Why Microwaves Always Mess Up Your Food

Ah, what a beautiful smell! A wonderful blend of spices is tickling the inside of your nose. You have just finished your meal preparation for the week and are proud of your little creations: Moroccan-spiced chicken with roasted broccoli, a fragrant Thai green curry with lemongrass, and a hearty Mediterranean quinoa bowl.

But somehow, a thought in the back of your head is taking away from your enjoyment. You have the hunch that the next time you take a bite out of this food, it won’t be quite the same — because you will need to heat it up using a microwave oven. Results may vary depending on the make and model of your microwave, but we have all experienced something similar: You are starving and desperately want to bring your food up to temperature, so you entrust it to the magical heating device. 2 minutes later you hear a satisfying “bling”, grab your dish and eagerly take a bite out of it.

Damn it: The bite feels as if you had just licked a metal pole in Siberian winter. Wondering if you forgot to turn on the microwave in your rush, you take another bite from a different section of the plate — just to discover that your tongue suddenly seems to reside in the core of the Sun.

So heat did go into the food after all, but somehow not the right place? I’ve never had such an experience while cooking food on a stove. Or at least I don’t remember the last time I found an ice cube in my otherwise sizzling stir-fry. If you think about it, the principle behind cooking hasn’t changed a lot over the past millennia: We put our food on some hot surface, the heat coming either from literal fire, as in a gas grill, or more recently, an electrical stovetop. Then, when your pan gets hotter, your food gets hotter — until it’s cooked, simple as that.

We love and trust this traditional way of cooking: The best pizza place in town is the one where they stick the pizza next to a bunch of burning logs and char it up a little — it doesn’t get more low-tech than that. But we have to admit that it’s nice to also have a tool for warming food up quickly when we are in a rush (and don’t find ourselves in the possession of an already fired-up wood oven).

But the microwave oven does not seem to work at all like any of the cooking methods I just described — there is no plate or other part of the device that gets extremely hot and heats up the food by being in contact with it. But then, how do we get the heat into our food (or at least into parts of it)?le who are not expecting AI in their book relying on these fake locations for their holiday plans.

Cooking with Light

That’s because microwaves essentially cook your food with light instead of heat. Yes, that’s right — light! But I’m not encouraging you to shine a flashlight at your food the next time you need to defrost something; that unfortunately won’t work. The magic lies in the precise type of light we use.

Light is an electromagnetic wave, which means it consists of a rapidly varying electric field, which creates a magnetic field, which then in turn creates an electric field again — allowing the wave to move through space. The most important property of any wave is its wavelength — for ocean waves for example, it refers to the distance between two consecutive wave crests. While the wavelength of all the light you are able to see is comparable to a hundredth of the width of a human hair, microwaves have a much larger wavelength, typically 12 cm or 5 inches. Our eyes are not adapted to perceive this kind of light, so it unfortunately remains invisible to us. But it’s funny to think that if you were somehow able to see microwaves wiggling around, you could measure their size using a standard household ruler.

The wavelength is important to us because the smaller it is, the more energy the light has. Think of it this way: If the wavelength is smaller, you can fit a lot more up-and-down movements in a given amount of space, which then means more energy is stored there.

But wait… doesn’t that mean that microwaves have a lot less energy than visible light? After all, they have a much larger wavelength. And if I can’t cook my food by shining a flashlight at it, surely I can’t cook it with light that is even less energetic (in fact about 250,000 times less energetic). No wonder my microwave food is causing frostbites — how are microwaves supposed to heat up anything at all?

Putting the Magic in the Microwave

To understand how microwave cooking works, we need to understand what this light is doing with your food. Many molecules in your food are what is called dipoles (greek for “two poles”). They have a slightly positive charge on one side and a slightly negative charge on the other. Take a look at water for example, which you can find in abundance in almost any type of food:

But this behavior is not limited to water — there are many types of fat, sugar, and proteins that are also dipoles. It turns out we need precisely these kinds of molecules for microwave ovens to work: When microwave light gets close to the dipole, the dipole feels its electric field. The dipole then tries to align itself with it, meaning its positive side points to where the electric field is negative and the negative side points to where the electric field is positive.

It’s very similar to how magnets behave: Imagine I put a magnetic compass on a desk. If I now come in with a strong bar magnet, the compass needle is going to align itself in the direction of the bar magnet. More precisely, the needle (which is actually a magnetic south pole) will point to the magnetic north pole of my magnet.

Okay, but just because the dipole is now aligned with the electric field, how is that supposed to cook the food? I don’t know about you, but neither can I tell what way the molecules are aligned when I eat my food, nor do I care. The key lies in how the microwaves behave inside the device: First, they are sent out from the right side. After moving through the interior, they hit the left metal wall, where they are reflected and start moving back towards the right wall. But at the same time, the oven also sends out new waves from the right.

This means there are now waves coming in from the left and from the right, all having the exact same wavelength. These now overlap and interact with each other, creating what is called a standing wave:

We call it a standing wave because it doesn’t appear to move through space. Instead, the yellow points stay completely still while the rest of the wave just bounces up and down in place, like a jump rope that is held at both ends. It is this rapid up-and-down motion that is key to getting the heat into the food: Every time the direction changes, all those dipoles in the food realign themselves. Mind you, this happens about 2.5 billion times per second, which is a lot faster than I personally am able to flip around a compass with a magnet.

