Physicists Finally Discover Why Grapes Ignite in the Microwave

By Nathaniel Scharping | February 18, 2019 5:14 pm

A conflagration waiting to happen. (Credit: Kaiskynet Studio/Shutterstock)

Consider the humble grape. Small, spheroid, with pleasantly taut skin, leaving a burst of sweetness on the tongue. Hardly a fruit you’d need to defend yourself against.

Put a gently touching pair in the microwave, though, and the inoffensive fruit turns into a literal firecracker. Within just a few seconds, microwaved grapes will begin sparking as if electrified, and in some cases they’ll even produce a flash of plasma bright enough to make the microwave glow from the inside out. (An alternate method involves cutting the grape in half, leaving a strip of skin to connect the hemispheres. Both produce the same effect.)

It’s been a popular Youtube stunt for years now, but the actual physics behind exploding grapes was elusive. It’s obviously not the same process that produces sparks from metals, and, oddly, the same thing doesn’t happen for other foods — say, peas. It also didn’t help that little legit scientific research had been done into the phenomenon.

But our long wait appears to be over. Three Canadian researchers think they know what turns grapes into fruity flash-bangs inside a microwave oven.

Grapes’ Wrath

The answer is grounded in the complexities of how electromagnetic waves behave when passing through various substances. It all comes down to the fact that grapes are essentially spheres filled with water, and that their diameters match up with microwaves’ wavelengths. Those conditions combine to create a kind of trap for them. The incoming microwaves get stuck inside the grapes, where the water helps to focus their energy, and they begin to heat up from the inside.

Alone, the resulting “hot spot” won’t accrue enough energy to touch off any combustion. But when you put two grapes together, explains study co-author Pablo Bianucci of Montreal’s Concordia University, the trapped microwaves can hop between them, creating a powerful electromagnetic field in between the grapes. If left in the microwave for more than a few seconds the heat will get intense enough to turn ions in the grapes’ skins into a plasma, creating an incandescent display.

Recipe for Pyrotechnics

It’s not only grapes, either. The researchers were able to reproduce the phenomenon in hydrogel balls soaked with salt water and even quail eggs. The point wasn’t to show off, of course, but rather to prove that it’s not just geometry that’s responsible for the sparking and exploding. The researchers first microwaved the eggs touching each other like the grapes, and confirmed that a hot spot existed. They then drained the eggs of their contents and set them together in the microwave as before — but this time, nothing happened. It was the water inside the eggs, in addition to their shape, that was making them hot.

The research appears in the Proceedings of the National Academy of Sciences.

The grapes (or quail eggs) don’t even need to be touching, the researchers found. They conducted tests with the grapes wrapped up in a cozy paper container and with 15 layers of paper between them. Even kept apart, the grapes could quickly heat up.

grapes in the microwave

Two grapes in the experiment. (Credit: Khattak et. al)

The researchers have given armchair (or microwave, as the case may be) scientists something to think about while causing kitchen conflagrations, but they say their work has some practical applications as well. The work could, in the future, find use in the field of nanophotonics, or be used to create new wireless antennas and lead to better microwave imaging technologies.

For the rest of us, the researchers also reveal that they got the same flaming effect with not only grapes, but grape tomatoes and large blueberries as well. In fact, any water-rich object about the size of a grape would work, they say.

Needless to say, be safe.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: physics
  • Mike Richardson

    I can’t imagine why you’d put fruit in a microwave, so this is the first I’ve heard about the phenomenon. As a kid, I guess I was a bit less curious about putting things in the microwave than others. I hope my parents appreciate that. Fascinating to think you can create a plasma with simple fruits and a household appliance, though.

    • Uncle Al

      Microwave a ripe banana, taking suitable precautions to catch the goodies. A dusting of brown sugar on the naked fruit moves things along

  • Michael LaRocca, Editor

    That’s an Ig Nobel Prize waiting to be awarded. It’s also proof that some people have way too much free time.

  • jonathanpulliam

    Nippon’s leader, very wise,
    Put in Trump for Nobel prize
    Shinzo Abe wished to know
    If Trump’s grape-sized brain
    Might be coaxed to grow;
    Reckon’d that a few more lumps,
    Set alongside that of Trump’s,
    When frittered by a micro-beam
    Trump might not so stupid seem
    But when the microwave engaged
    Stabile Covfefe became enraged
    Stamped his feet, began to pout
    Threw his remaining cabinet out.

    • Uncle Al

      Political Science and Scientific Socialism were empirically falsified in 2016. Consider them to be microwave rotational spectra of molecules absent dipole moments. The Donald does not labor under the Presidency, he rejoices within it, as do American citizen-nationalists. MAGA

      • jonathanpulliam

        Fred Trump Jr., Trump’s older brother who the family ganged up on and some say drove to drink himself to death was “the good Trump”, it seems to me. He wanted the life of a pilot but they wouldn’t hear of it. Donald John was the “brainpower disappointment” that got shunted off by Fred Sr. to military academy. LOL. It’s sad. Horrible. Horrible and sad. So sad. Horrible. A Horror show: “Multiple Attorneys Got Arrested”! Nyuk nyuk nyuk

        • OWilson

          Well, all prominent families have a scion or two.

          Jimmy Carter had Billy, the Kennedys had Teddy, and cousin Skakel. The Clintons have Roger, and Obama has George, of Nairobi.

          I would imagine we will find more skeletons, when the 20 or so Democrat contenders face detailed scrutiny by the voters and the Media.

          • Uncle Al

            It will be a Democrat Socialist graveyard – Pocahontas; Democrat Socialist Right Wingnut Bernie Sanders; Kamala Harris and her public privates; wholly insane Alexandrita Occassional-Kotex, situational racist Cory Booker; Amy Klobuchar screaming “GLOBAL WARMING!” in a blizzard; Beto O’Rourke rallying 200 people against The Donald’s 50,000 hugely overlfiowing a stadium; Sherrod Brown promising to dump automattic income on people who refuse to work; Joe Biden chucking his shotgun so Putin will flee; Kirsten Gillibrand vowing to bring Mexico into the US; Julián Castro, former head of dog’s breakfast cat hairball HUD; loudly gay Pete Buttigieg (with a surname like that…); Jay Inslee and Global Warming; Steve Bullock and universal Medicare; Eric Holder, an ambulatory sack of liquid hog waste; Michael Bloomberg, swearing to bring New York City to Washington; Andrew Yang and universal basic income….

          • OWilson

            Let’s hope that the next Democrat contender will at least be able to stand up unassisted, and travel without that high tech mobile hospital! :)

    • Fred Raaflaub

      That grape-sized brain has a 156 IQ.

      • Mike Richardson

        😂😂😂 Oh, that’s a good one! Tell us another. Like maybe he’s 6 foot 3 and under 300 pounds.

        • Fred Raaflaub

          That is also true.

          • Mike Richardson

            In the minds of the deluded, perhaps. Objective reality tells a different story.


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