You Should Never Reheat Coffee In The Microwave. Here's Why
Facing a morning without coffee seems like an unbearable task to many. Humans are so dependent on coffee that more than 2.25 billion cups are drunk every day worldwide (via PBS). Recent research has shown too that it's actually beneficial to your health as well.
Perhaps the one problem with coffee is the speed at which it gets cold. There are a number of steps that can be taken to avoid this dreaded outcome. Thermoses can be quite effective, keeping the coffee hot for hours (sometimes they're so efficient that they don't even allow the beverage to cool down to an acceptable drinking temperature). Battery-powered mug warmers are also making the rounds, and some of them can even plug in to the USB port of a computer (via The Full Moon Cafe). Or, you can submit to the cold coffee entirely, drop some ice cubes into it and turn it into an iced coffee.
But what about microwaving your coffee? Isn't this what pretty much everyone does to reheat their cup?
What happens to coffee when you microwave it
Unfortunately, despite how easy it is, coffee happens to be one of the many things you should never nuke. When you microwave a cup of coffee, you're changing the chemical makeup of the beverage.
Todd Carmichael, CEO and co-founder of coffee company La Colombe, didn't mince words on his feelings about microwaved coffee when talking to Tasting Table. "Coffee is a one-time use kind of deal. You make it, you drink it and if it gets cold, you make some more. Reheating reorganizes the chemical makeup of the coffee and totally ruins the flavor profile. Some things just don't work to reheat, and coffee is one of them. It's always best just to brew a fresh cup," he explained. "In dire straits, if you're absolutely out of coffee and staring at a cold pot from that morning... still don't reheat it. Drink it over ice. Trust me, it'll be better."
Even a cooling cup of coffee becomes bitter
Coffee contains more than a thousand odor compounds, which are responsible for the intoxicating smell of freshly brewed coffee (via The Kitchn). These begin to deteriorate over time (even if you don't microwave it) but the microwave accelerates the process. Over time, coffee also becomes more acidic which can lead to bitterness.
This means that if you've already left your coffee out for long enough to get cold, it's probably going to be bitter and acidic even if you do manage to warm it back up. Of course, if you're not a coffee connoisseur, you may not even notice the difference and a bit of acidity may be a small price to pay to rewarm your cup. But if you have a finely tuned tongue, you might be better off making a new batch.