The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation

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Written by Kevin Tian, AP 225, Fall 2011

Title: The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation

Authors: Amy C. Rowat, Kathryn A. Hollar, Howard A. Stone, and Daniel Rosenberg

Journal: Journal of Chemical Education, 2011, 88 (1), pp 29–33

Keywords: chocolate, science education, phase transition, emulsions, nucleation

Paper Summary

Since the paper is from the Journal of Chemical Education the ultimate goal of this paper is to develop an effective method of communicating the soft-matter science involved in Chocolate. Rather than novel research, this presents novel methods of communication of science to the general public, including children ages 6+.

The work was based on a 1-hour presentation that focused on three questions related to observable properties of Chocolate. Basic historical and practical motivations are used to set-up the presentation for the three questions. In order to answer the questions, a tasting packet was used to guide the audience through a series of taste experiments. Furthermore each child was given one of three different T-shirts in order to identify them as one of three major chemical components of chocolate. This was then fully explained in the presentation, thus generating enthusiasm and discussion about both the subject and science. The details of this presentation's structure, format, and strategies are described.

The Presentation

Table 1. Contains the main questions and the various ways this is related back to the scientific themes of the talk. Detailed are various other aspects of the talk and how they relate to each other.

In Table 1 we can see an outline of the essential aspects of the presentation, including the main questions that motivate the closer study of the various properties of chocolate. The three questions that are asked relate to the three questions:

  • Q1 - Why does chocolate melt usually melt in my mouth and not in my hand?
  • Q2 - Why does chocolate feel smooth?
  • Q3 - Why does chocolate appear glossy and snap when you break it?

In order to further motivate the questions, the authors decide to frame the presentation with the question, "How do you make a good chocolate bar?".

Chocolate Origins

Though the consumption of chocolate goes as far back as Aztec and Mayan civilizations, the problem has always been, and still is, how does one make the perfect chocolate bar?

In order to achieve a higher standard of chocolate, one must first look at the basic ingredients composing the typical chocolate bar. Figure 2A gives lists the basic ingredients of a standard chocolate bar (with nothing too fancy added). One notes that aside from sugar and a flavoring, there are three major ingredients essential to a chocolate:

  • Cocoa Butter
    • Figure 2B - Hydrophibic
  • Cocoa Mass
    • Figure 2C - Hydrophilic
  • Lecithin (in this case it is Soy derived)
    • Figure 2D - Amphiphilic (emulsifier)
    • This compound is naturally occurring in the cocoa bean, but more is added during production.

The attention of the audience is then drawn to the tasting experiment packet (Figure 1). In this packet are 4 experiments:

  • i) Raw Cocoa bean
  • ii) Dark vs. Milk Chocolate
  • iii) Smooth vs. Grainy Chocolate
  • iv) Tempered vs. Un-tempered

At this point experiment i) is conducted with the expected result that people dislike the taste of a raw cocoa bean (as it is rather sour/bitter). The general picture of chocolate production, from the fruit of a cocoa tree (cocoa pods) to the resulting cocoa mass and cocoa butter that is generated from the process. For summary purposes the details will not be described.

The remaining taste tests are then used to guide the audience towards making certain observations about chocolate, more specifically with regards to the three questions described previously.

Figure 1. The tasting experiment packet. Contents include: (A) Envelope for distribution to the audience; (B) Small plastic bags inside the envelope for each taste experiment (i-iv).
Figure 2. (A) The major ingredients of chocolate. The other three images are used to represent the essential ingredients that were the focus of the talk. (B) Cocoa Butter represented by the Triglyceride molecule (a component that is hydrophobic). (C) Cocoa mass represented by a serotonin molecule (a component that is hydrophilic). (D) Soy Lecithin represented by a lecithin molecule (the compound is amphiphilic and occurs naturally but is added when producing chocolate).
Figure 3. Depicted above are the main scientific themes with regards to the study of chocolate. (A) Phase transitions. (B) Emulsification. (C) Nucleation and Crystal Formation. These three themes are explored through the use of questions that can be explained by using these concepts.


Q1: Chocolate's Melting Temperature

Experiment ii) involves sampling small pieces of dark and milk chocolates. The audience is instructed to simultaneously place the chocolates on different sides of the tongue and observe which one melted first. A show of hand is then used to records the "results", making a clear demonstration that the milk chocolate melts first.

How this is then explained is by introducing the concept of phase diagrams. This is demonstrated in three ways:

Demo 1: Water, Ice and Steam - Temperature

    • Water is used as the system of interest to illustrate the three basic phases (solid, liquid, and gas), and how temperature can affect the phase of a substance.
    • A simulation (Figure 3A) is used to demonstrate the motion of molecules in each of the phases at different temperatures.
    • A group of children are then invited to the stage to act out this physically (going from solid->liquid->gas), thus illustrating the thermal motions in a different way.

Demo 2: Fats and Oils

    • The Melting of chocolate has much to do with cocoa butter (the major fat component)
    • Some fats are liquid at room temperature, whilst others are solid. This is explained by using molecular structure (kinked vs straight chains) and close-packing arguments.
    • Olive oil (unsaturated fatty acid, and thus kinked chain) is compared to butter (saturated fatty acid, and thus straight).

