When microwaving a bag of popcorn, why doesn’t the first kernel that is popped burn by the time the last kernel is popped?
In: Chemistry
A lot of the answers miss the fact that only a small patch in the middle of the bag is catching the majority of microwave energy, converting it to heat. The kernels don’t see much of it, despite moisture content.
As the kernels pop, they become less dense and float towards the top of the pile, while the unpopped ones generally fall to the bottom (where the heat patch and oil are). If left too long, it is the popped kernels nearest this patch that may burn first, along with some of the outliers at the corners of the bag.
Good question.
Microwaves heat up different things differently depending on how they absorb them. That’s why some containers are microwave safe and some will melt if you put them in.
Water absorbs microwaves really well, that’s what heats up when you microwave something mainly…that’s why it has a funky texture when it cools down. All the water is boiled out of it.
Kernels pop because they have moisture inside them and when it heats up it builds up pressure and that is what makes them pop. Once they pop they heat slower because the water is gone though. You can still burn them if you go too long though.
The process of popping a kernel absorbs more energy than it releases. When there are still lots of un-popped kernels, so much heat energy is absorbed by all these kernels that it prevents the temperature of the oils and steam in the bag from rising to the point where an already popped kernel would burn. Once most of the kernels have popped, the heat energy has nowhere else to go and the temperature goes up, eventually burning the popped ones.
This is why it’s almost impossible to get all the kernels to pop without burning some of them.
Kernels burn because they get too hot.
They get too hot because they get so much heat.
They get so much heat from water vapour.
Water vapour comes from exploded kernels and inside.
Exploded kernels help accelerate the rise in temperature outside as their water vapour is dissipated amongst all (neighbouring) kernels.
There is very little water vapour outside the kernels initially. All of it is inside.
Only the water inside the kernels can heat up due to the microwave.
Liquid water can only get to 100 deg C. Water vapour is not limited in temperature.
When there’s a lot more water vapour around as most kernels have opened, the outside temp can get very high and either directly burn opened kernels or the little butter/oil that was added for taste also starts heating up and burns them from below.
That’s why, Eli.
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