Steam explosions - the science of popcorn

31 May 2009

Ingredients

popping corn Popcorn A small saucepan Saucepan, preferably with a transparent lid
Pliers Pliers Vegetable Oil Oil

Instructions

Take a small handful of popcorn and crush it gently with your pliers; not into little pieces but just crack it.

Heat up some oil in the pan in the same way as you would to make normal popcorn.

Try putting in the damaged popcorn. Does it work?

Now try some undamaged popcorn. Is there a difference?

Warning: making popcorn involves things exploding in very hot oil. It is therefore really quite dangerous so be careful and keep the lid on until the pan has cooled down.

Result

The damaged popcorn is not very spectacular. Normally it just extrudes foam and sometimes makes very weak pops.

On the other hand, undamaged popcorn explodes incredibly violently. The video is first at normal speed, then 20x slower and finally 80x slower than real life.

Explanation

Popping corn is just a specially bred from of maize; a bit like a less sweet form of sweetcorn. Inside a seed there is lots of starch which the seed uses to survive until it has managed to grow enough leaves to harvest sunshine. This is protected by a very tough outer shell.

Popcorn Kernel Cross Section Popcorn structure
A popcorn kernel is mostly made up of starch and surrounded by a strong hull or shell

Popcorn goes pop because as you heat it up the moisture inside also gets hot and would like to turn into steam. However the hard outer coat of the kernel can support up to 10 atmospheres of pressure, stopping the water expanding into steam. The water keeps heating up above 100°C, building up more and more pressure. As it heats up the moisture in the kernel alters the starch stored inside forming a kind of jelly - similar chemistry to when you
thicken soups with cornflour (cornflour is ground maize).

Heating popcorn Popcorn rupturing
As the kernel heats up the starch molecules unwind in the water trapped inside to form a jelly. The water inside the kernel heats up and tries to boil, applying huge pressures to the shell. Eventually this ruptures and the water rapidly boils, forming a foam.

Eventually the temperature can increase to 180°C. This increases the pressure inside the shell enormously, causing the shell to split and release the pressure. The drop in pressure causes the water in amongst the jelly-like starch to boil immediately, expanding by a factor of 1-2000 or more. This blows the jelly into the foam which we call popcorn.

As all the water in the starchy jelly has now boiled off the starch dries out, becomes hard and can support itself even when the popcorn cools down and the water condenses again.

Popcorn foam
The foam expands violently forming what we know as popcorn. And as all the water is now a gas, the starch dries out and goes hard so the foam doesn't collapse as it cools.

If you break the popcorn shell the pressure can't build up so either the water just escapes slowly and it doesn't even form a foam (or it slowly extrudes out of a hole). Sometimes you have only weakened the shell and it will go pop but at a much lower pressure and therefore less violently than with an undamaged kernel.

Sometimes you find very small popcorn at the bottom of the bucket. These are probably the ones that were weakened on their way to you so they couldn't pop properly. Because these are smaller but came from the same sized piece of corn they are denser so sink to the bottom of the bowl.

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