Why does cheese melt?

Few things in cooking are as satisfying as cheese melting into a warm, gooey, stretchy layer over a pizza or inside a grilled sandwich. Yet not all cheeses melt the same way. Some flow smoothly and stretch beautifully, while others barely soften at all, or turn oily and grainy. Understanding why cheese melts reveals that this everyday kitchen event is really about the hidden structure inside the cheese.

At its core, cheese is built from three main things: protein, fat, and water. The most important protein in cheese is called casein. These casein proteins link together to form a kind of network or scaffold, a bit like a tangled web, and this network gives cheese its solid structure and shape. Tucked within this protein web are small droplets of fat and pockets of moisture. When cheese is cold, everything is held firmly in place, so the cheese stays solid.

When cheese is heated, two important things happen at once. First, the fat trapped inside begins to soften and turn liquid, since fat melts at fairly mild temperatures. Second, as the temperature rises further, the casein protein network starts to loosen and weaken. The bonds holding the protein scaffold together relax, so the structure can no longer stay rigid. With the protein web loosened and the fat now fluid, the whole cheese is able to flow and spread, which we experience as melting.

The way a cheese melts depends heavily on how it was made. Cheeses with higher moisture and fat content, like mozzarella, tend to melt smoothly because the fat and water help the loosened proteins slide past one another. Aged, drier cheeses can be more stubborn. Acidity also matters: cheeses made with a lot of acid, such as some fresh cheeses, have a tightly bonded protein structure that resists melting and may stay firm even when heated. This is why a cheese chosen for a pizza is very different from one meant to be eaten fresh and cold.

This article was written using information from Wikipedia.