You will come across the term emulsify when you are making Béarnaise sauce, hollandaise, mayonnaise, aioli, or salad dressing. These and other sauces are examples of emulsified foods. Emulsifications can be a thick liquid or a creamy semi-solid.
To emulsify means to combine two ingredients together which do not ordinarily mix easily. The ingredients are usually a fat or an oil, like olive oil, and a water-based liquid like broth, vinegar, or water itself. Oil and water do not mix naturally. The force of beating is used to combine these ingredients. They may form a temporary suspension with small droplets of oil in the water (or water droplets in the oil), but this can separate again quickly. Besides the force of beating, you need an emulsifier added to make it stable. Otherwise, it will separate, or break.
Emulsifiers can help make the suspension stable. An emulsifier keeps the oil particles dispersed throughout the liquid. They are particles with one end that is attracted to water and the other end to oil or have a surface area that can encapsulate the dispersed droplets. These can be proteins, diglycerides, monoglycerides, or tiny cell fragments.
Common emulsifiers include egg yolks (in which the protein lecithin is the emulsifier), butter (the protein casein is what makes it work), cheese, mustard, honey, tomato paste, catsup, miso, and garlic paste.
How to Emulsify
The traditional way to make an emulsion is that the liquids are combined very slowly, usually drop by drop, while beating vigorously. This suspends tiny drops of liquid throughout each other. A food processor or blender is an excellent tool for this task. You can also use a whisk or hand beater.
Acidic liquids like lemon juice help the process by changing the pH of the mixture. That's why you'll often find lemon juice or vinegar in recipes where you emulsify liquids.
Temperature is important when you are making an emulsion. If it is too low or too high the emulsion will break and separate.
Watch your emulsion carefully while you are whisking it. If it starts looking curdled, it is probably about to break and you need to take steps to stop the separation.
Fixing Broken Emulsions
These mixtures can sometimes split or separate if you combine them too quickly. If that happens, add a teaspoon of water and whisk the mixture, or blend it in a blender until it becomes smooth again. To fix a broken egg-based emulsion, such as mayonnaise, start with making the sauce again with an egg yolk and water or lemon juice. Then slowly add the broken emulsion and you should be able to rescue it. If you see mayonnaise developing oil on its surface, it needs a little more water, so whisk a spoonful into it.
To fix a broken vinaigrette, whisk it in a bowl or shake it vigorously in a closed container. Then use it immediately. These often have only a small amount of an emulsifier and so they are likely to separate when standing for any length of time.