JCE Classroom Activity: #79
Instructor Information
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Popcorn—What’s in the Bag?
Marissa B. Sherman and Thomas A. Evans* Dept. of Chemistry and Biochemistry, Denison University, Granville, OH 43023; *
[email protected] photo by J. J. Jacobsen, R. Wildman, E. K. Jacobsen
In this Activity students investigate microwave Bag contents popcorn, the process of microwave-promoted popping, and the materials involved: water, vegetable before popping; popped flake; flake oils, starch, and special packaging materials. popped in water.
Popcorn and its “popping” mechanism are well understood (1). Water is central to the popping process: a starch/water mixture inside an unpopped kernel becomes gelatinous at high temperatures, the vapor pressure of superheated water bursts the kernel, and the cooling effect of evaporating water transforms a liquid foam into a solid foam. Water’s role as a polar solvent is also demonstrated as a starch-based popcorn flake dissolves in water but vegetable oils do not. Related solubility behavior of polystyrene and cornstarch packing materials is the subject of a Classroom Activity (2). Microwave packaging for popcorn is influenced by the role of water. The aluminum susceptor (3) improves the efficiency of heating water in the kernel and the bag’s heat-sensitive adhesive allows water vapor to escape during popping.
Integrating the Activity into Your Curriculum This Activity supports discussion of thermal and electromagnetic energy, phase changes, intermolecular forces, patterns of solubility, and the structure of fats, oils, and starches. Nutritional issues related to these biomolecules can also be addressed. The health effects of trans fats are a prominent issue at this time.
perforated
About the Activity This Activity can be separated into three parts: Part 1, examining a package of microwave popcorn (steps 1–3); Part 2, examining the behavior of popcorn flakes in water (steps 4–5); and Part 3, quantitative measurements of popping (steps 6–10). The JCE Online Supplement contains a diagram of a kernel, a discussion of the starch–iodine test, additional questions to explore, and typical quantitative results.W Use microwave popcorn packaged with lots of vegetable oil for Parts 1 and 2. These products may have labels such as “Butter Lovers”; anything will work except “no fat” products. Currently, the vegetable oil is partially hydrogenated, but that may change because of concerns about transfats. “Low-fat” microwave popcorn was used for Part 3, but a “no fat” product should yield similar results. Part 1. Adhesives on the top and bottom of the bag appear equally effective before popping. However, when heated, the top adhesive fails, the package opens, and water vapor escapes. The popping instructions, “This Side Up,” assure that kernels rest on top of the susceptor (see step 2 on the student side) for more efficient heating. Solid vegetable oil is easily distinguished in the contents of the package. Part 2. Popcorn flakes (hydrophilic) coated with vegetable oil (hydrophobic) exhibit the expected solubility behavior when added to hot water. Fizzing is heard and gas bubbles are observed as the solid foam loses its structure and water replaces air in the foam. Povidone–iodine solution, an antiseptic wash product available in the first-aid section of drug stores, forms a blue-purple starch–iodine complex with starch in the flakes and in the water. Part 3. Although the popcorn should not be eaten, aiming for an “edible” product is desirable to meet the expectation that mass loss is accounted for by water loss. Popcorn does not necessarily lose all of its water during popping. There are several possible sources of error, so the fact that the % mass loss may deviate from the original 13–14% water content should not be overinterpreted. The susceptor in microwave packaging may serve a protective function, absorbing microwave energy that would otherwise be absorbed by already-formed flakes, causing them to dehydrate or burn.
Answers to Questions 1. The susceptor must be on the “down” side so that the kernels will rest on it during heating. Only the adhesive at the top opening of the bag weakens with heating, allowing water vapor to escape during popping. 2. Solid vegetable oil melts in hot water, floating on the surface rather than dissolving. The flakes lose their structure, accompanied by bubbles and fizzing. The blue-purple starch–iodine complex provides evidence for the presence of starch in the kernels and dissolved in the water from the kernels. Starch is soluble in water due to hydrogen bonding between polar water and starch. Fats and oils are non-polar and are insoluble in water. 3. The results should be consistent with the proposed role of water and the description of popping as a physical change. Popping lunch bag samples of kernels yield good linearity in a graph of flake mass vs kernel mass. Sources of error include unpopped kernels, which would lower the % mass loss, and flake scorching and burning, which would increase the % mass loss.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
References, Additional Related Activities, and Demonstrations 1. Sibley, L. K. Popcorn. ChemMatters 1984, 2 (3), 10–12. 2. Cook, P. A.; Hall, S.; Donahue, J. Pondering Packing Peanut Polymers. J. Chem. Educ. 2003, 80, 1288A–1288B. 3. The greatest discovery since fire. http://www.inventionandtechnology.com/xml/2005/4/it_2005_4_feat_4.xml, accessed Jan 2006. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 83 No. 3 March 2006 •
Journal of Chemical Education
416A
JCE Classroom Activity: #79
Student Activity
Popcorn—What’s in the Bag?
