A simple vinaigrette is made by shaking oil and vinegar together, but only makes a temporary emulsion (also called a colloidal suspension) that will separate in time as there is nothing to hold the two immiscible liquids together at the chemical level. On the other hand, mayonnaise is the same emulsion of oil and vinegar, but held together and made creamy with the help of egg yolk as an emulsifier.
The Science Behind the Technique
To create an emulsion, two things are required: an emulsifier and a force. Emulsifiers are the mediators that keep immiscible liquids held together, and emulsifying ingredients have molecules that have hydrophilic and lipophilic ends that attract water and oil respectively. The lipophilic ends attach to oil while the hydrophilic ends form a water-attracting layer around each globule of oil, allowing it to remain suspended in water.
Force is required to break apart the oil and disperse it evenly throughout the water, and is usually applied via whisking or blending. The emulsifier then prevents the oil particles from coming back together again.
Agitation, time and temperature can also make or break an emulsion. That’s why it’s important to add the oil in a very slow, thin stream when whisking together a mayonnaise so that the oil can be agitated enough and incorporated evenly into the vinegar component. And an egg-based emulsion will coagulate at higher temperatures and become lumpy and thin, while too much heat can also separate butterfat from butter and cause a beurre blanc to break.
The most common emulsifying agent is the egg yolk, the humble ingredient that has kept centuries of mayonnaise and Hollandaise sauces deliciously creamy and stable. Egg yolks contain a protein called lecithin, which is commonly extracted from soybean oil. Soy lecithin is an industry-standard additive used to stabilize not only food products that we find on supermarket shelves, but lotions and beauty products as well.
Commercial liquid lecithin or lecithin granules derived from soy or sunflower oil can be purchased at health food stores, larger supermarkets and online, but there are plenty of alternatives to be found in your pantry as well. Mustard, bottled mayonnaise and honey are easily accessible emulsifying agents that can be whisked into vinaigrette for a smooth salad dressing. Lesser known emulsifiers include tomato paste and garlic paste, the latter of which holds Spanish allioli together, creating a rich, creamy emulsion of garlic and oil.
Now that you know the science behind a successful emulsion, grab a whisk and try your hand at making some of these classic mother sauces of French cuisine or explore the technique in these recipes: