Alton Brown - I love you, but I think you're wrong!

On Good Eats tonight, he’s doing a show on crab. Fair enough. He’s explaining how microwave ovens work. Thanks, AB, that’s more information than I knew before. You’re so awesome. Then he gets to this statement:


“The microwaves make any asymmetrical molecules vibrate”
Well, maybe. I don’t know much about microwaves.

“The most common asymmetrical molecule is water… more stuff”

What? H-O-H. Check it out. There’s clearly a line of symmetry.

What’s the straight dope?

Your supposed to be concentrating on the crab!

I don’t really care much for crab, or lobster. I can eat them, sure, but they’re not my favorite food.

The current show on oysters interests me even less. I flat out don’t like clams or oysters. Scallops are ok, but I know that it’s just a muscle that I’m eating. I’ve had too much biology to be able to live with myself after eating clams or oysters.

I love Alton too, but he does get some science wrong once in a while. I recently heard him say that 2% milk is whole milk that has had 98% of it fat removed.

If you look at a ball-and-stick model of a water molecule, it is not symmetric. The two hydrogen atoms form an angle of approx. 104 degrees. See The Chemistry of Water.

I know that. My point is that you look at the plane that contains all 3 molecules (Look at the V sideways), and it’s symmetrical in that plane. Look at the V head-on, and draw a line straight down the middle, like this |/ and it’s symmetrical.

This, to me, is like saying that an isoceles triangle is asymmetrical, since you can draw a line somewhere on it that isn’t a line of symmetry.

That’s still symmetrical, divide it down the middle, 52 degrees on one side, 52 on the other. I vaguely remember hearing him say that, and thinking he was wrong too.

If it was perfectly symmetrical, it would have a neutral electrical charge. Since we are concerned about the effects of EM fields (microwaves) on molecules, the asymmetry of the water molecule is important and evident in its electrical field. One side has a positive charge and the other side has a negative charge.

The key here is that there is a significant assymetrical dipole moment with the magnetic field.

Symmetry was a poor choice of words, as it isn’t referring to the “fold over on the dotted line” type of geometrical symmetry.

I may be mistaken, but I thought microwaves worked because they excited the hydrogen atoms in a substance. My vague recollection is that the microwave wavelength is similar to the hydrogen bond length (or shares a resonant frequency). This is the first time I’ve ever heard the “asymmetrical molecule” explanation.

Someone please fight my ignorance.

First, for the kind of asymmetry we are talking about here, the molecule only has to be asymmetric along one axis, even if it is symmetric about the other two (like water).

As x-ray vision noted, you have a slightly polarized molecule. Now if you apply a magnetic field the molecule will align itself to the field. If the field is reversed the molecule will spin 180 deg. In a microwave this occurs at very high frequency causes all polarized molecules (primarily water) to spin like mad. These molecules bounce into others turning their rotational energy into kinetic energy. Kinetic energy on a molecular level is heat.

Side note: You have probably been warned that microwaved items can get hotter even once they leave the microwave. I believe (though am not cerrtain) that this is because the water molecules are still making the rotational/kinetic tradeoff.

Now I’ve been under the impression for some time (I think because of discussions here) that microwaves are essentially like any other source of light, and a thousand-watt light bulb would heat food up pretty effectively too. The advantage of microwaves is that food is generally semi-transparent to them, allowing them to penetrate an inch or two into the substance.

If this is the case, then it’s the same process that any other form of light - infrared or visible, for instance - can heat a substance up. It’s just a means to convey energy into the food.

Ok, well, if that’s what he means by “asymmetrical,” then I can accept his explanation. I still would have picked some different phrase, though.

Yes, just at a different frequency.

Nope, it would heat them in a totally different way. The lightbulb would mostly heat them through infrared and convection, the microwaves are at the right frequency to get the little water molecules jumpin’.

