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Nov 17

Microwave Grilling: How Does It Work?

by azmanam | Categories: chemical education, fun, materials chemistry, Uncategorized | (32325 Views)

Previous articles in the How Does It Work series:

If you have any topics where you’d like to know: How Does It Work, let me know in the comments :)

Today: how microwave grilling works. How does my Lean Cuisine Microwave Panini grill itself in my microwave? And how come when I microwave other things they just get soggy instead of crispy and grilled? It’s like using a toaster oven or George Forman, but not! What the heck is going on?

Panini

How’d you get so crispy?! Via Lean Cuisine

First, let’s briefly talk about the differences between a regular oven, a toaster oven, and a microwave oven. They all heat your food, but they do it by different means with different results.

In a conventional oven, you don’t want the oven’s burners to be on. You preheat the oven to make all the air inside the oven the same (really hot) temperature. Then you put your food inside and conduction causes the heat to transfer from the hot air to the food. Dry hot air around the food also causes moisture at the surface to evaporate and the surface to become dry and somewhat crispy – that’s why your bread gets crusty on the outside but is still moist on the inside.

In a toaster oven (or just a toaster, or when your oven is on broil) your food gets heated through infrared radiation. When things (like metals) get hot, they vibrate. When they vibrate they emit infrared radiation. The radiation gets absorbed by your food (particularly the outer layer which is directly exposed to the infrared radiation) and heats up and dries out… toasting your morning breakfast or making the perfect garlic bread. This is why you don’’t want to keep opening the oven to check on the food. If you do, all the hot air will escape and the oven will have to turn the heating coils back on to re-pre-heat the oven… this could potentially lead to unintentional toasting of your food.

In a microwave oven, the food gets heated through microwave radiation. Microwaves can penetrate food and they are absorbed by water and fats and sugars… but not absorbed by most plastics or glass or ceramics. The microwaves penetrate food fairly evenly. The foods absorb the microwave energy and convert the energy to heat. The outer layer doesn’t heat any faster than any other part of the food, so there’s no way to make a crispy crust.

So how are we able to grill in a microwave now? It’s seems like we made a toaster oven out of a microwave.  At first, I thought it might be some chemical on the outside of the bread that reacted under microwave radiation to create the sensation of a crust. Some special butter or something. Turns out, it’s much simpler. It’s not even really a chemistry answer, it’s more of a physics answer.

It all has to do with the silver Panini tray or the protective Hot Pocket sleeve. It is a really thin metalized film called a susceptor. The metal is usually aluminum. The metal absorbs the microwave radiation and gets really hot. If the susceptor is directly touching your food, it heats the outer layer through conduction like an oven, so it cooks more but doesn’t get too too crispy (this is common in microwave pot pies). If the susceptor does not have good thermal contact with the food, like if it’s behind a paper or separated by a layer of air, it emits the heat as infrared radiation. This toasts (or grills, or broils) the outer layer of your food making my microwave pizza nice and crisp and not limp and soggy.

Susceptor

You can see the susceptor in the grilling tray. via Lunchtime Review

4 comments

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  1. John Spevacek

    ” Microwaves can penetrate food and they are absorbed by water and fats and sugars… but not absorbed by most plastics or glass or ceramics.”

    Absorbed? No, not really. The alternating electromagnetic field rotates the dipolar molecules (most specifically water, but also others to a lesser extent), heating them in the process, but also indirectly heating the remainder of the food as this mechanical energy diffuses. Does absorption occur? Sure, as you can’t get all of this for free (just think! you could heat endless amounts of food once you’ve generated the initial microwaves), but direct absorption is not the basis for microwave oven heating. Direct absorption is the basis for a heat lamp.

    (It looks to me that you took much of your information from the Wikipedia article on microwave ovens. The article on dielectric heating is much more accurate.)

    1. azmanam

      Thanks for your corrections. Most of my info was from various HowStuffWorks.com articles, not Wikipedia. :)

  2. Anneliese

    There is an absorption, and that absorption of energy is what causes the water to rotate.

    That’s why when running reactions in microwave ovens you have to use solvents that can absorb the wavelength of the microwaves.

  3. adracamas

    So this layer on the paper/cardboard in the microwave tray is… Aluminum? That’s a little concerning. Judging from the cracks in it after use can we assume its being vaporized and infused in the food too?

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