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Pouring Agar for Simple Minds

( updated: April 15, 2008, at 02:43 PM )
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These instructions will guide you through the process of pouring agar without having heavy condensation on the petri dishes.

I stack the petris in 5 or 6-piece stacks in front of the hood. Then I prepare some cups and boiling water.
I let the agar cool down to around 35 °C, just before it solidifies. Then I pour the agar in the dishes, and immediately after a stack is finished I put a cup with the hot water on top of it.

What does this do? Water vapor condensates at cold surfaces. Now, when you stack the dishes, the top of each dish is kept warm by the bottom of the previous dish. Except the top one! Through the cold top one there is a temperature gradient and the top and to some extent also the dishes underneath get some condensation.

When you put a cup of hot water on the top dish, it prevents this condensation by heating the lid.
You will also notice I put a sheet of paper in front of the hood, so the dishes don't cool too fast from the side, which again contributes to less condensation.

I seal agar plates with a Glad Clingwrap, where I cut a small piece off the roll with a very sharp knife.
You can use any wrap, as long it's made from polyethylene. There are differences between brands though, some will work better than others.
Saran wrap is NOT suitable, since it's not made from polyethylene and is not gas permeable.
Sealing dishes prevents contaminants coming into the dish for whatever reasons, ads for easy handling since the both dish parts would otherwise come apart and allows gas exchange, since the polyethylene wrap is gas permeable.

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Page last modified on April 15, 2008, at 02:43 PM