The Science of Chocolate

As you bite into a piece of chocolate do you ever wonder why it melts in your mouth? Are you ever amazed by the crisp snap of a glossy, rich chocolate bar? Chocolate-making is an exact science that requires carefully controlled processing in order to deliver the smooth, warm taste that chocolate-lovers crave. The formation of chocolate is an ongoing field of research, and groups such as Princeton’s own chocolate club, the Institute for Chocolate Studies (ICS), use this research in the pursuit of the perfect chocolate bar.

The process of making the chocolate consists of several long and meticulous stages. According to ICS member Ming-Ming Tran ’15, the club splits up into several teams, each of which meets weekly to work on one of the following stages of production: roasting, winnowing, grinding, tempering, and product. After cacao beans are harvested, the first step is to prepare them for chocolate-making. This is done by drying and roasting the beans, which develops the flavor of the chocolate by producing a series of chemical reactions known as the Maillard reactions. A Maillard reaction is a reaction between amino acids and reducing sugars that causes browning and creates a desirable flavor.


In the next stage, the meat, or nib, of the cacao bean is separated from the thin papery shell and is ground into cocoa liquor. In order to winnow, or separate the nib from the shell, the ICS made an Automatic Winnower as part of the HackPrinceton hackathon and won $1000 for their creation. The nibs are ground in the winnower to produce cocoa liquor, at which point sugar may be added to make the bitter cocoa liquor sweeter. The human tongue can detect distinct particles down to a size of 30 microns in diameter. In order to pass this threshold and achieve that perfectly smooth consistency that is characteristic of chocolate, the cocoa liquor is ground using a melanger to an even finer consistency.

Finally, the chocolate must be tempered if it is to be turned into bars or solidified in molds. Tempering chocolate is a tricky but essential step in creating the perfect chocolate bar. It ensures that the correct polymorph of cocoa butter, a fat found in the cacao bean, is formed. Polymorphism is the ability of a material to exist in several distinct crystal structures. There are six forms of cocoa butter crystals that increase in stability and melting point from Form I to Form VI.

Form V is the preferred polymorph for chocolate. This polymorph has a melting point of 33 °C and melts easily in the mouth whereas Form IV (melting point of 28 °C) melts upon touch and Form VI (melting point of 36 °C) does not melt easily in the mouth and is therefore not suitable for consumption. Form V is also glossy and snaps cleanly unlike less stable polymorphs because of its packed crystalline structure. Its stability also confers a longer shelf life than Forms I-IV.

Tempering chocolate is a tricky but essential step in creating the perfect chocolate bar.

To produce this polymorph, ICS tempers the chocolate in four steps. First, they heat the chocolate above 43.3 °C to melt it and remove all crystals that exist in the chocolate. They then create seed crystals to induce the formation of Form V crystals. These seed crystals are reincorporated into the melted chocolate, resulting in a mixture which should be between 30.6 °C and 32.2 °C. The chocolate is then poured, covered, and allowed to solidify. Exact temperature is crucial in this process. Under-tempering causes the cocoa butter to move to the surface of the chocolate, resulting in an unsightly “fat bloom.” This “fat bloom” occurs when the chocolate passes through Form IV as it changes to Form V, and the proper crystals are not homogeneously distributed throughout the product. Over-tempering causes hardness, stickiness with reduced gloss, and darkening of the chocolate’s surface. After tempering, the chocolate is ready to be sold.

The chocolate bar that results from these steps will be dark brown and glossy and will make a satisfying snap when broken. Beginning with a raw cacao bean and ending with chocolate, the process of creating this sweet treat is surprisingly complex. However, an understanding of the chemistry behind chocolate making has allowed for the variety of chocolates seen today.

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