Fix a grainy cheese sauce with a squeeze of lemon.
A cheese sauce that has curdled can be revived with a spoonful of lemon, although it's best not to overdo it with the acidity.
Dairy-based sauces, such as Alfredo, cheese, or Mornay, are famously difficult to perfect. They can curdle at any point, leaving a grainy, lumpy texture, but that doesn't mean you have to throw the sauce away.
Simply chill the curdled sauce, add a tablespoon of lemon juice (or white wine, if you don't mind the flavor), and whisk vigorously. This trick works best if no acid has been added yet, because too much can have the opposite effect and cause the sauce to curdle even more. Balance is key.
Sauce often curdles when it gets too hot because intense heat denatures the cheese proteins, causing them to clump. Acids dissolve these clumped molecules, preventing the sauce from thickening further. However, too much acidity also causes curdling, as do sudden temperature changes, such as adding cold cheese to a boiling sauce.
To avoid this, heat and cool the sauce gradually, and let the cheese reach room temperature before adding the acid. Grate the cheese as finely as possible so it melts more quickly over low heat.
An emulsifier also helps, because it acts as a binder and makes it harder for proteins to clump together and separate. Mustard, for example, not only adds flavor but is also an emulsifier, and a pinch helps maintain consistency. The next time your sauce looks a little lumpy, a squeeze of lemon juice can restore its smooth texture.
