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Medical Biochemistry point of view

My biochemistry teacher said today that the problem with micros is that they do not kill all harmful bacteria. He proposed boiling food instead.

I think there are two reasons. Water can emit heat to bacteria from many more different directions than microwaves can. The heat frequency is changing all the time to the bacteria, since water is moving. Radio waves can be applied only from discrete directions. To make microwaves better, I think reflection and different materials on the walls should be considered

I am not sure which one is the stronger reason why micros cannot make good food:

  • sequentially different waves - probably not
  • or heating bacteria from different angles by mirror/reflection - I think this is the main reason why boiling and oven is better

There at least two types of micros - wide ones and more vertical ones. I have had an intuition that the vertical ones can be more effective. They can send signals more broadly from the bottom, while the wide micros can send them only from one direction - left or right. Also, the reflection technique is easier to apply to those vertical micros, since the roof can be circular, while in the other boxes it is not possible.

Medical Microbiology point of view

Murray's book, Medical Biochemistry, says for different diseases, like Listeria's epidemiology that "Disease can occur if the food is uncooked or inadequately cooked (e.g. microwaved beef and turkey franks) before consumption.

This suggests me that there is some point of view why microwaved food is called "soft-food". I will add pieces of evidence here when I explore more.

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    Interesting question, Masi! Do you have any documented research that microwave ovens don't kill all harmful bacteria when used to cook food? Can you please include it in your question? Thanks!
    – Kristina Lopez
    Feb 19, 2013 at 18:49
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    I'm confused. If you actually cook food in your microwave (not just reheat it), well, you are cooking it. It's the heat that kills things, so if you heat it as much as you would any other way, why would it not kill things? None of the reasons you speculate about sound like they have anything to do with microwave food safety, really.
    – Cascabel ♦
    Feb 19, 2013 at 19:15
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    @No'amNewman that would apply to all cooking methods though - if you've got heat-stable toxins in your food, you can't get rid of them with any cooking method. The only thing I can think of with microwaves specifically is the potential for uneven heating.
    – Matthew Walton
    Feb 20, 2013 at 13:29
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    I also think this depends on the type of food. Are we talking about a solid protein piece where the center of it is probably fairly sterile or is it ground beef, soup, mixtures of meat and veg, etc.
    – Brendan
    Feb 20, 2013 at 15:04
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    I'm not sure if it's intentional or just a language barrier, but this question seems to be engaging in some very obvious circular reasoning, by first explicitly labeling microwaving as "not cooking" and then going on to speculate about how it is somehow different from "cooking". But microwaving is cooking, so the whole discussion is moot. In fact, microwaves cook by heating the water in food, so anything that steaming can do, so can a microwave. It's less even cooking, as one of the answers says, and you need to be careful of that, but it's still cooking.
    – Aaronut
    May 12, 2013 at 19:44

3 Answers 3

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There's an interesting article on The Straight Dope that tests the question of how well a microwave kills bacteria on pizza. Here's a few quotes:

If I take a piece of pizza that's been sitting on the table awhile and microwave it, would that kill the bacteria, or am I just eating nice hot bacteria?

Will a microwave kill microbes? Sure. Microwave ovens use electromagnetic radiation to heat water molecules in food. It's the heat, not the microwaves, that's lethal here; the hotter you make your food, the more likely you are to kill the bacteria in it. (Some contend microwave energy itself is fatal to bacteria, but that's unproven.) The key is making the food hot enough uniformly enough for long enough. If it heats unevenly, a common problem in microwaves, some bacteria may survive.

After running some real-world tests and examining their petri dishes, they concluded:

  1. Heating the pizza for 30 seconds was relatively ineffectual.

  2. Heating it for a full minute killed most of the bacteria but not all.

  3. We didn't go in for another round of testing, but suspect that at least two minutes of microwaving would be needed to ensure 100 percent bacteria eradication, at the possible cost of rendering the pizza inedible.

Checkout the full (quite entertaining) article here:
http://www.straightdope.com/columns/read/2947/do-microwave-ovens-kill-bacteria

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  • This answer has the correct idea here. There is no evidence that the microwave energy itself is fatal to bacteria. It is the water that is necessary. Assume you have had your pizza long time on table. This would cause a uneven loss of water. Some parts of pizza will be dry i.e. no water and/or water gas perfusing the pizza when heating so not energy heat provided to some parts of the pizza. The little water multifocally means much higher temperatures in multifocal parts i.e. much faster carbonation of the pizza.
    – Léo Léopold Hertz 준영
    Mar 19, 2015 at 11:22
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    @Masi but cytoplasm is 80% water according to wikipedia? Will it heat up, i.e will a bacteria boild from inside like we do?
    – noncom
    Oct 19, 2015 at 21:12
  • What about water molecules inside bacteria?
    – juanmf
    Feb 16, 2017 at 18:44
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I'll go ahead and take a stab at answering this, even though the question is a bit vague. I assume by "cook" you mean "cook with a non-microwave method", like boiling, steaming, baking, frying, sauteeing, or anything else.

