When you make coffee, the water extracts oils and other compounds from the coffee beans. This process is called brewing. The longer the water is in contact with the coffee beans, the more it will extract. That’s why French press coffee can taste stronger than drip coffee.
During brewing, some of the compounds in coffee beans are degraded by heat. This gives coffee its characteristic aroma and flavor. But if the brew temperature gets too high, these same molecules will start to break down further, creating new molecules that have a bitter, burnt taste.
The ideal brew temperature for drip coffee is between 195 and 205 degrees Fahrenheit (91-96 degrees Celsius). If your coffeemaker doesn’t have a built-in thermostat, you can buy an inexpensive external one that you can attach to your pot. This will help ensure that your brew temperature stays within the ideal range.
If you find that your coffee tastes burnt even when you brew at the right temperature, there are a few other possible causes. One is using pre-ground coffee; because it’s been sitting around for who knows how long, it may have lost some of its flavor potential before you even brewed it. Always use freshly ground beans for best results.
Another possibility is that your coffeemaker itself is causing problems. If it hasn’t been cleaned recently, built-up oils and residue from previous batches of coffee can scorch easily and impart a burnt flavor to your current batch. Be sure to clean your machine according to manufacturer’s instructions on a regular basis – most recommend doing this every few months or so – to prevent this from happening.