The kitchen · a pressure vessel · a number with no gauge behind it

A Pressure Read Off a Steam Table

Popcorn pops because the hull is a pressure vessel: the water inside is heated past boiling without boiling, until the hull gives way. Explainers add that the pressure reaches about 135 psi. In every source we could read, nobody measured it. It is what a steam table gives for water at 177 °C, the temperature at which kernels popped in oil in a 1983 study, and the popping temperatures measured since run from 171 to about 189 °C, which by the same table is 118 to 177 psi. Slide along the steam curve and see where each popular figure sits.

A drop of water that is not allowed to boil

A popcorn kernel is a small sealed can. Inside is starch holding some water, about 20 milligrams of it per kernel by one estimate; outside is a hull, the pericarp, tough enough to hold pressure. In an open pan water boils at 100 °C, because that is the temperature at which its vapour pushes back as hard as the air does. Sealed inside the hull it cannot escape, so as the kernel heats the vapour pressure climbs, and the water stays liquid well past 100 °C. Just before the kernel bursts, most of the water is still liquid:

“In the conditions of pressure and temperature just before explosion, only a small part (less than 1 mg) is in the vapour phase, which means that there is also a liquid phase in the popcorn before explosion.”

Virot & Ponomarenko, J. R. Soc. Interface 12 (2015)

When the hull fails, the pressure drops at once, that superheated water flashes to steam, and the steam blows the softened starch into foam that sets as it cools. (The Popcorn Board's explainer says water “turns into steam” around 212 degrees, the open-pan boiling point in Fahrenheit. Inside a sealed hull it is the pressure, not the 212, that decides.)

The number, and the curve it sits on

For pure water there is exactly one pressure at which liquid and vapour sit together at a given temperature. Engineers tabulate it as the saturation curve, the line a pressure cooker or a power-station boiler lives on. Here it is, computed in your browser from the international standard for steam (IAPWS-IF97). Drag along it.

At 177 °C the table gives 135.7 pounds per square inch, 9.23 atmospheres. That is the famous number, and it sits on the curve to within a rounding. It is also an absolute pressure, counted from vacuum; measured from the air outside the kernel, the way a tyre gauge would, it is about 121 psi.

Where 135 came from

The figure goes back to a 1983 paper by R. C. Hoseney and colleagues, “Mechanism of popcorn popping”. We could not read it (the publisher's site refused our requests), so we read it the way most people now meet it, through a paper that cites it. In 1992 P. Johnny Wu and Henry Schwartzberg wrote, in a paper the journal marks as public domain:

“Hoseney et al (1983) reported that popping occurred at about 177°C, which for pure water is equivalent to a pressure of 931 KPa (135 psia) inside the kernel.”

Wu & Schwartzberg, Cereal Chemistry 69 (1992) 567, introduction

“For pure water is equivalent to.” What was observed was a temperature. The pressure was looked up. Wu and Schwartzberg go straight on to say why the lookup is not quite right:

“His approach is not completely correct, because he assumed that vapor that formed inside the popcorn kernel was saturated and neglected the volume occupied by dry solids in the popcorn.”

Wu & Schwartzberg 1992, introduction

None of the later sources we read reports a gauge, a sensor or any direct reading of pressure inside a kernel. Byrd and Perona's 2005 kinetics paper works with “the internal aqueous vapor pressure”, again a quantity of a model. Virot and Ponomarenko in 2015 came at it from the other side: from the hull's strength (about 10 megapascals) and its thickness against the kernel's radius they estimated the pressure the hull can hold, “leading to p c ∼ 10 bar”, and then read the temperature off the steam relation, getting about 180 °C. Their arrow points the other way along the same curve, and it is still a calculation.

How the number travelled

A converted figure carries its temperature with it, or it should. Here are five places the pair turns up, with what the steam table says at each one's own temperature.

The first and last sit on the curve because each was made from it. The middle three are the number on its travels: 135 psi kept, its temperature drifting. Wikipedia pairs it with 180 °C, where the table says 145.4; the Popcorn Board with 347 °F (it gives no unit, but 347 °C would char a kernel), where the table says 129.4. Each pair is near enough to be harmless and none is a reading. The steam table puts 135 psia at 176.8 °C.

What was actually measured

Temperatures were measured, with thermocouples sealed into holes drilled in kernels, and they do not agree on one number. Wu and Schwartzberg popped 125 kernels one at a time in 200 °C air and recorded each one's temperature at the moment it burst:

They also report that in Hoseney's hot oil “most popping occurred between 171 and 177°C”, and that Roshdy and colleagues in 1984 “measured an average temperature of 187°C at the center of kernels at the instant of popping in a continuous hot air popper.” Virot and Ponomarenko, warming batches of 50 kernels in an oven, found “only 34% of popcorn are popped at 170°C (17 out of 50). Instead, 96% of popcorn are popped at 180°C (48 out of 50)”.

