A popcorn kernel contains moist starch trapped inside a hard, dry hull. You can scorch this hull by cooking the corn in hot oil but not when you cook it in a microwave oven. How can the microwave oven pop the corn without risk of overheating the hull?

Microwave ovens are a common kitchen appliance used to heat foods quickly. They work by emitting microwaves, a type of electromagnetic radiation, that interact with food molecules. When you place a popcorn kernel in a microwave, the microwaves specifically target the water molecules inside the kernel. These water molecules start to vibrate rapidly, creating friction and heat.

The microwaves are absorbed by the water molecules, causing them to become excited and generate heat internally. This process is highly efficient for heating up foods that have water content because microwaves penetrate deeply and heat evenly. The benefit of using a microwave oven for popcorn is that the hull of the kernel does not directly absorb microwave energy. This means the heat is generated inside the kernel, rather than on its surface, reducing the risk of burning the hull.

Thermal conduction is the process by which heat is transferred through a material. When making popcorn on a stovetop with hot oil, thermal conduction primarily occurs. The hot oil heats the outside of the popcorn kernels first.

As heat is conducted through the hull to the moist starch inside, the hull can get very hot and even scorch. The heat travels from the higher temperature oil to the lower temperature kernel. Since microwaves heat from inside out, thermal conduction within the kernel is less of a concern. This internal heating helps prevent the hull from scorching, making microwave popping a gentler method for the kernel's outer shell.

When you microwave popcorn, the water molecules inside the kernel turn to steam because of the heat generated by microwave energy. As steam forms, it creates pressure inside the kernel. This pressure builds up as more and more water converts to steam.

The kernel's hard, dry hull traps this steam and pressure until it reaches a critical point. When the pressure becomes too great, it forces the kernel to pop, turning it inside out and creating the fluffy popcorn we enjoy. Because the hull doesn’t absorb the microwave energy, it doesn’t burn. The efficient conversion of water to steam is crucial to the popping process, ensuring the kernel pops without causing damage to the outer shell.