John gave me a scare with his link to a paper claiming that microwaving destroys 97% of flavonoids in broccoli:
Research suggests that microwaving causes 97% loss of flavonoids in broccoli. Boiling caused 66% loss, high pressure boiling 47%. Steaming had minimal effect. [1]
Now I would be very reluctant to give up use of the microwave. We do a lot of cooking on weekends – soups, stews, lasagna, a pot of sweet potatoes, and the like – and eat those foods through the week. Stovetop warming is impossible at work, takes more time at home, and necessitates cleaning pots; so microwave warming is a central part of our lifestyle. We also make a pot of green tea every morning and use the microwave to re-heat cups through the day.
A Hard Result to Believe
The idea that flavonoids are so fragile that they are quickly destroyed by microwaves is extremely surprising. Flavonoids are partly responsible for the color of berries:
Berry colors are due to natural plant pigments, many of which are polyphenols such as the flavonoids, anthocyanins, and tannins localized mainly in berry skins and seeds. [Wikipedia, “Berry”]
Yet I’ve never seen berries lose their color in the microwave. The cranberries in my Neo-Agutak (as Melissa dubbed it) may have darkened a bit, but they remained visibly red:
But who am I to believe my own eyes when published, peer-reviewed science says otherwise? So let’s look at the science.
The Vallejo et al paper
In the paper John cited, they submerged 1.2-inch diameter bunch of broccoli florets in tap water and cooked them “at full power (1000W) for 5 min.” [1]
The broccoli was then withdrawn from the cooking water, freeze-dried, powdered, after which 2 g of powder was homogenized three times in 50 ml of 70% methanol, and the homogenate filtered through cheesecloth to remove all solids. The resulting solution was evaporated in a vacuum at 30°C until it was reduced to 1 ml, after which they added 4 ml water. At this point the supernatant (the liquid on top) was taken and the bottom portion, containing any heavier solids, was discarded. The supernatant liquid was then filtered through a 0.45 µm polyethersulphone filter to remove any molecules or solids larger than about 4,000 atoms in diameter. [1]
Now I’m not a food chemist, and am not qualified to evaluate this procedure. But it seems clear that anything resembling “food” is rigorously removed from the water before measurement. There are many steps at which flavonoids could be lost.
The remaining filtered liquid was then analyzed with a Liquid Chromatography machine:
Phenolic acids were quantified as chlorogenic acid (5-caffeoylquinic acid; Sigma, St Louis, MO, USA), flavonols as quercetin 3-rutinoside (Sigma), and sinapic acid derivatives as sinapic acid (Sigma). [1]
The table of data for sinapic acid derivatives is detailed enough that we can get some idea of the variation across samples. Here is their data table:
Of the raw data – the top 9 rows and the 7 columns under “Sinapic and feruloyl derivatives” – 34 of the 63 data points read “0.0.” Every method of cooking has at least one 0.0 entry for edible parts, and for the watery part only 2 of the 28 data points are non-zero. Since each data point represents an average of three samples, it’s clear that the great majority of samples had no detectable nutrients at all by the time they reached the Liquid Chromatography machine.
Since nutrients dissolved in water were entirely lost, and the filtering and supernatant extraction were designed to remove everything that wasn’t dissolved in water, I think “0.0” is precisely what we should expect. If that’s the case, it’s very hard to be confident that the non-zero data points are measuring nutrients in the original food.
Here is their discussion of the results:
Microwave treatment led to the highest losses of phenolic compounds, similar to those found for glucosinolates.19 According to this, 97, 74 and 87% losses of total flavonoids, sinapics and caffeoylquinic derivatives respectively (Fig 2), were reached, probably owing to the high level of evaporation of water in which the leached compounds were dissolved. This flavonoid loss rate does not agree with that previously reported by other authors for microwaving.15,17 Also in this treatment, no compounds were detected in the cooking water (Fig 2). As previously mentioned in this work, it is suggested that components in the tissue are more stable than those in water, where they are more degraded. Probably a new microwave cooking treatment without water would lead to lower percentage losses, as previously reported for ascorbic acid,22 but it would also result in some flavor differences. [1]
I’m not a chemist, but it seems odd that evaporation of water would cause loss of flavonoids. Also, if the flavonoids were leached en masse from the broccoli into the “cooking water,” which then evaporated (turned to steam) due to microwave heating, then the microwaving must have been far more intense than most people’s kitchen heating.
These authors have also estimated flavonoid loss in other stages of food delivery:
- 60% loss of flavonoids in storage and shipping. [2]
- 84% loss of flavonoids in a model of intestinal digestion. [3]
So if our cranberries lost 60% of flavonoids in the supermarket, 97% in the microwave, and 84% in digestion, only 0.2% were left for the body. Perhaps.
What Do Other Researchers Find?
Fortunately for us, other groups have studied what happens when you microwave food.
Indeed, it seems that due to commercial products such as dried fruit and flavonoid supplements, there has been active research into microwaves as a means of drying berries and extracting flavonoids from various plants.
In these studies microwave heating often does better than other methods at preserving flavonoids!
