HOW COME?: Why some foods make us pucker up

Bite into a slice of lemon, and there it is: that involuntary grimace. But ever had a similar reaction to the first swig of a fizzy soda? Turns out, there's a connection.

Scientists say there are at least four basic tastes - sweet, salty, sour and bitter. Some add a fifth - umami - the savory taste in soy sauce and other foods, courtesy of an amino acid called glutamate. Human beings are primed to like sweet and salty tastes. But it's a more complicated story when it comes to bitter and sour.

Take babies who haven't had the chance to associate bitter with too-strong coffee, or sour lemons with lemonade. When given a taste of something bitter, infants make an expression of disgust and turn their faces away. But when tasting something sour, like lemon juice, a baby's reaction is often milder. Lips pucker, nose wrinkles, eyes narrow and the infant may close his mouth or frown. But wait a few months, and the same baby may actually grin at a lemony taste.

Why the difference between bitter and sour? Bitter can indicate a poison, so the brain is hard-wired to reject the taste. A sour taste may mean a food is spoiled (and possibly full of harmful bacteria). But it can also mean "Here comes a delightful vitamin C-packed lemon or lime" or a tart cup of fresh yogurt. So while we may scrunch up our faces at the first taste of a sour gummy worm, we may also decide that it's harmless and tasty. Even though our knee-jerk reflex is to clamp our lips, we soon learn what's safe to eat . . . and what should be poured down the drain or tossed in the trash.

Still, some people enjoy sour tastes more than others do, and studies show that the preference is actually inherited from our ancestors. Researchers gave water spiked with varying amounts of citric acid (the acid that makes lemons sour) to pairs of twins. They found that some people were a thousand times as sensitive as others to even a hint of sourness. And identical twins, who share the same genes, were much more similar in their sour sensitivity than were fraternal twins.

It wasn't until 2006 that researchers announced they'd found the sour taste receptors on the tongue. And in 2009, researchers found another piece of the sour mystery, which may explain why carbonated soda can make us grimace a bit, too. In studies done with mice, they discovered that we taste carbonation thanks to a protein on the surface of the tongue's sour receptors. The brain decodes the signals from the activated sour receptors, and the resulting sensation combines a trace of sourness with that fizzy feeling.

Surprisingly, it's not the carbon dioxide bubbles bursting that creates the fizz. Even in a pressure chamber, scientists say, where the bubbles don't burst, the sensation is the same. So for an extreme activation of your tongue's sour-taste receptors, try a lemon-lime soda. And be prepared to pucker up.