Can Science Explain Why Microwaving Coffee Tastes So Awful?

Hot coffee is meant to be hot, and cold coffee should stay cold — that's the rule. The issue with hot coffee is that you can't enjoy the whole cup while it's at the perfect temperature unless you're willing to down it quickly. So, when it cools down, you have a few options: drink it cold, reheat it, or just dump it and brew a fresh pot.

These three options are either completely acceptable or utterly barbaric, depending on who you ask. For some, cold coffee is a nightmare, while for others, reheating it makes it undrinkable. But try telling that to someone who microwaves a cup of nine-hour-old Folgers every afternoon at 4 to power through the rest of the day, ignoring any judgments from coffee purists.

Research on why reheated coffee tastes so different is minimal, but it's likely tied to our sense of smell. Humans struggle to separate taste from smell, and coffee hits all five flavors detected by our taste buds: sweet, salty, bitter, sour, and umami. Your personal sense of smell plays a big role in how coffee tastes to you, no matter its temperature or how that temperature was reached.

The chemical makeup of coffee is incredibly intricate. While its reputation largely stems from its caffeine content, the flavor of coffee actually comes from over 1,000 distinct chemical compounds. Additionally, the final taste of the coffee you savored this morning was shaped by a myriad of factors, including the temperature and climate conditions during the beans' growth, the timing of the harvest, as well as the methods of drying, storing, roasting, grinding, and brewing. While the compound 3-methylbutanal might lend your coffee a caramel-like flavor, and ethyl nonanoate could impart fruity notes, each stage of the process uniquely influences the expression of these compounds.

Now, let's talk about reheating.

"Reheating coffee, in theory, can be a perfectly acceptable way to enjoy a flavorful drink," says Christopher Hendon, a postdoctoral fellow in the Chemistry Department at MIT and author of *Water for Coffee*, a book exploring coffee's interaction with water. "In reality, however, this rarely works as intended because people tend to reheat their coffee in ways that cause the loss of essential volatile compounds. The process of heating, cooling, and reheating causes the aromatic and tasty compounds to evaporate, leaving behind a less desirable cup."

Hendon notes that coffee aficionados tend to favor a brew that's been made within the last five to 20 minutes. Interestingly, most people prefer their coffee when it cools to around 150°F (65°C). This preference is linked to how our taste receptors respond to temperature in food. If something we consume is too hot or too cold, we can't fully detect the flavor-enhancing compounds. Given that coffee contains a wide range of these compounds, the temperature of the drink plays a significant role in its taste.

Brewing coffee brings out specific aromatic compounds. However, whether the cooling process alters the coffee's chemistry remains debated; Hendon believes it’s "absolutely benign," while others argue it increases the acidity as the coffee oxidizes when exposed to air. Reheating the coffee to its original temperature may restore the perfect taste, but it could also trigger chemical reactions that change the flavor further. And if you’re reheating coffee with milk or sugar, additional chemical complexities will affect the taste.

While many coffee purists will insist that once coffee cools, it’s hopeless, others argue that reheating it slowly is key to avoiding extra chemical changes. There are even tips for reheating coffee in the microwave. You might want to share this advice before your next heated coffee debate.