sterilization in microwave oven

[protozoan]

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.

[mith]

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.

[canalon]

quote protozoan:

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 😡 ). 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.

Patrick

[b_d_41501]

Microwaves are good for “cleaning” CD’s. 😀

[victor]

I don’t think it’s good because microwave can heat the thing so I think it can be melted down.. 😉

[DevGrp]

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?

[canalon]

quote DevGrp:

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).

Patrick

[mith]

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

[canalon]

quote mithrilhack:

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.

Patrick

[t00dy]

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

[brzezinski2]

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

[canalon]

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).

[brzezinski2]

Yes I stand corrected.

It is disinfection not sterilisation.

I do not wish to invite discussion about adequate care of contact lenses.

What I described was from a time long ago when thermal disinfection was the only approved method of contact lens disinfection and does not apply now with many modern methods available.

There are so many different types of contact lenses, possibly more than a thousand now that require different methods of care for different individuals, that a person wearing contact lenses should always ask their personal eye care professional to guide them on everything.
Gaining ideas off the net will only lead to trouble!!!!

There is a FDA approved technique for thermal disinfection of contact lenses and this is used by thermal disinfection unit makers when designing their thermal disinfection units for adequate contact lens disinfection. These units do not boil the lenses as is commonly believed. They all work at much lower levels of temperature than the boiling point of saline. Often around 70 to 80C. Some of them would have worked around 90C for a few minutes as I described to achieve adequate disinfection.

The FDA approved technique for thermal disinfection is also used as a guide for people who boil their lenses in their contact lens case in boiling water over the stove. This is a very effective method for people who want to disinfect there contact lens case as well as their lenes.

The FDA recommendations are a sliding scale and from memory its 71C for 30 minutes, 80C for 10 minutes, and the time reduces as the temperature increases up to putting the lenses in an autoclave for minutes or even a fractions of a minute at 121C depending if you are trying to sterilise the lenses or only disinfect the lenses to an adequately level.

Yes all methods of contact lens disinfection are all just “adequate disinfection of the lenses” and not sterilisation in order to achieve a low pathogenic microbe level which is much lower than is present in the eye normally or even on the cleaned delivery hand or finger.

I have to disagree with the above authors statement that microwave ovens can super heat water above 100C.

I believe that water in a microwave at one atmosphere can only reach a temperature of 100C unless it is in a confined container. Of course I do not recommend that anyone heat any enclosed container of anything using any method unless you know what you are doing.

I do not agree that what I described is what happens in an autoclave or a pressure cooker.

That was the point of the piece that I wrote was to describe how heating steam in a microwave was different to other methods of conductive heat like autoclaves or pressure cookers or element heaters.

Autoclaves work under pressure but before they pressurise them selves they actually evacuate the containing air so the resulting gas medium in the autoclave after heating is predominantly saturated water. This medium is more effect at sterilisation than just dry heat or a saturated air/water mix.

What I have described is the super heating of the steam in a microwave achieved without the accompanying pressure rise and is actually achieved at not much more than one atmosphere. The steam in the microwave is hotter than 100C because the water molecules in the steam are being hit by the microwaves first which result in heating them way above 100C compared to the water below. The water may actually be below 100C.

Its this super heated steam, which I advocate as a medium that is a more effective method of disinfecting or sterilising than just boiling in water. It depends on the time as to what you may achieve.

You can not extrapolate what happens in a microwave with the times used in an autoclave to achieve different levels of disinfection or sterilisation because an autoclave has a much more effective sterilisation medium being the saturated steam.

Also I have to disagree that what I described is similar to a pressure cooker. The difference is, in a pressure cooker the water or liquid medium would be at the same temperature as the steam. This pressure cooker steam would not achieve the same temperature than that of the super heated steam in a microwave so would not be as effective at sterilisation.

My point is that steam in a microwave can and does reach well above 100C even when the pressure is at one atmosphere because the microwaves hit the steam first before the water below which elevates the steams temperature well above 100C.

Apart from melting plastic there are many useful applications for this and is a more effective method of disinfection/sterilisation than just boiling something at 100C.

An affective method of super heating steam for disinfection or sterilisation would be to trap or capture the steam in an inverted plastic lid permeable to microwaves, Doing this would greatly elevate the steams temperature and enhance its effectiveness.

