Why Does Cheese Melt, and Why Do Some Cheeses Refuse To?
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Heat a slice of cheddar and it slumps into a glossy pool; heat a slab of halloumi and it browns, squeaks and keeps its shape. Both are cheese, both are mostly protein and fat, and the difference between them is written into how each was made. Melting is what happens when the net of protein holding a cheese together is loose enough to flow, and cheesemakers control that looseness with acid, calcium and time.
A net of protein around drops of fat
Cheese is milk with most of the water removed, and what remains is a scaffold of casein, the main milk protein, holding pockets of fat and a little water. In milk the casein floats as tiny clusters. Cheesemaking makes those clusters join into a network, either with the enzyme rennet, which snips off the part of the casein that keeps the clusters apart, or with acid, which neutralises the charge that repels them. Salt, pressing and ageing then tighten or loosen the result.
When cheese is heated, two things happen in order. Around 30 degrees Celsius the milk fat inside the pockets turns from solid to liquid, which is why a cheese left in a warm room sweats. Then, somewhere between about 55 and 80 degrees depending on the cheese, the bonds between casein clusters weaken enough for the network to slide over itself. The cheese flows, and the liquid fat lubricates the flow.
Calcium, acid and the stretch
What holds the casein network together is largely calcium, which forms bridges between the protein clusters. A cheese with plenty of calcium bridges has a tight, strong network; one with fewer has a looser one. The amount is set by acidity during cheesemaking, because acid dissolves calcium out of the curd. A moderately acidic curd, like that of mozzarella or young cheddar, keeps enough calcium for a network that softens and stretches when heated but still holds together. That is the pizza pull: long casein strands, aligned during the stretching of the fresh curd, sliding past each other without breaking.
Mozzarella stretches best when fresh. As a cheese ages, enzymes chop the casein into shorter pieces, and short pieces cannot form long strands. An aged cheddar or parmesan therefore melts into a smooth but unstretchy mass, and if pushed too hard it breaks: the protein clumps, and the fat, no longer held, runs out as oil. A grilled cheese made with a two-year-old cheddar will pool grease for exactly this reason, while a mild young one stays creamy.
The cheeses that will not melt
Some cheeses hold their shape in a pan because their network was built differently. Paneer, ricotta, queso fresco and most fresh goat cheeses are set with acid rather than rennet, and acid strips out so much calcium that the casein forms a dense, tangled mass rather than a flexible net. Heat it and there are no bridges left to loosen; the protein simply firms up further, as an egg does. Halloumi is a rennet cheese, but its curd is cooked in hot whey during manufacture, which sets the protein in a rubbery structure that browns without flowing. Feta is too acidic and too salty to melt cleanly and crumbles instead.
Very old hard cheeses sit at the other extreme. Parmesan melts, but reluctantly and with a tendency to go grainy, because its protein has been broken down for years and its moisture is very low. The classic behaviour of each type:
- •Fresh mozzarella, young cheddar, Gruyere: melt and stretch
- •Aged cheddar, Gouda: melt smoothly, stretch little, can separate
- •Parmesan, pecorino: melt slowly and can go grainy
- •Halloumi, paneer, ricotta, feta: hold their shape or crumble
How a sauce stays smooth
Melting cheese into liquid is where most kitchen disasters happen. Cheese stirred into hot milk often splits into rubbery strings and oil, because the casein clumps together as the calcium bridges reform in the liquid. Traditional recipes prevent it with a starch base, a flour-thickened white sauce for macaroni cheese or cornflour in a fondue, whose starch molecules get between the protein clusters and keep them apart, and with acid, which is why fondue contains wine and lemon juice.
Processed cheese and the smoothest modern sauces use a chemical trick. Sodium citrate, an emulsifying salt, grabs the calcium that would otherwise bridge the casein and swaps in sodium, which does not. The network can no longer clump, and the cheese melts into a glossy, stable liquid at any temperature. A teaspoon of sodium citrate in a cup of water will turn almost any cheese into a perfect sauce, which is the whole secret of the slice on a burger.
The takeaway
Cheese melts when heat liquefies its fat and loosens the calcium bridges holding its casein network, letting it flow; young rennet cheeses stretch, aged ones melt without stretching and can split, and acid-set or whey-cooked cheeses such as paneer and halloumi have no bridges to loosen and hold their shape. Starch, acid or an emulsifying salt keeps the protein from clumping in a sauce.