Discovery in fizzy science: Tongue, not bubbles, gives soda ‘bite’

Los Angeles TimesSeptember 2, 2013 

Paul Wise is a sensory psychologist at the Monell Chemical Senses Center.

MONELL.ORG

What gives carbonated drinks their zesty bite? Bubbles might be your first guess – but you’d be wrong.

In fact, bubbles aren’t needed to experience the burning sensation, but they do enhance it, according to a study published in the journal PLOS ONE. The secret behind soda’s sharp taste is the acid that forms on your tongue when you take a sip.

Carbonated beverages are produced by dissolving carbon dioxide in liquid, typically under high pressure. Popping open a can or bottle of the liquid reduces that pressure, releasing the carbon dioxide in the form of bubbles. Enzymes in the mouth convert the carbon dioxide into carbonic acid. The acid stimulates nerve endings, activating pain mechanisms that cause a mild irritation, or “bite.”

The earliest clues that carbonic acid, not bubbles, were responsible for this sensation came from mountaineers’ accounts of the “champagne blues.” After pouring a bottle of celebratory champagne at the summit, mountaineers who had taken medication for altitude sickness disappointedly remarked on the drink’s flat taste – despite its fizziness. It turns out the medication blocked the enzyme that converts carbon dioxide into carbonic acid.

Still, popular belief continued to hold that the bubbles were primarily responsible. To settle the debate, scientists at the Monell Chemical Senses Center in Philadelphia took advantage of the inability of bubbles to form above a certain pressure level.

The researchers seated 12 healthy adults in a pressurized chamber and asked them to rate the bite intensity of carbonated water once while under normal pressure – at which bubbles could still form – and a second time at higher pressure that made bubble formation impossible. There was no difference in the bite ratings reported in the two conditions.

But the bubbles may have still influenced that perception. Earlier studies have shown that light touch can soothe pain – much like how rubbing a mosquito bite can relieve itching – so the researchers expected the bubbles to suppress the sting of carbonation.

To test this hypothesis, they asked 11 adults to rate the bite intensity of carbonated water when a filter pumped bubbles just below their tongues and again when another filter pumped the bubbles from further away.

To the scientists’ surprise, the volunteers rated the bite as stronger when the bubbles were directly hitting their tongues – suggesting that bubbles enhance the bite of carbonated drinks. How the bubbles do this remains a mystery. Perhaps they “stir” the liquid in the mouth, continually drawing in fresh carbon dioxide to convert into carbonic acid, said study leader Paul Wise, a sensory psychologist.

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