We set up an experiment to find out.
An emulsion is a mixture of two liquids that ordinarily resist one another, such as the proverbial oil and water. (Vinaigrettes and mayonnaise are two classic examples.) Emulsions are creamier and thicker than nonemulsified sauces, which helps them coat and cling to food. We set up an experiment to demonstrate the mechanics at work in an emulsion.
We made two batches of the butter sauce that accompanies our Buttery Spring Vegetables. For the first batch, we followed the recipe: We reduced the leftover cooking water with shallots, vinegar, salt, and sugar and then whisked in cold butter 1 tablespoon at a time to create an emulsified sauce. For the second batch, we simply combined the same ingredients in a saucepan over low heat until the butter was melted.
The first (emulsified) sauce had a thick, velvety consistency, while the second was thin and separated, with watery shallots below and liquid butter speckled with milk solids on top. When we dipped radishes into the two sauces, the emulsified sauce coated and clung nicely, but the second sauce slipped right off.
There are two types of emulsions: water-in-oil and oil-in-water. Solid butter is a water-in-oil emulsion—fat with tiny droplets of water suspended throughout. When butter simply melts, as in our nonemulsified sauce, the water and fat separate from each other so that the resulting sauce feels slippery and greasy and resists clinging to the surface of foods. But if you gradually whisk cold butter into hot liquid, as in our emulsified sauce, you can actually transform the sauce into an oil-in-water emulsion.
Here's how it works: The water droplets in butter contain remnants of the cream from which it was made—proteins. These proteins act as emulsifiers, coating and separating tiny fat droplets as they disperse into the liquid when the butter melts. Because these fat droplets are now separated by water, the resulting sauce is more viscous than either melted butter or water alone. It also clings to moist vegetables because the fat droplets are surrounded by water (remember, water is attracted to water but resists fat).
Emulsifying might be a little extra work, but it comes with a big payoff. Emulsions are cohesive and creamy, which helps them to cling to food more effectively.