Why does food cooked in the microwave heat inside-out?

Question

The other day I was heating a left over casserole that was placed in the freezer for 24 hours before I decided to reheat it. I heated it in the microwave for 4 min. What I noticed was the outside portion of the casserole was still cold but there was steam coming from the middle. I tried heating other items as well and noticed that even soup heats faster in the middle. What i find is that food reheated or cooked in the microwave often splatters and in some cases explodes. Why does this happen or is my microwave the only one?

Answer 1

Your microwave isn't exactly cooking your food from the inside out. Instead, what is happening is that some parts of your food that happen to be on the interior are being heated faster than those parts of the exterior that you observe. This sort of uneven heating is intrinsic with how microwaves work.

Microwave ovens cook unevenly because a pattern of standing waves forms inside the oven chamber, and the pattern creates an array of hotspots throughout the oven's volume. An operating frequency of 2.45 GHz will produce a wavelength of around 12.25 cm, and the regions of maximum intensity (hotspots) will be at half-wave points, or every 6.125 cm, but in a complex 3D pattern

From here

So, in your microwave the waves happen to be arranged just so the inside of your casserole heats up faster than the outside.

In microwave ovens that do not have a rotating carousel, this effect is even more pronounced, although even using a carousel is not a panacea. Many microwave oven recipes often include instructions to rotate and/or stir the food intermittently as a means of distributing the heat more evenly for the same reasons; changing the position of the food within the oven has the effect of shifting the hotspots around in the food as it cooks.

So, my suggestion is to try changing the position of the food in the oven over time, particularly if it's being heated over several minutes, as that should help it heat more evenly. If you can, it is also a good idea to stir the food intermittently as well. Personally, after the microwave finishes the allotted time, I also will stir and then wait a few minutes before serving to let the hot spots naturally dissipate a bit via conduction.

Answer 2

Ice does not absorb microwaves very well.

When ice melts, it turns to water.

Water absorbs microwaves very well and heats quickly.

So when the least dense part of the frozen food melts, it absorbs more microwaves and starts to heat up. It can heat enough for the food to start cooking even though the food around it is still frozen, hence you get 'hot spots' and 'cold spots'.

Stirring several times during re-heating will even out the hot and cold spots.

Before freezing, you can try to make sure the ingredients are as evenly spread out - e.g. avoid (for example) having most of the meat clumped together at one side of the tub.

Answer 3

Microwave oven does NOT cook from inside-out or more accurately it does NOT cook food containing water from inside-out.

Microwave waves (EM radiation) are strongly absorbed by liquid water, reflected off of conducting materials (e.g. metals), and pass through a variety of other materials.

This passing of waves through most of other materials is the reason some people think microwaves heat from the center outwards. Of course if you have a thick crust (that contains little water) and a juicy center it would seem as if the heat comes from inside. In fact the heat comes from the surface of the juicy center because most of the waves are rather quickly absorbed. This can be easily tested with a cold meatball that you microwave for a short time and then cut in half to touch the center and the surface of the ball.

Now I am not sure of the state that the casserole in question was in at the time but I would guess the top layer was relatively dry thus leting the waves pass unabsorbed.

The soup example was most likely wrongly "measured" as the convection is pretty fast in distributing heat no matter where the heat originated. This is easpecially true if the soup was moved around.

Food exploding in microwaves is due to heating of trapped water to the point of evaporation where the water vapor's pressure (expanding water vapor) overcomes the "trap" and the sudden equalization of pressure results in an explosion.

I would like to add that the most upvoted anwser at the time this was written is/was wrong. It claims that the reason for uneven heating are standing waves. This is absolutely incorrect and the source given is incorrect on this as well (not sure about other claims in the source).

It is possible to construct a box in which one would get standing microwave waves however this would require a certain amount of precision in at least the dimensions of the box (whole number multiple of half the wavelenght). Given the different sizes of microwaves using (presumably) the same microwave wave lenghts this condition is clearly not met for the overwhelming majority of microwaves (most likely all). And this only applies to an empty box. Given that food/water is a strong absorber of microwaves standing waves would not form even in our perfect box since there would be no reflecting waves to form the standing ones.

Also the reason why food is rotated in a microwave is because the source of the microwaves is usually located only in one point of the interior. And since we established that food does not get heated from the center out not rotating your meatball would result in one side of the ball to be very hot while the other would still be cold.