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How the miracle fruit changes sour into sweet

Pop a “miracle berry” into your mouth, and you might wonder if it was named by an overreaching marketing department. The small red fruit tastes of very little – it has a “mildly sweet tang… [like] a less flavorful cranberry”. But it’s not the taste of the fruit itself that matters. To understand why the berry gets its name, you need to eat something acidic. The berries have the ability to make sour foods taste deliciously sweet. Munch one, and you can swig vinegar like it was a milkshake, or bite lemons as if they were candy.

The secret to the fruit’s taste-transforming powers is a protein called miraculin. Now, Ayako Koizumi from the University of Tokyo has discovered just how the protein acts upon our tongues.

Two groups of scientists independently isolated miraculin from miracle berries in 1968, but people have been experiencing its effects for far longer. West Africans have chewed miracle berries (Richadella dulcifica) before their meals for centuries, to get those unusual sweet hits from otherwise sour food. Europeans became aware of the fruit in 1725, when French explorer Chevalier des Marchais described its use.

In the 1970s, an American company called Miralin tried to develop miraculin as a simple way of getting a sugar rush without having to gorge on cakes and sweets. Their attempt was nixed by the US Food and Drug Administration, who decided to classify miraculin as an additive, dooming it to years of further testing. Miralin folded, amid suspicions of foul play from the sugar industry. The miracle berries never quite hit the big time, but they do make occasional appearances at high-class events known as “flavour-tripping parties”.

While party-goers thrilled at the miracle fruit’s tongue-teasing trick, scientists were equally intrigued. Various groups have pored over miraculin’s properties in great detail. Thanks to their efforts, we know a lot about the protein’s structure, and how acidity, temperature and dose affect its taste-altering powers. By comparison, we still know very little about how it actually works.

The general idea is that miraculin changes the shape of proteins on our tongues called sweet receptors. These proteins are normally set off by sugars, but when miraculin disfigures them, they respond to acids too. Suddenly, sour mouthfuls are registered as sweet ones.

According to Koizumi’s experiments, this explanation is not quite right. She showed that miraculin does stick directly to sweet receptors, and it latches on more strongly than do other conventional sweeteners like aspartame or saccharin. In neutral conditions, neither acidic nor alkaline, miraculin stops these other sweeteners from getting a hold on the sweet receptors. It actually represses the receptors, stopping them from doing their job. Under acidic conditions, the opposite happens – miraculin supercharges the sweet receptors. It distorts them into an active shape, while also making them extra-sensitive to sweeteners like aspartame.

Here, then, is what happens when you chomp on a miracle berry. Miraculin sits on your sweet receptors for an hour or so. For most of that time, it silences the receptors, which is why the fruit itself tastes of very little. Whenever you take a bite or swig of something acidic, miraculin gains a few extra protons and changes shape. In doing so, it also changes the shape of the sweet receptors it has stuck to, sending them into a signalling frenzy.

Koizumi also looked at a second taste-changing protein called neoculin. This one comes from the Malaysia lumbah plant; it also converts sourness to sweetness, and it also sticks to sweet receptors. But the similarities stop there.

Amazingly, miraculin and neoculin are completely unrelated. They have different shapes and sizes, they are made from different chains of amino acids, and they stick to different parts of the sweet receptor. Unlike miraculin, neoculin tastes sweet in its own right, and it transforms sour tastes into an even stronger sweet buzz. The miraculous aspect of these two proteins is not that they turn sour into sweet, but that they have evolved to do the same thing through two completely different means.

Reference: Koizumi, Tsuchiya, Nakajima, Ito, Terada, Shimizu-Ibuka, Briand, Asakura, Misaka & Abe. 2011. Human sweet taste receptor mediates acid-induced sweetness of miraculin. PNAS http://dx.doi.org/10.1073/pnas.1016644108

Image by Hamale Lyman

17 thoughts on “How the miracle fruit changes sour into sweet

  1. Do we know anything about these berries in their natural environment? How are they dispersed? The effect is extremely interesting, but I would be even more interested in its ecological context and evolutionary history.

    Edit: Following up on that, it would be good to see what these berries taste like with the protein inactivated. Someone needs to make some miraculin-knockout miracle berries and eat them. If they taste terrible, it might suggest synthesising miraculin to artificially sweeten the berries is less costly for the plant than producing heaps of real sugar would be.

