Why does aluminum foil react with food leaving a metallic deposit?

Answer 1

1) When aluminum and a dissimilar metal are in contact in the presence of moisture, an electrolytic reaction may occur causing a breakdown of the aluminum. To prevent this reaction, use plastic wrap to cover metal containers (silver, stainless steel, or iron used to store food. 2) If the aluminum foil was not used with a dissimilar metal container, the reaction was probably a chemical one. It is possible for heavy concentrations of salt, vinegar or some other acidic compound, or highly spiced foods to cause the foil to disintegrate. The product of either of these reactions is an aluminum salt. It does not harm the food but you will want to scrape any deposit off the food as it may impart an undesired flavor and color.