Does Coffee Lose Caffeine As It Sits Out?

The tantalizing taste and aroma of freshly brewed coffee are unquestionably some of life's greatest simple pleasures. However, leftover coffee is also a fact of life, and although coffee snobs may shudder at the mere thought, the remains of your morning brew can be a lifesaver when you need a quick afternoon boost. One critical factor to consider when deciding whether to reheat that old cup of coffee is the caffeine content. (Some would argue that when it comes to coffee, the caffeine is far more important than the flavor.)

Considering that leftover coffee tends to taste significantly worse than the freshly brewed stuff after just a few hours (especially when reheated), it seems logical to think that coffee might lose its mysterious caffeine powers over time along with its flavor. While it's true that coffee can lose some caffeine as it sits out for long periods if stored incorrectly, the caffeine in brewed coffee is relatively stable and breaks down at a much slower rate than, for example, the flavor compounds. In other words, day-old coffee will probably taste slightly stale, but it should give you more or less the same buzz as a fresh pot.

To preserve both flavor and caffeine, it's best to store your leftover brewed coffee in an airtight container in the fridge. This minimizes exposure to environmental factors like light and air that speed up the degradation of both caffeine and flavor compounds. When stored properly, leftover coffee can be safely consumed for around three to four days, after which it's best tossed (and probably tastes quite unpleasant anyway). 

The question of how best to reheat coffee is a matter of fierce debate. The most convenient and quick option is simply sticking it in the microwave, but some prefer gently reheating it on the stovetop. In any case, reheating that old cup of joe won't decrease the caffeine content — in fact, reheating may even slightly increase the caffeine concentration as some of the water evaporates. If you're looking for other great ways to use up extra java, try adding leftover coffee to your overnight oats for the ultimate breakfast upgrade, or freezing it to make magical coffee ice cubes.