Sodium citrate. Which I believe is there to allow the cheese to survive pasteurization, but it results in the texture and melting properties that make it the objectively correct choice for cheeseburgers. I still stand by provolone for cheesesteak sandwiches though.

And in that vein if you're sufficiently adventurous you can americanize your cheeses of choice

That's interesting. I would imagine the same with magnesium citrate would be more of a uh liberating experience.