I was unaware of the fact that green beans spark in the microwave. Since I'm forcing my child to eat something that I myself don't eat, I've recently learned this. Also, no forcing is involved. He loves the stuff.
There is nothing in a “green bean” that sparks or metallic. The spark you are getting occurs when there is not enough resistance from the magnetron. Microwaves heat water molecules, which speeds them up, and if there is not enough water, then bad things happen. This is why the owner’s manual instructs you not to run the microwave empty. This occurred with your green beans because of the small amount. Green beans do not have the density like many of the other “baby food” you microwave. You will likely get the same or similar results if you microwave nothing! -DWH