The Food Lab: Better Home Cooking Through Science

easily mix to form a homogeneous mixture. In cooking, this
most often occurs with oil and water (and, for all intents and
purposes, vinegar or lemon juice can be considered water,
as they behave in the same way). You can put them in a
container together and stir them up, but eventually, like cats
and dogs, they will separate and stick with their own kind.
There are a couple of ways around this. The first is to
disperse one of the two—the oil, say—into fine-enough
droplets that water can completely surround them. Kind of
like putting a single cat inside a ring of dogs—there’s no
way for it to escape and rejoin its feline friends. A common
example of this kind of emulsion is homogenized milk, in
which whole milk is forced at high pressure through a fine
screen, breaking up its fat molecules into individual droplets
that are suspended in the watery whey. This is called an oil-
in-water emulsion, because the fat molecules are separated
and completely surrounded by water molecules. Most
familiar culinary emulsions are of this type, the most
common exception being butter, which is a water-in-oil
emulsion: tiny drops of water are completely suspended in
butterfat (of course, once you incorporate that butter into a
hollandaise sauce, you’ve converted it into an oil-in-water
emulsion; see here for more on hollandaise).
Simply mixing oil and vinegar forms an extremely
unstable emulsion—no matter how thoroughly you mix
them, no matter how much you separate the oil molecules,
eventually they regroup and your emulsion will break. In
order to form a stable emulsion, you need to add an
emulsifying agent known as a surfactant.
Remember that cartoon CatDog? The one with the head