Kernels blow up bigger when cooked at low pressure.
Next time you go to the movies, look out. If the popcorn vendors have read this article, your cup of popcorn might contain fewer pieces than it used to. That's because the pieces could each be up to twice the volume they were previously.
On the other hand, that's good news if you make your own popcorn at home, because you'll be able to get two cups' worth from the number of kernels that previously gave you just one. What's more, there will be fewer of those annoying, crunchy unpopped kernels.
How is it done? The trick is simple: just pop the kernels at a lower pressure. Paul Quinn of Kutztown University in Pennsylvania and Joseph Both of the Stanford School of Medicine in California achieved this by fitting a simple vacuum pump to the pressure cooker in which they popped their corn1.
Quinn started wondering how to get more pop from his corn in 1999, when he was a graduate student under Daniel Hong of Lehigh University in Pennsylvania, who died two years ago. As Hong and Quinn sat in a session on the physics of food at a meeting of the American Physical Society, they realized that the characteristics of popcorn hinge on the behaviour of hot gases.
When the kernels are heated, the moisture they contain turns their starchy interior into a jelly-like substance. The moisture eventually turns into water vapour, which is trapped by the hard shell of the kernel, called the pericarp.
The hotter it gets, the more the internal pressure rises, and eventually it ruptures the pericarp. Then the water vapour expands rapidly, blowing out the starch jelly into a foam. This material continues to expand until the pressure of the hot vapour is the same as the pressure of the surrounding air.
So to make the expansion go on longer, Hong and Both, another of his graduate students, predicted that you just need to lower the air pressure. But this prediction was based on a very simplified picture of what corn kernels are like. When Quinn and Both decided to put the theory to the test, "I didn't think it was going to work," admits Quinn.
But it did. They found that their makeshift popping apparatus gave nearly double the volume of popcorn for the same number of kernels.
And because the pressure difference between the inside and outside of the pericarp was greater, it was more likely to rupture - so fewer kernels failed to pop. There were six times fewer unpopped kernels than when they made popcorn without the vacuum pump. This could help the popcorn industry to make considerable savings by reducing waste, as well as giving movie-goers a better product - even though there's less of it per cup.
QuinnP. V., HongD. C. & BothJ. A, Increasing the size of a piece of popcorn, Preprint xxx.arxiv.org/cond-mat/0409424 (2004).
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Ball, P. Popcorn gets poppier. Nature (2004). https://doi.org/10.1038/news040927-6