Have you ever noticed that meat or vegetables or ice cream left too long in the freezer
develop a layer of frosty ice on their surface?
And when eaten, they're kind of dry and icky?
This is freezer burn, and it happens because ice in the frozen food sublimates - that is,
the water changes directly from a solid to a gas rather than first melting and then evaporating.
Unlike regular frost (also called "hoar frost"), which forms when water vapor that's already
in the air freezes into crystals on top of something cold, the frost in freezer burn
comes from frozen water in the food itself.
That's why freezer burn happens less to flour and nuts than to vegetables and meat and ice
cream, which contain more water.
And the ultimate effect is that water is transferred from inside the food to outside the food,
leaving it dry and frosty, but still safe to eat.
You're basically freeze-drying it, although the food might also oxidize and turn brown.
Of course, ice by itself can also sublimate, which is why ice cubes shrink in their trays
over time.
And the frost on the walls of a freezer comes partly from water that was once in ice cubes
and food in the freezer.
Sublimation happens more easily when the air is dry and when the ice is close to the melting
point of water, so you can avoid having your food get freezer burn by keeping your freezer
consistently cold and preventing air from contacting the food in it - for example, by
sealing food in plastic freezer bags without any air pockets.
What's more, freezer burn ISN'T just limited to freezers - it happens in nature, too.
Frost-flowers on sea ice, for example, form when water vapor sublimates from warm ice
into air too cold to hold it.
Homeless, the vapor deposits back onto the surface as icy lilies - sublime sublimation.