Posted on behalf of Susan Young.
Meet the miracle fruit, the berry of the West African plant Richardella dulcifica. Its surprising ability to make sour foods taste sugary has spawned ‘flavour-tripping’ parties among the culinary chic as well as scientific curiosity about how the trick actually works.
The effect has been compared to a hallucinogen for the taste buds: once the flesh of the bright red berry coats the tongue, sour tasting foods, such as lemons, are magically transformed into sweet delights.
Now, in a study published online today inThe Proceedings of the National Academy of Sciences, a team of Japanese and French researchers offers an answer. They find the protein that is responsible for the taste shift behaves differently depending on the acidity of it surroundings.
That protein — known as ‘miraculin’ — was first identified some 40 years ago. But without an objective method to measure the sour-to-sweet taste modification, it was difficult to clarify the molecular mechanisms of miraculin’s effects, says Keiko Abe, a biochemist at the University of Tokyo and a co-author on the study.
The team had previously described another sweet-to-sour-converting molecule called neoculin. Its amino acid sequence bears no obvious resemblance to miraculin. Yet, neoculin activates the sweet receptor in acidic conditions and inhibits the receptor at neutral pH — so the authors wondered whether miraculin works in a similar way.
To explore the question, the team cultured human kidney cells to produce sweet receptor proteins and treated the cells with fluorescent molecules that lit up when the receptors were activated. The setup allowed the researchers to measure the degree to which the sweet receptors responded to miraculin. What they uncovered were three distinct effects that varied according to pH level.
“At neutral pH, miraculin is flat in taste,” says Abe, and it inhibits the activation of the sweet receptors by familiar sweet molecules such as aspartame and sucrose. On the other hand, in weakly acidic conditions, miraculin boosts the response of the receptors to other sweeteners. At still higher levels of acidity, it activates the receptors all on its own.
This is the first case of a protein that seems to possess all three properties, says Liquan Huang, a taste-sensation biologist at the Monell Chemical Senses Center in Philadelphia. “It will be very interesting to know how exactly pH alters the structure of this protein.”
Beyond sheer fascination in the miracle berry’s taste illusion, its taste-bud trickery could have practical applications. “Industrially, we are interested in a large-scale production of miraculin because it has a good, sucrose-like taste and combines a non-caloric property, since developing a safe sweetener for anti-diabetes and anti-obesity uses is of pressing importance,” says Abe.
Photo Credit: Hamale Lyman