I was speaking to a friend the other day about microwave
cooking and he told me that he was under the impression
that during microwave heating of food the molecular structure
is changed to a state which may be either carcinogenic or
somehow otherwise unhealthful for human consumption.

This is true of any process that heats food!


I understand that molecules are vibrated during microwave
exposure...

As they are when heated in a regular oven. Or home on the range.


and I think that I've heard that if you heat something until
it is a hard unrecognizable crunchy lump then maybe that
could be true. But for general reheating of food or beverages
is there a concern?

Probably not. Microwave heating (you can't very well call it "cooking") has
been around for 60 years. If there were a risk, it probably would have been
discovered by now.

I particularly like the way microwave ovens handle bacon.