Why Does Popcorn Pop?
– Every Kernel Is a Tiny Pressure Cooker
A hard yellow kernel suddenly goes "pop!" and turns into a white, fluffy puff. There is no gunpowder inside. What makes it burst is a tiny amount of water. When that water heats up while trapped in a hard shell, the pressure inside the kernel climbs to about four times that of a car tyre.
You put popcorn kernels and a little oil in a frying pan, put the lid on and turn on the heat. For a while, nothing happens.
After a few minutes, one kernel goes "pop." Then pop, pop, pop-pop-pop-pop... it gets lively all at once, and the popcorn swells until it lifts the lid.
If a child asked you, "Why does it pop?", how would you answer? "Because it's hot" isn't an answer. A kernel of grilled corn never pops, however long you cook it.
There are only two reasons it pops
Even a kernel that looks dry holds water, about 14% of its weight. As the water heats up, it tries to turn into steam and pushes hard outward from inside the kernel.
The shell of a popcorn kernel is hard and has almost no gaps. With nowhere to go, the steam pressure keeps building until, past its limit, the shell tears open.
In other words, each kernel is a sealed "tiny pressure cooker." The difference from a real pressure cooker is that it has no safety valve. So in the end, it has no choice but to burst.
How many atmospheres are inside the kernel?
Water in an open pot boils at 100°C and escapes as steam. Water that is shut in is different. The steam can't get out, so the hotter it gets, the harder the steam pushes (its pressure).
Look at Figure 1. The horizontal axis is the heating temperature and the vertical axis is the pressure inside the kernel. At 100°C the pressure is 1 atmosphere, the same as the surrounding air. As the temperature rises, though, the curve climbs ever more steeply to the upper right. At about 180°C it reaches around 10 atmospheres, and that is where the shell is thought to give way.
The important point is that this curve is not a straight line. From 100°C to 140°C, a span of 40°C, the pressure rises by only about 2.5 atmospheres. From 140°C to 180°C, another 40°C, it rises by more than 6 atmospheres. The hotter it gets, the more a small bit of extra heat raises the pressure. That is why the first few minutes are quiet, and then at some point the kernels all start popping at once.
Why doesn't ordinary corn pop?
Grilled corn and the corn in corn soup also contain water. What they lack is the "lid" that traps the steam.
Look at Figure 2. The popcorn kernel on the left (a type called flint-like "popping corn") has a thick shell with almost no gaps. The shell of ordinary corn on the right is thought to let steam leak out when heated. If the steam keeps slipping away, pressure never builds. The kernel just scorches or shrivels.
The white fluff is a "foam" that sets in an instant
Most of the inside of a kernel is starch. Exposed to hot steam, the starch turns soft and gluey. Then the shell tears, and the 10-atmosphere steam is suddenly released into the 1-atmosphere outside.
The steam expands all at once and pushes the gluey starch outward from inside, like a foam. The volume is thought to grow to dozens of times that of the original kernel. The swollen foam is cooled by the outside air and loses moisture, so it quickly sets into a crisp white lump. The white part of popcorn is a mass of fine bubbles made of starch.
In 2015, French researchers studied popcorn with a high-speed camera and sound recordings. They reported that the "pop" seems to come not from the moment the shell breaks, but from the steam inside rushing out. The kernel's jump is also thought to happen because the starch that bursts out acts like a "leg" and pushes off the pan.
Popcorn kernels are said to puff best when their moisture is about 13 to 14%. If you leave a bag open and let them dry out, there isn't enough pressure and more kernels stay unpopped. To store them, keep them in a sealed container.
Summary
Popcorn pops because the water inside a kernel turns to steam and has nowhere to go inside a hard shell. The pressure of the trapped steam rises steeply with temperature, reaches about 10 atmospheres at around 180°C, and tears the shell. The released steam puffs up the softened starch, and when that sets, you get the white fluff.
It isn't the heat that makes it pop, but the water.
Because the shell acts as a "lid," the water becomes the force of an explosion.
The way confinement changes a boiling point is the exact reverse of the story in Why doesn't rice cook well on Mt. Fuji? (high on a mountain the pressure is low, so water boils below 100°C). For other stories of heat changing what's inside food, see Why does a raw egg set when you boil it? and Why does bread rise?
Living things also use the power of trapped water suddenly turning to steam. How can a bombardier beetle shoot hot gas from its rear? describes how this insect boils a liquid with the heat of a chemical reaction to defend itself.
- Prepare two sets of 10 popcorn kernels. In one set only, make a small hole in each shell with a needle.
- Heat the two sets separately in a covered frying pan in the same way, and compare how many pop. The kernels with holes should pop less easily, because the steam leaks out.
- Crack open some kernels that didn't pop. Look to see whether the shell is damaged and whether the inside is too dry.
Don't lift the lid while heating, and keep your face away. Popped kernels and oil can fly out. If you use a microwave, use a container or product made specifically for popcorn.
For those who want more – terms, formulas and links to textbooksEach part is labelled by level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics and chemistry
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity specialist material (thermodynamics, food physics) not taught in high school
- ResearchNot taught even at university as settled knowledge; researchers are still studying it
Junior highTerms: this phenomenon has names
- Popping corn (flint-type "bakuretsu-shu"): the group of corn varieties used for popcorn. The shell (pericarp) is hard, and the kernel pops when heated.
- Vapour pressure (saturated vapour pressure): the pressure of steam when water and steam are in balance in a closed container. It rises steeply as the temperature goes up.