Demo 3: Melting of Fats

    • Children are brought up from the audience to act out saturated vs. unsaturated fats, illustrating how kinked chains (bending at the waist) cannot be packed as closely as straight chains (standing straight).
    • The statement is then made that cocoa butter has a transition temperature of <math>97^{\circ}</math> (this is essentially body temperature). The temperature of one's hand is <math>80-90^{\circ}</math>, thus illustrating how molecular composition affects the melting temperature, and also explaining why chocolate melts when it does.


Q2: Chocolate's Texture

Table 2. A basic guide to the descriptive terms used in the presentation, "Hydrophilic", "Hydrophobic", and "Amphiphilic". This table clarifies the origins/roots of the words and eases one into the use of the words.

Audience members are then asked to perform taste experiment iii), which compares European-style chocolate (smooth) vs. Mexican-style chocolate (grainy), with special attention to the sensation on the tongue. Observations such as "rough", "smooth, "bumpy" were made. The audience was then invited to make suggestions as to why this might occur.

Chemical composition is then put forth as important to texture. The concepts of hydrophobic and hydrophilic substances is then introduced, by relating the groupings to the ingredients of chocolate (Figure 2). One demonstration is the shaking of a flask with oil and water, thus showing the tendency of hydrophobic and hydrophilic substances to separate. A detergent is then added to the same mixture to introduce the idea of an amphiphilic substance. The resulting mixture is called an emulsion, and the amphiphilic substance (detergent) is an emulsifier.

Familiar examples are used such as the emulsifiers used (such as egg or mustard) to create stable emulsions of oil and vinegar. This is then used to introduce the purpose of soy lecithin, which is to promote the mixing of cocoa solids and cocoa butter. This results in the smooth texture that chocolate is well known for. This approach can thus be used to illustrate the chemical concepts of hydrophobic, hydrophilic and amphiphilic substances.


Q3: Chocolate's Appearance and Mechanical Properties

The example used to introduce this topic is concerns chocolate that has been melted and re-solidified. Oftentimes this chocolate is described as having a "moldy" appearance and a crumbly texture. Emphasis is placed on the formation of solids from liquids, which introduces the idea of crystallization.

First a computer simulation is used (Figure 3C) to visualize the solidification process of a liquid, showing particles moving slower as temperature is decreased. Eventually the particles form "ordered clumps". These ordered clumps illustrate the idea of a crystal structure. This is further described as a way in which solid particles can pack together, and the way in which this occurs is not unique. More importantly, the crystal structure heavily influences mechanical properties.

It is known that in chocolate, the fat molecules can crystallize into 6 different forms upon solidification. Only two of these crystal structures are desired (smooth and glossy), with the others resulting in more crumbly textures. The method through which Chocolatiers control the crystal size/type forming via steady heating and cooling is known as "tempering". The tempering process involves the formation of seed crystal that encourage the formation of the desired particular crystal structure. The results of this process are then illustrated with the final taste experiment (iv).

Discussion

Suggestions for Engagment

According to the authors, taste experiments have proven very effective as the questions and observations that follow are made by the audience members, rather than the presenter/lecturer. It is also encouraged to have audience participation at 8-10min intervals; a pace that is critical for keeping younger children engaged. Asking questions regarding topics intermittently throughout the presentation ("Where do cocoa beans grow?", "What did you notice about the chocolate you just ate?", etc.) is one such method of doing this. Calling volunteers to help with tabletop demonstrations or to act out the illustrations has also been used to great effect.

Alternatively using the local history can also engage the audience. One such example is that at Harvard, the authors noted to the audience that one of the first successful chocolate mills in the US was co-established by Harvard graduate, James Baker, in the 1750s in Dorchester, MA. The authors suggest collaborations with local chocolate companies for their expertise (the authors collaborated with Taza Chocolate for example).

Adaptation to the Classroom

Table 3. This provides a mapping of all the lecture/presentation components to the National Science Education Standards. For convenience I have grouped the NSE standards that are equivalent across grades in a single colored box (arbitrary colors).

As is noted in Table 3, much of the presentation can easily be adapted for students of all pre-college grade levels. The table identifies how the National Science Education Standards are easily met by the presentation. As such the conversion of the presentation described above can be adapted as a one hour lesson plan or in a more modular fashion.


Conclusions

The very interactive nature of the presentation on the science of chocolate employs various methods that have been demonstrated to be effective in science education, in the hopes of integrating as many effective strategies as possible. This includes the following:

  • The presentation of scientific concepts in the context of familiar a material that "everyone knows and loves" (chocolate!)
  • Engaging the individuals in making their own observations and ask questions (through the use of tasting experiments)
  • Demonstrations that involve acting out the science being described (children acting out the physical response of molecules)

The comment cards that were received

Before reading this article it had not occurred to me that publications on the topics of science education existed. It was certainly an interesting read, regarding the content and the methods of presentation that were utilized. The relation to topics of soft-matter are quite apparent in the discussion of the various phases of chocolate, it's crystal structure and intermolecular interactions. Though being at forefront of scientific research is important, being aware of the necessity, and having the ability to, convey that science to the public is equally important (if not more-so given the current state of affairs).