Here is a sampling of papers. First, microwave drying preserved more anthocyanins in cranberries than hot air drying:
[C]ranberries were dried by vacuum-microwave drying (VMD), freeze-drying (FD), or hot air-drying (AD), to compare the effects of different drying processes on both physical changes as well as the retention of bioactive components in dried samples…. In general, vacuum-microwave drying and freeze-drying resulted in similar retention of anthocyanins and antioxidant activity, which were both relatively higher (P < 0.05) than that recovered from cranberries dried by hot air drying. [4]
There’s been a lot of work on green tea. Microwaving does better than standard methods for the extraction and preservation of catechin and epicatechin from green tea:
In this work, for the first time, microwave-assisted extraction (MAE) followed by CE was developed for the fast analysis of catechin and epicatechin in green tea. In the proposed method, catechin and epicatechin in green tea samples were rapidly extracted by MAE technique, and then analyzed by CE. The MAE conditions and the method’s validation were studied. It is found that the extraction time of 1 min with 400 W microwave irradiation is enough to completely extract catechin and epicatechin in green tea sample, whereas the conventional ultrasonic extraction (USE) technique needs long extraction time of 60 min…. The proposed method has good recoveries, which are 118% for catechin and 120% for epicatechin. The proposed method was successfully applied to determination of the catechin and epicatechin in different green tea samples. The experiment results have demonstrated that the MAE following CE is a simple, fast and reliable method for the determination of catechin and epicatechin in green tea. [5]
Another study used both low-power microwaves and conventional heating to keep green tea at 80°C for 30 minutes. Microwaved tea had a higher content of catechins, including the flavanol epigallocatechin gallate, than conventionally heated tea:
[M]icrowave-assisted water extraction (MWE) … appears more efficient than CWE [conventional water extraction] at both 80 and 100 degrees C, particularly for the extraction of flavanols and hydroxycinnamic acids. [6]
Microwaving tea leaf residues to temperatures as high as 230°C may not destroy polyphenols:
Microwave heating was used to produce aqueous-soluble components from green, oolong, and black tea residues. Heating at 200-230 degrees C for 2 min extracted 40-50% of polysaccharides and 60-70% of the polyphenols. [7]
In green tea, microwaves do destroy a natural chemical: the polyphenol-degrading enzyme polyphenol oxidase. Loss of polyphenol oxidase leads to higher levels of phenols and catechins. As a result, microwave pre-treatment of green tea leaves creates a tastier, higher quality tea:
Parched and sun-dried teas contained the lowest levels of total phenols and catechins, and their infusions were dull in color with a slightly burnt odor. Microwave-inactivated and-dried teas showed the highest levels of total phenols and catechins, and their infusions were bright in color and sweet in taste with a subtle pleasant odor. [8]
Onions are another frequently studied food. Microwaving onions causes about half the flavonoids to leach into water and half to remain in the onion. Only about 2% are lost:
[A]t optimized power (500W) [m]icrowave extraction resulted significant yield (81.5%) with 41.9% of flavonol contents, with better retain of remaining flavonoids (55.9%) in residues of onions. [9]
In a Japanese study, onions retained the highest fraction of flavonoids and vitamin C if they were cooked in a microwave, rather than fried or boiled:
We selected quercetin conjugates, total phenol compounds, and ascorbic acid to estimate the amount of flavonoid ingestion from onion. We examined the following cooking methods: boiling, frying with oil and butter, and microwave cooking. Microwave cooking without water better retains flavonoids and ascorbic acid. Frying does not affect flavonoid intake. The boiling of onion leads to about 30% loss of quercetin glycosides, which transfers to the boiling water. [10]
Vacuum microwaving seems to be the optimal way to dry and preserve strawberries:
This study has demonstrated that vacuum-microwave drying, especially at 240 W, can produce high-quality products, with the additional advantage of reduced processing times, compared to other processes such as freeze-drying. [11]
Microwaving causes no loss of anthocyanins from pigmented potatoes:
The analyses were carried out both on fresh tubers and after cooking treatments (boiling and microwaves)…. For the pigmented potatoes, the heating treatment did not cause any changes in the phenolic acids content, while anthocyanins showed only a small decrement (16-29%). The cv. Highland Burgundy Red showed anthocyanins and phenolic acid concentrations close to 1 g/kg and more than 1.1 g/kg, respectively. [12]
Olive oil polyphenols survive best under microwave heating:
Virgin olive oils were subjected to simulated common domestic processing, including frying, microwave heating, and boiling with water in a pressure cooker.… Microwave heating of oils for 10 min caused only minor losses in polyphenols, and the oil degradation was lower than that in thermoxidation assays. [13]
A Chinese group did a nice study comparing the flavonoid content of nine vegetables after microwaving, frying, boiling, and stewing. For flavonoid preservation, microwaving did better than boiling and stewing, worse than frying. Missing flavonoids were not destroyed, but transferred to the soup:
METHODS: Nine kinds of vegetables obtained from Tianjin market were cooked by frying, boiling, stewing, microwave cooking respectively, then the contents of flavonoids in vegetables and soups after cooking were determined by HPLC.
RESULTS: The reserving rates of flavonoids after frying, boiling, stewing and microwave cooking ranged from 54.6% to 115.6%, 33.6% to 107.8%, 31.7% to 100.5%, and 43.1% to 109.6% respectively. Parts of flavonoids were also transfered to the soup after cooking. The transferring rate ranged from 1.4% to 55.8%. [14]
Summary
I looked at a lot of papers and the Vallejo et al paper was the only one that found a substantial loss of nutrients in microwave cooking. A sizeable literature reports no significant loss of nutrients from microwave cooking, though microwaving like other cooking methods may cause significant leaching of flavonoids into cooking water or oils.
