Published March 1, 2013. From Cook's Illustrated.
These stainless-steel capsules regulate the temperature of hot beverages, provided that you use them in a travel mug.
Freshly brewed coffee can start out too hot to drink—and can too soon end up cold. Enter Coffee Joulies (“JOO-lees”), stainless steel capsules shaped like oversize coffee beans and filled with a proprietary material. The product promises to cool your coffee quickly to the ideal temperature (140 degrees) and keep it there.
We bought a set of Joulies for $49.95, which includes five Joulies and a carrying pouch, with instructions to use one “bean” for every 4 ounces of coffee. Then we brewed hot coffee and compared its temperature with and without Joulies in our favorite travel mug and in an ordinary ceramic coffee mug.
In the regular coffee mug, results were unremarkable (the temperature immediately dropped to a drinkable 140 degrees with the Joulies but then cooled as fast as the coffee without Joulies). In the travel mug, however, the Joulies made a big difference. The mug with the Joulies rapidly cooled to 140 degrees and held steady for 2 hours. The other mug gradually lost heat, starting out too hot and taking slightly more than 2 hours just to reach 140 degrees. Then both mugs slowly cooled at a constant rate.
So how do they work? The “phase change” material encapsulated within each Joulie has a melting temperature of 140 degrees. When it’s placed in an environment above its melting temperature, it absorbs heat, cooling the area around it until it is completely liquid. Then it slowly releases heat back into the environment until it becomes solid again. It works just like ice in a drink, which cools the drink to 32 degrees—the melting temperature of water—and then maintains this temperature until all of the ice is gone. Physics aside, the Joulies may work as advertised, but only if you’re sipping your brew out of a travel mug—and $49.95 is a lot to spend to address a small woe.