There's something totally entrancing about watching cheese melt, as it changes from a solid block into a delicious, dripping glop of cheesy goodness. This transformation isn't magic, though. The reasons why some cheeses melt better than others is all based in science. And though it's easy enough to melt cheese, the actual science of melting cheese is somewhat complicated, and there are few factors that will determine if a cheese will melt or not. But in order to best understand how cheese melts and what factors affect the way in which cheese melts, you first need to understand the structure of cheese.
"Cheese is mostly protein, fat, and water," explains David Montgomery, outreach specialist and assistant coordinator at the Center for Dairy Research in Madison, Wisconsin, adding, "You can kind of think of cheese as a sponge." The protein strands, also known as casein, form the spongey-part of the sponge, the structure that gives the cheese its shape. The gaps in between the strands—or the "holes" in this proverbial cheese sponge, if we're continuing with Montgomery's metaphor—are filled with fat and water, the other two main ingredients in cheese. So when a cheese is heated up, the protein structure breaks down, releasing the fat and the water, and that is what causes cheese to melt.
"So with composition," says Montgomery "in a generic sense, a higher fat, higher moisture cheese tends to melt more." That's because the more fat and water there is in the cheese, the weaker the protein structure is, "so it tends to break down faster." An example of this is Havarti cheese or a young Gouda. It's high-fat, high-moisture, and it melts like a dream.
The acidity of a cheese will also have an impact on how well it melts. Cheeses with a neutral acidity, meaning 7.0 on the pH scale or a little lower, don't tend to melt. "It's the same thing with a really, really low pH cheese, so very high acid," explains Montgomery, giving feta cheese and cottage cheese as examples. "At a very low pH," he continues, "the proteins are really, really tightly bound together," which means that the protein structure won't collapse and cause the cheese to spread and melt. If you're looking for melt, you want cheese with a pH right between neutral and acidic—between 5.0 and 5.5 on the scale. Both mozzarella and cheddar cheese fall into that range, which is one of the reasons that both cheeses melt so well.
But just because a cheese melts well, doesn't mean it'll stretch—and there is a very real, scientific distinction between the two terms. If melt is a cheese's ability to spread out into a puddle, stretch is its ability to maintain its structure and not break even when pulled apart. If you're having a hard time picturing the difference, imagine a cheese pull; that's stretch. And in order to get stretch, the stands of protein need to be as long as possible. That way, the structure of the cheese stays together even as pressure is applied in opposing directions.
This is where the age of the cheese comes into play. As a cheese gets older, the protein strands start to break down, which makes it less likely to stretch. Instead, the cheese will snap as you pull it apart. But an aged cheese can still melt well. Montgomery gives the example of cheddar cheese. A young cheddar cheese both melts and stretches, because it's got relatively long protein strands, a medium-pH level, and relatively high levels of fat and moisture. An aged cheddar cheese still has the same pH level and comparable levels of fat and moisture—but the protein strands have broken down. So when you heat it, the aged cheddar cheese will still melt. "But because your protein structures have broken down, you won't get stretch," explains Montgomery, unlike the younger cheddar cheese.
There are, of course, exceptions to every rule, especially in the wonderful world of cheese, and it can sometimes be hard to tell which of a cheese's characteristics will override the others. For instance, many Latin American cheeses have a high moisture and fat content, which makes you think they'll melt, but they don't because the acidity is too high. But, in general, the moisture, fat content, and acidity of a cheese will determine how well it melts, and the fidelity of the protein structure will determine the cheese's ability to stretch.
So take this knowledge and experiment with melting cheese—all in the name of science, naturally.