Indeed, if anything microwaves seem to be better at preserving nutritional value than most other cooking methods.
The scientists studying freeze-drying and polyphenol extraction seem to have settled on low-power settings – typically 200 to 500 W – as optimal. This fits with Anand’s suggestion to try low-power settings. However, I saw no evidence that high-power microwave settings destroy substantial amounts of nutrients.
Although they are a bit older, dating back to the 1980s, several review papers have been written on the effect of microwaves on the nutrient value of foods. Here are their conclusions:
Overall, the nutritional effects of microwaves on protein, lipid, and minerals appear minimal…. [T]here are only slight differences between microwave and conventional cooking on vitamin retention in foods. In conclusion, no significant nutritional differences exist between foods prepared by conventional and microwave methods. Any differences reported in the literature are minimal. [15]
Based on the information available in the literature, nutrient content and retention of microwave-cooked or reheated foods is equal to or better than the same product prepared conventionally or held hot in a foodservice operation. [16]
Conclusion
I’ve learned an important lesson from these papers. If you want your food to have a high nutrient content, DO NOT:
- Freeze dry it after cooking;
- Grind it to a powder and dissolve it in methanol;
- Filter the solution through a cheesecloth to remove all solids;
- Evaporate the filtered liquid in a vacuum;
- Dissolve any remaining food in water and extract the upper part of the water, leaving behind the lower part of the water which might contain anything heavy;
- Filter the remaining water through a 0.45 µm filter.
If you do these things you may find the nutrient content of your food has been diminished by 97% or more. So ignore any cookbooks that feature these steps at the end of recipes!
Finally: Congratulations, Jaminet family microwave!
You will live to re-heat again.
References
[1] Vallejo, F. et al. Phenolic compound contents in edible parts of broccoli inflorescences after domestic cooking. Journal of the Science of Food and Agriculture. Oct. 2003; 83(14):1511-1516 http://onlinelibrary.wiley.com/doi/10.1002/jsfa.1585/abstract.
[2] Vallejo F et al. Health-promoting compounds in broccoli as influenced by refrigerated transport and retail sale period. J Agric Food Chem. 2003 May 7;51(10):3029-34. http://pmid.us/12720387.
[3] Vallejo F et al. In vitro gastrointestinal digestion study of broccoli inflorescence phenolic compounds, glucosinolates, and vitamin C. J Agric Food Chem. 2004 Jan 14;52(1):135-8. http://www.ncbi.nlm.nih.gov/pubmed/14709026.
[4] Leusink GJ et al. Retention of antioxidant capacity of vacuum microwave dried cranberry. J Food Sci. 2010 Apr;75(3):C311-6. http://pmid.us/20492285.
[5] Li Z et al. Microwave-assisted extraction followed by CE for determination of catechin and epicatechin in green tea. J Sep Sci. 2010 Apr;33(8):1079-84. http://pmid.us/20175087.
[6] Nkhili E et al. Microwave-assisted water extraction of green tea polyphenols. Phytochem Anal. 2009 Sep-Oct;20(5):408-15. http://pmid.us/19609884.
[7] Tsubaki S et al. Microwave heating of tea residue yields polysaccharides, polyphenols, and plant biopolyester. J Agric Food Chem. 2008 Dec 10;56(23):11293-9. http://pmid.us/18998700.
[8] Gulati A et al. Application of microwave energy in the manufacture of enhanced-quality green tea. J Agric Food Chem. 2003 Jul 30;51(16):4764-8. http://pmid.us/14705910.
[9] Zill-E-Huma et al. Clean recovery of antioxidant flavonoids from onions: optimising solvent free microwave extraction method. J Chromatogr A. 2009 Nov 6;1216(45):7700-7. http://pmid.us/19800069.
[10] Ioku K et al. Various cooking methods and the flavonoid content in onion. J Nutr Sci Vitaminol (Tokyo). 2001 Feb;47(1):78-83. http://pmid.us/11349895.
[11] Wojdy?o A et al. Effect of drying methods with the application of vacuum microwaves on the bioactive compounds, color, and antioxidant activity of strawberry fruits. J Agric Food Chem. 2009 Feb 25;57(4):1337-43. http://pmid.us/19170638.
[12] Mulinacci N et al. Effect of cooking on the anthocyanins, phenolic acids, glycoalkaloids, and resistant starch content in two pigmented cultivars of Solanum tuberosum L. J Agric Food Chem. 2008 Dec 24;56(24):11830-7. http://pmid.us/19053373.
[13] Brenes M et al. Influence of thermal treatments simulating cooking processes on the polyphenol content in virgin olive oil. J Agric Food Chem. 2002 Oct 9;50(21):5962-7. http://pmid.us/12358466.
[14] Xu J et al. [Effect of various cooking methods on the contents of major flavonoids in vegetables] Wei Sheng Yan Jiu. 2007 Mar;36(2):223-5. http://pmid.us/17555108.
[15] Cross GA, Fung DY. The effect of microwaves on nutrient value of foods. Crit Rev Food Sci Nutr. 1982;16(4):355-81. http://pmid.us/7047080.
[16] Klein BP. Retention of nutrients in microwave-cooked foods. Bol Asoc Med P R. 1989 Jul;81(7):277-9. http://pmid.us/2775405.
via this link:
Yeahhh. Good news. I would starve without my microwave.
