The Science of Chocolate: interactive activities on phase transitions, emulsification, and nucleation
Entry by Andrew Capulli, AP225 Fall 2011
A. C. Rowat, K. A. Hollar, H. A. Stone, and D. Rosenberg, "The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation," J. Chem. Educ. 88 (1), 29-33 (2011).
A change in a system being studied can be generally viewed as a phase change and although much of this sort of physics studied in academia seems like far-out irrelevant work, as we all know, its really not... in fact, it can be related to common materials and phenomena we see everyday (remember to Cheerios Effect!?... see a couple wiki entries ago). And what better, in terms of explaining 'science' than food? Stone et al. use chocolate as their 'everyday' material to explain some fundamental physics to the community (ages 6+); chocolate being a very complex food that deliciously illustrates the authors' points (discussed below). But even to the graduate student, the demonstrations used by the authors and the seemingly simple physics being covered is easily recognizable as an opening of the door to a far more complicated subject. In the past few weeks in Soft Matter we have covered some introductory material on surfactants and phase diagrams/transitions and this 'paper' addresses those subjects while further demanding more investigation of the reader. Because this paper is more so an instructive layout on how to teach/demo the subjects to a general audience, I'll focus my wiki not only on what the paper had to say, but some further investigation into the phases of chocolate.
Summary: Educational Points of the Paper
This paper, unlike the typical methods or experimental account we're used to reading and discussing, is an instructive summary on how to address some relatively simple but very important explanations (the physics) behind some common phenomena we see everyday. It should be noted that while the 'take homes' from the summarized demonstrations seem fundamental... even as graduate students we re-learn the concepts. The authors aim to address three main concepts:
- Phase Changes (Solid, Liquid, Vapor)
- Crystallization (More Phase Change)
1) Phase Changes: To frame this topic, the authors pose the question: "why does chocolate melt in your mouth and not in your hand?" Using their demonstration the authors show that dark chocolate melts 'slower' than milk chocolate. Dark chocolate has less cocoa butter (fat) and consequently melts at a higher temperature. Cocoa butter is a saturated fat (a straight chained molecule) that packs closely and crystallizes at temperatures below room temperature (which is about that of your hand). For a more complete discussion, the authors briefly describe that unsaturated fats are 'kinked' molecules that cannot as closely pack and therefore are liquids at room temperature (olive oil for example). It just so happens that the crystal structure of the saturated fat cocoa butter melts at about 97 degrees F which is approximately body temperature (the temperature in your mouth). Here is the fundamental idea of phase change. Given a constant concentration of a material--chocolate in this case--as temperature is varied, different phases (solid, liquid, vapor) are observed. The vapor phase of chocolate is not demonstrated or discussed... most people would consider this phase of chocolate to be too hot for any interest/use.
2) Surfactant/Emulsification: Although the authors do not use these words per say, this is the subject they address by asking the question, "Why does chocolate feel smooth in your mouth?" While this texture may be partially a result of cocoa solid particulate size, much of it is due to the emulsification process. As discussed by the authors, cocoa powder and cocoa butter (the two main ingredients of chocolate) do not readily mix; however, as evident by the mixing of hot chocolate or as the authors mention, chocolate milk, cocoa powder mixes well with water. This is reminiscent of last week's topic and wiki entry: surfactants. The cocoa powder is hydrophilic which, as we have discussed, does not mix well with fats/oils (cocoa butter). The emulsifier used in may chocolates is soy lecithin which is amphiphilic and thus a stabilizes the cocoa powder in the cocoa butter. Much of lecture considered the the mixing of oil and water but we also discussed the solid-oil interaction with oil soluble surfactant (image from lecture slide below). Here, the surfactant is soy lecithin, the solid is cocoa powder particles, and the oil is cocoa butter. As Stone et al suggest and as we have discussed, the adsorption onto the surface of the solid (cocoa powder) is driven by high head group/solid affinity while the stabilization of the system results from the addition hydrocarbon chain affinity with oil. It is the amphiphilic nature of the soy lecithin that makes for a smooth chocolate (similar to the smooth ice cream proposed in my previous wiki entry via the amphiphilic crescent shaped microparticles).
3)Crystallization: This is the section of the paper that most applies to the current soft matter topic (more detail in the next section). Here the authors discuss the crystallization of fats and in particular, the crystallization of cocoa butter in chocolate. The authors address this topic by posing the question, "Why does chocolate snap when you break it?" Here the authors discuss how cocoa butter can crystallize in different ways (6 different ways) of which two are the best for making chocolate. A process called tempering is used by chocolate makes to achieve the crystal structure they want. Tempering chocolate is similar to tempering metals; the chocolate is heated and cooled slowly to promote seed crystal formation and growth. While the authors leave the discussion at this point for their more general presentation, it is at this point where the physics of chocolate starts to become interesting. The different crystal structures (phases!) the cocoa butter can form dictate the flavor and feel of chocolate. There is a reason why some chocolates are very expensive while others are not. For the same reasons, there are proper storage temperature ranges for chocolates despite the taste of some eaters (like myself). The phase changes between the crystal structures of chocolate is where the authors leave off but is where Soft Matter comes in to play (see below: Connection to Soft Matter).
Another wiki entry on this article by Kevin Tian (The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation) discusses the demonstrations of the three topics presented by the authors in more detail. Rather than repeating whats already written, see his entry for a more detailed account with images from the paper (well laid out and easy to follow). Below I continue the investigation of phase transitions in chocolate and the implications thereof (something not directly covered in the paper itself but what I beleive to be the connection to our soft matter discussions).