We explain the science behind aluminum foil and acidic foods.
Aluminum is what is called an “active metal.” That means that it’s relatively easily transformed by outside influences. For instance, it dissolves when it comes into contact with acid, such as the tomato sauce in your lasagna. But tomato sauce couldn’t dissolve enough aluminum foil overnight to result in holes, according to our science editor. The reaction you observed, he said, is not chemical but electrical.
We were intrigued. Aluminum atoms have a weaker hold on their electrons than do other metal atoms, like iron or steel (an alloy of iron and other elements), he continued. Given a conductive medium for electrons to travel across, electron-gripping iron atoms can steal aluminum’s more weakly held electrons, converting them into electrically charged ions that are soluble in water (or tomato sauce). Acidic foods like tomato sauce, fruit juices, and wine reductions are conductive enough to provide such a pathway. So when you see holes in the foil covering a pan of tomato sauce, you are looking at areas where the pan has stolen electrons from the foil, converting the aluminum atoms into a substance that can dissolve in the sauce. The tomato sauce is serving as the getaway car in an electron heist masterminded by the steel pan. If you put acidic food in a glass, plastic, or aluminum container and cover it with foil, it won’t develop holes because those materials don’t have sticky fingers for electrons.
So was your lasagna safe to eat? Absolutely. The amount of aluminum that dissolved from the foil was just a few milligrams or less. People typically consume more than that amount every day from both aluminum pans and natural sources, like food.
THE BOTTOM LINE: If you are storing acidic foods in (nonaluminum) metal, wrap them in plastic wrap, not foil.