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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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Re: sterilization in microwave oven

Postby brzezinski2 » Sun Sep 06, 2009 6:32 am

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.
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Postby justmeonlyme » Sat Oct 24, 2009 5:26 pm

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.
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Re: sterilization in microwave oven

Postby JackBean » Sun Oct 25, 2009 7:50 am

canalon wrote: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 ).

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 ;)
http://www.biolib.cz/en/main/

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Postby canalon » Mon Oct 26, 2009 12:30 am

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.
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any proof. (Ashley Montague)
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Re: sterilization in microwave oven

Postby microwaveguru » Thu Jan 14, 2010 4:58 pm

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 _at_ 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.
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Re: sterilization in microwave oven

Postby sciencenaturally » Tue Jan 19, 2010 2:17 am

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.
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Re: sterilization in microwave oven

Postby microwaveguru » Thu Jan 21, 2010 3:59 pm

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.
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Postby mkwaje » Fri Jan 29, 2010 1:19 pm

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 :)
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Postby AndyCheung » Mon Mar 01, 2010 12:51 pm

You can did it
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Postby mmoir125 » Thu Mar 25, 2010 1:23 am

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
But there's more old drunks than there are old doctors, So I think that I'll have another round!
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Re: sterilization in microwave oven

Postby Davy » Wed Jul 21, 2010 5:48 pm

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 :D
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Postby topshoebag » Sat Aug 21, 2010 4:43 am

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