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I was reading "Szlachetne Zdrowie" (No 7/2019) which is a Polish health magazine released by "Nasz Dziennik", which is a Catholic, very conservative newspaper of questionable quality. I read the chapter about microwaving written by Barbara Zielonka. She is a food technologist and claims that:

Microwaves frequency cause changes in organic compounds structure (isomers of those compounds might be created) and disintegration of many of them, with new, unknown chemical compounds unknown to nature being created. Food heated in microwave contains particles that are not created during conventional heating of the food (conduction, convection, radiation), where heat is transmitted from outside to the inside of the product.

(my own loose translation from Polish to English)

Also she claims that in The Lancet, there was some research showing that when the milk was microwaved, the amino acid proline in the milk changed its form from L-proline to D-proline and created so called "cis isomers". D-proline might be toxic, she claims. She also claims that the article in The Lancet is stating that "conversion of trans forms to cis forms might be dangerous, because cis amino acids are embedding themselves into peptides and proteins instead of trans isomers".

I was unable to locate that article in The Lancet.

So the bottom line is: is it true that microwaving food can create some kind of chemical compounds that are not created when heating the food in traditional ways? If yes, then should we be worried? Could any be dangerous, such as the mentioned D-proline? I thought that microwaving is safe because it is just making water molecules vibrate and thus warming up the food. On the other hand I am very sceptical of the mentioned source where I found this article, but it mentioned The Lancet and it got me interested.

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    On the other hand, oven cooked food is more likely to burn, and was the topic of a previous Skeptics question: Does burning your food increase your risk of getting cancer? – Weather Vane Oct 22 at 18:17
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    "conventional heating of the food (conduction, convection, radiation), where heat is transmitted from outside to the inside" - a small experiment people need to do: take a large beef joint (or a roasting joint - basically any large single piece of meat), put it in the microwave and cook it on high for 5 minutes. Take it out of the microwave and cut it down the middle. Look at how it has cooked - you might be surprised... – Moo Oct 23 at 0:12
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    Moo, could you elaborate what would suprise us? You mean, that it would be undercooked inside? – Learner Oct 23 at 6:58
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    @Learner Yes I also grew up in the 80s learning that microwaves "cook food from the inside"; but if you do Moo's thing you get a very obviously raw joint with a cooked layer on the outside. Which is obvious if you think about it --- microwaves coming from outside get absorbed on their way to the centre, fewer and fewer can interact the deeper you go --- but somehow this myth hangs on. – user3445853 Oct 23 at 9:55
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    @SZCZERZOKŁY: No; a chemical compound can be new even if it doesn't contain any new elements. New chemical compounds are created all the time; new elements are created very rarely. – ruakh Oct 24 at 17:14
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The English abstract of Zur Frage der Aminosäureisomerisierung im Mikrowellenfeld Ergebnisse eines Modellversuches mit Standardlösungen [The question of amino acid isomerization in a microwave field Results of experiments with standard solutions] Zeitschrift für Ernährungswissenschaft September 1992, Volume 31, Issue 3, pp 219–224 is:

Aqueous standard-solutions of L-alanine, L-glutamic acid, and L-proline do not reveal any increase of D-enantiomers after 30 min heating - neither by the conventional method on a hotplate, nor ina standard microwave oven. A specific "microwave effect" and, hence, a special consumer risk is, in contrast to recent assumptions, not detectable. Effects on the amino acids which were observed in conventionally heated samples are explained by higher heat-exposure during the treatment of these samples.

In the body of the article, it explained that the research is particularly to test the claims made by the Lancet article:

Können durch Erhitzen von Nahrungsmitteln im Mikrowellenherd D-Aminosäuren entstehen? Diese Frage wurde anlässlich einer im Dezember 1989 erschienenen Kurzmitteilung im Lancet (5), in welcher über Isomerisierungen der Aminosäuren Prolin und trans-Hydroxyprolin in erhitzter Milch berichtet wird, zur Diskussion gestellt.

Where reference "5" is Lubec G, Wolf Chr, Bartosch S (1989) Amino acid isomerisation and microwave exposure. Lancet Nr. 9:1392-1393

The conclusion of the German article is:

Halbstündiges Sieden der Aminosäuren L-Alanin, L-Glutaminsäure und L-Prolin in Wasser hat unter den beschriebenen experimentellen Bedingungen keine nachweisliche Zunahme der D-Enantiomere zur Folge. Ein spezifischer ,Mikrowelleneffekt' ist nicht erkennbar. Maximaltemperaturen von 102-104 °C reichen unter normalen Kochbedingungen unter Atmosphärendruck bei neutralem bis schwach basischem pH-Wert (7-7,5) demnach nicht aus, um signifikante Isomerisierungsreaktionen an den verwendeten Aminosäuren, auch nicht an Prolin, auszulösen.

which roughly translates (someone help with German please):

Half-hourlong boiling of the amino acids L-alanine, L-glutamic acid and L-proline in water results in no demonstrable increase of the D enantiomers under the experimental conditions described. A specific 'microwave effect' is not recognizable. Maximum temperatures of 102-104 °C under normal cooking conditions under atmospheric pressure at neutral or weakly basic pH levels (7-7.5) is therefore not sufficient to trigger significant isomerization reactions on the amino acids used, not even on proline.

So, there is no such effect from proline under neutral pH conditions.

