Save Coffee Grounds For One Simple But Powerful Use Around The Kitchen

Microwaves become haunted by the ghosts of dinners past. Burnt popcorn, reheated fish, scorched cheese, and whatever exploded unnoticed onto the ceiling can leave the appliance reeking of the condensed history of recent meals. If your microwave still smells like last night's dinner, the remains of this morning's coffee could help clear the air. Combined with a little water, heated in the stinky microwave, spent coffee grounds can deodorize the enclosed space.

Start by wiping the inside of any obvious splatters or burned-on food, since a bowl of coffee grounds can only do so much when the source of the stink is hanging like a bat in a cave from the roof of the microwave. When it's fairly visually clean, combine about two tablespoons of used coffee grounds with ½ cup of water in a microwave-safe bowl. Heat the mixture up for a few minutes, allowing the mixture to get hot enough to produce plenty of steam, then leave the door closed briefly before carefully removing the bowl. Once the interior is cool enough to handle, wipe down the walls, ceiling, turntable, and door. The grounds have served one last useful assignment, and the hot water does some of the dirty work for you — steam softens any persistent, crusty food splatters, so you can easily wipe them away instead of scrubbing with elbow grease. 

For particularly persistent smells, there's another coffee-ground trick worth trying, and this one doesn't require turning the microwave on at all. After brewing your coffee via your chosen method, spread the spent grounds out and allow them to dry thoroughly so you don't get mold growth; you can place them on a plate or small tray for this. Put about a cup of the dry grounds in an open bowl or container, set it inside the cleaned microwave, and shut the door. Leave everything alone overnight, with the microwave off. The next morning, remove the bowl before using the appliance. This low-effort treatment will diffuse a fragrant microwave long after the offending meal is gone. 

There's some material science behind coffee grounds' reputation as a cooking odor absorber. Smells are produced by volatile compounds that make their way into the air and eventually up your nose. Researchers studying spent coffee grounds have found that their surface can absorb volatile organic compounds, meaning the airborne molecules adhere to the material. In a 2019 study published in the Journal of UOEH, which involved 14 different VOCs, used grounds adsorbed every compound tested to varying degrees. The researchers weren't studying microwaves or the specific compounds responsible for last night's bourbon-glazed salmon, and the findings don't establish coffee as a universal odor eraser. But they do help explain why leaving grounds exposed inside a small, enclosed space can be useful for more than replacing one strong smell with another.

If the microwave continues to smell after you've removed the food residue and tried deodorizing it, the problem may require more than another round of coffee. Some over-the-range microwaves contain charcoal filters designed to capture cooking odors, and those eventually need to be replaced. For ordinary malodorous apparitions, though, yesterday's brew may be just the exorcist you need.