Hmm, the oldest food in any of my packs presently is a half dozen Hoo-ah bars I absconded from Baghdad with. They are wrapped up together and bound tightly with nylon straps. The bars have deformed from the compression, but they otherwise cannot displace inside the straps, so they and their wrappers remain intact, despite being in my pack since mid-2005. I opened up the wad the other day and ate one of them. They are about the same as I remember them being.

Those Crystal light plastic cannisters I have would be pretty darned handy for food storage, in fact, that is what I have my Smuckers single serving peanut butter tubs loaded into. The lids are tight enough (not water tight on their own, but plenty secure) that a wrap of duct tape around the top and lid, and then a couple wraps longitudinally overlapped would fairly seal those cannisters, and they are just rigid enough they are not going to be incidentally squished (you'd have to do something like fall on or sit on your pack to compromise them, and even then they might be strong enough to resist if they are full and taped up).