Whenever you’re popping corn, whether it’s in a microwave, or on the stovetop, your ears play a crucial role. At the start, the kernels are quiet, not a lot of going on and at some point, your start hearing popping sounds. The frequency of popping will ramp up quickly until it’s reached its peak, after which it dies down again slowly. Even though we find it obvious that popcorn pops, it took scientists a while to truly understand why popping popcorn makes that sound. Turns out, why popcorn pops is pretty fascinating.
What happens when you pop popcorn
Before we dig into why popcorn pops, we need to know how popcorn is made. You can make popcorn in a pot on a stovetop, in the microwave, in a special popcorn popping machine, or, as was likely done centuries ago, on a fire. No matter the method, the mechanism is pretty much the same in all: Heat the popcorn until it is hot enough to pop.
Of course, there are some complicating factors. You don’t want the kernels to burn while you’re popping corn (which is why on the stovetop you have to shake your pan regularly). Also, you need to make sure the popping corn doesn’t pop everywhere. So you need a device that can handle both the heat and the expanding volume of the popcorn. while ensuring the kernels don’t burn.
Corn kernel composition
Now let’s have a look at a corn kernel. Corn is made up of various layers. The most important ones for us are shown below. The outer layer of corn is what we call the pericarp, or outer hull. Within that hull, there’s the endosperm, which contains a lot of starch, and the germ. The germ is what would form the start of a new plant if planted.
Both the endosperm and germ contain moisture. A good popcorn corn kernel contains about 14% moisture in this entire kernel. Not just any corn is suitable for making into popcorn, only popcorn corn is!
Heat causes evaporation
While you’re heating popcorn, you’re heating up all components, but most importantly, you’re heating up water. In fact, you’re heating it up so much that it starts to evaporate. It’s similar to what happens when you’re baking cakes or bread, a part of the rise of the cakes and breads. When moisture evaporates, transforms from a liquid into a gas, it expands. The same mass of moisture will take up a lot more volume. However, the firm pericarp around the kernel won’t let that happen. It will keep all the moisture in.
The pressure builds up
If the volume of the gas can’t increase though, something else will happen: the pressure within the kernel increases. This can be described using the ideal gas law (n*R*T = P*V). In a corn kernel the number of molecules doesn’t change. If in such a scenario the temperature increase, but the volume stays the same (because the outer layer doesn’t bulge), the pressure will have to increase.
Until it pops!
A popcorn kernel hull is so strong that it can actually hold on to pressures up to 10 bar! This is a lot of pressure! However, at some point, the hull won’t be able to resist the pressure anymore. The popcorn pops!
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Because of the enormous pressure build up the sudden expansion of the gas will puff up the starches within the kernel. If the pressure wasn’t allowed to build up, the moisture would just have escaped and wouldn’t have had the ‘force’ to puff up the corn!
Once the gas has escaped the popcorn cools down quickly. As a result, the starch turns solid again and holds onto its shape. The starch undergoes very rapid cooking and gelatinization.
Factors that influence the ‘pop’ of popcorn
Not every corn will pop. You need the right variety and store it in the right way to make popcorn. We’ve discussed those varieties before. Researchers have investigated the optimal properties of a corn kernel for popping and they found several important factors.
First of all, the water content. The corn should contain enough moisture to reach those high pressures upon heating. However, it should not contain so much water as to soften the outer hull, which would limit the pressure it can withstand.
Why can’t popcorn be popped in boiling water?
It is important for the corn full to remain strong and relatively dry, to withstand those high pressures. Simultaneously, you need temperatures well above the boiling point of water to actually pop the corn.
As you can imagine, the strength of the outer hull of the corn kernel is very important. The hull should be strong enough to hold onto these high pressures, however, it should not be so strong that it never breaks. Scientists found that a hull in which the molecules are structured orderly will give a better pop to the corn.
Even though the moisture and hull determine whether a kernel puffs, it’s the starch that forms the final popped corn. As such, the type of starch is essential. Scientists describe these starches as being either hard or soft. Again, it’s a balance, the starch needs to be soft enough to expand into this beautiful fluffy consistency. However, it should then also be hard enough to hold onto its shape.
Why does popping popcorn make a popping sound?
Somehow it sounds obvious that popcorn pops. We’ve so associated that sound with the popping of corn. However, those loud pops have puzzled researchers for a long time. How can such a small corn kernel make these pretty loud noises?
By recording both the sounds and slow motion images of popping corn, researchers were able to identify which step makes this popping sound. You might have expected the breakage of the hull to cause the popping sound, but that is not it. Instead, it’s the escape of water vapor that makes the sound. That sound is then amplified and repeated by bouncing around within the popcorn kernel and the pan in which it’s popped!
The origin of sound is, and this might surprise, actually very similar to that of a champagne bottle being unpopped!
Modifying shape and size of popcorn
You might be wondering how you can optimize the size and shape of your popcorn. However, once you’ve bought your kernels, there’s not a lot you can do here. The process you use has a negligible effect on the final popcorn size and shape. Instead, it’s the kernel itself that impacts most just how that popcorn turns out.
For instance, a stronger hull makes a larger popcorn. The stronger hull can hold onto more pressure, so once it’s released, the starch will expand more. Also, smaller kernels expand relatively more than larger ones do. A lower moisture content (up to a limit of course), will also increase size. If there is too much moisture the hull will break at a lower pressure, causing it to puff less.
There are roughly two main shapes of popcorn: round vs. butterfly. Which shape your popcorn will take on really mostly depends on the properties of the corn you’re using. Which type you use will depend on your application, as we discuss when making caramel popcorn.
Looking for the optimal popcorn?
Now that you know why popcorn pops, you might want to go just one level deeper. Designing your optimal popcorn. Yes, it will be yours, because everyone has their own ideal popcorn properties :-). Nevertheless, if you want to take a researchers advice on this, two papers claim to have found a ‘best’ way to pop corn: this one and this one.
Joshua Foer, The Physics of . . . Popcorn, Jan-19, 2005, link
Sabri Gökmen, Effects of moisture content and popping method on popping characteristics of popcorn, Journal of Food Engineering, Volume 65, Issue 3, 2004, Pages 357-362, ISSN 0260-8774, https://doi.org/10.1016/j.jfoodeng.2004.01.034
Mark Peplow, Popping Perfect Popcorn, 25-Feb, 2004, Nature, link
Phys.org, The chemistry of popcorn: It’s all about ‘pop-ability’, April-14, 2005, link
Jess C Sweley, Devin J. Rose, David S. Jackson, Hybrid and environment effects on popcorn kernel physiochemical properties and their relationship to microwave physiochemical properties and their relationship to microwave popping performance popping performance, Faculty Publications in Food Science and Technology. 115, link
Virot, Emmanuel and Ponomarenko, Alexandre, 2015, Popcorn: critical temperature, jump and sound, J. R. Soc., Interface. 12 20141247, http://doi.org/10.1098/rsif.2014.1247