Why does medium turn yellow fast during the intensive synthesis of protein?

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AllenChiu
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Why does medium turn yellow fast during the intensive synthesis of protein?

I noticed that in the case of overexpression of exogenous protein in 293T by transfection,the medium will accelerately turn to yellow.
What is the cause that turn medium to yellow or that lower the pH in general?
Is it the by-product of metabolism?Is it lactic acid?
Will the intensive synthesis of exogenous protein product huge mount of such things?
Cuz after transfection of 3 vectors(25ug each 10cm dish of 293T) in lentiviral package,the medium can’t hold on more than 24 after the change of medium 8 hours after transfection before it begin turn to yellow,which prohibit me from collecting more virus,like 48hours after.So can I add some buffer like HEPES to stop it from become yellow so soon?
Well,can you give me some advises?