Popcorn has been a popular snack for thousands of years. Remnants of the tasty treat have been found in Mexico dating back to 3600 BC. Popcorn pops because each popcorn kernel is special. Here's a look at what makes popcorn different from other seeds and how popcorn pops.

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Why It Pops

Popcorn kernels contain oil and water with starch, surrounded by a hard and strong outer coating. When popcorn is heated, the water inside the kernel tries to expand into steam, but it cannot escape through the seed coat (the popcorn hull or pericarp). The hot oil and steam gelatinizes the starch inside the popcorn kernel, making it softer and more pliable.

When the popcorn reaches a temperature of 180 C (356 F), the pressure inside the kernel is around 135 psi (930 kPa), which is sufficient pressure to rupture the popcorn hull, essentially turning the kernel inside-out. The pressure inside the kernel is released very quickly, expanding the proteins and starch inside the popcorn kernel into a foam, which cools and sets into the familiar popcorn puff. A popped piece of corn is about 20 to 50 times larger than the original kernel.

If popcorn is heated too slowly, it won't pop because steam leaks out of the tender tip of the kernel. If popcorn is heated too quickly, it will pop, but the center of each kernel will be hard because the starch hasn't had time to gelatinize and form a foam.

How Microwave Popcorn Works

Originally, popcorn was made by directly heating the kernels. Bags of microwave popcorn are a bit different because the energy comes from microwaves rather than infrared radiation. The energy from the microwaves makes the water molecules in each kernel move faster, exerting more pressure on the hull until the kernel explodes. The bag that microwave popcorn comes in helps trap the steam and moisture so the corn can pop more quickly. Each bag is lined with flavors so when a kernel pops, it strikes the side of the bag and gets coated. Some microwave popcorn presents a health risk not encountered with regular popcorn because the flavorings are also affected by the microwave and get into the air.

Does all corn pop?

Popcorn that you buy at the store or grow as popcorn for a garden is a special variety of corn. The commonly cultivated strain is Zea mays everta, which is a type of flint corn. Some wild or heritage strains of corn will also pop. The most common types of popcorn have white or yellow pearl-type kernels, although white, yellow, mauve, red, purple, and variegated colors are available in both pearl and rice shapes. Even the right strain of corn won't pop unless its moisture content has a moisture content around 14 to 15%. Freshly harvested corn pops, but the resulting popcorn will be chewy and dense.

Sweet Corn and Field Corn

Two other common types of corn are sweet corn and field corn. If these types of corn are dried so they have the right moisture content, a small number of kernels will pop. However, the corn that pops won't be as fluffy as regular popcorn and will have a different flavor. Attempting to pop field corn using oil is more likely to produce a snack more like Corn Nuts, where the corn kernels expand but don't break apart.

Do other grains pop?

Popcorn is not the only grain that pops! Sorghum, quinoa, millet, and amaranth grain all puff up when heated as the pressure from expanding steam breaks open the seed coat.

Professional Development Modules

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The Science of Microwave Popcorn: A Matter of Understanding Physical Changes

Grades: 5-8
Author: Tony Gosmer

Abstract

Module Description

This unique module of science and technology will give teachers the opportunity to explore the mysteries of microwave popcorn and the technologies that make it possible. In this process they will venture through a series of events in the investigation of the microwave popcorn bag. They will make inferences and hypothesis based on observations and prior knowledge. The participants will have a better understanding of scientific investigations in the areas of science and technology.

Objectives

• Participants will explain how technology influences our daily lives.
• Participants will explain how decisions about the use of products can result in desirable and undesirable consequences.
• Participants will keep clear and accurate records while recording and interpreting data.
• The following NSES indicators will be addressed in the engagement activities of this module:
• Identify a physical change (popcorn).
• Properly use appropriate tools and instruments (scale, thermometer) .

Materials

• triple bar scale-1 per team (to be used during engagement)
• thermometer (to be used during extension activities)
• a variety of microwaveable popcorns (regular, fat-free, extra butter, unsalted, kettle corn etc.)
• heat protective gloves (cotton or leather) 1 pair per team
• eye protection (1 pair optional for each participant)
• plastic tray (1 per team)
• paper towels or handy-wipes (enough for each participant)
• microwave (depending on the number of participants)
• Fuji film canister (white) with lid (1 per team)
• 1/4 tablet of Alka-Seltzer per team
• about 50mL of water per team
• 1 calculator per team
• 2-3 trash cans throughout the room
• containers to collect recyclable materials (plastic water bottles, aluminum cans etc.)
• if possible, locate a vermicompost container to place discarded popcorn
• hand lens or magnifying glass (1 per team)
• 1 writing journal per individual

Procedures

The first two activities included in this module are designed to get the participants excited and engaged for the day's lesson. The first activity will demonstrate a simple physical science model that will parallel the physics inside the popcorn kernel. The second engagement activity will demonstrate how a physical change takes place inside the popcorn bag. Both activities can be used as either engagement activities or extension activities. Then, following the two engagement activities the lesson will move toward the Microwave Popcorn lesson.

