Have you ever wondered why a chocolate bar that has melted and resolidified doesn’t have the same shine or snap that it did before? We are going to take a quick detour from talking about how atoms can group together to talk about some of the messy, interesting, and delicious chemistry of chocolate.
Chocolate is actually a fermented food, like sourdough bread or beer. To make chocolate, you start by taking the fruit pods of the cacao tree and letting them sit for several days, so that the wild yeasts which are on the surfaces of most fruit can start digesting the sugar in the fruit. After the fermentation, the pods are dried, and the seeds are removed from the dried and fermented fruit flesh; these seeds are called cocoa beans. The beans are roasted, then removed from their shells, and finally ground into a thick paste from which the dissolved fat, cocoa butter, can be removed (or more can be added!). Vanilla, sugar, and other flavors can also be added at this point, but much of the character of the chocolate comes from the initial fermentation and the roasting. Once the flavors are as desired, the chocolate is ready to be tempered!
Tempering is the process of heating a material to a specific temperature and then letting it cool into a solid, where the temperature is chosen so that the solid will have certain properties. It’s important in chocolate because the fats in chocolate have six different crystal phases, that is, different configurations in which they can solidify. This is called polymorphism, and it’s not unique to chocolate! The orientation of the fat molecules relative to each other, and their bonding, determine how easy it is to break apart the solid. This means that the melting temperature for each crystal phase is different, with some phases being easier to break apart than others. The six phases of chocolate are generally numbered with Roman numerals in ascending order of melting temperature, and it’s phase V that has the glossy sheen and satisfying snap that we are used to in commercial chocolate. And as an added virtue, its melting temperature is very close to human body temperature, so that it can melt in your mouth!
So how can chocolate be preferentially solidified in phase V? It has to be heated above the melting temperature of the unwanted phases I-IV, so that none of those crystal structures can form. But if it’s heated above the melting temperature of phase VI, it may resolidify in that phase, which is exactly what happens when you leave a bar of chocolate in the sun and it melts and resolidifies. Thus, the usual tempering procedure in chocolate manufacture is the following: first, heat the chocolate above all the melting points, to get rid of existing crystal phases. Then cool it to around the phase V crystallization temperature, and keep it there for awhile so that that regions of phase V begin to form. If you stir during this process, you will get many small crystals which can act as seeds for the desired crystal phase. Finally, cool the chocolate, causing the phase V seeds to spread so that the entire chocolate bar has the desirable crystal phase.
Chocolate is a great example of how different material properties result from the arrangement of matter at a small scale. And if you’re interested, you can always learn more about the history, manufacture, and usage of chocolate at its wikipedia page.