Freezer burn is a condition that occurs when frozen food has been damaged by dehydration and oxidation, due to air reaching the food. It is generally induced by substandard (non-airtight) packaging.

Freezer burn is not a food safety issue. It appears as grayish-brown leathery spots on frozen food, and occurs when air reaches the food's surface and dries out the product. This can happen when food is not securely wrapped in air-tight packaging. Color changes result from chemical changes in the food's pigment. Although undesirable, freezer burn does not make the food unsafe. It merely causes dry spots in foods.

The condition is primarily caused by sublimation. Water evaporates at all temperatures, even from what appears to be solid ice. When the constantly oscillating water molecules in the meats and vegetables stored in the freezer migrate to the surface, crystals of ice outside of the solid food are formed. The parts now deprived of moisture become dry and shriveled and look burned. In meats, air can cause fats to oxidize.

This process occurs even if the package has never been opened, due to the tendency for all molecules, especially water, to escape solids via vapor pressure. Fluctuations in temperature within a freezer also contribute to the onset of freezer burn because such fluctuations set up temperature gradients within the solid food and air in the freezer, which create additional physical motivation for water molecules to move from their original positions.

The best way to avoid freezer burn is to keep food in vacuum sealed in air-tight vacuum sealer bags and vacuum sealer canisters.  FoodSaver, Seal a Meal, and other food vacuum sealers have become many customers' choices in keeping food fresh up to 5 times longer than just freezing alone.