Why does popcorn pop? – Exploring the science of popcorn

Updated: 16-Feb-2017

Just the other day, when making stovetop popcorn at home, I was listening to the popping sounds of kernels transforming into fluffy popcorn. It got me thinking: why does popcorn pop? How do these stiff hard kernels transform in such a soft product?

 

To answer those questions, we will dive into popcorn corn. Because yes, not just any corn is popcorn corn! From there we’ll be able to answer the question, why does popcorn pop and how does it do that?

Popcorn corn

Popcorn cannot be made with just any type of corn, you need a special type of corn to make popcorn. Generally speaking, you want sweet corn to be sweet and moist, whereas popcorn should be dry and firm, which the exact right moisture content. A good popping corn can roughly be described using 3 characteristics:

  • Water content: A well popping popcorn has a water content of approx. 14%. (Actual scientific research has looked into this, how cool is that?!) Since moisture content is so important, the popcorn corn should be stored free from moisture, but also protected from drying out.
  • Strength of the hull: As you can see below, a corn kernel is surrounded by a hull, made up mostly of cellulose. The strength of the hull influences the quality of the pop. Scientists found that the stronger and more ordered the molecular structure of the outer hull, the better the corn pops.corn kernel structure - why does popcorn pop
  • Composition of the endosperm: The endosperm contains a large amount of starch. This starch can be either soft or hard (I haven’t found a more chemical definition). Corn for popping requires enough of the soft starch to expand, but also enough of the hard starch to provide structure to the kernel.

Why does popcorn pop?

Let’s first quickly recap how popcorn is made (read a more detailed description here). Oil is heated in a pan after which the kernels are added. By continued heating of the corn (with the lid on top!) they will start popping. Once it has turned quiet, all corn has popped!

Science of why popcorn pops

Now let’s look what happens during popping.

The kernels are heated to a temperature well above the boiling temperature of water (100°C). That’s why oil is used, it can be heated to far higher temperatures. By heating the corn so intensely, moisture in the kernel will start to evaporate. However, the thick hull on the outside won’t let the water gas (vapour) escape. This causes pressure to build up in the corn kernel (can be up to 10 bar!).

When the pressure becomes too high, the hull breaks and the popcorn pops! This happens around 180°C. Once the hull breaks, the gas will expand the starch in the kernel. The high heat and sudden puff will cause the starch to expand, before cooling down again and forming the fluffy structure. The starch has gelatinized in the process.Below you can see that happen in slow motion.

Why does popping popcorn make a popping sound?

Researchers have been thinking about just that question as well. They found out that the popping sound you hear while making popcorn is due to the rapid escape of water from the kernel. The shape of the popcorn is such that it can amplify the sound.

The size & shape of popcorn

Apparently there exist roughly two shapes of popcorn: butterfly & mushroom (spherical). The mushroom variety will give a round popcorn whereas the butterfly will have ‘wings’ sticking out on all sides. It seems like no one really knows why certain varieties turn out round and other butterflyish, there are just a few known factors of influence (or people know, but don’t publish it).

The uses of the different types are more clear. The round versions are generally prefered for flavoured popcorn, they don’t break as easily. Whereas sellers preferring a high volume with a low mass will prefer the butterfly shape.

round popcorn, with caramel and black pepper
The popcorn at the top of the post is an example of butterfly popcorn. This popcorn (a flavored version with caramel and black pepper) is clearly a round variety.

Making larger popcorn

A lot of research has been done on how to obtain the largest volume of popcorn from a kernel. The larger a kernel becomes after popping, the less kernels you need to fill a bowl or bag of popcorn! Thus it’s cheaper for the manufacturer.

Again those three characteristics mentioned at the start are of this post are important:

  • A stronger hull makes for a larger popcorn since it will be able to withstand a higher pressure and cause a larger expansion.
  • A smaller kernel will expand relatively more.
  • A higher moisture content causes the hull to break at a lower pressure, thus it’s a smaller piece of popcorn.

Q&A on popcorn science

Why is popcorn salted?

Besides the fact that popcorn without any salt or sugar tastes pretty bland, studies did show that adding some salt to the popcorn would help it pop. However, only a small quantity is enough, adding more and more won’t help.

Can butter be used instead of oil?

Yes, that is possible. However, butter will burn faster. The reason butter or oil is added is to transfer the heat to the kernels more efficiently. So you should add enough to coat the kernels, without them becomines greasy.

What is the best popcorn?

Yes, also that has been researched. Two different projects each came up with their own ideal composition (this one and this one). However, corn remains a natural product so it will depend on the variety how it will react and what is the best to use.

Why can’t popcorn be popped in boiling water?

As we mentioned before, the moisture in the popcorn should evaporate and this hould happen quite quickly. When popcorn is placed in boiling water this will not happen. Instead, the hull might cook and soften long before the pressure has been able to build up.

Where does popcorn come from and how can I make it?

Those are two questions that require longer answers. So, I’ve written two separate posts on the topic: one on the history of popcorn and one on how to make stovetop popcorn.

Sources

Most sources have been mentioned in the article, but this article got me really excited! Here’s some more background reading on what people want in popcorn. And even Nature talks about popcorn.

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