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.
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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.
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.