Foods rich in citric acid -- often those containing citrus fruits or their juices -- may discolor tin foil. Because of its acidity, citric acid can cause small holes to form in the foil. In some cases, it may also change color. While it looks unappealing, this reaction does not make your food unsafe to eat.
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Acids and Aluminum
Acidic foods, such as tomato sauce, vinegar and citrus fruits and their juices, can sometimes react with aluminum foil. This reaction will not always happen -- and it can be tricky to figure out when it will and when it will not happen -- but the acids cause the aluminum to break down into aluminum salt. Aluminum salt is harmless and is safe to eat.
How to Avoid Reactions
To avoid the risk of your aluminum foil reacting to citric acid, you can brush your foil with a thin coating of oil before placing it on foods rich in acids. In some cases, such as with foil packets for individually baked fish fillets, you can use parchment paper in place of foil. If you are merely covering your food for storage, use plastic wrap in place of the foil. Foil is more likely to react to citric acid when it is exposed to it for long periods of time or in high concentrations.
Not Just Citrusy Foods
Citric acid is not the only element that can react with aluminum foil. Salty and spicy foods can also cause a similar reaction. As well, aluminum, when exposed to other metals and in the presence of moisture, can also develop small pinholes and discoloration. Avoid using foil if you are using silver, stainless steel or iron dishes. Instead, use glass, ceramic or aluminum cookware if you must use tin foil.
Cooking With Citrus Fruits and Foil
If you must use both tin foil and citrus fruits together, when you add the citric acid can have affect if a reaction with the foil occurs. The longer the foil is exposed to citric acid, the greater the chance of a reaction. Because of this, consider adding your lemon or lime juice at the end of the cooking process. This helps keep your citrus flavor bright and in the foreground of the dish and also minimizes the chances of a reaction. If you need to cook for long periods of time with citrus and foil, arrange your food so there is a buffer. For example, if you are making foil packets of fish fillets with lemon and orange slices, place the fish at the bottom, the citrus slices on top and cover those with onions so the citrus does not touch the foil directly.