Does the protien mix for the shakes go bad after opening the jar?

I have been doing a protien shake for breakfast and lunch everyday for the last few months. I use the Pure Whey Stack by Champion. I am starting to wonder how long the mix stays fresh after the jug has been opened. I put the cap back on tighly after each use, but I do have 2 different flavors that I switch between. I don't want to keep buying the 5lb. jugs for the money savings if the protien is going to go bad. Does anyone know if the protien can go bad in the storage jar?

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