Melting cheese into sauce

Why

Cheese is a network of the milk protein casein that holds fat and water in place. Gentle heat softens the fat and loosens the network so the cheese flows smoothly; overheat it, though, and the proteins tighten into rubbery clumps and squeeze out fat that ends up floating on top. Hard, aged cheeses with little moisture cope worst, young supple cheeses melt more willingly, and grated cheese melts quickly and evenly, so it has no time to overheat. Starch – from flour in the sauce base or dusted over the cheese – gets in between the proteins and fat droplets and stops them clumping; acid from wine or lemon binds the calcium that holds the casein network together, which is why cheese fondue is made with wine.

How to do it

• Grate the cheese finely and let it come to room temperature.
• Before adding the cheese, take the sauce off the heat or turn the heat to minimum; it must no longer bubble.
• Add the cheese a handful at a time and let each one melt before adding the next.
• Use a sauce thickened with roux as the base, or toss the grated cheese with a teaspoon of starch beforehand.
• Add a splash of white wine or a few drops of lemon juice to a cheese sauce without flour.
• Once the cheese has melted, don't boil the sauce again; just keep it warm.

If it goes wrong

Take a slightly grainy or greasy sauce off the heat, add a few drops of lemon juice or a spoonful of cold cream and whisk hard or use a stick blender. A teaspoon of starch mixed with a little cold water or wine works more reliably: stir it in and warm the sauce briefly, stirring constantly, until it comes together. Cheese that has seized into a rubbery ball won't melt smoothly again – fish it out, and next time grate finely and melt over lower heat.