Why do grapes in a microwave oven produce plasma? Some of you may know this experiment (Grape + Microwave oven = Plasma video link):


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*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 phenomenon happens, I have found either laconic or partial answers to that question.
In broad strokes, this what I understand :


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*Microwaves seem to create an electric current in the grape because of ions.


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*Why do grapes contain ions ?


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


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*How is that link broken ?


*In parallel, the grape is warmed up and a gas is released from the grape


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*What is this gas made of? Water? Sugar?


*The combination of the electric arc in that gas creates a plasma


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*What is the necessary condition for a gas crossed by an electric arc to create plasma?
Is that correct? 
Are there any relevant parameters (microwave frequency, grape size, grape orientation) that make it work?
Any idea of the 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!)?
 A: As of early 2019 a research paper has been published to explain this phenomenon as due to resonance of the microwaves:
H.K. Khattak, P. Bianucci, and A.D. Slepkov, "Linking plasma formation in grapes to microwave resonances of aqueous dimers," Proc. Natl. Acad. Sci. USA, vol. 116, no. 10, pp. 4000-4005, Mar 5, 2019. DOI: 10.1073/pnas.1818350116
Here I quote an excerpt of its abstract:

By expanding this phenomenon to whole spherical dimers of various grape-sized fruit and hydrogel water beads, we demonstrate that the formation of plasma is due to electromagnetic hotspots arising from the cooperative interaction of Mie resonances in the individual spheres. The large dielectric constant of water at the relevant gigahertz frequencies can be used to form systems that mimic surface plasmon resonances that are typically reserved for nanoscale metallic objects. The absorptive properties of water furthermore act to homogenize higher-mode profiles and to preferentially select evanescent field concentrations such as the axial hotspot.

And an excerpt of its summary:

[W]e have shown that the popular-science phenomenon of forming plasma with grapes in a household microwave oven is explained by MDR behavior. Grapes act as spheres of water, which, due to their large index of refraction and small absorptivity, form leaky resonators at 2.4 GHz. Mie resonances in isolated spheres coherently add when brought together such that the aqueous dimer displays an intense hotspot at the point of contact that is sufficient to field-ionize available sodium and potassium ions, igniting a plasma.

A: Plasma is a state of matter which is composed entirely of charged particles or ions. By this definition, a flame is a plasma in itself because it consists of charged particles. This can be demonstrated by placing a flame in a uniform electric field. The flame bends to a side in presence of a field. It may also be due to the fact that an electric field ionizes some constituents in the flame, causing it to be affected by the electric field itself. In either case, it is not wrong to call a flame some sort of a "cold plasma". If a grape bursts into flame when put in the microwave, I do not think that people are wrong to call it plasma by any standards. As far as your question is concerned, you already have your answer. 
A: 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.
A: Youtube video: http://www.youtube.com/watch?v=vCNNqgKqnaQ
There are quite a few questions here:
1a) Because any living organism contains ions. (Scientist can not answer the question "why" they answer the question "how").
1b) From the video, it seems to be working for any size of grape (you can try at home). The microwave frequency used in oven is the water molecule rotation frequency.
2a) Water molecules absorb the microwaves and get heated. "A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (GHz) —a wavelength of 122 millimeters (4.80 in)— through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e., as both potential energy and kinetic energy of atoms), is heat." http://en.wikipedia.org/wiki/Microwave_oven
That must weaken the link.
The arc can be up to a few thousands volt.
3a) The gas is probably mainly water plus some biological molecules.
3b) An electric arc through a gas is a plasma. Plasma temperature varies from a thousand degrees to several millions. Joule effect and microwave heating are responsible for this temperature.
A: Usually, the microwaves are too weak to heat anything to a plasma state. However, if you bypass the manufacturer’s instructions and turn on the microwave with nothing in it to absorb energy, the microwave energy density will be much higher than normal.
Now, the reason you cut the grape in half is:
"Now it turns out that grapes are just the right size to further focus the electric fields in the oven. Two grapes placed close but not quite touching will produce a large field between them, sometimes enough to make a plasma." 
source: https://van.physics.illinois.edu/qa/listing.php?id=818
