The Sweetest Engineering? Chocolate

With Valentine's Day just around the corner, forgetful romantics will soon be scrambling for last minute gifts. But that will not be the case for the 15 students who attended the Feb. 11 Chocolate Workshop in Yale's Center for Engineering Innovation & Design (CEID).

Led by CEID design fellow Ngoc Doan, SEAS senior research scientist Larry Wilen, and CEID design aides Genevieve Fowler and Catherine Jameson, the workshop provided compelling evidence that making chocolate is perhaps just as suited to engineers as it is to chefs. “It's actually a lot like tempering steel,” Wilen said.

The reason is chocolate’s ability to exist in more than one crystal structure — an ability known as polymorphism. “The stability of a particular crystal structure, or polymorph,” said Wilen, “is related to the temperature at which that structure melts. But while that phase transition is often thought of as happening at a specific temperature, a liquid does not actually become a solid the instant it hits that temperature. For example, water can be supercooled below zero degrees Celsius before it becomes ice.”

Instead, different polymorphs form at different temperatures, with chocolate’s less stable structures forming at lower temperatures; when cooled down quickly, all the structures form at similar rates and the chocolate ends up being not only soft at room temperature but also less tasty. The best chocolate therefore consists of only the most stable crystals.

To perfect their polymorphs, workshoppers melted chocolate over a double boiler, then cooled it in an ice bath. Just when the chocolate began to solidify, it was carefully heated to 89 degrees — just below the melting point for the most stable polymorph. Finally, the tempered chocolate was poured into custom plastic molds that the students had designed using 3D printed symbols and laser cut wooden letters, then fabricated on the CEID’s vacuum former. The designs included snowflakes, Valentine’s Day hearts, and even CEID logos.

But though for at least one team, four simple letters — Y A L E — were all they needed to get chocolate perfection. “I like the idea of YALE chocolate,” said freshman Stuart Monk, who helped create his team's design. “Who wouldn’t want a Yale Bar?”