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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.

ErikE
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Learner
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    Regarding mentioned The Lancet article and D-proline, it seems that it is explained here: https://www.abc.net.au/science/articles/2006/03/23/1597903.htm It was not a peer-reviewed article and milk was exposed to much higher levels of microwaves than at home microwaving device – Learner Oct 22 '19 at 15:26
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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?](https://skeptics.stackexchange.com/questions/4714/does-burning-your-food-increase-your-risk-of-getting-cancer) – Weather Vane Oct 22 '19 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 '19 at 00:12
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    Moo, could you elaborate what would suprise us? You mean, that it would be undercooked inside? – Learner Oct 23 '19 at 06:58
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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 '19 at 09:55
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    @Learner that the basis for the claim is dubious... – Moo Oct 23 '19 at 19:43
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    It's rather rare to find a health magazine that actually tries to promote anything healthy. It's mostly about urban myths, things pulled straight out of thin air and getting revenue from "health" ads. Just compare what you read in health magazines with e.g. WHO reports. – Luaan Oct 24 '19 at 12:00
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    "with new, unknown chemical compounds unknown to nature being created" - So anyone with microwave can add new elements to Mendeleev tables? – SZCZERZO KŁY Oct 24 '19 at 13:35
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    @SZCZERZOKŁY: No; a [chemical *compound*](https://en.wikipedia.org/wiki/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 '19 at 17:14
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    @Luaan WHO reports tend to be much the same, just written to attract funding from other parties. – jwenting Oct 25 '19 at 03:24
  • I'll note that people have a tendency to microwave food in plastic containers but generally do not do so when using other cooking methods. The [USDA](https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/appliances-and-thermometers/cooking-safely-in-the-microwave/cooking-safely-in-the-microwave-oven) says plastic containers are fine if and only if the label/packaging says it is safe. Many such containers are labeled, "microwave safe" and then packaged with a warning, "Not safe for cooking. Only microwave for reheating." – Brian Oct 25 '19 at 13:09
  • "just making water molecules vibrate and thus warming up the food" That is not a good picture of what is happening. The microwave is bombarding your food with electromagnetic radiation at a frequency which is readily absorbed by many foods. The food absorbs the radiation, therefore the radiation is increasing the energy in the food, increasing its heat. Hot molecules are vibrating yes... that's part of what heat is, not what is heating the food. Radiation is heating the food, unlike conventional cooking which uses conductive heating (pot/pan to food). – Aaron Oct 25 '19 at 16:52

3 Answers3

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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.

