The Science Behind Coffee Joulies
Coffee is hot, obviously. But coffee also contains a lot of thermal energy, better known as heat. Water, the main ingredient in coffee (duh), has the highest specific heat of any liquid at 4.18 joules per gram per degree celsius (J/g°C). In fact, water has the third highest specific heat of any substance, beaten only by hydrogen gas at 14.3 J/g°C and helium gas at 5.3 J/g°C.
Liquid water even has a higher specific heat than itself as ice at 0°C or steam at 100°C, both 2.01 J/g°C. Compared to other common substances such as granite at 0.79 J/g°C and iron at 0.45 J/g°C, water is truly remarkable.
Fortunately, we have a weapon: latent heat of fusion. You experience this property when you put ice cubes in your drink to cool it down. The ice requires a large amount of thermal energy to break its crystalline bonds (aka melt) to turn it into liquid water, 334 joules per gram (J/g), the highest of any substance. This heat is absorbed from your beverage, cooling it close to the melting point of the ice itself, 0°C or 32°F.
Coffee Joulies work the same way as ice cubes in your drink, but at a higher temperature. Our phase change material was chemically engineered to melt at 60°C (140°F), and with a latent heat of fusion of 220 J/g it is 66% as effective as ice at absorbing thermal energy. This is the best modern science can do, folks.
Here's some test data with Coffee Joulies in a high-quality vacuum-insulated thermos. We preheat the Joulies in the thermos with hot tap water while brewing our coffee so they are closer to the melting temperature of the PCM, letting it do its job better.
This is how it works:
- Coffee is served at 82°C (180°F), hot enough to scald and cause third degree burns, which is what happened in the infamous McDonalds lawsuit. Tea is served up to 96°C (205°F), making it even more dangerous.
- Coffee Joulies' PCM absorbs the excess thermal energy from the coffee or tea through the breaking of its crystalline bonds, turning from a solid into a liquid while sealed inside the stainless steel shell. The PCM stays at a constant 60°C (140°F) during this process, until it becomes fully molten.
- Once the coffee or tea reaches 60°C (140°F), the process reverses. The PCM begins solidifying, reforming its crystalline bonds, again remaining at its melting temperature of 60°C (140°F), slowly releasing all the stored thermal energy back into the coffee, keeping it at the optimal temperature of 140°F significantly longer.
Because Coffee Joulies work using physical chemistry instead of batteries, this process works over and over, with no recharging necessary. The PCM never wears out, and Coffee Joulies literally last forever.
We have found through testing that coffee is safe to sip starting at 68°C (155°F), and reaches that point in just a few minutes with Joulies, versus almost two hours without Joulies. The specialty coffee community considers 63°C-57°C (145°F-135°F) to be the ideal temperature range for coffee flavor. Using Joulies more than doubles the length of time you get to enjoy your coffee in this temperature range. Our favorite way enjoy coffee & tea is the Glass Mug Gift Set.
