You may want to satisfy a childhood curiosity on whether stainless steel can go in the microwave. Or you want to get the facts straight before you put a travel mug full of coffee in.
Stainless steel is not microwave-safe. Like other metals, stainless steel bounces off the microwaves generated by the device, causing sparks and even fire. The shape and size of the container, the length of heating time, and its distance from the microwave’s walls affect the risks.
Here are the details on what happens when stainless steel heats in a microwave. After that, you can also read on to understand why steel causes issues, how they develop, and other heating options you can use.
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The response many expect is sparking from the metal item being heated, or even the start of a fire hazard. And it does happen. Stainless steel reflects heat energy, which can spark or damage the microwave.
The risk of sparks and fire has to do with four factors. The risk factors include:
- The Shape of the metal. Round objects, such as spoons, are the least likely to react, while a ceramic mug with a metal trim–the trim being what’s elongated rather than round–is more likely to cause problems.
- Size. Larger objects are more stable than smaller objects.
- Distance from the microwave’s walls. The closer the metal is to the wall of the microwave, the more likely bounced heat can damage it.
- Duration of the heating. The longer you heat steel, the more you risk a fire or explosion.
Danger aside, heating stainless steel can change the flavor and texture of your food, if not outright burn it.
The odds of steel reacting in a microwave also depend on the model of the microwave. Every make uses slightly different wattage and power.
When you heat food in a microwave, you want it to absorb the heat. However, there’s no heat absorption; water molecules in the food absorb the energy created by microwaves, vibrate, and add friction, which generates heat.
Items like metal that don’t have water molecules available need direct contact with something with water before they warm. And even then, they still reflect energy.
If they reflect too much energy too fast, they’ll:
- Cause a fire
Shape matters as well. The reason why rounded shapes are safer is because of how they reflect energy compared to long shapes. Narrow and sharp-edged items react faster because they have a smaller surface at a given angle for the microwave to bounce off.
Metal trim around a ceramic is even worse. It has that narrow surface, but it also loops around. That looping can create a dangerous electric arc.
The microwaves that give the appliance its name are a radiation frequency between radio and infrared waves. Fat, carbohydrates, and water in the food absorb this radiation in the form of heat.
However, different angles or obstructions limit this absorption, so microwaved food heats unevenly. The food molecules also need room to loosen and vibrate, further contributing to uneven heating, as thawed parts heat faster than frozen parts.
Opposite of fats, sugars, and water, is metal. Nothing reflects microwaves quite like metal. The waves can’t pass the first layer of electrons and instead charges them. Those charges release as sparks and explosions.
Aluminum and tin tolerate the microwave better than stainless steel, though it still comes with risks. Foil laying flat, not touching the floor, ceiling, or walls, and heated briefly is considered safe by some. Even so, you would still need to keep an eye on it, and you may feel the risk isn’t worth it.
Foil can still be dangerous when wrinkled. The sharp angles of the wrinkles create many small surfaces for energy to concentrate on and reflect off. Thus, they can potentially become a fire hazard.
You may wonder why many prepared meals in grocery stores to be heated in the microwave have an accompanying piece of metal. Notice that these pieces are flat or smooth and face the food.
This flat shape causes the microwaves to have a large surface area that doesn’t concentrate as much. But it still grabs energy. The metal catches energy and directs it to the food for even heating rather than bouncing at the appliance.
Reflection is also why foil should not wrap the entire food. It would reflect all the energy, and your food would stay cold. The foil needs the correct position to catch energy by bouncing it into the food. In particular, it needs to reflect heat into the core of the food, so it can heat as fast as the top and edges of the food, the latter of which you want to block so they don’t overcook.
The walls, floor, and ceiling of a microwave are metal because they contain microwaves. This metal is part of the reason why having extra metal inside reflecting or touching escalates the fire risk.
Spoons are safer than forks because spoons have more bulk and roundedness to disperse energy. Meanwhile, the individual prongs of a fork have small, angular surfaces that will overcharge and eject heat more quickly. These utensils also create electric arcs from those multiple angles of bouncing energy.
Another contrast is between a flat surface and a pan. A single, flat surface has the entire space to disperse energy and reflect in one direction. A pan, however, has walls, giving it a few more angles for reflecting energy, including back and forth onto itself.
Shorter time frames are less likely to cause sparks and fires than longer heating times. For example, thirty seconds of a flat metal sheet or a spoon heating in the microwave will likely not cause problems.
You can guarantee that you safely heat your food in the microwave, particularly for longer than thirty seconds. Many other materials work better.
Wax paper is a safe and effective option for wrapping food heating in a microwave. It will still keep the moisture and heat evenly and for an extended period. Dedicating some ceramic, glass, or BPA-free plastic containers for microwave use are also a good idea.
While stainless steel is a bad contender for heating food in a microwave, it is excellent for:
- Heating in an oven
- A toaster oven
- On a stove
It can tolerate up to 350°F (176.7°C), but only here.
The factors involved in microwaves will cause steel to spark even at lower temperatures. The heat from stoves, ovens, and toaster ovens has none of these factors. There’s no intense radiation to concentrate on the steel or reflect off it. Flame and conventional heat soak into metal. It’s slower but more thorough.
Stainless steel and microwaves do not mix. While metal doesn’t immediately spark, cause fire, explode or do so every time, it’s always a potential hazard. If you have to heat metal, remember that flat surfaces for a short time are better than narrow, edgy surfaces for an extended time. Aluminum also fairs better than stainless steel.
But there are safer alternatives. You can heat in a microwave using ceramic containers, plastic, or wax paper. Also, you can use stainless steel but heat with a stove, oven, or toaster oven.