There's no doubt about it--emulsions are tricky. They're confusing to understand and they're confusing to make. Sometimes even the most seasoned chef can have trouble getting their sauces to emulsify! But if you can start to get a feel for the science behind the scenes, you'll feel more in control and confident next time you decide to whip up a hollandaise sauce for Sunday brunch.
Ready? Here we go!
At its most basic, an emulsion is a suspension two liquids within each other that would not naturally mix.
Think of a liquid--a cup of vinegar, for instance--as made up of millions of tiny droplets. If you pour oil into the vinegar, at first the oil will float on the top of the vinegar because it's less dense. However, if you whisk them together, the tiny droplets forming each liquid start to mix together and become suspended within each other. This is an emulsion.
However, this simple vinaigrette will eventually separate back into vinegar and oil because, at a chemical level, there is nothing holding the drops of each liquid together except for the temporary confusion of having been whisked together.
To get a stable, permanent emulsion, you need to use something to hold the drops of opposing liquid together and prevent them from separating. This "something" is called an emulsifying agent. And this agent is like a mutual friend who holds the oil-based liquid in one hand and the water-based liquid in the other. It creates a chemical bond with each liquid and becomes a bridge between them.
The most common emulsifying agent is an egg yolk, as in mayonnaise and hollandaise sauces. Two others are the casein found in butter and the fine particles of ground dry mustard.
Up next week: why sauces break and how to fix it!
(Photo Credits: Fresh Finds, Emma Christensen, and Kathryn Hill)