LMAO, love the conclusion. I usually avoid the microwave at home but at work there is no other means of heating my food. I’m glad this has cleared up its use for me.
Many Thanks
James
Thanks for your post. I don’t own a microwave. I have never trusted their safety. Don’t ask me for a rational explanation, it’s just one of those preferences we all have. 🙂 Instead, we use a toaster/convection oven. (we have a Cuisinart) Cooking time is of course longer than a microwave, but other than that, it works quite well for us. The other kitchen tool that we wouldn’t be without is a slow cooker. We make some wonderful broths, soups, and stews in our slow cooker. We coat the slow cooker with coconut oil before cooking, and it makes for an easier cleanup. Yes, we still have to attend to some cleanup afterward, but there are trade offs in life. 🙂
Paul, my engineer brother who is suspicious of new gadgets (he still uses a slide rule for accuracy checks) has a microwave in his house. ‘Nuf said.
Thanks for this post, Paul.
After ‘hearing’ for years from supposedly savvy alternative nutrition ‘experts’ about how microwaving food destroyed the nutrients, this is good news. Now I don’t have to feel guilty for possibly damaging my food and thus myself when I use it.
BTW, the Neo-Agatuk I made for yesterday’s fast was an interesting experience: tangy, creamy, unlike any other meal I can remember. After the newness wore off, I was really enjoying it by the end of the bowl. Ate it about 1:30 after having had nothing but a Tbsp or two of coconut oil previously, then continued the fast until about 4pm when I had a couple of light beers prior to supper (as it used to be called, now dinner).
Great post Paul, your rigorous diligence in finding scientific data is very commendable! Keep up the great work.
In health,
Brandon
Hi Paul. Although you present one side pretty convincingly (well, some of those studies you quote are supported by the industry), there are plenty of studies that show that microwaves are not good on food nutrition or one’s health. To list a few I got from Mercola:
[1] Davis D R. (February 1, 2009) “Declining fruit and vegetable nutrient composition: What is the evidence?” American Society of Horticultural Science
[2] Kidmose U and Kaack K. Acta Agriculturae Scandinavica B 1999:49(2):110-117
[3] Song K and Milner J A. “The influence of heating on the anticancer properties of garlic,” Journal of Nutrition 2001;131(3S):1054S-57S
[4] Watanabe F, Takenaka S, Abe K, Tamura Y, and Nakano Y. J. Agric. Food Chem. Feb 26 1998;46(4):1433-1436
[5] George D F, Bilek M M, and McKenzie D R. “Non-thermal effects in the microwave induced unfolding of proteins observed by chaperone binding,” Bioelectromagnetics 2008 May;29(4):324-30
[6] Quan R (et al) “Effects of microwave radiation on anti-infective factors in human milk,” Pediatrics 89(4 part I):667-669.
[7] Lee L. “Health effects of microwave radiation-microwave ovens,” Lancet December 9, 1989 (Article)
[8] “Microwave ovens: A danger to your health?” (January 26, 2010) Nutritional and Physical Regeneration
[9] Villablanca E (December 19, 2007) “Ionizing and non-ionizing radiation: Their difference and possible health consequences”
[10] Kakita Y, Kashige N, Murata K, Kuroiwa A, Funatsu M and Watanabe K. “Inactivation of
Lactobacillus bacteriophage PL-1 by microwave irradiation” Microbiol. Immunol. 1995;39:571-576.
However, my stance on this is probably pretty similar with Cecil, The Straight Dope. And anyone who can work “ponderomotive” into an article, should get read.
http://www.straightdope.com/columns/read/2595/does-microwaving-kill-nutrients-in-food-is-microwaving-safe
Until that day when we know more about the full effects of microwaves, I’ll ere on the side of caution–just like you do with quinoa and such.
A bit off topic, Paul, but since you mentioned drinking a pot of green tea a day—do you worry about the fluoride that is apparently concentrated very efficiently by Camellia sinensis? Among other concerns, those of us trying to replete iodine are urged to avoid toxic halides as much as possible. A quick look suggests to me that normal levels of tea drinking could expose a person to daily fluoride at levels comparable to or greater than that from ingesting fluoridated municipal water. I love my green tea, too!
Hi Poisonguy,
Thanks for the links/cites. I will read them when I have time.
I agree that my post is hardly the last word on the effects of microwave cooking. I purposely restricted myself to flavonoids since that came up in the Neo-Agutak post and there were plenty of papers just about flavonoids.
Even this post, however, notes destruction of polyphenol oxidase by microwave heating. So I am not disputing that some food compounds can be destroyed.
The question is how does microwaving compare with other kinds of cooking? Microwaving does tend to cook less evenly than some other kinds of cooking, and so to achieve an average temperature of say 80 C you might have some molecules exposed to 100 C or higher. Thus, nutrients that degrade under heat may be more likely to be lost in microwave cooking.
I am open to the possibility that microwave cooking may need to be avoided in some situations. However, I think for most applications, such as re-heating already cooked food, microwaving is safe and the effects on nutrients are probably inconsequential.
Best, Paul
Jim – I’m glad you like the Neo-Agutak! I think it’s a very healthy way to finish a fast.
Bill – I’m not too concerned about flouride. Most flouride goes to bones/teeth or is excreted.