    But in that case, the question becomes: why haven’t more plants hit upon the trick?

  2. I think the reason “more plants haven’t hit upon the trick” is that fruit sugars serves a dual purpose – to aid in seed dispersion and also to provide nutrients for the seeds once they land in the soil. While miraculin is great for the former, I’m not sure it will do too much for the latter.

  3. Cathy – Is that true? I would have imagined very little of the sugar survives the passage through the animal’s digestive tract. One would think that any starch or sugar for the seed to use for its own growth would be in the endosperm rather than the flesh of the fruit. The faeces of the animal would of course contain lots of nitrogen and other helpful organic matter, though.

    Also: is this article available online? The DOI isn’t working and I can’t find it on the PNAS site (or elsewhere).

  4. A great article. You always find the best stuff out there, Ed. I love cases of convergent evolution. Interestingly, following Cathy’s comment, if neoculin is sweet by itself, then it should be better at seed dispersion than miraculin which needs to be eaten together with something sour to elicit the perception of sweetness, am I correct?

    (Yes, DOI is not working, and I couldn’t find it on PNAS either, probably because of the embargo; it should be online later this week, I think)

  5. Really cool! This is the first that I have heard of a protein that changes conformation after binding to the receptor (due to changes of the environment) and effectively leading to receptor activation. Wonder how long this effect lasts?
    Anyhow, one potential use that pops up to my mind is “drug delivery” of some sour/acidic oral drugs.

  6. Yes I have experience the effect of this miracle berry. Sour green mangoes!and limes, just turned very very palatable sweet. We only got 2 berry and for.all.of us to.experience we share it among 6 of is.
    What was very interesting observarion was that.the.length.of time each one of us to experince the sweet.taste sensation varies.
    The plant is easy to maintain and the seed is easy to propagate. You can grow it in a pot.

  7. I wonder whether there’s some advantage to the plant in getting its seeds spread by having these proteins in the fruit. Do animals eat it for the same reason people do? It’s interesting that the two proteins have nothing in common too. It’s a nice piece of convergent evolution if there is an advantage to the plant in having a special effect on sweet receptors.

  8. Besides the wonderful biochemical trick of the miracle fruit, I can’t stop thinking about the philosophical question it entitles. Is reality what we experience with our senses, or what exists without an observer. In a few words, just like some drugs, the miracle berry changes reality. Every human should do the exercise of eating the berry before and after eating limes, so that we’d be convinced that every person experiences life differently depending on the context and that doesn’t make it less valid.

  9. Veronica,

    Your philosophical question is way overdone. Taking LSD and changing what you experience with your senses does not change reality. There is nothing philosophical about this. Now the evolutionary question this fruit entitles… THAT is interesting.

  10. I am wondering what is the evolutionary benefit – to humans or animals – of using the berry to consume acidic foods that we might not otherwise consume. To the best of my knowledge, our taste buds are there for a reason. We notice the acidity and perhaps consume less of the food (hello, ulcer people). So I’m just wondering, other than the sheer fun of doing it, is there some physiological benefit to the body of making acidic/sour foods taste sweet so that we might consume more of them?

    That’s what I’d like to know…

  11. @TiddlyM – I see this is a bit of an old post, but you bring up a good point. For me, the point is not to consume more acidic foods, but to enjoy sweet low calorie foods. For example, you can take a glass of soda water and add a few drops of fresh lemon and lime juice. Eat a miracle fruit and you have a delicious lemon-lime soda that is all natural and nearly zero calories (yes, all of the major soda makers R&D departments have worked with me to can this, but they can’t get past the fact that after drinking the soda, it will modify the flavor of other foods). You can eat strawberries and they taste like they are dipped in sugar. You can make a vegetable platter and drizzle it with lime juice and kids will eat it like a box of chocolates…

    Also, this is currently being used by cancer patients to consume foods they normally would not be able to eat during chemotherapy.

    As far as evolutionary benefit to animals, I don’t know, but I can tell you all animals seem to LOVE the fruit (from squirrel to birds). If you leave a plant outside here in Florida, the wildlife will quickly eat all the fruit.

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