18
$\begingroup$

A commonplace empirical observation is that when a microwave oven stops, unpopped kernels are very hot (it's physically painful to touch them) and popped kernels are not.

Is there an elementary (or not) exposition of the physics involved?

$\endgroup$
3
  • 3
    $\begingroup$ I have to say that I've never noticed this effect, though I don't eat that much popcorn. Are the unpopped kernels actually hotter, or do they just feel hotter to the touch? There might be issues with thermal conductivity of the popped vs. unpopped kernels in play. $\endgroup$ Jan 8 at 18:43
  • 2
    $\begingroup$ @MichaelSeifert : It is physically painful to touch an unpopped kernel just after the microwave stops. $\endgroup$ Jan 8 at 18:58
  • $\begingroup$ See latent heat... $\endgroup$
    – Jon Custer
    Jan 9 at 16:37
41
$\begingroup$

Popcorn pops in a microwave oven due to the microwaves interacting with the moisture in the popcorn kernel raising its internal temperature and pressure. Once the pressure increases enough the kernel pops and the moisture escapes and cools. The moisture in the un popped kernel remains hot.

Hope this helps.

$\endgroup$
8
  • 1
    $\begingroup$ Also, unlike other methods of popping, the microwave heats only the moisture. Once a kernel pops and most of the moisture escapes, the microwave might not be very effective at heating popped kernels, unlike for instance air poppers or pans on the stove, which heat everything. (Note: I haven't actually tested this, since I don't pop my popcorn in a microwave. Should be easy to do an experimental test, though: just put some popped kernels in a microwave and see how well they reheat.) $\endgroup$
    – jamesqf
    Jan 9 at 4:47
  • $\begingroup$ Hmm, correct me if I'm wrong, but didn't packets of microwave popcorn also include a sheet of tinfoil (or similar) inside it? I always thought that it heated that sheet instead of the kernels themselves. But I could be wrong since I don't own a microwave and have only seen the process a few times in my life. $\endgroup$
    – Vilx-
    Jan 9 at 11:08
  • $\begingroup$ @jamesqf Yes, the microwave might not be very effective at heating popped kernels is their moisture content is too low. That is one of the reasons for the use of a special sealed bag with a metalized film that absorbs microwaves and reradiates the energy as infrared thermal energy. See my comment below to Vilx $\endgroup$
    – Bob D
    Jan 9 at 14:22
  • 2
    $\begingroup$ @jamesqf Although the microwaves will continue to interact with the dipolar molecules of the steam giving them rotational kinetic energy, those molecules are so much farther apart from one another than in the liquid state, the rotational kinetic energy will not be as easily randomized into translational kinetic energy by collisions as it is in the liquid state. This makes the temperature of the steam pretty much capped at the saturation temperature (100C). The rotational KE is low. It's when its randomized into translational KE that raises the temperature. Hope that makes sense to you. $\endgroup$
    – Bob D
    Jan 9 at 18:57
  • 2
    $\begingroup$ It's the overall electric polarity of the water, rather than one of the bonds specifically (those are usually in the IR spectrum, rather than microwave). The frequency is the same regardless of physical state, but as mentioned, it's turning "spin fast" into "move fast" that means steam will not heat as much compared to liquid. $\endgroup$
    – Nij
    Jan 9 at 21:30
31
$\begingroup$

How hot (or cold) something feels is not just down to temperature, but to the rate of heat transfer, or thermal conductivity. Popped corn is a good insulator, having a foam-like structure. As a result, little heat escapes to the hand. Unpopped corn is a much better conductor of heat, and transfers heat to the hand much more quickly, so that it feels much hotter.

Similarly, at normal temperatures, a metal may feel cold, because it conducts heat away from the hand. Expanded polystyrene or wool at the same temperature feels warm because it insulates against heat loss.

$\endgroup$
2
  • $\begingroup$ I have qualms about this answer because eating a large amount of the popcorn within seconds after it's taken out of the microwave doesn't seem to cause problems. $\endgroup$ Jan 9 at 19:54
  • $\begingroup$ This agrees with my experience making popcorn on the stovetop, in oil. The popped corn never feels all that hot after, but unpopped kernels can retain unpleasantly high levels of heat for a while. I'd expect the release of the steam from the kernel assists, but thermal conductivity seems important as well (same way picking up a penny that's been baking in the sun on the sidewalk can hurt briefly, even though touching the sidewalk it's on is only warm). $\endgroup$ Jan 10 at 2:53
19
$\begingroup$

Winging it:

Microwaves heat the corn kernel which behaves like a tiny pressure cooker. Due to increased pressure, water inside will remain liquid at higher temperatures than at atmospheric pressure. So, temperature and pressure rise.

If it doesn't pop, then... well, it'll stay hot for a while.

If it pops, the pressure vessel is broken, so pressure inside the kernel drops to atmospheric. With dropping pressure, boiling point of water drops down to 100°C, thus the water is much hotter than its boiling point (ie, superheated) so it flashes to steam. This creates a BLEVE (Boiling Liquid Expanding Vapor Explosion) and makes lots of steam bubble inside the kernel, which expand and turn it into foam, ie popcorn. It is no different from a boiler explosion.

The amount of steam generated depends on temperature. All the water may not boil, some may remain liquid. In order to turn into steam, water needs an amount of energy known as Enthalpy of vaporization that is taken from the thermal energy. This means temperature drops until water stops boiling. When water boils, the steam takes away energy, not just its own latent heat but also its enthalpy of vaporization. It will return this energy when condensing on an object (since we're in the kitchen, think steam cooking broccoli).

Anyway. When they pop, kernels release steam that takes away the latent heat of vaporization, but it doesn't stop there. The popped kernel is still hot and has lost its shell, so any water that hasn't turned to steam will tend to evaporate, further cooling it. On the other hand, unpopped kernels didn't release any steam, and water can't evaporate through the hard shell, so all the heat is still trapped inside.

If you measure the temperature with your finger instead of an infrared thermometer, then what you're measuring is how much the object is heating your finger. If the object has low thermal mass and low thermal conductivity compared to your finger (like foam or a popped corn) then.. if it is hot your finger will cool the surface and take away very little heat from it, which you won't feel as hot. The core may remain hot, but you won't feel it.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.