None of this is new.

There are plastic food warmers used to enhance the effectiveness of microwave ovens when cooking food and I’m sure many of you people have one. Well think about why they work more effectively and cook your food faster. Yes they keep the heat in by keeping the steam close to the food but the steam is also climbing above 100C. There are temperature indicator tabs or tapes, which you can use to check this.

There are also similar devices for disinfecting baby utensils, which is also similar to what I have described. It was not that long ago that baby utensils were boiled in water at 100C. The new method is much more effective.

[justmeonlyme]

http://findarticles.com/p/articles/mi_q … ntent;col1
Sterilization is very effective, just so that people trust it i guess we will have to use it in combination with some other methods such as UV.

[JackBean]

quote canalon:

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 😡 ).

Patrick

Well,if you have too large volume, it won’t be heated enough inside in 20 minutes, so you must prolong your sterilization time or autoclave smaller volumes 😉

[canalon]

Yes, the volume to be sterilized must be heated to 120°C for 20 minutes. Which is different from the object must stay 20 minutes at 120°C…

As for justmeonlyme’s answer. The paper you are pointing out does not prove sterilization in a microwave. Just decontamination. The difference is not trivial.

[microwaveguru]

Sorry, but there are some errors in your statement:
1. Superheating steam – microwaves are not captured by steam – the water molecules are too far apart, making steam essentially non-lossy, i.e. invisible. For example, when there is steam in a waveguide the transmission of microwaves is nearly 100 % over a long length.

2. Superheating water at 1 atmosphere – it does happen – we’ve seen it in my laboratory and I reported on it in 1983 in Microwave World – temperatures well over 110 C. The reason it can occur at 1 atmosphere is because the superheating occurs inside the volume of water, far from the edges of the container. It is at the edges that bubbles, which are required for boiling to occur @ 100 C, usually form. However, since the air in the microwave oven is cold (ambient temperature) the edges are cooled. Unless the water is highly aerated, these edges are the only source of bubbles, so the internal temperature can exceed 100 C before the edges do. Hence, superheating. This is especially dangerous because, when a bubble finally forms and is able to overcome interfacial tension so it can become buoyant, it encounters the superheated water and expands explosively (water expands about 1700 times when converted to steam). This has been the source of numerous injuries from microwaved liquids.

[sciencenaturally]

It appears the microwave method can superheat enough to kill even the highly persistent Bacillus cereus.

"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. […] Like many other bacterial spores, Bacillus cereus spores are quite resistant to radiation, heat and toxic chemicals, and they are notoriously difficult to kill.

The researchers also found that microwaves were effective in decontaminating syringes, but that it generally took far longer, up to 12 minutes for Bacillus cereus spores. The researchers also discovered they could shorten the time required for sterilization by placing the syringes in heat-trapping ceramic bowls.

Using a dose of Bacillus cereus dried on an envelope as a substitute for mail contaminated by anthrax spores, Bitton said he found he could kill 98 percent of the spores in 10 minutes by microwaving the paper – suggesting, he said, one possible course of action for people who fear mail might be contaminated. However, more research is needed to confirm that this approach works against actual anthrax spores, he said.

Link submitted earlier by a poster on this board, but content apparently overlooked.

http://www.sciencedaily.com/releases/2007/01/
070122143050.htm

Microwave Oven Can Sterilize Sponges, Scrub Pads

ScienceDaily (Jan. 22, 2007) — Microwave ovens may be good for more than just zapping the leftovers; they may also help protect your family.

University of Florida engineering researchers have found that microwaving kitchen sponges and plastic scrubbers — known to be common carriers of the bacteria and viruses that cause food-borne illnesses – sterilizes them rapidly and effectively.

That means that the estimated 90-plus percent of Americans with microwaves in their kitchens have a powerful weapon against E. coli, salmonella and other bugs at the root of increasing incidents of potentially deadly food poisoning and other illnesses.

“Basically what we find is that we could knock out most bacteria in two minutes,” said Gabriel Bitton, a UF professor of environmental engineering. “People often put their sponges and scrubbers in the dishwasher, but if they really want to decontaminate them and not just clean them, they should use the microwave.”

