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When your workday suddenly fills with back-to-back meetings and approximately 347 unread emails to return, it can be all too easy to forget about that cup of coffee sitting on your desk. When that happens (and if you’re still craving a caffeine hit), the solution seems obvious: Reheat it in the microwave.
Sure, it’s a quick and easy way to turn that lukewarm liquid piping hot, but the final result might not be quite as delectable as you hoped: Nuking your coffee (or espresso, macchiato, cappuccino, Americano, or any other kind of “-o”) can cause chemical changes that could heighten the bitterness, Masha Zanozina, a Los Angeles–based regional sales and account manager for Counter Culture Coffee and a coffee championship judge, tells SELF.
But the microwave is only part of the story. Coffee is a complex beverage that contains more than 2,000 compounds, including carbohydrates, fats, amino acids, vitamins, and minerals. When you brew it, you extract compounds from the grounds, and those, in turn, “create that flavor of the coffee,” Zanozina says—like how bitter, acidic, and astringent it is.
Water temperature matters a lot for that extraction. Too cold? Too few compounds will be released (underextraction). Too hot? Too many compounds will be released (overextraction). Either way, the result likely won’t taste too good, so it’s important to find a happy medium, Zanozina says. You want the final temp to fall somewhere between 195 and 205 degrees Fahrenheit, according to the National Coffee Association—7 to 17 degrees below the boiling point. If your home coffee maker doesn’t offer the option to set it yourself and you’re in the market for one that does, Zanozina recommends the Fellow Aiden precision coffee maker, which comes out in October but is available for preorder now ($365, fellowproducts.com), Moccamaster KGBV Select ($359, amazon.com), xBloom Studio ($499, xbloom.com), and—her personal fave—the Ratio Six coffee machine ($339, amazon.com). For a cheaper option, we like the Cuisinart glass carafe coffee maker, which experts say it consistently gets your brew hot enough ($75, amazon.com).
Still with us? Good, because water temperature and a microwave field trip aren’t the only factors that can influence the palatability of coffee: Letting yours sit out for an extended period of time will itself hurt the taste as it becomes “more or less the [coffee] equivalent of stale,” coffee expert Christopher Hendon, PhD, an associate professor in the department of chemistry and biochemistry at the University of Oregon, tells SELF.
One reason is that volatile compounds, the stuff responsible for that distinct coffee smell, dwindle with time, temperature, and air exposure—“just evaporation, basically,” Dr. Hendon says. Simultaneously, another, slower process known as oxidation can also be occurring. “It takes the sort of sweet, fruity flavors and sort of blunts them,” Dr. Hendon says. (To keep from falling into this trap, try to drink coffee within an hour of brewing it, according to Zanozina.)
So by the time you stick your mug in the microwave, “a lot of the positive flavors and smells that we like have left the cup already,” Dr. Hendon says. Then that sudden rush of heat can potentially spark another series of chemical reactions. For example, chlorogenic acid breaks down into caffeic acid and quinic acid, both of which can intensify the bitterness, Zanozina says. What’s more, any previously added milk can scald, which “would further change the taste of your coffee beverage,” she adds. (One thing the microwave won’t mess with? Your cup’s caffeine content—that’ll stay stable.)
Not every coffee responds in the same way—dark-roast and cheaper varieties are more susceptible, for example—but the overall effect typically isn’t a positive one, according to Dr. Hendon. Regardless of roast color or coffee quality, coffees “do tend to taste a little funny and a little different when you reheat them” whether due to volatile loss, oxidation, or the reheating process itself, he says.
So how do you avoid ending up with a beverage that won’t make you gag when you take a sip? Prevention, as always, is better than cure: Make sure your coffee stays warm from the jump, according to Zanozina. One option is to invest in a self-heating mug like the Ember mug, according to Dr. Hendon. (For what it’s worth, SELF’s director of editorial operations is “madly in love” with his own.) For a no-cost alternative, try preheating a mug at home by pouring a little hot water in and out before you fill it up with the joe, Zanozina adds.
But if you’ve already nuked your coffee and don’t have the time or inclination to brew a replacement, you’re not completely sunk. Stirring in some milk, creamer, sugar, or any other add-in should help take the bitter edge off. Or you can try counteracting it with a little chemistry of your own. Acidity is on the opposite end of the spectrum as bitterness, so take a page out of the Italian book and try squeezing in some lemon juice.
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