Physicists explain fireballs erupting from grapes in microwave oven

Physicists explain fireballs erupting from grapes in microwave oven
Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1818350116

A trio of researchers with McMaster, Concordia and Trent Universities has solved the mystery of why pairs of grapes ignite into fireballs when cooked together in a microwave oven. In their paper published in Proceedings of the National Academy of Sciences, Hamza Khattak, Pablo Bianucci and Aaron Slepkov claim that the fireball is not the result of heat from the outside of the grapes making its way in, but instead comes about due to hotspots that form in both grapes.

Back in 2011, impressive videos of grapes igniting in microwaves went viral on YouTube. All a person had to do was cut a in half, leaving the two halves connected by a bit of skin at the bottom, and heat them in a —within seconds, a tiny fireball would appear between them. Making things even more exciting was that nobody could explain it. Since that time, many armchair scientists have presented possible explanations—one of the more popular was the suggestion that the grapes somehow form an antenna directing the microwaves across the skin bridge. In this new effort, the physicists in Canada ran multiple tests on the grapes and other similar objects to learn the true reason for the formation of the fireball.

The tests consisted mostly of using to capture the action as the grapes were heated and running simulations. They also tested other similarly sized fruit and plastic balls filled with water.

The researchers found that the formation of the fireball was the result of a simple process. As the microwaves enter the grapes, hot spots form in both pieces at the points where they are closest to one another due to a bond between them. As the hot spots grow hotter, surrounding electrolytes become supercharged, resulting in the formation of a burst of in the form of a small fireball.

The researchers note that the same effect could be produced using similarly sized fruit or water-filled balls. They also found that it is not necessary to maintain any sort of physical connection between the two pieces—all that is required is that they be no more than three millimeters apart.

Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1818350116
Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1818350116

Explore further

Genomic study reveals clues to wild past of grapes

More information: Linking plasma formation in grapes to microwave resonances of aqueous dimers, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1818350116

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Feb 19, 2019
I am going to go try this experiment now.

Feb 19, 2019
Oh for crying outloud!
Now we're going to have to put up with more of the prattling claims plasmannoying loons claiming that here is proof of a plasma universe or some such drivel.
That the vast Uranus Conspiracy had hidden from Mankind for centuries.
Until the courageous efforts of youtubers exposed the previously, completely unknown, officially unacknowledged & never published, repressed in publication of the existence of plasma.
Conveniently ignoring the scientists, using computer modeling & running recorded experiments to explained this effect.

Cause good lord, I have always been concerned that a grape I have bit into might explode into a plasma fireball!
Though come ti think of it?
That would be a pretty neat party trick!
Especially if you first had a slug of cheap, single filtered vodka. The fusel oil would ignite an impressive flame.

Has the drink "Dragon's Breath" been trademarked?
If not?
I'm staking a claim.

Feb 19, 2019
Saw the most recent explanation on YouTube yesterday. Something about attenuation of the MW
wavelength matching the size of the grape. Veritasium channel.

Feb 19, 2019
I see that the EO [Electric Oven] idea is alive and well; too bad about the usual smelly mess it leaves.

"Now in sour grapes!"


Feb 19, 2019
A trio of researchers . . . has solved the mystery . . . claim that the fireball is not the result of heat from the outside of the grapes making its way in, but instead comes about due to hotspots that form in both grapes.


That is more of an observation, not an explanation. We can see the hotspots with our eyes, but not everything heats up that much, so the question is what is causing grapes to be so susceptible to this phenomenon? I think Whydening Gyre's comment may be getting warmer, pun intended. :-)

Feb 19, 2019
This is really cool. I did cut up a few grapes and each time was a different "fireworks" display. Awesome! Just the burnt smell was a little offensive. I touched the grapes soon afterwards each time and none were especially hot to the touch. Just warm.

My guess is that the wavelength of the microwaves (approx. 12cm) is causing large currents to flow between each cut half thus setting in of fire. That portion was always burnt with ashes.

Feb 19, 2019
Saw the most recent explanation on YouTube yesterday. Something about attenuation of the MW
wavelength matching the size of the grape. Veritasium channel.

Yes, I watched it too.
A better explanation too.

Feb 19, 2019
Saw the most recent explanation on YouTube yesterday. Something about attenuation of the MW
wavelength matching the size of the grape. Veritasium channel.

Yes, I watched it too.
A better explanation too.

Feb 19, 2019
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Looks like Anonyms Physorg account has been hacked....

Feb 20, 2019
@Bart_A
This is really cool. I did cut up a few grapes and each time was a different "fireworks" display. Awesome! Just the burnt smell was a little offensive. I touched the grapes soon afterwards each time and none were especially hot to the touch. Just warm.

I was wondering if the fireworks are big enough to destroy the microwave? I don't want to try it if it destroys my oven. Or maybe placing a glass over them can keep the sparks from getting too big.
My guess is that the wavelength of the microwaves (approx. 12cm) is causing large currents to flow between each cut half thus setting in of fire. That portion was always burnt with ashes.

One video showed the wavelength is 12 cm, but in water (the medium of the grape) its shorter. It ends up being about the size of the grape, so you get a standing wave inside the grape.

Feb 20, 2019

Looks like Anonyms Physorg account has been hacked....


Reported, already twice ;)

Feb 20, 2019
has anyone tried this experiment comparing seedless grapes with those still including seeds?
Did it make a difference for the plasma discharge?

has anyone a multi-spectrum camera to measure differences in discharges?

All I have are seedless grapes.

Feb 21, 2019
I tried it with seedless grapes. Worked fine each time.
Arcmetal, there was no damage to my microwave. Just flames shooting up from the center. All I had to do was air out the room to get rid of the burnt smell.

Feb 21, 2019
I tried it with seedless grapes. Worked fine each time.
Arcmetal, there was no damage to my microwave. Just flames shooting up from the center. All I had to do was air out the room to get rid of the burnt smell.

From my understanding of the Veritasium video, you didn't even need to cut the grape to have it develop a plasma in it....
I'm out o grapes. I wonder if blueberries would work?
Does it just burn the skin? Or the whole face of the cut area?

Feb 21, 2019
Just tested it.... Blueberries just boil...

Feb 25, 2019
The same effect can be seen when defrosting frozen blueberries. Why..,oh why is all of this important?

Feb 25, 2019
@Bart_A
Yes, it did not burn down my oven.

It works. I was worried my grapes were too big and it would just cook them, but no, it produced flames bursting out between the two cut halves of the grapes.

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