Edit: As noted in the comments, I’m testing these under a situation which is not the recommended method from the Joulies company. I am, however, testing them in the real-world situation in which I would most likely use the Joulies. I wanted to add this disclaimer to be fair to the guys struggling to turn this into a company. I stand by my results, but leave you to make the decision about how well the Joulies would meet your needs for the product.
Abstract: I recently received my set of Coffee Joulies (http://www.joulies.com/) after putting some money into their Kickstarter project. Excited to review and test them out, I came up with a simple experiment. I tested the Joulies vs. an empty control mug with just hot water vs. a mug with rocks of roughly equivalent size to the Joulies. Both the Joulies and the rocks dropped roughly 30 degrees in temperature immediately and then lost steadily at the same rate. The cup with just water didn’t see the immediate drop, but lost temperature steadily. I saw no significant or noticeable difference between the rocks and the Joulies. Given that the Joulies cost $50 and the rocks cost me walking out into the front yard, I’m skeptical about the value of the Joulies.
After finding 3 mugs of identical size and shape, I went out in the yard to find a handful of rocks which closely matched the volume of the Joulies. Following this, I set all 3 mugs up on the same wooden coffee table surface. I left the mugs, rocks, and Joulies overnight to allow them to equalize to room temperature.
The following day I boiled enough water to fill all three mugs in an electric tea pot. Once finished, I began the testing. Filling the first of the three mugs (the empty one). I started my stopwatch. I measured the temperature immediately using a digital meat thermometer. My method for obtaining temperatures was thus: place the thermometer in the water in the corner opposite the handle (all mugs were oriented the same way), then watch the temperature until it flattened out and began dropping. I measured the temperature as the maximum before the drop. Typically it only wavered by a degree or so.
Thirty seconds later I poured the next mug (with the rocks) and repeated the measurement for it, I then did the same with the 3rd mug (with the Joulies). By doing it thus, I was able to measure all of the mugs at the same elapsed times. I then continued this process at intervals until reaching 1 hour.
As usual, a graph is worth a thousand words. I’ve broken them up into two different graphs to prevent distortion from having changed from measurements every 2 minutes to every 4 minutes at the 20 minutes mark.
No that we can see the behavior of the 3 mugs and their contents, let’s examine some claims from the Joulie website.
1) “Their polished stainless steel shells are full of a very special phase change material (an ingredient in food) that melts at 140°F. When you put them in your coffee this PCM begins melting, absorbing a LOT of heat in the process and cooling your coffee down much faster than normal.”
I can’t argue with this, they did indeed rapidly cool the hot water down. However, the rocks and the Joulies had roughly the same effect in this regard.
2) “When your coffee reaches 140°F (the perfect drinking temperature) the molten PCM [Phase Change Material] begins solidifying again, releasing all that energy back into your coffee to keep it at a comfortable and delicious drinking temperature. The more heat you feed your Joulies, the longer they’ll keep your coffee warm.”
The Joulies behaved almost identically to the rocks and as such do not seem to have had any sort of magical technology. In fact, after the initial drop, the rocks maintained a higher temperature for a longer period of time. Also worth noting, the slope of the rocks and Joulie lines were very similar, showing a large initial drop and then a steady decline.
Obviously, this is a fairly limited experiment and it would be helpful to perform with various other size containers and materials. However, I do think that it is enough to show that the Joulies are not some sort of magical material. While there may indeed be some sort of special materials inside, the effects they had were not enough to be noticeable in a real life situation, and therefore do not justify the cost of the Joulies over simply buying a more massive mug.
Edit: A friend just sent me this similar post on testing and reviewing of Joulies by Marco Arment
Edit 1/15/12: Here’s another interesting article on the subject.