An emulsifier prevents droplets from joining together. Mustard or vegetable puree prevents a vinaigrette from separating.
An emulsifier (also known as an emulgent) is a substance which stabilizes an emulsion by increasing their kinetic stability. One class of emulsifiers are known as surface active substances, or surfactants. Examples of food emulsifiers are egg yolk (where the main emulsifying chemical is lecithin), honey, and mustard, where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers; proteins and low-molecular weight emulsifiers are common as well. Soy lecithin is another emulsifier and thickener. In some cases, particles can stabilize emulsions as well through a mechanism called Pickering stabilization. Both mayonnaise and Hollandaise sauce are oil-in-water emulsions that are stabilized with egg yolk lecithin.