The Science of Melting Cheese

Have you ever gotten all excited to make a grilled cheese and then...your cheese just doesn't melt? Or maybe it breaks, separating into a tough, stringy mass surrounded by a pool of oil? Yup, it's a crappy feeling. Luckily, there are ways to know what you're getting into before that happens. Science ways.

Want to reliably tell whether the cheese you're buying is gonna dominate the melting game or stay nice and solid for cooking or grilling? Wish you could make any melting cheese melt better? Here's what you need to know.

On Curds and Whey: How Milk Becomes Cheese

Milk contains a whole slew of molecules, from bobbing fat globules to dissolved sugars, vitamins, and minerals. But when it comes to cheesemaking, milk's most crucial component is casein: a type of protein that clumps together in families called micelles. Picture them as little bumper cars, each holding several passengers. Calcium and hydrophobic (water-repelling) bonds act like the seat belts and doors of the cars, keeping everyone inside and secure. In milk's liquid state, the micelles have a slight negative charge, which makes them repel each other, just like bumper cars.

Turning milk into cheese is a matter of changing the properties of those proteins, causing the caseins to coagulate into milky curds and leaving behind the cloudy, watery byproduct known as whey. There are two ways to effect this change: with rennet (an enzyme found in the stomach of unweaned grazing animals like calves and lambs) and with acid (typically lemon juice or vinegar), both of which do so in combination with heat.

When milk is heated and treated with an acid, the electric charge of the casein bundles reaches a tipping point. Instead of veering away from each other, the micelles all fall apart—those seat belts and car doors disintegrate and all the passengers huddle together, clinging to each other for dear life and squeezing out everything in between. This forms a curd that's creamy but not particularly elastic, best-suited to fresh cheeses like queso fresco, chèvre, and paneer.

But when milk is heated with rennet instead, the effect is more nuanced: instead of the bumper cars disappearing altogether, they simply stop bouncing apart—the electricity shorted out and now the cars can get as cozy as they want. Our cars are in gridlock now, but instead of losing their cool, everybody's reaching out the windows and holding hands. Isn't it so sweet? Our caseins have formed a gelled network that traps fats and liquids in its web, rather than squeezing them out. These are the curds responsible for the vast majority of cheeses—the rest is a matter of aging, storage conditions, and the presence or addition of live cultures that help develop flavor and texture traits.

So what does all of this have to do with melting? Let's get specific.

When Cheese Gets Hot: Why Cheese Melts, and Breaks

Technically speaking, cheese is an emulsion of dairy fat and water, held together by a network of proteins. In cooler temperatures, that dairy fat remains a solid; let it warm to around 90°F and the fat reaches a liquid state and the cheese becomes more pliable—you may even notice some cheeses begin to bead with "sweat" if they're left out at room temperature. Raise the temperature by another 40 to 90 degrees and all the bonds that joined your caseins together start to break, allowing the entire protein structure to sag and stretch into an increasingly loosey goosey, lava-like puddle.

What determines a good melting cheese from a bad one has a lot to do with how well it can maintain its emulsion when that protein network begins to collapse, which in turn has to do with the ratio of water to fat, as well as the strength of that protein network.

First and foremost, the balance of water and fat has to be more or less maintained—otherwise the fat molecules will slip free and draw together. That's why younger, high-moisture cheeses like mozzarella, Taleggio, brie, Gruyère, Emmental, and Jack are such reliable melters, while drier grating cheeses like Parmesan or Pecorino-Romano which have already lost much of their moisture to evaporation, often separate into clumps or even break.

Those aged cheeses have a further disadvantage: as cheese ages, its proteins tend to form tighter and tighter clumps, making them less effective at binding fat and water together in a smooth matrix. That's why even some cheeses aged in a water-tight barrier (like gouda that's aged in a wax shell or some types of waxed aged cheddar) will have a tough time melting smoothly, despite their high moisture content.

Even with excellent melting cheeses, excessive or long-term heat will gradually cook off and reduce their water content until the milk proteins shrink and brown and the fat molecules—no longer trapped by a firm protein network or suspended in water—escape and pool together. Suddenly, instead of a smoothly melted cheese, you have a tough ball of casein proteins floating in grease: the protein structure shrinks so much it simply can't contain the fat. Think the greasy slick and tough curds on top of an overcooked pizza or a lasagna.

Which Cheeses Melt Best

Your most reliable melting cheeses will be young, moist, rennet-set cheeses that are easily sliced at room temperature. We even have a list of less-common, full-flavored melters that are perfect for pizza.

But what if you're craving the intense flavor of a harder, aged cheese but want the melting capabilities of a young cheese? Well we've got some workarounds up our sleeves.

Turning Bad Melters Good

One of the most common ways to help cheese melt smoothly is to add starch: this is a process that works in sauces like mornay, or nacho cheese sauce. Starches and other thickeners will physically impede fat molecules from joining up into larger droplets, as well as making the water phase of the cheese more viscous. All of this helps to prevent fat from coalescing and breaking out.

Adding acid, like the tartaric acid you'll find in a tart white wine, can also help prevent the cheese's casein proteins from clumping together and turning stringy, like in a classic fondue (our recipe uses a touch of starch for further protection from breaking).

But when even that's not enough, it's time for the big guns: more liquid. This is where Kenji's genius steaming technique comes into play. Steaming a cheese doesn't just heat it more evenly and gently than, say, a toaster oven, but it reintroduces water that's otherwise lost in more traditional heating methods. You can even go the whole nine yards and stir in a combination of milk or cream and starch like Gold Medal Wondra Flour—this method's perfect for topping burgers or filling your next grilled cheese. Check out the full story or go one step farther and learn how to make American-style cheese slices from scratch out of nearly any cheese! (Hint: it involves a food processor, gelatin, and a whole lot of goo).

The Super Melters: Processed Cheese

Process cheeses, including "American" cheese, are cheese products designed to be extremely meltable and nearly impossible to break. This is accomplished by starting with real cheese (usually a young cheddar-style), and blending it with extra milk (for the added water), extra milk protein micelles (for the strong protein structure), and some form of chemical salt that helps prevent proteins from tightening. Because American-style cheeses have so much excess liquid added to them, they have extremely low melting points, making for extra-gooey grilled cheese sandwiches or cheeseburgers. This meltability, of course, comes at the cost of less intense flavor.

Acid-Set Cheeses: The Grillers and Fryers

There's one type of cheese that no amount of tweaking will melt: Acid-set cheeses, like fresh goat cheese, quick farmers cheese, paneer, queso fresco, and ricotta, just can't do it. That's because acid, unlike rennet, actually "dissolves the calcium glue that holds the casein proteins together in micelles," explains McGee in On Food and Cooking. "So," he continues,"when an acid curd is heated, the first thing to be shaken loose is not the proteins, but water." Continue to heat an acid-set cheese, and its proteins will get closer and closer together and increasingly more water will be cooked off, but without that calcium holding everything together, you're not going to get any melting action.

Sometimes that's a great thing—just check out all the cheeses you can cook with and even grill.