For flourine to measurably impact thyroid function, you have to take 5 mg/day and be iodine deficient (http://www.slweb.org/galletti.html). Doses from tea are probably well below that, and I think if you supplement iodine (as we recommend) then flourine shouldn’t be a problem.
Very nice. Finally a view on microwaving based on more than just one paper.
Also a very solid reminder to me that assumptions can easily lead to inadequate information.
I didn’t have the full text of the Vallejo et al. paper, only Anthony Colpo’s interpretation in chapter 16 of his “The Great Cholesterol Con”. So, based on Stephan Guyenet’s recommendation of that book, Stephan’s comment “Every time I’ve looked up one of Colpo’s references, what he claimed about it has been correct”, and Chris Masterjohn’s favorable review, I assumed that Colpo’s microwave information would probably be an accurate representation of the paper, and perhaps more widely applicable. My bad. Please let it be clear that I’m in no way whatsoever blaming Stephan or Chris for this. I was the one who didn’t do a proper reference check.
Paul, I apologize for giving you a scare. You seem to have recovered from it very well 😉
Thanks for putting the Vallejo et al. paper into the right context. Next time I’ll try to be less sloppy.
John
Hi John,
No need for apologies! I am grateful for your comment. It’s a topic a lot of people are interested in, and there’s enough ground for concern for that it warranted investigation.
Anthony did accurately cite the paper; they do say that microwaves diminish flavonoid content by 97%. A cite is not an endorsement, and I have no reason to think Anthony did anything wrong.
I cite papers all the time on this blog without critically examining their methods or reviewing all the relevant literature. Unfortunately, erroneous results appear in journals all the time. Anyone who writes about science knows that cited results may not be entirely correct. It’s a risk of the trade.
Best, Paul
As the function of water in a pot is to conduct heat from the pot to the vegetables, microwave cooking does not require the addition of water as the microwave energy generated is absorbed by the vegetables and generates heat within them.
I never add water when I microwave vegetables. Therefore, no nutrients are lost in water (though some are destroyed by heat).
I see Natural News have just sent an article through about the cancer causing effects of microwaved food.
“Why and How Microwave food causes cancer”
http://www.naturalnews.com/030651_microwave_cooking_cancer.html
There are some references including this article / study The Hidden Hazards of Microwave Cooking
http://chetday.com/microwave.html
How the science stacks up and whether this is just scaremongering – I have no idea.
Now you see Paul, this kind of research is the reason why I’ve stuck you both in the top 10 people to cure obesity… in the world.
You have a lot of work to do!
http://www.fatfiction.co.uk/solving-obesity/10-people-who-can-solve-the-obesity-crisis/
Mike
Speaking as a physicist, I’ve never seen a compelling discussion as to how microwaves can directly damage biological molecules, aside from heating them. Microwaves are very low energy photons that do not have enough uompf to break even weak hydrogen bonds.
Thanks Paul!
I use a microwave to reheat, but was concerned when a representative from a company I had purchased coconut oil from was floored when I even suggested that I had heated the solid oil in order to liquefy it for certain purposes. She claimed it destroyed the benefits of the oil, though without any specifics or references for her claims. I will rest easier now, though I realize there probably is no true final word on the subject.
Thanks again for your expertise!
Hi Nigel,
Makes sense. We rarely cook anything in the microwave the first time, we mostly use it for re-heating.
Hi julianne,
An emailer sent me links to those earlier, I’m afraid it doesn’t sound the least bit plausible to me.
Hi Mike,
If we’re supposed to cure obesity in the world, that will be a lot of work!
Thanks for putting us in such distinguished company.
Hi Robert,
Yes, as a physicist too that’s my prejudice. The mechanism has to be exciting water molecules, and energetic water molecules slam into other molecules to break their bonds. Or else some kind of electrical heating, if the food in question is conductive.
It is true that many biological molecules are heat-fragile. On the other hand, those are precisely the molecules that are likely to be destroyed in conventional cooking or in digestion. In most cases, destruction of molecules is a plus — it gets rid of toxins, and gives the body the basic lipid/amino acid/sugar/mineral building blocks it needs.
There are some cases in which we want heat-fragile molecules to survive. The case of immunogenic compounds in human mothers’ breast milk is a good example; ideally breast milk should never exceed body temperature. That’s hard to achieve in a microwave.
But for most applications, it’s hard to see how microwaving would cause a problem.
Best, Paul
Robert, you should read the links that julianne posted. They are hysterical.
“Technically produced microwaves are based on the principle of alternating current. Atoms, molecules and cells hit by this hard electromagnetic radiation are forced to reverse polarity 1 to 100 billion times a second. There are no atoms, molecules or cells of any organic system able to withstand such a violent, destructive power for any extended period of time, not even in the low energy range of milliwatts.”
Thanks for covering this topic. I’ve often wondered about the safety of microwaving and so far nothing I’ve read has turned me off using them. The only change I’ve made is to stop using any type of plastic containers when cooking/reheating. I use glass or ceramic instead.
Paul,
Off topic, but hopefully useful to some readers. From CNN: “Doctors slash patients’ lab-test costs” is about a MD who did something about the need for low cost lab tests for folks without adequate health insurance. It’s at http://www.cnn.com/2010/HEALTH/12/08/low.cost.lab.tests/index.html?hpt=C2
One doc said it’s like an Amazon.com for lab work and they are at http://www.prepaidlab.com/
Hope this helps someone.
