It is not everyday that we have the opportunity to have a post on grilled cheese sandwich, so we were glad to see that The Kitchen as Laboratory: Reflections on the Science of Food and Cooking opens with Jennifer Kimmel’s chapter “The Science of a Grilled Cheese Sandwich.”
Here is the chapter in its entirety:
Why do certain varieties of cheese make great grilled cheese sandwiches? The secret lies in understanding how the molecules within cheese influence the ooey-gooey melted goodness that is the essence of a perfect grilled cheese sandwich.
It all begins with the cow (or goat or sheep). After all, cheese, no matter the variety, gets its start from milk. Even though milk is made of 80 to 90 percent water (in most hoofed species), it is still a very good source of proteins (casein and whey), carbohydrates (lactose, or milk sugar), and minerals (especially calcium). These three components, along with milk fat, are the essential ingredients for making cheese. Proteins (primarily caseins) give cheese its structure and allow the fat and a small amount of moisture to be retained while the majority of the water is removed. Lactose provides a food source for the growth of bacteria, which lend individual cheese varieties their distinctive flavor. The calcium in the milk determines how the proteins interact and this interaction ultimately dictates the softening, melting, and stretching characteristics of the heated cheese in a grilled sandwich.
Before milk is converted into cheese, the casein proteins are arranged in individual clusters, called micelles, which are suspended in what is known as the aqueous phase. They contain two-thirds of the milk’s total calcium and have a net negative charge that prevents them from aggregating together. To convert milk into cheese, however, the proteins must aggregate and form a curd, trapping both fat and water. To achieve aggregation, the negative charge must be eliminated from the casein micelles. This is accomplished either by adding acid and neutralizing the negative charge or by adding an enzyme and cleaving the portion of the cluster that contains the negative charge. While the transformation from protein aggregation to cheese is complicated, the main steps include cooking the curd and draining the whey, followed by salting and pressing the curds together. Aging is the final step, which allows for structure and flavor formation.
The ideal cheese characteristic needed to make a grilled cheese sandwich is melt. Who does not love to cut into a hot grilled cheese sandwich and see smooth, creamy melted cheese oozing from between the slices of grilled bread. But why do some cheeses melt better than others? Why do certain varieties melt as homogeneous molten masses, while others as oily lumps? Again, we go back to the molecular interactions within the cheese, primarily the interactions between the casein proteins and the calcium. The casein proteins are held together in the micelles by calcium bridges, and the number of these bridges is influenced by the acidity of the cheese. As cheese ages, more of the lactose is converted to lactic acid, causing the pH of the cheese to decrease and become more acidic. This, in turn, causes a dwindling in the number of calcium bridges within the casein micelles as the calcium solubilizes and moves from its position among the proteins to the entrapped water within the curd. The fewer the number of calcium bridges, the greater the mobility of the proteins as their connections give way.
The loss of the calcium bridges allows for the casein proteins to become more soluble (that is, less held together in the micelle), which also helps to better bind the fat originally trapped in the cheese. Therefore, when the cheese is heated, the protein molecules are able to flow, resulting in a nice even melt. Moreover, the soluble protein molecules are able to interact with the oil droplets, preventing them from leaking out of the cheese and causing an oil slick to form on top. However, if the pH continues to decrease, then too much of the calcium is solubilized and the caseins collapse on one another as a function of their low-pH insolubility. This results in a curdy melt with free oil leaking from the cheese. If the pH is too high, then not enough calcium is solubilized and the casein proteins are held too tightly together by the greater calcium bridging, resulting in cheese that does not melt or flow. An appropriate pH for the good melting of cheese is in the vicinity of 5.3 to 5.5. It of course depends on how the cheese is made, but in general, this pH range balances the different protein interactions and allows for a good melt.
Examples of cheeses with good melting properties include Gruyere, Manchego, and Gouda. These varieties balance the final cheese pH to achieve both soluble calcium and soluble protein, resulting in a cheese mass that melts and flows upon heating while keeping the fat trapped within the matrix. The pH can be both perfectly balanced and too acidic in a single cheese variety. Take mild versus aged cheddar. While the mild cheddar melts evenly and maintains the fat within the matrix, the aged cheddar, because of its lower, more acidic, pH, will melt into lumps, releasing free fat.
Manufacturers of processed cheese have developed a method to decrease calcium bridging. They use salts (specifically, citrate and phosphate salts) to bind the calcium from within the casein micelles. To make processed cheese, natural cheese is cooked with these salts, decreasing the calcium bridging from within the casein micelles as the overall pH is increased. The same thing happens to cheese fondue when wine is added. The tartaric acid in wine binds calcium from within, affecting the bridging process.