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The wikipedia article on microwave ovens says

Another misconception is that microwave ovens cook food "from the inside out", meaning from the center of the entire mass of food outwards.

It further says that

with uniformly structured or reasonably homogenous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers.

However, on more than one occasion I've microwaved a stick of butter, and the inside melts first, then the outside caves in releasing a flood of butter. (It may be relevant that my microwave turntable does not turn - but since I've done it more than once, I would not expect it to be a fluke of placement in the standing wave. And, the resulting butter-softness seemed very strongly correlated with depth, more than I'd expect from accident.) That sure seems consistent with the food absorbing more energy on the inside than on the outside. Given that this takes place over 30 seconds or so, I'd not expect much heat exchange to occur with the butter's environment (nor inside the butter itself), so that would forestall explanations of "the air cools off the outer layer of butter", unless I'm seriously underestimating the ability of air to cool off warm butter. So what's going on?

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    $\begingroup$ I can't imagine how the radiation is affecting the inside more than the outside, but I agree it is also not air efficiently cooling the outside. Given those two pieces of information, I think it might be that the heat generated inside has nowhere to go. So it's not that the air efficiently cools the outside butter, just that it provides some non-zero cooling, the inside has zero cooling, effectively trapped heat hence melts first $\endgroup$
    – dllahr
    Feb 21 at 3:48
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    $\begingroup$ I always interpreted the "cooking from the inside out" claim as a layman's-language way to distinguish microwave cooking, in which at least some of the microwave energy is directly deposited internally, from other methods of cooking which all rely on external heating, whereby energy can only reach the interior indirectly through conduction from the outer layer. In other words, the claim isn't really that the heating originates inside, just that there is some direct internal heating unlike other cooking methods. $\endgroup$
    – TypeIA
    Feb 21 at 14:04
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    $\begingroup$ could it be that internal reflection as the microwave is leaving the butter plays a role? $\endgroup$ Feb 21 at 17:31
  • $\begingroup$ @TypeIA Sounds like an answer; The correct answer. $\endgroup$
    – DKNguyen
    Feb 21 at 19:37
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    $\begingroup$ For what it's worth my microwave's turntable works just fine and still blocks of butter collapse in the same way as you describe. $\endgroup$ Feb 22 at 10:35

7 Answers 7

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A microwave oven's microwaves have a half-wavelength of about 6 centimeters. While there are microwaves bouncing around in all directions and with all manner of offset origins inside the mirrored (for microwaves) interior of the oven, there's still a significant contribution to the energy deposited in the target that's from the primary beam.

enter image description here

A stick of butter is around 12 centimeters long. Therefore, it's highly likely that a randomly positioned stick of butter has exactly one maximum amplitude point located somewhere between the end points. This point will receive the most energy and melt first. Because butter has a very low melting point, it only needs to receive a little bit more energy to melt first, and because it's not in a rigid container, the melted part can puddle down and expose the part that was previously shielded from the radiation by the outer layer to the same maximum amplitude point.

This does not in any way contradict the claim that microwaves cook from the outside in. They simply cook from the outside-in slightly more where the primary wave direct from the magnetron is bigger. A working turntable helps to reduce this effect to make for more even heating across the whole target.

Air cooling is almost entirely irrelevant.

As an aside - you can do a crude measurement of the speed of light if you line up two sticks of butter in a row, measure the distance $\lambda / 2$ between the places where they first start to melt, and multiply by twice the frequency $\nu$ listed by the manufacturer of the oven. $c = \lambda \nu$

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    $\begingroup$ That measurement of $c$ works well with marshmellows, apparently. $\endgroup$
    – Gert
    Feb 21 at 11:56
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    $\begingroup$ "the melted part can puddle down and expose the part that was previously shielded from the radiation by the outer layer" - except, the inside is melting before the outside. I do wonder if it's still a peak-amplitude thing, just maybe in 3-dimensional space. $\endgroup$
    – Erhannis
    Feb 22 at 4:40
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    $\begingroup$ @Gert Yes: youtube.com/watch?v=ejKCiBZwB34 $\endgroup$
    – richardb
    Feb 22 at 22:40
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    $\begingroup$ @Gert "A microwave oven has a half-wavelength of about 6 centimeters." How is that possible if the microwave oven is at rest. Are you using debroglie wavelength. Or are you referring to the electromagnetic waves in the microwave oven. $\endgroup$
    – john
    Feb 23 at 11:14
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    $\begingroup$ @John That is false/misleading. The wavelength used by microwave ovens is around 12 cm, not 30cm. $\endgroup$
    – g s
    Feb 23 at 17:19
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A beam of microwaves penetrates into a food item, creating heat as it goes. For a thick item like a roast, the further the beams penetrate, the weaker the heating effect becomes as the beam gets progressively absorbed. This means that most of the heating is occurring within an inch or two of the surface.