Hey GeeBee,
I’m with you on the plastic issue. Our options are narrowing though: the new Consumers Report has a long article about how mishandling glass containers can result in dangerous breakage, including projectile fragmentation. We’re talking about the familiar brands here. Some incidents have been reported in which no mishandling was involved.
Thanks, Jim. I’m sure some readers will find this useful. From the CNN story:
For example, a lipid panel (cholesterol test) in Lefton’s area can cost as much as $148 for an uninsured person. The same test is available for less than $18 through the site.
Here’s how it works: Patients needing lab work can go to the medical society’s website and click on the big yellow box in the middle of the page. From there they choose the tests their doctor says they need, give the doctor’s fax number, pay with a credit card and print out the order. They then take the order to any LabCorp location in 47 states and have the work done. Results are sent securely to the patient and the doctor, often within 24 hours.
“It’s like using Amazon.com to buy your lab tests,” Lefton said.
Hi Paul,
I don’t think you can blame them for extracting the flavonoids. If the sample looked “anything like food” by the time they injected it, they’d probably ruin their column and maybe their detector.
At a minimum you need to get rid of the proteins, unless you have a column designed to separate proteins, because they are so big they’d clog up and destroy most columns. The method has to sufficiently purify the analytes, be able to mathematically account for any changes in concentration from the original sample to the final injected preparation, and be delicate enough not to destroy or alter the analyte in the process.
When we measure vitamin E, for example, we have to saponify the sample with potassium hydroxide, then extract it with hexane, then evaporate the hexane, then redissolve the sample in methanol/ethanol. Numerous additions like vitamin C, BHT, and nitrogen gas are necessary at various steps to protect the vitamin E from the rough processing. If we didn’t saponify, we would miss a lot of the vitamin E bound up in phospholipid membranes and our results would dramatically underestimate the true value. If we didn’t add the antioxidants and didn’t dry the hexane under nitrogen, the vitamin E would oxidize. If we didn’t attempt to purify the vitamin E with a few other fat-soluble substances, we’d destroy an $800 column and a $3000 electrochemical detector.
Although it’s possible their method is damaging the analytes somehow, you’re probably best off trying to figure out what the microwaving is doing to the analytes under these conditions. The flavonoids can’t just be disappearing. Is their solubility in methanol changing? Are they dimerizing? Are their attached sugars being hydrolyzed? Most likely, they are undergoing some type of chemical change and the end product is something that the researchers aren’t measuring. For example you quote them as measuring quercetin 3-rutinoside but not plain old quercetin. I don’t have time to read the paper or familiarize myself with the background, but in all likelihood the flavonoids are undergoing chemical changes that may or may not affect their biological activity, and the biological activity of whatever they are getting converted into might be woefully understudied.
Hope that helps,
Chris
Hi Chris,
Although procedures like these may be necessary in liquid chromatography, they seem to be incompatible with preservation of these particular compounds.
Why did all the water samples show 0.0? Other papers show that roughly 50% of the flavonoids are extracted to water under any cooking method. So we know that the “water” samples from all four cooking methods all began with flavonoids / sinapic acids and all lost them (or the tested derivatives) in the course of sample preparation.
Like you, I don’t have time to try to figure out what went wrong; even if I did, I don’t have the expertise or the equipment to run experiments. I’m guessing it wouldn’t be easy, else the peer reviewers or the authors themselves would have done it.
Since this was apparently part of a group of 3 papers and came out a year after the other two, I’m guessing they had some trouble getting it published. I’m sure if they could easily have fixed their methods, they would have. So it seems that procedures for liquid chromatography just aren’t consistent with preservation of these nutrients.
As you say, it is a likely explanation that some chemical changes took place that rendered the nutrients undetectable. I think if they could figure out what happened in the water samples, they’d probably know what happened in the edible food samples to make microwave cooking look inferior.
Best, Paul
Hi Paul,
That makes sense, but I think one explanation could just be what they were measuring. I’d have to look at all the papers and don’t have time for it, but if other papers finding them in the water were measuring different flavonoids, that could be the problem. So it could be procedure or it could just be interpretation.
Chris
I have a recipe that specifically calls for evaporating the filtered liquid in a vacuum. I’m at a loss for how I should substitute that step during preparation. :/
I hate to pipe up as the cynical voice, but do we have an idea how clear these pro-microwave papers are of connections to the microwave industry?
I gave up having a microwave when I moved from a house with a built-in mic to a house with very little counter space, at a time when I was kind of feverish about getting rid of any potential toxic elements. I decided I didn’t want that thing blasting me and my family at short range multiple times a day, and that frankly, I couldn’t trust that the microwaves were contained. Not to mention uncertainty over what they do to the food.
So we’ve been quite happily microwaveless for many years now. The only thing I missed at first was reheating food easily, but now we store leftovers in glass containers that can go right into the oven, so it’s just as easy. Don’t miss it at all.
Hi David,
Overnight on the moon should work just fine!
Hi Nell,
Now that we have cell phones and WiFi it looks like there’s no getting away from microwaves. Hopefully they’re harmless … I guess we’ll know in 50 years.
Paul said: Although procedures like these may be necessary in liquid chromatography, they seem to be incompatible with preservation of these particular compounds.
