Why Does Reheated Coffee Taste Bad?

It has nothing to do with the microwave and everything to do with the volatility of aromas and chemicals in coffee. 

Coffee is all about science. The drink has over 1,000 aroma compounds, plus a complex profile of proteins and acids.

Several studies point out that the intensity of the 'roasty-sulfury' odor of coffee plays a key role in how it tastes. The odor decreases rapidly after initial brewing as it loses those aromatics to evaporation, resulting in a loss of taste.

Also, some of the different compounds in coffee fall out of solution as it cools. Proteins tend to sink to the bottom while oils float to the top, leaving a skim of oil on top of old coffee. The acidity also increases, and the taste becomes sourer. Essentially, the flavor profile breaks apart and can't be put back together again—just like Humpty Dumpty.

So what's the best way to reheat coffee?

Experts and regular folk alike agree. Avoid the microwave. Store hot coffee in an airtight thermos. Or better yet, start from scratch when you want a hot cup.

Some people in surveys have said that reheating coffee in the microwave to "warm it just enough" to drink is better than reheating it to very hot...but this may depend on how cool your coffee has gotten in the first place, or how much of its original flavor profile was lost before reheating.

I hope this helps you enjoy your next hot cup of coffee!