Freezer. If you don't lactobasillus will take over in about an hour and it will be a slimy stinky mess. In fact there's a sigline quote about me and Llama Spit, which is all about my experience with spent grain and Llamas.
I once got nearly attacted by a pack of LLamas for trying to feed them few days old, stored in the freezer, spent grain, that started to thaw on the hour drive in the summer.
I almost died, I swear......