Does anyone remember when it was claimed that microwave ovens cooked food from the inside out?

[Paulsrocket]

Well they do not do this, never did.  So why when this was said didn't the physicist who designed the ovens put a stop to the rumor?

[Bufofrog]

1 hour ago, Paulsrocket said:

Well they do not do this, never did.  So why when this was said didn't the physicist who designed the ovens put a stop to the rumor?

It's hard to put a stop to an urban legend.  For instance the majority of the republicans in the USA think the last presidential election was fraudulent.  Never underestimate the stupidity of people.

[exchemist]

1 hour ago, Paulsrocket said:

Well they do not do this, never did.  So why when this was said didn't the physicist who designed the ovens put a stop to the rumor?

 No I have never heard this ballocks. But then I bought my first microwave oven decades after leaving university, so I would have been fairly impervious to such myths.   

[pzkpfw]

There's also an element of "lies to children". The average person doesn't need a completely scientifically accurate explanation of something to the smallest detail. They just need to know "enough".

Like the friend of mine in college (= US high school) who put his motorcycle gloves in a microwave to "dry" them, and hurt his hands when he put them on.

It's like the difference between "education" by pop-sci youtube videos, vs University.

[MigL]

Cooking involves applying heat to foodstuffs.
Most always externally, as with fire, electrical heating elements, or even infrared radiation.

A microwave oven works by pumping microwave radiation ( which is NOT hot ) into the foodstuffs.
This microwave radiation is at the right wavelength/frequency to stretch/bend the intramolecular bonds of water ( and few other materials ), thereby heating the water, which then cooks/boils the foodstuffs.
IOW, no internal water content ... no cooking.
Haven't you ever wondered why the glass tray inside your microwave doesn't heat up ?

That would seem very much like 'internal' cooking to me.
Hard to dispel a 'myth' that happens to be true.

[exchemist]

13 minutes ago, MigL said:

Cooking involves applying heat to foodstuffs.
Most always externally, as with fire, electrical heating elements, or even infrared radiation.

A microwave oven works by pumping microwave radiation ( which is NOT hot ) into the foodstuffs.
This microwave radiation is at the right wavelength/frequency to stretch/bend the intramolecular bonds of water ( and few other materials ), thereby heating the water, which then cooks/boils the foodstuffs.
IOW, no internal water content ... no cooking.
Haven't you ever wondered why the glass tray inside your microwave doesn't heat up ?

That would seem very much like 'internal' cooking to me.
Hard to dispel a 'myth' that happens to be true.

I think it will be rotation. You would need IR to excite stretching and bending vibrational modes, surely?

[J.C.MacSwell]

3 hours ago, Paulsrocket said:

Well they do not do this, never did.  So why when this was said didn't the physicist who designed the ovens put a stop to the rumor?

Never?

The microwave wavelengths are chosen for good penetration into the food. Although the food nearest the surface does tend to absorb more than food further, the food further can actually get hotter as the thermal energy has further to go to escape and may also, depending on the microwave design, and the geometry and any lack of homogeneity of the food, actually absorb more than a similar volume near the surface. So the temperature can build up more toward the middle depending on quite a number of factors..

So sometimes the food does actually cook from the inside out, even if this wasn't the explanation given to you as a non scientist, by an advertiser non scientist.

 

Edited January 14 by J.C.MacSwell

[MigL]

16 minutes ago, exchemist said:

I think it will be rotation. You would need IR to excite stretching and bending vibrational modes, surely?

I'm no Chemist, but I do believe you are right.

[swansont]

1 hour ago, MigL said:

Cooking involves applying heat to foodstuffs.
Most always externally, as with fire, electrical heating elements, or even infrared radiation.

I will add to this that all of these methods transfer heat, from outside-in.

Microwaves do not do this, as MigL and JCM have described. It’s not outside-in, but it is depositing energy to the interior, and somebody probably decided to describe that as inside out. Which it kinda-sorta is, but not necessarily center-out.

The poor pop-sci description persists because zombie descriptions are hard to kill, when they are so easy to repeat.

[exchemist]

I've been looking into this a bit today. It seems that in the liquid phase, pure rotations of water are quenched by the transient weak intermolecular bonding,  but a broad band of microwave absorption is still present due to the breaking and forming of these weak bonds. 

Also it is not just water that absorbs, since other polar molecules can also be affected by the radiation. I imagine this is important in defrosting food, since molecules in ice itself will not be able to absorb, as they are bound in fixed positions. However, there may be lattice vibrations that are low-lying enough to be excited.

