The Science of Pizza: The Molecular Origins of Cheese, Bread, and Digestion Using Interactive Activities for the General Public

We describe a presentation on the science of pizza, which is designed for the general public including children ages 6 and older. The presentation focuses on the science of making and digesting cheese and bread. We highlight 4 major scientific themes: (1) how macromolecules such as carbohydrates and proteins are composed of atoms and small molecules; (2) how macromolecules interact to form networks in bread and cheese; (3) how microbes contribute to the texture of bread; and (4) how enzymes break down macromolecules during digestion. Using live demonstrations and interactive exercises with children in the audience, we provide simple explanations of the scientific principles related to these themes that are essential for understanding how to make pizza, and what happens when we eat it. This general approach can be adapted to a variety of informal and classroom settings focused on sharing the excitement of scientific discovery and understanding with students and the public. Introduction This article describes a presentation we developed on the science of pizza that is geared toward nonspecialists and the general public including children ages 6 and older. The presentation is structured around 4 major scientific themes related to the science of making and eating cheese and bread (Table 1): building larger molecules from atoms and small molecules, the formation of networks consisting of larger molecules bonded or linked together, the role of microbes in food texture, and digestion and enzymatic degradation. Since pizza, cheese, and bread are popular foods, this lecture builds on the common experience of individual audience members; moreover, breads and cheeses of various types are present in many cultures, making these topics readily adaptable to diverse audiences. This approach can be used either as a lecture for the general public or as a series of classroom activities for students. Audience participation is a critical part of our approach and is programmed into the hourlong lecture at 8 to 10 min intervals (Taylor 1988); this pacing is critical for keeping young children engaged in the presentation. We begin by engaging the audience, asking questions such as “Where does cheese come from? Why MS 20091171 Submitted 11/23/2009, Accepted 5/11/2010. Author Rowat is with Dept. of Physics, Harvard Univ., Cambridge, MA 02138, U.S.A. Authors Rowat, Hollar, and Stone are with School of Engineering & Applied Sciences, Harvard Univ., Cambridge, MA 02138, U.S.A. Author Rosenberg is with Faculty of Arts & Sciences Lecture Demonstrations, Harvard Univ., Cambridge, MA 02138, U.S.A. Author Stone is presently with Dept. of Mechanical and Aerospace Engineering, Princeton Univ., Princeton, NJ 08544, U.S.A. Direct inquiries to author Rowat (E-mail: [email protected], [email protected], [email protected], or [email protected]). does bread have holes?” Furthermore, we call for individual volunteers to help with table-top demonstrations and have children from the audience act the behavior of individual molecules during short role-playing segments. To help children identify their role, each child receives 1 of 4 different t-shirts at the beginning of the lecture, which has a unique color and design representing a simple sugar, water molecule, enzyme, or network former (Figure 1A to 1D). Moreover, each person takes part in individual taste experiments. We believe that this interactive format engages people in science; studies have shown that kinesthetic learning is an effective tool for increasing understanding of science and engineering (Felder and Silverman 1988; Gage 1995; Ebert-May and others 1997; Handelsman and others 2004). Furthermore, engaging parents in the learning process of their children helps advance their scientific understanding and affects long-term changes in family learning behavior (Ostlund and others 1985; Gennaro and others 1986). Audience feedback obtained by written questionnaires indicates that we accomplished our goal of generating enthusiasm and discussion about science in an informal setting that extends out into the community (Table 2). Sparking Curiosity about a Piece of Pizza Ask any group of children (or adults) to name a favorite food, and pizza is a likely answer. Pizza’s universal appeal, as well as the interesting physical properties and origins of its major ingredients, provides a rich and engaging platform for introducing scientific concepts. Simply looking at a piece of pizza with a high magnification lens raises many interesting questions: Why is melted cheese greasy? Why does the crust have holes? By making these observations, we motivate the audience to ask what gives cheese and 106 Journal of Food Science Education Vol. 9, 2010 c © 2010 Institute of Food Technologists® doi: 10.1111/j.1541-4329.2010.00101.x Pizza science: interactive activities . . . Table 1–Main questions and related scientific themes. Theme Demonstration/audience participation Main scientific concept Networks Formation of alginate gel in calcium chloride solution; children link hands to form a network. Networks form when molecules link together. Microbes Children play the role of yeast cells and cause the network to expand; yeast inflate balloon that is sealed on top of bottle. Microbes alter the texture of bread. Digestion Children act as simple sugars that link together to form chains of complex carbohydrates; “enzymes” come along and break the chains of children apart. Enzymes break down macromolecules into simple sugars for energy. Figure 1–Major players