Imagine I am preparing food -- just about to put it into a microwave oven -- and some of it falls on the floor. Assuming it got some bacteria or other organisms (viruses?) on it, will the microwave processing disinfect it? What about worm eggs? As I understand, any living organism, given enough exposure to microwaves, degenerates. Is this true?

  • $\begingroup$ That amount of usual pathogen does not matter unless you clean up the floor with food... Yes microwave will disinfect the food, it is used for pasteurization. $\endgroup$ – inf3rno Oct 17 '15 at 9:02
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    $\begingroup$ cooking.stackexchange.com/questions/32055/… $\endgroup$ – Nathan Oct 19 '15 at 19:39
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    $\begingroup$ A little off topic, but I'll share anyway I have some old artifacts of wood with evidence of active worms. After putting them in a 1200 watt oven for 10-12 minutes all evidence of wood worms disappeared, that was several years ago. My assumption was that the water in the live worms was heated to a lethal temp. Obviously size was a constraint, although a commercial oven should be easy to find. Not for painted objects probably. $\endgroup$ – Greg Whittingham Jul 21 '17 at 21:09

Microwave ovens can indeed kill bacteria in food by heating them to high temperatures. For example, this article found that microwave heating could kill all of the Salmonella bacteria in a chicken thigh sample:

The effect of microwave heating on Salmonella Enteritidis inoculated on fresh chicken was investigated using a microwave oven (800 w) to determine the destruction of Salmonella Enteritidis isolated from chicken carcasses, in relation to the time of heating at two power settings: high (power level 10) and medium (power level 6); The relationship between heating time and temperature was also been studied. The destruction was 6.4 log cycles at time 95 sec for the high power level, and 5 log cycles at time 140 sec for medium power setting. After 110 sec for higher power level, no survival of Salmonella Enteritidis was detected in samples (100g), but at 140 sec for medium power level, these food pathogens were still present.

However, the main issue is balancing the amount of microwave exposure. Heating the food for too long will result in the food becoming dehydrated or burned, and heating it for too short a period of time will result in food poisoning.

This paper shows that insufficient microwave exposure can and will lead to food poisoning if there is a large amount of bacteria inside the food:

Leftover samples of a savoury rice dish consumed by all six ill persons contained 6×103/gm Salmonella enteritidis PT4. The rice salad comprised boiled rice, raw carrots, eggs, cheese and curry powder. The curry powder and remainder of the pack of six eggs were negative on microbiological analysis. The rice dish had been prepared by heating in a 500 W microwave oven with a rotating turntable on full power for 5 min.

  • $\begingroup$ So in essence, microwaves don't kill bacteria / viruses. Heating does. $\endgroup$ – Dmitry Grigoryev Sep 14 '20 at 8:13

I'm no expert, and this is more of a physics answer rather than biology. The microwaves will not reach all of the food. This is why you get hot and cold spots. The rotating plate mitigates this to some extent, but not all of the food will receive a blast. However heat will slowly spread throughout the food. If the contaminated bits are hot enough for long enough it should be OK, but I wouldn't rely on this.

  • $\begingroup$ Yeah, the warming of the food is extremily non-uniform, but I thought that at least the surface area gets enough radiation. And still - how much does it take to kill a bacteria? A colony of them? Is that heat that can kill them, or do microwaves affect them? ...and I bet if I asked this on Physics SE, they'd said, this one is for Biology :D.. Somehow my questions always fall in between SE sites :/ funny thing.. $\endgroup$ – noncom Oct 15 '15 at 18:37
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    $\begingroup$ Not even the surface will be covered. The microwaves don't have a problem penetrating the surface, the problem is due to the wavelength only some areas get their molecules excited by the waves. This explains it better than I can: what-if.xkcd.com/131 $\endgroup$ – Robin Salih Oct 15 '15 at 18:41
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    $\begingroup$ Can you provide citations to back up your claim? $\endgroup$ – March Ho Oct 16 '15 at 12:14

Keep in mind that microwaves kill organisms via heat, and in particular, heating mainly water. This is contrast to ionizing radiation, for example as obtained from the cobalt used in commercial food radiation.

If the pathogen does not contain, or is not sufficiently adjacent to liquid water, or is not sensitive to heat, it may persist.

As an explanation, a microwave generates heat because microwave oven peak frequency emissions (~2.45 x 10^9 Hz) (reference) are able to excite water molecules. The physics of that are actually quite complex, but at a high level, water doesn't really exist appreciably as H20. Rather, it is better described at STP as HOH (hydrogen hydroxide) or more accurately, H3O+ OH- (hydronium hydroxide). All those ionic and van der Waals' forces (reference) allow water to have a high specific heat (i.e. a small bit of water can contain a lot of heat). That water heat (via predominantly conduction) heats the rest of the food.

The importance of this is that in order for a microwave oven to sterilize anything, including food, both of the following must be true:

  1. The target must be raised to a sufficiently high temperature.
  2. That temperature must be maintained long enough to adequately disrupt the homeostasis or (in the case of a virus or prion) structure of the pathogen.

And these predominantly depends on water being present (though some fats and sugars can also efficiently absorb microwaves). If there is insufficient liquid water present, the microwaves would inefficiently heat the dry food, and as a consequence could fail to achieve a sufficient kill temperature. (In contrast to liquid water, ice has a much lower resonance with microwaves, which is why the defrost cycle of a microwave uses lower energy and switches on and off to allow heat conduction to even things out and minimize hot and cold spots).

Some pathogens would not likely be killed by a consumer microwave oven:

  1. Bacteria which form spores often require very high heat, above the boiling point of water, so in a microwave, those spores (e.g. Bacillus anthracis, Clostridium botulinum) would be unlikely to reach those temperatures (as additional microwave energy would convert the water to steam at STP, rather than raise its temperature).
  2. Prions are also notoriously difficult to destroy. In fact, when a patient with CJD has neurosurgery, typical hospital policy is to throw away the entire surgical instrument tray, because there is no adequate sterilization procedure available, period. (You're unlikely to find prions in your kitchen unless your name is Hannibal Lecter, but I include them for completeness.)

Finally, there are two main routes by which pathogens in food make you ill. The first is an active pathogen (e.g. live bacteria or replicating virus) acting within your body.

The second mode is that of preformed enterotoxins. While the OP asked about food on the floor (presumably of short duration), the title is directed to disinfecting food. From a food safety perspective, many common bacteria (most classically Bacillus cereus) produce toxins which persist even after they have been killed and the toxins can survive high heat. Microwave radiation and the expected heat produced would not destroy these molecules, and would not render the food safe. This would also apply for many chemical substances potentially present on a typical kitchen floor.

All that said, in 2014 microbiology research done at Aston University suggested that there is actually a decreased risk of eating food dropped on the floor if you follow the five-second rule.


Yes, you are right. If the food just fall on the floor and you microwave it, maybe that could kill most of bacteria or disinfect virus. bacteria are living organism, normally can't tolerate over 100 ℃, and virus' protein coats are not stable too.

However! maybe there are Spores which are generated from gram positive bacteria, the power of microwave or oven can't disinfect all of them. And also, the toxic chemicals on the floor, which might from your floor cleaners or be generated by bacteria, maybe microwave or oven are not strong enough to denature them. In biological lab, we use autoclave, 121℃ and 1.2 kg / cm² pressure over 15 mins, to kill bacteria spores. And it can't clean toxic chemicals. To be noted, there is not pressure function in normal housse using oven and microwave!

  • $\begingroup$ What do you mean by "spot"? Did you mean "spores"? $\endgroup$ – March Ho Oct 18 '15 at 1:55

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