My middle school science students really enjoyed this week-long Moldy Bread Experiment. All that's required is four kinds of bread, some sandwich baggies, tap water, cups and eye droppers.
As an added benefit, fun lab can "run in the background" while the class pursues other state-required science explorations. This lab can also be started on a Friday and left over the weekend to shorten its actual class-time duration.
The added jigsaw readings from the FDA are purely optional but I highly recommend you use them. The "Poison Squad" tells about an experiment where people ate things to see if they got sick! The "Preservatives" story discusses the use of preservatives in our foods and will be eye-opening for both you and your students.
The "Preservatives" story is also a stepping off point for more discussion and research, as several scientists have suggested many food allergies may actually be allergies to the preservatives in the food. Similarly, there are also some suggestions by pathologists that the preservatives collected in our body tissues over a lifetime are slowing the decay of bodies after we die!