Coffee joulies are bean-shaped capsules of phase change material, or PCM, that regulates the temperature of hot beverages, including tea, cider and soup. The PCM, which is covered in a silver stainless steel shell, stores heat from hot coffee and releases it once the drink falls below 140 degrees Fahrenheit.
The interior PCM begins as a solid substance and melts with the heat of the hot drink. The PCM liquid stores the heat and slowly emits that energy to warm the beverage. The use of multiple joulies in an insulated container with a lid increases the effectiveness of keeping beverages at a drinkable temperature, according to the company's official website.