Not actually. Microwaves hit the right frequency to start the little buggers spinnin around like maniacs. When people say that the microwaves are absorbed by the water, what they mean is that the water constantly tries to reorient itself to the ever changing direction of the electric (not magnetic, my mistake earlier, though EM radiation has both) field. This is rotational energy which becomes kinetic once they bounce off each other. Here is an excellent site describing it.

Yes, it was an unfortunate choice of words.

This part was BS. The correct reason for this is described here.

I meant a hypothetical light bulb that only produces visible light. The idea that infrared light is somehow especially connected to heat is a misperception. While an incandescent light bulb produces considerably more infrared light than visible light, any radiation that’s absorbed by an object will heat it.

The association that people have between infrared light and heat is due to black-body radiation, which is emitted by all objects that are warmer than absolute zero. Everything radiates away heat. The radiation can be represented by a curve, and at normal temperatures that humans deal with, that curve’s maximum intensity is in the infrared - thus the use of infrared detection to see body heat, and so forth. Black-body radiation is exactly the same process that causes hot objects to glow red- or white-hot. Heaters that work by emitting radiant heat simply produce very bright infrared light, and if there was an easy way to produce similarly bright green light, it would heat whatever it hit as well. Of course, objects tend to be more transparent to infrared, which means that it penetrates and can warm the inside as well, while visible light would be more likely to just burn away the surface.

If we were some form of hypothetical being living in a much hotter world, with ambient temperatures hot enough to cause objects to glow in the human-visible spectrum, then those colors would be perceived as particularly involved in heat transmission.

I’ll have to read more about microwaves, since I’ve of course heard the common explanation of microwaves having a particular ability to heat water, I’ve also read several times that this is incorrect, and it’s just normal absorption of radiation that’s at work.

You are of course right about general absorption of light becoming heat. The amount of visible light required to act as a heater would probably instantly blind you before it warmed you up though.

I spent a great deal of time finding a good source for the physics behind microwaves so i suggest you read my first linked article. The first paragraph or so is good enough for the concepts, but then they get into the juicy equations and go all out.

Ah, garbled physics. Gotta love it. Here’s the full story:

Any molecule has a certain number of vibrational modes, each with a given frequency. Water, for example, has three vibrational modes: one where the molecule “breathes”, and the hydrogen atoms move towards & away from the oxygen in sync; one where the hydrogen atoms move towards & away from the oxygen out of sync; and one where the molecule “bends”. See here for pictures (scroll down & mouse over the diagram.)

Now, if electromagnetic radiation (such as microwaves) hits a molecule, it can absorb the energy of the radiation and start vibrating if (a) the radiation has the same frequency as one of the vibrational modes of the molecule, and (b) that vibrational mode would cause the electric (not magnetic) dipole moment of the molecule to change. In general, the second condition is the one that determines whether or not a molecule will get heated in a microwave. For example, all three of the vibrational modes of water cause its dipole moment to change, and so it can absorb microwaves at three different frequencies. Oxygen or nitrogen molecules, on the other hand, only have one vibrational mode, and because these molecules are symmetric their dipole moments don’t change when the molecules stretch. So they don’t absorb microwaves. In general, the more symmetric a molecule is, the less likely its vibrational modes are to change its dipole moment, and the less likely it is to be able to absorb radiation via its vibrational modes.

That’s essentially true. Hydrogen bonds tend to resonate at a lower frequency (in frequencies we usually call microwave) than bonds between heavier atoms, which tend to vibrate in the infrared instead.

MikeS: one thing I’ve read - and I can’t remember where, so I claim no authority, but it made sense to me - was that if microwaves did match a particular resonant frequency of a water molecule, then only the very outermost portion of the food could be heated, as the water molecules would be such efficient absorbers of the radiation that it would never penetrate inside. What I’ve read was that microwaves are specifically not attuned exactly to the vibration of water molecules, which is why they penetrate a couple inches into food to heat it.

It’s OK to think the Guru is wrong. I think he’s totally in the wrong with his grilled cheese sandwich technique.