I think that 10 Kgy would suffice to the prepared plates of culture media. Concerning the pH lowering this minor dose would conserve better the biochemical properties of the culture media, maybe it would be necessary to improve the buffer used in the media, i.e. its molar concentration if it is desired to maintain for longer periods.

15 Kgy is very high dose for culture media plate. I think Gabriel is right in suggesting a lower dose.

I have not had experience of exposing plates to such high dose. Even very thick Aluminium sheets (8-10 inches thick) are sterilized by exposure of 12-15 Kgy.

We had exposed one white colored food grade powder to 15 Kgy radiation, the result was discoloration. The powder turned out to be slightly pale after exposure. So I would also recommend you to expose your plates at somewhere 8-10 Kgy radiation

High doses tend to generate oxidation, that's why things appear smoky. I agree that 10 kGy would suffice for the vast majority of contaminants, but keep in mind that there are organisms that resist extremely high doses. The dose that kills 90% of a cell population (LD10) for Deinococcus radiodurans, for example, can be anywhere between 10 kGy to 15 kGy. I would still use 10 kGy and only change if a contamination problem appears, which is not expected.

Thank you for your response & advice. Appreciate it !

As per following paper, sterility of the irradiated plate can be obatained at 14-15 Kgy dose.

Radiation Sterilization of Two

Commonly Culture Media Used for

Bacterial Growth

Hala N. El-Hifnawi 

Link: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/39/120/39120356.pdf

hello everyone,

Need more data

its my observation that pH of media plates drops down (becomes acidic) after few months of irradiation.

Has anyone else also has same observation?

if so do reply.

Awaiting for your rsponse

At higher radiation doses everything is altered in the culture media, not only the pH. Have you ever make a protein or saccharides -and specially vitamins- probe? Maybe this irradiated culture media is no longer useful. I suggest, if the purpose is the longer period of utilization of the prepared culture media, to previously pasteurize your culture media components, or ultrahigh filtration of the entire media. Gaurav tell us that even aluminum sheets (8-10 inchs thick !) are sterilized at 12-15Kgy, Figure out the biochemical dammage within your culture media. To preserve for use during months your previously prepared media, it would be better to conventionally sterilize it, lyophilize your media and hydrate it with sterilized water -or whatsoever medium- immediately before use it. It would be useful after years! (at least five years). after hydration you can check your desired pH and adjust it well with a few drops of sterilized (if necessary) acidulated solution, or simply a convenient buffer.

Ivan Glaucio points out on the oxidation promoted by radiation.  In organic substances the polimerization also takes a very important signicant role to denature  them. In every case a calibration process is mandatory  to know the extension of the biological denaturation of the organic substances within the culture media. I have irradiated only plastic material for cell culture at 10-15 Kgy. The polycarbonate materials and the polyacrilic materials become smoky and brittle. Then I left the massive generators of radiation at the National Institute of Nuclear Research (ININ-Mexico) for a more convenient local cobalt radiation pump.