Why do onions spark when microwaved?

Question

This question was asked in the cooking section last year, but it wasn't really resolved. Last night, I was making guacamole and all I had were frozen onions and I didn't want to take the time to thaw them naturally, so I microwaved some chopped onion on a paper plate. A couple of bits started sparking and popping so much that they were a little charred. The onions were not in the microwave for long (30 seconds). Does anybody have any idea what causes this?

Answer 1

Very interesting question. The mechanism at work is the same that causes sparks when you put metal objects in a microwave.

Microwave ovens create an electromagnetic field, which heats water molecules inside food. Objects at different locations within the field exist at different electric potentials. Although air is normally an electric insulator, when the electric field potential between two objects exceeds the insulation capacity of air (3,000 volts/millimeter), a spark jumps between the objects.

Your onion bits spark when adjacent bits have such very, very small gaps of air between them, that electric charges can jump from bit to bit. The sparking isn't dangerous, but you can probably reduce it by spreading the onion bits out more.

The onion bits being frozen may also contribute to the sparking. Although liquid water absorbs microwaves well, frozen water doesn't absorb microwaves well at all. (which is why microwaving frozen things tends to burn them on the surface, which thaws from the outside, while leaving the interior frozen). If your onions bits were entirely frozen, then I think (but am not certain) that larger charges would build up on their surfaces, which would increase the likelihood of sparking. If you're curious, try thawing the onion bits first, then see if you get as much sparking.

I think a few other posts here are answering a different question. Microwaves cause objects (like corn kernels) to pop because they convert internal water, enclosed in a confined space, to steam. The steam creates sufficient internal pressure to explode the object.

Answer 2

My thought was that as you cut an onion especially on a hard ceramic surface like a plate, very small pieces of the knife blade "chip" off and since this is steel, the micro wave causes them to spark. I consider myself a scientist but this is just a guess on my part.

Answer 3

not sure scientifically... but i'd say it has to do with the amount of water stored inside the onion is in such a small space... maybe or the fact that they are just little... ever put beans? (refried for me) Everytime i put beans in the micro, they pop or explode after only about 30 seconds too.

Answer 4

It wouldnt of been the onion its probably the paper plate

Answer 5

I microwave onions all the time, but that has never happened to me.

Answer 6

The noino chemical found in the onion reacts with your eyes when you look at the onion causing the noino to blend with the snoi and create a very very dangerous spark that in some rare cases burn your eyes out of the socket and melt onto your brain when you look at it.

Answer 7

Miss Anne, They are filled with moisture !

Answer 8

its the water molacules bursting from the microwaves

Answer 9

thats an interesting question and I hope you will get reasonable answers

Answer 10

Probably because these cook very fast -