However, that internally-generated heat has no easy escape path from the inside of the roast except to diffuse deeper into the cooler interior of the roast, since the outer layers of the roast are hotter.

This means the inside of the roast is being heated both by microwaves and by conduction inwards from the hotter outer portions. In contrast, in a conventional oven the only heating mechanism for the inside of the roast is by conduction from the hotter outside of the roast. This means that people say that a microwave cooks the roast "from the inside out", which as we see here is not entirely correct- but because the heat is generated inside the roast, it will indeed cook through faster than it would in an ordinary oven.

By the way, microwave ovens all contain either a turntable or a rotating "mode mixer" at the mouth of the feed horn so the microwaves will fill the oven cavity more or less evenly and thereby bathe all surfaces of the roast more or less evenly, thereby preventing any local hot spots from building up.

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    $\begingroup$ If I have understood, you are saying that heat diffuses faster into the centre, because it is cooler there. But that doesn't explain the OP's observations. As soon as the centre reaches the temperature of the outside layer, this process will stop. So how can it end up hotter than the outside layer? $\endgroup$
    – TonyK
    Feb 21 at 12:22
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    $\begingroup$ I think the heat conduction you're relying on is too small to play a significant part here. The thermal diffusivity of water (e.g.) is only about $1.4\times 10^{-7}\,\mathrm{m^2s^{-1}}$. Cooking a chicken to core temperature of $\approx 70^{\circ}\mathrm{C}$ with an oven at $\approx 200^{\circ}\mathrm{C}$ takes about $1.5\,\mathrm{h}$. $\endgroup$
    – Gert
    Feb 21 at 12:32
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    $\begingroup$ You should also take into account that the outermost layer of butter is exposed to air which will provide some cooling. I don't have sources for this but I expect that this cooling, even if it's minor, will cause the outermost layer of the butter to be slightly cooler than the layer just below that. So the heat profile as you go from outside to inside will first go up, then reach a maximum and then slowly taper off as you go towards the center. If the outermost layer is even slightly cooler it will cause the butter to melt inside out even though the center might not be molten yet. $\endgroup$ Feb 21 at 12:58
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    $\begingroup$ Have you been trying to roast food in your microwave? $\endgroup$
    – Joe
    Feb 21 at 13:01
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I think there are two major avenues of heat loss, which surprisingly have not been considered by others:

  1. Evaporation: Both the water and oils in the butter can evaporate off the butter stick, taking away a significant amount of heat from the surface. This can only cool the outer layer.

  2. Radiation: Although the microwaves would impart more heat to the outer layers than inner layers, the radiation given off by the outer layers escapes more easily than that given off by the inner layers.

Add to that air-cooling (mostly by convection) of the outer surface, and you get the hottest being somewhere inside rather than on the outside.

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When people say "It's a myth that a microwave heats from the inside out", they are trying to correct the impression that heating starts in the very middle of a large item like a roast, and works its way out. For something thin or narrow like a stick of butter or a single serving of pasta, pretty much the whole item consists of "outer layer", so heating is fairly uniform throughout except for variation caused by the microwave variability (which means that it is actually not uniform at all).

The answer by g s explained why the most melting would be between the two ends of the stick, but I have the impression that you are seeing the interior melt before the top surface. What I suspect is happening is something like this:

  1. Somewhere along the stick is a region of maximum amplitude
  2. In that region, there may be an amplitude peak inside the stick, but heating may be fairly uniform on that small scale of 2-3 cm.
  3. If there is an amplitude peak inside the stick, then that will explain your observations. But also I am not sure that g s is right about air cooling being irrelevant. Plus there is radiant cooling. (And evaporative cooling, maybe most important of all, props to user21820.) Heat deposited in the interior of the stick cannot dissipate as quickly as heat deposited on the exterior. Considering how close the stick is to melting already, it would not be surprising for the inside to get slightly hotter than the outside and melt first.

Added later: I did some research. If you google "microwave penetration depth" you will find different sites giving reasonably consistent values. For water at room temperature it's around 1 to 1.5 cm, but it's more for cooked meat.

It's also important to note that there is no sharp cutoff. The penetration depth is defined as the depth at which the power is $1/e$ of the level at the surface, or about 37%. If you go the same distance further in, it will be 37% of 37%, and so on.

