Popcorn Science: Edible Explosions

By Nicole Rhodes, Lead STEAM Tutor

What is hard as a rock, explodes under pressure, but is delicious drizzled with butter? Popcorn! Read on for some history and science of popcorn, then put on your science hat to practice taking measurements and math with a yummy ending. Finally, attract birds to your yard with popcorn-seed balls! 

History & Science of Popcorn

January 19th marks National Popcorn Day–and has since about 1988, for no known reason except to celebrate this ubiquitous snack! Popcorn originated from wild grasses bred by Indigenous Americans. The oldest ears of popcorn are about 5,600 years old and were found in New Mexico. It is now a staple food, providing over 20% of the world’s nutrition!  Europeans first encountered popcorn when exploring the Great Lakes region of the United States and Canada. There is a record of French explorers observing Haudenosaunee people popping corn with heated sand in 1612. European colonists quickly embraced popcorn as a delicious snack and often ate it with sugar and cream for breakfast. Indigenous American folklore told of spirits inside corn kernels, who would grow angry and shake their houses as they were heated. When they were mad enough, they would burst out with a puff of steam. This story can certainly describe what we see when corn is popping–but what is the science behind it? 

As the corn kernels heat up, they begin to shake. This is from water molecules trapped inside the kernels. As water molecules heat, they begin to move around, causing the popcorn to move. When it gets hot enough (about 180 degrees Celsius, or 355 degrees Fahrenheit), the pressure from the steam inside the kernel hull, or “pericarp” is so high, it bursts open. Soft starch from the endosperm is carried out with the steam as a gelatinous goo. Once the popcorn cools down, the gelatinous starch hardens and forms the popcorn we are familiar with. 

The Science Behind the Sound

We typically hear a “pop” sound as popcorn is being made, but it turns out that scientists did not know where it was coming from! You might think that the pop sound is made when the hull bursts open, or maybe from hitting something as it launches into the air, but slow-motion video footage proves that the sound does not happen during either of those events. Recently, researchers at the Polytechnic School in France discovered that the sound is actually from the water vapor released. Here is a fun video from their research, showing the popcorn’s lift-off, puff of steam, and acrobatic spin after bursting out a tiny starchy “leg” from which it propels itself, much like a gymnast doing an aerial somersault.

How big does popcorn get?

It’s pretty spectacular that so little corn can make a pile of popcorn. This is a great opportunity to practice math and measuring, and you can eat your results! Break out your old air popper, or use the recipe below to make popcorn the stovetop way. Either way, follow the methods below to measure your beginning and ending volumes (be sure to write them down), and give your kids a hands-on experience to visualize ratios and percentages! 

Materials

• Measuring cups
• Unpopped popcorn
• Large pot
• Vegetable oil
• Large bowl
• Pencil & paper
• Bakers scale (optional)

1. Measure out a small amount of unpopped popcorn by taking a cool cooking pot and pouring the unpopped corn into the bottom in a single layer. Then pour the corn into a measuring cup and record the unpopped corn’s initial volume. [Optional: Place the corn on a small scale and record its mass in grams]. 
2. Then, add just enough vegetable oil to the bottom of your empty pot, and place over medium-high heat. Add three kernels of corn to the pot, and cover it with a lid. 
3. When your first three kernels pop, add the rest of your popcorn to the pot and replace the lid. Put the pot back over medium-high heat. 
4. Stir the kernels. Have an adult hold the lid in place while wearing hot mitts and gently shake the entire pot about every minute, until the popping sound slows to less than one pop per second. 
5. Remove from heat and carefully remove the lid while wearing hot mitts. Be careful–the steam is hot! Notice steam and water vapor on the pot lid – where did it come from? Allow the popcorn to cool. 
6. Measure the cooled popcorn with your measuring cups. Record the popcorn’s volume. [Optional: Place the popped corn on a small scale and record its mass in grams]. 

Compare your initial and final measurements. 

Change in volume:

The change in volume can be written as a ratio, for example–

Final volume: Initial Volume

or

10 cups:  0.5 cups

The change in volume can also be written as a fraction, for example–

Final Volume       or    10 c

Initial volume 0.5 c

You can use this ratio to scale up or down. Say you are having a party and want 20 cups of popcorn. How much corn should you pop? 

If you had a popping ratio of 10 cups/0.5 cups, then you would need to multiply the numerator and denominator by 2, so you would need to add 1 cup of corn to your pot to get a final volume of 20 cups. 

Percent Yield: Were there any unpopped kernels?

If you add 1 cup of corn to the pot, will you get exactly 20 cups of popcorn? You might not, especially if some of the kernels were too old or dry to pop. If you measure the volume of the popcorn and compare it to the expected volume (20 cups), then you can calculate your percent yield. A percent yield is comparing what you got, versus what you expect to get. Chemists do this when they are trying to see how well their reactions are working. 

For example, if you got 18 cups of popcorn from 1 cup of unpopped corn, then your percent yield is:

18 cups/20 cups = 0.90 cups, or 90%

What might cause your percent yield to be less than 100%

What might improve your percent yield?

Winter Popcorn Seed Balls for Birds

Now do you have a ton of popcorn on your hands? Put aside plain, unsalted popcorn and turn it into wild bird food! This can be a nice supplement of starch for songbirds during cold winter months, but popcorn should not be more than 10% of a bird’s diet, so it’s best to mix it with birdseed. Here is how to make birdseed balls to hang outside for your feathered neighbors. 

Materials

• 6-7 cups of air-popped popcorn
• 1 cup of wild birdseed
• ¼ cup of dried, unsweetened berries 
• 4 tablespoons of unsalted peanut butter (naturally flavored with no added sugars)
• Cooking oil 
• Large mixing bowl
• Twine or Sisal rope
• Wooden spoon 

Instructions

1. Make plain popcorn without any toppings or seasonings (especially no salt).
2. Heat the peanut butter in a small saucepan, or on low heat in a microwave for 10 seconds at a time. You just want the peanut butter to be a little runny to make it easier to incorporate with the other ingredients. 
3. In a large bowl, combine plain, unsalted peanut butter and popcorn. Mix carefully using your wooden spoon.
4. Add the bird seeds and the berries to the bowl while slowly mixing the ingredients, so the popcorn does not break apart too much. 
5. Allow the mixture to cool before forming them into balls. The mixture should be cool enough when you touch it. Line a baking sheet with parchment paper.
6. Coat your hands with vegetable oil to make sure that the mixture will not stick to your hands.
7. Gather handfuls of the mixture and shape them into tennis-sized balls. Place on the parchment paper to dry for about 5 hours, or overnight. 
8. Use the twine to wrap around the ball and tie to it a tree or your bird stand.
9. You can store extra popcorn balls in an airtight container in the refrigerator for a week, or in the freezer for longer. 

Check out Cornel Lab of Ornithology’s Bird Guide to identify some of your backyard birds.