What Makes Popcorn Pop? The Science of Your Favorite Snack

Popcorn: Fun and Physics
By: Kelsey Tenney of appeasingafoodgeek.com 

Hello virtual friends! My name is Kelsey Tenney, and I’m a graduate food science student at Penn State. I’m guest posting on Chews-Worthy’s blog; just stopping by to pass on some food chemistry and physics your way! More information about me if you’re interested…hmm…I dabble in many cookbooks at a time, baking and cooking my way through, I love anything William’s & Sonoma, and you can usually find me munching on cookie dough while I sip red wine and watch Netflix.

Well friends, it’s March! In my dread of the impending spring rains here in State College, I have to have fun snacks on hand. It’s a must. And you know what my idea of the perfect fun snack to make is? Popcorn!

Just think about it. It’s super easy and fast. And cheap. And healthy (in its pure form…). And it’s a blank canvas. You can literally do anything to it and it will probably taste amazing. It’s like vanilla ice cream but better…because it makes cool noises and jumps around.

I think my love of popcorn began with my dad who used to insist on popcorn every weekend for our movie nights. My brother and I always got our own personal bowls of popcorn! PS my dad never burnt it—a distinct talent I think. My mom further fueled my popcorn appreciation when she introduced homemade caramel corn to me while we watched chick flicks in her bed. Do you notice that movies are always associated with my fond popcorn memories? Yeah no wonder I am obsessed with movies still.

So make these popcorns for your House of Cards binge. Or while you wait for the next episodes of Togetherness and Girls! But first—the fun part. What exactly is happening when you listen to those popcorn kernels popping in your microwave or on your stovetop (the “fancy” way)? Investigators in France, Emmanuel Virot and Alexandre Ponomarenko, have recently released a paper (http://emmanuelvirot.free.fr/virot2015popcornjump.pdf) documenting the ins and outs of popcorn and why it behaves as it does. They hope to encourage the teaching of basic physics through the simple equations that explain how popcorn pops.

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When the temperature heats up around the popcorn kernels and exceeds 100⁰C, the water inside of the popcorn kernel begins to boil and change to water vapor. As the temperature continues to increase, the water vapor molecules move faster and faster until the vapor pressure on the kernel becomes too great.

When the steam inside of the popcorn kernel reaches the maximum pressure that the kernel hull can take, the hull begins to split. This sudden release in pressure causes the steam to escape really quickly. It is hypothesized thought that the steam release is what you hear when the popcorn “pops.”

This recent work by Virot and Ponomarenko has identified the critical temperature for this maximum vapor pressure is 180⁰C. They have modeled this phenomenon using the properties of water vapor, an ideal gas. Solving this relation justifies why popcorn all pops at around the same temperature. That makes it easier for us to know when the popcorn is done simply by noise.

Secondarily, the steam release heats the starch inside of the kernel and causes it to expand and cook extremely quickly. That is what causes the soft and spongy texture of a popcorn kernel. Directly after the steam release, the starch expansion pops out of the fractured popcorn kernel. This starch “leg” has so much energy following release that it causes the kernel to jump and flip. It is during this flip that the rest of the starch pushes its way out of the fractured kernel hull essentially turning the popcorn kernel inside out. Virot and Ponomarenko have demonstrated a rotation angle of 490⁰. They are certain that this leg of starch following the steam release causes the flip because the steam ejection itself does not cause the effect. They have compared the movement of a popping kernel to a gymnast concluding that the kernel has a better somersault. The gymnast only flips with an angle of 300⁰.

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The investigators have mapped this jump energy using three terms—horizontal, vertical, and rotary kinetic energy. They believe that the jump could actually have greater kinetic energy, but that the starch leg inelasticity, the energy sound release from the “pop,” and the actual fracture of the hull reduce the energy output of the flip.

The popcorn kernel is approximately 170 mg and has a jump energy of 20 μJ and an initial acceleration of 200m/s2. To put this in perspective, a flea is 1 mm in size and jumps at an acceleration of approximately 1000m/s2. If you want to see pictures of the phenomenon and the equations explaining this, click here (http://emmanuelvirot.free.fr/virot2015popcornjump.pdf), and if you want to see a slow-mo video, click here! (http://www.nytimes.com/2015/02/17/science/why-popcorn-also-jumps.html?smid=tw-nytimes&_r=0)

Virot and Ponomarenko demonstrated the popping sound by carefully monitoring the popping kernel. A microphone can pick up the popping sound prior to any flipping movement. This means that the steam release is responsible for the noise and not the starch expansion. They believe that the sound is caused by the sudden drop of pressure upon the steam release. The cavities inside of the kernel act as an acoustic resonator. This phenomenon can be experienced in the popping of a champagne cork.

So now that you know, pop that popcorn! Here are some recipes to keep the fun times rolling.

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Nutella Puppy Chow Popcorn

Adapted from Food52 (https://food52.com/recipes/33817-nutella-popcorn-puppy-chow)

Serves 4

1 tablespoon butter

1 cup dark chocolate chips

¾ cup Nutella

½ teaspoon vanilla extract

1 bag salted microwave popcorn (no butter please!)

1 cup cocoa puffs or other chocolate cereal

1 ½ cups powdered sugar

1. Melt butter, chocolate, and Nutella together. Microwave in 30 second increments, stirring between, until smooth. Add the vanilla extract and stir to combine.

2. Pop the popcorn and be sure not to burn! Pour into a large bowl followed by the chocolate mixture. Mix gently until all of the popcorn is coated. Add the cereal and mix to combine. (Also if you act now, cocoa puffs are stars and moons! Cute much?!)

3. Add the powdered sugar and mix to coat.

Honey Butter Popcorn

Adapted from The Faux Martha (http://www.thefauxmartha.com/2012/02/13/sea-salt-honey-butter-popcorn/)

Serves 4

3 tablespoons butter

3 tablespoons honey

2 cups cinnamon toast crunch or other cinnamon cereal

1 bag salted microwave popcorn (no butter please!)

Salt to taste

1. Pop the popcorn and be sure not to burn! Pour into a large bowl. Add the cereal and mix to distribute.

2. Melt the butter in the microwave. Add the honey and microwave for 15 seconds to make sure it will be easy to toss the popcorn in.

3. Pour the honey butter mixture into the popcorn/cereal. Add a few pinches of salt and mix to coat evenly.

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