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Microwave heating of food-safe or unsafe

Whenever I tell people that my PhD thesis was on microwave heating of foods, they look at me with awe and ask endless questions. One of the common question is simple but very touch. They ask me whether warming food in a microwave oven is safe. I respond with resounding Yes.

Microwave oven is a modern convenience appliance in every kitchen in the developed countries. However, it is not the case in many homes in developing countries, but the use of microwave ovens is picking up at a tremendous rate. All of the users of microwave oven are always interested in knowing how it works.

In this article, I will outline how microwave oven works.

How Microwave Oven works
Microwave oven uses electromagnetic waves called micro-waves to heat food. No wonder the name of the appliance is; microwave oven. The micro-waves oscillate at a very high speed, normally 2450 times per second.

When food is placed in a microwave oven, various food ingredients behave differently. The main ingredient that enables food to be heated by micro-waves  is water. The higher the water content in food, the faster the heating rate. Water in molecular level behaves exactly like a magnet. Water has two oppositely charged ends due to presence of positively charged 2 Hydrogen atoms and a negatively charged Oxygen molecule. Therefore, water in food behaves like a magnet. If a bar magnet is held above another bar magnet, and you rotate the held magnet,  the other one also rotates. Similarly, due to two different poles in water, when micro-waves oscillate the water molecules rotate. This is because the negatively charged end of water is attracted to positively charged end of micro-wave, while the positive end of water is attracted to the negative  charged end of micro-waves.

The micro- waves rotate at extremely high speed of 2450 time per second. This means for every second a micro-wave rotates,  the water molecule also rotates 2450 times. This extremely high rotation rate causes water molecules to collide with each other at very fast rate. This creates friction between water molecules. This friction generates heat. The heat flows through the food by conduction, convection or radiation. Therefore food warms up. That is simply how microwave heats food.

Does micro-waves remain in food after warming food?

No. Microwave are never embedded in food therefore microwaves are not left in food following warming of food.

Is Microwaved food safe?

Yes. Microwaved food is safe for human consumption.

Common uses of microwave oven
-Thawing of frozen foods such as meat, bread etc.
-Warming ready-to-eat foods.
– Warming chilled foods

Foods that should not be heated in a microwave.
Some foods explode or generate sparks when heated in microwave ovens.

Foods that explode when heated in a microwave include eggs. So, they should not be heated in a microwave.

Items that should not be used in a microwave

Never put any metallic object e.g spoon or metallic dishes in the microwave oven. Metals reflect micro-waves and hence generates sparks.

Use only microwave-safe plastics when warming food in a microwave. Some plastics when heated in a microwave, the compounds leach into the food and maybe dangerous to health

Can micro-waves leak through the glass door?
No. The glass door helps to see inside the oven while it has a mesh to prevent micro-waves from leaking out.

Questions.
If you have any question, please feel free to leave it in comments section below.

{ 53 comments… add one }
  • Elda Tata Masake May 6, 2010, 6:58 pm

    Good information. I'v always wondered if warning food via microwave was safe. Atleast u'v addressed the questions i used to have but never asked.

    • Joshua Arimi May 7, 2010, 9:31 am

      You are welcome Elda Tata. Actually many people still wonder if microwave oven is safe. If you know any of these people, link them to this article.

  • Japhet Muchai Mutungi May 7, 2010, 6:57 am

    Very well simplified and applicable. Interpretation for the layman facilitated. We all been at some point unsure of the safety of microwave warmed foods. Well done

    • Joshua Arimi May 7, 2010, 9:38 am

      You are welcome Japhet. I tried my best to make it as simple and understandable as possible.

  • Alligator May 7, 2010, 8:35 am

    Does heating food using microwave lower the nutritional value of the food?

    • Joshua Arimi May 7, 2010, 9:36 am

      Hi Alligator, thanks for your question. Microwave has been shown to affect some specific compounds in some foods e.g. allicin in garlic. But, so far there is no evidence to show it affects the main nutrients in food. Mirowave warmed food can get dry on the surface but generally nutrients remain fairly unaffected. Actually some microwave heated food retain more nutrients due to short heating time than conventionally boiled foods.

