Q: Sometimes I’ve seen people use aluminum foil to cover frosted cakes. After a while, tiny holes begin to appear in the foil where it touches the frosting. What’s happening, and is it dangerous?
A: It’s common to see those little holes when you use foil to cover foods that are high in acid, particularly tomatoes or tomato sauces. It can leave dark spots on lasagne as well as holes in the foil.
Other things can cause the reaction, too, particularly salt and citrus fruit. Sugar is a little acidic and could cause the reaction.
The acid isn’t actually eating away the foil. It’s creating aluminum salts, such as sodium aluminum phosphate, which can get in the food. While studies haven’t shown a connection between aluminum salts and health effects, some people are concerned about exposure to aluminum and avoid it.