Chemistry for Everyone The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation Amy C. Rowat* Department of Physics and School of Engineering & Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States *[email protected] Kathryn A. Hollar and Howard A. Stone† School of Engineering & Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States Daniel Rosenberg Faculty of Arts & Sciences Lecture Demonstrations, Harvard University, Cambridge, Massachusetts 02138, United States † Current address: Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, United States Here we describe a presentation we developed on the science To address the question of how to make good chocolate of chocolate for the general public, including children ages 6 and up. and motivate our scientific themes, we begin by explaining the Chocolate is a complex material, yet nonetheless demonstrates ingredients in a typical chocolate bar (Figure 2A). We identify basic scientific concepts that we guide the participants to under- the major components listed on the label: cocoa beans or mass, stand through a series of inquiries and simplified explanations. cocoa butter, and soy lecithin (an emulsifier) (Figure 2B-D). We structure the 1-h presentation around three simple questions Each member of the audience is then prompted to taste a pure related to observable properties of chocolate (Table 1). To cocoa bean: because to most participants the bean tastes sour, answer these questions, we provide each audience member with bitter, and “disgusting”, this observation prompts the question of a tasting packet (Figure 1) and guide them through a series of how these beans are transformed into a delicious chocolate bar. taste experiments. This format is a highly effective way to engage The path to tasty chocolate begins with the cocoa tree that people in science, as each person takes part in individual taste bears fruit in the form of giant cocoa pods (14, 15). These pods experiments (1-4); indeed, audience feedback indicates that we contain white pulp in which the cocoa beans are embedded, and accomplished our goal of generating enthusiasm and discussion are akin to familiar fruits such as peaches or plums with pits or about science. Each child receives one of three different T-shirts, apples with bitter seeds. The pods are harvested, split open, and identifying them each as one of the major chemical components the pulp becomes a food source for natural yeast and bacteria. of chocolate.1 To explain scientific concepts, we use basic Over the course of several days, the pulp is digested; this chemical experiments as well as interactive demonstrations with fermentation process is also essential to develop the flavor profile children playing the parts of molecules. The material that we of chocolate, as chemical reactions initiated by the microbes help describe can be adapted as a lecture for families and the general to make the beans taste less bitter. The beans are then picked out public or in the classroom. Because chocolate is a popular subject of the pod and dried. The next step of roasting the beans is also for science education (5), general science journals (6, 7), as well as important for developing the characteristic flavor of choco- active scientific research (8-14), there is a wide variety of late (5). Thereafter, the beans are crushed into small pieces or resources that can be used to complement and supplement the pressed to extract the cocoa butter fat from the cocoa solids. This material we present here. process of extraction is similar to how olive oil is pressed from olives, or juice is extracted from fruits like apples or oranges. In The Origins of Chocolate this way, the two main ingredients of chocolate, cocoa solids and cocoa butter, are generated. We motivate the questions framing our presentation with the challenge of how to make a good chocolate bar. The consump- Questions about Chocolate tion of chocolate dates back to ancient Aztec and Mayan civilizations. Since the industrial revolution, many people have We address specific questions about the physical properties of tackled the problem of how to produce good chocolate. As a chocolate by guiding the audience through a series of three result, many famous chocolatiers have pioneered processes additional taste experiments (Figure 1, Table 1). Taste exper- critical to manufacturing a successful chocolate bar including iments require that each audience member is engaged in making Cadbury, Hershey, Lindt, and Nestle. All of these chocolatiers observations about chocolate: What does it look like: glossy or asked the same question inspiring this lesson: how to make the dull? What does it feel like: