It’s just the sodium citrate. You can order that stuff and make any cheese melt nicely with just a little bit of it thrown in. You don’t have to mix American cheese in to make nice melty cheese. I now make a delicious queso (a decent copy of the one from Moe’s), and another similar recipe but instead of pepper jack and cheddar I use Monterey and cheddar and add rosemary. Spreads great cold on crackers like a Brie would. American cheese would not pair well with either of these.