Some people guzzle their morning coffee, emptying their mugs and infusing themselves with caffeine as soon as possible. Others luxuriate, taking sips of the steaming brew as they wake up, have breakfast, and start the workday.
I’ve always been firmly in the latter camp: I want my beloved morning coffee to last as long as possible. But I admittedly take the savoring a little too seriously sometimes—and find myself with a still-half-full cup of room-temperature coffee by the end of my morning meeting.
At this point, a slow coffee drinker like myself might be tempted to pop the mug in the microwave for a few seconds to heat it back up. It’s a move that coffee aficionados scorn—but what impact does microwaving really have on coffee? I asked Senior Science Editor Paul Adams to find out.
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Why Lukewarm Coffee Tastes So Bad
Let’s get the bad news out of the way first: Coffee that has gone lukewarm or cold is never going to be as good as it was straight out of the pot.
In fact, the moment coffee is brewed, Adams told me, its flavor begins to slowly degrade.
As your coffee sits, many chemical transformations take place that alter the flavor. For example, the compound furfuryl thiol, a primary component of that fresh, roasty aroma you smell when you walk into a good coffee shop, begins to conjugate: It sticks to other compounds in the brewed coffee, forming larger molecules that don’t have an aroma.
At the same time, the coffee’s mild-tasting chlorogenic acid lactones break down into chlorogenic acid, which lends the brew an unpleasant acidic taste.
These transformations can’t be reversed, and they also can’t be prevented by keeping your coffee insulated (coffee’s flavor degrades because of time, not because of temperature loss). So if you’ve let your coffee go cold, the rest of your cup won’t taste the same as the part you drank when it was hot.
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Does Microwaving Make Lukewarm Coffee Even Worse?
That being said, it seems a shame to toss a half-full mug of coffee. So how bad is it, really, to reheat your mug in the microwave?
The truth is that a gentle warm-up in the microwave, just to a standard drinking temp of 140°F, actually won’t do anything to further compromise your coffee’s flavor—the damage is already done. In other words, nuking your coffee doesn’t have a negative impact on the way it will taste. (Be sure that your coffee doesn’t get hotter than 140°F, though, or you’ll start to evaporate flavor.)
The warmer temperature makes the flavor loss more obvious than it is in cold coffee, but if hotness is your main goal, go ahead and pop it in the microwave—it won’t taste quite as smooth and robust as it did at first sip, but sometimes the quick caffeine jolt is worth sacrificing some flavor.