  • beth May 17, 2010, 9:08 am

    A querry
    some people sat that microwave cause cancer when you warm your food often.How true is this?

    • Joshua Arimi May 17, 2010, 10:22 am

      Thanks Beth for your question.
      Warming food in a microwave does not cause cancer. There is no evidence to show that, but people continue to think that way. People think micro-waves get absorbed and embedded in food. Micro-waves are never embedded in food and hence people do not consume any radiation by eating microwave warmed food.

      However, never expose any part of your body to direct microwave radiation. The way microwave oven is built, with metallic walls it is safe. However, if your microwave oven breaks down, especially the door. Throw it away. Do not attempt to repair it. This is because if the microwaves leaks and get intouch with your body, it is extremely lethal.

  • Steve May 29, 2010, 5:26 am

    Very well simplified and applicable. Interpretation for the layman facilitated. We all been at some point unsure of the safety of microwave warmed foods. Well done

  • HILDAH June 24, 2010, 9:39 pm

    thanx for the advise joshua, it has solved all the question that i had.

  • Elda Tata Masake June 27, 2010, 10:02 pm

    Hi Joshua! This is very good information. Quick question, why is it that, so it seems to me, that microwave warmed food cools faster than open fire warmed food?

  • Joshua Arimi June 27, 2010, 10:42 pm

    @Elda, quite interesting. The most plausible explanation reason could be that, in a microwave, it heats mainly water (mark you boiling temp of water is 100C).Once at 100C there is minimal increase in temp due to latent heat of vaporisation. Once all the water has been heated and evaporated, it is normally difficult to heat the food further. On the other hand, in open fire/hot plate, with or without water food will continue to heat. Therefore food in microwave can generally achieve lower maximum temps compared to open fire/hot plate foods.Normally it is advisable that If you warm food in a microwave to piping hot, leave it in there for a few minutes for heat to distribute evenly. This way you minimise the chance of some hot and cold spots in your food which could contribute to cooling effect.Another reason for quick cooling is surface area. Normally you warm small quantity of food in a microwave compared to wedding cooking sufurias. The smaller the amount of food, the larger the surface area to lose heat.I hope this provides some explanation. If not satisfied, please do not hesitate to pose another question.

  • Joshua Arimi June 27, 2010, 10:42 pm

    @Elda, quite interesting. The most plausible explanation reason could be that, in a microwave, it heats mainly water (mark you boiling temp of water is 100C).Once at 100C there is minimal increase in temp due to latent heat of vaporisation. Once all the water has been heated and evaporated, it is normally difficult to heat the food further. On the other hand, in open fire/hot plate, with or without water food will continue to heat. Therefore food in microwave can generally achieve lower maximum temps compared to open fire/hot plate foods.Normally it is advisable that If you warm food in a microwave to piping hot, leave it in there for a few minutes for heat to distribute evenly. This way you minimise the chance of some hot and cold spots in your food which could contribute to cooling effect.Another reason for quick cooling is surface area. Normally you warm small quantity of food in a microwave compared to wedding cooking sufurias. The smaller the amount of food, the larger the surface area to lose heat.I hope this provides some explanation. If not satisfied, please do not hesitate to pose another question.

  • Elda Tata Masake June 28, 2010, 6:24 am

    Thanks alot. Your explanation is very elaborate.

  • pradeep August 6, 2010, 12:27 pm

    I am eager to know,what kind of waves are there in microwave ovens.

    • Joshua Arimi August 7, 2010, 9:43 pm

      Thanks Pradeep for your question. The waves in Microwave oven are called microwaves. Their frequency range from 300MHz-30GHz with a wavelength of 0.3 -30 com.

      I hope this answers your questions. If you need more help, please feel free to contact me.

  • Kilungu September 1, 2010, 3:08 pm

    Well good information. It demystifies many theories, rumors and speculation around microwaves in a very simplified manner.

  • Eve September 9, 2010, 9:21 am

    I would like to know the plastics that are recommended for microwave use, most of the ones sold in Kenya do not have riders indicating if they are safe

  • beaty obinge November 4, 2010, 11:53 am

    thanx 4 ur gud teachings Karimi its today i came across ur web n the whole day i hav been there ,the next thing is 2 go practice my many questions hav been answered but pliz tell me how safe it is to use a pressure cooker n if it realy has effects on the tast of food be blest.

