A broken sauce is a such a sad sight. Instead of a thick cream, suddenly you have grainy bits of fat floating in a bowl of watery liquid. Not very appetizing! So what happened?
There are a couple of reasons why your sauce might have given out on you:
The fat was added too quickly. If you add it too fast, the emulsifying agent (the egg in mayo and hollandaise) gets overwhelmed and has trouble forming that bridge between the fat and the liquid. Whisk the oil or butter into your sauce one teaspoon at a time, especially at the beginning. Once it thickens up a bit, you can add the fat in larger amounts.
The sauce got too hot. At high temperatures, the eggs in a hollandaise will start to coagulate and lose their ability to hold the emulsion together. At this point, the eggs are actually starting to scramble! When flour-based sauces like bechamel and veloute get too hot, the starch molecules also start to lose their ability to thicken properly. Since the eggs start to coagulate around 180-degrees, keep egg-based sauces well below boiling. Flour-based sauces can be heated a little more and can be kept at a simmer.
The sauce was kept warm for too long. Sauces are best served as close to serving time as possible. If you have to wait a while, it's usually best to let the sauce cool to room temperature and then very gently re-heat it while stirring with a whisk or spoon.
Your sauce was refrigerated. Unfortunately, these finicky sauces will also separate when they're cooled below room temperature. This is because the fat solidifies and breaks the emulsion.
And if your sauce does break after all this, there's still hope! Up next week: How to fix a broken sauce...
(Photo Credit: Taunton--Fine Cooking)