Engagement

So what makes the popcorn pop?

Engagement activity #1

• Distribute one white Fuji film canister and one lid to each team and 1/4 tablet Alka-Seltzer.
• Each team will need approximately 20mL of water. Fill the film canister 2/3 full of water.

Engage a discussion as to the physics taking place inside the popcorn kernel.

Now place the canister with the water in it on a napkin with the open end of the canister facing up.

• Drop the 1/4 tablet into the water and quickly secure the lid and step back from the table.
• Observe what is happening (CO2 gas begins to accumulate inside the canister creating pressure. Once the pressure exceeds the lid/seal capacity and the outside air pressure, the CO2 gas escapes creating an auditory 'pop'.)

Assessment: Discuss and record observations in journal. How do you think this activity relates to popcorn? What changes are taking place inside the microwave popcorn package?

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Exploration

You will need one triple bar scale for each team and one package of unpopped popcorn and explore the physics inside a popcorn bag.

• Remove the cellophane wrapping around the package and place on scale.
• Record the mass (grams) of the unpopped package and discuss the variables that create the mass (salt, popcorn kernels, butter, packaging etc.)
• Now place the package in the microwave for 3 minutes on HIGH.
• Observe and record the process (from a safe distance).
• After 3 minutes remove package from microwave (have gloves as a option).
• Place popcorn package on scale and record the mass (grams)
• Engage discussion comparing and contrasting the data generated.
• What changes took place in the package to generate differing numbers?

Assessment: Discuss and record any changes in the popcorn or popcorn package.

Microwave popcorn lesson procedures:

• Open a discussion involving unique technologies in the construction of the popcorn package (paper, plastic etc.).
• Proceed to procedure A in the popcorn lesson. After participants determine the mass have them note and record variables that contribute to the mass of the package and compare this data with other teams.
• Now, using the same brand as in procedure A place the package in the microwave for 3 minutes on HIGH, open the package as described in procedure B and investigate the number of kernels. Have the participants observe the seal at the open end of the package. Continue through procedure B.
• In procedure C we will discover the layers in the construction of the microwave popcorn package. Using the popped bag from procedure B. This portion of the procedures will provide an interesting inquiry investigation of the bag construction.
• Follow the procedures in the Conclusion section of the lesson. Here you will need the calculators and writing journals.
• Lesson assessment should be ongoing throughout the lesson by means of observations, journal review/sharing out, questioning, antidotal notes and discussions.

Explanation

Review the teacher notes of the lesson for basic information involving polymeric technologies in the construction of microwave popcorn packages prior to your presentation. Discuss examples of how technology influences our daily lives (popcorn popping changed when the microwave oven got popular) and that sometimes technologies have undesirable consequences (How many times have you opened the popcorn bag and found lots of duds?). Have each participant think of ways that technology influences our lives an informal assessment and to demonstrate knowledge of the new material. For further information on polymers and popcorn see the credit/ references section of this module. See also Explanation of Science section below.

Elaboration

The microwave popcorn lesson provides us with a unique opportunity that encourages the participants to plan lessons that utilize inquiry investigations and the use of technology for learning. The participants will be able to elaborate thoughts and opinions through reflective writing. Give participants time to plan an inquiry-based lesson to deliver to their students - see lesson plan template.

Overall Assessment

Monitor participants throughout the session as outlined in the lesson for students.

Engage conversations throughout the lesson allowing for a question and "What do you think?" session.

Have participant's share journal reflections, thoughts and opinions. "How could we improve this lesson?"

Rationale

This module will give opportunities for various teaching methods; one such method is inquiry-based learning. This module will guide the participants through a series of simple activities that will allow for inquiry-based learning in the classroom where both the teacher and students have a positive experience. The engagement activities will provide simple opportunities to attempt inquiry learning, which can be accomplished at varied levels of skills and background knowledge.