    • Joshua Arimi November 7, 2010, 10:52 pm

      Hi Beaty,
      Pressure cooker is safe and rarely affects the taste of food unless if you overcook. Pressure cooker just increases the pressure within the cooking pot and lowers the boiling point of water hence allowing food to cook over a short period.

      All the best.

  • Lani November 12, 2010, 6:42 pm

    Joshua,
    Will reheating food in a microwave more than one time change its nutritional value (damage) the food ? How does reheating food in microwave compare with doing it in a conventional oven.
    Thanks for a nice explanation on microwaving food.

  • Bindu raj November 24, 2010, 9:56 am

    Please provide me the following details?

    Type of containers used for warming food?

    Is lid is necessary at the time of warming, If yes why?

  • Dnyanesh January 4, 2011, 10:15 am

    Hi sir,
    I am working on “Industrial Microwave Conveyor Dryer” using 208 nos. magnetrons of 1.5KW each(National make).I want to know how to fix the size of cavity and waveguide? Each cavity will acomodate 16 magnetrons.

    • Joshua Arimi January 22, 2011, 1:56 pm

      Hi Dnyanesh,
      Unfortunately, I am not an Engineer, I am a Food Engineer, thus I do not design equipments but just a use them. I regret that I might not be in a position to advice you.
      Good luck in your search for the solution.

  • reshma January 19, 2011, 3:06 pm

    does food from microvave effect me r my child during pregnancy and moreover will the rays from microwaveoven effect the foetus(does standing infront of the foetus effect the developing foetus

  • yvonne March 7, 2011, 6:30 am

    I love to get a answer to this problem.. i have use microwave oven for 10 years ..and never had any problem’s..my microwave LG is about a year old..one day i heat up some soup and it had a terrible taste..like a mix of chemicals and metallics..hard to explain..i trew the pot were i made the soup away..with the soup..thinking maybe it was the aluminum pot.. and i continue to use the microwave over until one morning i use it to heat a cup of coffee and there it was again..that taste..i realise it was the microwave..and when i open the door and look at the back and smell were the motor is i could smell what i have been tasting ..I don’t use it anymore and i might not buy another one…what do you think is wrong….

    • Joshua Arimi March 13, 2011, 10:55 pm

      Hi Yvone,
      Thanks for your question. It seems the microwave might have had a problem in the magnetron that generates microwaves. The main issue could be with the age of the microwave. With time the performance decreases and also they can malfunction. 10 years seems are very long period for it to still function properly.

      I hope this explains the effect you saw.

  • John August 5, 2011, 8:06 pm

    Dear Joshua,

    I use microwaves in a laboratory to replicate Pasturised and retorted products on an industrial scale. How do microwaves’ heat transfer in oily/creamy products like salad creams compare to less creamy/higher water containing products like tomato sauces?

    • Joshua Arimi August 14, 2011, 7:15 pm

      Hi John,
      Water is dipolar compared to oil. Therefore heats better in microwave than oil. But oil has lower heat capacity compared to water. Therefore temperature rise in oil products might also be very fast. You can read further in a book by Buffler.

  • kassim August 22, 2011, 9:50 am

    Hi joshua,
    just wondering if its advisable to leave the food open when its in the microwave…some people tend to think that if you leave it open the radiations might affect the food..
    please clarify,
    thanks
    kassim

    • Joshua Arimi August 23, 2011, 9:41 am

      Hi Kassim,
      First, there is no radiation in the microwaves.
      Second, it is advisable to cover the food while heating in a microwave. This is to reduce soiling of the microwave oven cavity by the bumping food. It also helps in uniform distribution of heat by trapping steam. There is no radiation in the microwave oven, therefore, the idea of radition doesnot come in. The mechanim by which microwave heats food is not by radiation but by dipole rotation.

  • Ann November 8, 2011, 9:30 am

    I like your explanation about microwave food and its safety. That’s reassuring but i wish you would include in your web an opportunity to share this information with our friends on facebook. I love all the articles you have as i love reading health stories.

    Ann.

