From pizza to panini and from quiche to quesadilla, melted cheese plays a starring role in countless culinary classics. But why do some cheeses melt into ooey-gooey deliciousness, and others simply don’t? Forget black holes or the origins of life—this is the sort of question that science was created to address.
“Cheese melting is a very complex phenomenon,” says Prateek Sharma, a food scientist at Utah State University. “When people eat cheese, they should think about what may be going on inside the cheese at the molecular level.”
At that level, the process by which all the action happens involves a protein called casein. In milk, casein delivers the vital nutrients calcium and phosphate to a calf, lamb or baby. But the cheese-making process turns casein into a network held together by weak bonds and studded with molecules of water and fat.
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In general, the dissolution of that network—the melting of the cheese—plays out in stages as the cheese is heated, Sharma says. First, beginning as cool as around room temperature, the cheese’s assorted fats will begin to melt and seep out of the network, floating to the surface (this is why cheese gets sweaty). Next, as the cheese reaches the temperature of water from a hot tap, the proteins within it start losing water, shrinking and softening. By around 160 degrees Fahrenheit, about the temperature of thoroughly cooked meat, the melting process is complete, Sharma says. “Everything is molten,” he adds.
But thoroughly melted cheese doesn’t act like typical melted substances. When ice, butter and chocolate melt, they become liquid: they flow and drip. In contrast, mozzarella oozes and stretches, making it the poster cheese for good melting.
The magic happens thanks to casein—more specifically, as a function of the loose interactions between casein molecules. The casein network needs to be flexible enough to move but rigid enough that the cheese hangs together. Increasing the amount of water or fat locked into the casein network can encourage melting, but so can modifying the casein itself.
For a satisfying melt, cheese needs a Goldilocks level of bonds between molecules of calcium phosphate embedded in the casein that lock together the protein network. “If you have too much calcium content, cheese will not melt,” Sharma says. “If there is too little calcium, then it will melt very fast, so it will be runny.”
Cheese makers’ key tool in adjusting the number of these bonds is acidity, says John Lucey, a food scientist at the University of Wisconsin–Madison and director of the Wisconsin Center for Dairy Research. In cheese made at a relatively neutral pH, there are enough calcium bonds that casein molecules are stiffly bound to each other.
Add some acid, and things start to loosen up in the resulting cheese. “That dissolves some of the calcium and phosphate bonds—just enough of them that the system becomes more fluid,” Lucey says. But add too much acid, and so many calcium bonds dissolve that charged areas on casein molecules can attract each other directly and continue to interact tightly enough to prevent stretch in the finished cheese. A good melt requires just the right amount of acid.
A second phenomenon affecting the way a cheese melts is the aging process. As cheese matures, enzymes and microorganisms inside of it can snip the casein molecules themselves. With more of this snipping, the cheese melts more easily but is less able to stretch and more likely to become soupy.
All this emphasis on the role casein plays in how cheese melts also helps to explain why vegan cheese—with no casein at all—is relatively soft even when chilled and often downright unappetizing when heated. “Plant-based cheeses, they start off as cold Jell-O, and then they become hot Jell-O,” says Neil Cunningham, a materials scientist at the Center for Industrial Rheology in England, who has compared these cheeses with dairy ones.
Ricotta is another poor melter, even though it isn’t vegan. That’s because it’s made from whey, the liquid left over from making cheeses. This starting material means that ricotta contains a little casein but mostly consists of whey proteins that build a very different type of structure—one that is characterized particularly by permanent bonds between sulfur molecules, Sharma says. Those permanent bonds make it much harder to truly melt ricotta; it merely softens.
But when casein dominates, understanding how acid and aging have affected the protein structure goes a long way in explaining the different melting characteristics of different cheeses. Compared with mozzarella, “something like feta is much more brittle and chalky and crumbly,” Lucey says. That’s because of the high acid content. On the other end of the spectrum, panela won’t melt—there’s not enough acid. “It’s just held together too robustly—it’s not fluid,” Lucey says. A blue cheese may start out quite acidic and hard to melt but become less so as it ages and microbes break down the acids. “Eventually,” he says, “you might be able to melt some of that into a kind of liquid pool.” Lower moisture levels make Parmesan less conducive to melting.
For Lucey, this is the marvel of cheese-making: most varieties are the same milk treated in slightly different ways, all built on centuries of experimentation. “That’s why you get hundreds of different types of cheese,” he says. “It gives you artistic license to change anything that you want to and see how it turns out.”