A vinaigrette (as well as the mayonnaise and the aïoli) is an emulsion. That is, a mixture of two liquids that are otherwise difficult to combine. In this case, that’s the vinegar, which is mostly water, with the oil. With very specialized lab equipment that’s pretty impractical for a home cook, it is actually possible to make a stable emulsion (one that won’t separate) with only the vinegar and the oil.

For all intents and purposes, though, we need to use an emulsifier to glue the two phases together. Emulsifiers contain molecules that have both a water loving (hydrophilic) and oil loving (oleophilic) end, called surfactants. These surfactants coat the molecule of one kind, so they can sit more easily in the rest of the mixture. In the case of this vinaigrette, the surfactants present in the mustard coat the water from the vinegar so it doesn’t separate from the oil.

The quality of the emulsion also depends on the size of the dissolved droplets. The smaller the dissolved droplets, the smoother and more stable the emulsion. At one extreme, if the oil and water are completely separated we have in effect two “mega droplets” and, obviously, no emulsion at all.

Once we get the droplets reasonably small, we wind up with something resembling a sauce because the droplets are small enough we can’t distinguish the different parts from one another. For a great sauce, however, we want to make the droplets as small as possible so that it doesn’t come back apart later.

We can do that by making the droplets smaller. In part, our goal is making it difficult for the water molecules from the vinegar to touch each other, because if they touch each other they’ll stick to each other, and at a certain point we’ll have a big enough blob of water molecules we’ll notice it when we pour our sauce over our food.

In the emulsion, the water molecules are kept apart from one another with the surfactants in the mustard. But this layer is only ever going to be one molecule thick, so the smaller we can make the vinegar droplets, the less likely their molecular “cage” is to break. Think of trying to keep liquid contained in a plastic bag. If we only put a few drops of water in the bag, there’s virtually no chance the bag breaks apart, and the water is completely contained. If we fill the bag well beyond its capacity, the smallest disturbance could cause it to break and let the water out.

Whether they know it or not, this is why many people suggest that while making a vinaigrette or other emulsified sauce you very slowly pour the oil into the rest of the sauce while whisking. They’re hoping that by adding only a very tiny amount of new material, your vigorous whisking will be distributed among a smaller amount of liquid, thus breaking it up into smaller droplets.

This process is fiddly, and it’s not very effective. Even the most athletic person can’t move the whisk that quickly, and the tines of a whisk are pretty blunt. It would be impractical if you risked cutting your hand open picking up the whisk the wrong way around. Because it’s so hard to create small molecules, the risk of failure is high, however judicious you are with slowly adding the oil while whisking.

Instead, get yourself an immersion blender. The blades are very sharp, perfect for slicing through the oil and water to create really small droplets. The high-speed motor also operates at speeds far above anything practically achievable with your arm. As a result, we get a much more consistent, robust process. It’s good enough you don’t even need to worry about slowly drizzling oil into the mustard and vinegar: dump everything into a container and go.

Ingredients

  • 40 g Dijon mustard
  • 60 g red wine vinegar
  • 160 g neutral oil
  • Salt
  • Black pepper

Method

Combine the mustard, vinegar, and oil into a narrow vessel just wide enough for an immersion blender. Blend the mixture together until it’s thick, smooth, and homogeneous. Add salt and pepper to taste. Use immediately, or refrigerate for up to a week in an airtight container.