Also, the more efficiently it heats, the shallower it penetrates. An analogy might be carrying a plate of hors d'oeuvres into a large crowded party. Water is like a hungry crowd that takes snacks quickly (but still takes fewer if they see the plate is getting empty, so you get that exponential decay). So you don't make it far into the crowd, but they get filled up quicker, just like water heats up quicker. Meat is like people who are too busy talking so not as many people grab the snacks. You make it farther in but the crowd is less satiated. And glass is like a room full of kids who can't even see what's on the tray as you heartlessly walk all the way across the room.

Butter is a mixture of water and fat, so the penetration depth will probably be greater than 1.5cm, but I don't know what to guess.

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  • $\begingroup$ Of the given answers so far, this one best matches both my understanding of the topic and the evidence I observe. It still feels like it doesn't quite fit - it didn't look like some arbitrary point in the butter was melting first; but rather the entire center. I wonder if Lucas Baldo's comment about internal reflection is important here. (1/2) $\endgroup$
    – Erhannis
    Feb 22 at 4:49
  • $\begingroup$ And while I still sorta feel intuitively like radiation/conduction/convection would be too small and slow on these timescales and temperatures, I've become willing to consider it a decent possibility. In order to really test it, I'd need to e.g. microwave butter in hot air in a microwave, and that sounds like too much work to set up, haha. If I ever do more investigation into the matter, I'll come back and give an update and possibly change my answer, but for now I'm selecting this one. (2/2) $\endgroup$
    – Erhannis
    Feb 22 at 4:49
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The whole reason why microwave ovens have turntables is that they always heat food unevenly because standing waves form inside them:

enter image description here

If a wave's antinode (where most energy is released) happens to be in the middle of your butter stick, the butter will melt in that place first, and may even explode if the temperature inside reaches the boiling point of water (there's often some water in the butter) before the sides of the stick have a chance to melt.

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    $\begingroup$ To repeat my comment from the question: my microwave's turntable works just fine and still blocks of butter collapse in the same way as described in the question, with the inside melting first. $\endgroup$ Feb 22 at 19:20
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    $\begingroup$ Turning won't help if you have an antinode close to the axis of rotation: it stays inside the butter no matter the position. Try placing the butter stick away from the axis (on the side of the plate) and see if it still melts from the inside. $\endgroup$ Feb 22 at 19:24
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    $\begingroup$ Yes, it does, I always place stuff as far away from the axis as possible. $\endgroup$ Feb 22 at 19:28
  • $\begingroup$ It is good that you turned my comment into a reply but I saw it just as a one possibility or factor for explaining the phenomenon. You can also expand the idea of (total) internal reflection inside the butter illustrated in my second comment. This one will cope with the functional turntable. ... What is the refractive index of microwaves in butter? $\endgroup$ Feb 23 at 11:59
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False.

Simple, common sense answer without introducing unnecessary concepts like speed of light or the meaning of $1/e$ in the context of this question.

The microwaves have to penetrate the object in order to heat it. As they penetrate the object, they gradually lose energy the deeper they penetrate. It is just that the penetration depth before any significant energy loss occurs is much more than the thickness of even the thickest dish that could fit into a microwave oven, and the wavelength of microwaves is also comparable to the dimensions of objects being heated. Nonetheless, still, the microwaves are the least attenuated, and thus the strongest, at the outside of the object. No such a technology exists that would allow to EM waves to somehow penetrate inside an arbitrary object and have that object absorb more energy deep within itself rather than on the surface. Therefore, it is fantasy that microwave ovens cook from inside out.

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    $\begingroup$ You say penetration depth is >> "even the thickest dish that could fit into a microwave oven". But it's actually only a couple centimetres into raw meat (mostly water), which is why a microwave won't uniformly heat a big piece of meat like roast beef or a pork shoulder. (That's not the only reason for not using a microwave to cook such cuts of meat; browning the surface and/or slow cooking to break down collagen into gelatin is important... But if you did, you'd get a cooked layer of a few cm at the surface, and just warm on the inside, not cooked, if you used full power.) $\endgroup$ Feb 23 at 23:37
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You guys are complicating the whole thing. Butter melts at a relatively low temperature. So too, the difference between "solid" and "liquid" is very small. Microwaves penetrate from around outside of a food stuff about ½". How thick is a stick of butter? Just over an inch. If microwaves pass into food ½" then they pass in from all sides ½". So where do they meet, combine or cancel each other out? In the center of a stick which is about 1" in cross section.

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