What you see: Small holes in the aluminum foil you used to seal something up and silver-colored blobs on your food
What it is: A chemical reaction! The foil is reacting with something in the food to form a type of aluminum salt.
Eat or toss? Eat! Well, not the foil, or the weird globs, but the rest is fine.
This was really quite alarming. I had finally found a new use for the giant block of processed cheese we had acquired during a macaroni and cheese, uh, emergency. I took the block out of the fridge, removed the foil I had wrapped it in, and there, staring back at me were some frightening-looking silver-colored spots. Sort of like globs of metallic nail polish unceremoniously deposited on our food.
I took a closer look at the aluminum foil itself and saw tiny nibbly holes, which, of course, matched up perfectly to those silverly globs. It would appear that the cheese had been eating the foil.
I was certain something toxic, frightening and poisonous had happened, but it turns out that the situation was not so dire. The foil simply reacted with something in the processed cheese to form a type of aluminum salt. Most likely, says Matt Hartings, a chemistry professor at American University, the salt in the cheese reacted with the foil to form aluminum chloride crystals.
Aluminum is reactive
Aluminum is a particularly reactive metal, Hartings says, so if it finds itself in the company of certain chemicals, like, say, table salt, these reactions can occur.
"Whenever I dry brine meat, I never cover it with aluminum foil for this reason," he says, noting that if the foil touches the salt rubbed onto the meat, it could form aluminum chloride crystals like those on my cheese.
Salt isn't the only thing you should be careful with around foil. Given aluminum's reactivity, a variety of chemicals can cause different types of aluminum salts to form. Acids, like vinegar, can react to form aluminum acetate. Logically, high acid foods, like tomatoes and berries, may also react with foil.
So, eat or toss?
So, back to my silver-spotted block of processed cheese. Metal is not food and aluminum is not a mineral that our bodies need (unlike, say, iron), though it is used in very limited quantities in vaccines , antacids and some aspirins. There are also conflicting reports about whether excessive aluminum exposure can be harmful. So I concluded that I didn't want to eat the silvery globs. The rest, however, looked fine.
Hartings, author of Chemistry in Your Kitchen, confirms that aluminum chloride would only be present in areas where those silver-colored spots are visible. I cut off the chunk where the reaction had occurred and confidently ate the rest.
Matthew Hartings. Associate Professor. Department of Chemistry. American University. Author of Chemistry in Your Kitchen.
Aluminum. Toxic Substances Portal. Agency for Toxic Substances and Disease Registry. Centers for Disease Control and Prevention.
What makes aluminum foil pit or disintegrate? Foodsafetysite.com
Klotz, Katrin et al. “The Health Effects of Aluminum Exposure” Deutsches Arzteblatt international vol. 114,39 (2017): 653-659.
Holy moly, foiled again!