Datrex claims a one year shelf life for their rations once the outer bag is open, so long as the individual wrapping is intact. I don't know why they wouldn't keep nearly that long- they're relatively dry, so AG is very low.

I broke up one block of Datrex bars and sealed each individually, but my sealer is a commercial chamber vacuum machine. Not sure if a Foodsaver would work as well.
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman