Different cheeses melt in different ways. Some melt smoothly. Others don’t melt at all. Moisture plays a role, but the state of a cheese’s proteins affects it most. There are two main stages in the cheese-making process that affect the protein structure and, eventually, the cheese’s meltability: coagulation and the process of aging.
All cheeses can be categorized into two groups based on how they are coagulated: with acid (such as vinegar or lemon juice) or with an enzyme known as rennet (which can be animal- or plant-derived).
When rennet is added to a milk mixture in order to help formulate curds in the cheese-making process, enzymes present in the rennet work to strip down the bundles of protein in the milk, called casein micelles. These stripped-down proteins can then link together to form a matrix, surrounding fat and water. This matrix, though solid, is not too tightly bound. When it comes to melting, rennet-coagulated cheeses melt in two stages: First, their fat globules change from solid to liquid, which makes the cheese more supple. Then, as the temperature continues to rise, the tightly bonded casein proteins loosen their grip on one another and the cheese flows like a thick liquid. Rennet-coagulated cheeses include cheddar, Monterey Jack, and mozzarella.
Cheeses coagulated with acid resist melting because the acid dissolves the calcium ions between the casein proteins and alters their electrical charge, both of which cause the proteins to link up tightly and clump. Heat then makes the proteins bond together even more tightly, which squeezes out the water and causes the cheese to dry out and stiffen. Acid-coagulated cheeses include cottage cheese and ricotta.
In freshly made cheeses, casein proteins are in tightly wound clusters, allowing for little interaction with one another. As cheese ages, it goes through a process called proteolysis, in which bonds within individual casein molecules are “snipped,” allowing the clusters to unwind and bind with other casein molecules, forming a matrix. Early in this process, the matrix is flexible, allowing young cheeses to melt smoothly. With time, the proteins bond together more tightly, forming a stronger network that requires more heat to melt and is less flexible when melted. This can result in more separated fat and clumps.
Young cheeses like mild cheddar and mozzarella melt really well. They turn from a relatively soft solid to a smooth, thin flowing mass without much heat, because their protein is held together by fairly weak forces. Young cheeses also contain high levels of water that separate the clusters of casein micelles and keep them from bonding too tightly.
Hard cheeses that have been aged, like parmesan, extra-sharp cheddar, and pecorino Romano, are difficult to melt. When they age they lose a lot of water, which allows the casein clusters to come closer together and form stronger bonds. These hard cheeses also contain a lot of fat trapped within the protein network. This fat can melt well before the protein begins to flow, resulting in separation of the fat and protein—a problem known as “breaking.”
This is excerpted from Cook’s Science: How to Unlock Flavor in 50 of our Favorite Ingredients, by the editors at America’s Test Kitchen and Guy Crosby, Ph.D. For information about the Cook’s Science Live Burger Tour, where you can purchase the book, see here.