Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated soft drinks.
Carbonation can also describe a chemical reaction, one example of which is a key step in photosynthesis.
Carbonation also describes the incorporation of carbon dioxide into chemical compounds. Our carbon-based life originates from a carbonation reaction that is most often catalysed by the enzyme RuBisCO. So important is this carbonation process that a significant fraction of leaf mass consists of this carbonating enzyme.
In 1767, chemist Joseph Priestley stood in his laboratory one day with an idea to help English mariners stay healthy on long ocean voyages. He infused water with carbon dioxide to create an effervescent liquid that mimicked the finest mineral waters consumed at European health spas. Priestley's man-made tonic, which he urged his benefactors to test aboard His Majesty's ships, never prevented a scurvy outbreak. But, as the decades passed, his carbonated water became popular in cities and towns for its enjoyable taste and later as the main ingredient of sodas, sparkling wines, and all variety of carbonated drinks.
Missing from this nearly 250-year-old story is a scientific explanation of how people taste the carbonation bubbling in their glass. In this week's issue of the journal Science, researchers at the National Institute of Dental and Craniofacial Research (NIDCR), part of the National Institutes of Health, and their colleagues from the Howard Hughes Medical Institute at the University of California, San Diego (UCSD) 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, 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, Ph.D., a senior author of this study and an NIDCR scientist. "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."
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.
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," he said. "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."
