A coffee maker with coffee in the pot, against a white background.
The Important Reason You Can't Let Used Grounds Sit In Your Coffee Maker
Not only do leftover grounds in your coffee machine get sticky and more difficult to remove over time, but the damp organic material also becomes a hotbed for microorganism growth.
However, University of Arizona professor of microbiology Charles Gerba, PhD points out that “Mold has a better chance of growing in there than bacteria."
Instead, harmful bacteria colonies have a higher chance of being found in the coffee machine’s water reservoir and on the pot’s handle, according to the results of one test.
If all that wasn’t bad enough, coffee oils can collect in improperly cleaned places and will inevitably turn rancid, which will make your next fresh pot of coffee taste bad.
All coffee makers should be thoroughly cleaned after each use. Espresso makers need their steam wands wiped, and heads and filters should be brushed free of coffee grounds.
Drip trays and waste containers should also be emptied when full. Professor Gerba recommends washing removable parts with soap and water, then drying them with paper towels.