Why grapes create plasma in a microwave oven: scientists discover that they trap electromagnetic radiation

Scientists have explained a pyrotechnic tour featuring grapes catching fire in the microwave.

The tour, popular in science fairs and on YouTube, is as follows: Cut a grape into two halves joined by a small piece of skin and place it in a household microwave oven.

Turn on full power and enjoy the show while a white light is burning at the point where the two halves of grapes are touching each other.

While many have sought to provide a scientific explanation behind the phenomenon, few have understood – until now.

Scroll for the video

A new study by a team of Canadian scientists shows how the phenomenon is triggered by charged molecules containing sodium and potassium.

Scientists have used a combination of thermal imaging and computer simulations of electromagnetic fields to explain the physics behind the formation of sparks from grapes in the microwave.

Experiments with water-based hydrogel beads and computer simulations revealed that the piece of skin that joins does not play an important role, as previously thought.

Electromagnetic resonance has led to concentrated points where "ionized" molecules have generated a plasma, a super hot electrically conductive gas.

The ability of water to absorb microwaves has contributed to the formation of "hot spots".

In simple terms, the microwaves were trapped in cavities between the grapes.

"If you take two of these grapes and you bring them closer together, you do not have an independent system anymore, but you have a system in which these two – we call them cavities – talk to each other, "Thomas Volz, associate professor at Macquarie University in Sydney, told New Scientist.

A study by a team of scientists shows how charged molecules containing sodium and potassium have triggered the phenomenon of grapes generating microwaved plasma

Experiments with water-based hydrogel beads (illustrated) and computer simulations revealed that the piece of skin that joins does not play an important role, as previously thought.

By simply placing two grape or hydrogel beads close to each other, they have the appropriate size and shape to trap the microwaves, create a "hot spot" and generate plasma

Plasma self-perpetuates and separates from the "dimer" – the pair of identical source objects, in this case two halves of grapes, said the scientists.

Dr. Aaron Slepkov, of Trent University in Ontario, and his colleagues wrote in the journal Proceedings of the National Academy of Sciences: "The observation of a fruit inflamed in a microwave oven. Wave is exciting and memorable.

The emission spectra of grape plasma suggest that potassium and sodium species, abundant in grape skin, are ionized by a high concentration of electric field near the point of contact.

"The ions themselves resonate with the microwave motor radiation and can generate an ionization cascade in the air, forming a microwave – heated plasma that develops and becomes independent of the dimer," added Slepkov. .

Two grapes trap the microwaves and warm them up. By bringing together two grapes, they are the perfect size to trap microwaves, creating a "hot spot" and generating a plasma

In addition to hydrogel beads and grapes, scientists discovered that the effect could be replicated with gooseberry, quail eggs and blackberries.

The key to unleashing a fireball is an object the size of a sufficiently moist grape.

Scientists have also tested the effect on small quail eggs, which have been shown to produce hot spots in the microwave.

The emptied eggs did not react in the same way, but when the eggs were filled with water, the hot spots returned.