when I used my protein powder, I mixed it with milk, and that would go bad after a few hours (walking to class, hour and a half class, walking to gym, workout, then consume)
I ended up tossing a few ice cubes in it on my way out the door in the morning, and it was never a problem after that.
As for the protein breaking down after being mixed... I wouldn't think so. The typical extent of breakdown by enzymes is to amino acid, which would still be usable by your body if you drank them. But my guess is that it's fine.