Miracle fruit secret revealed

Berry sensitizes tongue's sweet sensors to acidic flavors

Scientists have finally explained how a little red berry makes just about anything, from the sourest lemon to the bitterest beer, taste as sweet as honey. A protein found in the fruit tickles the tongue’s sweet-sensing machinery, its effects intensifying in the presence of acidic flavors like citrus and carbonated drinks.

Researchers and foodies alike have long known the effects of the miracle fruit (a.k.a. Richadella dulcifica). At flavor-tripping parties, guests will pop a berry then chew, chew and chew some more, letting the masticated fruit linger on the tongue. Then the sampling begins: Guinness tastes like a chocolate shake, Tabasco loses its sting and pickles their mouth-pinching tang. Lemons and limes gush with sweetness.

While the active ingredient in miracle fruit — miraculin — has been known for decades, it hasn’t been clear exactly how the protein confers its sweetness. Now scientists in Japan and France report that miraculin’s interaction with the tongue’s sweet sensors depends on the acidity of the local environment. At a pH of 4.8 (water is neutral with a pH close to 7), the sweet-tasting cells respond twice as vigorously to miraculin than they do at a less acidic pH of 5.7. At closer-to-neutral pH levels of 6.7 and higher, the protein seems to slightly shift shape, blocking the sweet sensors but not activating them. This explains why under certain conditions sweet foods may taste less flavorful after eating the berry, researchers led by Keiko Abe of the University of Tokyo report online September 26 in the Proceedings of the National Academy of Sciences.

The weird thing about miraculin’s ability to tango with human sweet sensors is that in general proteins — think of a nice ribeye — aren’t known for tasting sweet. Plants usually pack their fruits with sugars to please the palates of animals, which gobble up and distribute the seeds inside. But, in a trick of nature, some plants use protein instead of sugar to deliver a sweet punch, says sensory biologist Paul Breslin of Rutgers University in New Brunswick, N.J.

“Many of these small little berries are fruits that probably can’t generate a whole lot of sugar,” says Breslin, also of the Monell Chemical Senses Center in Philadelphia. “But they can make a protein that can trick us into thinking there is sugar.”

Miraculin is one of a handful of such plant proteins (as is monellin, named after the research institution.)

While miraculin packs a sweet wallop, the U.S. Food and Drug Administration hasn’t approved the protein’s use in food. And since it is a protein, it probably would fall apart if heated, making it a poor candidate for baked goods anyway, says Breslin. For now, it must make do being the limelight of parties.

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