When food is improperly packaged and sits in a freezer, the surface dries out, making it look “burnt.” Depending on how much of the food is exposed, the process can take anywhere from a few days to a few weeks, says Barry Swanson, PhD, an expert on food shelf life with the Institute of Food Technologists. “The texture will be very dry and difficult to chew, it will look lighter in color, and if there is fat present it will oxidize,” he says.

The oxidized fats are what cause the gross freezer flavor and smell we associate with freezer burn. “When you oxidize fats, you produce volatile compounds in the air. They are what give that wet-paper, cardboardy smell,” says Swanson.

When food is frozen, all of its natural moisture converts to ice crystals. Robert L. Wolke, professor emeritus of chemistry at the University of Pittsburgh and author of several kitchen-science books, explains that if food is poorly wrapped, water molecules can escape from the surface of these crystals. That’s because molecules at the surface of an ice crystal aren’t bound as tightly as they are in the crystal’s interior. Since they still have some energy, there is a certain probability that they will pop off, leaving a dehydrated spot on your ground beef.

Swanson says that better freezer technology and packaging has made freezer burn less of a problem. Technically, it’s safe to eat freezer-burnt foods, but he says it can be difficult to differentiate between freezer burn and microbial contamination. “Nothing is ever totally safe, because you don’t always know the history of food,” he says. “If you can’t identify it as freezer burn, it’s better to discard it.”

To avoid those nasty dried-out spots, see the suggestions on freezing from the National Center for Home Food Preservation.