Science Standards

National Content Standards A: Science as Inquiry; Content Standard B: Physical Science; Content Standard E: Science and Technology

The following indicators based on NSES content will be addressed through a series of engagement activities (see engagement section in procedures, see also extension activities)

• Describe that in a physical change (e.g., state, shape and size) the chemical properties of a substance remain unchanged.
• Distinguish between observation and inference.

Professional Development Standards

• Involve teachers in actively investigating phenomena that can be studied scientifically, interpreting results, and making sense of findings consistent with currently accepted scientific understanding.
• Build on the teacher's current science understanding, ability, and attitudes.
• Incorporate ongoing reflection on the process and outcomes of understanding science through inquiry.

Best Teaching Practices

• Technology
• Inquiry

Time Frame

2 hours (+/- 20 minutes)

Preparation

Organize room to accommodate groups of no more than 2-3 participants.

Have room arranged to accommodate the expected number of participants. I would suggest having no more than 2-3 participants in each group as this will cut down on unnecessary chit-chat and annoying conversations that do not pertain to the module. Provide your name and contact information. Kindly ask all participants to place their cell phones on vibrate or place in off position, as this is another annoying disturbance that so often occurs during professional development sessions.

Do not place more than one microwave in any given electrical outlet. Contact the building supervisor for recommendations. Have paper towels and/or handy wipes available after handling popcorn.

Safety

Be conscious of extreme heat after removing popcorn from microwave (optional gloves). Make goggles or eye protection an option for the participants. If possible place all recyclable items in recycling bins.

Assessments

Monitor participants throughout the session as outlined in the lesson for students.

Engage conversations throughout the lesson allowing for a question and "What do you think?" session.

Have participant's share journal reflections, thoughts and opinions. "How could we improve this lesson?"

Explanation of Science

The film canister in engagement activity #1 represents the popcorn kernel's pericarp. When the Alka-Seltzer is dropped into the water, carbon dioxide gas forms. The enclosed canister allows the gas pressure to increase causing the lid to "pop", whereas the popcorn kernel's pericarp acts similar to the film canister. The heat forms steam inside the kernel, the pressure builds and the kernel "pops". The kernel contains a tiny amount of water, which undergoes a physical change from the heat (liquid to gas) as does the starch granules change into gelatinized globules (physical change).

The microwave popcorn bags vary in construction. One has a paper outer layer with a PET (same polymer that is found in the two-liter bottle) inner layer. The PET layer prevents the fat and oils to be absorbed by the paper outer layer. Paper is very inexpensive. There is a low melting adhesive for the ends so that when the popping cycle is almost complete, the seal melts in the steam, and opens the ends for gases to escape. The special layer for reradiating energy is located under the paper layer and inside the inner layer of a polymer film. This layer is called the susceptor and it is a metallized PET film. Aluminum is sprayed onto the film in a vacuum chamber to prepare this metallized film. All producers of these packages now use a susceptor in one geometry or another. They do not have to cover the entire package. The susceptor is placed on the bottom of the microwave oven with the corn resting on top. Other packages might use OPP (oriented polypropylene) for an overwrap which costs less than PET. The package is still evolving since manufacturers are striving to minimize unpopped kernels.

Popcorn contains mostly the translucent endosperm as opposed to the opaque endosperm. This endosperm is penetrated by the high pressure steam while the popcorn is heating. The starch granules change into gelatinized globules and then expand into thin fusible bubbles as the pericarp is burst by the pressure of the water vapor. The pericarp is the protective layer on the outside of the kernel. It breaks at about a pressure of 9 atmospheres. (ChemMatters October, 1984)

Handouts

N/A

Extensions

Place a thermometer inside the popcorn bag immediately following removal from bag (be sure it is a thermometer suitable for extreme heat). Try this with various brands and flavors.

Determine the mass of a popped package of popcorn 1 day after popping (you would want to determine the mass after popping to compare).

Go to http://www.jollytime.com/kids_corner/ for more information and activities.

Lesson Implementation Template

Download Lesson Implementation Template: Word Document or PDF File

Equity

Take into consideration the diverse learning styles teacher as well as students have. Consider that you may have technical/confluent learners as well as precise/sequential learners of a variety of backgrounds. All of which interpret and process differently.

Resources

None available for this module.

References

The original Microwave Popcorn lesson was written by Mary Harris, Missouri Polymer Ambassador, Copyright 1998.

For more information on popcorn or polymers see these interesting websites:

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