Q: What does it mean when a recipe says a sauce will “break”?
A: It means a cook’s hopes have been shattered.
Actually, no, that’s a short way of saying that a sauce has curdled, or has lost the ability to be emulsified.
To make most sauces, you create a thick, smooth texture by suspending molecules of starch and fat in a liquid, creating an emulsification. That’s what you’re doing when you cook flour in fat to make a roux and then whisk in hot stock, or when you whisk melted butter into egg yolks to make hollandaise.
If the starch, fat and liquid separate, we say the sauce has broken: Something has interrupted the emulsification. The most common reason is overheating the sauce or trying to keep it warm too long. Sauces thickened with cornstarch, such as Asian stir-fry sauces, are particularly prone to breaking if you overheat them.
Sometimes you can pull a sauce together again. You can whisk in more melted butter or a little hot water to fix a broken hollandaise, for instance.