Bloat (more appropriately GVD= gastric volvulous and dilation) is a random occurrence and truly cannot be "prevented" by anything other than surgery.
Raw foods don't prevent bloat at all, but it may decrease the chances of GVD from happening. I think this may be true because kibble expands in the stomach which causes more gas to be produced which can trigger torsion of the stomach.
As a foster home for the local Dane rescue, we are well informed and have plenty of experience dealing with GVD. I will tell you from experience that you could do all the tricks in the book to help prevent a dog from bloating, but in the end it doesn't work very effectively.
If you are worried about GVD, I would get a gastropexy done to prevent it 99%. Totally worth it in my opinion. You will spend $500-1000 on a preventative surgery depending on where you live but if you're dog bloats, you're looking at a $5K+ surgery and a very sick dog for weeks or quite possibly a dead dog after that. There's no option to me, a gastropexy is just as important as a spay/neuter on any large/giant breed dog or any other dog that is prone to GVD.