There's an interesting discussion of "What's in iron rations, anyway?" over at The Twenty-Sided Tale. (Please note that they are discussing fantasy/medieval rations, not the emergency rations of World War I.)
General consensus is that whatever they are, they taste horrid, much like modern-day MREs.
Freak that I am, the MREs I've eaten have actually been pretty tasty (though the dehydrated pork patty needed some substantial "modification"). It might have something to do with the fact that for whatever reason, I don't have much of a taste for salt (you have to POUR it into food for me to detect it -- like fast food french fries -- and if the mashed potatoes are undersalted I'll never know).
But stay away from the Chicken a la King, though. That shit is nasty.
And the less said about Chipped Beef on Toast, the better.