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.
To combat oxygen, they packed each sandwich in an air-sealed package with an oxygen scavenger, which is a packet of iron filings that sucks oxygen from ambient air and traps it in a layer of rust.
However, the best part of the development is that our soldiers seem to enjoy them, as shown in the BBC’s video report.