It's Best Not to Freeze Popcorn
You probably have more storage space in your cupboards or pantry than in the freezer, so it's easy to store popcorn at room temperature. And storing the kernels at room temp maintains the right conditions for maximized popping. It's not unsafe to store unpopped popcorn in the freezer, but it may affect the popping ability of the kernels. Stored in an airtight plastic or glass container, popcorn stays fresh for up to two years at room temperature.
If you store unpopped popcorn in the freezer, the kernels can become dehydrated if not wrapped extremely well, and could loose the small amount of moisture they contain that's necessary for popping.
The Science of Popcorn
Even though popcorn looks and feels completely dry, it actually contains about 13.5 percent water, along with starches. When you heat up the kernels, the water heats to its boiling point and turns to steam, which takes up more space than water. The steam exerts pressure on the inside of the kernel, and at anywhere from three to six minutes, the starches in the kernels can't hold in the pressure any longer. At that point, the kernel explodes, exposing the fluffy, dry starches to the air and providing you with a tasty snack.
What Happens in the Fridge and Freezer
When the water molecules freeze, they migrate to the coldest part of their environment, so the water inside each popcorn kernel moves to the outer edges and toward the sides of the freezer where it is coldest. If the popcorn isn't wrapped in an impenetrable way, the water molecules escape, leaving the kernels dry and unable to pop or to pop only slightly. When temperatures fluctuate in your freezer, the water molecules move around even more.
Rehydrating Popcorn Kernels
If your popcorn sat in your cupboard for more than two years and is leaving you with lots of unpopped kernels, you can experiment with adding more moisture to the kernels. Add about 1 tablespoon of water to 4 cups of popcorn, and stir the popcorn to ensure that all the kernels are moistened. Keep the popcorn in an airtight container, and stir it again every few hours for one day. Let the popcorn sit for another two to three days, and try popping it again.
Already Popped Popcorn
Once you've popped popcorn or bought an already popped product, you can store it in the freezer as long as you do your best to prevent water molecules in the product from escaping. Wrap or double-wrap the popcorn in freezer-grade plastic or foil to minimize the chances of water molecules escaping. The popcorn will stay fresh for two to three months in the freezer.