in the science of pizza. To demonstrate the major scientific themes in making cheese and bread, children play the role of individual molecules and act out processes from network formation to enzymatic breakdown. We use the following icons on the childrens’ t-shirts as well as throughout the lecture: (A) simple sugars; (B) networks; (C) water; and (D) enzymes. These components are essential for understanding how (E) the major ingredients of milk, as listed on the label of a milk carton, are transformed into cheese. bread these properties. Moreover, where does pizza come from and how is it made? These questions begin our exploration of the science behind making pizza; this narrative style is an effective way to engage audiences in understanding scientific concepts (Taylor 1988; Kapon and others 2010). Atoms to Molecules to Macromolecules to Networks Ultimately all the major ingredients of pizza—tomatoes, cheese, and bread—come from the sun and the soil. Using energy from the sun, plants convert water and carbon dioxide into sugars and longer chained molecules of sugars, producing oxygen in the process. In turn, plants are consumed and digested by humans as well as cows, goats, or sheep, which produce milk. Highlighting the process of photosynthesis shows how small molecules are converted into larger molecules (Figure 2). Small molecules such as carbon dioxide and water assemble together to build other simple molecules (sugars) that in turn link together to form larger macromolecules (carbohydrates, proteins). Long molecules that consist of many molecules or repeating units are called polymers. Macromolecules or polymers can assemble into even higher order structures or networks. We use a demonstration with beaded necklaces (Demo 1A in Appendix A) as well as with alginate (Demo 1B in Appendix A, Shakhashiri 1983) to illustrate cross-linking and network formation. Since alginate is a negatively charged polymer, it interacts with positively charged atoms or molecules. However, only species with 2 positive charges Table 2–Summary of audience evaluation. Feedback was obtained through a total of 39 written questionnaires. Motivation Question Participant response To engage public using interactive activities and live demonstrations. What did you like about the presentation? Interactive demonstrations (“Going down to participate;” “The kids demonstrating the ideas;” “Interpretive fun for adults and children”) Live demonstrations (“Seeing the milk under the microscope;” “Seeing the acid added to the milk;” “Videos and live action”) To educate the public about scientific concepts in food: (1) foods are composed of smaller constituent parts; (2) microbes play an important role in generating the What was the coolest thing you learned today? How cheese is made (“What whey and curds are;” “Adding vinegar/acid to milk makes it clump up, when you squeeze out the whey, you are left with cheese”) texture of foods; (3) food consists of macromolecules that are broken down by enzymes during digestion. Role of microbes in food (“Yeast farts out oxygen;” “The holes in swiss cheese come from bacteria!”) Role of enzymes in digestion (“Enzymes eat sugar”) Available on-line through ift.org Vol. 9, 2010 Journal of Food Science Education 107 Pizza science: interactive activities . . . Figure 2–Origins and cycle of pizza. Plants use energy from the sun together with small molecules such as water and carbon dioxide to produce larger molecules that other animals such as cows transform into larger molecules. We transform the raw ingredients of flour, milk, and tomatoes into pizza that we eat. During digestion, enzymes break down larger molecules into smaller molecules including simple sugars that give us energy. are cross-linkers that can bring together 2 parts of the alginate polymer chain; for example, the divalent cation calcium (Ca2+), which results upon dissociation of calcium chloride (CaCl2), is an effective cross-linker of alginate. This scientific concept of crosslinking and network formation is crucial for understanding how cheese and bread are made. The Science of Making Cheese Cheese is central to many cultures around the world, making it an accessible food for engaging people in science. All cheese starts as milk. Studying the label of a milk container reveals that there are many components in milk, including fat, protein, and carbohydrates (Figure 1E). Many common fats are in the form of oil, such as canola or olive oil; proteins are molecules found in high concentrations in food sources such as meat, fish, eggs, nuts, and tofu; carbohydrates are large molecules consisting of simple sugars, which constitute pasta as well as other common plant materials such as wood. Another common word on a package of milk is “pasteurized,” a word describing the process whereby milk is heated to a high temperature to kill harmful microbes that might cause illness or spoilage of milk (Figure 3). A simple way to turn cheese into milk is to add acid, such as vinegar or lemon juice. Vinegar is poured into a beaker of milk, causing the milk to separate into curds, which is the network of coagulated casein particles, which entrap the fat droplets; the remaining liquid is known as whey. Using a strainer, the curds are removed from the whey and drained to remove the excess water. This basic cheese is commonly made in India and is known as paneer. Similar types of fresh or unripened cheeses are found in other cultures. To understand the mechanism underlying the formation of solid materials in milk, we begin by describing the structure of milk (McGee 1984). Using a microscope to investigate the small-scale structure (Figure 4) of milk reveals that this liquid consists of many small droplets, which are oils (fats) (Figure 5, Demo 2A in Appendix A). Upon adding acid to milk, large aggregates of droplets are observed (Figure 5B). To explain what is happening at the molecular level, we present sketches showing how the fat drops are interspersed among tiny particles consisting of casein (Figure Figure 3–The technique of pasteurization was invented by the renowned scientist Louis Pasteur (1822 to 1895), a scientist who made many fundamental discoveries in the fields of chemistry and microbiology. Figure from Creative Commons. 