Microwave and Dish Shapes

Is there an optimal dish shape for microwave cooking? Plates allow the food to spread out, whereas bowls provide (I think) more points of contact with the hot surface. Does it depend on the type of food being cooked?

It’s the food itself that heats up, not the plate or bowl it’s on. So it’s the shape of the food that matters.

If you just have a blob of food, it’s the center that will stay cold, while the edges get hot. So get rid of the center: shape your food like a donut. That will let the food heat more evenly.

What hot surface? Microwaves heat the food not the container. They work by agitating water molecules to create friction.

The shape of the container may have some effect, but I don’t believe that it is very significant. I guess that food spread over a large flat area would receive more radiation than the same food concentrated on a bowl, but I doubt that for practical purposes it is something to worry about.

A bundt cake pan shape would probably be the most effective.

But like Bob++ said, it’s probably not of any practical effect in actual use.

a deep plate or casserole dish spread things out and still have volume.

Round is better than square
Height equal to width is better than thin and spread out
More liquid heats more evenly

Why is this?

Microwaves cook from the inside out but from the outside in. That’s probably a bit confusing, but this is what I mean. Microwaves are radio waves in the microwave radio band (roughly 2.4 GHz). Despite commonly referring to “nuking” food, there’s no nuclear type (ionizing) radiation involved at all. The radio waves penetrate into the food and excite different molecules, generating heat (this is the inside out part). But the microwaves only penetrate so far (this is the outside in part).

Not all molecules are heated by the microwaves. Its mostly polar molecules like water, sugars, and fats. If your food is too thin, what can happen is the hot spots get too hot and the food starts to burn before the heat can be transferred by conduction to the rest of the food. On the other hand, if your food is too thick, the microwaves don’t penetrate all the way to the inside and the middle of the food remains cold.

Microwaves also heat unevenly, partly because they only heat certain molecules and partly because they set up a standing wave inside the box, which means that you have hot spots and cold spots in a pattern all throughout the inside of your microwave.

Like this:

This is why most microwaves these days have a rotating platter at the bottom. This rotates your food through the hot spots and cold spots and helps it cook more evenly.

Liquids, besides generally conducing heat fairly well through conduction, will also move around due to convection currents and heat more evenly. This is why you don’t end up with hot and cold spots inside your soup or coffee.

One other type of heating that is done in microwaves is those gray pads for things like microwave pizzas and hot pockets and such. That gray pad is actually a metalized film. The microwave radio waves induce eddy currents in the metal film, causing it to get hot. This heat then cooks the crust of the pizza, hot pocket, whatever, which allows the crust to get nice and crispy. Without that metalized pad the crust would be gooey and not so pleasant to eat.

I knew someone would have a more technical answer than mine:)

I dispute your point about not having hot spots in a liquid though. I regularly heat up soup (a fairly thick liquid) in a carton. It takes 6 minutes in my M/wave and I take it out for a stir half way through. At that point the outer liquid is, literally, boiling, although when I stir it, the overall temperature is tepid.

I worked for a catering equipment company in the 70s when M/waves were just being introduced, mainly in catering. I was helping with the breakdown after an exhibition, when some guys from another stand bought over a box of pies and asked if we could use the machine we had on display to heat them up.

None of us had any experience, but we agreed that 3 minutes sounded about right and a pie was put in. When it pinged, the guy took it out, touched the pastry crust and said that it was just warm. He then took a big bite, promptly followed by a scream, as the boiling contents of the pie scalded his mouth. I last saw him being led off, in considerable pain, to hospital for treatment. A valuable lesson learned.

thick soups don’t allow convection like a liquid.

sugary liquids can heat much higher than is safe to have in contact with your body. the crust can pass some microwave and heat to the filling without the crust burning.

Even if contact area were important, the OP has it backwards: For any given volume, a bowl will have much less contact area than a plate. The minimum surface area per volume is a sphere, and bowls are closer to spherical than plates are.

Friction is not involved. Heat is not, as it were, created by moving molecules rubbing together. The movement of the molecules (caused by the electromagnetic microwaves) is the heat.