The Mystery Ingredient Revealed
Chocolate melts because it contains cocoa butter, the fat extracted from the cocoa bean. Cocoa butter is a complex fat with polymorphic properties. That is to say, cocoa butter is comprised of six different crystal forms that become fluid when exposed to heat. Once chocolate is removed from heat, the crystals in the cocoa butter will "reform," permitting the chocolate to solidify -- even at room temperature.
The Art of Tempering
Melting chocolate is often referred to as "tempering." The melting point of cocoa butter is around 36 degrees Celsius (97 degrees Fahrenheit), which is right around body temperature. To avoid scorching chocolate, most cooks melt chocolate in a double-boiler, not above 117 degrees Fahrenheit. If chocolate is melted too fast at a high heat (above 120 degrees Fahrenheit), the fats and the solids will separate, and the chocolate will darken and become crumbly.
Obstacles to Tempering Chocolate
There are two things that may slow the tempering process. The first is the ratio of cocoa butter to cocoa solids. A dark "gourmet" chocolate bar may contain up to 80% cocoa solids. The addition of milk also affects the point at which chocolate begins to melt. While milk chocolate appears more pliable than dark chocolate; the crystals in milk fat are incompatible with those in cocoa butter and melt at a different rate.