Answer to Question #22762 in Microbiology for thooma

It depends on the purpose. In a clinical microbiology lab, where quality control is important, media should not be used past the expiration date for diagnostic purposes, but it may be used for less critical tasks.& If it is checked with the culture organisms and its working, be sure this media is not contominated with other compounds and microorganisms and then it can be used for not critical tasks. It is easier and cheaper to simply make fresh medium.
The recommended expiration date of prepared culture media varies greatly. Screw-capped tubes can be stored for 6 months or longer at low to ambient temperatures. Plated media may be stored inverted in a plastic bag or other container in a refrigerator for 1–2 weeks or longer. Store all media away from light