The secret to mixing oil and vinegar is to use lots of force. The secret to keeping them separate is to use an edible binder or glue. Both of these principles come into play when you make a vinaigrette emulsion, or a mixture of two liquids that don't combine easily.
Oil and vinegar are hard to mix, and separate easily, because their molecular structures repel each other: Fat molecules in oil are hydrophobic, meaning that they are not attracted to water; and the water molecules in vinegar are hydrophilic, meaning that they are attracted to only water.
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Once you forcibly break the molecules apart, the tiny droplets combine. Lecithin, a natural emulsifier found in egg yolks and other foods, keeps the liquids combined. Commercial manufacturers use xanthan gum or other substances that have the same effect as lecithin.
You have options when it comes to making a classic vinaigrette, typically made with 3 parts oil and 1 part vinegar. These three methods blend the liquids together for only a few minutes before they separate again:
- Place the oil and vinegar in a jar with a tight-fitting lid and shake the jar vigorously for 30 seconds.
- Whisk the two liquids in a bowl with a wire whisk.
- Blend the oil and vinegar using an immersion or countertop blender or a food processor.
Because oil and vinegar without an emulsifier separate immediately, dress your salad with a homemade vinaigrette just before serving and right after shaking or whisking the oil and vinegar together and before it actually stops moving. The ingredients will re-emulsify every time you give them a vigorous shake.
Secrets of Success
Using emulsifiers and adding the ingredients in a specific order help keep oil and vinegar emulsified for up to 30 minutes or more:
- Use an emulsifier, such as Dijon mustard or mayonnaise, mixed into the vinegar before you start adding the oil. Even with mustard, the oil and vinegar will separate, but will re-emulsify when you mix it again.
- Add the oil to the vinegar in a slow stream, waiting for the oil and vinegar to begin thickening before adding a little more oil.
If you plan to make hollandaise sauce that contains egg yolks in addition to fat, vinegar and water, keep the temperature of the yolk over a double boiler as low as you can. If the yolk gets too hot, the mixture curdles and the butter separates. Remedy the curdling with more whisking or by adding an additional yolk.