RedLocks:
If the temperature begins to rise in your bag either from the weather, body heat or maybe a bike motor, the aroma becomes more apparent if not airtight. Call myth busters?
That'd be a fun episode!

bikeTripper:
Use Tupperware, a jar or some similar airtight container inside of the backpack.
That's a good idea. After I made the post it occurred to me that there's a freshly empty Metamucil bottle laying around, which is kinda funny if you think about it. "Oh look, I'm just constipated."