Reheating coffee makes it taste bitter due to the breakdown of chlorogenic acids into quinic and caffeic acid, which have a more pronounced bitter and astringent flavor. According to Emily Rosenberg, director of education and training operations at Stumptown Coffee, green coffee contains chlorogenic acids that, when roasted, break down into these bitter compounds. Freshly brewed coffee balances bitterness with sweetness and acidity, but reheating encourages further production of quinic and caffeic acid, resulting in a gnarly taste.
The Science Behind Bitter Reheated Coffee
Michael Phillips, director of coffee culture at Blue Bottle Coffee, explains that volatile compounds in coffee easily break down when reheated. These compounds are responsible for the great taste and smell of freshly brewed coffee, but reheating causes them to disappear, leaving behind more bitter components. Additionally, tiny particles in coffee, especially from a French press, continue to brew when reheated, extracting unwanted flavors.
Rosenberg compares reheating coffee to cooking a finished product: "You wouldn't put a cake back in the oven because it'll dry it out and totally change the flavor. In a similar way, coffee is a finished product." Coffee pots that keep coffee warm on a hot plate also cause bitterness, as the applied heat brings out metallic flavors. This is why old diner coffee pots fell out of fashion.
Does Roast Type Matter?
All roasts become more bitter upon reheating, but dark roasts are more affected because they contain more quinic and caffeic acid from the longer roasting process. Rosenberg notes that dark roasts have a more pronounced bitterness to begin with, which intensifies when reheated.
Best Alternatives to Reheating Coffee
Instead of microwaving or using a stovetop, experts suggest using an insulated to-go cup. Rosenberg recommends pouring coffee into a thermal mug to maintain temperature longer, as regular mugs cool faster due to greater surface area. Phillips adds that a thermos keeps coffee piping hot for 30-45 minutes, though flavor is best when coffee cools to around 125 degrees Fahrenheit, where sweetness is more apparent.
Preheating your coffee pot and mug also helps. Pour hot water into the pot, swirl, and dump it out before brewing. Do the same with your mug to maintain temperature.
Use Coffee to Break Up Your Day
Brewing smaller amounts of coffee several times a day can provide needed breaks while working from home. Rosenberg says, "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." She finds the brewing process comforting and a nice moment to be present. Coffee is simple—just two ingredients—and the stakes are low compared to baking bread.
Rosenberg admits she doesn't own a microwave, so the option of reheating coffee isn't on the table for her.
