Wednesday, February 15, 2012

What is freezer burn?

Hamburger with freezer burn
Have you ever removed an old item from the freezer and found it dry and discolored? Or covered in ice crystals? If so, you've been burned by your freezer.

According to Everyday Mysteries, water migrates to the coldest place they can find. If the shelves and walls of your freezer are colder than your steak, the water leaves the steaks and builds frost elsewhere, drying out the food, leaving space for Oxygen molecules to move in, which discolors the food. Wikipedia calls this sublimation, an odd process of going from a solid (ice) to a gas while skipping the liquid in-between state. Water molecules move about, find themselves near the food surface, freeze, and then sublime into water vapor and refreeze elsewhere.

The recommended way to prevent this is to tightly wrap food in layers. This slows the water migration but can not stop it. There is a time limit for storing foods and eventually you'll have ice crystals on the meat, inside the wrapping, instead of on the walls. Great wrapping (better yet vacuum sealed) will block oxidation and discoloration.

An odd cause of freezer burn is a freezer that is TOO warm, above 0 degrees F. In the 0 to 32 degree range water will freeze and "evaporate" from sublimation as described above. This process is actually encouraged with low presssure to "freeze-dry" items.

Bottom Line

With the exception of freezer burn damage, food can last indefinitely in a freezer. Freezing to 0 °F inactivates any microbes — bacteria, yeasts and molds — present in food. The microbes sleep, but they are not dead! Once thawed, the microbes again become active, multiplying under the right conditions to levels that can lead to foodborne illness. Treat thawed items the same as you would any perishable food.

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