The Science of Chocolate: interactive activities on phase transitions, emulsification, and nucleation

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Entry by Andrew Capulli, AP225 Fall 2011

Reference

A. C. Rowat, K. A. Hollar, Howard Stone, and D. Rosenberg, "The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation," J. Chem. Educ. 88 (1), 29-33 (2011).

Key Words

Phase Change, Emulsification, Surfactant, Phase Diagram, Metastable, Spinodal, Critical Point, Phase Rule

Introduction: Motivation

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:

  1. Phase Changes (Solid, Liquid, Vapor)
  2. Surfactant/Emulsification
  3. 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).

Choc 1.jpg

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 what's 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).

Connection to Soft matter

The crystalline structure of chocolate is crucial to its look, texture, and taste. Cocoa butter has a number different crystal structures that can be formed given time and temperature fluctuation. The figure below (III.b) is from Verma et al at the University of Buffalo and can be found by clicking the following:[[1]]. The Gamma crystal structure results from fast "quenching" like cooling of the chocolate from the liquid melt. As the cooling process is slowed and there is more time for nucleation and see crystal formation, different, more densely packed lattices form including the alpha, beta prime, and beta phases as shown (and also sub-phases within these phases, 1 and 2). The beta two phase is required for a proper chocolate as it results in a densely packed fat that gives the chocolate its uniform texture and gloss. Less dense lattices like the gamma and alpha phases result in chocolate that crumbles due to poor and non-uniform nucleation and lattice formation.

Choc 2.jpg

Below is an image (yellow background) from Kevin Smith and Kees van Malssen's "Cocoa Butter Crystallization" which can be found by clicking the following:[[2]]. This is an approximate phase diagram for cocoa butter crystal phase given as a function of temperature and curing time. As the authors discussed, curing or tempering of the chocolate is a delicate process. Proper time needs to be given for adequate nucleation and growth of the cocoa butter crystals to form the beta 2 phase of the cocoa butter which results in a uniform 'snap' of the chocolate and smooth texture that melts at body temperature. As can be seen in the phase diagram below, rapid cooling results in alpha or gamma phases which do not have the texture desired for a good chocolate. A slow and delicate cooling needs to occur to achieve the beta phase desired (notice that the beta phases are to the right of the diagram where cooling is a slower process). These beta phases also have a higher melting temperature because of their denser packing which further helps to explain why chocolate melts in your mouth and not your hand; not only does the beta phase taste better, it melts at a higher temperature! Interestingly, many people like cold chocolate, or maybe its just a New England thing, but I know I'm a fan; there are numerous gas station and convenience stores around that sell chocolate bars in the refrigerator next to the drinks. To anyone who studies the 'proper' making of chocolate, this would be high treason. Storing chocolate at low temperatures (such as those in convenience store refrigerators) results in a phase change of the chocolate from the desired beta phase to a less dense mix of alpha and beta phases (or even just alpha phase). This is clearly the case given the phase diagram below. Furthermore, the very unsightly 'bloom' may occur if storage isn't appropriate for the given chocolate (see the wiki on bloom at: http://en.wikipedia.org/wiki/Chocolate_bloom).

Choc 3.jpg

More Thoughts

As a senior undergrad last year I had the chance to take a very interesting course called Food Engineering. Among the many topics covered was chocolate making and during the few weeks of this topic, much of what the authors of this paper discussed was examined a little more closely. The most interesting point we discussed, however, was the crystal phases of cocoa butter and the necessity of adequate tempering and crystal development to form the correct lattice. Cold storage of chocolate, although it may be somewhat popular, ruins the uniform beta phase of the chocolate bar as discussed and has further implications on the mechanical integrity of the chocolate. In addition, milk chocolate and dark chocolate were discussed and even put to the test. As many know, dark chocolate has more cocoa powder but it also has more cocoa butter (percentage) than milk chocolate does; milk chocolate has milk fats in it as cocoa butter substitute to lower price... but also quality. With more cocoa butter, the dark chocolate has a complete and uniform beta phase (if made correctly) while milk chocolate has other fats that disrupt the desired uniform lattice of cocoa butter. We took a three point bending apparatus in the lab and were able to make some simple stress-strain graphs of dark and milk chocolate:

Choc 4.jpg

As you can see, the modulus and UTS of milk chocolate is much lower than that of dark chocolate (primitive but I would say indicative data). This is because the cocoa butter phase of milk chocolate is not as precisely defined as in dark chocolate (because of the inclusion of disruptive milk fats).

In lecture the metastable or spinodal region of a phase diagram was discussed. Chocolates are clearly unstable mixtures given the various phases of the cocoa butter discussed. By examining the phase diagram discussed above at, say, room temperature (20 C), we can see there are a number a phases that can exist. Does this imply that chocolate as we know it is in a metastable region of its phase diagram... the comfort food we all know and love that stabilizes us isn't so stable itself? Perhaps a chocolate bar has 12% cocoa butter; this may fall into a metastable portion of the phase diagram where, if given enough time, the cocoa butter and other ingredients would separate out into a portion of highly concentrated cocoa butter and a highly concentrated portion of cocoa powder (and other ingredients). Because of emulsifiers and the complexity of the food, this may take a very long time but the thought experiment is interesting. At reasonable temperatures such as room temperature, are there many mixtures that are truly stable? We can even consider other foods or materials we make... they are often assembled at high temperatures. Take a chicken soup for example: it is delicious when newly made and hot but if you take the leftovers out of the refrigerator the next day you see the fats have separated out into other (more solid) phases. There was recently a lot of debate over the leaching of BPA out of plastic polycarbonate water bottles. At high temperatures when the plastic is melted and being formed, the BPA and poymer mixture must be in a stable phase (assuming the plastic is formed at very high temperatures). However, at room temperature the mixture perhaps falls into a metastable region where the two components separate (thus the BPA leaching into water within the bottle). This is just postulation from someone who isn't a polymer chemist but it seems like it could be likely. Its also possible that BPA is more soluble in water and the leaching is a result of increased solubility and slow diffusion.

...I think we may live in a very metastable world...