First of all, no, I can't think of any reason why microwaves would be worse than any other cooking method. If you fully cook something in a microwave, it is as safe as fully cooking it any other way. If the food reaches the same temperatures, and is held there for the same amount of time, the bacteria will be just as dead no matter what the heat source is. The heat supplied by a microwave is fairly similar to that supplied by steaming, and no one claims that steaming is an unsafe cooking method. The only thing I can think that would be unsafe is if you don't actually fully cook the food, but just heat it to the temperature that you want to eat it at. But that'd be a problem for any cooking method; dangerous undercooked meat is dangerous because of the temperature, not the cooking method. (And of course, not all food needs to be cooked to be safe.)

The arguments you suggest don't really make much sense to me, either. For the first, I don't have any idea what you mean by heat frequency or sequentially different waves; heat is not a wave, so I'm not sure how to respond. No matter how you're cooking something, heat is being transferred into the food. Heat caused by absorption of microwaves is no different from heat supplied by contact with boiling water, steam, pan, or the hot air in an oven. It's still heat transfer, and it makes the temperature go up. Different methods may heat more or less evenly, or at different speeds, but heat is heat.

As for your second suggestion, reflection of microwaves, well, that happens already. You may have noticed that the food in a microwave ends up hot on all sides; this is especially noticeable if it's a big solid piece of food like a casserole. It'll be hot on the top, the bottom, and the sides. The microwaves are reflected around; there's really no other way for it to be. If they weren't reflected they'd have to either be absorbed (meaning your microwave would heat itself up) or pass through the walls (meaning standing next to a microwave would be dangerous). And it'd be horribly inefficient, on top of that.

Of course, all the stuff about reflection and heating from all sides is still a moot point in terms of food safety. When you cook something in a pan on the stove, as long as you fully cook it, it's fine, even though it's only being heated from the bottom. All that matters is the temperature the food reaches.

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    I think a standard microwave is something like a 12cm wavelength and this is why very small pieced of food (i.e. single kernels of popcorn) can be put in the nuker for several minutes without heating significantly. But heat is heat, microwaves just happen to be very effective on water and oil but if it gets to a sufficient temp I see no reason why bacteria won't be killed but the texture is probably going to be crap.
    – Brendan
    Feb 20, 2013 at 0:40
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    @Brendan Popcorn heats slowly because it doesn't have much moisture, I think. The wavelength does cause hot and cold spots in some microwaves, but they're not the full wavelength - all the reflections smooth things out - and turntables essentially make them a non-issue. With respect to texture, it completely depends on what you're making. You can even get crispy bacon.
    – Cascabel ♦
    Feb 20, 2013 at 1:36
  • I just finished reading the microwave section of the Modernist Cuisine at Home book and they were talking about popcorn specifically. They used it as an example because of all the trouble we all have with the last few kernels that never pop at the bottom of the bag. I can't link it since it's in a book by it's page 40 in the book, very interesting stuff actually.
    – Brendan
    Feb 20, 2013 at 2:30
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    @Jefromi, size matters, try cutting e.g. cheese in 0.5cm, 1cm, 2cm and 4cm pieces and heat in micro, they will heat differently.
    – Stefan
    Feb 20, 2013 at 2:45
  • @Stefan Okay, maybe size does matter here (possibly just because of heat transfer within the food), but if we want to talk about that, someone should just post another question, since it doesn't have anything to do with my answer.
    – Cascabel ♦
    Feb 20, 2013 at 3:42
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Microwave ovens do not cook food very evenly. This is improved by the turntable, but unless the food is stir-able and you stir it, the food will have hot and cold spots. Most people seem to overcook food and then let it rest for the heat to even out

Most other cooking methods are slower than microwave cooking, so this give time for heat to conduct through the food and give a generally even heat that given time will render most surface bacteria safe (not that you should rely on this). With microwave ovens, the high speed of cooking, the uneven heating, and also insufficient time for heat to conduct through the food without seriously over cooking your food, means you cannot rely on this

You should not cook food that requires heat to kill bacteria, it is always a non-perfect process, regardless of the equipment you use so it is risky, microwave oven or not

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    Actually, if you don't stir while cooking traditionally, you end up just the same. The only argument here is that stirring in traditional cooking is more common.
    – SF.
    Feb 20, 2013 at 8:54
  • @SF There are some traditional dishes, such as rice, were it is recommended not to stirr.
    – J.A.I.L.
    Feb 20, 2013 at 11:06
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    @TFD, that is why most microwave (and regular cooking) recipes usually include stirring at specific intervals in the instructions. Even my microwave lunches specify either mid-cycle stirring or a post-nuking rest in the microwave oven of 1-2 minutes to ensure even and thorough heating. I do agree with your comment about any cooking method being potentially risky (if not done properly).
    – Kristina Lopez
    Feb 20, 2013 at 23:07
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    @SAJ14SAJ Most forms of cooking take much longer than microwave ovens, so with them there is sufficient time for the heat to migrate through the food. Exception are like grilling a steak which shows slow heat migration, and usually a short cook time. The difference is the exterior surface of the steak is fully cooked, while in the microwave only parts of the surface will have been fully cooked (yummy, microwave steak!)
    – TFD
    Feb 20, 2013 at 23:16
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    @tfd Now I am picturing herds of migratory heat, stampeding across the microwave plains....
    – SAJ14SAJ
    Feb 20, 2013 at 23:21

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