One way or another, you have to insure that all bacteria (particularly Clostridium botulinum) are killed when canning. There are two basic methods- using acidity or heat+pressure treatment. Heat+pressure is required in combo because botulinum does not die at 212. Pressure allows the water to get to a higher temperature to kill the buggers.
If done properly and the seal remains, the food will be usable for a good while. It will slowly deteriorate in flavor and nutrients, but still be safe beyond recommended "eat by" claims.
If done properly and the seal remains, the food will be usable for a good while. It will slowly deteriorate in flavor and nutrients, but still be safe beyond recommended "eat by" claims.