People have seen videos of fruits bursting into flames in a microwave oven, but could not find the reason behind this phenomenon.
DIY science enthusiasts cut a grape into two pieces, connected by a strip of skin at the bottom and place it a household microwave. Blitz appear in the microwave for few seconds.
In a paper published in the Proceedings of the National Academy of Sciences, Trent University researchers explain why grape hemispheres when cooked in a microwave oven ignite into fireballs.
The researchers placed thermal cameras to observe a hot spot between the grapes. The appliance pushes microwaves into the two grape halves, where the waves bounce around and add constructively to focus the energy to a spot on the skin. Both grape halves focus the energy to the same tiny point in the centre. That intense energy jostles the atoms and molecules at that spot, heating them up so much that they can no longer hold onto their electrons, which turns them into a plasma—and boom, fireball.
The Trent researchers noted that the same effect could be produced using gooseberries, large blackberries, and quail eggs or similarly sized fruit or water-filled balls. They also found that the pair of grapes ignite into fireball happens even if the fruit is cut, but kept separate within 3 millimeters of each other.