Our electric kettle broke. So this morning I used the microwave to boil water for the first cup of coffee.

Only thing is - the water in south Africa is not really the best of quality when you get it from the tap. I presume using a kettle kills of some of the bacteria that is still in the water.

So my question is:

Is it safe to boil water in the microwave and then consuming it in coffee? What other complication might boiling water in the microwave bring other than from a bacterial point of view?

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    140F is enough to kill salmonella and staph but not enough to destroy the toxin that staph produces. Likewise, it will kill botulina but not the toxin that it produces. If you want to go further, you can heat your water to 240F in a pressure cooker. You may also want to invest in a reverse osmosis filter. Besides bacteria, local water can have things like mercury, lead, cadmium...even discarded prescription drugs. A simple RO filter can get your water's TDS (total dissolved solids) down to 10PPM or lower...that is, most all the rubbish is gone.
    – user36802
    Sep 10, 2015 at 10:58
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    @ChefBrooksie Oh, good. Now you really scared our OP...
    – Stephie
    Sep 10, 2015 at 11:07
  • 2
    @PorkChop The different tastes from different boiling methods are most likely due to the water being at slightly different temperatures. Boiling water is boiling water but the kind of container it's in will influence how much it cools between the time you stop heating it and the time you make coffee with it.# Sep 10, 2015 at 11:19
  • 4
    Didn't mean to scare people. Let me just add this and I'll shut up. I drink tap water as well as RO water. My tap water has a TDS of 240. One thing about RO water...it's essentially distilled meaning all the good minerals are stripped off as well as the bad ones. If you are in a hot climate and drink lots of water you may be better off with tap. You don't want to flush out all the minerals and salts your body needs. Ironically, the fancy bottled waters are often run through RO filtration first and then specific minerals are added back in.
    – user36802
    Sep 10, 2015 at 11:28
  • 2
    The different taste can also be depending on the vessel it was heated in, and the effects of the heating method on said vessel. Heated in a (metal) kettle where the (metal) kettle gets quite hot, you likely have a bit of metal dissolving into the water - not enough to be particularly harmful, but possibly taste-able (depending on what metal the kettle is made from in particular). You also likely don't clean your kettle as carefully as you clean your cup - so there are likely dissolved solids from the water it's boiled before in it.
    – Joe M
    Sep 10, 2015 at 20:23

5 Answers 5


There is one very different issue to be kept in mind - water in a microwave can overheat and "explode" once it is disturbed.

Another poster had exactly this problem a short while ago: Water exploded in Microwave

So follow the usual precautions, e.g. putting a wooden toothpick or a small, very clean stone (chemists have them in their labs) in your vessel. In a pinch, a spoon will do, but not all microwaves handle metal objects well.

If you are worried that heating your water in a microwave might not be sufficient to kill all "nasties", remember that killing bacteria is a function of time and temperature, so you might feel safer if you not only bring your water to a boil, but continue boiling it for another minute or so. I would assume that this is mostly for your psychological benefit, but it certainly won't hurt. The temperature reached is identical for different heating devices as physics dictates the boiling point of water and it can't exceed that as long as it's liquid.

  • 7
    A submerged spoon is no problem. It still prevents superheating by providing a nucleus for bubble forming, but there's no air-metal surface where sparks can form.
    – MSalters
    Sep 10, 2015 at 8:55
  • 36
    I didn't think ANY microwaves handled metal well. Don't put metal in the science oven! Sep 10, 2015 at 11:05
  • 7
    @MaxWilliams - that isn't true. Most microwaves handle metal objects without any sharp edges a cracks (such as spoons) perfectly well. Sep 10, 2015 at 13:40
  • 6
    As far as I know, distilled water can overheat and explode when impurities are introduced. Regular old non-distilled water will just start boiling (because it already has impurities in it). Sep 10, 2015 at 16:28
  • 12
    @mikeTheLiar But there are enough practical counter-examples where regular tap water overheated. The linked post being one of them, my personal experience another.
    – Stephie
    Sep 10, 2015 at 18:02

There is no difference. Whether you boil water in a kettle or in a microwave, it reaches a temperature of 100 °C/212 °F at sea level. Not only that, but no method that doesn't involve pressure will get the water to reach a temperature of over 100 °C/212 °F. Water boiled in a microwave is just as safe as water boiled in a kettle.

