If your coffee usually tastes bitter when it comes out of the microwave after reheating, what you’re noticing is all the ways your coffee is changing on its own.

Your coffee’s taste begins to degrade the moment you pour it into the mug — as it cools. However, it can take half an hour or more before the change in the flavor is noticeable.

Allowing freshly brewed coffee to sit causes its compounds to break down into acids, resulting in more bitterness. Furthermore, oxidation degrades your coffee’s flavor and aroma.

If coffee is reheated past 120-140 degrees Fahrenheit, the high temperatures can restart the brewing process, causing more bitter-tasting compounds to be released.

While there’s no real way to stop brewed coffee from losing its flavor, a bitter cup of coffee is still safe to drink even if these sorts of chemical reactions have taken place.