So it's actually quite complicated, apparently. I have found, the hard way, that some crockery can heat up very rapidly in the microwave oven, to the point of cracking the glaze, whereas other (white) crockery stays cold. I suspect some of the dyes in coloured glazes may absorb, or something.    

[TheVat]

Some ceramics retain a bit of water inside, even after firing.  I've had such that cracked when nuked.  Which is why they need to say Microwave Safe on the bottom, or I don't put them in.

If I heat a burrito, it definitely heats from inside out, since the moist interior (with those rotating h2o molecules) heats up quickly while the dryer tortilla wrapping is still cool.  Empiricism!  

[iNow]

39 minutes ago, TheVat said:

while the dryer tortilla wrapping is still cool.  Empiricism!  

Empán-iricism, as it were 

[exchemist]

41 minutes ago, TheVat said:

Some ceramics retain a bit of water inside, even after firing.  I've had such that cracked when nuked.  Which is why they need to say Microwave Safe on the bottom, or I don't put them in.

If I heat a burrito, it definitely heats from inside out, since the moist interior (with those rotating h2o molecules) heats up quickly while the dryer tortilla wrapping is still cool.  Empiricism!  

I'd like to experiment with an ice cube. As that is only frozen water, in theory it should only absorb a few very low-lying lattice vibrational excitations. But I imagine once it starts to melt it will go a lot faster.  

[TheVat]

The bad pun tribunal has just been empan -eled.

[swansont]

Just now, exchemist said:

I'd like to experiment with an ice cube. As that is only frozen water, in theory it should only absorb a few very low-lying lattice vibrational excitations. But I imagine once it starts to melt it will go a lot faster.  

That's the problem of cooking something that's frozen. One section gets defrosted first and the heating gets much more efficient and cooks while the rest is still frozen.

[exchemist]

11 minutes ago, swansont said:

That's the problem of cooking something that's frozen. One section gets defrosted first and the heating gets much more efficient and cooks while the rest is still frozen.

Yep, I don't use it much for defrosting, except things like soup where you get it started and then tip into a pan to heat through.

Interestingly I once bought some frozen quails stuffed with foie gras at Picard, the frozen food chain in France. These they recommended you cook in a pan from frozen! By the time the outside is golden and the quail is cooked, the foie gras in the centre is nicely defrosted, without getting too hot and all melting out! They were in fact delicious. Also bought some frozen Burgundy snails, to be cooked in the oven on the same principle. Only the French.............  

Getting them back from France before they defrosted was the challenge. Possible if bought at Calais with a cool box and you jump on the shuttle. Not feasible now that we go on the ferry from St Malo (8hr crossing). 

Edited January 15 by exchemist

[MigL]

6 hours ago, exchemist said:

I imagine this is important in defrosting food, since molecules in ice itself will not be able to absorb, as they are bound in fixed positions.

It was for exactly this reason (food defrosts in a microwave ) that I assumed it was bond stretching and bending, rather than translational and rotational.
Thanks for looking into it.

2 hours ago, exchemist said:

quails stuffed with foie gras

No wonder French people drink so much wine; they need the alcohol to cut the fatty foods 😄 .

[exchemist]

18 minutes ago, MigL said:

It was for exactly this reason (food defrosts in a microwave ) that I assumed it was bond stretching and bending, rather than translational and rotational.
Thanks for looking into it.

No wonder French people drink so much wine; they need the alcohol to cut the fatty foods 😄 .

What I am still not sure about is the lattice vibration thing, i.e. phonons. The article I read mentioned absorption in the far IR, which shades off into the microwave region. I presume that in a large crystal there will be a huge range of states, the longer wavelength ones having a pretty low excitation energy. But whether microwaves can excite these seems a bit hazy so I’m guessing a bit.
 

Regarding rotation, molecules in liquids can of course rotate, but each rotation will get interrupted by banging into neighbours so you end up with an incoherent mess of absorptions from all kinds of dipole-induced excitation that can’t really be said to due to any specific degree of freedom.  What I am fairly sure of, though, is that the classical stretching and bending modes of covalent bonds need photons of IR frequency to stimulate them. 

Edited January 15 by exchemist

[John Cuthber]

The penetration depth of "cooking" microwaves into food is variable but typically about 3 cm.
That for the IR from a grill is less than a millimetre.

So, for an item less than a few cm thick, it is heated "in the middle" by the microwaves.

The outside is heated more than the inside, but, unlike a conventional oven, heat is generated inside the item, rather than being conducted from the outside.