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Ask A Scientist

How does a kernel pop into popcorn?

Asked by: Samantha Koilikas
School: St. James Middle School
Grade: 7
Teacher: Mrs. Hantsch
Hobbies/Interests: Animals, drawing, writing, sports, knitting and making things.
Career Interest: Chef, actress, musician, or scientist

Answer from Douglas W. Green

Adjunct Lecturer, Binghamton University

Research area: Leadership and Instructional data analysis
Interests/hobbies: Playing my banjo, biking, golf and reading
Family: Daughter Lena, age 24, who is an animator in New York City

When a kernel of popcorn is heated, the pressure inside increases. This exerts pressure on the outer hull. It is important to know that as the pressure on a liquid increases, so does the boiling point of the liquid. When the pressure inside a kernel gets to about nine times the pressure on the outside, the water inside the kernel turns into steam. The pressure then cracks the hull and the material inside rapidly expands. The expansion is so rapid that the kernel explodes. This happens when the temperature reaches about 350o F or 175o C. Keep in mind that water usually turns to steam at 212o F or 100o C at normal atmospheric pressure. 


Not just any corn kernel will pop. The type of corn we eat or feed to animals does not have a strong enough hull or pericarp to contain pressurized water long enough. In addition to the hull, a kernel of corn contains a starchy material called endosperm and a central portion called the germ. A corn kernel is a seed and the germ is what grows into a new plant if the kernel is planted and watered. The starchy endosperm is there to feed the germ as it grows.

As a popcorn kernel is heated, the endosperm is penetrated by water heated above its normal boiling point. This turns it into hot gelatinized globules that form the bulk of a popped kernel. The remains of the hull are visible if you look closely at a popped kernel, which is about forty times its original size.

In order to pop, a kernel must have just the right amount of water. The ideal amount is around 14%. This means that popcorn growers must dry the kernels a bit before placing them in airtight packages so they won’t dry out too much. It is also important to protect the outer hull from damage. If it is cut or cracked during processing, the hot water will escape and the kernel will not pop. You can check this for yourself by making holes in some kernels with a pin prior to trying to pop them. You can also dry out some popcorn by baking it at 250o F for about thirty minutes prior to trying to pop it. This should lower the water content below the ideal 14%. Be sure you have an adult on board if you try any experimentation on this most interesting snack food.

Here is some additional material that you can include on the web site.

While we don’t know when popcorn was discovered, we do know that Mexicans used it for food before Columbus made his first trip to the Americas over 500 years ago. There was no corn of any kind in Europe at the time.

Popcorn is considered to be a snack food and as such it does provide some nutrition as long as you don’t load it up with a lot of salt and butter. This why hot air poppers that don’t use hot oil and microwave popcorn can be more nutritious.

Check this site nutrition information on popcorn http://www.nutritiondata.com/facts/snacks/5659/2.

For slow motion popping and the difference between damaged and undamaged popcorn check the following. http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/steam-explosions-the-science-of-popcorn/

Last Updated: 9/18/13