Polyethelene is what freezer bags are made of.

When you ask the manufacturers about "cooking" in freezer bags, they assume you mean boiling in a bag or using it in a microwave, which is how most people cook in plastic . Freezer bags certainly were not made for that. We need to specify that we are not cooking but rehydrating dried food by pouring hot (no longer boiling) water into the bag and letting it sit, inside an insulating cozy. This is quite different than boiling or microwaving the freezer bag!