Some of you may know this experience (Grape + Microwave oven = Plasma video link

Question

Some of you may know this experience (Grape + Microwave oven = Plasma video link):

take a grape that you almost split in two parts, letting just a tiny piece of skin making a link between each half-part.
put that in a microwave oven, and few seconds later, a light ball which seems to be a plasma appears upon that grape

Looking through google to understand why this phenomena happens, I have found either laconic or partial answers to that question.

In the big lines, this what I understand :

Micro waves seems to create an electric current in the grape because of ions.
Why do grapes contains ions ?

Suddenly the tiny link between the two half-parts is broken which creates an electric arc
How is that link broken ?

In parallel, the grape is warmed up and a gas is raised upon the grape
What is this gas made of ? water ? sugar ?

The combination of the electric arc in that gas creates a plasma
What is the necessary condition for a gas crossed by an electric arc to create a plasma ?

Is that correct ?

Are there any relevant parameters (microwave frequency, grape size, grape orientation) that make it works ?

Any idea of order of magnitude for the intensity involved, the voltage of such an arc, the temperature reached (I've read 3000 degrees !) ?

Has someone a complete explanation to provide (reference to physical principle would be appreciated !) ?

Explanation / Answer

There does seem to be a lot of mythology around about the "grape in a microwave" experiment. I have never see any publications on the subject in a respectable journal, however from chatting to other scientists there seems to be a consensus about what happens.

It's all rather boring really. The grape is the right size (about a quarter wavelength) and shape to act as an antenna that focusses the power in the middle. The skin joining the grape halves heats up, vapourises and bursts into flame.

If anyone feels in an experimental mood some obvious tests of this would be to change the grape size and shape, and see if that affects the flame. Less easy to do at home would be to try the experiment with a nitrogen atmosphere as that should prevent combustion.

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