As someone who works with the R&D department of our company on a daily basis, and have to use their data for my reports, and as a pharmacist with some experience with validation of analytical techniques, I can tell you with fair certainty that you are way off. The simple proof of this is that the studies you quoted in support of your position on microwave heating used similar analytical techniques and their recovery data was good (i.e. didn’t destroy the flavonoids). So, this process does not necessarily destroy anything. Maybe all studies presented accurate data. You can’t pick and choose which is reliable based on which study agrees with the position you’ve taken.
You said: I’m guessing they had some trouble getting it published.
Come on, Paul. I understand that you are just trying to justify your stance again, but that’s somewhat below the belt. Sometimes we’re just better off saying, “I don’t know,” which you used to admit too more often in the early days of your blog. I still think that you have a terrific diet (and the book is great too), but since the attack on LC/VLC diets with a mixed of good science and really questionable science (two people dying of stomach cancer while being on Dr. K’s diet is not an epidemic…no matter what a commenter on a blog says, etc), I’ve started double checking a lot more facts than I used to previously. Be fair. Be as impartial as you can. Just saying.
Great article. But it addresses only this particular paper. I think this whole fearing of Microwave arises because people don’t understand how heating happens in normal cooking methods. They get afraid of hearing the words electro magnetic radiation.
Lets talk about just heating the old way in a skillet.
So how does skillet transfer heat to food.
It uses what is called black body radiation. When we heat the skillet, it radiates the energy in multiple frequencies depending on the temperature. The higher the heat the higher the frequencies.
Food absorbs these electro magnetic radiations or photons. The absorption of these radiations causes the Food to get cooked. Light is actually more energetic than infrared which in turn is more energetic than microwaves.
Same with fire. Fire is actually created when the air gets hot enough to radiate in the visible spectrum. That is why fire is mostly red, because red is the least energetic of the visible frequency. To get a blue fire the energy in the fire will need to be very high.
Something similar happens in steam or boiling. The water molecules will radiate their energy (which mostly lies in the 2.4Ghz, same as the Microwave).
So how is microwave different. Microwave also uses the same electro magnetic radiation as the other forms of heating. Actually the only difference between microwave heating and other forms of heating is that microwave heating uses a single frequency, and provides a lot more power in a very short time.
The other problem is that you have no access to the food item and you cannot turn it around. It is like a skillet where you have just covered the food, and you do not touch it while it is cooking.
Notice if you cook food in the skillet at high and you don’t turn it around the food gets burnt. But it does not get burnt easily in microwave. The reason is that skillet can provide a lot more power than a microwave. Actually microwave provides much more even heating than possible in a skillet.
I think that Microwave is a much more healthy way of cooking compared to cooking in a skillet or over fire. The only methods which are better than microwave are boiling and steaming. Grok used to cook over fire. If he survived that, we will survive the microwave.
I was thinking that we need a response on this topic from Robert Mcleod, the physicist at entropyproduction.blogspot.com.
Paul, I just remembered that you are a physicist as well.
Why don’t you do a writeup with the physics behind cooking it might be a better argument.
Hi Poisonguy,
Well, there’s still a lot I don’t know, and I hope I acknowledge it. 🙂
I often speculate on this blog, and I hope people are able to recognize when I’m speculating. Usually when the sentence begins with “I’m guessing” that’s a clue.
In regard to this paper, it’s clear that their procedure removes a lot of flavonoids. As researchers, you have to hope it removes the same fraction from all your different samples, so the proportions remain the same. But simple arithmetic shows that if all samples started with something, but most samples end with 0.0, then the proportions DID NOT remain the same!
So it’s clear proportions did not remain the same across samples, and therefore there’s no reason to think that microwave cooking reduced flavonoids by 97%. What’s not clear is WHY differing amounts of nutrients were lost in the different samples.
I’m sorry you think of my criticisms of very low-carb diets as an “attack.” It’s not that emotional for me. I ate very low-carb for years and wanted it to work. But it didn’t, and I reluctantly had to accept that some carbs are beneficial. As I studied the science, it became clear that glucose is a very important nutrient.
My posts in the zero-carb dangers series are speculative / hypothetical. We have mainly anecdotal data and case studies to work with at this point. But all science begins as observation, hypothesis, and speculation.
Thanks for your comments and if you find any mistakes in your double-checking, please let us know!
Best, Paul
Hi Anand,
Thanks for your contribution. By the way, Robert McLeod did leave a comment above, here: http://perfecthealthdiet.com/?p=1315#comment-10098.
I actually discussed the physics of microwave heating in my original draft, but the post was very long so I removed it.
Basically, the photons have inconsequential energy, so the behavior is classical: oscillating electric fields cause molecules with a dipole moment like water to rotate, gaining kinetic energy which via collisions is diffused as heat.
Since water molecules are heated far more than any molecules that are in solid form or that lack a dipole moment, heating is uneven. This means the kinetic energy distribution is not Maxwellian and some molecules may be high in energy — high enough to break bonds.
But even under regular cooking, heating is uneven and the Maxwellian distribution produces some high-energy molecules. So the situation is not dramatically different from other cooking methods.
Best, Paul
Thank you Paul for covering this topic. I use microwaves to reheat my food too, although reluctantly:)
I really don’t know much about physics or biology for that matter, so I was wondering what about claims like these that I’ve seen quoted everywhere:
“microwaves damage the cell wall of foods to such a degree that the gut receptors are not likely to recognize microwaved food particles as food, resulting in an immune response.”