  • Elizabeth Anderson December 4, 2011, 9:54 pm

    Does the surface area of the food affect how quickly it is heated? In other words, will a greater surface area equal faster heating rates?

    • Joshua Arimi December 31, 2011, 7:55 am

      Hi Elizabeth, Definitely, surface area influences the rate of heating.

      • Elizabeth Anderson December 31, 2011, 9:16 pm

        thank you

  • Chandan December 25, 2011, 2:56 am

    Dear Joshua,
    Very interesting, basically I am a mechanical engr. now doing PhD in food drying. And today I go through your website and so absorbed in it that forgot my lunch.
    I have some query, In drying of food convection drying takes longer because at later stage of drying the moisture needs to be diffused from inside to the surface. So I am thinking incorporating microwave to increase diffusivity by heating up the food.
    now my question is if I apply microwave at later stage of drying when there is no moisture at the surface will the microwave damage the surface ?
    Or only the part where moisture is reside that part will heated up ?
    My writing is not very good. thanks for patients. I will be very happy if you answer and give me chance to make contact with you.

    Thanks
    Chandan
    QUT

    • Joshua Arimi December 31, 2011, 7:53 am

      Hi Chandan,
      It will depend on the type of food that you are drying. I know microwave is used to assist drying but it requires optimisation to avoid damaging (burning) the surface of the food being dried. Microwave heats by volumetric heating i.e. it heats the inside faster than the surface. Probably this would allow it to evaporate the moisture without affecting the surface. I would suggest you look at a few publications on microwave assisted drying from science direct and they will offer you more info. All the best.

      • Chandan January 2, 2012, 1:18 am

        thanks

  • rezania December 25, 2011, 6:46 am

    Does the surface area of the food affect how quickly it is heated? In other words, will a greater surface area equal faster heating rates?

    • Chandan December 28, 2011, 3:37 am

      Yes rezania,
      greater surface area definitely increase heat and mass transfer rate thus faster drying.
      Thank you

  • wan amin April 9, 2012, 6:49 am

    Dear Dr Joshua,

    Where can i found your thesis about microwave. I want to use it as my reference on my research about heating swamp eel in microwave. Hope you can reply it.

    Thank you.

    • lucy December 19, 2012, 8:01 am

      can’t write proper English yet you want a Phd Thesis. Sign up for an English for the average class first. nktest

  • tenny awosaya September 3, 2012, 9:55 am

    Hi Dr joshua,I wanted to ask if is good to buy used microwaves oven from different country to nigeria.And the particular one my mum bought was stated do not cover so don’t quit understand what it means by do not cover.thanks for your favorable answer.tenny

  • Lynn Lind September 13, 2012, 8:27 pm

    Thank you for this article on the internet. I have one friend who refuses to eat any food that is cooked (or reheated) in a microwave. Then I saw on one of my favorite TV shows “Perception” where the Ph.D. says, would I eat irradiated food? Meaning, of course, that he would not. This brought the question to mind. Is food reheated in a microwave the same as what I read about on the internet called “irradiated food”?

  • mark October 15, 2012, 2:12 pm

    If the microwave leak out the microwave radiation and that his dangerous to our health so how can the microwave be good to warm food when the microwave are lethal to our body

  • by Laura December 23, 2012, 7:15 am

    Is it safe to microwave open foods(foods without a cover/lid)?

  • TE Streeter January 12, 2013, 10:06 pm

    Is it safe to heat food in styrofoam? I have heard styrofoam gives off toxic substances when microwaved

  • karan January 24, 2013, 1:18 pm

    application of microwave heating

  • soner July 3, 2013, 10:59 pm

    hole diameter should be how to not come out of the oven wavelength?

  • Carol Mutahi July 5, 2013, 2:53 pm

    Very good information from one of our own.What bothers me is that if you have two plants and water one with tap water and another with water boiled in a microwave and let to cool,the later dies after 3 days.Please shed some light on this.

  • annet February 21, 2014, 1:40 am

    is it true that food warmed in a microwave increases stress and cholesterol in one’s body?

  • prosper emagboron August 2, 2014, 12:53 pm

    what kinds of plate to be use in microwave that prevent the radiations that is causing infect on food and body.like breakable firmly cover.or plastic plates

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