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, so a vigorous whisking is used to combine these ingredients until uniform. They may form a temporary suspension which can quickly separate again, or become a semi-permanent or permanent emulsion which will last longer. But no matter how much whisking there is, if there is no emulsifier added, the mixture will not be stable and will separate or break.
Emulsifiers can help make the suspension stable as they keep the oil particles dispersed throughout the liquid. Emulsifiers are particles where one end is attracted to water and the other end is drawn to oil. Or they 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 to combine the liquids 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 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 also important when you are making an emulsion. If the ingredients are too cold or too warm 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
Unfortunately, emulsions can sometimes split or separate if you combine them too quickly. But luckily, there are ways to repair them. In general, you should 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 and then slowly add the broken emulsion; this should rescue the sauce. If you are making mayonnaise and see developing oil on its surface, that means it needs a little more water; 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 in them and so they are likely to separate when standing for any length of time.