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Larry Farrell farrlarr at isu.edu
Thu Jul 5 15:02:37 EST 2001

Dilworth wrote:

> Any solution with agar has to boil about a minute before you cool it.
> If it's not totally melted it won't solidify.  Be careful at this stage,
> as it boils over and burns easily.
> I've only ever used premixed plating media with agar already in it, but
> it will turn clear when it's melted.  Use padded gloves and keep
> swirling it periodically while it's heating, unless you melt in an
> autoclave.  We used to make 2-3 liters at a time, so swirling this was
> pretty tricky. If you're using smaller amounts use a large enough flask
> (Ehrlenmeyers are great) so you can swirl, but not too big.

Am I simply overlooking something here or does this completely miss the need to
sterilize the medium?  Just because the agar has been melted and mixed into a
medium does not mean that the medium is sterile.  Even pre-mixed media containing
agar must be sterilized, and sterilization in an autoclave heats the medium long
enough and hot enough to melt the agar.  Of course, it still needs to be mixed
after removal from the autoclave, once it has cooled enough that it doesn't boil
over when you mix it, to assure that the molten agar is evenly mixed throughout
the medium.

Larry D. Farrell, Ph.D.
Professor of Microbiology
Idaho State University

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