I don't buy it in bulk, so therefore I just keep it tightly sealed in it's original bag. I've never noticed a taste difference. Some people on tv just keep it in containers, although I'm sure they change it out all the time. I've had mine for sometimes a few months and never tasted anything odd.
I just found out my local Sams sells PS in bulk. What is the best way to store it? I have noticed that a half used bag even stored in an air tight container for more then a week has a funny taste to it like its stale.
I buy my PS from Sams in bulk and store it in the bag it came in and put the bags into a rubber maid storage container. I have not noticed a "stale" taste to the PS at all. Before I started caking, I always had a box of PS and BS in the cabinet and they might have been there for a year with no change of flavor. I didn't think PS could go stale. Maybe someone else can provide an explanation. I am puzzled by this... 
Thanks for posting this. Hope we can figure this out. 
I too have noticed that my PS gets an "off" flavor at times. I do believe it is the type of processing that it goes through. The only kind that I buy that does not get that taste, is from Stop-n-Shop, a local store. It is pure cane sugar and does a much better job than even a name brand.
The only thing I can come up with is that maybe your airtight container isn't really airtight and that your ps is absorbing surrounding odors. I don't know that is possible, but I am afraid to keep an opened ps box for very long. Mine gets these tiny pellets, like maybe it is absorbing some moisture from the air or something. Good luck!
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