The texture and consistency of several classic sauces, including mayonnaise, vinaigrette, and beurre blanc, depend on a technique known as emulsification. This important bit of kitchen alchemy lets you combine two ingredients, typically oil and water (or other thin liquid), that wouldn’t ordinarily combine. When successfully formed, an emulsified sauce will be thick enough to coat foods and stable enough to last for anywhere from an hour to a week without separating. A poorly formed emulsion won’t thicken and will appear broken or separated, with the oil or melted butter pooling on the top. Here are a few useful tricks and principles:
• An emulsion needs vigorous whisking or beating to form. Use a whisk, a blender, a food processor, or in some cases a mortar and pestle.
• Start with the water or liquid part of the emulsion and add the oil or other fat to that. For example, when making a vinaigrette, begin with the vinegar. For a mayonnaise, it’s the lemon juice that acts as the liquid, and in beurre blanc, it’s the wine and vinegar reduction.
• Whisk in the oil (or butter) very slowly—drop by drop—to start. Once the sauce begins to thicken and you can see that the emulsion is taking hold, you can add the oil in a steady stream. Never dump in all the oil at once.
• Set the bowl on a potholder or folded dishtowel to keep it from spinning while you whisk.
• If a sauce seems to be getting too thick (small droplets of oil may begin to appear on the surface), it may be about to break, or separate. Immediately stop adding the oil and whisk in a few drops of water, vinegar, or lemon juice to thin the sauce somewhat before continuing.
• If a sauce does break, salvage it by beginning another batch in a clean bowl, but instead of adding the oil or butter, whisk in the broken sauce. Unfortunately, due to the delicate nature of a beurre blanc emulsion, it doesn’t respond to this trick.
• Many emulsified sauces contain some type of emulsifying agent or emulsifier—an ingredient that stabilizes the emulsification. The most effective is an egg yolk, which is why egg-based mayonnaise is so stable. Mustard acts as the emulsifier for vinaigrettes, and vegetable purées work to thicken pesto and other vegetable-based sauces.