In general, food should not be eaten if it is emitting light for reasons other than its temperature (which would be incandescence). Almost all safely edible substances do not have an intrinsic luminecence. For that matter, if your food is incandescent, it is probably not safe to eat because it would be very hot.
There are several diffent classes of luminescence which your food might be exhibiting. Each has a different level of probable safety associated with it.
If your food started glowing after you microwaved it, you probably have phosphorescent food on your hands. This could also happen after you've shined a bright light on your food or something similar. Somone could have spilled glow-in-the-dark paint onto or into your food, or put a huge amount of phosphorus into it. Worst Case: The most disturbing possibility is fermented urine was involved in its manufacture.
If your food was glowing in the microwave, or is glowing under your funky blacklight, but stopped when you took it out or turned off the blacklight, you have flourescent food on your hands. Quinine, a flavoring used in tonic water and other soft drinks, is one of the oldest known fluorescent molecules. If you're lucky, you might have some genetically-modified food into which a gene for green-fluorescent protein was inserted. This has been done with rabbits, so if your rabbit stew is glowing green under your blue (488 nm) lights, chances are it's GFP's fault. (see http://www.ekac.org/gfpbunny.html). Unfortunately, it is much more likely that a colony of flourescent bacteria is now calling your food home.
The most disturbing possibility is that your food is now inhabited by bioluminescent organisms: living things that can produce their own light (a firefly is an example). The problem is that these organisms usually only produce light while still alive -- meaning you've got an interesting dish in your hands. Some bioluminescent fungi are poisonus (like the jack-o-lantern), others are just slimy (like the honey mushroom). (see http://www.frognet.net/~bfc/Winter1999/Articles/mj13foxf.html) All seem to be unpleasant. Most bioluminescent fish are terribly poisonus as well.
Safely edible luminous foodstuffs:
- honey mushroom (Armilleriella mellea) -- commonly grows at the base of dying trees. (In fact, in many cases they are causing the tree's death.) Honey-colored cap with brown highlights. Not particularly tasty, and rather slimy. Cook it with something acidic (lemon juice, cooking wine, vinegar) to get rid of the slimyness.
- tonic water glows under black light due to the quinine: thus it is flourescent. Submitted by chinakow, who adds "after much experimenting mixed with gin I beleive that it is not dangerous."
Please /msg me with any others you know of.