When microwaves force the molecules to spin and flip rapidly in this way, they bump into neighboring molecules; these collisions then make the surrounding molecules move faster. Since temperature is defined as the average speed of the molecules in a material, we can heat up food simply by making molecules dance around more energetically. If you take a look at the animation again, you can see the first thing that can go wrong with heating up food: The yellow spots don’t receive any energy and will therefore stay cold. You can actually confirm this yourself at home, as engineer Bill Hammack demonstrated in a YouTube video. Just grab a tray of grated cheese and place it in your microwave oven. If you then make sure the tray is not rotating and microwave it for a few seconds, you will see that there are parts of the cheese that have melted and other places that haven’t changed at all.

If you measure the distance between two melted cheese spots (or two intact spots), you find it’s about 6 cm or 2.5 inches, which is half of the wavelength of the radiation. The way one usually tries to solve this problem is by continuously rotating the food, such that most parts of it hopefully experience enough changing electric fields to heat up.

Why Your Flashlight Can’t Cook Dinner

But the question remains: Why won’t a flashlight cook food, but a microwave will? The most important point to consider is the amount of light we are using. Yes, it’s true that each individual wave packet of microwave light has less energy than a wave packet of visible light. But we can more than compensate for that by just sending more wave packets at our food in general. While a typical flashlight operates at about 3 watts of power, a microwave uses up to 1200 watts of power. If you were to shine 1200 watts of normal, visible light at your stir-fry, you can be very sure it would heat up by a similar amount. The important difference is however that most of the heat would land directly in the outer layer of the food, charring your chicken or tofu to a crisp very quickly. This is similar to how you get a sunburn on your skin and not on the inside of the organs because most of the light is already absorbed after a few millimeters (at least I don’t know of anyone who has sunbathed for long enough to sunburn their liver). Microwave light on the other hand penetrates into food more deeply (up to a few centimeters or about an inch), which makes it a lot more useful for creating even heating inside the food and is the reason we use this wavelength in the first place. But even microwaves won’t reach layers deeper than that, which means anything sufficiently far below the surface will not be heated directly. Instead, this portion of the food will be heated indirectly by other parts that were struck by the microwaves and now share their warmth. All of this means it makes sense to spread out your food as much as possible such that every part of it can be reached well in the first place.

It is also useful to know that the larger the difference in temperature between two objects, the faster the heat flows between them — that’s also the reason why your freshly brewed coffee cools from 90°C (190°F) to 60°C (140°F) really quickly, but takes relatively long to cool from 60° (140°F) to 30°C (85°F). In the beginning, the difference between air temperature and coffee temperature is simply much higher.

This in turn means that if there are very large differences in heating of your food, simply letting it sit for one or two minutes will already even out the worst microwaving mishaps. You get bonus points if you stop the microwaving process somewhere in the middle and stir up your food, since that also helps distribute the heat more evenly. Unfortunately, there’s yet a third reason why the heating is not equal: Since water is such a nice dipole and therefore heats up quickly in the microwave, food will also cook with varying speeds depending on its water content. Vegetables, soups, and beverages and will heat up quickly, while dry foods like crackers or fat-rich foods like butter will take more time. So if it is possible, microwave foods together that are similar in their makeup: Don’t put dry bread right next to a soup, since one of them is going to take longer than the other one.

Really the trickiest thing to heat up is deeply frozen food. This is because in ice, the water molecules find themselves in a crystal lattice, which heavily restricts their movement. If you put two ice cubes in a microwave and place the same weight of liquid water right next to them, the liquid water actually starts to boil before the ice cubes have melted. And that is even though the liquid water needs to increase its temperature by 80°C or 180°F while the ice cubes would only need a few degrees. So once a frozen part of your food is finally liquid, it very happily absorbs microwave radiation and heats up quickly — all while the still frozen parts remain stuck.

Can Microwave Radiation Kill You?

Now that we know some home remedies for messed up food, we come to the “don’t try this at home” portion of our program. You may for example have heard that you shouldn’t put metal in your microwave. This is true —but only in part. If you think about it, the microwave is literally made out of metal, because that’s what reflects the microwaves and allows it to form the standing wave we discussed earlier.

The problem occurs when you accidentally build an “antenna” for microwave radiation, like when you put a fork (or any other spiky metal object) into the device. Different parts of the fork then experience very strong differences in the electric field, which are then evened out by electrons jumping from one part of the fork to the next in an arc reminiscent of a lightning strike. While certainly beautiful, this is unfortunately also a great way to create a fire in your microwave. Talking about cooking food unevenly, it probably doesn’t get worse than literally lighting it on fire… (though once everything is charred to dust, it’s technically evenly heated).

But can microwaves harm you directly as well? Actually yes: Since microwaves are great at heating anything up that has water (or other dipole molecules) in it, and you are mostly made up of water, it can potentially also heat you up to temperatures that are — to put it mildly — incompatible with life, provided that you are crazy enough to stick your head into it. But fortunately, microwaves are equipped with mechanisms that make sure they can’t be turned on while the door is open, which is probably why I haven’t yet heard about anyone getting microwave-burned. Though that certainly sounds like an exciting alternative to the boring old sunburn at the beach.

Is the Universe a Microwave?

I’d like to leave you with the thought that microwave light is found in other places than your kitchen at home — in fact, microwaves are found everywhere in the universe. If you point a telescope at the sky which is tuned to capture this type of light, you will measure what’s called the cosmic microwave background, a sort of baby picture of our universe:

This ancient cosmic radiation, leftover from just 380,000 years after the Big Bang, shares its nature with the waves heating your leftovers — connecting your mundane microwave oven to the birth of the cosmos. Perhaps the next time your food comes out unevenly heated, you can take comfort in knowing you are experiencing a phenomenon linked to the origins of everything we know. But that’s a story for another time.