The Physics of Chocolate

Most of us have experienced the joy of eating a well-made chocolate bar: the glossy appearance, the smooth mouthfeel, and the unique taste. I’ve always enjoyed eating chocolate, but as a graduate student in Applied Physics at Harvard, I saw an opportunity to pursue my interest further.

For my PhD qualification exam, I presented a set of experiments to investigate some of these aspects in more detail. There is plenty to be learned from studying chocolate from a material science perspective.

The pleasant cooling sensation of the chocolate bar in your mouth is caused by the cocoa butter crystals absorbing energy from your tongue as it melts. In the lab, we do a more controlled version of this using a machine called a differential scanning calorimeter, or DSC. With the DSC, we can increase the temperature of the chocolate in a controlled way and measure the amount of heat absorbed. When a phase transition occurs, like ice turning to water, a large amount of heat is absorbed.

Surprisingly, in chocolate, this happens several times as the chocolate melts, demonstrating that there are several crystal forms. Getting as many and as small crystals of the proper form is the goal of the tempering process, in which the chocolate is heated and cooled in a precise way before being formed into bars.

A high-quality chocolate bar breaks cleanly, whereas a bar improperly stored will crumble. This transition from a brittle fracture to a more flexible break is a common issue in many materials, including metals and plastics. Chocolate is a great material to study, since it melts just slightly above room temperature, in contrast to the much higher temperatures of many other materials.

The way that molten chocolate flows across your tongue can also be quantified in the lab. A device called a rheometer measures thethickness, more technically known as the viscosity, of the liquid chocolate. In my own work, I studied the change in the viscosity of the chocolate as the cocoa butter froze, to try to better understand how tempering affected the mechanical properties of the chocolate.

The viscosity of the chocolate is just one aspect of the mouthfeel of chocolate – the smoothness or grittiness is a result of the cocoa particles in it. Early chocolate bars had large particles, which can be perceived by the tongue. With the advance of tempering machines, invented by Randolph Lindt in 1879, the cocoa particles are more finely ground to the point that they cannot be felt by the tongue. The difference between these two types of chocolate can be clearly seen with a high-powered microscope called a Scanning Electron Microscope, or SEM.

                     

Images shown are photos of chocolate as seen through a Scanning Electron Microscope.

Despite having all this equipment, there is still no good way to fully quantify the “mouthfeel” of a material. The way a material breaks, dissolves, and releases flavor is a complex process and varies from person to person. The only way to fully evaluate a piece of chocolate will be to eat it.

Post written by:  Naveen Sinha , Applied Physics PhD candidate in the Harvard School of Engineering and Applied Sciences. You can ask him more questions about physics and food at nsinha@seas.harvard.edu.