Pique the Geek 20100530: Microwave Ovens

(10 am. – promoted by ek hornbeck)

First, The Geek apologizes for missing last week.  He had a throbbing headache brought on by allergies and felt neither like writing nor sitting to answer comments.  I am much recovered tonight.

This topic was suggested by Eldest Son who is a lurker here.  It turns out that he has an acquaintance who will not eat food cooked or warmed in a microwave oven, ostensibly because of that person’s belief that the food somehow has dangerous radiation remaining in it, or that the food has somehow been activated into radioactivity by the microwaves.

First, let us look a little into the fundamental transitions that matter can undergo.  The first is translation (no relation, LOL!), wherein atoms or molecules move in straight lines (until they hit another, like in air, for example).  The energy for this transition is supplied by the thermal background, and these transitions are for the most part continuous, in other words not governed by quantum mechanics.

The next higher energy transition is that of rotation, and is caused by energies in the microwave region.  The microwave region lies just above the radiofrequency and just below the infrared part of the electromagnetic spectrum.  These are relatively low energy transitions, but are generally quantized.  As a matter of fact, microwave spectrometry is often used to determine the geometry of molecules that are microwave active.  Atoms, diatomic molecules with both atoms the same, and some other highly symmetrical molecules are not microwave active, because there has to be an electric dipole with which for the microwaves to interact.  Water has a strong dipole, nitrogen gas has none.

The next higher energy transition is that of vibration.  This is not the vibration of entire molecules or atoms into and off of each other (this is really translation), but rather vibrations of atoms within a given molecule.  Hence, single atoms are not infrared active, but all molecules containing two or more atoms are.  You physical chemists will argue with me on that one, but it is more true than not.  Once again, an electric dipole must be present for interaction with the radiation, thus carbon dioxide is infrared active and nitrogen gas is not.  However, the nitrogen-nitrogen bond does vibrate, and this can be detected by Raman spectrometry.  In modern practice, high intensity laser light is impinged on a sample and inelastic scattering occurs, the resulting signals for vibrations corresponding to differences in wavelengths from the laser wavelength that are in the infrared.

The forth higher transition are electronic transitions.  These occur in the visible, ultraviolet, and X-ray portion of the spectrum.  This happens when an electron absorbs energy, going to a higher energy level.  When it drops back to its ground state level, light is emitted, which we detect as color.  In the UV and X-ray regions, the electrons can be promoted to an energy high enough to be ejected from its chemical bond (or, for single atoms, from its atomic cloud).  UV and X-rays are termed ionizing radiation, because an atom or molecules that has lost an electron becomes electrically charged and highly reactive chemically.  This is the reason that exposure to UV and X-rays should be limited insofar as possible, because when DNA is so ionized it becomes damaged, with the potential for carcinogenesis if the body can not repair the DNA.

The highest energy transitions are nuclear transitions, in the gamma ray portion of the spectrum.  When a gamma is absorbed by a nucleus, it can cause a rearrangement in the nucleus, and the nucleus can reemit the gamma or undergo other changes.  Gamma is extremely highly ionizing as well, since sometimes they interact with electrons like X-rays do.  As a matter of fact, the break in the spectrum between very high energy X-rays and gammas is arbitrary.

So you can see that microwaves are on the puny end of the electromagnetic spectrum, and in no way can ionize or otherwise disrupt chemical bonds.  In a microwave oven, the only thing that microwaves can do is to cause water molecules (or other molecules with a dipole moment) to rotate.  AS they rotate, they impart energy in the form of heat into the food being heated, and this heat is rapidly degraded to translational motion, making the food hot.  On a stovetop, the hot element causes translation motion in the cooking vessel, which in turn is communicated to the food.

The microwave oven was developed from radar research after World War II.  One engineer was working with a radar unit and got the munchies.  Reaching into his pocket for his chocolate bar, he found it to melted and warmer than his body temperature.  The only influence that could have caused it was the radar, which operates in the microwave range.  After more development, his invention was marketed in the United States by the Raytheon company under the trade name Radar Range.  They were very large and clunky (and expensive) at the time, and really did not go very far until the technology improved.  Raytheon bought Amana in the 1960s, and in 1967 Amana introduced the first “modern” Radar Range for home use (for about $500, over $2000 in inflation-adjusted dollars).

Many people believe that only water in food is heated by the microwaves, but that is not true.  Water just happens to be the major component of most foods, but fats and proteins have enough polarity to be heated as well. If only water could be heated, bacon would not cook crisp, since temperatures far in excess of the boiling point of water are required to crisp it.

