By David Joachim and Andrew Schloss
from Fine Cooking #114, pp. 38-39
What do mayonnaise, hollandaise, and vinaigrette have in common? They're all emulsion sauces, which means they get their luscious mouthfeel from fat suspended in water. But we all know that fat and water don't mix, so emulsified sauces are always on the verge of "breaking," or separating. Knowing the science behind that separation can help you prevent it.
Emulsion sauces are made by mixing two substances that don't normally mix. To do this, you have to break one of them into millions of miniscule droplets and suspend those droplets in the other substance by vigorously whisking, or better yet, blending them in a blender or food processor.
When two substances don't naturally mix, it's because the molecules of each are more attracted to themselves than to the others, so even the most thoroughly combined emulsion sauce will not stay combined for long. To prevent separation, a substance called an "emulsifier" is often mixed in. Emulsifiers, such as egg yolks and mustard, are made up of big, bulky protein molecules. When combined with fat, like oil or butter, and watery ingredients, like vinegar, lemon juice, and of course, water, these molecules get in the way, making it harder for like molecules to find and bind to each other. Therefore, there's a better change that the emulsion will hold.
Some of the most common emulsion sauces are vinaigrette (oil suspended in vinegar, sometimes emulsified with mustard), mayonnaise (oil suspended in lemon juice and water, emulsified with egg yolk), hollandaise (melted butter suspended in lemon juice and water, emulsified with egg yolk), and beurre blanc (butter suspended in white wine vinegar, emulsified by the milk solids in the butter).
Add ingredients in the right order. Begin with the watery ingredients mixed with an emulsifier; then whisk in the oil or butter.
Start slowly and whisk vigorously. It's important to make the network of fat droplets as fine as possible. To that end, you need to add the oil in a slow stream or the butter a tablespoon at a time, while whisking constantly and making sure each addition is fully incorporated before adding the next. The fat droplets will become interspersed with the emulsifier, which encourages them to stay in suspension and form a thick, stable emulsion. Once the sauce gets viscous, it's safe to add the remaining oil or butter more quickly.
Control the temperature. If an emulsion sauce that contains egg, like hollandaise, gets too hot, the egg proteins will coagulate and turn the sauce lumpy and thin. In an eggless emulsion, like a beurre blanc, too much heat will make the butterfat separate from the butter and leak out.
Sometimes, despite your best efforts, an emulsion sauce might still break on you. If this happens, you may be able to rescue it. See the chart below for some quick fixes.