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October 26, 2009
Scientists Discover Receptor for Carbonation Taste
Almost 250 years after chemist Joseph Priestley infused water with carbon dioxide to make the first artificially carbonated water, researchers have finally discovered how people “taste” that carbonation bubbling in their beverage.
Over the past decade, researchers have made tremendous progress identifying the basis for detecting the 5 taste qualities—sweet, sour, salty, bitter and savory. Scientists believe that our perception of flavors arise from this limited palate of tastes, along with input from other senses like touch and smell. Interestingly, the gas carbon dioxide also induces strong responses in taste nerves, but how it's detected by the taste system was unclear.
The laboratories of Dr. Nicholas Ryba of NIH's National Institute of Dental and Craniofacial Research (NIDCR) and Dr. Charles Zuker from the Howard Hughes Medical Institute at the University of California at San Diego previously teamed up to identify the components responsible for our sweet, bitter, savory and sour detection. In their new study, appearing in the October 16, 2009, edition of Science, they explored the taste of carbonation by recording the electrical responses of taste nerves in mice.
The team tested mice engineered to lack different types of taste cells and discovered that sour-sensing cells were responsible for detecting carbon dioxide. Further work revealed that the response is initiated by an enzyme called carbonic anhydrase 4 (CA-IV). CA-IV is one of a family of enzymes that helps convert carbon dioxide to carbonic acid, providing cells and tissues with a buffer to help prevent excessive changes in pH. CA-IV interacts with carbon dioxide on the surface of sour-sensing cells in taste buds, the researchers found, prompting sour cells to send a message to the brain.
“Of course, this raises the question of why carbonation doesn't just taste sour,” Ryba says. He says the answer likely lies in the somatosensory system, which transmits information about touch, pain and temperature to the brain. “We know that carbon dioxide also stimulates the mouth's somatosensory system. Therefore, what we perceive as carbonation must reflect the combination of this somatosensory information with that from taste.”
The body senses carbon dioxide on many levels—in the somatosensory system, smell, and in the brain and blood to control respiration. But why would mammals need to taste carbon dioxide? One possibility is that it might serve an important role—for instance, as a way to avoid fermenting foods, which have high levels of carbon dioxide.
“Although we don't know for certain,” Ryba says, “we favor the explanation that carbon dioxide detection by taste may be accidental, occurring simply because of the presence of CA-IV on the surface of the sour cells. We think CA-IV is there to maintain the pH balance and health of the taste buds rather than to act as a carbon dioxide detector.”
If this explanation turns out to be true, then the familiar, unique and attractive sensation we get when we sip a carbonated drink is just a fortunate consequence of a much more fundamental biological need.