Butter Sauces

Butter, like egg yolks, works as an emulsifier, albeit one that needs to be handled carefully to keep it from separating.

Butter contains about 25 percent water and about 5 percent protein, and the balance is fat. The protein is what keeps the fat and water in an emulsion. If you heat butter in a pan, the proteins are “denatured” and lose their ability to emulsify, and the butter separates into frothing proteins, water, and clear butterfat. But if butter is whisked into liquid, especially a liquid that is already an emulsion, such as a sauce thickened with flour, the emulsion is preserved. Keep two things in mind when making butter sauces: the sauce must never boil and the butter must be cold and whisked continuously until it emulsifies into the sauce.