An emulsion is a matrix made up of two liquids that don’t get along with one another. That is, liquids that won’t dissolve in each other in the way that say, sugar syrup and water do. In order to make an emulsion you need two liquids that retain their individual identities when combined. In the kitchen, that always means oil (fat) and water.
The neat thing, or should I say one of the many neat things, about emulsions is that they are reversible. You can have an oil-in-water emulsion, like a mayonnaise, or a water-in-oil emulsion like a vinaigrette. They’re like two stacked plastic cups: A can contain B, or B can contain A, whichever way you like.
Of course the big problem with emulsions is once you make one, it just wants to separate back out again. The tiny droplets of oil and water can hardly wait to fuse back together into larger and larger masses. But we can prevent this by a judicious application of emulsifying agents. These come in two basic types.
The first type is made up of two broad families of molecules: monoglycerides and diglycerides. What these multivarious cousins of fats (triglycerides) all have in common is that they have water-loving heads and fat-loving tails. When introduced to an emulsion they’re drawn to the fat droplets, where they immediately bury in their fatty little tails, their water-loving heads protruding. Thus the surface of the fat droplet becomes studded with “bumpers” that repel other fat droplets when they approach.
The second broad family of emulsifiers consists of various proteins. As you may remember, proteins as a rule are long, languid molecules made up of amino acids. Normally they’re clustered together in clumps, but when separated out and unwound in a liquid (or “denatured”) they disperse, generally interfering with other molecules’ attempts to bond to one another. Protein emulsifiers do this process one better with their own fat-loving and water-loving regions that function in very much the same way as the aforementioned pseudo-fats.
Yet no emulsifier can keep an emulsion steady by itself, which is why stabilizers like proteins, starch molecules, pectins and gums are usually employed to keep everybody in line (which is to say, separated). In fact it’s the gum from the seed coat of the mustard grain that acts as the primary stabilizer in vinaigrette. Very handy that it also tastes so good.