 Pasteurization process, so it's not the same kind of protein structure that's in the raw milk. And it doesn't go sour. It just absolutely rots. The smell of what's considered sour pasteurized milk is worse than the smell of sour raw milk. But the sour raw milk has been fermented, and it's extremely healthy. You may not want to drink it, but put it in your sauces. Put it in your soups.