Freezer burn is a condition that occurs when frozen food has been damaged by dehydration and oxidation, due to air reaching the food.[1] It is generally induced by substandard (non-airtight) packaging.
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Freezer burn is not a food safety risk. 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 any time 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.[2]
The condition is primarily caused by sublimation. Water evaporates at all temperatures, even from what appears to be solid ice. When the constantly vibrating 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 impetus for water molecules to move from their original positions.
It is possible to slow down freezer burn by filling plastic containers with water (leaving room for expansion) and leaving them open in the freezer to help maintain humidity. Proper packaging can also help delay freezer burn because small, air-tight packaging allows local homeostasis of humidity, and, to a lesser degree, temperature, although current available packaging cannot do this perfectly.
Meats and vegetables stored in a manual defrost freezer will last longer than those stored in automatic defrost freezers. That is because the temperature of a manual defrost freezer remains closer to 0 °F/-18 °C while the temperature of automatic defrost freezers fluctuates. Food with freezer burn, though dried and wrinkled, is safe to eat. However, food afflicted with freezer burn may have an unpleasant flavor. In most cases, it is sufficient to remove the parts affected by freezer burn.