Sure, freezer burn is an oxymoron, but not a bad one at that.
Take a good look at that ancient pork chop you’ve been keeping in the freezer for an emergency. Doesn’t its parched and shriveled surface look as if it had been seared? Believe it or not, “seared” doesn’t only refer to heat; it also means withered or dried out. And that’s exactly what the freezer has done to the meat: dried it out.
How does coldness make food dry out and wither? Please observe that the patches of “burn” on that forlorn pork chop are dry and parched looking, as if much of the water had been sucked out. And it has. But in what state or condition does the water exist in frozen foods?
Correct. It is indeed ice. So we are forced to the conclusion that while the hapless chop in question has been languishing in your freezer for much longer than you had ever believed possible, something has been spiriting away ice molecules (which are water molecules, of course) from its surface.
But how can water molecules, so firmly anchored in solid ice crystals, be wafted off to some other location?
It turns out that they will spontaneously migrate, if they can, to any place that offers them a more hospitable climate. To a water molecule, that means a place that’s as cold as possible, because that’s where its energy is lowest, and, other things being equal, Mother Nature will always try to find the lowest-energy conditions for her little particles. You can drive water molecules away by heating them, right?
So if the wrapping on the food isn’t absolutely water-molecule-tight, water molecules will migrate through it or around it, from the ice molecules in the food’s surface to any other location that’s colder, such as the walls of the freezer itself. The net result is that water will leave the food and be disposed of by the freezer’s defrosting mechanism. The food’s surface is left parched, wrinkled, and discolored.
This doesn’t happen over night, of course. It’s a slow process, and it can be slowed down to practically nothing simply by wrapping the food tightly in a material that is impermeable to wandering water molecules. Some plastic wraps do a better job than others.
Best of all are vacuum-sealed, thick plastic packages like Cryovac, because in addition to being quite impermeable to water vapor, they leave no space between the food and the wrapping. If there is any air space in a package, water molecules will migrate across it to the inner wall of the wrapping and settle there as ice, for the same reason that they’d go all the way to the freezer walls if they could. And you’ll still get freezer burn.
Moral Number 1: For long-range keeping of frozen foods, minimize freezer burn by: (a) using a wrapping material specifically designed for freezing because of its impermeability to migrant water molecules and (b) wrapping the food very tightly, leaving no air pockets.
Moral Number 2: When buying already-frozen foods, watch for any ice crystals in the space inside the package. Where do you think that water came from? Right. The food. So either it’s been “burned” by being kept too long or it’s been thawed, which releases juices from the food, and refrozen. Shop somewhere else.