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How and Why Does Popcorn Pops? Science & Structure of Popcorn Kernels

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From kids to elders, everyone loves popcorn. How do unappetizing hard kernels of corn become so delectable within two minutes? There is enormous science behind it. Every part of a corn kernel(popcorn seed) plays a role in making hard kernel into fluffy popcorn. There exist other kernels which pop when heated, like seeds of pigweed, quinoa, and sorghum, but popcorn is the biggest and best popper!


Parts of Corn Kernel

Hard, drop-shaped, popcorn kernels that we use to pop in the oven are actually seeds of the maize plant. They grow on a cob of the maize plant and are arranged in rows on the cob. Kernels or seeds which finally pop to become popcorn are made up of three layers.

  1. Hull, outer hard-skin (pericarp)
  2. starchy interior (endosperm)
  3. Embryo plant.

Endosperm(starch) acts as the food for the embryo when the seed is allowed to germinate. There are lots of varieties of the maize plant. Popcorn is a special variety with a hard hull or cover. It is not possible to pop other varieties of maize even after drying its kernels in the sun.


What Makes Popcorn Pop?

Popcorns are generally 'oil popped' or 'air popped', when kernels are placed into 'heated oil' or 'hot-air popper', water inside the kernel gets super-heated and the temperature rises above the boiling point of water. But because of the hard and stiff outer cover (hull), water can't evaporate, and fast-moving water molecules increase pressure inside the kernel from 100 kPa to 900 kPa (9-10 times normal atmospheric pressure). Under such tremendous pressure, water penetrates the starch structure and turns it into a gel form.

Gelatinization needs three key factors; starch, moisture, and heat. When hot water under intense pressure penetrates starch granules it transforms starch into a gel. The popping of popcorn is the combined effect of physical and chemical change on the kernel. On heating, tremendous pressure of water vapour inside the kernel and cracking of hardcover is a physical change. While gelatinization of starch is chemical.

At around 180°C pressure inside the kernel is enough to rupture the hardcover of the popcorn kernel. When it ruptures the pressure inside the starch gel instantly reduces to 100 kPa (normal atmospheric pressure). Reduction in pressure allows water to expand and evaporate as Boyle's law suggests. This change causes the starch in the kernel to expand 30-40 times its original size. this change is so fast that it makes the kernel explode and pop.

It has been observed that when the popcorn pops the first thing to pop out is a small flake at the bottom of the kernel, which looks like a limb coming out of the fractured hull. This limb gives the kernel a boost to jump in the air at the speed of 200m per second!

The popping sound while making popcorn is because of water vapor escaping the gel. The vacuum created by escaping water vapor is occupied by the surrounding air. The sudden rush of air in all the particles of the gel setup an echoing resonance, making all the cavities in the gel 'acoustic resonators'.

Concerning these facts, it is necessary to store unpopped kernels in an airtight container. Normally a kernel contains around 14 to 17 percent of water, which is around 20 milligram!. The hull of the kernel is not waterproof, which means it has very minute pores from which water could escape. If kernels are kept uncovered for a day in a dry climate, they can lose 1% of moisture. A loss of 3% or more will not be qualified enough to pop under the oven heat!

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Varieties of Popcorn

There any many varieties of corn in the market, they all are classified mainly into;

  1. 'Dent corn'
  2. 'Flint corn'
  3. 'popcorn'
  4. 'Sweet corn'

The specialty of the 'popcorn' variety is that its outer cover (hull) is hard and thick compared to other varieties. Many people have a misconception that; normal sweet-corn or other soft varieties of corn could be transformed into 'popcorn kernel' by drying it in the Sun. Popcorn kernel needs a hard and thick outer cover(hull) which is not possible in sweet-corn kernels.

Even popcorn variety divides into two main subcategories; butterfly and mushroom. The reason for these names is their shape of flakes after they pop. Butterfly flakes are soft and tend to melt in your mouth, on the other hand, mushroom flakes are stiffer and the hull on flakes surface is more noticeable. Mushroom flakes are mostly used in commercial packagings because they are more sturdy. They are also more preferable in confection because it is easy to coat them with chocolate and caramel.

Two kernels from the same cob can have two different flakes but the latest biotechnology has provided us with a hybrid that can produce 100% butterfly flakes or100% mushroom flakes.

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Varieties of Corn (Maize) Crop

Varieties of Corn (Maize) Crop

Varieties of Popcorn

Varieties of Popcorn

Varieties of Popped-Popcorn

Varieties of Popped-Popcorn

Varieties of Popcorn

  • Popcorn Varieties
    Did you know that popcorn comes in many different varieties? See some of the differences between popcorn varieties.

Different ways to pop popcorn

Popped popcorns have been found in 'bat cave' which was occupied 3 thousand years ago by humans practicing primitive agriculture techniques. This suggests that popcorn is not a recent discovery until the 1880's popcorns were confined to houses and families. Then Iowa was the first company to pop out their products of popcorn named 'big buster' and 'little buster' on the streets.

There are different techniques and machines to pop the popcorns. Generally, popcorns are heated through three different mediums.

  1. Oil
  2. Air
  3. Microwave

It is very important to heat all the kernels of the popcorn in the vessel at the same pace and time, otherwise, they are cooked unevenly. To achieve this kernels are covered by either hot air or hot oil which heat the kernel till it pops, this differentiates them into air-popped and oil-popped corns.

Microwave radiation of the oven interacts with dipole molecules present in the food(molecules with negative and positive separate charges). Water is also a dipole molecule and thus interacts with microwave radiation and vibrates approximately with the frequency of the waves. When corns are microwaved, the water inside the kernel gets super-heated and the popcorn pops. The novelty in this process is that neither oil nor hot air is used to pop the kernel.

Different types of popcorn machines

Different types of popcorn machines

Benefits of Air-Popped Popcorns

· Very low in saturated fat

· No cholesterol

· Very low in sodium

· Very low in sugar

· High in dietary fiber

Oil popped popcorn have 7% more fats and low dietary fibers compared to air-popped.

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Brad Masters from Southern California on May 16, 2012:

thanks for the interesting facts on popcorn.

The best tasting is the popped in coconut oil, and drizzled in melted butter.

The best for you as you have stated is the air popped corn.

rjsadowski on May 16, 2012:

All I know is that it tastes good but thanks for the explanation.

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