With miracle fruit, sour tastes sweet

Over the last few weeks, several people have asked me about so-called miracle fruit and its unusual properties. For about an hour or so after munching on its pulp, people find that any sour foods they eat taste remarkably sweet. Reportedly, the berry is even being featured at "flavor parties" -- where guests are handed a miracle fruit at the door and then turned loose at a buffet laden with tart fare.

While flavor parties may be a creating a stir, the fruit's odd properties have been known for a long time. It was nicknamed "miraculous berry" in the 1850s by European travelers who experienced its effects while traveling in tropical West Africa, and the fruit has been cultivated in Florida for more than 50 years.

Miracle fruit comes from a shrubby plant known to botanists as Synsepalum dulcificum or Richadella dulcifica. Its secret ingredient is a compound called miraculin -- a small protein that tricks the sweet receptors on the tongue into being stimulated by acids instead of sugars. This deception tells us that cider vinegar tastes like apple juice and that lemons taste like lemonade.

Normally, various tastes activate specific taste receptor cells housed within the bumpy papillae on the tongue -- sugars bind to sweet receptors, salt binds to salt receptors and so on -- sending messages to the brain that tell us what we are tasting is sweet or salty, bitter, sour or meaty. Miraculin doesn't bind to any taste receptors and, in fact, by itself elicits no flavor at all as long as there's no acid in the mouth.

But when sour foods are placed on the tongue just after the miracle berry is chewed, the shape of the sweet receptors are modified in such a way that miraculin binds firmly to them -- like a key in a lock -- stimulating the effect of a strong, sweet taste in the mouth. Because the compound binds so strongly, the effect can last a couple of hours.

Miraculin is just one of seven curious compounds found in a number of unusual tropical fruits. Scientists refer to them, depending on their particular properties, as taste-modifying proteins or sweet proteins. Taste-modifying proteins like miraculin are not sweet themselves, but are able to alter the taste of other foods. Sweet proteins don't alter taste, but they taste extremely sweet. One remarkable protein called curculin, found in the fruit of Curculigo latifolia (native to Malaysia), exhibits both properties -- it's more than 500 times sweeter than sugar and it makes water and sour foods taste sweet.