&
“carcinogens were formed from the microwaving of nearly all foods tested”
“microwaves are capable of extensively fragmenting and destroying viral DNA, something that cannot be accomplished by heating alone”
Is it complete nonsense or could there be any truth to it?
Hi Mia,
The things you quoted don’t make a lot of sense:
1. Gut receptors don’t bind to cell walls, they bind to individual molecules. Animal cells are fully digested to fat and protein, while the immune system doesn’t react to plant cells in the gut which never get past the intestinal barrier. We want an immune response to food that gets in the wrong places. It’s not the case that the immune system distinguishes between food and foreign things; food is a foreign thing.
2. Carcinogens are present in high quantities in non-microwave foods. Since two-thirds of all known compounds are carcinogens (by the Ames test), anything that modifies food will “create carcinogens.” This doesn’t mean it’s creating danger. In fact, cooking of any kind, including microwaves, at low temperatures reduces the carcinogen content of food by making it more digestible.
3. Heat can destroy viral DNA, so all forms of cooking can. Fragmenting and destroying viral DNA is desirable. So if microwaves are better at it, that’s a plus.
Poisonguy had a pretty good list of the anti-microwave literature in this comment: http://perfecthealthdiet.com/?p=1315#comment-10074. Aside from special situations, like human breast milk for a baby, it appears that microwave cooking is, like other cooking methods, largely beneficial — it helps us digest food.
I think there are better things than microwaves to worry about.
Thanks Paul!
I’m glad I don’t need to throw away my microwave just yet:)
From the procedure I am not understanding the intent of doing all these. First of all the brocolli is cooked for 5 mins immersed in water and then removed to be engaged in the further process. Had the water been checked,the lost content should have been identified at that stage itself.
I believe that staying away from prepared foods could be the initial step so that you can drop some weight. They could taste fine, but ready-made foods have got minimal vitamins and minerals, making you eat more just to have sufficient vigor to get over the day. If you’re constantly consuming these types of foods, transitioning to whole grains and other complex carbohydrates will assist you to have more electricity while ingesting less. Thanks alot : ) for your blog post.
Reading about gelatin and bone broths I came across this:
http://www.westonaprice.org/food-features/why-broth-is-beautiful
Whatever form of gelatin is used, it should never be cooked or reheated in the microwave. According to a letter published in The Lancet, the common practice of microwaving converts l-proline to d-proline. They write, “The conversion of trans to cis forms could be hazardous because when cis-amino acids are incorporated into peptides and proteins instead of their trans isomers, this can lead to structural, functional and immunological changes.”
I’m unable to access the original paper:
Lubec, G, et al. Amino acid isomerisation and microwave exposure, Lancet, 1989, 2, 8676, 1392-1393.
There has also been a discussion about it on PaleoHacks, but no one seems to be sure about the issue.
http://paleohacks.com/questions/66890/bad-to-heat-broth-in-the-microwave#axzz1b4DwLAHh
What is your opinion?
Hi Ole,
I’ll reserve that one for a future blog post. Can’t say anything specific without reading it, only that I microwave my bone broth and will continue to do so. Proline is mainly not present as isolated amino acids, and I strongly doubt that amino acids in protein are modified in such a way.
kudo!
being also a physicist, i have had a hard time believing that microwave is more harmful than others.
did you say that we should try to get “biggest bang for our money” first?
for me, excessive PUFA is the hardest to minimize cause it’s everywhere for a person who can’t avoid eating out. (harder than gluten).
regards,
Hi Pam,
Microwaves are very convenient so it would be hard to give them up. And they seem pretty harmless.
I agree about the PUFA! The best thing is to cook at home, but not everyone can do that.
Dr. Jaminete,
your reply comes so quick!
these are things i don’t do enough.
#1. get enough sleep!
#2. stress reduction (simplify my life)
#3. avoid PUFA
in order to avoid PUFA (+ gluten, HFCS), that means bringing leftover to work & microwave it. i prefer warm food in winter so food has to be heated)
naturally, following your advice, i’d rather focus my “money” on biggest bang (avoiding PUFA, since #1 & #2 are sometimes out of my control, alas!
stay healthy & warm,
Hi Paul,
One study that disturbed me a while back regarding microwave cooking was this one: http://www.springerlink.com/content/0yblxvnrnhmd8p4e/
I was unable to access the full paper, but the abstract stated:
“Significantly higher amounts of polar compounds, of triglyceride oligopolymers – known to have adverse effects on human health – and of oxidised triglycerides were found in microwaved than in conventionally heated oils, indicating a higher extent of oxidative degradation.”
Since reading that, i have avoided microwave cooking anything with more than a marginal fat content.
I posted about it on Tom Venuto’s blog http://www.burnthefatblog.com/archives/2011/07/what-science-really-says-about-microwave-ovens.php and he seemed to dismiss the study as an anomaly, but said that high PUFA foods do not fare well with ANY high heat cooking. I’m aware of this, and i avoid high PUFA intake anyway so thats a moot pint. However the oil used in the study was olive oil, so its not particularly highly unsaturated.
Hi Rob,
Thanks, I’ll look into that paper.
I’m not too much concerned about loosing nutrients with microwave heating, but do you think lowering the microwave settings would be better in terms of AGEs creation in the food?
How do you prefer to cook meat, for example?
Thanks for your post to clarify the facts. When I stumble on the claim, I wanted to do what you just did since I too, feels that what really matters are in the details of how the experiments are done. You did a wonderful job.