Community
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DavePhD
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    Maybe I do not understand, but you didn't really answer the question. – BЈовић Oct 23 '19 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 '19 at 12:27
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    @DavePhD: Yes, but you did not summarize the article's conclusion. – Brian Oct 23 '19 at 13:00
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    @Brian which article? The abstract summarizes the German article's conclusion. The Lancet hypothesis was just a comment, not a real article. – DavePhD Oct 23 '19 at 13:02
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    This is from the question: `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?` - you answered none. – BЈовић Oct 23 '19 at 13:07
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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 '19 at 13:09
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    annoying springer paywall – undefined Oct 23 '19 at 14:34
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    @undefined I have access to the full article, but only as image that doesn't OCR well, and it's in German (which I guess would be good for you, but not me). – DavePhD Oct 23 '19 at 14:38
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    Basically, in addition to just quoting the article text (translated or otherwise), and expecting the asker(s) to deduce the answer to their question from that verbiage, you should state the conclusion in plain layperson terms - something like "**No** - according to this study, microwaving does not have the effect claimed in the article you read.". Ideally perhaps even as the very first sentence of the answer. Be mindful of mental reference frames - the mental step to the conclusion probably looks clear and effortless to you, but that's because you're very used to taking steps like this one. – mtraceur Oct 23 '19 at 18:23
  • @mtraceur ok, see if it is better now – DavePhD Oct 23 '19 at 18:42
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    @mtraceur Thank you, this drives me crazy. We should **answer** the question, _then_ explain. Especially if it's a yes or no question. – user91988 Oct 23 '19 at 20:51
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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." – Him Oct 23 '19 at 23:13
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    @mtraceur "No - according to this study, microwaving does not have the effect claimed in the article you read." This statement is not deducible from the cited articles. – Him Oct 23 '19 at 23:16
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    @only_pro I would argue that stating the facts/studies first and then summarize it to get to the conclusion is the better way to do it. Cause it gives you the possibility to evaluate the content instead of being primed. Plus you can not say no for sure. – Kami Kaze Oct 24 '19 at 07:02
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    The quoted author, from the magazine mentioned in the question, thinks microwaving food is unhealthy. She gives her own vague explanation why and then offers some scientific evidence. This answer explains how the scientific evidence, when put to more thorough testing, actually demonstrated there was no danger. Therefore, the scientific basis for the author's opinion has been discounted and we're left solely with her own personal vague description, thus feeling less inclined to take her seriously. That seems like a textbook good answer to me. – Max Williams Oct 24 '19 at 14:30
  • @Scott You're absolutely right to fight for a proper understanding and application of scientific epistemology, what can actually be empirically known/answered, etc, and yet: Look at my comment. Look at *everything else* in my comment. Notice that almost every other word in that comment was more important than that example quote. Now look at the comment length limit. Notice all the costs of refining wording. The wording is a necessarily simplistic mirror of the general direction of the answer whose function is to communicate the shape of a suggestion orthogonal to what conclusion to make. – mtraceur Oct 24 '19 at 18:09
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    @mtraceur I am not trying to find fault with you. Please accept my apologies if offense was taken... I assure you none was intended. I'm not trying to be pedantic here, I'm simply trying to point out that "No" is not a valid conclusion of this answer. If there is a better way to "sum-up" this answer than with an incorrect conclusion of "yes" or "no", I support you entirely. – Him Oct 24 '19 at 19:27
  • @DavePhD "the OP can't find the Lancet article. So this answer helps by finding the article." Where do you see that? I see this answer finding an article that just cites the Lancet article. – Hasse1987 Oct 24 '19 at 23:21
  • @Hasse1987 I don’t have the full text of the Lancet article. I just mean that this answer identifies the particular Lancet article. https://www.ncbi.nlm.nih.gov/m/pubmed/2574327/ If someone has the full text that would be better. – DavePhD Oct 24 '19 at 23:25
  • Your answer is basically, "Person A said this, person B said that. Therefore person B is right." Why would you trust what the german article concludes in its replication versus the Lancet article ("there is no such effect from proline under neutral pH conditions.")? Particularly without even seeing the Lancet article, and in particular without comparing the methods? – Hasse1987 Oct 24 '19 at 23:28
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    @Hasse1987 because the Lancet article wasn’t a research article, but more like a brief letter describing a hypothesis. Then the Germans actually tested the hypothesis. But again it would be better if someone had access to the full text. And originally I didn’t have the conclusion, but eventually the above comments pushed me to add a conclusion. – DavePhD Oct 24 '19 at 23:32
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    +1 for expecting people to deduce the answer to the question from that verbiage, and not stating the conclusion in plain layperson terms, because the (*actual question*, not the title, people) isn't stated in layperson's terms. The title is too broad to even ask on SE in the first place. But I guess *L-proline to D-proline* won't get you on the HNQ.... – Mazura Oct 25 '19 at 01:43
  • A long text that doesn't provide an answer – Pedro Lobito Oct 25 '19 at 02:28
  • @mtraceur I added a second answer to better illustrate why I didn't just say "**No**". – DavePhD Oct 25 '19 at 12:16
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    @Hasse1987 actually, according to the book "Process-induced Changes of Amino Acids and Proteins" at page 12, the Lancet researchers did do some experimentation, heating milk for 10 minutes at 80 degrees C by microwave and water bath, but no one else could reproduce their results. – DavePhD Oct 25 '19 at 12:53
  • @DavePhD Is there any indication how many attempts at replication there were? – Hasse1987 Oct 26 '19 at 23:28
  • @Hasse1987 not in the book https://books.google.com/books?id=i3BKd-AdQv4C&pg=PA7&dq=%22Process-induced+Changes+of+Amino+Acids+and+Proteins&hl=en&sa=X&ved=0ahUKEwi3qMKeirvlAhVxg-AKHU7cBvMQ6AEIJzAA#v=onepage&q=%22Process-induced%20Changes%20of%20Amino%20Acids%20and%20Proteins&f=false still don’t have article full text – DavePhD Oct 26 '19 at 23:31
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    I don't understand why you need two answers, neither of them with simple conclusions: No, the cited study does not support the claims made by Zielonka, however, yes, there are certainly chemical differences in the results of different heating systems. – Oddthinking Oct 29 '19 at 12:39
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    @Oddthinking I agree I don't *need* two answers, one or even zero would be fine too. This answer is for the specific claim about proline enantiomerization. This answer has the conclusion "there is no such effect from proline under neutral pH conditions". The other answer is for the title question that encompasses all foods. It doesn't have a conclusion, but it would be "yes, ornithine is created when milk is heated, and the effect is greater for microwaving than for other heating, according to this study", repeating what I already boldfaced. – DavePhD Oct 29 '19 at 13:10
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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.

Laurel
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user3445853
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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 '19 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 '19 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 '19 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 '19 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 '19 at 15:09
  • @rackandboneman Yeah, don't try heating chocolate in a microwave oven. Or bacon. Fat gets heated much more quickly than water in a microwave, and can reach higher temperatures to boot. – Luaan Oct 25 '19 at 07:43
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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.”

DavePhD
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