By Harold McGee
In addition to the two incompatible liquids, a successful emulsion requires a third ingredient: an emulsifier. An emulsifier is a substance of some kind that coats the oil droplets and prevents them from coalescing with each other. Several different materials can serve this function, including proteins, cell-wall fragments, and a group of hybrid molecules (for example, egg-yolk lecithin) that have an oil-like end and a water-soluble end. To make an emulsified sauce, we add oil to a mixture of water and emulsifiers (egg yolk, ground herbs or spices), and break the oil up into microscopic droplets, which the emulsifiers immediately coat and stabilize. Or we can begin with a premade emulsion. Cream is an especially robust and versatile base for emulsified sauces.