I have some American Cheese with fine white crystals on the outside of each slice. They have a gritty texture. Is this a sign that the cheese is spoiled, or just lactic acid salts like aged cheese gets?
It is calcium lactate crystals. It is safe to eat. On regular cheeses it is a sign that it is aged well. For packaged, sliced american cheese I am not sure I would care for it. But it is a matter of personal preference.
When the protein in milk (casein) breaks down, the amino acid, tyrosine, will form crystals inside the cheese. These are common in some aged Gouda, cheddar, and, I think, manchego. These are perfectly harmless and actually indicate that the cheese is well aged. I don't know if this is the same as crystals on the outside, and American cheese is certainly not aged.