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sterilization in microwave oven

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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sterilization in microwave oven

Postby protozoan » Sun Jul 10, 2005 3:16 pm

Could be something like that possible? If i put a glass (pipettes, test tubes etc) to the microwave oven and set it to for example 10 minutes (may be more) and to maximal power (heat) couldnt be it sterile?..........just wondering.
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Postby mith » Sun Jul 10, 2005 9:37 pm

A microwave works by vibrating water molecules and the heat results from that friction. Maybe the water in bacterial cells would boil but I'm not sure about virii.
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Re: sterilization in microwave oven

Postby canalon » Mon Jul 11, 2005 1:10 am

protozoan wrote:Could be something like that possible? If i put a glass (pipettes, test tubes etc) to the microwave oven and set it to for example 10 minutes (may be more) and to maximal power (heat) couldnt be it sterile?..........just wondering.


No, really. That's why we still have autoclaves beside the microwave oven (which has many other uses) in every microbiology lab. The best reason is that to really kill everything you need to heat the stuff around 120°C for 15 to 20 minutes. And even though some of the toughest spores may survive (rare, but a pain when it happens and you have to throw away liters of culture medium :x ). In the other hand your microwave will never heat anything over 100°C.
Plus you may damage your microwave if there isn't enough water or anything to absorb the microwaves inside the oven. Definitely not a good thing to do.

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Postby b_d_41501 » Mon Jul 11, 2005 3:23 am

Microwaves are good for "cleaning" CD's. :D
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Postby victor » Mon Jul 11, 2005 12:07 pm

I don't think it's good because microwave can heat the thing so I think it can be melted down.. :wink:
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Postby DevGrp » Mon Jul 11, 2005 12:17 pm

How about using a microwave steamer?

We got one for babies bottles and I have been thinking about getting one for the lab for quick/ small scale sterilsation?
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Postby canalon » Mon Jul 11, 2005 3:13 pm

DevGrp wrote:How about using a microwave steamer?

We got one for babies bottles and I have been thinking about getting one for the lab for quick/ small scale sterilsation?


Nope again. Sterilization need temperature around 120ºC, and this cannot be attained in a microwave oven, because you need pressure. If you cannot afford a real autoclave just buy a pressure cooker, and sterilize in it (20 minutes after it starts whistling).

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Postby mith » Mon Jul 11, 2005 5:07 pm

How about UV generators? My chem teacher used those to sterilize our shared goggles.
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Postby canalon » Mon Jul 11, 2005 8:03 pm

mithrilhack wrote:How about UV generators? My chem teacher used those to sterilize our shared goggles.


Generally works great but:
- it is not easy to have all surface completely lighted and hence disinfected
- Some plastics really do not like UV in the long term
- Some bacteria are very resistant, so it can take some time to attain a proper disinfection, and I am not sure that spores will die.
- UV "bulbs" lose power over time, but you cannot see it without something to mesaure the enrgy of the bulb, so sterilization time lengthen.

That's why the cheapest way to sterilize is still autoclave/pressure cooker.

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Re: sterilization in microwave oven

Postby t00dy » Sat Sep 06, 2008 11:33 pm

what about this??? take a look

Microwave Oven Can Sterilize Sponges, Scrub Pads

The results were unambiguous: Two minutes of microwaving on full power mode killed or inactivated more than 99 percent of all the living pathogens in the sponges and pads, although the Bacillus cereus spores required four minutes for total inactivation.

source: http://www.sciencedaily.com/releases/20 ... 143050.htm
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Re: sterilization in microwave oven

Postby brzezinski2 » Thu Sep 03, 2009 12:01 pm

Hi People,
I was an Optometrist but am retired now.
Around 1983 that’s right 26 years ago I started using and recommending microwave ovens to sterilities contact lenses.
This was an extrapolation of an old original technique of using heat like boiling the lenses in saline in their case over a stove???? This all sounds primitive but soft contact lenses that were useable had only just been invented and started to become available to the general public. Most were made on a local mini lathe put in a vial and hand delivered in those days. How they were sterilised effectively was still a wild frontier. This all changed within a few years with the advent of a few contact lens sterilisation solutions.

Anyway what I found was you could sterilise above 120 degrees Celsius using a microwave. Those of you who think that water in a microwave only reaches 100 degrees following the normal laws of physics are wrong.

