Each grilled cheese sandwich enthusiast, being a gouda groupie or gruyère kinda guy, will agree that the gooier the better, when speaking about this student staple.
You will see the detailed science behind cheese and its amazingly gloopy properties in the following YouTube video by the American Chemistry Society channel Reactions.
The reason for the beautiful gooey mess is the calcium and acidity found in the cheese. The casein proteins in cheese are gathered in fatty spheres named micelles, held together by calcium. The micelles have the same outer charge, thus being repelled by each other.
If you leave a cheese to mature, it’ll become more acidic and sharper, since most of the sugar lactose is converted to lactic acid. If the cheese is more acidic, the calcium will hold the micelles together lesser, thus allowing for the proteins to separate, mingle and flow together.
Nevertheless, in case the pH levels are too acidic and low, the cheese upon heating will release all its oils resulting in a greasy mess.
So what types of cheese hit the sweet spot of the pH levels? Namely, it’s gruyere, gouda and manchego that have 5.3 – 5.5pH level. The well-known yellowy-orange slices of American processed cheese can as well do a good job, too. Usually, they are made of various cheese types and lots of phosphate salts and emulsifiers which again limit the calcium amount holding it all together.