Looking for the perfect cheese for the ultimately gooey, perfectly melted grilled cheese sandwich? Good news: Scientists with the American Chemical Society have put out a three-minute video from the society's "Reactions" series on YouTube all about that very subject. And the winners are ... gouda, gruyére, or manchego, with mild cheeses melting more beautifully than sharp. It's all about how casein proteins clump together (the resulting spheres are called micelles) when cheese melts. These micelles, which are held together by calcium and are gloriously fatty, are negatively charged, meaning those tiny balls bounce apart. Add lactic acid, however, which goes up the longer a cheese ages, and those micelles smack together, trapping water and getting all gooey, reports Consumerist.
But wait, there's more. The older the cheese, the more lactic acid, which in turn lowers the cheese's pH—thus sharper cheeses have a lower pH and mild ones a higher pH. The ideal pH, in fact, is right between 5.3 and 5.5, with cheese of lower pH releasing their oils when heated and getting clumpy. This is why gouda, gruyére, or manchego are ideal but on the milder side; if they age too long and become sharper, the pH dips too low and the cheese breaks down more easily, meaning you could end up with a curdled mess while heating it. (Check out why this cheese tests best for pizza.)