Photo by Flickr user Elsie Hui

Physics finally explains the best way to get hot-but-not-too-hot coffee

Matthew Kassel
November 09, 2016

An enterprising science blogger at Wired, Rhett Allain, took it upon himself to examine the physics of hot coffee. In particular, he looked into the most effective methods for cooling hot coffee, which, as it happens, is best consumed lukewarm, despite the high temperature at which it is usually served. (Excessive temperatures—hot and cold—mask flavor.) This is a worthwhile inquiry, especially if, like me, you consume your coffee black, in which case there is no chance to cool down your drink with cow, soy, almond milk or whatever in-vogue creamers the kids are using these days.

Allain, who also drinks black coffee, tells us that he has two techniques to cool his beverage, one of which involves taking the lid off and letting the steam blow away, while the other involves leaving the lid on and blowing occasionally into the drinking hole. (It is unclear if blowing occasionally into the drinking hole is a common strategy to cool off coffee—it’s never occurred to me—but taking the lid off certainly seems practical, given the circumstances. My method is way more basic: I drink the coffee, even if I'm burning my mouth in the process, until it reaches a suitable temperature, no blowing necessary.) To test which method works best, Allain poured hot water into three cups—one without a lid, one with a lid he’d blow into, and another (the control) with a lid he wouldn’t blow into—sticking one thermometer into each cup to ensure accuracy. Perhaps unsurprisingly, he found that the cup without a lid cooled most quickly.

But his analysis really gets interesting when he explains, in a physical sense, how coffee can cool off—through heat conduction (a transfer of energy to another object with a lower temperature, which isn’t so practical unless you like burning your hand) and cooling by evaporation (which lid removal facilitates). “As water on the surface of the coffee evaporates, it leaves behind lower energy water molecules, resulting in a lower coffee temperature,” Allain explains. “Without a lid, the cup provides a large surface area for evaporation—and thus cooling. If you leave the lid on, you could possibly increase the evaporation rate by blowing over the hole, but this clearly isn’t as effective as removing the lid.”

In a final experiment, Allain also tests out whether those plugs that fit into the drinking hole of your lid to keep heat from escaping (and, perhaps, to prevent your drink from spilling) really work. The verdict? Not really. Plugs be damned! So, there you have it. Of course, you could obviate the need to cool off your beverage at all by purchasing an iced coffee. But what’s the fun of that?

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