While on the subject of coffee and microwaves, I was recently shown another
unusual feature of microwave cooking. A colleague took a large sultana
grape and cut it almost in half, length wise, so that it was just held
together at the vine end by its skin. He placed it in the microwave and
"cooked" it at high power for 30 seconds. After ten seconds of cooking
large sparks (1cm) were emitted from between the two grape halves. This
continued until the grape skin separated from burning. What causes this to
occur?
I have tried the same experiment with different types of grapes with
varying success. Small grapes do not appear to work.