Drinking a carbonated beverage often causes a distinct, sharp tingle or sting in the mouth and throat, frequently described as a burn or a bite. This sensation is not caused by the physical bursting of gas bubbles, but rather by a complex chemical reaction. Carbonation, the process of dissolving carbon dioxide (\(\text{CO}_2\)) gas into a liquid under pressure, triggers molecular and neurological processes that cause this characteristic feeling.
The Chemical Culprit
The “sting” begins when carbon dioxide (\(\text{CO}_2\)) from the beverage meets the water and saliva in the mouth. \(\text{CO}_2\) gas readily dissolves in water, initiating a chemical reaction that produces carbonic acid (\(\text{H}_2\text{CO}_3\)). This reaction is represented by the formula \(\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3\).
Carbonic acid is a weak acid that only partially breaks down in the solution. When it dissociates, it releases free hydrogen ions (\(\text{H}^+\)), which lowers the overall pH of the surrounding liquid. This increase in acidity, not the mechanical pop of the bubbles, is the chemical source of the irritating sensation.
This conversion is accelerated by carbonic anhydrase, an enzyme present in saliva and on the surface of some sensory cells. The enzyme quickly catalyzes the reaction of \(\text{CO}_2\) and water into carbonic acid. This rapid production of acid near the sensory nerve endings generates a strong and immediate chemical signal.
Sensory Receptors and Detection
The body detects this rapid change in acidity through specialized nerve endings called nociceptors, which signal irritation. These sensory neurons, which innervate the oral, nasal, and respiratory epithelia, express a particular protein channel. The primary sensor for this chemical irritant is the Transient Receptor Potential Ankyrin 1 (\(\text{TRPA}1\)) ion channel.
The \(\text{TRPA}1\) receptor, sometimes called the “wasabi receptor,” detects chemical irritants like mustard oil. When carbonated liquid is consumed, \(\text{CO}_2\) molecules pass through the cell membranes of the sensory neurons. Inside the cell, the \(\text{CO}_2\) is converted into carbonic acid.
This intracellular conversion increases internal protons, causing localized acidification of the cell’s interior. This buildup of intracellular protons directly activates the \(\text{TRPA}1\) channel. Once activated, the channel opens to allow ions to flow into the neuron, generating an electrical signal that travels toward the brain.
How the Brain Interprets the Sting
The electrical signal generated by the activated \(\text{TRPA}1\) receptors travels along the sensory nerves, primarily via the trigeminal nerve, to the central nervous system. The trigeminal nerve relays sensations from the face, nose, and mouth, including pressure and irritation.
When the brain receives this signal, it interprets the stimulus as chemogenic pain or irritation (nociception). This is why the sensation is felt as a sharp tingle, pungent feeling, or burn, rather than a specific taste like sourness. Carbonation triggers the same protective neural pathway that alerts the body to mild chemical threats.
The intensity of the feeling relates directly to the concentration of \(\text{CO}_2\) and the resulting acidification level. The brain integrates this initial chemical signal of irritation with other sensations, such as cooling and bubble pressure, to create the unique “fizz” of a carbonated drink.