Aluminum foil is an incredibly useful material for storing leftovers, since it locks in the food's moisture and prevents odors from escaping and contaminating other food. However, after wrapping food in foil and storing it in the freezer, you may find that it's turned slightly blue once you unwrap it — here's what that blue residue really is.

The blue cast on your food is not likely to be mold, since blue mold is quite rare, and common food mold will be either white, black, or green, with a slimy or fuzzy texture. In actuality, the acids in your frozen food react to the alloyed aluminum in its foil wrapping to create this harmless blue residue, called aluminum salt.

Produce like tomatoes or blueberries or foods that are high in salt, vinegar, or spice are more susceptible to aluminum salt, and while this residue doesn't indicate spoilage and is harmless to eat, you can cut off if it bothers you. You can also stop it from forming it by wrapping your food in plastic wrap before covering it in aluminum foil.