Taste Is Chemistry
Julie Mennella, a biologist who studies the sense of taste in babies and toddlers, often records her experiments on video. When I visited her recently at the Monell Chemical Senses Center in Philadelphia, she showed me a video of a baby in a high chair being fed something sweet by her mother. Almost as soon as the spoon is in the baby’s mouth, her face lights up ecstatically, and her lips pucker as if to suck. Then Mennella showed me another video, of a different baby being given his first taste of broccoli, which, like many green vegetables, has a mildly bitter taste. The baby grimaces, gags, and shudders. He pounds the tray of his high chair. He makes the sign language gesture for “stop.”
Human breast milk contains lactose, a sugar. “What we know about babies is that they’re born preferring sweet,” Mennella said. “It’s only been a couple of centuries since the time when, if you didn’t breast-feed from your mother or a wet nurse, your chance of survival was close to zero.” The aversion to bitter foods is inborn too, she said, and it also has survival value: It helps us avoid ingesting toxins that plants evolved to keep from being eaten—including by us.
Food or poison? Vertebrates arose more than 500 million years ago in the ocean, andtaste evolved mainly as a way of settling that issue. All vertebrates have taste receptors similar to ours, though not necessarily in the same places. “There are more taste receptors on the whiskers of a large catfish than there are on the tongues of everybody in this whole building,” Gary Beauchamp, another Monell scientist, told me, indulging in a little hyperbole. Anencephalic infants, who are born with virtually no brain beyond the brain stem—the most primitive, ancient part—react to sweetness with the same joyful-seeming facial expressions I saw in Mennella’s video. The broccoli grimace is also primitive. In fact, although our tongues have just one or two types of receptor for sweet, they have at least two dozen different ones for bitter—a sign of how important avoiding poison was to our ancestors.