The miraculous miraculin
I've held off writing about the so-called miracle fruit. Partly because it got plenty of airtime on QI, the BBC/ABC show that steals so many of our good ideas for stories. But also because I have great trouble with words like miracle, unexplained and supernatural. As for the word 'fruit', don't get me started. Tomato anyone?
I chewed on my first miracle fruit a month or two ago, after a talk by Alan Carle on his tropical fruit and rainforest garden near Port Douglas called Botanical Ark. The talk was held further south, in the lovely Flower Temple in Melbourne's Artemis Lane.
Alan's wife Susan handed out the fruits and we had to chew them thoroughly before we were offered a piece of that green citrus fruit with the generally sour taste. Sure enough, the lime tasted sweet. Not pleasantly so I'd have to say. A kind of syrupy sweetness, with an after taste a little like those some artificial sweetners that you tricked into eating and drinking on occasions.
So what is the Miracle Fruit and why is it so miraculous? Botanically it is the fruit or berry - a fleshy structure bearing seed - of a West African shrub called Synsepalum dulcificum (or sometimes Richardella ducifica). The Wikipedia cloud tells me there are at least two other plants producing fruits that change our perception of sweetness - Gymnema sylvestrei and Thaumatococcus daniellii - but let's stick with the one QI and I favour.
The Miracle Fruit grows on the edge of forests and in farmland in tropical Africa. It appears to be a colonising species, ready to spring up after disturbance of some kind. The fruits, as you can see, are footy shaped and coloured, but a bit smaller - in fact only a few centimetres long.
Inside the chewy berry pulp is a relatively large single seed. One masticates the fleshy outside then spits out the seed (and maybe the remaining pulp). It's then time for the lime! I think this look on Lynda's face means the lime wasn't as a sour as she expected.
The chemical responsible for this miraculous feat is fittingly called miraculin. Yes really. It's a kind of glycoprotein. Another glycoprotein you might be more familiar with is mucin, in your mucous.
Miraculin isn't sweet and it doesn't block the sour taste. The best description I could find of its miraculous mechanism was in an ABC Science report called Miracle Fruit's Trippy Effects Explained (although Dr Karl Krusulnicki does a lovely job of reexplaining and contextualizing the whole business in a couple of his posts as well). Miraculin binds to the receptors on our tongue that sense sweet things, but only when the you add something acidic (e.g. suck a lime). And when miraculin binds, in the presence of acid, the receptors give you that sweet sensation.
By chewing the fruit you are covering the surface of your tongue with miraculin, ready to react as soon as an acidic solution enters the mouth. Even after you swallow the lime juice the miraculin remains bound to the sweet receptors for up to an hour- inactive, but ready to switch to sweetness when you suck your next lime.
An interesting application of this fruit, other than as a party trick, is for chemotherapy patients who often have their taste distorted by the treatment. According to the ABC Science Site, it's claimed the miracle fruit will improve the taste of foods for some patients.
Miraculin has been used in West Africa for centuries to make sour foods and drink palatable. Only in recent decades has mass production, for local as well as international markets, been considered. You can read more about the propagation of the Miracle Fruit in this paper from the African Journal of Botany.
In the meantime, unless you are undergoing chemotherapy, I recommend enjoying the unadulterated acidic taste of lemons, limes and the like. Save your sweet tooth for cakes and confectionery. I know sugar cane gets a bad rap, but miraculous sweetening isn't quite the same.