The older military meals - LURPs - were nothing more than a dehydrated meal that was rehydrated in the pouch with water then eaten. The puches appeared to me to be just regular heavy duty plastic bags. If you didn't have hot water you poured in cold then stashed the pouch under your shirt to rehydrate/get semi warm. I ate quite a few of these and don't think I suffered any long term ill effects. Some of the short term effects were pretty bad.

The older MREs had hockey pucks - beef or pork patties - that were similar. Reydrate in the plastic bag.