A cup of coffee has a predetermined lifecycle: You pour a steamy mug of joe, set it down next to your laptop to refrain from burning your tongue, and before you know it, an hour has passed and your coffee is stone cold. You pop it in the microwave to nuke it for a few seconds, press the cup to your lips and grimace. It’s bitter. Bitter in a way that makes you wonder if someone poured a Romeo and Juliet-style vial of poison into it.
Does this sound familiar? No matter how you’re trying to reheat your coffee ― in the microwave, on the stovetop, whatever ― you’ve no doubt shared this experience. Because however you do it, reheating coffee brings out compounds that make it taste decisively more bitter. We talked to experts who explained how this happens and offer a few realistic solutions to help you avoid falling into this pattern.
Now that you’re working at home more than ever, read up and never drink another bad cup of reheated coffee.
Why reheating coffee makes it taste bitter
Emily Rosenberg, director of education and training operations at Stumptown Coffee, explained to HuffPost that before your coffee beans are even roasted, their DNA is made up of acids and compounds that are just waiting to turn bitter when they’re heated up.
Green (unroasted) coffee contains chlorogenic acids, and the roasting process breaks those down into quinic acid (whose flavour you can associate with quinine in tonic water) and caffeic acid. While chlorogenic acid has a bitter taste, quinic acid and caffeic acid both have an even more pronounced bitter, astringent flavour.
“All coffee has some amount of bitterness,” Rosenberg said. “But in freshly brewed coffee, there is also plenty of sweetness and acidity that balance the bitterness and create a complex and delicious-tasting coffee.”
When you reheat your coffee, you encourage more production of that quinic and caffeic acid, therefore giving your coffee “even more bitter, astringent, gnarly flavour,” Rosenberg said.
Michael Phillips, director of coffee culture at Blue Bottle Coffee, elaborated: “It all comes down to two words: volatile compounds. And coffee is full of them. These are the things that make a properly roasted and prepared cup of coffee both taste and smell great. As you can see right in the name, however, they are volatile and easily fall to pieces. When you reheat coffee, all of the good stuff in the coffee starts to disappear and the resulting cup leans toward the more bitter components of coffee that stick around through the heating process.”
There are also tiny particles floating around in most cups of coffee ― especially if you’ve used a French press ― that continue to brew and get more bitter when you reheat your coffee.
“That coffee is sitting in there and swirling around, and it’ll almost continue to brew, essentially, and you’re extracting flavours that you wouldn’t necessarily want to continue to extract,” Rosenberg said.
Rosenberg put it into perspective: “You’re cooking an already finished product. You wouldn’t put a cake back in the oven, because it’ll dry it out and totally change the flavour and texture of it. In a similar way, you can think of coffee as a finished product. And if you continue to cook it, essentially, it’s gonna change the flavour of it.”
What about coffee pots that keep your coffee warm all day? Do those make coffee taste bitter, too?
“Yes,” Rosenberg said. “Any kind of brewer or carafe that applies heat to the coffee to keep it warm (rather than just insulating it to maintain temperature) will bring out that bitter, metallic flavour.”
Phillips agreed.
“This process is why those old-school diner coffee pots went out of fashion, because they kept the brewed coffee on hot plates after brewing,” he said. “While the hot plates kept the coffee hot, they also made it taste bad to the point of it becoming a diner signature.”
Rosenberg said brewed coffee will taste best if consumed within an hour or 90 minutes of brewing, no matter what.
Do all roasts of coffee ― dark, medium, light ― turn bitter upon reheating?
Every type of roast will taste more bitter upon reheating it, to a certain degree. But Rosenberg said a dark roast coffee’s bitterness will be even more pronounced.
It’s for the same reason we just talked about ― because a dark roast has had more heat applied to the beans during the roasting process, it’ll contain more of the bitter-tasting quinic and caffeic acids than lighter roasts in the first place.
What to do instead of reheating your coffee
Expert advice on this topic has ranged from “set your microwave to 80% power” to “heat it slowly on the stovetop.” But Rosenberg has a much easier suggestion, one you’ll be embarrassed you didn’t already think of.
“People who are working from home probably already have a thermal to-go cup or an insulated cup,” Rosenberg said. “When you’re drinking from home, you might not think to use it ― you usually drink out of your mug, which will cool down a lot faster because it allows more surface area to come in contact with the air ― so I’ll just put it in my to-go mug that I’d normally take out to go get coffee at a cafe.”
Phillips offered that same suggestion, with the caveat that a thermos won’t last you all day.
“The cup will still start to falter around 30 to 45 minutes in terms of the best flavour, but it will be piping hot the whole time,” he said. “The flavour of good coffee changes as it cools, and most professionals enjoy it most at lower ranges. For me, I like it best when the coffee has cooled to around 125 degrees because the sweetness is more apparent.”
Rosenberg said you should also make sure to preheat whatever container you’re brewing into, whether you’re doing a manual brew or a Mr. Coffee. Heat up some hot water in a kettle, then pour it into your pot, swirl it around a little and dump it out before you brew into it ― voila, your pot will be warm. The same goes for the mug you’re drinking out of. Take your mug and slosh a little hot water around in it to maintain that temperature even better.
Use coffee as a way to break up your day
While it might feel nice to brew a massive amount of coffee and be set for the day, keep in mind that at home, you’re likely not taking as many breaks as you would at the office. And making smaller amounts of coffee several times throughout the day can help build a break into your new daily routine.
“It can be a treat to have a small amount of coffee in the morning, go back into the kitchen at 11 a.m. and brew a little bit more,” Rosenberg said. “Brewing smaller amounts more frequently will help keep it warm and give you breaks through the day.”
“The process of brewing coffee is something that’s very comforting to me,” she said. “It’s a nice little moment to just set aside whatever else I was doing and just be present for a second. It’s something that I really enjoy and appreciate, especially in times like these.”
Rosenberg pointed out that brewing coffee is a lot less of a time commitment than the at-home sourdough bread-baking that’s been so popular during the coronavirus pandemic, with just as great a reward.
“Coffee is so simple. It’s just two ingredients, versus baked goods that use 10, or bread that you’re spending hours and hours doing,” she said. “With coffee, the stakes are pretty low.”
So does Rosenberg ever cave and use the microwave herself?
“I don’t own a microwave,” she said. “The option’s not on the table.”