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    Maybe I do not understand, but you didn't really answer the question. – BЈовић Oct 23 at 12:16
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    @BЈовић The OP is saying that the OP can't find the Lancet article. So this answer helps by finding the article. Then the OP asks if the isomerization of proline supposed described in the Lancet article actually occurs and is dangerous, so I found the German research which actually tested the Lancet hypothesis. – DavePhD Oct 23 at 12:27
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    @DavePhD: Yes, but you did not summarize the article's conclusion. – Brian Oct 23 at 13:00
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    @BЈовић answer says "L-proline do not reveal any increase of D-enantiomers after 30 min heating - neither by the conventional method on a hotplate, nor in a standard microwave oven. A specific 'microwave effect' and, hence, a special consumer risk is, in contrast to recent assumptions, not detectable." – DavePhD Oct 23 at 13:09
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    @only_pro, science, unfortunately, does not operate in a yes or no way. The answer to the question is either "yes" or "no", but no amount of sciencing the topic will reveal that answer. Especially if the answer is "no". The best we can generally do is "We looked pretty hard for evidence of "yes", and found none.... Therefore, not necessarily "no", but there currently exists no evidence to the contrary." – Scott Oct 23 at 23:13
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Not food, but microwave-vs.-conventional heating is a branch of chemistry: Comparison of Conventional and Microwave-assisted Synthesis of Benzotriazole Derivatives.

So the basic idea that different reactions happen is obviously true when cooking food: It tastes different, the crust (if there) feels different, etc. You specifically miss most Maillard reactions (=non-caramelisation browning) because microwaves don't get food sufficiently hot (the water turns to steam first). So you create different proportions of compounds as in ovens.

I don't know if there's really novel molecules (instead of more/less of otherwise-also occurring ones), e.g. old research showing volatile flavour compounds missing in microwave baking: Science: Why microwave cooking fails the taste test. I think you have to look for papers in this direction if you want to prove/disprove the underlying argument of the original post.

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    Within chemistry, the only effect a microwave has is a different heat distribution, resulting in a hotter solvent resulting in faster reactions. Sometimes, this is amplified by microwave vessels being sealed and thus the solvent being overheated and the pressure in the vessel being high. You don’t get different reactions in lab microwaves, only faster ones. – Jan Oct 23 at 10:31
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    This answer is incomplete. It contains interesting background information about what compounds do not form in a microwave, but nothing about those which only form in a microwave. "I think you have to look for papers in this direction" doesn't answer the question. – Philipp Oct 23 at 11:47
  • I have to agree with @Philipp on this one. This "answer" offers really good and useful supplementary information, but does not answer the question directly. – mtraceur Oct 23 at 18:27
  • You're talking about differences in cooking. The question is about warming the (conventionally cooked and subsequently cooled) food. – Luaan Oct 24 at 11:57
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    "because microwaves don't get food sufficiently hot (the water turns to steam first" .... unless there are grease pockets in the food, these can turn phenomenally hot in a microwave.... – rackandboneman Oct 24 at 15:09
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According to Free amino acid concentrations in milk: Effects of microwave versus conventional heating Amino Acids (1998) 15: 385

concentrations of glutamate and glycine increased more after water bath heating at 90°C (325 +/- 4 and 101 +/- 1 micromol/ L, respectively) than after microwave heating (312 +/- 4 and 95 + 1/micromol/L, respectively, p < 0.05) suggesting milk proteolysis. Moreover, the accumulation of ammonia observed at 90°C with the water bath together with increase Glu levels might reflect a degradation of glutamine. An ornithine enrichment, more evident with microwave heating, was shown and could be of interest as it is a polyamine precursor. Also, considering few variations of free amino acid concentrations and the time saved, microwave heating appears to be an appropriate method to heat milk.

enter image description here

(ornithine concentrations in cow milk before and after heating)

More generally, see Microwave Chemistry Remains Hot, Fast, And A Tad Mystical Chemical & Engineering News Volume 92, pp. 26-28 (2014) which explains:

new results have suggested that micro­waves might be inducing subtle thermodynamic effects that aren’t connected to a change in bulk reaction temperature. An acerbic debate erupted last year among some researchers in the field—including [C. Oliver] Kappe—on how to interpret the new findings

...

Kappe’s group has helped lead the way over the past decade in assessing the plausibility of nonthermal microwave effects and some proposed specific thermal micro­wave effects, debunking many claims of extraordinary results. His team has shown through careful reinvestigations that in most cases erroneous temperature measurements were to blame or the experimental conditions biased the results.

But some scientists have observed unexplained specific microwave effects. These effects appear to be a result of the rapid heating altering normal thermodynamic processes, leading to enhanced reactivity or enantioselectivity that is not connected to an increase in the bulk reaction temperature.

One of these examples was reported in 2012 by Gregory B. Dudley, Albert E. Stiegman, and coworkers at Florida State University. The researchers carried out a Friedel-Crafts benzylation of deuterated p-xylene in an open vessel at constant micro­wave power using a highly polar benzyl-containing pyridinium salt as the precursor. The xylene, which is nonpolar, doubled as a reactant and a non-microwave-absorbing solvent (Chem. Sci. 2012, DOI: 10.1039/c2sc01003h).

...

After discussing the research and this point with Yamada, Kappe says the results “remain unexplainable.” To be sure, he adds, his group or others would have to repeat the experiments to help determine whether there is a specific microwave effect of any kind involved.

“There have been few reports on microwave-assisted enantioselective reactions, and the complete understanding of microwave heating has never been clarified,” Yamada says. “We believe our observation could open a new paradigm and application of microwave-assisted synthetic chemistry.”

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