Hmmm, unusual question and isn't addressed by the guidelines other than to generally note that microwaving has no real role in home food preservation due to its uneven heating. Not only do microwaves affect liquids and solids quite differently but some foods would be quite prone to boiling over in the process is another potential problem that comes to mind.

So I guess I'd need to know why? And what food specifically?

Dave

No reason, the question came to mind when I was microwaving some sauce in a pint jar.