Since 1767, when chemist Joseph Priestley first infused water with carbon dioxide to produce carbonated water—the main ingredient of sodas, sparkling wines, and a variety of carbonated drinks—there has been no scientific explanation of how people taste the carbonation bubbling in their glass. Researchers at the National Institute of Dental and Craniofacial Research (NIDCR) and their colleagues from the Howard Hughes Medical Institute at the University of California, San Diego, report that they have discovered the answer in mice, whose sense of taste closely resembles that of humans. They found that the taste of carbonation is initiated by an enzyme tethered like a small flag from the surface of sour-sensing cells in taste buds. The enzyme, called carbonic anhydrase 4 (CA-IV), interacts with the carbon dioxide in the soda, activating the sour cells in the taste bud and prompting it to send a sensory message to the brain, where carbonation is perceived as a familiar sensation.
“Of course, this raises the question of why carbonation doesn’t just taste sour,” says Nicholas Ryba, PhD, a senior author of this study and an NIDCR scientist. “We know that carbon dioxide also stimulates the mouth’s somat0sensory system. Therefore, what we perceive as carbonation must reflect the combination of this somatosensory information with that from taste.” A somatosensory system transmits sensory information within the body from protein receptors to nerve fibers and onward to the brain, where a sensation is perceived. Common sensory information includes taste, touch, pain, and temperature. Dr. Ryba added that the taste of carbonation is quite deceptive. “When people drink soft drinks, they think that they are detecting the bubbles bursting on their tongue. But if you drink a carbonated drink in a pressure chamber, which prevents the bubbles from bursting, it turns out the sensation is actually the same. What people taste when they detect the fizz and tingle on their tongue is a combination of the activation of the taste receptor and the somatosensory cells. That’s what gives carbonation its characteristic sensation,” said Dr. Ryba.
The scientists found that if they eliminated CA-IV from the sour-sensing cells or inhibited the enzyme’s activity, they severely reduced a mouse’s sense of taste for carbon dioxide. Thus, CA-IV activity provides the primary signal detected by the taste system. As CA-IV is expressed on the surface of sour cells, lead author Jayaram Chandrashekar, PhD, and co-workers concluded that the enzyme is ideally poised to generate an acid stimulus for detection by these cells when presented with carbon dioxide.
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