sterilization in microwave oven

[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]

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.