How come when you eat sour candy you make weird faces?
How come when you eat sour candy you make weird faces? asks Danielle Vollono, a student in Brookville, NY.
It’s how we decode dinner: Scientists say there are at least four basic tastes — sweet, salty, sour and bitter. (Some add a fifth, umami, the savory taste provided by an amino acid in food called glutamate.) While we have generally pleasant reactions to sweet and salty tastes, it’s a different story with “bitter” and “sour.” “Bitter” may mean poison, so our brain is hard-wired to reactive negatively to the taste. “Sour” may mean a food is spoiled, and full of harmful bacteria.
When given a taste of something bitter, newborn babies make an immediate expression of disgust, and turn their faces away. But when tasting something sour, like lemon juice, an infant’s reaction is usually slower and milder. The lips purse and pucker, the nose wrinkles, the eyes narrow. Over a period of seconds, the baby may close her mouth and retract her lips, or frown.
But as babies get a few months older, some may actually smile at the taste of lemon juice. Human beings seem to have a love/hate relationship with sour tastes, even at 4 months old. Which is why some of us enjoy endless varieties of sour candy — and lemons — even as we pucker our lips and scrunch up our face.
Until the last few years, scientists hadn’t found the sour-taste receptors they suspected were lurking on our tongues. Then, in 2006, researchers announced that they had narrowed down the search to tongue cells containing two proteins, PKD1L3 and PKD2L1.
The proteins combine to create “ion channels,” allowing electrically charged calcium atoms (ions) to flow in and out of cells. This flow of ions, in turn, allows signals to travel to the brain, where they are decoded as different tastes. In the experiments, sweet, salty, and bitter solutions caused the channels to stay closed. But when sour-tasting acids were introduced, the channels opened.
The studies also uncovered a fascinating connection: One of the tongue’s sour-sensing proteins, PKD2L1, is also lurking in a group of neurons on the spinal cord. There, the protein may monitor the acidity of cerebrospinal fluid that bathes the spinal cord and brain. In a sense, the protein lets the spinal cord “taste” the fluid. (Scientists say the protein is a good example of evolution’s multitasking ways.)
A recent study by scientists at the Monell Chemical Senses Center in Philadelphia found that our reaction to sour tastes may lie in our genes. Researchers gave water with varying amounts of citric acid to pairs of identical as well as fraternal twins. (Fraternal twins are no more closely related than any two siblings in a family.) They found that some people were a thousand times as sensitive as others at detecting a slightly sour taste. Identical twins, who share the same genes, were much more similar in sour sensitivity to each other than were fraternal twins. The conclusion: Our reaction to sour tastes — where we fall on the Pucker Index — is “highly inherited.”
