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If microwave ovens heat food from the inside out, how come I always have cold spots in my spaghetti and meatball leftovers? And why is the cheese on top of my burger boiling while the center of my all-beef patty is just lukewarm? Well, microwave ovens are pretty darn convenient, but they don’t perform sorcery. And they certainly don’t cook food from the inside out.
Microwaves and Magnetrons
A microwave oven is a pretty simple device when you get right down to it. It contains a high-voltage power source (driven by your wall outlet) and a magnetron, all packaged in a steel box acting like a Faraday cage to contain and reflect all those microwaves bouncing around and heating your food.
What’s a magnetron? It’s that thing Science Patrol member Shin Hayata used to hold up in the air to turn himself into Ultraman. Nah, just kidding, that thing was the Beta Capsule. According to the dictionary, a magnetron is an electron tube for amplifying or generating microwaves. Well, that’s not very helpful, so let’s try a different tack.
There’s some complicated physics involved, but the components are fairly straightforward. A heated cathode beams out electrons toward an anode. But a powerful magnet redirects the path of all those electrons whizzing around. The anode contains cavities, which resonate when electrons fly by, but instead of creating audible sound waves like an oboe, they generate microwaves. A couple more physics tricks focus and aim outbound microwaves right at your frozen burrito.
The microwaves reheating your Mac and cheese are in the same general part of the radio frequency spectrum as many home electronics and networking devices. Remember all those 2.4 gigahertz marketing claims on slightly older wireless routers? Maybe that’s why watching TikTok videos can get a little wonky when you’re standing too close to the microwave oven while zapping your Hot Pockets.
How does your food get hot?
A standard oven definitely heats your food from the outside in by using convection. A microwave also heats your food from the outside in but in a different manner.
With a traditional oven, heating elements or gas flames generate heat, which warms the air inside the oven. The air surrounds your food, so heat is transferred to the surface of your dinner, eventually making its way to the middle of your pot roast.
You might have noticed the air in your microwave doesn’t get very hot or even warm. That’s because all those microwaves bouncing around inside aren’t hot, at least in the traditional sense. What does happen is that the microwaves beam at your food, attacking it from the outside in—they don’t magically find their way to the center of your frozen waffles, only to begin the heating work there. The microwaves beaming outside-in do get absorbed by certain substances like water, fats, liquids stuff in general and a few other substance types. As they get absorbed, water molecules (and similar, but we’ll stick with water as it makes the best example) go berserk as they are bipolar in nature with positive and negative charges. They spin, dance and get excited, and in that process, emit a by-product of … heat.
So, in a sense, microwave cooking does kinda, sorta go directly to work on your food after passing right through most containers made of paper, cardboard, Tupperware and the like, but they certainly do not find their way into the center of your food first. Starting from outside in, food laden with excitable water and other molecules generate heat in response to incoming microwaves. The more time your food is exposed to the bombardment, the deeper into the chicken pot pie the microwaves reach to continue the process and the more opportunity the already hot parts of the food have to transmit heat around via convection.
Why do microwave ovens suck at crispy and other tidbits
When a conventional oven creates hot air to surround your upcoming meal, it evaporates water and applies direct heat to exterior layers. Like most anything organic exposed to heat, that creates a dry, crusty exterior. Mmmm. Brown food tastes good!
With no ambient heat, a microwave oven doesn’t do much to create a scrumptious, flaky top to your pot pie. But food scientists have come up with somewhat of a lame solution. You might have encountered junk food designed and packaged for microwave cooking. Sometimes, especially for items with a crust, you’ll get a cardboard package with a slightly metallic reflective surface. That’s designed to heat up during the microwave process, hopefully generating a bit of evaporation and resulting crispy crust. Yeah, the food usually comes out soggy anyway. Conventional ovens have their place.
There’s universal panic over putting metal in the microwave. It’s not a particularly good idea, but not nearly as apocalyptic as the rumors suggest. That’s because metal doesn’t contain many excitable water molecules and reflects microwaves instead of absorbing them like your food. So, yes, some metal can create electrical arcs and damage your oven, but it’s not likely to blow up the neighborhood. After all, the inside of your microwave is made of … metal.
One more thing. Don’t worry about wearing lead overcoats while reheating leftovers with your microwave. The magnetron-generated radiation is non-ionizing, so it’s nothing like the nasty stuff detected by Geiger counters.