In the simplest terms, an emulsion is a stable mixture of two liquids - such as oil and water - that normally separate from each other. Emulsions can be temporary (measured in seconds or minutes), semipermanent (hours), or relatively permanent (days, months, and sometimes years). Mayonnaise is the best known and most widely consumed emulsified sauce. Basically, it is an emulsion comprising oil, egg yolks, and either lemon juice or vinegar. Related sauces include chantilly (mayonnaise mixed with whipped cream), gribiche (a piquant mayonnaise made with hard-boiled yolks), and remoulade (mayonnaise plus chopped pickles, mustard, and other flavoring agents).
Sauces are not the only emulsions. Whole milk, for example, is one, too. If milk fresh from the cow is left to stand, the emulsion breaks down and the cream (butterfat) rises to the top. Homogenization, a process that creates a relatively stable emulsion, prevents this separation.
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