What Is Emulsification and How Does It Work? Plus How to Fix Broken Emulsions

To emulsify is to force two immiscible liquids to combine in a suspension—substances like oil and water, which cannot dissolve in each other to form a uniform, homogenous solution. Although oil and water can’t mix, we can break oil down into teeny-tiny droplets that can remain suspended in the water. An emulsion happens when small droplets of one solution (the dispersed solution, which is often oil based) are dispersed throughout another (the continuous solution, which is often water based).

Two unlike substances won’t form an emulsion on their own—you need help, in the form of an emulsifier. The emulsifier coats the droplets, keeping them separate from each other, because when left to their own devices, the droplets will clump together, causing the emulsion to separate. Emulsifiers are molecules with a fat-soluble part and a water-soluble part. The fat-loving part sticks to the oil, and the water-soluble part sticks to the water, creating an effective barrier around the droplets. Emulsifiers come in many forms, including milk proteins called casein and the protein lecithin found in egg yolks.

• Milk is an emulsion of milkfat droplets suspended in water. Whole milk contains about 4 percent fat in the form of these droplets. (The word emulsify actually comes from the Latin word for milk.)
• Cream and buttermilk are also natural emulsions.
• Butter is a special case because unlike most natural emulsions, which are oil-in-water emulsions, its continuous phase is fat (80% by volume) with water droplets dispersed throughout, making it a water-in-oil emulsion. (Clarified butter, which has had its water removed, is not an emulsion.)
• Egg yolks, which contain fat suspended in water, are both an emulsion and a highly effective emulsifier, thanks to their high content of lecithin and other emulsifying proteins.

Emulsifying isn’t just useful for getting two unlike substances to hold together—when we take advantage of emulsification in the kitchen, we can create foods with properties unique from their starting ingredients.

Emulsions are especially important in creating thick, creamy sauces. Since oil molecules are larger and move slower than water molecules, when oil molecules are dispersed throughout water, they create a thicker consistency throughout the entire mixture. In addition to the famous emulsified sauces, like hollandaise and mayonnaise, countless other dishes rely on emulsification, including:

• Ice cream made with egg yolks—French- or custard-style ice cream—relies on emulsifying agents found in the yolk to keep the ice crystals very small, yielding a creamy, smooth texture.
• Bouillabaisse is an emulsified French fish soup. Boiling, the finishing step, breaks down the soup's olive oil into droplets, which then get coated with gelatin from the fish parts, giving the soup a creamy consistency.
• In Italian espresso, the high pressure used to force water through the ground coffee beans causes the oil from the beans to emulsify with the water, creating a smooth and creamy texture without any dairy.
• Ganache, made with chocolate and cream, is an emulsion of milk fat and cocoa fat suspended in a solution of sugar (from the chocolate) and water (from the cream).
• Water-in-fat emulsions such as vinaigrette, which usually contains three parts oil to one part vinegar. Vinaigrette is typically a temporary emulsion made by shaking or whisking extra-virgin olive oil and balsamic vinegar together at the last minute and serving immediately; though some vinaigrettes include an egg yolk to emulsify, such as Chef Thomas Keller’s recipe. When using vinaigrette for salads, make sure to dry your greens thoroughly, because water will repel the oily vinaigrette, forcing the sauce to the bottom of the bowl rather than coating the leaves. Learn how to properly wash and dry your salad lettuces with Alice Waters here.
• Hollandaise sauces like béarnaise are emulsified with butter. Beurre blanc is another French sauce made with butter, plus vinegar or white wine.
• Fat-in-water emulsions, such as mayonnaise, which contains 80% fat droplets by volume, suspended in a base of egg yolk, lemon juice, or vinegar, and sometimes mustard, used as a stabilizer.
• Pesto, which contains water from fresh basil leaves and plenty of olive oil, is broken down into a thick, creamy paste by partial emulsion.
• Many sauces and soups are partial emulsions, thickened by butter or cream at the very end of cooking.

• Add an emulsion stabilizer, such as mustard or tomato paste, to create an additional barrier between the oil and water in your emulsion.
• Smaller droplet sizes doesn't just help prevent the sauce from breaking, they also yield a thicker, more flavorful sauce, so use a blender or food processor to break the oil into tiny fat globules, or make sure you, or your stand mixer, is whisking vigorously.
• Add the dispersed phase very slowly, while blending or whisking vigorously, to prevent the oil droplets from sticking together.
• For cooked emulsions, such as hollandaise, don’t get the sauce too hot—heat causes molecules to move faster, increasing the chances that the oil droplets will run into each other and clump together.

Emulsions are unstable by nature, so it’s normal for them to separate or break. There are a few tricks for re-emulsifying a broken sauce:

• Try putting a broken emulsion in the blender, which can break down the dispersed phase into small droplets again.
• In a large bowl, start with a small amount of the continuous phase with an egg yolk and then gradually beat the broken sauce into it.
• In cooked egg emulsions, proteins can easily coagulate (curdle). If this happens, strain out the lumps before attempting to re-emulsify a broken cooked egg sauce.
• For overheated sauces, try adding a small amount of cool water (about a tablespoon) and whisk vigorously.

For broken hollandaise, combine a teaspoon of lemon juice and a tablespoon of the broken sauce. Beat until creamy and then add the rest of the sauce, half a tablespoon at a time, beating until creamy with each addition.

For broken mayonnaise, combine a teaspoon of mustard with a tablespoon of the broken mayonnaise (or one egg yolk plus a little lemon juice), beat until creamy, and then add the rest of the broken mayonnaise, one teaspoon at a time. If mayonnaise becomes oily on the surface, whisk in a tablespoon of water.