I recently asked a first-year student what the difference was between a rock and a mineral and he replied, “A rock is like a salad…” His immediate reply started me thinking about using food analogues to teach geological concepts. I subsequently found this approach has been widely studied and proven to be effective. For example, Baker et al. (2004) used the viscosities of common foods as analogues for silicate melts to help teach students about igneous processes. In this study, the viscosities of maple syrup, molasses, ketchup, and smooth peanut butter were all measured at 25 °C and 1 bar and compared with the viscosities of natural silicate melts. The viscosity of peanut butter turned out to be near to that of a rhyolitic melt with ∼2 wt% H2O at 800 °C, and the viscosity of ketchup near to that of an anhydrous, molten tholeiitic basalt at 1,200 °C. Students who had been taught silicate melt viscosities using food analogues retained their knowledge of silicate melt viscosities better than students who had not used food examples. Inspired by this, and seeking a new way to introduce concepts of rheology and phase stability in a class on Earth materials, I turned to a common food that is familiar to all: chocolate. Chocolate is a fascinating and complex material, displaying many properties analogous to Earth materials. And if your interest is piqued, read a more detailed description of the science of chocolate in Stephen Beckett's book of the same name, The Science of Chocolate (2008).
Chocolate is made by first mixing sugar with chocolate liquor (cocoa beans that have been fermented, roasted, and ground until they form a liquid of cocoa butter and cocoa solids). This mixture is then ground, and more cocoa butter is added along with the emulsifier lecithin, which makes the ingredients blend together. There may then be additives such as milk powder and/or fruits and nuts. Although we think of chocolate as a solid, it is normally a liquid (the liquor) and is only solidified just before it is ready to be packaged or eaten.
The rheology of the chocolate—how it deforms and flows under the influence of mechanical forces—is very important in producing its correct weight, appearance, and, most importantly, its taste! Chocolate is a non-Newtonian liquid, which means that its viscosity varies with the application of mechanical forces, such as shear stress. A simple experiment demonstrates this concept. Take a piece of chocolate and insert it into your mouth, letting it melt on your tongue. Next, press this highly viscous liquid against the roof of your mouth. Note how the application of the mechanical force decreases the viscosity of the chocolate so that it flows over the surfaces of your tongue and palate. The key to making the best melt-in-your-mouth chocolate depends on its crystal structure.
Of all the ingredients in chocolate, it is the cocoa butter that determines its crystal structure. To complicate matters, there are different ways that the individual molecules of cocoa butter can pack together: these different ways lead to six polymorphic forms, designated as I–VI or, alternatively, as γ, α, β2', β1', β2, β1. The cocoa butter polymorphs form at different temperatures, the rates of formation being dependent on temperature. The polymorphs greatly affect the taste and texture of a chocolate by controlling its melting point, how easily it snaps, as well as its strength, glossiness, and texture. For example, the thermodynamically most stable polymorph is VI (β1), which is visually unattractive and has a dull surface. It melts slowly to produce a sandy sensation when being eaten and has a soft texture similar to candle wax. The desired polymorph of chocolate is form V (β2), which displays a glossy surface, crisp hardness, and produces that pleasant melting sensation in the mouth. The challenge is to make the chocolate crystallize in this metastable, but preferred, form. And this is done by tempering.
Tempering involves melting chocolate to about 50 °C to erase all memory of existing crystalline structures and then cooling the chocolate to about 26–27 °C to form a mix of crystals. It is then warmed back up to 30–32 °C to remove the undesirable III and IV (β') polymorphs, leaving only the V (β2) crystals which have a melting point of 33–34 °C. In practice, some of the undesired polymorphs remain, so seed crystals of the desired polymorph are added to the partially molten chocolate to promote nucleation. This part of the process also results in a smaller particle size, which gives the chocolate a smooth appearance. Once fully tempered, however, your prized chocolate bar may still undergo undesired changes. Over time, the metastable V crystals will slowly transform into the thermodynamically stable polymorph VI (β1). Larger crystals of this polymorph will appear on the chocolate's surface to give it a greyish appearance, something often referred to as “chocolate bloom”. At room temperature, this reversible transformation limits the shelf life of chocolate to several months, but the shelf life can be extended if such chocolate is stored in a refrigerated environment. Unfortunately, as you may have discovered by leaving your chocolate in the sun or in a warmed-up car, the undesirable phase transition to polymorph VI (β2) happens quickly at higher temperatures. If this phase transition occurs, the hard work of tempering the chocolate is lost and you will have chocolate that is dull, soft and melts slowly in the mouth!
I hope you have enjoyed this brief tour of the material science of chocolate and you have identified many of the processes and properties that we also find in the less edible Earth materials. Some food for your geological thought.