Freezer burn is caused by the sublimation of frozen water (ice) into water vapor. Normally, when ice turns to steam, it melts into water first. Sublimation is when ice turns directly into water vapor, skipping the water phase. In a controlled environment, such as with freeze-drying, this process can preserve the shape and color of foods while removing its water content. But when it happens to meat in your freezer, the result is freezer burn—inedible dry swaths of meat that need to be trimmed before cooking.