A group of us, some scientists but mostly not, have just had dinner together at the New York Academy of Sciences. We've eaten well—broiled salmon with spears of asparagus, a leafy salad, goblets of green-tea ice cream, wine and coffee for those who want them—and settled down for the evening's lecture. The speaker, from Yale University, is Linda Bartoshuk, a specialist in human taste. She hands us each a small packet containing what looks like a Communion wafer. It's a piece of filter paper saturated with a compound called propylthiouracil, known in taste circles as PROP. We're instructed to put the papers in our mouths. As my saliva wets it, a nasty bitterness blooms. My neighbor, too, is making a face that says yuck. Then Bartoshuk asks for a show of hands. How many of us tasted something? How many of us didn't? Of course, Bartoshuk knows the punch line: Typically, a quarter of the audience tastes nothing. This routine never fails to impress an audience. Jaws go slack as hands shoot up in answer to both questions. How can something be tasteless to some people and so unpleasantly bitter to others? If we trust our senses to reflect the real world, the answer seems unnerving: We may think we all ate the same dinner tonight—but we don't all live in the same taste world.
Actually, researchers began suspecting as much in the 1930s, after a chemist who was making a batch of a compound called phenylthiocarbamide (PTC) let a puff of the crystals fly into the air. A lab colleague, who must have swallowed some of the airborne crystals, noted how bitter they were. The astonished chemist, who tasted nothing himself, became the first to describe "taste blindness" to the bitter compound. Of course, taste blindness to PTC or its chemical cousin PROP might just have been a scientific curio—so-called nontasters do respond to other types of bitterness. But, as it turned out, nontasters respond to all types of bitterness less intensely than tasters, and the degree to which people taste PROP can serve as a general indicator of their overall taste capacity.
Judging from family studies, the inability to taste PROP is genetic and most likely due to a recessive gene. That would fit rather nicely with Bartoshuk's finding that there exists a subset of PROP-tasters supersensitive to bitterness. She calls them super-tasters. Looking at the three groups, you see just the sort of patterns you'd expect for a recessive gene. Roughly 25 percent of people tested with PROP don't taste it, consistent with two recessive genes; 50 percent are tasters, consistent with one recessive and one dominant gene; and 25 percent are super-tasters, consistent with two dominant gene copies, one from each parent. The distribution is slightly sex-skewed. More women than men are super-tasters, perhaps because bitter is the skull and crossbones of the natural world, and evolution once favored mothers with superior poison-detecting systems.
What's fascinating, says Bartoshuk, is that tongue anatomy spells out the differences among these groups. Tasters have more taste buds than nontasters, and super-tasters have the most. Because taste buds are surrounded by nerve endings that sense not only taste but pain and touch, super-tasters, perhaps not surprisingly, have a more sensational taste repertoire. Bitter tastes bitterer, salt a bit saltier, sour sharper, and some sweets sweeter. Fat feels fattier, gums thicker, and alcohol and chili burn more fiercely. The inside of a nontaster's mouth makes up "a very small world compared to the super-taster's," says Bartoshuk, a non-PROP-taster herself. But because the super-tasters' sensory realms are so intense, they may avoid strong tastes—especially bitter ones like grapefruit, coffee, beer, or broccoli—and thus actually shrink their dietary horizons.
