h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I know that it's a myth that microwave ovens cook from the inside out but there seems to be an exception where softening hard butter is concerned.

Is it a) an optics phenomenon? Microwaves arriving at two or more faces of a corner would tend to refract toward each other and heat some spot inside. Possibly some internal reflection.

or b) A resonance thing where the whole body resonates with the energy?

or c) None of the above?

Since I'm too busy (lazy) to do the experiments I'm sure that the Nobel Prize for Physics has someone else's name on it.

Traveller in Time boiling water
"Think it is more of a lag phenomena, not unlike boiling water in a microwave.

It is already melted, though the structure looks still solid until a tiny disturbance makes the whole butter drop into liquid at once. "

: I don't think that's right...

from trials, preparing a new 250g block of butter for morning toast , If I get it wrong the lot just goes to ghee and is never the same again... However, get it right (about 10s in my microwave) and the middle is softened/melted while the outside becomes about the right consistency and, so long as I don't expose the middle too much, it remains butter throughout. (From the freezer it's, not surprisingly, much less predictable).

Buy easy spread butter. Sorted.

Easy spread... Yeah, used to make that on galley duty in the Navy - 25% butter, mostly margarine and Lots of water in a b i g mixer.

Expensive water!

Don't see much water here:



http://www.lovelurpak.co.uk/aboutLurpak/ourFamily/spreadableSlightlySalted/

...and in my experience about 5 time more expensive than proper butter.

Not quite but certainly more expensive:

Tesco on-line Lurpak Slightly Salted £3.04/kg, Lurpak Spreadable Slightly Salted £4.64/kg.

I sometimes wonder if I should get a life. What am I doing debating the comparative cost of butter

-a tiny disturbance makes the whole butter drop into liquid at once.-

Yes, I've seen that too but more often the phenomenon is liquid pouring from a hole in an otherwise cold block. Also, if I put small pats on my toast and nuke that, it takes forever to melt while a big block goes right away.

Orcus- It's infinitely more expensive than proper butter. However much you pay for it, it's still not proper butter.

"I know that it's a myth that microwave ovens cook from the inside out"

Myth?

"Not much water here"

Butter (69%), vegetable oil (25%), lactic culture, salt (0.9%).

Only about 5.1% nice little extra profit

I've seen Canola Oil mix butter in NZ. It's cold spreadable.
Unless, of course, the '25% veg oil' *is* that...

Someone can correct me here but my understanding is that the microwaves heat the food by being absorbed, usually by water molecules. They can penetrate some before that happens but still mostly cook from the outside in.

"I know that it's a myth that microwave ovens cook from the inside out"

It's not a myth...with the correct foods.

Where you have a bone in whatever you are cooking..e.g. meat...the bone absorbs the microwave energy more quickly than the meat itself. Therefore, the meat around the bone will start to be cooked before the meat on the outside of the joint.

If you haven't got a bone or something which preferentially absords the microwave energy faster than the bulk of the food then it will not cook inside out.

ReDot3- Thank you. I started with one exception and you gave another. If we get enough exceptions, the rule will be destroyed.

The original question was-in a homogenous block, why would more energy be absorbed inside ie. farther from the source of the microwaves. It's all butter, it should all absorb equally. Any energy absorbed by the outside is that much less available to the inside so it should still melt from the outside in. But it doesn't.

Microwaves can only penetrate so far into things, depending on frequency and the conductivity of the 'target'. Hence non-conductive plastic boxes are transparent to microwaves the conductive aluminium casing of the oven is opaque, and food containing water is somewhere in between. Typically, they will penetrate only 25mm into the food, where their energy is absorbed by (mainly) water molecules that try to align with the alternating waves. It's only this outside 'layer' that's being heated directly; the rest heats by straightforward conduction / convection.

Microwaves are also very directional and will tend to go in straight lines rather than spread out and occupy the oven cavity evenly. The oven cavity is also designed to resonate, which will cause standing waves. This is why you need the fan at the top to scatter them and the turntable to move the food around, but you still get hot and cold spots.

I reckon the butter phenomenon is a fairly simple combination of:

a) butter needing very little energy to melt (low specific heat capacity?);
b) microwave hot spots dumping a load of energy very quickly in one place (hence a big block is more likely to pick up a hot spot than a small pat);
c) butter having a low thermal conductivity (it is a very good insulator).

Therefore very localised heating causes the butter to melt in specific spots before the heat can be conducted to other areas, as it might have done with other foods.

However, it is possible that solid and liquid butter are similar to ice and water. In ice, it is harder to move the molecules, so it tends not to absorb much radiation. Once it starts to melt, the molecules become easier to move, so they absorb more. Bits of the food that have already melted absorb more radiation and get hotter still.

The explanation I have given is correct. Therefore, the 'rule' or 'urban myth' doesn't need destroying.

The rule should be:

All foods in a microwave cook from the outside in...apart from meat joints (and anything else with something internal(which I just cannot think of) i.e. one main exception

My oven has a fan, it has a turntable. How is it that the moving block of butter preferentially catches the hot spots inside the block?

Not preferentially, just *more likely* by virtue of being bigger.