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Microwave sterilization

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I was wondering if I could sterilize glass petri dishes and melted agar in a microwave oven and how long would I put it on for, if possible?

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Microwaving at least in normal ovens is comparatively inefficient. Cooking agar in it for instance is not much more (if at all) more efficient than normal cooking. The heating is too heterogenous. Also, for efficient sterilization you need heating above 100° C, which only work with some overpressure. You can for instance try to moisture your dishes and then cook it for around 20 minutes. Agar will dry in that time, though.

However, in all theses cases I still would only uses these to grow fast growing bacteria. It is very unlikely that they will be completely sterile.

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I was wondering if I could sterilize glass petri dishes and melted agar in a microwave oven and how long would I put it on for, if possible?

 

Sterilization of agar and plates is usually done in an autoclave machine, which reaches much higher temperatures than microwaves, and are designed for sterilization. If you're working at a research facility it's standard to have one.

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Actually an autoclave is not much hotter per se. The main trick is that it maintains overpressure allowing liquids to be heated up to 120° C. Essentially it is a steamcooker. Essentially you can use one to sterilize your media.

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Actually an autoclave is not much hotter per se. The main trick is that it maintains overpressure allowing liquids to be heated up to 120° C. Essentially it is a steamcooker. Essentially you can use one to sterilize your media.

 

The pressure also keeps the agar from solidifying until you're ready to make the plates.

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Ehm, not really. It prevents agar from overboiling during the cooldown. You usually open autoclaves between 80 and 90°C, temperatures at which agar is still very liquid. They solidify roughly ~40°C.

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A couple months ago I saw a series of different news reports citing a study that found microwaving a wet sponge for about five minutes pretty much sterilized it. Microwaving a sponge after sending it through a dishwasher along with one's dishes is apparently a very good way to keep sponges sanitary.

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If the bacteria have water in them wont it boil and kill them?

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In theory yes, however the field created within the oven is quite inhomogenous. Very small particles (as bacteria) can quite easily be missed.

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That's to Bad,

 

I know a pressure cookers can sterilize agar, but can pressure cookers sterilize glass petri dishes. If they can would it be better to sterilize the agar and the petri in two pressure cookers instead of letting one sit wile the other is cooking, or is doesn't matter?

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Ehm, not really. It prevents agar from overboiling during the cooldown. You usually open autoclaves between 80 and 90°C, temperatures at which agar is still very liquid. They solidify roughly ~40°C.

yeah... I'm not sure why I said that. perhaps I was thinking about temperature. :doh:

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If the bacteria have water in them wont it boil and kill them?

 

Not necessarily. If a fly is caught is entrapped in a microwave it might just fly out as if nothing happened after your dish is cooked. I'll let someone else explain the physics. Here's one link among many via Google:

 

http://answers.google.com/answers/threadview?id=265489

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I know a pressure cookers can sterilize agar, but can pressure cookers sterilize glass petri dishes. If they can would it be better to sterilize the agar and the petri in two pressure cookers instead of letting one sit wile the other is cooking, or is doesn't matter?

 

Well, to sterilize glass wares you got two options. Sterilize them in a steam cooker (I am not sure how much pressure a normal steam cooker can build up. It is possible that they do not sterilize completely), or you can just bake them at at least 180°C for ~12 hours.

Usually agar and dishes are sterilized independently, and then you pour the agar under sterile condition into the dishes.

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I did not mean sterilize agar and petri dishes in the same pot, I was wondering how long a steril petri dish can stay in open air before it is not steril anymore? Also, does anyone know a "cheap homemade" way to keep steril glass petri dishes steril? I was think jaring them or something like that.

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Well it depends how sterile the surrounding is and how the air movements are. If you want to store them you can put your petri dishes in a larger jar that can be sealed, sterilize them within the jar and then seal it after sterilization.

For short term you can sterilize the dish in an oven then let it stand in it until agar is done.

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Sterilization packets for steam/pressure sterilization are inexpensive and widely available and they work equally as well in a pressure cooker as they do an autoclave.

The indicators on the packets will give you some confidence that the sterilization was adequate. The packets (sealed prior to sterilization) should retain inner sterilization as long as the exterior of the packet is not compromised. However, I wouldn’t go longer than 6 months without repeating the sterilization before use.

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