Chocolate: gateway into the sciences

[by Naveen]

My gastroscience research for the past month has focused on the physics of chocolate. It's taken me a while to write this post since I've been a bit overwhelmed by all the science involved in one of my favorite foods. Below is just a small subset of the questions one can investigate:
  • Botany: where do cocoa plants grow?
  • Microbiology: what's the best way to ferment the cocoa pods?
  • Organic chemistry: what are the flavor molecules in cocoa particles?
  • Physical chemistry: what is the structure of the cocoa fat?
  • Rheology: what is the viscous/elastic behavior of chocolate at different temperatures?
  • Physiology: how do we taste chocolate?
  • Neuroscience: how does eating chocolate affect one's mood?
Since several books and numerous research articles have been written about the subject, in this post I'll just focus on the phase transitions in the cocoa fat. In elementary school I learned about phase transitions like ice melting or water boiling, but chocolate is way more complicated. One can't take a simplistic view when it comes to producing chocolate that has a smooth, glossy surface and that breaks cleanly (aka "tempering").

The fats in chocolate, like most foods, are triacylglyerides (TAGs), which have three fatty acids attached to a carbon backbone (#1 in the figure below). The fatty acids (labelled R1, R2, and R3), can be either saturated (straight) or unsaturated (with a kink), with the center fatty acid often being unsaturated (#2). The composition of fats changes depending on where the cocoa pods were harvested. Unlike the E-shape shown in the chemical formula, the fatty acids distribute themselves on opposite sides of the carbon backbone to form a chair or tuning fork shape (#3). These TAGs can stack in several different ways. In one of these forms, the chairs pack in a double length configuration (#4). A tighter packing is possible in a triple-length configuration (#5). This tighter packing has an observable effect, a chocolate bar can contract by 1 or 2% if it solidifies in this configuration.

The type of packing is determined by the temperature of the cocoa fat. The tighter packings require more energy to form, so their melting point is higher. In cocoa butter, there are six different types of packings (aka phases), labelled with either Greek letters or Roman numerals. The fifth form (Form V or Beta-1) is the goal when tempering the chocolate to achieve a nice, glossy appearance. In the tempering process, molten chocolate is cooled enough to allow the Form V crystals to form, but also to keep the temperature above the melting points of the undesired forms. Once enough Form V crystals have formed, the chocolate can be cooled all the way down to room temperature.

This is where things get complicated and I'm still working to sort out the details. The crystals can form over a range of temperatures and the crystal structures can transform among themselves in particular ways over times that can range from minutes to months. This is the cause of chocolate bloom, in which a dull, whitish coating can form on top of chocolate that is stored improperly. I've tried to show a rough idea of the types of crystals that form at different temperatures in the figure below, but I welcome the feedback from anyone with more authoritative knowledge on the subject.

This is just the tip of the iceberg when it comes to phase transitions in chocolate. The rate at which crystals nucleate, the effect of shearing the chocolate, the presence of other fats (like in milk chocolate), and numerous other variables can affect this process. There is certainly enough left to explore to fill an entire PhD dissertation.

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