure
Here's how they figured it out: In the first stage of their experiment, they took 12 healthy adults and sat them down in a hyperbaric chamber — a sealed room where the atmospheric pressure can be raised to twice as much as normal. At that pressure, the CO2 that's dissolved in a liquid can't form bubbles.
The researchers asked their subjects to rate the intensity of the "bite" produced by various concentrations of plain carbonated water, at normal pressure and at high pressure. The ratings were the same, whether or not bubbles were produced.
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Previous research has shown that carbonic acid has an irritating effect on nerve cells in the tongue, sending mild pain signals to the brain. The Monell experiment showed that the effect was pretty much the same whether or not there were actually bubbles in the beverage, as long as the carbon dioxide was turned into carbonic acid. But does that mean the bubbles are unnecessary?
To answer that question, the researchers set up another experiment: This time, the researchers added some extra air bubbles to the carbonated water. They expected that there'd be no difference, but were surprised to find that the air bubbles actually enhanced the bite of the carbon dioxide bubbles. Presumably, the "mouthfeel" of the bubbles added an extra dimension to the experience of fizz.
"We thought the touch of the bubbles would suppress the painful aspects of carbonation, much as itching a mosquito bite or rubbing a sore muscle does," Bryant said.
Flat vs. fizzy
One of Bryant's colleagues at the Monell Center, Paul Wise, told NBC News that the findings are in line with observations reported by mountain climbers. "They found that their 'victory beer' on top of the mountain tasted flat," Wise said. It turns out that climbers often take a drug known as acetazolamide, or Diamox, to counter altitude sickness. That drug blocks the conversion of carbon dioxide to carbonic acid — and thus dulls the beer's bite.
How about adding carbonic acid to a flat beverage? Would that give you the bite without the bubbles? That's an iffy proposition. "It just might be horrendously sour," Wise said.
"That's why CO2 is so wonderful," he added, "because it doesn't seem to stimulate taste nerves directly, but it goes into the tissue and produces the acid both inside the nerve endings and in the tissue surrounding the nerve endings."
He acknowledged that there's much more to a fizzy beverage than its bite. For instance, researchers have found that the carbon dioxide bubbles in Champagne transport volatile organic chemicals into the nose, stimulating all the olfactory sensations that are associated with a fine sparkling wine. The flavor of a beverage arises from the complex interplay of taste, smell and touch. "Ultimately, we're taking little simple pieces and working up to the more complex interaction," Wise said.
If you want to enhance the bite of your fizzy drink, here's a tip: Chill it down as much as you can. "There's a very strong interaction between temperature and carbonation bite," Wise said. "You get a much stronger bite when it's colder than when it's warmer."
Researchers haven't yet completely figured out why that is, but Wise said "it seems that somehow there are temperature-sensitive nerves that are involved in enhancing the bite of CO2." Just as hot-sauce lovers learn to yearn for the irritating sensation of capsaicin, fans of fizzy drinks get hooked on the nerve-tingling bite of a cold carbonated beverage.
More about the science of drinks:
In addition to Wise and Bryant, the authors of "The Influence of Bubbles on the Perception Carbonation Bite" include Madeline Wolf and Stephen Thorn. The study was funded by Anheuser-Busch InBev Inc. and Monell Chemical Senses Center.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.