smooth or soft or crumbly? What does perfect chocolate bar? it taste like: bitter or sweet? Details about the demonstrations, r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 1 January 2011 Journal of Chemical Education 29 10.1021/ed100503p_ Published on_ Web 10/27/2010 _ Chemistry for Everyone Table 1. Main Questions and Related Scientific Themes Motivating Question Taste Experiment Specific Question Result Answer Main Scientific Themes Why does chocolate Dark chocolate versus milk Which chocolate melts first in Milk chocolate melts at a Melting temperature Phases of matter, phase usually melt in my mouth, chocolate chips your mouth? lower temperature than depends on material transitions, lipid not in my hand? dark chocolate composition and on the composition shape of fat molecules; cocoa butter melts at around your body temperature Why does chocolate feel European-style chocolate How do the chocolates feel One chocolate feels rough or Texture depends on particles Hydrophobic, hydrophilic, smooth? versus Mexican-style different in your mouth? bumpy compared to the size; amphiphilic emulsification chocolate other molecules or emulsifiers also help make chocolate feel smooth Why does chocolate snap Tempered versus How do the appearance and Tempered chocolate has Cocoa butter molecules need Phase transitions, when you break it and untempered chocolate texture of the chocolates sheen and snaps when to pack in the right way crystallization, have sheen? differ? broken; the untempered nucleation chocolate looks spotty or moldy, and crumbles chocolate that has been with different texture, onebean; smooth (ii) and a piece the of dark other andplastic grainy; milk bags chocolate; (iv) (iii) for two each a pieces of of piece thedistributed chocolate of following to taste each experiments: (i) audience aFigure member. raw 1. (B) cocoa The Tasting envelope contains experiment small packet and contents: (A) The envelope in different states, such as solidtemperatures, and we liquid. introduce Because the chocolate concept is of phases: a materials exist clear that the milk chocolateof melts hands first. to recordright side the and observe results which chocolate ofon melts this first. the We use left a show sideinstruct of each their person to tongue simultaneously andchocolate, place the each the dark of milk chocolate chocolate which on melts the at a differentHand? temperature. We Why Does Chocolate Melt in Your Mouth and Notinformation. in Your experiments, and simulations arechocolate. provided in thenaturally found supporting in theor cocoa emulsifier bean is and represented as is lecithin,component also an of cocoa amphiphilic added that compound is when that largely hydrophilic. producing is (D)hydrophobic. Amphiphilic (C) molecule Cocoa mass is representedsented as as a a triglyceride serotonin molecule, a molecule, component a ofchildren's cocoa butter that is largely (emulsifier), each of which are representedlecture, by the we following focus icons on onmass), the the sugar, cocoa cocoa beans, butter,illustrates cocoa and the butter, soy major and lecithin.Figure ingredients soy For 2. lecithin in the chocolate: Major purposes ingredients cacao of in bean chocolate. the (A) (or Label of cocoa a chocolate bar To explain why different types of chocolate melt at different We begin by sampling small pieces of dark and milk ” T-shirts and throughout the lecture: (B) Cocoa butter is repre- “ untempered ” and a tempered control sample. “ experiment ” , making it 30 r Journal of Chemical Education _ Vol. 88 No. 1 January 2011 _ pubs.acs.org/jchemeduc _ 2010 American Chemical Society and Division of Chemical Education, Inc. Chemistry for Everyone Table 2. Guide to Hydrophilic, Hydrophobic, and Amphiphilic Greek Word Root Meaning Common words Hydro- Water Hydrofoil, fire hydrant, hydroponic, hydroelectricity -Phobic Fear Arachnophobic, claustrophobic Amphi- Both Amphibian, amphitheater -Philic Love Philadelphia, philanthropy, chocophile Using children from the audience, we demonstrate how the different molecular structures of saturated and unsaturated fats result in different melting temperatures (Demo 3). The children who are “saturated” fats stand up straight and gather close “ ” Figure 3. Main scientific themes in chocolate. (A) Phase transitions: why together; the unsaturated fats adopt a kinked shape by bending does chocolate melt in your mouth? We use a computer simulation to at the waist, and thus cannot stand as close to each other. As illustrate the behavior of molecules as the temperature increases and the the temperature is increased, we instruct the children to act out phase transitions from solid to liquid