Cheese sauce adds flavour to dishes from the humble mac and cheese to the elegant scallops Mornay. If your cheese sauce curdles, or separates, it can ruin the entire recipe. Prevent a sauce from curdling by grating the cheese as finely as possible, adding a starch --- such as flour or cornstarch --- to the cheese before melting and cooking the sauce over very low heat. If, despite your best efforts, your cheese sauce does curdle, you may still be able to save it.
Remove the sauce from the burner immediately. Cheese begins to separate at about 71.1 degrees C; too-high heat will cause your cheese sauce to curdle. Even after you remove the pan from the burner, the residual heat will continue to curdle the cheese sauce; if possible, pour the sauce into another pan.
Whisk a spoonful or two of lemon juice into the sauce. Curdling occurs when heat causes the milk proteins in the cheese to separate from the cheese. The acid in lemon juice interferes with this separation and can detangle the protein molecules.
Mix in a few spoonfuls of cream if you don't have lemon juice on hand. The fat in the cream stabilises the sauce, and the lower temperature of the refrigerated cream will help bring the sauce's temperature back below the curdling point. Don't add too much cream, as it can affect the flavour and texture of the cheese sauce.
Adding white wine instead of lemon juice to the sauce may also stop it from curdling.
Never add both lemon juice and cream to the sauce; the acid in the juice will react with the cream and make the problem worse.