|MadSci Network: Microbiology|
Wow, 4 week old bread with no mold - that's pretty well preserved I would say. One of the most widely used preservatives in breads is propionic acid. This is usually used in a calcium or sodium salt form to improve the solubility in the food system. Propionic acid is an organic acid that shows antifungal properties, but does not affect the growth of yeast. This is why it is used in bread - it will inhibit mold growth but allow the yeast fermentation to go unabated. Propionic acid has its best antifungal activity at low pHs, so that may be why you see some growth on your "high pH" bread. Molds, in general, will grow over a wide range of pHs. In fact, to isolate mold from a food sample the medium we grow it on may be have the pH lowered to inhibit bacterial growth and allow mold growth. I would suggest that you retry this experiment with some fresh bakery or home-made bread that does not contain preservatives. It should only take about a week before you see good mold growth on these. I would also suggest that you dust the surface very lightly with some soil to make sure you are incoulating the bread with mold spores. Also, keep the bread moist by putting each slice in a container along with a damp piece of paper towel - molds need some water to grow. Hope that helps and good luck in the science fair. Reference: Modern Food Microbiology. 1996. James M. Jay. 5th Edition. Chapman & Hall. New York.
Try the links in the MadSci Library for more information on Microbiology.