A Spanish study published in Frontiers in Microbiology examined 30 microwaves from homes, offices, and laboratories and identified 747 different types of bacteria living inside them.
While that sounds alarming, experts say it’s not entirely surprising.
Senior researcher Morten Poulsen from the Technical University of Denmark explains that microwaves are designed to warm food evenly, not to disinfect.
The radiation causes water molecules in food to vibrate, generating heat. But the waves don’t have the strength to destroy bacterial cell structures directly.
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That means bacteria can survive on inner surfaces or in spots where food doesn’t heat enough, especially around corners or splattered residue that never gets cleaned.
Dry spots help bacteria survive
According to Susanne Knøchel, professor of food microbiology at the University of Copenhagen, dryness plays a key role in bacterial survival.
Microorganisms are far more heat-resistant in dry conditions.
When bacteria dry out, they lose much of their internal moisture, which makes them more resistant to the heat generated by microwaves.
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This also helps explain why reheating often feels uneven: one part of your meal steams while another stays cool.
Susanne Knøchel notes that the inside walls of a microwave contain little free water, so they rarely heat up unless steam builds up during cooking, creating a perfect environment for certain microbes to linger.
Keeping it clean matters most
Most of the bacteria found in microwaves are harmless types that live on skin or kitchen surfaces.
The real risk arises if these fall into underheated or reheated food and multiply afterward.
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Experts recommend wiping the interior regularly with a clean, damp cloth.
A quick cleaning routine reduces bacterial buildup, and keeps your “quick meal” appliance a little safer.
Sources: Forskning.no, and Frontiers in Microbiology.
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