Bitton, an expert on wastewater microbiology, co-authored a paper about the research that appears in the December issue of the Journal of Environmental Health, the most recent issue. The other authors are Richard Melker, a UF professor of anesthesiology, and Dong Kyoo Park, a UF biomedical engineering doctoral student.

Food-borne illnesses afflict at least 6 million Americans annually, causing at least 9,000 deaths and $4 billion to $6 billion in medical costs and other expenses. Home kitchens are a common source of contamination, as pathogens from uncooked eggs, meat and vegetables find their way onto countertops, utensils and cleaning tools. Previous studies have shown that sponges and dishcloths are common carriers of the pathogens, in part because they often remain damp, which helps the bugs survive, according to the UF paper.

Bitton said the UF researchers soaked sponges and scrubbing pads in raw wastewater containing a witch’s brew of fecal bacteria, viruses, protozoan parasites and bacterial spores, including Bacillus cereus spores.

Like many other bacterial spores, Bacillus cereus spores are quite resistant to radiation, heat and toxic chemicals, and they are notoriously difficult to kill. The UF researchers used the spores as surrogates for cysts and oocysts of disease-causing parasitic protozoa such as Giardia, the infectious stage of the protozoa. The researchers used bacterial viruses as a substitute for disease-causing food-borne viruses, such as noroviruses and hepatitis A virus.

The researchers used an off-the-shelf microwave oven to zap the sponges and scrub pads for varying lengths of time, wringing them out and determining the microbial load of the water for each test. They compared their findings with water from control sponges and pads not placed in the microwave.

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.

Bitton said the heat, rather than the microwave radiation, likely is what proves fatal to the pathogens. Because the microwave works by exciting water molecules, it is better to microwave wet rather than dry sponges or scrub pads, he said.

“The microwave is a very powerful and an inexpensive tool for sterilization,” Bitton said, adding that people should microwave their sponges according to how often they cook, with every other day being a good rule of thumb.

Spurred by the trend toward home health care, the researchers also examined the effects of microwaving contaminated syringes. Bitton said the goal in this research was to come up with a way to sterilize syringes and other equipment that, at home, often gets tossed in the household trash, winding up in standard rather than hazardous waste landfills.

The researchers also found that microwaves were effective in decontaminating syringes, but that it generally took far longer, up to 12 minutes for Bacillus cereus spores. The researchers also discovered they could shorten the time required for sterilization by placing the syringes in heat-trapping ceramic bowls.

Bitton said preliminary research also shows that microwaves might be effective against bioterrorism pathogens such as anthrax, used in the deadly, still-unsolved 2001 postal attacks.

Using a dose of Bacillus cereus dried on an envelope as a substitute for mail contaminated by anthrax spores, Bitton said he found he could kill 98 percent of the spores in 10 minutes by microwaving the paper – suggesting, he said, one possible course of action for people who fear mail might be contaminated. However, more research is needed to confirm that this approach works against actual anthrax spores, he said.

Story Source:
Adapted from materials provided by University of Florida.

[microwaveguru]

There is a significant danger sterilizing sponges in a microwave oven. While it can be done under controlled conditions, it is not advisable for consumers. There are too many variables in microwave power output; launch location; sensitivity of various ovens to load size (most ovens deposit only small percentage of full power into small loads, such as sponges. but I have an oven in my lab that delivers 100% of its 1000 watts into small loads); presence or absence of the turntable (the turntable is an important "matching" device to deal with small loads in microwave ovens – many consumers have broken and not replaced their turntables – a VERY dangerous condition); much more. As to the sponges themselves, being cellulosic, they are subject to not only absorbing microwaves when hot & dry, but then heating faster the hotter they become (thermal runaway) and that can easily lead to a fire.