If you can imagine a contact lens case made of thermal resistant plastic with its lid slightly unscrewed then you have how this is done. What happens is the microwaves emanate from the magnetron at the top of the oven and travel down. The first water molecules they hit are the ones in the air space above the contact lens saline solution. These molecules reach temperatures well in excess of 100C. In effect they are super heated. This heat is then transferred into the surrounding case and solution below containing the lens. If you time it right usually a matter of only about 20 seconds the solution goes way above 120C. What I aimed for was to take the temperature to just below the melting temperature of the contact lens case. Don’t ask me what that was because I don’t know and I determined this by destroying hundreds of cases. It probably was above 300C.
I then rotated the contact lens case lid down and sealed the case with the lens sitting in the solution until it cooled. About 30 minutes.
At that time there was a standard for sterilising contact lenses using heat and from memory it was to keep the lens above 84C fore more than two minutes or a temperature of 92C for one minute. This standard was determined for electrical heating units that could heat the case while you slept. I’m not saying this standard was correct, its just what was considered to be OK at that time.
I did measure the temperature of the solution in the contact lens case after 4 minutes after toping the microwave and it was still above 92C.
Far better chemical sterilisation solutions became available and I stop using this heating method around 1990.
My point is microwave oven can take steam, which is confided up to temperatures well above 100C even though there is still water, which is still in liquid form bellow.
You can control this temperature by regulating how much pressure or steam is released or vented off while heating.

Don’t believe me. Them find a plastic container, which you can seal about 10 to 30ml internal volume.
Put a little water in it about half full and seal it. Place in the microwave with a super dry tea towel around it.
PUT YOUR SAFTY GLASSES ON AND STAND WAY WAY WAY BACK FROM THE OVEN.
Turn the microwave on and be ready for one hell of a BIG BANG.
SEE, the container has melted more so where the steam area was.
The temperature must have reached above 100C or the melting temperature of the plastic even though there is water in the tea towel indicating that all the water did not turn to steam.
Lastly try and boil the same container in water over the stove to see if you can get it to melt or even get soft.
So the laws of physic can be bent. Your professor was slightly wrong.
Now all you clever people go out and think of ways of sterilising things like holding items in the super heated steam above a tray of water with a lid on top to capture the steam.
I note that there are already baby utensil sterilisation devices you can put in the microwave which work as I have described.
Still not happy then e-mail me, may by I will reply. Brzezinski2@bigpond.com
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Postby canalon » Thu Sep 03, 2009 3:14 pm

I do not agree fully with what is above, but I think that there are a few points that are woth mentioning:

- what you are describing (92ºC for 2min) is closer to pasteurisation than sterilisation. What happens is that you kill most of the microorganism, and since this is repeated daily, and considering that the eyes can deal with limited contamination it is probably safe enough, but the contact lenses are probably not sterile.

- Microwave can and do superheat water above 100ºC, but this is neither reliable nor safe. Not reliable because it is impossible to predict accurately what temperature will be reached before it starts boiling (usually "explosively") nor how long you can maintain the temperature. Meaning that for sterilization, you can't guarantee that enough time and heat has been applied to your device to be truly sterile. And as I said above it is not safe because handling superheated liquid might be fun, but create massive/explosive bubbling that can cause burn.

- In your last paragraph what you are describing is actually exactly what happens in an autoclave or a pressure cooker, less things that are designed for safety. The microwave as any other source of heat can and will heat water well above 100ºC if you keep it in an enclosed compartment, which will increase pressure inside the vessel. The difference between that and the pressure cooker is that the pressure cooker has a safety valve and walls made of thick metal. The walls are able to withstand the pressure, your plastic might not (just as you point out) and the safety valve has 2 essential roles: it prevent the pressure to build up to unsafe levels even with the walls, and more importantly it controls the steam temperature (which is proportional to the pressure). Then you can have the certitude that whatever you wanted to sterilize has been exposed to enough heat to kill all bacteria and viruses.

So to wrap this up, microwave will allow the disinfection (reducing the amount of bacteria/viruses to safe levels) of what you put in them, but they are neither safe nor reliable for sterilization (destroying all living organisms on the surface).
Patrick

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