Credit: Slug Signorino

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I work at the deli counter at a natural-foods store and recently one of my customers chided me for suggesting she warm up her food by microwaving it. She said microwaving changes the molecular structure of food and makes it dangerous. Being skeptical, I researched this online. Your May 2005 column said the jury was still out on this question. Any new information? —Bet P., Charleston, S.C.

The jury was still out, although if my eyes don’t deceive me they just filed back into the courtroom. More on that in a moment, but first a word. Next time you get some fragile eggshell mind telling you that “microwaving changes the molecular structure of food,” look them in the eye and reply as follows: “You’re 100 percent correct. THIS IS CALLED COOKING, YOU NITWIT.”

Sorry, needed to vent. Back to the jury. Their verdict comes in the form of an article published last year entitled: “Microwave Effects in Organic Synthesis: Myth or Reality?” The answer, not to kill the suspense, is: myth.

To recap, people who freak out about microwaves because they use radiation fail to grasp that there are two kinds of radiation: (1) ionizing, the high-energy kind produced by nuclear bombs and radioactive elements, and (2) non-ionizing, the relatively low-energy type we encounter daily in the form of light, heat, and radio waves. Microwaves are located between radio and heat (infrared) on the non-ionizing end of things. When we speak of “nuking” something in the microwave, that’s not what we’re actually doing.

Microwave heating is different from conventional heating because, whereas infrared energy warms up pretty much any molecule it plows into, microwaves only affect molecules having polarity—that is, positive and negative ends, which rotate rapidly back and forth as the microwaves go by. A common type of polar molecule is water, which is distributed fairly evenly throughout many foods.

So while ordinary heat gets absorbed by the outer layer of a food and only slowly penetrates the interior, microwave energy passes through most of the food and heats up the water, and to a degree the sorta polar fats and sugars, which in turn heat up everything else. The food thus cooks uniformly and in much less time.

But let’s be clear: Heating is heating. The mainstream view is that microwaves do what conventional heating does, only faster. A few scientists, however, think there may be what are known as nonthermal microwave effects of possibly ominous significance. Since precision microwave ovens have become widely available in labs, an opportunity to settle this longstanding controversy is now at hand—or so it seemed in 2005.

Which brings us to the article cited above, published last year by three chemists, C. Oliver Kappe, Bartholomäus Pieber, and Doris Dallinger, in the journal Angewandte Chemie. Having reviewed the literature and done some experiments, they report as follows:

Everyone agrees microwaves are far too low-energy to break molecular bonds and cause chemical reactions. (Cooking, whether done conventionally or with microwaves, unbends or “denatures” proteins, changing their shape much as one might unbend a paper clip, so in that sense it changes molecular structure. But it doesn’t turn the molecules into something else.)

That said, laboratory microwave ovens do things that are difficult or impossible to replicate with conventional heating. A reaction that might take five hours to complete if the starting mixture were simply boiled can be accomplished in one second using a microwave to superheat the stuff in a sealed vessel. The fact remains: These are still thermal effects.

Claims of nonthermal microwave effects continue to show up in the scientific journals, but in the opinion of Oliver, Bart, and Doris, these are mostly due to chemists not really getting how microwaves work and experimental error. A common problem is inaccurate temperature monitoring.

For instance, a scientific team led by one Dudley reported it had heated a chemical mixture to 100 degrees Celsius using both conventional and microwave heating. However, after 30 minutes, the reaction in the conventionally heated mixture was only 25 percent complete, whereas in the microwaved mixture it was 90 percent. Since the temperature of the two mixtures was the same, Dudley and friends contended, this was evidence of a nonthermal microwave effect.

Kuhscheisse, riposted our three skeptics. Team Dudley had used sensors that measured the surface temperature, not the internal temperature of the mix. The Austrians reran the experiment using an internal probe and found the reactions in the microwaved and conventionally heated mixtures occurred at exactly the same rate.

One article won’t end the argument. But I’m inclined to agree with Oliver, Bart, and Doris: Mysterious microwave effects (and presumably their attendant dangers) are a myth. —Cecil Adams