The Army is upgrading the Meal, Ready-to-Eat (MRE):
[F]ood scientists started with ingredients like sugar (in jams or jellies for instance), salt, or honey that contain moisture but also retain it, keeping it out of contact with other ingredients. Think about a fresh tomato; on a sandwich, it will quickly cause the bread to become soggy as water from the tomato soaks into the bread. But jelly or honey on toast, though moist, doesn’t impart its moisture to the bread. Using ingredients that lock their moisture inside was key to the process. Perhaps more difficult is keeping oxygen away from the sandwich. To do so, each one is packed in an air-sealed package with an oxygen scavenger–a small packet of iron filings that pulls oxygen from the ambient air and locks it up in a layer of rust. This keeps oxygen away from things like bread, where it could feed a reaction resulting in mold and decay. Devoid of oxygen and water, a sandwich can last a long time–two years in this case.
BBC report here.
The Dish 