Q: Does a teeny bit of rust inside the can at the seam mean big problems? Just tell me—am I going to be responsible for major illness? There was no time to get another can of coconut milk for the cake.
—Faye Hess, New York, N.Y.
A: No news is good news so I'm assuming all turned out well with the cake. Whether what was inside the can was actually "a teeny bit of rust" or not, I think the best advice comes from Christopher Rosche, a spokesman for the Can Manufacturers Institute, a trade association in Washington, D.C. — when in doubt, use caution.
"A small amount of rust on just the surface of a food or beverage can does not normally present a serious health threat," he said in an email. "A heavily rusted can, however, has probably gone far beyond the expiration date and the contents of the can or the safety lining itself may have degraded. In addition, significant rusting could compromise the integrity of the can's lining and lead to bacterial growth. Heavily rusted cans should definitely be discarded."
"Rust on the inside of a can would indicate that the hermetic seal has been broken or compromised," Rosche added. "If it truly is rust, this was likely caused by a severe dent or puncture, or the can was opened at some point. Any contact with air would allow bacteria to grow and significantly increase the potential for contamination."
The U.S. Department of Agriculture's Food Safety and Inspection Service spells it out further in an article of shelf-stable food safety found on its website, fsis.usda.gov: "Cans that are heavily rusted can have tiny holes in them, allowing bacteria to enter. Surface rust that you can remove by rubbing with your finger or a paper towel is not serious. You can keep these canned foods. If you open the cans and there is any rust inside, do not eat the food. Rust (oxidized iron) is not safe to eat."
What to do? Any suspect cans recently purchased should be returned to the store, Rosche said, while older cans should be discarded.
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