[mkwaje]

I’ll add my thoughts into the fire:

1. Sterilization means COMPLETE absence of all life forms. There is no such thing as semi-sterile or 99% sterile. Its either sterile or non-sterile. So, if you can reduce 99% of the population using the microwave oven, that doesn’t constitute sterilization. Disinfection, yes but not sterilization.
2. Regarding the article, I haven’t read the exact article yet published in the journal, but some wastewater organisms are notoriously difficult to cultivate in the lab due to previous association with some detritus in the wastewater. And the 1% remaining organisms can still cause a lot of mischief. If you think your sponge is absolutely safe because 99% of the organisms are gone, think again.
3. During sterilization in an autoclave, its the steam and the temperature (121C) that kills the organisms, not the pressure. The pressure is only essential to raise the temperature higher than 100 C. Dry sterilization should be done in ovens at 250-350 C for a few hours to kill the spores.
4. I will only believe that microwave can sterilize if you will dip 2 sponges in a mixture of organisms (wastewater will do), microwave it and then put into a nutrient broth (TSB or NB) and FTM (for anaerobic). Incubate them for 48 hours. If there are no turbidity in both medium, I will gladly eat my words and congratulate you. (sounds like a challenge, he he). Of course your work has to be reproducible and you have to put some pics here for evidence.

good luck and prove me wrong 🙂

[AndyCheung]

You can did it

[mmoir125]

microwaves will kill off living bacteria, but any spore’s are endo/ enterotoxins will not be effected unless it is for a prolonged time, i think

[Davy]

I’ve sterilized jars for home preserves by sealing them with a damp piece of cheesecloth inside. Not a foolproof method, maybe, but there is increased pressured in the jar when steam forms. In any case, the chutney didn’t spoil 😀

[topshoebag]

I don’t think it’s good because microwave can heat the thing so I think it can be melted down..

[FrederickCarson]

quote protozoan:

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.

Yes. Despite what some people have said on this forum it is possible to sterilize in the microwave.
Many people use it for kitchen sponges.
The article below from researchers at the university of florida showed that its even possible to sterilize syringes killing even Bacillus cereus spores, its just it takes 12 minutes to do so.
http://news.ufl.edu/2007/01/22/zap-the-bugs/

[canalon]

If I am sure that the microwave can and will destroy most bacteria, and that a proper protocol might increase its efficiency I find what you give us is a PR note that do not link to any primary source (not surprising, but annoying, even more so knowing that PR dept are known for often arranging the result to make them more interesting…).
The second thing is that there are norms and protocols to prove that a sterilization protocol is working, and interestingly, they are not used nor mentioned in this. For one thing, the common tests are based on the survival of Bacillus stearathermophilus, not Bacillus cereus. And usually cultures with known numbers of bacteria are used which allow to track the efficiency of the sterilization over time.
So decontamination (reducing the bacterial load to safe level) in a microwave, sure. Sterilizing, not so much.

[alchilito]

conditions for sterilization have been established clearly for a long time now

I highly doubt any kind of microwave treatment would effectively kill spores.

[jaypark]

An interesting one for the classification of the microwave, we shall determine it by making some aspect that could be use to see the function. It was the function that should be the way to use it for a long period. It was highly recommended to make some action to know the result if they trying to sterilize.

[canalon]

And how would you translate that in english?

[fdgsr]

I am a microbiologist, now retired. I have used autoclaves for years. Many of you may not know of the older ways of sterilization when autoclaves were not available or too expensive for the purpose. Nurses used to use a boiling water bath to ‘sterilize’ needles for reuse and to ‘sterilize’ surgical instruments. It is innocence pure and simple with all authority for sterile in the title, NURSE. (Back in those days nurses were kept sterile by not allowing them to marry. Teachers and other professional women could not get a job if married because their husbands were their owners and could veto any order by a super seer.) Of course anthrax spores and spores of common contaminants were not killed. But, the rare spore of a pathogen on an instrument after it was thoroughly washed and dried along with the improbability of a pathogen in the air at the moment of contact rendered those methods relatively safe and traditional. Dentists often did not do more than wash instruments in soap and water. The method of ‘inspissation’ came into use after the turn of the 20th century. A culture medium would be heated to boiling and then held at room temperature or even incubated for a few hours to a day or two. The heat killed all vegetative forms, and the spores germinated to vegetative forms. The culture medium was then heated to boiling again. That could be done as many as three times to obtain almost absolute sterility. Most early bacteriology was done with media ‘sterilized’ in that manner. Microwave could be used in the same way to ‘inspissate’ by intermittent microwaving. It would kill vegetative forms and if spores germinated, they could be killed by a second or third microwaving.

[Author]

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