The Gibco DMEM with which I have been working for quite some time is turning yellow while it is in 4 degC and also has froth. A similar medium was prepared with FBS earlier than this one and it is pink in colour and has no froth. An unopened sealed bottle of DMEM also turned jelly-like and is appearing yellowish.

Is this a sign of contamination or something else? If Yes, which type of contamination it is?

Also, I prepare the medium by adding Pen-Strepto to it and 10% FBS and then store it at 4 degC.