Originally Posted by ajdelange
Chalk is calcium carbonate. Gypsum is calcium sulfate.
I'm guessing that it is the interaction of sulfate with hops that is responsible - not anything happening to the gypsum. It's just calcium sulfate - very stable stuff. Your walls are made of it. I suppose it's possible that some mold has started to grow in it but then the powder itself would look or smell funny.
I experienced this first hand on Saturday. I opened a previously-opened tub of wall-board compound (gypsum-based). I did a little stir and noticed that it was much darker below the surface, not white like usual. Then the smell hit me, sulfur, rotten eggs. Clearly, the sulfur-reducing bacteria had invaded the wall-board compound. I still used it though!
I would be surprised if that same activity could occur in powdered gypsum since it should take more water to foster the bacterial activity. Maybe there is enough moisture in the air??