The Biggest Reason Some Popcorn Doesn't Pop
No matter how hard you try to perfectly time your popcorn in the microwave, some kernels always seem to be left rolling around in the bottom of the bag. Those who use a stovetop popcorn popper or even pop their own popcorn in a cast iron skillet are likely to also encounter the same problem. But the reason popcorn sometimes doesn't pop has less to do with the way the corn kernels are treated and everything to do with the kernels themselves.
According to popcorn company America's Favorite Popcorn, the kernels are forced to pop open into popcorn when the amount of pressure caused by steam inside of the kernel is too much to contain. But for that steam to build up and force the corn kernels to pop, there must be enough moisture inside them to begin with. But how to tell if your kernels are duds (besides spending the time trying, and failing, to pop them)?
This is the exact amount of moisture the kernels need to pop
The Art of Doing Stuff explains that popcorn kernels that are easily dented with your fingernail are too wet, while those that are so hard you can't even leave a mark are too dry. So clearly it requires a very specific amount of moisture to successfully pop.
According to America's Favorite Popcorn, corn kernels need between 14% and 15% moisture to pop open into popcorn. If the moisture content is too far above or below that threshold, it just won't work — and there is very little margin for error. In a test The Art of Doing Stuff completed as an example, only 78% of kernels with 10% moisture popped, while almost none at 20% moisture did. The perfect moisture content is absolutely vital to create the right amount of pressure for popcorn to pop.
One other important takeaway from The Art of Doing Stuff's test? Even with the correct moisture level older kernels will result in "smaller, more dense" popcorn, while newer kernels will be picture perfect and fluffy. So the next time you're shopping for popcorn kernels, be sure to check the expiration date since the fresher the kernels the better your popcorn will pop. And if you plan to stick to the bagged stuff, you might need to go ahead and accept the fact that you'll likely end up with un-popped kernels at the bottom of your bag.