Reheating coffee, whether in a microwave or on the stovetop, can lead to a distinctly bitter taste. This phenomenon is due to chemical reactions that occur when coffee is subjected to heat after its initial brewing.
According to Emily Rosenberg, director of education and training operations at Stumptown Coffee, coffee beans contain acids and compounds that turn bitter when heated. During the roasting process, chlorogenic acids in green coffee beans break down into quinic acid and caffeic acid. While chlorogenic acid has a bitter taste, quinic and caffeic acids possess a more pronounced bitterness and astringency.
"All coffee has some amount of bitterness," Rosenberg explained. "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 coffee is reheated, the production of quinic and caffeic acid increases, resulting in a more bitter flavor profile.
Michael Phillips, director of coffee culture at Blue Bottle Coffee, added that volatile compounds, responsible for coffee's aroma and flavor, degrade when coffee is reheated. This leaves behind the more bitter components that are less susceptible to heat.
Furthermore, tiny particles in coffee, particularly from methods like French press, can continue to brew and extract more bitter flavors when reheated. Rosenberg likened reheating coffee to putting a finished cake back into the oven, altering its flavor and texture.
Traditional coffee pots that use hot plates to keep coffee warm also contribute to bitterness. Rosenberg noted that this is why old-school diner coffee pots, which kept brewed coffee on hot plates, went out of fashion. Ideally, brewed coffee should be consumed within an hour or 90 minutes of brewing for the best taste.
While all roasts will become more bitter when reheated, dark roasts may exhibit this more prominently. Dark roasts have undergone more heat during their roasting process, leading to a higher concentration of bitter-tasting quinic and caffeic acids from the outset.
To avoid bitter reheated coffee, experts suggest alternative methods for maintaining warmth. Rosenberg recommends using insulated to-go cups or thermoses, even when at home. While these will not keep coffee at its ideal flavor indefinitely, they maintain temperature longer than a standard mug.
Phillips noted that while a thermos can keep coffee hot, its optimal flavor may start to change after 30 to 45 minutes. He personally prefers coffee when it has cooled to around 125 degrees Fahrenheit, as the sweetness becomes more apparent.
Another strategy involves preheating brewing equipment and mugs with hot water before use. This helps to maintain the coffee's temperature. Additionally, brewing smaller batches of coffee multiple times throughout the day can serve as a natural break and ensure a fresher, better-tasting cup.
Rosenberg described the process of brewing coffee as a comforting ritual that can provide a moment of presence, especially for those working from home. She contrasted this simple two-ingredient process with more time-consuming activities like sourdough baking, highlighting the low stakes and rewarding nature of brewing coffee.