When aluminum foil stays in contact with acidic foods such as tomatoes, lemon, vinegar, and pomegranate molasses for a long time, its surface corrodes; it can leave a metallic taste in the food and develop tiny holes and gray spots. Brief contact does not cause a problem; the difference is mainly noticeable during long cooking or overnight storage. For these foods, use a lidded glass container or parchment paper.
Aluminum has a thin, invisible protective layer on its surface; this is what makes foil durable in everyday use. Acid slowly wears away this layer and exposes the metal underneath. Salt speeds up the same process. As contact continues, gray, dull spots appear on the surface; sometimes the foil develops holes and small particles may get into the food. The aluminum salt that forms is harmless to eat, though you can trim that part off for appearance. Heat speeds up this change, so it is more noticeable with acidic foods cooked in the oven for a long time. The same change also continues, more slowly, when acidic food is stored in the refrigerator.
The first sign in the kitchen is taste. When a tomato-sauce dish is cooked for a long time under foil or wrapped in foil and stored overnight in the refrigerator, it can develop a slightly metallic, sharp taste. The same applies to fish with lemon, vinegar marinades, and pomegranate-molasses sauces. Brief contact does not cause this taste; a dish wrapped in foil and baked within half an hour or a plate covered during serving will not cause a problem.
That is why other solutions are used for acidic foods. Use a lidded glass or ceramic dish in the oven; if there is no lid, first place parchment paper over the dish, then cover the paper with foil so the foil does not touch the food. Follow the same order for storage: transfer acidic food to a lidded glass container first, and do not wrap it in foil. Do not cover pickles, tomato paste, or lemon sauces with foil; store them in lidded jars. Do not cover an opened can with foil either; transfer its contents to a glass container.
Foil has other limitations unrelated to acid. Do not line the bottom of an oven with foil; it reflects heat from below, puts strain on the lower heating element, and can damage the oven floor. Do not use it in a microwave. Foil can be useful on a grill or barbecue, but acidic marinades cause the same corrosion there. The difference between the shiny and dull sides of foil does not matter for cooking; the two sides work the same way. When acid is involved, it makes no difference which side is used.
Proper use comes down to a simple rule: do not leave foil in contact with acidic food for a long time. If wrapping food in foil, cool it completely first, place parchment paper between the food and foil if possible, and eat it soon. Replace foil with gray spots, holes, or a metallic taste, and store that food without foil. You do not need to discard acidic food that has touched foil, but if you notice a change in taste, use a different container next time. Making lidded glass containers a habit instead of foil eliminates this problem completely.
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