Cheese's combination of savory flavors and rich creaminess make it one of the world's most widely loved foods. It can be soft or hard, fresh and mild or aged and sharp, and served cold or heated. For many cheese lovers, it's best of all when hot and golden-brown from the oven or broiler. The color and rich flavor of toasted cheese both come from the same chemical reaction.
Fresh milk is a highly nourishing food, filled with protein and calcium, but it's also very perishable. If that milk is curdled with acids or rennet from a calf's stomach, and if the curds are well rinsed and salted, the resulting cheese can be stored for much longer than raw milk. That was the initial motivation for making cheese, and -- like bacon or ham -- it's still made today simply because it's delicious. Cheese contains much of the fat, protein and calcium from milk, along with varying degrees of moisture. The proportions of fat, protein and moisture determine how soft a cheese becomes when it's heated.
When cheese is warmed to about 90 degrees Fahrenheit, the milk fat begins to soften and melt just as it does in butter. Your cheese will "sweat" small droplets of oil, giving it a greasy appearance. As the temperature rises, the protein structure that gives the cheese its shape will begin to collapse. Most cheeses will begin to melt and flow at temperatures between 130 F and 150 F. As the heat continues to build, moisture will evaporate from the surface of the cheese and leave behind a thin, dry layer at the surface. This "skin" is where the browning takes place.
The browning process is referred to as a Maillard reaction, after the French scientist who first described it. The surface of the cheese is initially cooled by the evaporation of moisture, but once a thin layer of dried cheese forms, the heat quickly rises. Once the cheese's temperature passes 230 F, the amino acids that make up its proteins begin to break down under the heat. Those amino acid fragments re-form into a number of larger, more complex molecules, changing the color of the cheese and creating a number of subtle flavors. That's why browned cheese is so tasty.
Your choice of cheese for browning will depend on the dish you're making. For example, dense Greek haloumi cheese softens but doesn't melt when it's heated, so you can pan-fry it golden in a hot skillet. The mozzarella on a pizza takes a relatively long time to brown, because its high moisture content takes time to evaporate. It's often mixed with other cheeses to speed browning. Relatively dry cheeses, such as Parmesan, brown readily because there's correspondingly little water to evaporate. Chefs often melt and brown small mounds of Parmesan on a silicone mat to form golden, flavorful cheese "crackers" that can be shaped when warm, but become crisp as they cool.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
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