Keeping agar in liquid form - problems? - (Feb/02/2010 )

I teach a high school micro class and I am trying to find the easiest way to make agar plates as I need them. Pouring plates usually makes more than I need, so I came up with the idea of dispensing 12 mL (enough to fill a 60mm plate) into screw capped tubes, autoclaving them, then leaving them in a 45-50 C incubator to stay liquid until I need to pour plates. I would then take the # of tubes I need, which is usually only a couple at a time, and pour plates.
Is there any foreseeable problem in keeping agar (just nutrient agar or TSA) in liquid form on standby for weeks/months on end?
Thanks in advance.

You can even keep them at room temperature for some time. I used to keep them at RT and then warm them in the microwave.

Now we just prepare the plates and keep them at 4 ºC . (You just have to be carefull with the antibiotics)

thanks. i just have a problem with plates drying out here in New Mexico. I am using non-vented plates now to prevent dryout.
so no problem at that temp with nutrient breakdown in the agar?

Actually I am not sure about the 50ºC, but I would keep the medium with the agar at RT or 4ºC better than at 50ºC.

Store pre-made plates for several months in the refrigerator, sealed in plastic bags. Discard if they show any sign of growth. Antibiotics are stable for at least two months.

I'm not sure about keeping them for extended periods at 50C -- my gut tells me no, but I don't have any empirical or literature-based evidence of that.

I would either find a way to re-melt the small aliquots (waterbath, microwave, steam box, etc.), or pour them all and keep the plates sealed tightly at 4C.

If drying is a problem even when the plates are stored in tightly sealed plastic bags, you could probably re-hydrate them with sterile water (pour a puddle of sterile water on them and let them soak it in for a period of time, decant the excess, and allow the plates to dry at room temperature overnight), or keep the bags of plates in a larger sealed secondary container which includes wet paper towels that you can change periodically -- a humidity chamber, if you will.

I have used humidity boxes for storing things like Ouchterlony plates, but I've never tried re-hydration of agar plates.