  • 8
    I agree, however, you still have the very small chance of superheating (en.wikipedia.org/wiki/Superheating).
    – Mien
    Sep 10, 2015 at 7:48
  • 1
    That water will not exceed 100C/212F is well known. Excess heat is bled off as steam. In order for the temp to exceed those values, it must be under pressure as it would be in a pressure cooker. That's why pressure cookers are used to kill bacteria that plain old boiled water can't. There is another scenario here where the temp can exceed the max while not being under pressure...boiling water in a microwave. The microwave heats the water faster than the heat can be bled off as steam. The temp exceeds 212 and you risk an explosion.
    – user36802
    Sep 10, 2015 at 10:05
  • 5
    @ChefBrooksie: Well-known perhaps, but wrong. Steam forms when bubbles can form, which require nuclei. This isn't an issue when cooking food - a potato is quite sufficient - but it is dangerous when boiling water.
    – MSalters
    Sep 10, 2015 at 10:08
  • 2
    fda.gov/Radiation-EmittingProducts/… There are many scientific explanations available. Use an old (scratched) cup or add an object to the cup to aid the formation of bubbles. Avoid metal in a microwave as it creates SWR that can damage the klystron.
    – user36802
    Sep 10, 2015 at 10:13

Even If i put on the tin foil beanie and assert that microwaves are dangerous to food, the microwave remains an ideal way to boil and heat water. Leaving aside the steam dangers which are mostly common to steam regardless of heat source (steam is dangerous) the most dangerous chemical reactions possible from high energy ionization is to produce mild base (HO ions), mild acid (H ions), Hydrogen gas, Oxygen gas, and most of the base will be neutralized by the acid forming water, and the hydrogen and oxygen are not enough to measure. This is actually the safest use for a microwave. It is even slightly more effective at killing bacteria, as some bacteria are also sensitive to microwave radiation as well as heat, but for good practice I would still heat to the recommended temperatures.

  • 1
    Could you please describe how a microwave can ionize water, and produce oxygen and hydrogen gases? I don't think this is true. Jul 19, 2017 at 16:18
  • Actually getting oxygen gas in a typical microwave operation is low probability although theoretically possible in that eddy currents could induce electrolysis. Hydrogen gas would also occur in such a case, but again low probability. Mostly in the parts per million to parts per billion range, and mostly quickly re-reacted to form water. Not a concern nor an effective method for generation, particularly as rotation disrupts stable eddy currents. Presence of Hydrogen ions however is much more common as it occurs at room temperature on the counter in any water, more at heat, and also -- cont ...
    – hildred
    Jul 20, 2017 at 0:50
  • . . . cont. more in the presence of radiation. It makes water an even better solvent of ionic solids however since the positive and negative ions are in balance it self neutralizes quickly. The whole thing amounts to less than a tempest in a tea pot, unless you leave in a fork, in which case you get sparks and ozone too. As I said even at the most paranoid edges of rational thought using a microwave to heat water is safe.
    – hildred
    Jul 20, 2017 at 1:00

Boiling water will kill any live bacteria or viruses that might be in your water. The only thing that can "survive" are bacterial spores. Spores are like seeds or eggs which can hatch to live bacteria. Interestingly, this process is what is used to make salt risen bread...a type of bread leavened by hydrogen-producing bacteria instead of carbon dioxide producing yeast.

The only way to kill spores is the use of a pressure cooker. As another answer mentioned, this does nothing to the chemical dangers, just the biological dangers. If the water is so filthy that you expect there to be actual toxins in the water, purification by RO or distillation is the best bet.

The most significant danger is the super-heated water phenomena. the microwave heats water so gently, that it can actually remain liquid above 100 degrees C. However, the addition of coffee grounds will provide places for steam bubbles to begin to form. The water will then immediately and violently begin to boil, and you could receive some serious burns. If you place a small pinch of coffee into the water while it is in the microwave, it will promote boiling and prevent superheating.


From a bacterial standpoint, boiling water in your microwave is probably sufficient to kill everything of concern.

The time needed to disinfect water depends on the temperature: for example, according to Water Disinfection for International and Wilderness Travelers, five minutes at 60°C, three minutes at 65°C, or one minute at 70°C is sufficient to kill E. Coli. Normally, simply heating the water to boiling and letting it cool is sufficient, but if you've got an exceptionally powerful heat source or unusually cool air, the water may not spend long enough at high temperature. Because of this, the same source recommends holding the water at a boil for one minute as a safety measure.

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