Run every one of those temperatures through the same table Hoseney's 135 came from and the pressure at popping runs from 118 psia (171 °C) to 177 psia (188.8 °C). One standard deviation either side of Wu and Schwartzberg's mean, 175.8 to 191.2 °C, is already 132 to 187 psia. The curve is steep here: each degree adds roughly three to four psi. So the honest version of the famous sentence is: kernels burst somewhere around 170 to 190 °C, which for pure saturated steam would mean roughly 120 to 180 psi, about 8 to 12 atmospheres. And the one study we read that went past the steam table lands lower still. Having objected that the vapour inside need not be saturated, Wu and Schwartzberg end their paper with their own estimate: the hull “ruptures when the internal pressure becomes excessive, i.e., when internal pressure is roughly 110-120 psia and differential pressure across the pericarp is 95-105 psi.” That is below the table's figure at every popping temperature they measured, and it is a calculation too. Three ways of working it out, three answers between about 110 and 180 psi, and no gauge.

Why the water matters, and old maids

The table also explains why moisture matters. Wu and Schwartzberg found that between 11.4 and 16.0 percent moisture, drier kernels popped hotter: “samples with lower moisture content pop at higher temperature.” Less water has to be pushed further up the curve before the hull gives. And too little water makes a feeble pop. For one variety (Metzger and colleagues, 1989), the most popcorn by volume came at 13.54 percent moisture in oil and 14.03 percent with hot air:

“Oil popping produced a maximum volume of 43.6 cm3 /g of dry matter at a moisture content of 13.54%. Air popping produced a maximum volume of 55.3 cm 3 /g of dry matter at a moisture content of 14.03%.”

Metzger, Hsu, Ziegler & Bern, Cereal Chemistry 66 (1989) 247, p. 248

(Those are wet-basis percentages; Wu and Schwartzberg's table is on a dry basis, so 14.8 there is about 12.9 wet.) A hull that leaks lets the steam out before the pressure can build, and a kernel that has dried out has too little to build it with: these are the “old maids” at the bottom of the bowl. In microwave popping, one 2005 study found the number of unpopped kernels “highly correlated (r = 0.826, p = 0.011) with pericarp enthalpy values”, the heat given off by a change in the hull's cellulose that the authors link to crystallisation, and so to how well the hull holds moisture. That is a correlation across a handful of hybrids, not a demonstration of cause.

The pop is the steam, and the jump is a leg

Virot and Ponomarenko filmed kernels on a hot plate at 2,900 frames a second with a microphone beside them. The sound comes before the kernel leaves the plate, so it is not the landing, and most cracks in the hull make no sound, so it is not the fracture. Their conclusion is put as a proposal: “we propose that the familiar ‘pop’ sound of the popcorn is caused by the release of water vapour.” The noise lasts “approximately 50 ms, without a clear dominant frequency”.

The jump is not jet propulsion either (“no rocket effect”). The first starch to burst out forms a “leg” that is squashed against the plate and springs back, and the popcorn “jumps a few millimetres high to several centimetres high”, with a jump energy they estimate at about 20 microjoules. Popped, a kernel is about “two times larger and eight times less dense”.

What this page does not show

We did not read Hoseney's 1983 paper; what it says comes here through Wu and Schwartzberg's summary, and a search-engine snippet of its abstract that we could not check at the source. We did not read the full text of Byrd and Perona, only its abstract. We searched for a direct measurement of pressure inside a popping kernel and did not find one; that is a statement about what we could find, not proof that none exists, and if you know of one, the door is open. The steam table is for pure water in equilibrium with its own vapour; a kernel is starch, protein and water heated in seconds, so the curve gives an estimate, not the pressure, which is Wu and Schwartzberg's point. Nothing here was popped: every figure is either quoted from a source or computed from the steam equation.

Sources

1. P. J. Wu and H. G. Schwartzberg, “Popping behavior and zein coating of popcorn”, Cereal Chemistry 69(5) (1992) 567 to 573, PDF (marked public domain). Introduction and Table I.
2. R. C. Hoseney, K. Zeleznak and A. Abdelrahman, “Mechanism of popcorn popping”, Journal of Cereal Science 1 (1983) 43 to 52, doi:10.1016/S0733-5210(83)80007-1 (not read; cited through Wu & Schwartzberg).
3. E. Virot and A. Ponomarenko, “Popcorn: critical temperature, jump and sound”, J. R. Soc. Interface 12 (2015) 20141247, doi:10.1098/rsif.2014.1247 (PMC4345489).
4. J. E. Byrd and M. J. Perona, “Kinetics of popping of popcorn”, Cereal Chemistry 82 (2005) 53 to 59, doi:10.1094/CC-82-0053 (abstract only was read).
5. D. D. Metzger, K. H. Hsu, K. E. Ziegler and C. J. Bern, “Effect of moisture content on popcorn popping volume for oil and hot-air popping”, Cereal Chemistry 66 (1989) 247 to 248, PDF.
6. A. S. Tandjung, S. Janaswamy, R. Chandrasekaran, A. Aboubacar and B. R. Hamaker, “Role of the pericarp cellulose matrix as a moisture barrier in microwaveable popcorn”, Biomacromolecules 6 (2005) 1654 to 1660, doi:10.1021/bm049220l (abstract only was read).
7. The Popcorn Board, “What makes popcorn pop?”, read 5 October 2026; Wikipedia, “Popcorn”, read 5 October 2026; L. Sibley, “Popcorn” (American Chemical Society), as quoted in a 2003 classroom handout, PDF.
8. Steam: IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (2007), equation 30, coefficients from its table 34 and check values from its table 35; E. W. Lemmon et al., “Thermophysical properties of fluid systems”, NIST Chemistry WebBook, saturation table for water, 170 to 190 °C.