How Much Rust Is Too Much For Canned Food?

I remember when I learned about botulism in middle school. My science teacher warned us to "throw out any dented and rusted cans." This lesson was happening during a recession in the early '90s when I lived in a blue-collar household. My family was struggling like most living in the "Rust Belt" of America. So that's probably why my mom practically roared when she saw me throwing her canned food into a garbage bag. I told her what my science teacher said. She told me to put it all back. "A little rust ain't hurt nobody," she said.

Years later, I found out she was right. The U.S. Food and Drug Administration put out a factsheet that says rust is okay if you can rub it off with your finger. Rust is that red, sometimes orange or brown stuff that appears on metal. It's a chemical reaction called corrosion, and a sign that the can may be compromised. The reaction happens when air and water come in contact with the metal of the can. Rust on the surface that hasn't penetrated the metal is the type that will wipe off. If the rust is stubborn, try rubbing it off with a paper towel. According to the FDA, the can underneath should be fully intact, with no holes, damage, or sharp points. Those cans are good to use. Just be sure to clean off all the rust before opening to prevent contamination of the food as you empty the can.

Michigan State University Extension calls this allowable rust "surface rust." MSU warns people to throw out cans that are "heavily rusted." This includes spots of deep rust that have obviously broken the surface of the can. When you try to wipe away this rust, you will see more rust and feel a bumpy, damaged surface underneath. Heavily rusted cans also have holes. Some are dented with sharp, rusty points and edges. Look for cans that are leaking through the rusty parts, too (and if your can is bulging, throw it out). The USDA also warns you to check the inside of the can for rust. If you see any rust on the inside of a can, discard the contents that came out of the can. They have already been exposed to rust and possibly dangerous bacteria.

So, my mother was right. Rust on a can is okay if you can wipe it off with your fingers or a paper towel. The problems only begin when the rust doesn't budge.