I baked a ham. After dinner, I covered it with foil and placed it in the fridge overnight for sandwiches today. When I uncovered it, the foil had somehow left "metal" where it actually was touching the meat? What is that?
Found my own answer. Caused from the salt in the ham (I even soaked in plain water for 10 hours to extract some salt). The deposits are not actual metal but rather aluminum salts. Supposedly harmless. A new one on me!
Tomatoes and other foods with high acid content also cause reactions--sometimes eating holes in the foil.
Putting a layer of parchment paper between the food and the foil creates a barrier so the foil will not react.
I don't wrap food directly in foil or plastic wrap anymore--I always use parchment paper first.