Bacon is pretty much the exception in that it is one of the few foods that will brown up well in a microwave oven.  Breads do not, nor does other meat. To get around this difficulty, frozen food manufacturers put microwave dense coating in the inside of the packaging for their products if they need to be browned.  For example, in pot pies the dish is dense, and the top of the box is as well, so that the crust will crispen properly.

Before I go to some cautions about using a microwave oven, let me repeat that microwave cooking in no way adversely affects the food insofar as radiation goes.  As I pointed out before, no nuclear transitions whatsoever are involved, and the term “to nuke” food is unfortunate and quite incorrect.

There are a couple of precautions to follow with a microwave oven.  First, if you drop one, get it checked for radiation leakage around the door.  Any repair shop can check for that.  While microwaves are not nearly as dangerous as half an hour in the sun, local heating of your tissues is possible if you stand near a leaky one.  I have never seen a case of injury from that, but in theory it is possible.

Second, do not put metal into the microwave, with the exception that I will tell you in a bit.  Metal, especially with large, flat surfaces, like bowls, foil, and cutlery, become electrically charged and emit sparks under microwave irradiation, and while that will not hurt you, it can damage the oven itself.

Third, be extremely cautious when heating water in a microwave oven.  It is possible to superheat the water.  This means getting the water above the boiling point without it boiling.  This is most common when using very smooth glass or porcelain vessels.  I personally saw a cup of water erupt, almost in the former Mrs. Translator’s face, after she heated a cup of water for tea.  She was lucky, and received only a minor scald on her hand.  Water can be superheated when there are no defects or scratches in a vessel to provide sites for nucleation that allows boiling to proceed smoothly.  Remember your high school chemistry classes where you put boiling chips in a test tube to prevent bumping.  The chips provide nucleation surfaces that prevent superheating.  If in doubt, put a couple of very clean pebbles or a few grains of rice in the vessel.

Another hazard is trying to use a microwave oven to dry out damp cloth.  Middle Son almost lit up a damp washcloth doing that.  It started to smoke and smolder, but did not burst into flame, but probably would have if he has let it go much longer.

These cautions are pretty much all that there is.  Of course, you can burn food in a microwave oven just like on the stovetop, but with timers that are standard that is almost unheard of, but it can happen.  The only really dangerous one from a personal standpoint is the one about the superheated water.

I told you that I would give an exception to putting metal in a microwave oven.  If the metal surfaces are of a suitable geometry and size, they are invisible to microwaves.  For example, some microwave ovens have metal racks in them, and they do fine.  That is because the racks are designed in such a manner that the microwaves bend around them rather than interact with them.  Here is an exception that anyone can use.  I gave to give Alton Brown credit for this on in his book, I’m Just Here For the FOOD.

Home made microwave popcorn

(By the way, popcorn was the first food intentionally cooked with microwaves, after the accidental affair with the chocolate bar).

1/3 cup popcorn

Butter (optional)

Salt or popcorn salt to taste

Other toppers that you like to taste

Put the popcorn in a brown or white paper lunch bag.  Fold the shut, folding it twice, 1/2 inch at a time.  Take a regular office stapler and staple the folds shut, spacing the staples around 2 or 3 inches.

Place bag in oven and blast it on high until it pops around every five seconds.  (You will have to experiment to see how long your oven takes, and remember that different batches of popcorn have different moisture levels).

Remove, pour into a bowl and add the butter, salt, or whatever seasoning that you like and enjoy.

The reason that this works is that the staples are too small to interact with microwaves, like the rack.  They just bend around the staples.

This is a lot cheaper than buying commercial microwave popcorn, and has the advantage that you can control the amount of salt and fat in the final product, where you are stuck to whatever the manufacturer puts in their bags.  Try one batch with no seasonings as a breakfast cereal.  Sweeten the milk in your bowl first, and sprinkle in a couple of spoonsfull of popcorn and eat.  It gets soggy fast, so it is better to keep adding it as you eat it so it is crisper.

The microwave oven is now an essential part of a modern kitchen.  They do a few things extremely well, and many things very well.  For other things, not so good.  When you can use one, they are unsurpassed concerning energy efficiency.  Since only the food (and any container) is heated, there is very little waste of energy.  The oven and the stovetop are very much less efficient, since you are heating metal and air in addition to the food and its container.  In addition, in the enclosed environment of the microwave oven, little heat is wasted by convective transfer to the room.

Well, you have done it again!  You have wasted many einsteins of perfectly good photons reading this post.  And even though Rep. Issa admits that his crowing about the Sestak “job offer” is bogus when he reads me say this, I always learn much more than I could possibly hope to teach whilst writing this series.  Please keep those comments, questions, corrections (especially) and other thoughts coming.  Remember, no science or technology item is ever off topic in the comments to this post.

Warmest regards,

Doc

Crossposted at Dailykos.com