I'm not sure how the process of freeze drying actually works, but I'm confident there's more to it than chucking something in a freezer. The moisture's got to go somewhere and there's not a lot of air volume in a typical home freezer to take it away and keep it away from the item you're freeze-drying.
i've pulled some meat out of the freezer that was burnt pretty bad. all grey and dry.
you have to leave it open to the air. just hang it from a rack in the freezer.