Try This You will need: two bags of “high-fat” microwave popcorn; microwave oven; scissors; small container; large bowl; toothpicks; clear, colorless, or light-colored cup; hot water; paper plate; povidone–iodine solution; two bags of “low-fat” microwave popcorn; four brown paper lunch bags; balance with capacity of ca. 100.0 ± 0.1 g; marker; graph paper. __1. Remove the plastic overwrap from a bag of “high-fat” microwave popcorn. What purpose might the overwrap serve? Examine the adhesives that seal the top and bottom of the bag. Record your observations. Tear or cut open the top of the bag. Pour the bag contents into a small container. Record your observations of the contents. __ 2. Examine the inner and outer surfaces of the bag. Locate the “This Side Up” and “Open This End” instructions on the outside of the bag and a dark rectangle (the susceptor), on the inside of the bag. The susceptor is vacuum-deposited aluminum on a polyester film. What purpose do you think the susceptor serves? __ 3. Using a microwave oven, pop a second, unopened bag of “high-fat” microwave popcorn according to the package instructions. Carefully touch the susceptor; what do you notice about its temperature? Examine the adhesives that seal the top and bottom of the bag. Pour the contents into a large bowl. Record your observations. __ 4. Fill a clear, colorless, or light-colored cup with hot water. Add about a dozen popcorn flakes from step 3, one at a time, to the water. Listen! Look! Mix the water and flakes with a toothpick. Record your observations. __ 5. While wearing gloves and goggles, add a few drops of povidone–iodine solution to the flake/water mixture. Stir the mixture with a toothpick and allow it to stand for a few minutes. Starch turns blue-purple in the presence of iodine. Record your observations. Transfer flakes to a plastic bag before disposal in a trash can. Pour the remaining liquid into a sink for disposal. __ 6. Determine the % mass loss when a package of “low-fat” microwave popcorn is Be Safe! When popcorn is heated in popped. Remove the plastic film overwrap. Weigh the package of popcorn before a microwave oven, the bag and its conand after popping. Allow the bag to cool before the second weighing. tents get very hot. Use caution when handling the bag. Heating popcorn too % mass loss = (mass loss/mass of unpopped bag) ⫻ 100% __7. Prepare samples using the contents of a second package of “low-fat” microwave long in a microwave oven can cause popcorn. Divide the contents into four samples of approximately equal weight. the popcorn and/or the bag to burn. Place each sample in a labeled brown paper lunch bag. Loosely fold the top of the Wear gloves and goggles when hanbag over twice. Tear a notch at each end of the fold and twist the notch to hold the dling the povidone–iodine solution. fold shut. Measure and record the mass of each bag with its sample. __8. Pop each sample separately in a microwave oven. Use settings appropriate for a regular bag of microwave popcorn but turn off the oven when the popping becomes infrequent to avoid burning the popcorn and bag. __9. Allow the contents to cool before measuring and recording the mass. Examine the contents for unpopped kernels, evidence of oil, and burned flakes. _10. Graph mass of popped sample bag vs mass of unpopped sample bag. Draw the best-fit straight line and calculate its slope.
Questions 1. How are the instructions, “This Side Up” and “Open This End” related to the goals of the packaging design, which are to heat water efficiently and let water vapor escape during popping? 2. What happened to the flakes and vegetable oil when the flakes were added to hot water and when you added the iodine solution? Why do the starch and vegetable oil behave so differently? 3. The amount of water in popcorn kernels is usually closely controlled to be between 13–14%. Are your results consistent with the expectation that only water is lost when a kernel pops? Explain. What are sources of uncertainty in your results?
Information from the World Wide Web (accessed Jan 2006) Popcorn. http://www.chemistry.org/portal/a/c/s/1/acsdisplay.html?DOC=vc2%5C1rp%5Crp1_popcorn.html Revealing Trans Fats. http://www.fda.gov/fdac/features/2003/503_fats.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
416B
Journal of Chemical Education
• Vol. 83 No. 3 March 2006 •
www.JCE.DivCHED.org
photos by J. J. Jacobsen, R. Wildman, E. K. Jacobsen
Nearly everyone has popped popcorn in a microwave oven, producing the familiar tasty popped kernels, sometimes referred to as flakes. Microwave heating is the result of the friction created as polar molecules, such as water molecules inside an unpopped kernel of popcorn, oscillate at the microwave frequency. When a kernel bursts in response to the vapor pressure of superheated water, the hot gelatinous starch inside the kernel first expands and then cools to form a solid foam as water evaporates. In this Activity you will investigate microwave popcorn, including its packaging, the behavior of popcorn flakes, and the popping process itself.
