Let's assess the risks of those rock-hard leftover bits.
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It's a bitter, silent standoff between your friend and his two roommates—someone left explosive red sauce all over the microwave two months ago and no one's made a move to clean it or fess up. Food filth is now snowballing: Remnants of frozen Teriyaki chicken bowls, nacho cheese, soup stains and an unidentifiable oily film cover the microwave corner to corner. The last time your friend threw his chicken parm leftovers into the crime scene for a quick zap, he swears he saw a food particle—probably from June—dislodge from the ceiling and fall into his bubbling cheese.
However revolting that food splatter looks, your friend's pretty sure he's heard all bacteria dies when you nuke something new. So even if that gnarly old crumb mixed in with his dinner, there'd be no health risk because it's heat sterilized—right?
Microwaves (inside and out) are swarming with germs, both from leftover food bits and our own skin contact. A study led by microbiologist Charles Gerba from the University of Arizona found that the door handle on a microwave is one of the dirtiest surfaces in a shared office space (yarg, reach for soap and water before handling that hot sandwich).
More bad news: The heating power of a microwave doesn't sterilize its contents as well as you might think—a recent study published in Scientific Reports found that microwaving a sponge actually leaves behind plenty of germs that can spread and make the sponge disgusting again. Back to the chicken parm—if the new food you're nuking is thick or an odd shape, it might cook unevenly (even with a turntable) leaving behind cold pockets for bacteria to thrive in, says microbiologist Julie Torruellas Garcia, associate professor in the Department of Biological Sciences at Nova Southeastern University.
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The Worst That Can Happen
You'd have to be really negligent (and practice an aversion to plates) to get in cross-contamination trouble here. If your friend thaws raw, frozen chicken right on top of the turntable in the microwave, or there's splattering and raw chicken juice spills, bacteria like E. coli or salmonella could linger and contaminate the next food that sits on the turn table, even if you zap it. Another bad sitch: If your friend uses the restroom, doesn't wash his hands, and passes E. coli, aka poop, on to the inside of the microwave, your food could make contact with the bad bugs, triggering a bout of food poisoning or the stomach flu.
What Will Probably Happen
"If your food's on a plate or inside a container, there's almost no risk," says Ben Chapman, associate professor and food safety extension specialist at North Carolina State University. (There are no legit studies about the dangers of dirty microwaves, but most students survive their heinous college apartment microwaves, which means we can relax.)
Plus, food particles that explode onto the microwave ceiling (and could actually fall into your food weeks later, though that's unlikely) are usually tiny (we've never seen grime that's inches thick—fingers crossed it's not out there) and when you turn the microwave on, all the water is sucked out of those small suckers, wiping out pathogens that would give you any potential trouble, Chapman says.
What You Should Tell Your Friend
Not ready to break the cleaning deadlock? Plate and cover your food so it doesn't mingle with last week's beef taco bits and you're good to go. When your friend's roommate fesses up to the marinara mess, suggest he nuke a cup of water for a few minutes (the steam will soften those hardened pieces of food so they can be wiped off easily), then use a rag dipped in a cleaning solution with ten percent bleach to end bacteria on the inside and out, Garcia says.
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