The Science Oven

For five months straight, I lived in a hotel without a kitchen. Everything I ate was cooked in a microwave. At first, I craved the meals that required a convection oven. But by the end, I found I could make almost anything in a microwave. My favorite concoction was Nutella mug cake — a sinful, gooey, moist chocolate cake cooked in my favorite coffee mug in just under two minutes.

But how does a microwave actually work? And what makes it so different from a convection oven? The microwave was invented, accidentally, by American engineer Dr. Percy LeBaron Spencer. When testing a magnetron, a vacuum tube that generates microwaves, he realized the candy bar in his pocket had melted. After seeing what happens when other foods (popcorn and eggs) were placed in front of the magnetron, he quickly realized that microwaves could cook foods rapidly.

A microwave oven has three main parts: a magnetron (located on the side of the microwave) that makes microwaves, a waveguide (located within the wall of the microwave) that directs the microwaves from the magnetron to the food, and a compartment (where you place your food or beverage) to enclose both the food and the microwave radiation. Microwaves — just like radio waves, X-rays and visible light waves — are a kind of electromagnetic wave. But they have a unique property that makes them particularly useful for cooking food.

All electromagnetic waves operate at different wavelengths. Microwaves are wedged right in between radio waves and infrared light. At this frequency, something interesting happens. Water molecules, which are made up of one oxygen atom and two hydrogen atoms, are, by nature, polar — the oxygen atom steals more of the negative electrons than the hydrogen atoms do, making the oxygen slightly more negative and the two hydrogen atoms slightly more positive. Food is full of these slightly polar water molecules. When left alone, they stay still.

As a microwave travels, its electrical and magnetic fields change rapidly. When the wave travels near a water molecule, the molecule will try to align with the electrical field. Since the field is constantly changing, the water molecule rocks back and forth, generating energy in the form of heat. When the waveguide directs the microwaves, generated from the magnetron, into the compartment, all the water molecules are excited at once. This produces a lot of heat and cooks the food.

Frequency is the key to this effect. If you pass radio waves or infrared light through food, the water molecules will remain still as if nothing happened. But microwaves are at just the right frequency to excite the water molecules and generate heat — something other forms of electromagnetic waves can’t do.

In a traditional oven or stove, heat moves slowly from the outside in. But because microwaves travel straight through the food, exciting all the water molecules at once, a microwave oven cooks food evenly throughout. When heating food in a plastic or glass container, the microwaves pass straight through those containers with no interaction, making them microwave safe. But when heating food in a metal container, the metal will reflect the microwaves, causing sparks and sometimes a fire. So, experiment with your microwave. Go crazy. Make Nutella mug cake or your own version. But listen to Irving in American Hustle and never put metal in your science oven.