Why popcorn explodes when heated

Popcorn is a favorite snack for people of all ages, especially during movie nights. But have you ever wondered why only certain corn kernels pop while regular corn does not?

Food experts say the answer lies in the kernel’s unique structure moisture and the way it reacts to heat.

Not all corn can turn into popcorn. A special type of corn called Zea mays everta is used because its kernels have a hard outer shell and a soft starchy center with a small amount of water inside.

Although popcorn kernels look dry they contain about 13–15% moisture. When heated, the water inside turns into steam.

Since the outer shell is very strong the steam cannot escape causing pressure to build inside the kernel.

As the pressure keeps increasing the kernel acts like a tiny pressure cooker. Eventually the shell bursts with a popping sound.  The hot steam quickly escapes and the soft starch inside expands. Within seconds it cools and forms the fluffy crispy popcorn we enjoy.  A popped kernel can become 30 to 40 times larger than its original size.

Sometimes a few kernels do not pop. Experts say this usually happens because they do not contain enough moisture to create the pressure needed to burst.

Tiny cracks in the outer shell can also let steam escape too early preventing the kernel from popping.  Heating the kernels at a temperature that is too low may also cause them to dry out before enough pressure builds up.

Popcorn can be made in several ways including on the stove in an air popper or in a microwave.

Microwave popcorn bags and the oil inside them help spread heat evenly while oil on the stove helps heat the kernels more consistently.

Air poppers on the other hand use only hot air and do not require oil. According to food scientists the secret behind perfect popcorn is the combination of a strong outer shell the right amount of moisture steam pressure and the natural expansion of starch when heated.