| MadSci Network: Microbiology |
This sounds like a cool experiment!
One possibility would be that a 30 minute exposure time was not long enough to insure that all the pieces were inoculated with at least one mold spore from the air. One solution to this problem would be to scrape a piece of already moldy bread with a toothpick, and then use the toothpick to touch each piece that you want to inoculate.
Another possibility would be that the bread didn't contain enough moisture to support mold growth. You can fix this problem by sprinkling a few drops of water in each plastic bag.
Good luck!
Try the links in the MadSci Library for more information on Microbiology.