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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.

Comments
  • 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.

  • 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

  • Alligator May 7, 2010 8:35 am

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

  • 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?

  • 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.

  • 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.

  • 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.

  • 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….

  • 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?

  • 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

  • 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?

      • 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

      • 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

  • philomina udofia September 2, 2016 7:21 pm

    can breakable plate be used to warm food in microwave

  • Derrick September 10, 2016 8:11 am

    Hello, thanx for that insight.

    I’ll like to know first of all, is it ok/safe to heat food in a microwave without covering it and secondly can you use ceramics in microwaves to reheat food?

    Thank you.

  • Nicole March 2, 2017 8:08 am

    I’m just wanting to ask a question that may seem silly, however I am newly pregnant and our microwave spraked while on… we have replaced it after seeing that some of the paint had come off in an area on it which must have only just happened as have used this microwave for quite some time with no issues, but with the scratch sized amount of paint missing the metal under the paint was exposed causing a large spark, being in the first stages of pregnancy am now concerned if that could have any serious or negative effects to our unborn baby? Deeply concerned mother to be and am hopefully just stressing with it hopefully not being necessary….please respond when can! Thank you

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