- Gelatinisation: starch loosening in water and heat and turning soft and gluey. Cooking rice involves the same change.
Junior highHigh schoolChecking with a formula: what is the pressure inside a kernel at 180°C?
We estimate the force of the trapped water's steam (vapour pressure) from the temperature. Starting from the fact that it is 1 atmosphere at 100°C, we calculate the pressure at 180°C.
| In symbols | P2 ＝ P1 × e to the power of (L ÷ R) × (1/T1 − 1/T2) |
| In words | Pressure at high temperature ＝ pressure at 100°C × e to the power of "(heat needed to turn water to steam ÷ gas constant) × (difference of the reciprocals of the two absolute temperatures)" |
| Where it comes from | The Clausius–Clapeyron equation, which follows from the condition that water and steam are in balance (phase equilibrium). The more heat it takes to make steam, the more steeply the pressure changes with temperature. |
| Symbol L ÷ R (the heat to turn water to steam, divided by the gas constant) | About 4900 K |
| Symbols T1 and P1 (the ordinary boiling point and its pressure) | 373 K (100°C) at 1 atmosphere |
| Symbol T2 (the temperature at which popcorn pops) | About 453 K (180°C) |
| Difference of the two temperatures | 453 − 373 ＝ 80 |
| Product of the two temperatures | 373 × 453 ＝ 168969 |
| Difference of reciprocals (1/T1 − 1/T2) | 80 ÷ 168969 ≒ 0.000473 |
| The exponent | 4900 × 0.000473 ≒ 2.32 |
| e to the power of 2.32 | About 10.2 |
| Pressure inside the kernel at 180°C | 1 × 10.2 ＝ 10.2 atmospheres |
| Air pressure in a car tyre | About 2.5 atmospheres |
| How many times the tyre pressure | 10.2 ÷ 2.5 ≒ 4.1 |
The pressure inside the kernel reaches about four times that of a car tyre before it pops. In experiments, the pressure inside a kernel just before popping has been estimated at about 9 atmospheres, which roughly agrees with this estimate. A thin shell holds up under this much pressure right up until the moment it bursts.
High schoolHigh school+The boiling point is set by the surrounding pressure
High schoolWater boils at the temperature where its vapour pressure equals the surrounding pressure. With 1 atmosphere outside, that is 100°C. Conversely, if the outside pressure is higher, the boiling point rises. Water inside the shell can't get out, so it keeps getting hotter without boiling. A household pressure cooker, which can cook at around 120°C, works on the same principle.
High school+The instant the shell tears, the surrounding pressure drops suddenly from 10 atmospheres to 1. For water at 180°C, the boiling point at 1 atmosphere, 100°C, is far below its temperature, so the water boils all at once. This abrupt boiling is the force that whips the starch into foam from the inside.
UniversityPhase equilibrium, and how the starch foam sets
The formula above is the Clausius–Clapeyron equation, derived from the thermodynamic condition for phase equilibrium (equal Gibbs energy). Here we treated the heat needed to make steam as constant. In reality it becomes slightly smaller at high temperatures, so the result comes out a little higher than the measured vapour pressure. After gelatinisation, when starch loses water and cools, it changes from a soft state to a hard, glass-like one (the glass transition). The crispness of popcorn is thought to arise when the walls of the bubbles move into this state.
📖 Derivation of the formula and further reading: Clausius–Clapeyron equation (Wikipedia, in Japanese) ／ Starch and gelatinisation (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- Why do some kernels stay unpopped? There are several explanations: tiny flaws in the shell, too little moisture, and differences in how the fibres of the shell are arranged. How much each one matters varies with the variety and the storage conditions, and it hasn't been settled.
- The detailed mechanism of the sound and the jump. A 2015 high-speed imaging study reported that the sound comes from the steam rushing out and the jump from the starch "leg." But it doesn't explain the differences from one kernel to the next.
- Why the best moisture is around 14%. It is easy to see why too little water means too little pressure. Why too much also makes the puff worse is said to come down to a balance between the softness of the starch and the strength of the shell, and it isn't clearly understood.
So the contents of this article, too, are "an explanation as far as we understand today." Even a single kernel of an everyday snack still leaves room for research.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: changes of state (water becomes larger in volume as steam) | The water in the kernel turning to steam and expanding |
| High school | Chemistry: vapour pressure and boiling point | The curve in Figure 1; the boiling point being set by surrounding pressure |
| High school+ | Chemistry: phase diagrams and the idea of boiling-point elevation | The sudden boiling the instant the shell tears |
| University | Thermodynamics (phase equilibrium); food physics (starch gelatinisation and glass transition) | The Clausius–Clapeyron equation; how the foam sets |
| Research | Food engineering; fluid mechanics | Unpopped kernels; studies of the sound and the jump |
| ― | Links to daily life | Pressure cookers; storing kernels (keep sealed, don't let them dry out) |
- Virot, E. and Ponomarenko, A. (2015) Popcorn: critical temperature, jump and sound. Journal of the Royal Society Interface, 12.
- Hoseney, R. C., Zeleznak, K. and Abdelrahman, A. (1983) Mechanism of popcorn popping. Journal of Cereal Science, 1, 43-52.
- Ziegler, K. E. (2001) Popcorn. In: Hallauer, A. R. (ed.) Specialty Corns, 2nd ed., CRC Press.
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (saturated vapour pressure of water)
*This article is a general-audience science explainer. The numbers given are rough guides to help you understand how things work. Do any cooking with an adult watching, and follow the instructions for your cookware and products.