| MadSci Network: Agricultural Sciences |
Molds are fungi, just as mushroom are fungi. Like mushrooms, there is a wide variety of edibility in molds: some are poisonous, and some are not; some are tasty, and some are not. The black, spotty mold on bread is usually Rhizopus, and somewhat toxic. The most common mold on nuts (especially pistacchios) is Aspergillus, several species, including Aspergillus flavus, produce Aflatoxin, the most potent carcinogen known. The blue-green mold on fruits and bread and cheese is Penicillium, which comes in a variety of species with different charcteristics.
Some species of Penicillium are used medicinally for their production of antibiotic and antifungal agents: like P. chrysogenum which is the source of Penicillin, or P. griseofulvum which is a source of fungicide. Most species of Penicillium are not toxic unless you are allergic to penicillin - the allergy is actually against common protein contaminants in the antibiotic preparation, so some non-penicillin producing species can cause an allergic reaction. Penicillium species in general don't taste very good, but some species, like P. roqueforti and P. candidum, have a distinctive flavor that goes well with cheese.
Several other molds are used around the world to add flavor and texture to aged cheeses, and in each case, the species used has been cultivated for years for its unique taste, just as several mushrooms and truffles are regularly cultivated for their unique textures and tastes. In general, eating unidentified mold is like eating unidentified mushrooms: it probably won't taste very good and could easily be toxic, so its safer to simply discard moldy bread than wonder if the mold is edible.