Why Your Cooled Hot Coffee Tastes Awful

Riddle us this: iced coffee or cold brew (and yes, there are differences!) are delicious, but lukewarm coffee cooled down from hot is almost always terrible. That chilly-sweet coffee drink fresh from the fridge will cool you down and taste like angels dancing softly on your taste buds, but the Styrofoam cup you abandoned before your morning meeting might as well be battery acid. Logically, it doesn't make sense. Isn't all coffee the same?

Turns out that the answer is no. There is a difference between coffee that's been rapidly cooled, like iced coffee — or which was never hot at all, like cold brew — and coffee that has been allowed to cool naturally over a period of time, like that day-old sludge in your Stanley mug with the newly-replaced lid. The reasons behind this lie in the chemicals naturally contained in coffee, namely how they break down into unpleasant-tasting acids as well as the ways that our bodies detect and process them. Coffee science, like your ultra-customized Starbucks order, is actually kind of complicated.

Craft coffee roaster Dan McLaughlin told USA Today that chemicals called lactones are present in coffee, and the cooling process forces a chemical reaction that makes these organic bodies break down. In doing so, they release "carboxylic and chlorogenic acids [which] present themselves via a bitter/acidic flavor in the cup." Additionally, coffee oxidizes in ambient air, which adds to the nastiness. When hot coffee is forcibly cooled by ice, lactones don't have a chance to kickstart the unpleasant reaction.

But funky, nitpicky chemistry-class minutiae isn't the only culprit at fault. The quality of coffee you're using might also be to blame. Hot temperatures affect humans' ability to fine-tune their sense of taste. Bottom-shelf coffee might seem palatable when its natural notes are masked by steaming temps, but, as it cools, your tongue and sense of smell are able to more adeptly discern disagreeable flavors. Interestingly, the same goes for very cold temperatures. This would explain why the best brewing methods may salvage instant coffee and even produce decent iced coffee, but will still result in undrinkable swill when brought down to room temperature over several hours. High-quality coffee, on the other hand, is less likely to taste bad even after being left out in the open for a long time. As is usually the case, you get what you pay for.