Stupid question of the day: why do carbonated beverages taste so good? What is it about CO2 dissolved in water that so delights our taste buds?
At least I thought it was a stupid question, or perhaps it’s more correct to say that I feared it, that I was afraid there was a very obvious answer that I couldn’t see. Thankfully for my ego, it turns out that there’s nothing easy or obvious about it. Science has yet to completely explain this. Science is pretty clueless, in fact. This is probably because perception is so complex. It won’t do to simply study the chemical reactions between food and taste neurons in the tongue, for instance, since the brain massages this raw data and combines it with other perceptions, such as those coming from pain and olfactory sensors, in order to create what we subjectively experience as “taste”. In a completely unscientific anecdote, I know a woman, a neighbor of my parents, who has lost her sense of smell almost completely. She can hardly taste anything, either, because of the way the senses of taste and smell are linked.
That’s not to say that we don’t know anything about how this works. For a long time, the best hypothesis around was that carbonation simply stimulates our pain receptors, giving us a sweet pain reminiscent of the one spicy food generates. This does little to explain why we would find pain to be pleasant, though. Are we all masochists? Brian Palmer writes in Slate:
You can’t really “taste” carbonation. You feel it the same way you feel pain. When soda exits the pressurized environment of the can or bottle and strikes your tongue, carbon dioxide gas rushes out of solution. It then mixes with water and carbonic anhydrase (an enzyme that helps your body move carbon dioxide into and out of cells) to form carbonic acid. When the concentration of carbonic acid reaches a certain level, nerve endings called nocireceptors send pain signals to the brain. This is the reason soda leaves a tingling sensation in your mouth after you swallow it—the carbonation is gone, but the carbonic acid is still around.
This was written in July, 2009. As it turns out, that isn’t really true. We do really taste carbonation. This was demonstrated in October last year, in a paper called The Taste of Carbonation. (Much to my delight, I discovered that you can read Science papers that are more than a year old, but published after 1997 with a free subscription. I’ve been complaining about the lack of free access to science for years. Full paper here. It’s quite readable.) The paper doesn’t really explain why carbonation tastes good, but it does demonstrate that we’re capable of sensing it independent from our pain receptors, and it’s fascinating in its own right.
Even if you aren’t interested in the chemistry of the tongue, reading a paper like this one is a great way to see how modern science is really done. You see scientists summarizing a somewhat broad area, and then defining a relatively specific question they want to answer. They start out by describing their setup, but they don’t go into details about everything: instead, they reference papers that describe the knowledge and techniques they’re building on, going into detail only when describing the specifics of what they’ve done that’s different from what other people have done. They start out with various hypotheses, use them to generate predictions, and engineer experiments that will test these predictions while separating the various variables involved.
The researchers describe a series of experiments using genetically engineered mice. First, they created a series of sourless, sweetless, etc. mice, i.e., mice populations where one type of taste receptor was disabled, and tested whether they could still sense carbonation (by studying action potentials sent to the brain). As they had expected, the mice who couldn’t taste sour things couldn’t detect carbonation. Now sure that the carbonation-sensing was happening in the sour taste neurons, they compared messenger RNA (mRNA) from sour-sensing taste buds with mRNA from taste buds that aren’t sour-sensitive. This led them to a gene called Car4, which, in addition to being highly specific to the sour-sensing cells, also encodes carbonic anhydrase 4, “a member of a large family of enzymes implicated in sensing, acting on, and responding to CO2 in various systems, including chemosensation.”
Carbonic anhydrase catalyzes a chemical reaction that turns carbon dioxide (CO2) into hydrogencarbonate (HCO2) and free protons (H+). Slight interjection of high school chemistry: H+ is the essence of an acid. According to the Brønsted definition of acids and bases, an acid is something that’s capable of transferring a hydrogen ion, and, since hydrogen is simply an electron and a proton, when you remove the negative electron to get a positively charged ion, all you’re left with is a proton. So free protons, or the transfer of protons, is really at the core of what acids are all about. Anyway, the researchers guessed that if carbonic anhydrase was responsible for sensing carbonation, the following two things would hold: “(i) pharmacological block of extracellular carbonic anhydrases should abolish CO2 taste responses, and (ii) a knockout of Car4 should selectively affect CO2 taste detection.” They found that inhibiting carbonic anhydrase on the surface of sour cells also inhibited CO2-detection. Next, they tested mice without the Car4 gene, and they showed less response to carbonation, but could still detect it. As it turns out, the small residual response even without carbonic anhydrase 4 was due to other carbonic anhydrases not coded for by the Car4 gene, and inhibiting these also completely removed the response to carbonation.
Hydrogencarbonate doesn’t stimulate our taste buds, which implicates the lowly proton, the other thing that carbonic anhydrase turns carbon dioxide into, as the signal our cells look for to detect delicious fizziness. This isn’t surprising: after all, the proton is what distinguishes acids, and acids are what make things sour. In addition, the researchers tried blocking sour cells from sending messages to the brain, and the result, as expected, was that the mice couldn’t detect either sour things or carbonation.
Thus it would appear that, yes, we do actually taste carbonation. In short, the Car4 gene encodes an enzyme that sits on the edge (membrane) of sour-sensing cells and turns CO2 into acid, which the sour-sensing cells in turn detect and send along as sour sensations to the brain. But, as the researchers note:
Although CO2 activates the sour-sensing cells, it does not simply taste sour to humans. CO2 (like acid) acts not only on the taste system but also in other orosensory pathways, including robust stimulation of the somatosensory system; thus, the final percept of carbonation is likely to be a combination of multiple sensory inputs.
Back to the problem I noted in the beginning: perception is complicated, because many different sensory systems report to the brain, and the brain then puts these into a sensory stew for which we don’t know the recipe, and that stew is what gives us the final, subjective impression of sensing carbonation or spiciness or anything else.
While researching this post, I came upon this, an example of the brilliant way evolution reuses its inventions. We’ve established that sour taste buds detect acidity (protons), which is an indication of pH level. The same protein that serves this function in taste buds “is also found along the entire length of the spinal cord in nerve cells that surround and reach into the central canal.” There, it detects minute changes in pH level, thereby helping our body monitor the health of the cerebrospinal fluid. Human bodies: they’re pretty awesome, right?
Oct 29, 2010