5C to 5D). The addition of acid lowers the pH and causes the casein particles to aggregate, thereby forming a network in which the fat droplets are trapped. To produce cheese, the water needs to be squeezed out of the network to remove the excess liquid. The 3 basic steps for making cheese are thus: (1) start with milk (blobs of fat and protein in water); (2) add acid (vinegar or lemon juice); (3) squeeze out the water. We then invite about 20 to 30 children from the audience to play the role of macromolecules forming a network and “squeeze” out the water molecules from their midst, thereby demonstrating the process of making cheese (Demo 3 in Appendix A). This style of participation by the children and acting out of basic molecular ideas is popular with the audience. 108 Journal of Food Science Education Vol. 9, 2010 Available on-line through ift.org Pizza science: interactive activities . . . Figure 4–The concept of scale. A major challenge in communicating science to students and the public is to convey a sense of scale. Units such as micrometers or nanometers have little meaning in the context of everyday life. To demonstrate the scale of fat droplets in milk, or the dimensions of a single yeast cell, we compare these objects to the width of a human hair. For example, 15 yeast cells or 75 fat droplets can fit across the width of a single human hair of typical diameter 0.000075 meters. Shown above is a piece of hair imaged by scanning electron microscopy. Brightfield images of yeast cells (left) and fat globules in milk (right) are processed, sized to the same scale as the hair, and superimposed on the image. Figure 5–How milk turns into cheese. (A) Milk viewed with a light microscope reveals tiny fat droplets. (B) Upon adding acid in the form of vinegar, these droplets are observed to coagulate. (C) At the molecular level, this schematic illustration shows that milk consists of fat droplets and protein (casein) particles. (D) Upon addition of acid, the casein particles stick together forming a network in which the fat droplets are trapped. The coagulation of milk upon decrease in pH demonstrates a simple way to make cheese from milk. Developing the texture and flavor in more complex types of cheese relies on enzymes to induce formation of the curds, and microbes that help to lower the pH and create distinct flavors. While paneer is one of the simplest cheeses to make, it is also relatively straightforward to make mozzarella cheese using enzymes (Appendix B). The Science of Making Bread Central to daily life, bread is found in different forms in cultures around the world ranging from bagels to rye bread to pizza dough. All bread products derive from flour, making the label of a bag of flour a natural starting point for a discussion of bread. Investigating the label reveals that carbohydrates and proteins are major ingredients in flour. We reintroduce these words that were first discussed in the context of milk. We describe a simple way to make bread from flour, by adding water. A mixture of flour and water is easy to prepare in the classroom, or at the individual level with students. Baking this flour–water mixture yields a flatbread that is very dense and has no holes. We image the “flatbread” by electron microscopy, which reveals a dense aggregate of starch granules (Figure 6A); in between the starch granules is a protein network of gluten proteins, which is schematically illustrated in Figure 6C. Clearly, pizza dough has holes and a lighter texture. To demonstrate how we can form holes in bread, we introduce yeast, which is a microbe that produces gas (Figure 4). The yeast cells consume sugar and produce carbon dioxide: filling a bottle with yeast and sealing the top with a balloon shows that the balloon inflates over time as the yeast cells produce gas (Demo 4A in Appendix A). Using children from the audience, we Available on-line through ift.org Vol. 9, 2010 Journal of Food Science Education 109 Pizza science: interactive activities . . . Figure 6–Yeast cells alter the texture of bread. (A) Mixing together flour and water produces a basic bread that remains flat when baked. Imaging by electron microscopy reveals aggregates of starch granules. (B) Adding yeast to this mixture yields a bread with bubble and holes, as yeast eat simple sugars and in turn produce gas. (C) Schematic illustration showing the starch granules in a network of flour proteins, which consists largely of gluten proteins. (D) Yeast form holes in the network, giving bread a lighter, airy texture. demonstrate how a network can expand when yeast in the network produce gas (Demo 4B in Appendix A). We show that the equivalent flour–water mixture rises due to the activity of yeast in the dough. Comparing the electron micrograph shows there are large holes in the bread’s protein network structure (Figure 6B to 6D). These demonstrations on the science of bread build upon the theme of creating networks of macromolecules and show the role of microbes (yeast cells) in developing the texture of bread.